The Effect of Agricultural Management on Soil Microbial Necromass ：A Hierarchical Meta-Analysis

Soil microbial necromass plays a key role in soil organic carbon sequestration and nutrient cycling as an important part of the stable carbon pool. However, little is known about the response of microbial necromass to different agricultural management practices. In this study, 63 studies (344 observations) worldwide were synthesized to evaluate the impacts of tillage system (TS), cropping system (CS), fertilization system (FS) and straw mulching (SM) on total microbial necromass (total amino sugars, TAS), fungal necromass (glucosamine, GluN) and bacterial necromass (muramic acid, MurN) and explored their responses to different management and environmental factors using a hierarchical meta-analysis. Our results showed that the effect sizes of TS and CS on soil amino sugars were negative, whereas the effect sizes of FS and SM were positive. Fertilizer application, monocropping and removal of straw mulch enhanced the negative effect of tillage system on amino sugars. No-tillage treatment enhanced the negative effect of the crop rotation system on amino sugars. No-tillage and straw mulching enhanced the positive effect of fertilizer management on TAS, but led to further depletion of MurN. Furthermore, straw mulching had a better restoration effect on amino sugars in cultivated soil. Both meta--regression and multivariate analyses suggested that initial soil organic carbon (SOC) content and mean annual temperature (MAT) are the key factors affecting the change of amino sugars under different agricultural management system. The random forest model showed that the reduction in amino sugar depletion depends mainly on the soil nutrient content and climatic conditions as well as the combination between various management practices. This research emphasized the importance of developing a reasonable management system that not only minimise the consumption of microbial necromass, but also protect the arable land from degradation and maintain the stability of SOC in agroecosystems.