Stage-specific response of litter decomposition to N and S amendments in a subtropical forest soil

Nitrogen (N) and sulfur (S) deposition are important drivers of global climate change, but their effects on litter decomposition remain unclear in the subtropical regions. We investigated the influences of N, S, and their interactions on the decomposition of 13C-labeled Pinus massoniana leaf litter. An orthogonal experiment with three levels of N (0, 81, and 270 mg N kg−1 soil) and S (0, 121, and 405 mg S kg−1 soil) was conducted. We traced the incorporation of 13C-litter into carbon dioxide (CO2), dissolved organic C (DOC), and microbial phospholipids. Over the 420-day incubation, litter decomposition did not respond to low N and S additions but increased under high levels and combined amendments (NS). However, litter-derived CO2 emissions were enhanced during the first 56 days, with a positive interaction of N × S. N additions promoted fungal growth, while S stimulated growth of Gram-positive bacteria, fungi, and actinobacteria. Increased decomposition was related to higher litter-derived DOC and fungi/bacteria ratio. Inversely, N and/or S amendments inhibited decomposition (N > NS > S) from day 57 afterwards, possibly due to C limitation and decreased abundances of Gram-negative bacteria and actinobacteria. These results suggested that N deposition interacted with S to affect litter decomposition, and this effect depended on N and S deposition levels and litter decomposition stage.