Are there links between extracellular enzymes and soil carbon or nitrogen content under straw return? A meta-analysis

Straw return is an important agricultural practice with the potential to impact soil carbon (C) and nitrogen (N) cycling. Extracellular enzyme activity (EEA) is crucial for soil organic matter degradation. However, it remains unclear how soil EEA respond to straw return or whether these responses can predict changes in soil C and N pools induced by straw return. By synthesizing 135 field studies worldwide, we showed that straw return significantly increased the activities of C-acquisition (C-acq), N-acquisition (N-acq), phosphorus-acquisition (P-acq), and oxidative-decomposition (OX) enzymes by 26.5%, 20.3%, 17.8%, and 9.1%, respectively. Notably, we found a significant positive relationship between the logarithmic response ratios (lnR) of the activities of OX and C-acq and C pool, suggesting that changes in combined EEA might serve as indicators of soil C pool under straw return. Interestingly, the changes in EEA due to straw return were unrelated to increases in soil total N (TN). Moreover, the positive effects of straw return on the activities of C-acq and P-acq negatively correlated with the amount of straw applied. Some abiotic factors, such as soil pH and soil C: N ratio, influenced the straw return-induced changes in soil C and N pools. These findings highlight the importance of considering key abiotic factors in understanding the microbial mediation of soil C and N cycling under straw return. Overall, our results suggest that the EEA can be used to monitor changes in the primary C and N sources for soil microbes, providing mechanistic insights into soil C and N cycling.