Biol Fertil Soils DOI 10.1007/s00374-012-0666-5
SHORT COMMUNICATION
Soil microbial biomass as affected by groundcover management in a Fraser fir (Abies fraseri [Pursh] Poir) plantation after 1 year Paligwende Nikièma & Pascal Nzokou & David E. Rothstein & Mathieu Ngouajio
Received: 27 July 2011 / Revised: 13 January 2012 / Accepted: 17 January 2012 # Springer-Verlag 2012
Abstract We investigated soil microbial biomass response to incorporating a non-leguminous [perennial ryegrass (Lolium perenne)] and two leguminous [Dutch white clover (Trifolium repens) and alfalfa (Medicago sativa)] cover crops into a newly established Fraser fir (Abies fraseri) plantation. Groundcover treatments consisted of growing each cover crop in the interspaces of the plantation, mowing the aboveground biomass every 3 weeks, and leaving the plant residues on the ground to decompose. Conventionally managed plots were used as a control. Soil total C, total N, and microbial biomass carbon (SMB-C) and nitrogen (SMB-N) were assessed at the 0–15-, 15–30-, and 30–35-cm soil depths. Soil total C was unaffected by groundcovers at any depths, whereas soil total N was significantly (P00.031) higher in the cover crop treatments than in the conventional system at the top soil layer. Groundcovers increased SMB-C and SMB-N by 20–50% and 35–80%, respectively, in the top soil layer relative to the control. These results suggest that groundcovers could potentially improve soil fertility and be a good strategy for sustainable fir tree production. P. Nikièma : P. Nzokou : D. E. Rothstein Department of Forestry, Michigan State University, 126 Natural Resources, East Lansing, MI 48824, USA M. Ngouajio Department of Horticulture, Michigan State University, A428 Plant and Soil Sciences, East Lansing, MI 48824, USA Present Address: P. Nikièma (*) Department of Soil Science, University of Manitoba, 362 Ellis Bldg, Winnipeg, MB R3T2N2, Canada e-mail:
[email protected]
Keywords Cover crop . Christmas tree . Microbial biomass
Introduction Soil microbial biomass, the living constituent of soil organic matter (SOM), plays key roles in maintaining soil productive functions in both managed and unmanaged ecosystems. Although soil microbial biomass represents a small portion (≈5%) of SOM, it has been well documented that any changes in the microbial size is closely related to fluctuations in soil fertility (Powlson et al. 1987; Mäder et al. 2002). This is in part because soil microbial biomass is not only involved in key soil-forming processes and aggregation but also is a labile source of soil C and plant essential nutrients such as nitrogen (N), phosphorus (P), and sulfur (S) (Dalal 1998). Additionally, soil microbial biomass has extensively been reported to respond rapidly to soil disturbances such as crop rotation, soil amendment/fertilization, tillage practices, and chemical control of weeds (SalinasGarcia et al. 1997; Balota et al. 2003; Laudicina et al. 2011) and has, therefore, often been suggested as a reliable tool for measuring early changes in soil fertility (Powlson et al. 1987; Dalal 1998). However, there is still a lack of information regarding soil microbial biomass response to groundcover management, especially in Christmas tree plantations. Christmas tree farming is an important agricultural activity (≈150,000 ha of land in production) in the USA (Vilsack and Clark 2009). Trees in Christmas tree farms have traditionally been grown in monoculture, using intensive production techniques (i.e., fertilization, irrigation, and control of competing weeds) to obtain vigorous, good quality trees. With regard to nutrient management, inorganic N fertilizers are the most common fertilizer types used for enhancing the tree performance, and applied N rates are generally determined based on
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the initial soil fertility levels and tree ages (Hart et al. 2004; Pedersen et al. 2006). For instance, for Fraser fir (Abies fraseri [Pursh] Poir), one of the most popular but also nutrientdemanding Christmas tree species, it is recommended to keep soil N level around 120 kg Nha−1 during the first 2 years of the plantation and gradually increase the rate as the stand develops and tree size increases. Mid-rotation (5–6 years) aged fir trees may require nearly 200 kg Nha−1 year−1 (Koelling and Dornbush 1992). Controlling weeds throughout the rotation cycle is another important aspect of Christmas tree farming as uncontrolled weed growth can negatively affect the growth and quality of the trees. Weeds are usually suppressed using common herbicides such as glyphosate, simazine, and amitrole. Such practices may increase soil erosion, nutrient losses through runoff and leaching, lead to development of weed resistance to the herbicides, and be expensive for growers. Because of these environmental and economic considerations, there is an urgent need to find alternative groundcover management practices with the potential to improve profitability while at the same time achieving sustainability of Christmas tree farming. Cover cropping is a practice that has been widely recognized and promoted as a practical way to enhance soil productivity and environmental quality (Walsh et al. 1996; Baumann et al. 2001). Groundcover management generally involves the use of mowed, tilled, or killed cover crops, to add organic matter to soil, conserve soil humus, reduce soil erosion, increase soil organic matter levels, and steadily release available nutrients for associated/succeeding crop uptake as the organic matter breaks down (Broughton 1977; Sanchez et al. 2007). In this process, the action of soil microorganisms is a major determinant of nutrient cycling and plant growth. Planting either grass [e.g., perennial ryegrass (Lolium perenne) or legume cover crops (e.g., alfalfa (Medicago sativa) and Dutch white clover (Trifolium repens)] in the interspaces of tree crops increases plant residue inputs to soils and therefore may stimulate soil microbial activity (Mendes et al. 1999; Dinesh et al. 2004). Because these cover crops have the abilities to produce a significant amount of biomass, thrive when repeatedly mowed, fix atmospheric N through symbiotic association with N2-fixing bacteria, and/or scavenge excess N left in the soil that would otherwise be lost by leaching, they are considered as ideal candidates for “living mulch” systems (Clark 2007; Brunetto et al. 2011). Moreover, many studies have reported plant-induced quantitative and qualitative variations in N and C flow to the soil, and different plant species may maintain a different microbial biomass and activity (Haynes and Francis 1993; Groffman et al. 1996; Mullen et al. 1998). It has also been well documented that agricultural management practices that leave plant residues on the soil surface, such as no tillage and cover cropping, often result in higher concentrations of soluble organic C compounds, which may greatly influence soil microbial
populations and activities (Alvarez et al. 1998; Zhou et al. 2011). This study was part of a research project designed to investigate whether soil biological and chemical properties as well as tree performance could be improved through incorporating cover crops into Fraser fir Christmas tree plantations in Michigan. The objective of the present study was to examine the effects of different groundcover management practices on soil microbial biomass in a Fraser fir plantation after the first growing season following the plantation establishment. Because soil microbial biomass responds quickly to soil environmental changes, this approach may be better suited to investigate short-term soil microbial biomass and soil fertility changes following the introduction of new management practices. The introduction of these cover crops into the fir plantation was expected to enhance the amount of soil organic matter (SOM) and easily metabolizable C-substrates which are key drivers of microbiological processes. Therefore, we hypothesized that the overall site chemical and biological conditions, particularly soil microbial biomass, would be increased in the cover crop–Fraser fir intercropping systems relative to a conventional fir system. Additionally, we anticipated that, because residues from leguminous cover crops are more labile (having higher N content and smaller C/N ratios) than non-legumes, plots with the N-fixing cover crops (i.e., alfalfa and clover) would result in higher soil microbial biomass than plots with the grass cover crop (i.e., perennial ryegrass).
Materials and methods Site description The field experiment was established in the spring of 2007 at the Tree Research Center (42.67°N, 84.46°W) on the campus of Michigan State University in East Lansing, Michigan. The local climate is characterized by mean temperatures of 15.5°C and −6.6°C during summer and winter periods, respectively. The annual mean rainfall is about 850 mm, precipitation is evenly distributed throughout the year. The soil type of the studied site is a fine loamy, mixed, active, mesic Aquic Glossudalf (USDA/NRCS-MAES 1992). The general soil chemical and physical characteristics are provided in Table 1. Prior to establishing the experiment, the site had primarily been used for maize production with occasional rotations of wheat and soybeans for the past three to four decades. Experimental design, plant materials, and management The experiment was laid out in a randomized complete block design with three replications. The experimental field,
Biol Fertil Soils Table 1 Physical and chemical characteristics of the soil at 0–15-, 15– 30-, and 30–45-cm soil depth of the study site prior to establishing the Fraser fir plantation and growing the cover crops, in East Lansing, MI, USA Parameters
whole field was protected with an electric fence to prevent deer browsing. Soil sampling and analysis
Depth (cm) 0–15
15–30
30–45
Sand (%)
65
na
na
Silt+clay (%)
35
na
na
pH (soil/water ratio of 1:1) Soil organic C concentration (%)
5.86 2.14
5.96 1.60
6.08 0.65
Soil total N concentration (%) Mehlich-III P (mg kg−1) Extractable K (mg kg−1) Extractable Ca (mg kg−1) Extractable Mg (mg kg−1)
0.18 49.1 200 1,260 180
0.14 31.7 110 1,190 160
0.06 14.1 80 1,230 180
Data on soil texture (proportion of sand, silt+clay) were obtained from Rothstein (2005), while data on soil C and nutrient concentrations were derived from Nikiema et al. (2011) na data not available
blocks, and plots measured 1,633 m−2 (32.4 × 50.4 m), 436 m−2 (10.8×50.4 m), and 155.5 m−2 (7.2×10.8 m), respectively. Fraser fir transplants (plug+2) were obtained from a local commercial nursery (Peterson’s Riverview Nursery), and they were machine-planted (Whitfield planter) at a spacing of 1.8×1.8 m into a chisel-plowed and dragged field soil on 8 May of 2007. Pre-inoculated seeds of common Dutch white clover, alfalfa (SS 100 brand), and perennial ryegrass (VNS) were purchased from Michigan State Seeds (Grand Ledge, MI, USA) and hand-seeded 22 May of 2007. The rhizobium inoculant types used for alfalfa and white clover to obtain nitrogen fixation were Sinorhizobium meliloti and Rhizobium leguminosarum biovar trifoli, respectively. The seeding rates used were 28 kg ha−1 for clover and alfalfa and 16 kg ha−1 for perennial ryegrass. Once the cover crops were fully established, mechanical mowing was performed at 3 cm above the ground every 3 to 4 weeks (i.e., 2 July, 26 July, 21 August, and 18 September of 2007) to control cover crop growth, minimize the competition with the trees, and add green manure to the soil surface. The control treatment was managed conventionally by growing no cover crop on the plots and completely suppressing weeds with glyphosate (active ingredient concentration01.1 kg ha−1). Glyphosate was sprayed twice during the growing season (i.e., 8 June and 26 July 2007) to suppress weeds in the control plots. The area between seedlings in the border rows of each rectangular plot was used as a buffer and not included in measurements, therefore restricting data collection to the remaining interior area of each plot. We did not apply N fertilizer to any of the plots during this first year of plantation establishment as it may injure first-year seedling roots (Koelling 2002). The
In each plot, 15 randomly selected soil subsamples per plot were collected from 0–15-, 15–30-, and 30–45-cm depths with 5.4-, 3.8-, and 2.5-cm diameter PVC corers, respectively, and composited into one sample per plot. Soil samples were collected in mid-October of 2007, which correspond to the end of the growing season. Samples were placed in double plastic zip-lock bags, securely tied, and kept on ice in a cooler before transporting them to the laboratory at Michigan State University for analysis. Soil moisture content was determined gravimetrically on oven-dried samples (at 105°C for 48 h). Soil pH (soil/water ratio of 1:1) was measured from the airdried soil (passed through a 2-mm sieve) with a HORIBA pH/COND meter (model D-54; Spectrum Technologies, Inc. Japan). Soil total C and total N were determined by combustion with an elemental analyzer (Model ECS 4010, Costech Analytical, Valencia, CA). Microbial biomass analysis Soil microbial biomass C (SMB-C) and biomass N (SMB-N) were assessed from the fresh soil samples by the chloroform fumigation–extraction method described by Brookes et al. (1985) and Beck et al. (1997). Soil solutions obtained from the fumigated and non-fumigated samples were analyzed for total dissolved C and N by oxidative combustion–infrared analysis and oxidative combustion–chemiluminescence, respectively (Shimadzu models TOC-V CPN analyzer and TNM-1 unit, Kyoto, Japan). Soil microbial biomass C and SMB-N were calculated as the difference between C and N in the fumigated and non-fumigated samples using 0.45 and 0.54 as correction factors for SMB-C and SMB-N, respectively (Brookes et al. 1985; Beck et al. 1997). Data analysis Soil chemical properties and microbial biomass data were analyzed using a general linear model-repeated measures analysis of variance in SAS to assess the significance of the effects of groundcover treatments, sampling depth, and their interactions on soil chemical and microbial biomass data. The statistical model thus included one random factor (block) and two fixed factors: groundcover treatments and depths, with the latter variable considered as repeated measurements. The potential effect of depth on all parameters in the various groundcover treatments was initially assessed using an analysis of variance/covariance structure. Contrast vectors in SAS were used to compare main effects of cover
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crop species treatments with the control. Treatment differences were accepted as significant at the 0.05 level.
Results and discussion Soil chemical properties Soil total C showed no measurable main effect of groundcover treatment, nor did the effect of the interaction between treatment and depth (Table 2). As expected, however, soil total C substantially (P
