Long-term green manuring increases soil carbon sequestration via decreasing qCO2 caused by lower microbial phosphorus limitation in a dry land field

Organic fertilization in agroecosystems is an efficient fertilization management for sequestering carbon (C). Microbial metabolism affects C sequestration by influencing the rate of soil C mineralization, however, the responses of microbial metabolism limitation and soil respiration to different organic fertilizations remain unclear. Here, an experiment started from 1988 in a dry land of Northwest China was used to explore the mechanisms of C sequestration under five typical fertilization managements, namely no fertilization (CK), mineral nitrogen fertilizer (NF), wheat straw (WS), cattle manure (CM), and green manure (GM). The results indicated that the WS, CM, and GM treatments increased C sequestration rates by 28.3 %, 119.4 %, and 72.1 % in the 0-60 cm layer, compared with NF, whereas C sequestration efficiency was the greatest in the GM treatment. In addition, organic fertilization treatments increased the contents of soil nutrients across all the soil layers to varying degrees, and the activities of C, nitrogen (N), and phosphorus (P) -related enzyme in 0-60 cm soil layer also increased by 219.1 similar to 236.9 %, 236.7 similar to 774.5 %, and 2.5 similar to 36.5 % compared with NF. Enzymatic stoichiometry indicated that the soil microbial metabolism across all soil layer was mainly limited by P. Organic fertilization alleviated the microbial P limitation and decreased the microbial metabolic quotient (qCO(2)) in 0-20 cm soil layer. And GM treatment showed the lowest P limitation and qCO(2) in all soil layers. The partial least square path model demonstrated that microbial P limitation was the main driver affecting qCO(2). Alleviating the microbial P limitation decreased unnecessary C loss during microbial catabolism, converting more C to MBC, thereby increasing CUE and lowering qCO(2). In summary, green manuring is the most efficient practice to benefit C sequestration across all fertilization managements in this study.