Decomposition of trembling aspen leaf litter under long-term nitrogen and sulfur deposition: Effects of litter chemistry and forest floor microbial properties
Introduction
Litter decomposition plays a critical role in carbon (C) and nutrient cycling in forest ecosystems (Melillo et al., 1982) as carbon dioxide (CO2) and nutrients are released through litter decomposition in the soil (Berg and McClaugherty, 2003, Wood et al., 2006). Litter decomposition is affected by abiotic and biotic factors such as climate, soil physico-chemical properties, litter chemistry, nitrogen (N) availability, and activities of soil organisms (Berg, 2000, Lavelle et al., 1993, Swift et al., 1979). Among those, the effect of N availability on litter decomposition has received particular interest as increased N deposition alters litter decomposition pattern through changed litter chemistry (Berg and Matzner, 1997) as well as changed soil microbial community composition and enzyme activities (Berg, 1986, DeForest et al., 2004, Fog, 1988). For example, N deposition has been shown to increase N concentration of litter, leading to decreasing C to N (C/N) and lignin to N (lignin/N) ratios, which are key parameters associated with litter decomposability; i.e., litter with low lignin/N ratio is known to decompose faster than those with high lignin/N ratios (Berg and Matzner, 1997, Carreiro et al., 2000). Studies have also shown that N deposition can increase microbial biomass and enzyme activities and enhance decomposition by increasing soil N availability (Lv et al., 2013, Sinsabaugh et al., 2002).
Many studies reported that N addition increased litter decomposition rates (Carreiro et al., 2000, Hobbie, 2000, Hobbie and Vitousek, 2000, Hunt et al., 1988). However, lack of effect (Hunt et al., 1988, Pastor et al., 1987, Prescott, 1995) or suppressive effects of N addition on litter decomposition (Carreiro et al., 2000, Magill and Aber, 2000) have also been reported. The inconsistent results are ascribed to the variations in litter type, the chemical composition of N added (NH4+ or NO3−), the N addition rate, and the experiment duration (Thirukkumaran and Parkinson, 2000). In addition to the effect of N deposition, the effect of sulfur (S) deposition on litter decomposition has also been investigated; S deposition may inhibit (McKinley and Vestal, 1982, Traaen, 1980), enhance (Lee and Weber, 1983, Roberts et al., 1980), or have no effect (Killham et al., 1983) on litter decomposition. A recent study has also found that the reduction of S deposition can limit C sequestration in Europe and the USA (Fernandez-Martinez et al., 2017). However, how the co-occurrence of N and S deposition might affect litter decomposition by altering litter chemistry and soil microbial activities (e.g., enzyme activities) is poorly understood. This is an important research question as forest areas with heavy industrial activities are often subject to elevated levels of deposition of both N and S.
This study investigated the effect of N and S addition on changes in litter chemistry and soil microbial properties and the subsequent effect of changed litter chemistry and soil microbial properties on the rate of litter decomposition. We hypothesized that (1) external N addition will decrease litter C/N and lignin/N ratios by increasing litter N concentration in a N-limited forest; (2) external N and S addition will increase soil microbial activity as N and S are required macronutrients for microorganisms; and (3) lowered C/N and lignin/N ratios and increased soil microbial activity will enhance litter decomposition.
Section snippets
Research site and experimental design
To study the effect of N and S deposition on forest ecosystems in the oil sands region in northern Alberta, research plots were established in 2006 in a mixedwood boreal forest stand (56.1° N 110.9° W), located about 100 km southeast of Fort McMurray, a major city in the Athabasca oil sands region (AOSR) in western Canada (Jung and Chang, 2012). The background N and S deposition rates in the study site were measured between September 2014 and August 2016, with precipitation samples collected
Chemical properties of litter and forest floor
Long-term addition of N and/or S did not change the C concentration of the litter, but increased N and N addition decreased lignin concentrations, led to lower C/N and lignin/N (P = .009 and 0.012 for N and S addition, respectively) ratios (Table 1). When N and S were added together, lignin concentration increased (P < .001) but N concentration did not change compared to N and S added alone. Neither N nor S addition affected the concentrations of other elements including S and micronutrients.
Ten years of N and S addition altered litter and forest floor chemistry and microbial properties
Decreased litter C/N and lignin/N ratios via increased N and decreased lignin concentrations by 10 years of N and S addition (Table 1) support the first hypothesis. Litter N concentration is affected not only by the duration of N addition but also by the rate of N addition (Bergkvist and Folkeson, 1992, Jung et al., 2017, Jung and Chang, 2012, Tamm, 1991, Tamm et al., 1974). In previous studies that were conducted at the study site, 4 years of N addition did not change litter N concentration (
Conclusions
Ten years of N and S addition changed litter chemistry by decreasing C/N and lignin/N ratios via increased N concentration in the litter, and the changed litter chemistry, in turn, affected litter decomposition. Though 10 years of N and S addition in the field also altered microbial biomass and enzyme activities in the forest floor, such changes were not related to rates of litter decomposition. We conclude that litter chemistry rather than microbial and enzyme activities were the driver of
Acknowledgements
This research was partially funded by the Land Reclamation International Graduate School (LRIGS), which was funded through a CREATE (Collaborative Research and Training Experience) grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) and by a Discovery grant from NSERC to SXC.
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