Changes in soil total nitrogen induced by crop residue return: A meta-analysis

Residue return has a profound effect on soil total nitrogen (STN) content and thereby the global soil N cycle. However, a specific understanding of the key factors influencing STN changes after residue return is currently lacking. A meta-analysis of 502 comparisons from China based on 152 published papers was performed to quantify the changes in STN after residue return and assess the effect sizes of the predictor variables. Results showed that residue return significantly increased STN in 0–20 cm by 11.1% compared to no residue return. The STN accumulation accompanying return of crop residues was greater in areas with higher latitude (>40°), lower rainfall (<700 mm) and lower average annual temperature (<5 °C) such as in Northeast China. Crop management, climatic factors, soil properties, residue management and fertilizer addition all significantly affected STN after residue return. Land use and mean annual precipitation (MAP) were the most influential predictors of the changes in STN following residue return. The results of structural equation model showed that the duration of residue return, the amount of residue, and the initial STN content of the soil had direct effects on final STN following residue return. In particular, the initial STN content both directly affected the changes in STN and was a transitional factor indirectly affecting changes in STN influenced by MAT, MAP and N fertilizer addition. More generally, land use, climatic attributes, the duration of residue return and the initial STN content are important aspects which should be given more consideration in the process of developing management strategies for returning residue.