Abstract:

This study aims to quantify the overall effect of green manure incorporation on crop yields in China， clarify the yield response differences under various green manure and nitrogen fertilizer combination regimes， and identify the main factors influencing responses. The results provide a scientific basis for accurately evaluating the potential of green manure to increase yield and reduce nitrogen fertilizer application in Chinese farmland， as well as for optimizing green manure utilization to achieve synergistic benefits of yield enhancement and chemical fertilizer reduction. Using fields without green manure as the control and green manure incorporation as the treatment， we collected 238 publicly published studies from 2000 to July 2024 and established a database containing 2528 paired observations. A meta-analysis was conducted to quantify the effects of green manure on crop yield and identify factors influencing responses. The Random Forest model was employed to analyze the dominant factors controlling the yield response to green manure， while linear regression was applied to examine the relationship between the substitution ratio of chemical nitrogen fertilizer by green manure and the corresponding yield effect. On average， green manure incorporation increased crop yield by 4.68% in Chinese farmland. Specifically， the combination of green manure with nitrogen fertilizer significantly enhanced this effect， yielding an average increase of 8.86%. While this integrated approach consistently demonstrated yield benefits across various main crop management practices and green manure cultivation/utilization methods， the magnitude of improvement varied within a range of 5.32% to 17.03%. Random Forest analysis identified duration of green manure application， nitrogen fertilizer application rate， and green manure species as the dominant factors influencing responses. The highest yield gains were observed under conditions of long-term green manure use， lower nitrogen fertilizer inputs for main crops， and mixed sowing of green manure species. In contrast， substituting nitrogen fertilizer solely with green manure did not exhibit a significant overall yield effect. The yield response under substitution strategies varied widely （-20.53% to 13.26%） across different crop management and green manure practices. Random Forest modeling revealed that the substitution ratio and crop type were the primary factors determining the yield effect. The most positive yield outcomes occurred under low substitution ratios and in cropping systems dominated by wheat or single-season rice. There is a significant negative correlation between the yield effect of green manure substituting chemical nitrogen fertilizer and the substitution ratio. A synergistic effect of yield increase and fertilizer saving can be achieved when the critical substitution ratio reaches 40.5%. The nitrogen reduction thresholds varied across crops， in the following order： single-season rice （54.8%）， wheat （43.8%）， early rice （39.9%）， late rice （34.7%）， maize （21.5%）. The yield effects under different substitution ratios exhibited distinct temporal patterns： At 0-40% substitution： Sustained long-term yield increase and nitrogen reduction； At 40%-60% substitution： Achieved long-term nitrogen reduction while maintaining yield stability or moderate increase； Above 60% substitution： Resulted in short-term yield reduction with potential transition to yield stabilization without reduction over extended periods. Green manure incorporation demonstrates significant potential for simultaneously increasing crop yields and reducing nitrogen fertilizer use in Chinese agricultural systems. However， optimizing these benefits requires integrated management strategies that consider interactions among main crop management practices， green manure cultivation methods， and utilization techniques. This study provides quantitative evidence for accurately predicting yield responses to green manure application while offering a theoretical framework and technical guidance for maximizing the green manure dual benefits of yield enhancement and fertilizer reduction.

Key words: green manure, crop yield, meta-analysis, Random Forest