Differential effects of nitrogen addition on the organic carbon fractions of rhizosphere and bulk soil based on a pot experiment

Purpose The biogeochemical cycling of carbon (C) is essential for maintaining plant productivity and thus plays a vital role in soil carbon sequestration. However, limited information is available for addressing the effects of N input on SOC sequestration and discriminating between the differential effects on bulk soil and the rhizosphere. The aims of this study are to explore the responses of SOC fractions to different N addition levels and their potential environmental drivers. Materials and methods A 20-month pot experiment was conducted to investigate the responses of soil organic C (SOC) fractions to varying levels of N addition (0, 25, 50, 75, 100, and 150 kg N center dot ha(-1)center dot year(-1)) in rhizosphere and bulk soil by using a nylon mesh. The key roles of bacterial community composition and C-acquisition enzyme activities were also deciphered as the driver factors of SOC fractions. Results and discussion Carbon levels (i.e., particulate organic C (POC), light and heavy C fractions, dissolved organic C, and easily oxidizable organic C) in the rhizosphere increased with increasing N addition. Greater variation was observed in the response of POC to N addition, when compared to other SOC fractions, indicating relatively higher sensitivity of POC to elevated soil N levels. In the rhizosphere, N addition increased bacterial diversity and the activities of C-acquisition enzymes but only at lower levels (<= 50 kg N center dot ha(-1)center dot year(-1)). The effect of N addition on rhizospheric SOC was strongly associated with increased root biomass and inorganic N content. Meanwhile, in bulk soil, changes in SOC fractions were largely driven by soil NO3--N level. Conclusions The results of the present study indicate that N addition increases rhizospheric SOC content by regulating soil N levels and bacterial activities and highlight the differential responses of SOC fractions from rhizosphere and bulk soil to N addition, which will be helpful to understand the role of SOC sequestration in response to N deposition.