Abstract
BackgroundForest litter decomposition is a major component of the global carbon (C) budget, and is greatly affected by the atmospheric nitrogen (N) deposition observed globally. However, the effects of N addition on forest litter decomposition, in ecosystems receiving increasingly higher levels of ambient N deposition, are poorly understood.Methodology/Principal FindingsWe conducted a two-year field experiment in five forests along the western edge of the Sichuan Basin in China, where atmospheric N deposition was up to 82–114 kg N ha–1 in the study sites. Four levels of N treatments were applied: (1) control (no N added), (2) low-N (50 kg N ha–1 year–1), (3) medium-N (150 kg N ha–1 year–1), and (4) high-N (300 kg N ha–1 year–1), N additions ranging from 40% to 370% of ambient N deposition. The decomposition processes of ten types of forest litters were then studied. Nitrogen additions significantly decreased the decomposition rates of six types of forest litters. N additions decreased forest litter decomposition, and the mass of residual litter was closely correlated to residual lignin during the decomposition process over the study period. The inhibitory effect of N addition on litter decomposition can be primarily explained by the inhibition of lignin decomposition by exogenous inorganic N. The overall decomposition rate of ten investigated substrates exhibited a significant negative linear relationship with initial tissue C/N and lignin/N, and significant positive relationships with initial tissue K and N concentrations; these relationships exhibited linear and logarithmic curves, respectively.Conclusions/SignificanceThis study suggests that the expected progressive increases in N deposition may have a potential important impact on forest litter decomposition in the study area in the presence of high levels of ambient N deposition.
Highlights
Combustion of fossil fuels, nitrogen (N) fertilizer use, cultivation of N2-fixing crops, and other human activities have substantially altered the global N cycle and greatly accelerated the formation and deposition of reactive forms of N [1,2]
Conclusions/Significance: This study suggests that the expected progressive increases in N deposition may have a potential important impact on forest litter decomposition in the study area in the presence of high levels of ambient N deposition
Atmospheric Nitrogen Deposition High rates of wet N deposition were observed at the investigated sites (82–114 kg N ha–1 year–1, mean of 95 kg N ha–1 year–1), which is significantly higher than the average N deposition rate across China (15.8 kg N ha–1 year–1 [38]) and the average N deposition rate that was reported across 50 forest sites in China (16.6 kg N ha–1 year–1 [24])
Summary
Combustion of fossil fuels, nitrogen (N) fertilizer use, cultivation of N2-fixing crops, and other human activities have substantially altered the global N cycle and greatly accelerated the formation and deposition of reactive forms of N [1,2]. Alteration of the N cycle has greatly affected the cycle of carbon (C) on a global scale [3]. In many N addition studies conducted in forest ecosystems, plant growth (C fixation) nearly always responds positively [4], whereas the effect of N addition on litter decomposition (C release) varies considerably [5]. Several studies have reported significantly lower rates of litter decomposition in the presence of N additions [6,7,8,9]. Forest litter decomposition is a major component of the global carbon (C) budget, and is greatly affected by the atmospheric nitrogen (N) deposition observed globally. The effects of N addition on forest litter decomposition, in ecosystems receiving increasingly higher levels of ambient N deposition, are poorly understood
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