Soil labile and recalcitrant carbon and nitrogen dynamics in relation to functional vegetation groups along precipitation gradients in secondary grasslands of South China

Soil labile and recalcitrant carbon (C) and nitrogen (N) are strongly controlled by plant inputs and climatic conditions. However, the interrelation of labile and recalcitrant pools with changes in plant functional groups (i.e., C3 and C4) along precipitation gradients is not fully understood. Here, we investigated the soil organic C and N (SOC and SON), labile C and N (LC and LN), recalcitrant C and N (RC and RN), and their isotopes (δ¹³C, and δ¹⁵N) in relation to C3 and C4 plant inputs from 20 sites across a 600-km precipitation gradient in secondary grasslands of South China. The SOC content decreased first slightly and then increased along precipitation gradients, largely due to the increase in C4 plant C inputs in the lower precipitation regions. In contrast, the SON content increased with increasing N inputs from C3 plant at higher precipitation regions. The LC and LN contents increased with increasing precipitation, whereas RC and RN did not change with precipitation. The LC and LN were correlated with plant C and N contents, as well as the mean annual precipitation, respectively. Increases in LC and LN stocks were tightly related to enhanced plant C and N inputs influenced by precipitation, suggesting stronger sensitivity of labile pools to both plant functional groups inputs and precipitation compared to the recalcitrant pool. Moreover, the δ¹³C values in RC declined with precipitation, while the δ¹⁵N values of both labile and recalcitrant N increased with increasing precipitation, further revealing that soil labile and recalcitrant C and N pools closely related to the shift in the C3 and C4 plant along precipitation gradients. Overall, our findings indicated that soil labile and recalcitrant fractions should be considered in context of precipitation under which plant inputs takes place in predicting soil C and N dynamics.