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.

中文翻译：

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华南次草原降水量梯度下土壤不稳定和顽固碳氮动态与功能植被的关系

土壤的不稳定和顽固性碳（C）和氮（N）受植物投入和气候条件的强烈控制。然而，不稳定的和顽固的库与植物功能组（即C3和C4）沿降水梯度的变化之间的相互关系尚未完全了解。在这里，我们研究了土壤有机碳和氮（SOC和SON），不稳定的碳和氮（LC和LN），顽固性碳和氮（RC和RN）及其与C3和C有关的同位素（δ13C和δ15N）。来自华南次草原600公里降水梯度的20个地点的C4植物投入。SOC含量先略有下降，然后沿降水量梯度增加，这主要是由于较低降水区C4植物碳输入的增加。相反，在较高的降水区域，SON含量随C3植物氮素输入的增加而增加。LC和LN含量随降水增加而增加，而RC和RN则不随降水变化。LC和LN分别与植物C和N含量以及年平均降水量相关。LC和LN存量的增加与受降水影响的植物碳和氮输入的增加密切相关，这表明不稳定的库对植物功能组输入和降水的敏感性高于顽固库。此外，RC中的δ13C值随降水量而下降，而不稳定和顽固氮的δ15N值随降水量的增加而增加，进一步揭示了土壤不稳定和顽固的碳和氮库与C3和C4植物沿降水梯度的变化密切相关。总的来说，我们的发现表明，在降水的背景下应考虑土壤的不稳定和难降解部分，在这种降水下植物的投入发生在预测土壤碳和氮动态的过程中。