To reduce reliance on synthetic nitrogen (N) fertilizer and sustain food production, replacing synthetic N fertilizer with animal manure as an effective method is widely used. However, the effects of replacing synthetic N fertilizer with animal manure on crop yield and nitrogen use efficiency (NUE) remain uncertain under varying fertilization management practices, climate conditions, and soil properties. Here, we performed a meta-analysis of wheat (Triticum aestivum L.), maize (Zea mays L.), and rice (Oryza sativa L.) based on 118 published studies conducted in China. Overall, the results indicated that substituting synthetic N fertilizer with manure increased yield by 3.3%−3.9% for the three grain crops and increased NUE by 6.3%−10.0%. Crop yields and NUE did not significantly increase at a low N application rate (≤120 kg ha−1) or high substitution rate (>60%). Yields and NUE values had higher increases for upland crops (wheat and maize) in temperate monsoon climate/temperate continental climate regions with less average annual rainfall (AAR) and lower mean annual temperature (MAT), while rice had higher increases in subtropical monsoon climate regions with more AAR and higher MAT. The effect of manure substitution was better in soil with low organic matter and available phosphorus. Our study shows that the optimal substitution rate was 44% and the total N fertilizer input cannot be less than 161 kg ha−1 when substituting synthetic N fertilizer with manure. Moreover, site‐specific conditions should also be considered.
Summary
Carbon–nitrogen (C–N) interactions in terrestrial ecosystems regulate climate–C cycle feedbacks. How additions of N affect soil C sequestration and then regulate climate change, however, are not fully understood. Previous studies have assessed effects of N on bulk soil organic C (SOC) but have not yet carefully examined its effects on different SOC fractions, which determine how fast the N‐regulated C cycle feeds back to climate. Here we synthesized data from 36 studies with 296 observations by a meta‐analysis to evaluate the responses of SOC fractions to N additions. We hypothesized that additions of N might increase both labile and non‐labile C fractions. The SOC that was separated by density fractionation increased by 18.3% for free light fractions and by 3.0% for heavy density fractions without change in the occluded light fraction under N addition compared with the control. The SOC that was separated by aggregate fractionation increased by 4.4% for macroaggregate‐associated fractions and by 6.5% for aggregate mineral‐associated fractions without change in the microaggregate‐associated fraction under N additions. When bulk SOC was separated by chemical permanganate oxidation, the oxidizable fraction increased by 10.4%, whereas the unoxidizable fraction increased by 4.5%. The changes in different soil C fractions were related to the mechanisms of SOC stabilization. Because of the significant increases in non‐labile SOC fractions, our findings suggest that on a global scale, increased additions of N might promote SOC accumulation and slow down climate change in the long term. Our results will be useful in model development for better prediction of SOC sequestration under additions of N.
Highlights
Effects of N deposition on soil C fractions were addressed using meta‐analysis.
Nitrogen additions significantly increased labile SOC fractions.
Non‐labile SOC fractions were also increased under N additions.
Nitrogen additions might promote SOC accumulation in the long term.
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