Response of organic carbon mineralization and microbial community to leaf litter and nutrient additions in subtropical forest soils

Microorganisms are vital in soil organic carbon (SOC) mineralization. The deposition of atmospheric nitrogen (N) and phosphorus (P), as well as leaf-litter addition, may affect SOC mineralization and microbial community structure by changing the availability of soil nutrients and carbon (C). In this study, we added leaf-litters labeled by 13C (Pinus massoniana and Michelia macclurei) and nutrients (ammonium chloride and monopotassium phosphate) alone and in combination to soils collected from a coniferous forest in subtropical China. We aimed to investigate the effect of leaf-litter and nutrient addition on SOC mineralization and soil microbial community. CO2 production was continuously measured during 120-day laboratory incubation, and CO2 sources were partitioned using 13C isotopic techniques. The addition of P. massoniana and M. macclurei leaf-litters increased SOC mineralization by 7.4% and 22.4%, respectively. N and P addition alone decreased soil respiration by 6.6% and 7.1%, respectively. Compared with P addition, N addition exerted a higher inhibitory effect on SOC mineralization induced by leaf-litter addition. Leaf-litter addition stimulated soil microbial activity and decreased the ratio of bacteria to fungi as a result of greater promotion on fungal growth. Moreover, 16:0 and 18:1ω9c phospholipid fatty acids (PLFAs) had greater amount of 13C incorporation than other PLFAs, especially in nutrient-addition treatments. These results suggested that increased C input through leaf litter can stimulate SOC mineralization, whereas atmospheric N and P deposition can reduce this stimulatory effect and promote soil C storage in subtropical forests. Our results also illustrated that the use of 13C-labeled leaf litter coupled with 13C-PLFA profiling is a powerful tool for determining the microbial utilization of C.