Soil extracellular enzyme activity and stoichiometry in China’s forests

1. Ecoenzymatic stoichiometry links microbial decomposition with nutrient
mineralization and improves our understanding of nutrient cycling in
terrestrial ecosystems. Microbial C:N:P acquisition in the topsoil
converged at a ratio of 1:1:1 in global ecosystems. However, whether the
ratio of microbial acquisition is stable in forest soils, and is
applicable among different soil depths remain unknown. 2. Based on
large-scale soil sampling in China’s forests, we examined the patterns and
environmental drivers of the eight most-widely measured enzyme activities
and the relevant stoichiometry. 3. We found that the ratio of C:N:P
acquisition significantly deviated from 1:1:1. The specific enzyme
activities (g SOC-1) did not change significantly with latitude except
those for xylosidase and acid phosphatase. Similarly, only the C:P
acquisition ratio increased with latitude. Vertically, the specific
activities of C-acquiring enzymes mainly increased, N-acquiring enzymes
decreased, and P-acquiring enzymes did not change with soil depth.
Moreover, all ratios of microbial acquisition decreased, and the
percentage of recalcitrant C increased significantly with increasing
depth. Our study also showed that temperature and soil C:N ratio were the
important factors in explaining the variations in specific enzyme
activities and microbial nutrient acquisition. 4. Our results indicated
that no constant microbial C:N:P acquisition ratio can be widely
recognized, and that SOC quality changed from labile to recalcitrant with
depth. We highlight that depth-dependent enzymatic processes should be
considered in future SOC dynamic models.

1. 生态酶化学计量（ecoenzymatic stoichiometry）将微生物分解与养分矿化相联结，有助于深化我们对陆地生态系统养分循环的理解。全球陆地生态系统中，表层土壤的微生物碳氮磷获取比收敛于1:1:1。然而，微生物养分获取比在森林土壤中是否保持稳定，且能否适用于不同土层深度，目前仍属未知。
2. 本研究基于中国森林的大规模土壤采样，对8种最广泛测定的酶活性及其相关化学计量特征的分布模式与环境驱动因子进行了探究。
3. 研究结果显示，微生物碳氮磷获取比显著偏离1:1:1。除木糖苷酶（xylosidase）与酸性磷酸酶（acid phosphatase）外，其余酶的比活性（以每克土壤有机碳计，g SOC⁻¹）均未随纬度发生显著变化；类似地，仅碳磷获取比随纬度升高而增加。在垂直维度上，碳获取酶的比活性整体呈上升趋势，氮获取酶的比活性则有所下降，而磷获取酶的比活性未随土层深度发生明显变化。此外，随着土层深度增加，所有微生物养分获取比均有所降低，难降解碳占比则显著升高。本研究同时表明，温度与土壤碳氮比是解释比酶活性及微生物养分获取比变异的关键因子。
4. 我们的研究结果指出，不存在可被广泛认可的恒定微生物碳氮磷获取比，且土壤有机碳（SOC）质量随土层深度从易降解转向难降解。本研究强调，未来的土壤有机碳动态模型应纳入依赖于土层深度的酶促过程。