Plant Soil (2010) 335:289–298 DOI 10.1007/s11104-010-0415-1
REGULAR ARTICLE
Correlation between leaf litter and fine root decomposition among subtropical tree species Hui Wang & Shirong Liu & Jiangming Mo
Received: 4 January 2010 / Accepted: 28 April 2010 / Published online: 12 May 2010 # Springer Science+Business Media B.V. 2010
Abstract Elucidating the processes of leaf litter and fine root decomposition has been a major research focus, while how the correlation between leaf litter and fine root decomposition is unclear. We studied the in situ decomposition and N dynamics of leaf litter and fine root of four subtropical tree species (Pinus massoniana, Castanopsis hystrix, Michelia macclurei and Mytilaria laosensis) to determine whether leaf litter and fine root decomposition is correlated across species as well as which factors influence decomposition above versus below ground. Decomposition rate of leaf litter was related to that of fine root across species. The strong correlation between leaf litter and fine root decomposition rates arose largely for several
Responsible Editor: Alfonso Escudero. H. Wang : S. Liu (*) Key Laboratory of Forest Ecology and Environment, China’s State Forestry Administration, Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, No.1 Dongxiaofu, Haidian District, Beijing 100091, People’s Republic of China e-mail:
[email protected]
reasons. First, soil moisture had the similar influences on both leaf litter and fine root decomposition rates. Second, traits (i.e., initial Ca concentration) important to both leaf litter and fine root decomposition rates showed significant similarity among species. Third, initial P, N and aromatic C concentrations, and C/N ratio were uniquely important for fine root decomposition rate, while no unique traits for leaf litter decomposition rate. This also could account for the strong correlation between leaf litter and fine root decomposition rates. Our study suggests that among these subtropical trees, species effects on in situ decomposition rates of leaf litter and fine root are very similar. Thus, species differences in decomposition rates may be as large as they would be if faster decomposition of leaf litter was correlated with faster decomposition of fine root. N immobilization rate of leaf litter was unrelated to that of fine root across species. Our results help explain some important mechanisms by which tree species influence litter in situ decomposition. Keywords Microclimate . Plant traits . Species effects . Litter decomposition . Nitrogen dynamic
H. Wang e-mail:
[email protected]
Introduction J. Mo South China Botanical Garden, the Chinese Academy of Sciences, Dinghu, Zhaoqing, Guangdong 526070, People’s Republic of China
Litter decomposition is a fundamental ecosystem process, and a rich history of research shows that climate and litter chemistry strongly control rates of
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litter decay (Meentemeyer 1978; Hobbie 1996; Adair et al. 2008; Cusack et al. 2009). For example, soil water and temperature are the most important controls on litter mass loss in the model constructed by Jansson and Berg (1985). Initial litter chemistry, including N, P, Ca and lignin concentrations, the lignin/N and C/N ratios, not only affects rates of mass loss, but also determines rates of nutrient cycling (McClaugherty and Berg 1987; Ryan et al. 1990; Grabovich et al. 1995; Gijsman et al. 1997; Scott and Binkley 1997). Decomposition of leaf litter and fine root has been a major research focus during the past decade (Silver and Miya 2001; Xu and Hirata 2005; Hobbie et al. 2006; Guo et al. 2006a), while how the correlation between leaf litter and root decomposition is not well understood. Hobbie et al. (2010) found decomposition and N immobilization rates of leaf litter were unrelated to those of fine root across eleven temperate tree species in a common site, one of the plots dominated by Acer pseudoplatanus. They focused on the effects of litter chemistry on leaf litter and fine root decomposition, excluding the effects of microclimate under forests among tree species. However, the in situ decomposition of each litter occurs under its original forest. In this situation, the effects of tree species on litter decomposition do not only contain the effects of litter chemistry, but also the effects of microclimate under forests, whereas it remains unclear whether there is correlation between leaf litter and fine root in situ decomposition among tree species. In addition, estimating rates of root decomposition is challenging because roots are hidden from view (Bloomfield et al. 1996) and belowground productivity can be of similar magnitude to foliar productivity (Norby et al. 2004). Therefore, understanding the correlation between above- and belowground in situ decomposition is important for identifying the strength of tree species effects on in situ decomposition, which is influenced by microclimate and litter chemistry. Plantations are becoming a key component of the world’s forest resources and playing an important role in the context of overall sustainable forest management. Well-designed, multi-purpose plantations can reduce pressure on natural forests, restore some ecological services provided by natural forests and mitigate climate change through direct C sequestration (Paquette and Messier 2010). In China, the total
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plantation area recently reached 5.33×107 ha, accounting for 30% of the total forest area of China (SFA 2007) and 29% of the world’s total plantation area (FAO 2007). Subtropical China is a suitable region for developing plantations, due to the abundance of sunlight and water resources. In subtropical China, the plantation area accounted for 63% of the total plantation area of China (SFA 2007). Many plantations of indigenous tree species are established in subtropical region to supply valuable timber and enhance biodiversity and ecosystem services (Carnevalea and Montagnini 2002; Liang 2007). Thus, studies on litter decomposition of the main tree species for afforestation can provide a good opportunity to find out the patterns of C and N cycling in subtropical plantation ecosystems. In this study, we compared leaf litter and fine root (< 2 mm diameter) in situ decomposition of four subtropical tree species in southern China. We focused on fine roots because with their more rapid turnover rates (Eissenstat et al. 2000; Gill and Jackson 2000; Guo et al. 2008) compared to coarse roots, fine roots represent a substantial proportion of total tree root productivity. Our objectives were to determine: (1) whether decomposition rates or maximum N immobilization of leaf litter and fine root are correlated among tree species, and (2) whether such correlations are driven by the factors that influence leaf litter and fine root decomposition.
Materials and methods Site description The study area is located at the Experimental Center of Tropical Forestry, the Chinese Academy of Forestry (22°10′ N, 106°50′ E), Pingxiang City, Guangxi Zhuang Autonomous Region, China. Annual rainfall is approximately 1,400 mm, occurring primarily from April to September. Annual mean temperature is 21°C. The soils were formed from granite, classified as red soil in Chinese soil classification, equivalent to oxisol in USDA Soil Taxonomy (Liang and Wen 1992; State Soil Survey Service of China 1998; Soil Survey Staff of USDA, 2006). The soil texture is sandy in this study. Four adjacent monospecific plantations were selected based on their similar topography, soil texture,
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stand age, and management history. One coniferous plantation was composed of Pinus massoniana, and the three broadleaf plantations were Castanopsis hystrix, Michelia macclurei and Mytilaria laosensis. These four tree species are the main indigenous species for afforestation and are not N fixing species. The four plantations were established in 1983 after a clear cut harvest on a Cunninghamia lanceolata plantation site, located at the elevation of 550 m, with the respective area from 2.5 to 6.8 ha, stem density from 404 to 470 trees ha−1, DBH (diameter of tree measured at breast height) from 22.9 to 25.8 cm, and tree height from 17.2 to 22.6 m. Historically, the study site was occupied by a subtropical evergreen forest, and then a C. lanceolata plantation was established in 1950s after a clear cut harvest. In each plantation type, five plots (each 20 m×20 m) for sampling were delineated randomly. Leaf litter and fine root decomposition experiments Leaf litters of P. massoniana, C. hystrix, M. macclurei and M. laosensis were collected using litter traps and nylon mesh placed on the forest floor under the trees in respective plantation in August 2008. Meanwhile fresh fine roots (< 2 mm diameter) of P. massoniana, C. hystrix, M. macclurei and M. laosensis were collected from the top 10 cm of soil in respective plantation. We opted to use fresh roots because they best represent roots that have not yet begun to decompose as the description by Hobbie et al. (2010). All leaf litter and fine root were air-dried to a constant weight (Mo et al. 2006). The litterbag method (Crossley and Hoglund 1962) was applied to obtain decomposing litter samples (leaf litterbag size: 1 mm-mesh polyethylene bag, and 250 mm×250 mm; fine root litterbag size: 0.3 mm-mesh polyethylene bag, and 100 mm×100 mm). Each leaf litterbag was filled with 12 g of air-dried mass and each fine root litterbag was filled with 1 g of air-dried mass. In the in situ litter experiment, leaf litters produced in a plantation were placed back into the original site, and fine roots collected in a plantation were buried back in a small 45° angle slit in the top 10 cm of soil in the original site to allow for good contact with the soil surface (Ostertag et al. 2008), on 15 September 2008. Litterbags were randomly retrieved at 3-month intervals from September 2008 to September 2009.
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At each time point, 8 replicate leaf litterbags and 10 replicate fine root litterbags were collected randomly in each plot. The decomposing leaf litter and fine root fragments in the litterbags were picked out with tweezers to remove contaminated soil particulars as carefully as possible and dried in an oven at 50°C for 48 h before weighing (Ostertag et al. 2008). Samples were ground in a mill to pass through a 0.25 mm sieve before chemical analysis. Subsamples of the original and decomposing leaf litter and fine root were dried to 105°C, and all results are reported on 105°C basis. Soil surface temperature and moisture in each plantation were measured at the same hour of the same day of each month during decomposition period. Soil temperature was measured using digital thermometers. Volumetric soil moisture was measured simultaneously using MPKit which consists of three amplitude domain reflectometry (ADR) moisture probes (MP406) (Tang et al. 2006). Chemical analysis of litter samples Subsamples of litter samples were analyzed for (1) C by dichromate oxidation method (Nelson and Sommers 1996); (2) N by the Kjeldahl method (Bremner 1996); (3) P, Ca, and Mn by inductively coupled plasma (ICP) mass spectrometry analysis (IRIS Intrepid II XSP, Thermo Electric Co., USA); (4) alkyl C, O-alkyl C, and aromatic C by solid-state 13 C cross polarization with magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) on a Bruker AVANCE III 400 spectrometer (Schmidt et al. 1997). Statistical analyses The model for constant potential mass loss (Olson 1963) is represented by the following single exponential model, X/X0 = e−k t, X/X0 is fraction mass remaining at time t, X the mass remaining at time t, X0 the original mass, e the base of natural logarithm, k the decomposition coefficient, and t the time (3, 6, 9, 12 months). The maximum N immobilization is the maximum N immobilized into leaf litter or fine root during decomposition period. To assess differences among the four tree species, leaf litter and fine root kvalues, leaf litter and fine root maximum N immobi-
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Results Changes in C and N concentrations of litters during decomposition The leaf litter and fine root masses of P. massoniana, C. hystrix, M. macclurei and M. laosensis decreased with incubation time in a similar pattern and about 75% of initial leaf litter masses and 55% of initial fine root masses were lost after 12 months of incubation, respectively (Fig. 1). The N concentrations of leaf litter and fine root of the four tree species were increased with incubation time (Tables 1 and 2). The C/N ratios of leaf litter and fine root of the four tree species decreased with incubation time (Tables 1 and 2). Microclimate and chemical drivers of decomposition rates There were significant differences in leaf litter k-values (P
