Nitrogen additions reduce rhizospheric and heterotrophic respiration in a subtropical evergreen broad-leaved forest

Abstract

Increasing global atmospheric nitrogen (N) deposition has a considerable impact on soil respiration. Due to different carbon dioxide (CO2) resources of different components of soil respiration, there may be different mechanisms by which soils respond to N additions. The aims of this study are to investigate the effects of N additions on total soil respiration (RST), rhizospheric respiration (RSR) and heterotrophic respiration (RSH), and to elucidate the potential causal mechanisms. An artificial N addition experiment was conducted in an evergreen broad-leaved forest at the western edge of the Sichuan Basin in China. The effects of three N treatment levels (+0, +50, +150 kg N ha−1 yr.−1) were measured, with the N application initiated in April 2013. Trenching method was used for partitioning RST into RST and RSR. The values of RST and its components were measured monthly from May 2015 to April 2016. The mean annual RST was 1.92 ± 0.18 μmol CO2 m−2 s−1, with RSH and RSR contributing 75 ± 1% and 25 ± 1% to RST, respectively. Nitrogen addition significantly reduced RST and its two components, as well as the contribution of RSR to RST. Microbial biomass carbon (C) and pH in bulk soil decreased significantly after N application. Nitrogen addition had no effect on rhizospheric soil biochemical properties. RSR was significantly positively correlated with root biomass of Castanopsis platyacantha, while RSH was significantly positively correlated with the concentrations of microbial biomass C (MBC) and microbial biomass N (MBN) for bulk soil. The positive correlation between RSR and root biomass indicates that N application reduced RSR by reducing belowground C allocation and thus C inputs to the rhizosphere. The value of RSH decreased primarily due to a reduction in microbial activity and soil organic matter decomposition in root-free soil after N was added. The presence of plant roots may mitigate the effect of N inputs to the rhizosphere via alteration of root morphology and exudates.