Changes of labile and recalcitrant carbon pools under nitrogen addition in a city lawn soil

Carbon (C) flux is largely controlled by the highly bio-reactive labile C (LC) pool, while long-term C storage is determined by the recalcitrant C (RC) pool. Soil nitrogen (N) availability may considerably affect changes of these pools. The aim of this study was to investigate the effects of N treatments on soil LC and RC pools. A field experiment was conducted in a city lawn soil for 600 days with three N treatments, i.e., the control (0 kg N ha(-1) year(-1)), low-N (100 kg N ha(-1) year(-1)), and high-N (200 kg N ha(-1) year(-1)) treatments. As the N source, NH4NO3 solution was added to soil surface monthly. Measurements of LC, RC, and other soil biochemical properties, including pH, soil respiration rates, microbial biomass, and enzymes activities, were taken during the experiment period. The low-N and high-N treatments increased 6.3 and 13% of the LC pool, respectively, which was caused by decreased microbial biomass and soil respiration rates under the N treatments. By contrary, the low-N and high-N treatments decreased 5.9 and 12% of the RC pool, respectively. The N addition treatments enhanced phenol oxidase activities. The enhanced oxidase activities decreased new RC input and the increased dissolved organic C stimulated RC pool decomposition. The LC and RC pools were highly influenced by the N treatments, whereas effect of the N treatments on soil organic C was not significant. The N addition treatments also caused soil acidification and reduced bacterial biomass proportion in the soil microbial composition. The N addition increased the LC pool but decreased the RC pool in the soil. These changes should greatly impact soil long-term C storage.