Influences of nitrogen addition and aboveground litter-input manipulations on soil respiration and biochemical properties in a subtropical forest

Atmospheric nitrogen (N) deposition has rapidly increased in subtropical ecosystems and may have altered the input of aboveground litter to soil, which substantially impacts soil carbon (C) and nutrient cycling. But how the soil processes and properties respond to N deposition under uncertain fresh litter input is poorly understood. In order to examine the responses of soil respiration and biochemical properties to N addition and aboveground litter manipulation, a field N addition and litterfall manipulation interaction experiment was performed in an evergreen broadleaf forest on the western edge of the Sichuan Basin in China. Three levels of N addition, including an N control (CN, ambient N input) and low N (LN, + 50 kg N ha−1 year−1) and high N (HN, + 150 kg N ha−1 year−1), and three levels of litterfall manipulation, including intact litter input (L0, no litter alteration), litter reduction (L−, reduced by 50%) and litter addition (L+, increased by 50%), were conducted monthly starting in January 2014 and August 2015, respectively. Soil respiration was measured monthly from January 2016 to December 2017. Soil samples were collected four times, in October 2016 and January, April and July 2017, to measure soil biochemical properties. The results showed that: (1) short-term N addition did not significantly alter the aboveground litter input in this forest; (2) soil respiration decreased with elevating N input and was associated with amount of litterfall input; (3) N addition increased the total organic C (TOC) concentration in topsoil in subplots without litterfall alteration but did not affect TOC in subplots with increased or decreased litter-input; (4) N addition decreased soil pH and did not affect soil microbial biomass regardless of whether litterfall was altered or not; (5) short-term litter manipulation did not affect any soil properties in the N control plots, but both litterfall reduction and addition tended to reduce surface soil TOC concentration in the N-added plots; and (6) both N addition and litterfall manipulation showed stronger effects on organic soil than on mineral soil. These findings indicated that elevated N input increased the surface soil C content by reducing soil respiration mainly via enhancing stabilization of soil organic matter rather than by reducing soil microbial biomass, and that altered litterfall may mitigate the N-induced increase in soil C. Because of temporal lag, long-term experimentation is needed to investigate the response of soil to altered litter input under different N addition conditions.