Environmental Science and Pollution Research https://doi.org/10.1007/s11356-017-0877-2
RESEARCH ARTICLE
Insight into litter decomposition driven by nutrient demands of symbiosis system through the hypha bridge of arbuscular mycorrhizal fungi Xiangshi Kong 1 & Yanyan Jia 2 & Fuqiang Song 3 & Kai Tian 1 & Hong Lin 1 & Zhanlin Bei 1,4 & Xiuqin Jia 1 & Bei Yao 1 & Peng Guo 5 & Xingjun Tian 1 Received: 23 April 2017 / Accepted: 29 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017
Abstract Arbuscular mycorrhizal fungi (AMF) play an important role in litter decomposition. This study investigated how soil nutrient level affected the process. Results showed that AMF colonization had no significant effect on litter decomposition under normal soil nutrient conditions. However, litter decomposition was accelerated significantly under lower nutrient conditions. Soil microbial biomass in decomposition system was significantly increased. Especially, in moderate lower nutrient treatment (condition of half-normal soil nutrient), litters exhibited the highest decomposition rate, AMF hypha revealed the greatest density, and enzymes (especially nitrate reductase) showed the highest activities as well. Meanwhile, the immobilization of nitrogen (N) in the decomposing litter remarkably decreased. Our results suggested that the roles AMF played in ecosystem were largely affected by soil nutrient levels. At normal soil nutrient level, AMF exhibited limited effects in promoting decomposition. When soil nutrient level decreased, the promoting effect of AMF on litter decomposition began to appear, especially on N mobilization. However, under extremely low nutrient conditions, AMF showed less influence on decomposition and may even compete with decomposer microorganisms for nutrients. Keywords Litter decomposition . Arbuscular mycorrhizal fungi . Extracellular enzymatic activities . Microbial biomass Soil nutrients
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-017-0877-2) contains supplementary material, which is available to authorized users. * Peng Guo
[email protected] * Xingjun Tian
[email protected] 1
School of Life Sciences, Nanjing University, Nanjing 210023, People’s Republic of China
2
Huaiyin Institute of Agricultural Sciences in Xuhuai Area of Jiangsu, Huaian 223001, People’s Republic of China
3
College of Life Sciences, Heilongjiang University, Harbin 150080, People’s Republic of China
4
College of Biological Science and Engineering, Beifang University of Nationalities, Yinchuan 750021, People’s Republic of China
5
Hebei College of Industry and Technology, Shijiazhuang 050091, People’s Republic of China
Introduction Arbuscular mycorrhizal fungi (AMF) form associations with more than 80% of terrestrial plants (Smith and Smith 2011). They improve plant fitness by increasing the uptake of micronutrients, such as nitrogen (N) and phosphorus (P) (Kiers et al. 2011; Tao et al. 2016). Since Hodge et al. (2001) reported that AMF accelerates organic material decomposition and directly acquires N from decomposing litter, the positive effect of AMF on litter decomposition has been increasingly studied (Atul-Nayyar et al. 2009; Cheng and Baumgartner 2006; Leigh et al. 2009). On the other hand, decomposing leaves that were colonized by AMF have been documented (Aristizabal et al. 2004; Posada et al. 2012). Given that AMF does not have the saprophytic capacity (Hodge et al. 2001), research on the mechanism of this promoting effect mainly focuses on the relationship between AMF and other soil microorganisms. Cheng et al. (2012) demonstrated that the exclusion of AMF from soil reduces
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decomposition rate by reducing substrate supply to soil freeliving decomposers. Most recently, Gui et al. (2017) showed that AMF can interact with soil microbial communities and inhibit the development of soil fungal and bacterial groups at the late stage of the litter decomposition (180 days). However, the mechanisms are still unclear. AMF may exhibit stimulatory or inhibitory effects on soil microorganisms (Christensen and Jakobsen 1993; Wamberg et al. 2003) or may intensify microbial competition for nutrients (Cavagnaro et al. 2006). Jannoura et al. (2012) showed that the decomposition of added maize residues is significantly reduced by mycorrhizal peas due to insufficient N supply to soil microorganisms. Inoculation with AMF may inhibit the growth and activities of enzyme-producing saprotrophs through releasing C-rich exudates, which cause other rhizosphere microorganisms to immobilize nutrients (Lindahl et al. 2010). Extracellular enzymes are the ways by which soil microorganisms degrade complex organic compounds and can reflect the magnitude and direction of various soil biochemical processes (Yang and Wang 2001). Enzyme secreting pattern, in which different enzymes are secreted under different conditions, also reflects the resource allocation of the decomposer community (Mooshammer et al. 2014). It is reported that AMF can enhance the activities of soil enzymes, such as βglucosidase (Armada et al. 2016), alkaline phosphatase (Zarea et al. 2011; Zhang et al. 2011), and urease (Zhang et al. 2011). However, few researches investigate the enzyme activities of litter decomposition in the presence of AMF. The role that AMF plays in litter decomposition is complexly affected by hostplantspecies, soilnutrient level,and bacterialand fungal species (e.g., saprotrophs). Kleikamp and Joergensen (2006) reported that the positive effects of AMF on plant nutrient acquisition decrease under conditions of unlimited nutrient availability. On the other hand, van Diepen et al. (2010) showed that with N addition, intra- and extraradical AMF biomasses and total microbial biomass decrease by 36, 41, and 24%, respectively. To get a good colonization status, most researchers culture AMF use soil and quartz sand as media (Hodge et al. 2001; Schroeder-Moreno et al. 2012) and in natural or artificially controlled conditions (Nuccio et al. 2013). However, the Fig. 1 Pot culture and litter decomposition system. Pot culture system (a) and the hypha bridge of arbuscular mycorrhizal fungi (b)
effects of soil nutrients, the level of which greatly affects AMF performance, on AMF-mediated litter decomposition are rarely taken into consideration. Most studies in this field focus on the N transfer role of AMF, from decomposing litters or organic matter to host plants. Nuccio et al. (2013) used 70 days to investigate the effects of AMF on N cycling during litter decomposition. The duration of these studies is relatively short (between 1 and 3 months) (Herman et al. 2012; Leigh et al. 2009). As for nutrient transfer research, short-term experiment is sufficient and has a good representation (Cheng et al. 2012), which, however, may not adequately reflect the actual influence of AMF on litter decomposition. Usually, at the early stage, colonization percentage of AMF increases nearly exponentially and peaks within 2 months; the afterward effects are more important with regard to decomposition. As decompositions of litters in tropical forests (Makkonen et al. 2012), temperate zoon, and boreal area (Handa et al. 2014) all exceed a year, longer-term study is required to obtain a clear view of AMF effects on litter decomposition. In the current work, litters of Quercus variabilis, a dominant plant in Zijin mountain, were selected. Plantago asiatica L., which is widely found in Zijin mountain (Nanjing, China), was selected as the host plant. P. asiatica is a perennial flowering plant of genus Plantago that belongs to the Plantaginaceae family (Choi et al. 2008). Rhizophagus irregularis has a good colonization on the genus Plantago (Hodge et al. 2001). It is reported that P. asiatica could be colonized by AMF (Li et al. 2005). In our preliminary experiment, we found the good colonization of P. asiatica by R. irregularis. Thus, as a commonly used AMF species for decomposition study (Hodge et al. 2001; Nuccio et al. 2013), R. irregularis was selected. By using the nested pot method (Fig. 1), we set different soil nutrient levels for the host plants to test the effect of nutrient demand of symbiosis system (outer pot) on litter decomposition (inner pot) through AMF. We hypothesize the following: (1) Litter decomposition will be accelerated when AMF is involved; (2) the lower soil nutrient level in the symbiosis system, the faster litter will decompose, as the demand of the symbiosis system for nutrients Decomposition system
Symbiosis system
Litter soil
Plant soil
A
B
AMF hypha bridge
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intensifies; (3) enzymatic activities involved in N cycling will be enhanced after AMF inoculation, especially when the symbiosis system is under lower soil nutrient conditions.
Methods Experiment design In October 2013, fresh fallen leaves of Q. variabilis were collected from the forest floor of Zijin mountain (32° 4′ 7.81″ N, 118° 51′ 45.28″ E), which is located in Nanjing, China. The top 5-cm forest soil (slightly acidic Humic Cambisol with a pH of approximately 5.0) was collected for the AMF-plant system and litter decomposition. The initial chemical characteristics of Q. variabilis leaves and soil were determined (Tables 1 and S2). The leaves were oven dried at 60 °C for 1 week to achieve a constant weight and were cut into 0.5-cm2 fragments. R. irregularis BGC JEB07D (synonym, Glomus intraradices) was obtained from the Bank of Glomeromycota in China (BGC) in Beijing Academy of Agriculture and Forestry, China. Seeds of P. asiatica were scarified with 20–30 °C warm water and surface-sterilized with 0.3% K2MnO4 for 3–4 h. Then, the seeds were covered with four layers of moist gauze for germination, which was conducted at 20–30 °C for 60 h in an incubator. On April 1, 2014, the germinated seedlings were pre-cultured in sterile sands and watered with Hoagland solution. After 1 month, seedlings with similar height were selected and transplanted to the experimental pots, with six plants each pot. The nested pot system contained two compartments: AMFplant symbiosis compartment (or symbiosis system, outer pot) and litter decomposition compartment (or decomposition system, inner pot, Fig. 1). The plants were grown in the outer pots (160-mm top diameter, 110-mm bottom diameter, and 140-mm height) with or without AMF inoculation under different soil nutrient levels (plant soil): (1) forest soil without R. irregularis inoculation (AMF−4:0), negative control; (2) forest soil with R. irregularis inoculation (AMF+4:0), positive control; (3) 3:1 (v/v) of forest soil/sand and with R. irregularis inoculation (AMF+3:1); (4) 2:2 (v/v) of forest soil/sand and with R. irregularis inoculation (AMF+2:2); and (5) 1:3 (v/v) of forest soil/sand and with R. irregularis inoculation
(AMF+1:3). All culture media were autoclaved at 121 °C and 0.1 MPa for 1.5 h prior to transplantation. In the inoculated treatments, each pot was inoculated with 50-g inoculum, which consists of thoroughly mixed rhizosphere samples containing spores (80–100 spores per 10 g of inoculum), hyphae, and mycorrhizal root fragments. In the AMF−4:0 treatment, each pot received 50-g autoclaved inoculum (121 °C, 0.1 MPa for 1.5 h), plus 50 mL inoculum filtrate (< 20 μm), which was used to provide a general microbial population that was free of AMF propagules. The bottom of each pot had a tray to prevent exogenous microbial infection. The inner pot (60-mm top diameter, 50-mm bottom diameter, and 70-mm height) was set for decomposition of Q. variabilis leaf litters. Both the inside and outside of the wall and the bottom of the inner pots were enveloped with a 20-μm nylon mesh to allow the passing of AMF hypha but prevent the root and plant soil. A 2-mm atmospherical interlayer between the two layer meshes was set to minimize the effects of soil nutrient diffusion. Each inner pot received 60-g forest soil (litter soil) and 1-g Q. variabilis leaf litter fragments. Leaf litters were buried 1 cm in the litter soil to give a good contact with R. irregularis hypha. Each treatment had four replicates, and a total of 140 pots were randomly arranged in a greenhouse (16-h photoperiod, 25/18 °C day/night temperature, and 60% relative humidity). Deionized water was sprayed to the outer pots three times a week. Plants, litters, and the litter soil were harvested every month. Dry mass determination, mycorrhizal analysis, and enzymatic assay were conducted every month.
Dry mass determination, mycorrhizal analysis, and enzymatic assay When harvested, leaf litters and host plants were cleaned with deionized water and then oven-dried to constant weight under 60 °C; the dry mass was then determined. Roots were washed free of soil, cut into 1-cm fragments, cleared with 10% KOH in 90 °C water bath, and stained in acid fuchsin by the method of Phillips and Hayman (1970). The percentage of colonized root was calculated by gridline intersection method (Mcgonigle et al. 1990). Leaf litters were also checked for AMF colonization after the 7-month decomposition. R. irregularis extraradical hypha in litter soil was extracted
Table 1 N contents (%) of the initial and finial Q. variabilis leaf litters, litter soil, and the shoot and the root of P. asiatica. Values are mean and standard error (± SE, n = 4). Different lowercase letters in the same row indicate significant difference at the 0.05 level
Leaf litter Litter soil Shoot Root
Initial
AMF−4:0
AMF+4:0
AMF+3:1
AMF+2:2
AMF+1:3
0.75 ± 0.14c 0.22 ± 0.11c – –
1.61 ± 0.05a 0.29 ± 0.01a 1.13 ± 0.07ab 0.71 ± 0.04c
1.48 ± 0.06ab 0.26 ± 0.03ab 1.34 ± 0.06a 0.76 ± 0.03bc
1.29 ± 0.14b 0.27 ± 0.01ab 1.15 ± 0.11ab 0.78 ± 0.02abc
1.23 ± 0.10b 0.25 ± 0.02b 1.22 ± 0.09ab 0.85 ± 0.01a
1.33 ± 0.06ab 0.28 ± 0.01ab 1.04 ± 0.05b 0.82 ± 0.02ab
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100
Mass loss (%) a ab
0.08
bc
80
cd
d
40
0.02
20
0.00
0 +
F AM
AM
F
+
1 +
3:
F AM
AM
F
+
0 -
1: 3
0.04
2: 2
60
4: 0
0.06
4:
At the end of the 7-month decomposition, litters decomposed approximately 40%. Leaf litters decomposed faster with R. irregularis inoculation than the control (P < 0.05). Litters in AMF+4:0 (k = 0.0636) decomposed slightly faster than those in AMF−4:0 (k = 0.0619), whereas in inoculation and nutrient treatments, litter decomposition in AMF+3:1 (39% mass loss; k = 0.0728) and AMF+1:3 (38% mass loss; k = 0.0702) were significantly faster than in AMF + 4:0 (P < 0.05). Especially, the leaf litters of AMF + 2:2 decomposed (42% mass loss; k = 0.0783) 17.5% faster than
k value
Mass loss (%)
Results
0.10
F
The decomposition coefficient (k, month−1) was determined by fitting a negative exponential model following Olson (1963): k = − ln(x0/xt)/t, where k is the litter decomposition coefficient (month−1), x0 is the initial dry mass of leaf litter, xt is the mass remaining at time t, and t is the sampling time in month. Data were checked for deviations from normality and homogeneity of variance prior to analysis. Analysis of variance and post hoc Tukey’s honestly significant difference test were applied to assess the significant difference among the various treatments. All statistical analyses were performed using SPSS (version 17.0). Graphs were drawn by GraphPad Prism 6. To evaluate and visualize the relationship between enzymatic activities and each treatment plus the decompositioncoefficient, principalcomponentanalysis (PCA), an indirect gradient analysis, was carried out using CANOCO (version 5.0 for Windows).
AM
Data analyses
AMF +4:0 (36% mass loss, k = 0.0636) and 19.9% faster than AMF−4:0 (35% mass loss, k = 0.0619) (Figs. 2 and S4). In all cases, P. asiatica roots were successfully prevented from entering the litter compartments. Significant difference (P < 0.05) in plant colonization was observed between AMF + 2:2 (37% of colonization percentage) and other R. irregularis treatments (around 10%) at the first month. Colonization percentage increased dramatically during the first 3 months and reached the peak at the fourth month (between 80 and 90%), which showed no significance among the inoculation treatments (P > 0.05). All of the AMF treatments exhibited a good colonization of R. irregularis on P. asiatica roots (Fig. S1); no colonization was observed in AMF−4:0 plants. Meanwhile, after the 7-month decomposition, the decomposing leaves had been colonized by R. irregularis and spores with high densification were clearly observed (Fig. S2). AMF hyphae in the litter soil at the first month revealed extremely low density (0.2 m hyphae g−1 dry soil) and did not show significant difference among treatments (P > 0.05). At the fourth month, obvious differences emerged among the treatments; the AMF+2:2 had a higher (P < 0.05) hyphal length density than other treatments, especially for AMF+4:0 and AMF+1:3. At the final month, AMF hyphal length density in AMF+2:2 (2.15 m hyphae g−1 dry soil) was significantly (P < 0.05) higher than other treatments, whereas it was nearly the same in AMF + 4:0 and AMF + 3:1. Meanwhile, AMF hyphal length density in AMF+1:3 was the lowest throughout the decomposition period (Fig. 3). After the 7-month decomposition, litter N contents in the inoculated treatments (AMF+4:0, AMF+3:1, AMF+2:2, and AMF+1:3) were lower than those of AMF−4:0 (1.48%), especially for AMF+3:1 and AMF+2:2, which was significant (P < 0.05). Nevertheless, litter N contents in all treatments were enriched, from an initial 0.75 to 1.23% (AMF+2:2, Table 1). With AMF inoculation, litter N content were
k values (month -1)
using a modified membrane filter technique (Hanssen et al. 1974), stained with acid fuchsin, and checked at × 200 magnification with at least 60 fields using the gridline intercept method. AMF hyphal length density (m hyphae g−1 soil dry weight) was calculated. The litter, litter soil, and host plant were harvested every month. Total organic carbon (C) and N contents at the finial month were determined with elemental analyzer (Elemental Vario MICRO, Germany). Soil microbial biomass was estimated via substrate-induced respiration method (Beare et al. 1990). Briefly, the soil (1 g) was incubated with glucose (10 mg of glucose g−1 soil dry weight) at 25 °C for 1 h; CO2 production was measured with an infrared gas analyzer twice between 4 and 6 h after glucose addition (Bailey et al. 2002). Enzymatic activities of litter soil involved in the cycling of C (cellobiohydrolase, β-glucosidase, β-xylosidase, phenol oxidase, and peroxidase), N (nitrate reductase and urease), and P (acid phosphatase and alkaline phosphatase) were determined spectrophotometrically (Table S1). Litter soil samples were stored under 4 °C, and enzymatic activities were assayed within 1 week after sampling.
Fig. 2 The decomposition coefficient (k value) and mass loss of Q. variabilis leaf litters under different treatments during the 7-month decomposition. Values are mean and standard error (± SE, n = 4, P < 0.05). Different lowercase letters indicate significant difference among the treatments
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+
AMF 3:1
1.5
b
+
AMF 2:2 +
1.0
AMF 1:3
a ab b
b
0.5 0.0
1
4 Time (month)
7
Fig. 3 AMF hyphal length density in the litter soil over 7 months under different treatments. Values are mean and standard error (± SE, n = 4). Different lowercase letters indicate significant difference at the 0.05 level
enriched 1.97, 1.72, 1.64, and 1.77 times in AMF+4:0, AMF+3:1, AMF+2:2, and AMF+1:3, respectively. Especially, litters in AMF−4:0 were enriched with the highest N, which was 2.15 times that of the initial N content (Table 1). In addition, N content in the litter soil also increased (initial N content was 0.22%). The litter soil N content in AMF−4:0 was the highest, followed by AMF+1:3, AMF+3:1, AMF+1:0, and AMF+1:1. AMF+2:2 was enriched with the lowest N not only in leaf litter, but also in its litter soil. No significant difference in nutrient contents (C, N, and C:N) in litter soil was observed between the full soil (AMF−4:0 and AMF+4:0) and the nutrient-reduced soil (AMF+3:1, AMF+2:2, and AMF+1:3) treatments (P > 0.05, Tables 1 and S2). The shoots of the plants in AMF+4:0 had the highest N content (1.34%). However, the N contents of the roots in all inoculated treatments, except AMF+4:0, were significantly higher (P < 0.05) than those in AMF−4:0 (0.71%), especially under the lowest nutrient condition (AMF+2:2 and AMF+1:3, Table 1). Carbon contents of decomposing litters were significantly higher (P < 0.05) in inoculated treatments than those in AMF−4:0; C content was significantly higher than the initial C content in litter soil (P < 0.05, Table S2). During the 7-month decomposition, the average microbial biomass (SIR) between AMF+4:0 and AMF−4:0 did not show significant difference (P > 0.05). However, when host plants grew under lower nutrient conditions and with AMF inoculation (AMF+3:1, AMF+2:2, and AMF+1:3), the microbial biomass of the litter soil was significantly increased (P < 0.05, Figs. 4 and S4). Nitrogen cycling-related enzymatic activities of litter soil in AMF-inoculated treatments were higher (P < 0.05) than those of AMF−4:0, especially when the symbiosis system was under low nutrient conditions (AMF+3:1, AMF+2:2, and AMF+1:3). With AMF inoculation, urease activity in litter soil was significantly higher (P < 0.05) than that of AMF−4:0 (Figs. 5a and S4). The activities of nitrate reductase, which was more active, in AMF + 3:1, AMF + 2:2, and AMF + 1:3 were
b
c
c
AMF-4:0
AMF+4:0
60
a
a
AMF+2:2
AMF+1:3
40
2
0 AMF+3:1
Fig. 4 The mean substrate-induced respiration (SIR) of litter soil over 7 months under different treatments. Values are mean and standard error (± SE, n = 28). Different lowercase letters indicate significant difference at the 0.05 level
significantly (P < 0.05) higher than those of AMF+4:0 and AMF −4:0, whereas AMF+2:2 exhibited the greatest values (P < 0.05, Figs. 5b and S4). Alkaline phosphatase activity did not show significant difference (P > 0.05), while acid phosphatase activity in low soil nutrient conditions (AMF+3:1, AMF+2:2, and AMF+1:3) increased with AMF inoculation, especially for AMF+2:2, which was significant
Nitrate reductase activity (1 g NO2- released min-1g-1 soil)
Hyphal length density (m hyphae g-1 soil)
AMF 4:0
SIR ( l CO2 h-1 g-1 soil)
ab
+
2.0
80
a
-
AMF 4:0
15
a
A c
10
bc
b
b
5
0
Urease Activity (1 m g NH3 released h-1g-1 soil)
2.5
1.0
AMF-4:0 AMF+4:0 AMF+3:1 AMF+2:2 AMF+1:3
B b
ab
a
a
a
0.5
0.0
AMF-4:0 AMF+4:0 AMF+3:1 AMF+2:2 AMF+1:3
Fig. 5 The mean activities of a nitrate reductase and b urease during the decomposition of Q. variabilis leaf litters over 7 months under different treatments. Values are mean and standard error (± SE, n = 28, P < 0.05). Different lowercase letters indicate significant difference among the treatments at the 0.05 level
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(P < 0.05, Figs. S3A, B and S4). Enzymatic activities involved in C cycling, such as β-glucosidase and β-1,4-xylosidase (data not shown), did not show significant difference among the nutrient treatments (P > 0.05), except for cellobiohydrolase in AMF+2:2, which was significantly higher (P < 0.05) than that in AMF+4:0 and slightly higher than that of other treatments (Figs. S3C, D and S4). Furthermore, the lignin degradationrelated enzymatic activities (peroxidase and phenol oxidase) were downregulated in AMF+4:0, which were significantly lower than those in AMF+3:1 (P < 0.05, Figs S3E, F and S4). In PCA, the first two axes explained 67.83 and 23.41% of the variation (Fig. 6). PCA ordination characterized and partly separated the treatments. The first axis mainly isolated the Blow-nutrient treatment^ (AMF + 3:1, AMF + 2:2 and AMF+1:3) from the Bnormal nutrient-treatment^ (AMF+4:0 and AMF-4:0). Most of the enzymatic activities and k values were highly correlated with PC1. Low-nutrient treatments led to higher enzymatic activities than normal-nutrient treatments; especially, N-cycling-related enzymatic activities (nitrate reductase and urease) displayed the same. In addition, k displayed consistency with most enzymatic activities.
Discussion
1.0
The presence of plants accelerates soil organic matter mineralization by stimulating soil microbial activity, and this
BX
ALP
AMF+4:0
URE
AMF+2:2 AMF+3:1 AMF+1:3
NR
BG
k
ACP POD
-0.6
AMF-4:0
CBH1 PO
-0.6
1.0
Fig. 6 Principal component analysis (PCA) ordination of enzymatic activities (arrows), decomposition coefficient (k), and the five treatments (triangle: AMF+4:0, AMF−4:0, AMF+1:3, AMF+3:1, and AMF+2:2). The traits were centered and standardized prior to ordination. BX βxylosidase, ALP alkaline phosphatase, URE urease, NR nitrate reductase, ACP acid phosphatase, BG β-glucosidase, POD peroxidase, CBH1 cellobiohydrolase, PO phenol oxidase
phenomenon is known as the rhizosphere-priming effect (Cheng 2009; Cheng et al. 2003). This effect can be induced by several mechanisms including root exudates, root litters (Haichar et al. 2008), and AMF (Kaiser et al. 2015). In the current study, no obvious priming effect was observed under normal nutrient conditions. Similarly, Shahzad et al. (2015) did not observe the rhizosphere-priming effect of AMF on Poa trivialis and Trifolium repens. Nottingham et al. (2013) reported no increase in leaf litter mass loss by the presence of AMF hypha only. However, when AMF symbiosis lived under low nutrient conditions, litter decomposition rates were significantly accelerated. No significant differences in litter soil nutrients between the full and reduced soils were observed, suggesting that our designed decomposition environment did not change with the nutrient level of plant soil. The mesh successfully prevented the effects from roots and plant soil. Therefore, the effect was likely driven through AMF by the host plants, which were induced by nutrient depletion. In addition, organic C content in the decomposing leaf litters increased significantly with the presence of AMF, implying that the mycorrhizal-priming effects may exist. In the decomposing leaf litters, R. irregularis grew well and had highly dense spores, which implied that AMF in the leaf litters could obtain sufficient carbohydrates from the host plants. Surprisingly, the treatment under moderately low-nutrient condition (half of the normal soil nutrient) had the highest decomposition rate. Meanwhile, AMF hyphal length density in this treatment was the highest along the decomposition periods. In order to simulate the nutrient heterogeneity in natural soil, we used different soil-to-sand ratios. This way may alter the soil aeration and water-holding capacity of the plant soil, thereby affecting AMF growth. However, the AMF hyphae in litter soil of all treatments were under the same soil conditions which showed little influence by plant soil. In the lowest-nutrient treatment, extraradical hypha content was the lowest. It could be attributed to the limited C allocation by host plants due to less nutrients absorbed from the soil in this treatment. Leifheit et al. (2015) reported that AMF directly (via localized nutrient removal or altered moisture conditions) or indirectly (by providing an alternative C source) inhibits the activity of decomposers and significantly reduces the decomposition of wooden sticks. In the current work, we detected the downregulation of peroxidase and phenol oxidase activities when AMF symbiosis was under normal nutrient conditions. At the early decomposition stage, litters contained a large amount of labile C and N. N enriched in the leaf litters indicated that the growth of decomposer microorganisms was not likely N-limited. However, with the progress of decomposition, N might be transferred from the leaf litter to the surrounding environments. AMF might become the competitor for N with other soil microorganisms. In this stage, the decomposition process would be limited by N. In Leifheit et al. (2015) study, N content in wood sticks was only 0.1%, which had become the limiting factor for stick decomposition. The great
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absorption capacity of AMF on N would enhance the limitation. As a result, at the later stage of litter decomposition, the effect of AMF on decomposition may turn to inhibition from promotion. In the present work, urease activity increased after AMF inoculation. Nitrate reductase activities showed the same trend and revealed the highest values in moderately low-nutrient condition. It meant that AMF revealed the greatest promotion effect on N mineralization in this condition. Hodge et al. (2001) demonstrated that AMF colonizes leaf litter mainly for N, which was released through microbial mineralization. Given that, up to now, no report documented that AMF has the saprophytic capacity and the genome of AMF lacks many genes that encode carbohydrate-active enzymes involved in degradation of plant cell walls (Tisserant et al. 2013). However, in soil, many microorganisms produce urease, such as the genus Rhizobium (Humphry et al. 2007; Valverde et al. 2006), Streptomyces (Ikeda et al. 2003), and Pseudomonas (Karmali et al. 2004), and nitrate reductase, such as the genus Streptomycetales (Huang et al. 2017) and many species belonging to ascomycetes (Jargeat et al. 2000; Stolz and Basu 2002). These microorganisms may directly or indirectly be involved in litter decomposition or may exist just as a resource receiver. The upregulated activities of urease and nitrate reductase may be due to the stimulation on these microorganisms by AMF. In the current study, after AMF inoculation, N content in host plants and soil enzymatic activities related to N cycling increased; by contrast, N content in decomposing litters decreased. N is absorbed by AMF mainly in the form of ammonium (NH4+) and/or nitrate (NO3−), in particular NH4+ (Cheng et al. 2012; Gachomo et al. 2009; Govindarajulu et al. 2005; Johansen et al. 1996). Fellbaum et al. (2012) reported the downregulation of a fungal NO3− transporter in response to the increased C supply. After being absorbed into the hyphae, N was transported as glutamine and arginine (Jin et al. 2005; Smith and Smith 2011). NO3− can be reduced into amine in hyphae and incorporated into amino acids, which will consume C obtained from the host plants. In our experiment, when host plants grew under low-nutrient soil condition and with AMF inoculation, nitrate reductase activity increased dramatically (especially in moderately low-nutrient soil), indicating that NO3− might have been reduced to NH4+ by soil m i cro org ani s m s bef or e b ei ng abso r be d by A M F (Govindarajulu et al. 2005), as NH4+ uptake should be less energetically expensive for the AMF (Hawkins and George 2001; Hodge and Storer 2015). The nutrient level of plant soil significantly affected the impact of AMF on litter soil microbial biomass. After providing adequate nutrients, the presence of AMF cannot improve soil microbial biomass. However, under low-nutrient conditions, but not too low, host plants provide a large amount of photosynthates to litter-decomposing microorganisms via AM hyphae, which could nourish more microorganisms (Toljander et al. 2007). Mycelial exudates containing
carbohydrates can increase bacterial growth and vitality (Toljander et al. 2007). Significant C can flow from AMF to the soil microbial communities (Nottingham et al. 2013). Rapid turnover of AMF extraradical hypha can also provide a large quantity of labile C (Staddon et al. 2003). During the decomposition process, because of the growth and reproduction of microorganisms and the degradation of other components (labile C), N was enriched (immobilization) in the decomposing leaf litters, which was a global-scale phenomenon (Parton et al. 2007). Compared with the control, we found the inoculation of AMF weakens the extent of this immobilization, especially under moderately low-nutrient condition. Litters were decomposed by microorganisms, and nutrients were released or transferred by fungal hyphae to the surrounding soil (Frey et al. 2003), leading to the increase in soil nutrient contents and decrease in litter nutrient immobilization. In our experiment, N contents in all the litter soils were significantly increased. It was impossible due to N diffusing from the mycorrhiza system. We speculated that at the early decomposition stage, both N immobilization and N release occurred. In addition, we found that the magnitude of increment in N content of the litter soil was smaller after AMF colonization. Under moderately low-nutrient condition, the increment was the smallest. On the contrary, N contents increased slightly in shoots and significantly in roots after AMF colonization, especially in roots under moderately low-nutrient condition. All these results suggested that both litter N and litter-associated soil N were transported to the host plants through AMF extraradical hypha. However, after AMF inoculation, N contents of plants did not significantly decrease under extremely poor soil condition. This indicated that N from decomposing leaf litters and its surrounding soil could compensate for the nutrient deficiency of plant soil. However, this compensation effects were limited because AMF did not restore the N level of host plants in low-nutrient condition tothat level ofnormal nutrient condition.DespitehighN content in plant roots, low N content in shoots was observed. High root N level might be attributed to the high intraradical hypha content, particularly under low-nutrient conditions. Extraradical hyphae of AMF absorb nutrients from decomposing litters and their surrounding soil. Then, they transfer these nutrients to their host plants through intraradical hypha to alleviate the soil nutrient limitation on plant growth (Avio et al. 2006; Read and Perez-Moreno 2003). The lower the soil nutrient level, the higher the dependency of the host plants on mycorrhizal pathway to scavenge nutrients (Lambers et al. 2008; Smith and Smith 2011). However, how much contribution of plant nutrient demands from this dependence and the costs of the investment are not clear, which involve the balance between cost and resource allocation. In the current work, we found the lowest aboveground N content of plants and the lowest density of hyphal length in litter soil in the poorest-nutrient treatment. The soil N of plant soil might be too low to support the normal growth of mycorrhizae.
Environ Sci Pollut Res
Conclusions The effects of AMF on their environment are dependent on host plants, and AMF-mediated litter decomposition is dependent on host requirement on soil nutrients. Through regulating the activities of litter decomposition-related enzymes (e.g., nitrate reductase) and proliferation of AMF extraradical hypha, AMF promotes the decomposition of leaf litters. Simultaneously, AMF enhances the absorption and transfer of nutrients, especially for N. Nutrient deficiency could strengthen these processes, especially under moderately lowsoil-nutrient conditions. AMF weakens the extent of nutrient immobilization by accelerating nutrient transfer. Absorbed N from decomposing leaf litter can limitedly compensate for the nutrient deficiency when plants grow in low-nutrient soil. As nutrient deficiency promotes AMF-mediated decomposition of leaf litter, nutrient enrichment (for example, N deposition) would suppress this effect, which needs investigation. Our data provide a theoretical basis and instruction for future green agriculture recycling agrowaste as fertilizer in improving N use efficiency in crop production. Funding information This study was financially supported by the National Key Research and Development Program of the Ministry of Science and Technology of China (No. 2016YFD0600204); the State Key Program of National Natural Science Foundation of China (No. 31530007); the Sanxin Forestry Project in Jiangsu Province (No. LYSX[2016]46); the specimen platform of China and the teaching specimens sub-platform (2005DKA21403-JK); the Natural Science Foundation of Hebei Province, China (No. C2016417004); and the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2012ZX07204-004-003).
References Aristizabal C, Rivera EL, Janos DP (2004) Arbuscular mycorrhizal fungi colonize decomposing leaves of Myrica parvifolia, M-pubescens and Paepalanthus sp. Mycorrhiza 14:221–228. https://doi.org/10. 1007/s00572-003-0259-0 Armada E, Lopez-Castillo O, Roldan A, Azcon R (2016) Potential of mycorrhizal inocula to improve growth, nutrition and enzymatic activities in Retama sphaerocarpa compared with chemical fertilization under drought conditions. J Soil Sci Plant Nut 16:380–399. https://doi.org/10.4067/S0718-95162016005000035 Atul-Nayyar A, Hamel C, Hanson K, Germida J (2009) The arbuscular mycorrhizal symbiosis links N mineralization to plant demand. Mycorrhiza 19:239–246. https://doi.org/10. 1007/s00572-008-0215-0 Avio L, Pellegrino E, Bonari E, Giovannetti M (2006) Functional diversity of arbuscular mycorrhizal fungal isolates in relation to extraradical mycelial networks. New Phytol 172:347–357. https:// doi.org/10.1111/j.1469-8137.2006.01839.x Bailey VL, Peacock AD, Smith JL, Bolton H (2002) Relationships between soil microbial biomass determined by chloroform fumigationextraction, substrate-induced respiration, and phospholipid fatty acid analysis. Soil Biol Biochem 34:1385–1389. https://doi.org/10. 1016/S0038-0717(02)00070-6
Beare MH, Neely CL, Coleman DC, Hargrove WL (1990) A substrateinduced respiration (Sir) method for measurement of fungal and bacterial biomass on plant residues. Soil Biol Biochem 22:585– 594. https://doi.org/10.1016/0038-0717(90)90002-H Cavagnaro TR, Jackson LE, Six J, Ferris H, Goyal S, Asami D, Scow KM (2006) Arbuscular mycorrhizas, microbial communities, nutrient availability, and soil aggregates in organic tomato production. Plant Soil 282: 209–225. https://doi.org/10.1007/s11104-005-5847-7 Cheng L, Booker FL, Tu C, Burkey KO, Zhou LS, Shew HD, Rufty TW, Hu SJ (2012) Arbuscular mycorrhizal fungi increase organic carbon decomposition under elevated CO2. Science 337:1084–1087 Cheng WX (2009) Rhizosphere priming effect: its functional relationships with microbial turnover, evapotranspiration, and C-N budgets. Soil Biol Biochem 41:1795–1801. https://doi.org/10.1016/j.soilbio. 2008.04.018 Cheng WX, Johnson DW, Fu SL (2003) Rhizosphere effects on decomposition: controls of plant species, phenology, and fertilization. Soil Sci Soc Am J 67:1418–1427. https://doi.org/10.2136/sssaj2003.1418 Cheng XM, Baumgartner K (2006) Effects of mycorrhizal roots and extraradical hyphae on N-15 uptake from vineyard cover crop litter and the soil microbial community. Soil Biol Biochem 38:2665– 2675. https://doi.org/10.1016/j.soilbio.2006.03.023 Choi SY, Jung SH, Lee HS, Park KW, Yun BS, Lee KW (2008) Glycation inhibitory activity and the identification of an active compound in Plantago asiatica extract. Phytother Res 22:323–329. https://doi. org/10.1002/ptr.2316 Christensen H, Jakobsen I (1993) Reduction of bacterial-growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis-Sativus L). Biol Fertil Soils 15:253–258. https:// doi.org/10.1007/BF00337209 Fellbaum CR, Gachomo EW, Beesetty Y, Choudhari S, Strahan GD, Pfeffer PE, Kiers ET, Bucking H (2012) Carbon availability triggers fungal nitrogen uptake and transport in arbuscular mycorrhizal symbiosis. P Natl Acad Sci USA 109:2666–2671. https://doi.org/10. 1073/pnas.1118650109 Frey SD, Six J, Elliott ET (2003) Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface. Soil Biol Biochem 35:1001–1004. https://doi.org/10.1016/S0038-0717(03) 00155-X Gachomo E, Allen JW, Pfeffer PE, Govindarajulu M, Douds DD, Jin HR, Nagahashi G, Lammers PJ, Shachar-Hill Y, Bucking H (2009) Germinating spores of Glomus intraradices can use internal and exogenous nitrogen sources for de novo biosynthesis of amino acids. New Phytol 184:399–411. https://doi.org/10.1111/j.14698137.2009.02968.x Govindarajulu M, Pfeffer PE, Jin HR, Abubaker J, Douds DD, Allen JW, Bucking H, Lammers PJ, Shachar-Hill Y (2005) Nitrogen transfer in the arbuscular mycorrhizal symbiosis. Nature 435:819–823. https:// doi.org/10.1038/nature03610 Gui H, Hyde K, Xu JC, Mortimer P (2017) Arbuscular mycorrhiza enhance the rate of litter decomposition while inhibiting soil microbial community development. Sci Rep-Uk 7. https://doi.org/10.1038/ srep42184 Haichar FE, Marol C, Berge O, Rangel-Castro JI, Prosser JI, Balesdent J, Heulin T, Achouak W (2008) Plant host habitat and root exudates shape soil bacterial community structure. Isme J 2:1221–1230. https://doi.org/10.1038/ismej.2008.80 Handa IT, Aerts R, Berendse F, Berg MP, Bruder A, Butenschoen O, Chauvet E, Gessner MO, Jabiol J, Makkonen M, McKie BG, Malmqvist B, Peeters ETHM, Scheu S, Schmid B, van Ruijven J, Vos VCA, Hattenschwiler S (2014) Consequences of biodiversity loss for litter decomposition across biomes. Nature 509(7499):218– 221. https://doi.org/10.1038/nature13247 Hanssen JF, Thingstad TF, Goksoyr J (1974) Evaluation of hyphal lengths and fungal biomass in soil by a membrane-filter technique. Oikos 25:102–107. https://doi.org/10.2307/3543552
Environ Sci Pollut Res Hawkins H-J, George E (2001) Reduced15N-nitrogen transport through arbuscular mycorrhizal hyphae to Triticum aestivum L. supplied with ammonium vs. nitrate nutrition. Ann Bot 87:303–311. https:// doi.org/10.1006/anbo.2000.1305 Herman DJ, Firestone MK, Nuccio E, Hodge A (2012) Interactions between an arbuscular mycorrhizal fungus and a soil microbial community mediating litter decomposition. FEMS Microbiol Ecol 80: 236–247. https://doi.org/10.1111/j.1574-6941.2011.01292.x Hodge A, Campbell CD, Fitter AH (2001) An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material. Nature 413:297–299. https://doi.org/10. 1038/35095041 Hodge A, Storer K (2015) Arbuscular mycorrhiza and nitrogen: implications for individual plants through to ecosystems. Plant Soil 386:1– 19. https://doi.org/10.1007/s11104-014-2162-1 Huang M-J, Rao MPN, Salam N, Xiao M, Huang H-Q, Li W-J (2017) Allostreptomyces psammosilenae gen. nov., sp. nov., an endophytic actinobacterium isolated from the roots of Psammosilene tunicoides and emended description of the family Streptomycetaceae [Waksman and Henrici (1943) AL] emend. Rainey et al. 1997, emend. Kim et al. 2003, emend. Zhi et al. 2009. Int J Syst Evol Microbiol 67:288–293. https://doi.org/10.1099/ijsem.0.001617 Humphry DR, Andrews M, Santos SR, James EK, Vinogradova LV, Perin L, Reis VM, Cummings SP (2007) Phylogenetic assignment and mechanism of action of a crop growth promoting Rhizobium radiobacter strain used as a biofertiliser on graminaceous crops in Russia. Antonie Van Leeuwenhoek 91:105–113. https://doi.org/10. 1007/s10482-006-9100-z Ikeda H, Ishikawa J, Hanamoto A, Shinose M, Kikuchi H, Shiba T, Sakaki Y, Hattori M, Ōmura S (2003) Complete genome sequence and comparative analysis of the industrial microorganism Streptomyces avermitilis. Nat Biotechnol 21:526–531 Jannoura R, Kleikamp B, Dyckmans J, Joergensen RG (2012) Impact of pea growth and arbuscular mycorrhizal fungi on the decomposition of 15N-labeled maize residues. Biol Fertil Soils 48:547–560. https:// doi.org/10.1007/s00374-011-0647-0 Jargeat P, Gay G, Debaud J-C, Marmeisse R (2000) Transcription of a nitrate reductase gene isolated from the symbiotic basidiomycete fungus Hebeloma cylindrosporum does not require induction by nitrate. Mol Gen Genet 263:948–956. https://doi.org/10.1007/ PL00008695 Jin H, Pfeffer PE, Douds DD, Piotrowski E, Lammers PJ, Shachar-Hill Y (2005) The uptake, metabolism, transport and transfer of nitrogen in an arbuscular mycorrhizal symbiosis. New Phytol 168:687–696. https://doi.org/10.1111/j.1469-8137.2005.01536.x Johansen A, Finlay RD, Olsson PA (1996) Nitrogen metabolism of external hyphae of the arbuscular mycorrhizal fungus Glomus intraradices. New Phytol 133:705–712. https://doi.org/10.1093/ jxb/erw383 Kaiser C, Kilburn MR, Clode PL, Fuchslueger L, Koranda M, Cliff JB, Solaiman ZM, Murphy DV (2015) Exploring the transfer of recent plant photosynthates to soil microbes: mycorrhizal pathway vs direct root exudation. New Phytol 205:1537–1551. https://doi.org/10. 1111/nph.13138 Karmali K, Karmali A, Teixeira A, Curto MM (2004) The use of Fourier transform infrared spectroscopy to assay for urease from Pseudomonas aeruginosa and Canavalia ensiformis. Anal Biochem 331:115–121. https://doi.org/10.1016/j.ab.2004.04.020 Kiers ET, Duhamel M, Beesetty Y, Mensah JA, Franken O, Verbruggen E, Fellbaum CR, Kowalchuk GA, Hart MM, Bago A, Palmer TM, West SA, Vandenkoornhuyse P, Jansa J, Bucking H (2011) Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis. Science 333: 880–882. https://doi.org/10.1126/science.1208473 Kleikamp B, Joergensen RG (2006) Evaluation of arbuscular mycorrhiza with symbiotic and nonsymbiotic pea isolines at three sites in the
Alentejo, Portugal. J Plant Nutr Soil Sci 169:661–669. https://doi. org/10.1002/jpln.200620638 Lambers H, Raven JA, Shaver GR, Smith SE (2008) Plant nutrientacquisition strategies change with soil age. Trends Ecol Evol 23: 95–103. https://doi.org/10.1016/j.tree.2007.10.008 Leifheit EF, Verbruggen E, Rillig MC (2015) Arbuscular mycorrhizal fungi reduce decomposition of woody plant litter while increasing soil aggregation. Soil Biol Biochem 81:323–328. https://doi.org/10. 1016/j.soilbio.2014.12.003 Leigh J, Hodge A, Fitter AH (2009) Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material. New Phytol 181:199–207. https://doi.org/10. 1111/j.1469-8137.2008.02630.x Li LF, Yang A, Zhao ZW (2005) Seasonality of arbuscular mycorrhizal symbiosis and dark septate endophytes in a grassland site in southwest China. FEMS Microbiol Ecol 54:367–373. https://doi.org/10. 1016/j.femsec.2005.04.011 Lindahl BD, de Boer W, Finlay RD (2010) Disruption of root carbon transport into forest humus stimulates fungal opportunists at the expense of mycorrhizal fungi. Isme J 4:872–881. https://doi.org/ 10.1038/ismej.2010.19 Makkonen M, Berg MP, Handa IT, Hattenschwiler S, van Ruijven J, van Bodegom PM, Aerts R (2012) Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient. Ecol Lett 15:1033–1041. https://doi.org/10.1111/j. 1461-0248.2012.01826.x Mcgonigle TP, Miller MH, Evans DG, Fairchild GL, Swan JA (1990) A new method which gives an objective-measure of colonization of roots by vesicular arbuscular mycorrhizal fungi. New Phytol 115: 495–501. https://doi.org/10.1111/j.1469-8137.1990.tb00476.x Mooshammer M, Wanek W, Hammerle I, Fuchslueger L, Hofhansl F, Knoltsch A, Schnecker J, Takriti M, Watzka M, Wild B, Keiblinger KM, Zechmeister-Boltenstern S, Richter A (2014) Adjustment of microbial nitrogen use efficiency to carbon: nitrogen imbalances regulates soil nitrogen cycling. Nat Commun 5. https:// doi.org/10.1038/ncomms4694 Nottingham AT, Turner BL, Winter K, Chamberlain PM, Stott A, Tanner EVJ (2013) Root and arbuscular mycorrhizal mycelial interactions with soil microorganisms in lowland tropical forest. FEMS Microbiol Ecol 85:37–50. https://doi.org/10.1111/1574-6941.12096 Nuccio EE, Hodge A, Pett-Ridge J, Herman DJ, Weber PK, Firestone MK (2013) An arbuscular mycorrhizal fungus significantly modifies the soil bacterial community and nitrogen cycling during litter decomposition. Environ Microbiol 15:1870–1881. https://doi.org/10. 1111/1462-2920.12081 Olson JS (1963) Energy storage and the balance of producers and decomposers in ecological systems. Ecology 44:322–331. https://doi.org/ 10.2307/1932179 Parton W, Silver WL, Burke IC, Grassens L, Harmon ME, Currie WS, King JY, Adair EC, Brandt LA, Hart SC, Fasth B (2007) Global-scale similarities in nitrogen release patterns during long-term decomposition. Science 315:361–364. https://doi.org/10.1126/science.1134853 Phillips JM, Hayman DS (1970) Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. T Brit Mycol Soc 55:158–161. https:// doi.org/10.1016/S0007-1536(70)80110-3 Posada RH, Madrinan S, Rivera EL (2012) Relationships between the litter colonization by saprotrophic and arbuscular mycorrhizal fungi with depth in a tropical forest. Fungal Biol 116:747–755. https://doi. org/10.1016/j.funbio.2012.04.003 Read DJ, Perez-Moreno J (2003) Mycorrhizas and nutrient cycling in ecosystems—a journey towards relevance? New Phytol 157:475– 492. https://doi.org/10.1046/j.1469-8137.2003.00704.x Schroeder-Moreno MS, Greaver TL, Wang SX, Hu SJ, Rufty TW (2012) Mycorrhizal-mediated nitrogen acquisition in switchgrass under
Environ Sci Pollut Res elevated temperatures and N enrichment. GCB Bioenergy 4:266– 276. https://doi.org/10.1111/j.1757-1707.2011.01128.x Shahzad T, Chenu C, Genet P, Barot S, Perveen N, Mougin C, Fontaine S (2015) Contribution of exudates, arbuscular mycorrhizal fungi and litter depositions to the rhizosphere priming effect induced by grassland species. Soil Biol Biochem 80:146–155. https://doi.org/10. 1016/j.soilbio.2014.09.023 Smith SE, Smith FA (2011) Roles of arbuscular mycorrhizas in plant nutrition and growth: new paradigms from cellular to ecosystem scales. Annu Rev Plant Biol 62:227–250. https://doi.org/10.1146/ annurev-arplant-042110-103846 Staddon PL, Ramsey CB, Ostle N, Ineson P, Fitter AH (2003) Rapid turnover of hyphae of mycorrhizal fungi determined by AMS microanalysis of C-14. Science 300:1138–1140. https://doi.org/10. 1126/science.1084269 Stolz JF, Basu P (2002) Evolution of nitrate reductase: molecular and structural variations on a common function. Chembiochem 3:198– 206. https://doi.org/10.1002/1439-7633(20020301)3:2/33.0.CO;2-C Tao L, Ahmad A, Roode JC, Hunter MD (2016) Arbuscular mycorrhizal fungi affect plant tolerance and chemical defences to herbivory through different mechanisms. J Ecol 104:561–571. https://doi.org/ 10.1111/1365-2745.12535 Tisserant E, Malbreil M, Kuo A, Kohler A, Symeonidi A, Balestrini R, Charron P, Duensing N, dit Frey NF, Gianinazzi-Pearson V (2013) Genome of an arbuscular mycorrhizal fungus provides insight into the oldest plant symbiosis. Proc Natl Acad Sci U S A 110:20117–20122. https://doi.org/10.1073/pnas.1313452110 Toljander JF, Lindahl BD, Paul LR, Elfstrand M, Finlay RD (2007) Influence of arbuscular mycorrhizal mycelial exudates on soil
bacterial growth and community structure. FEMS Microbiol Ecol 61:295–304. https://doi.org/10.1111/j.1574-6941.2007.00337.x Valverde A, Igual JM, Peix A, Cervantes E, Velazquez E (2006) Rhizobium lusitanum sp. nov. a bacterium that nodulates Phaseolus vulgaris. Int J Syst Evol Microbiol 56:2631–2637. https://doi.org/10.1099/ijs.0.64402-0 van Diepen LT, Lilleskov EA, Pregitzer KS, Miller RM (2010) Simulated nitrogen deposition causes a decline of intra-and extraradical abundance of arbuscular mycorrhizal fungi and changes in microbial community structure in northern hardwood forests. Ecosystems 13:683–695. https://doi.org/10.1007/s10021-010-9347-0 Wamberg C, Christensen S, Jakobsen I, Muller AK, Sorensen SJ (2003) The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum). Soil Biol Biochem 35: 1349–1357. https://doi.org/10.1016/S0038-0717(03)00214-1 Yang W, Wang K (2001) Advances on soil enzymology. Chin J Appl Environ Biol/Zhongguo ke xue yuan Chengdu sheng wu yan jiu suo zhu ban 8:564–570 Zarea MJ, Karimi N, Goltapeh EM, Ghalavand A (2011) Effect of cropping systems and arbuscular mycorrhizal fungi on soil microbial activity and root nodule nitrogenase. J Saudi Soc Agric Sci 10: 109–120. https://doi.org/10.1016/j.jssas.2011.04.003 Zhang HS, Wu XH, Li G, Qin P (2011) Interactions between arbuscular mycorrhizal fungi and phosphate-solubilizing fungus (Mortierella sp.) and their effects on Kostelelzkya virginica growth and enzyme activities of rhizosphere and bulk soils at different salinities. Biol. Fertil Soils 47: 543–554. https://doi.org/10.1007/s00374-011-0563-3
