Divergent Effects of Straw and Straw-Derived Biochar on Soil N Transformation and N2o Emissions: A Global Meta-Analysis

Abstract

Inorganic fertilizers are widely used to provide nutrients to crops, but lead to the release of excess reactive nitrogen (N) from the soil into the environment, threatening ecosystem balance and service functions. The application of straw and straw-derived biochar has been considered a promising solution to regulate soil N cycle and reduce N efflux. However, whether returning straw directly to the field or pyrolyzing it into biochar is more conducive to N retention, N supply and N2O abatement has not been thoroughly investigated. Here, we conducted a meta-analysis using 1477 paired observations from 378 publications to compare the divergent effects of straw and its biochar application on N transformation and N2O emissions in different agroecosystems and to identify the key driving factors of soil N2O emissions. Regardless of agroecosystem types, straw addition significantly promoted the N mineralization and immobilization rates, and inhibited the autotrophic nitrification rate in paddy soils, whereas biochar addition increased the DNRA rate in upland soils and reduced the autotrophic nitrification rate in paddy soils. The effect of straw addition on N2O emissions differed between the two agroecosystems, with N2O emissions increasing in uplands (76.4%) and decreasing in paddy fields (12.2%). However, biochar application significantly mitigated soil N2O emissions in uplands (28.8%) and paddy fields (27.5%). In addition, the effects of straw and its biochar application on N2O emissions were strongly influenced by different factors. High doses of rice straw biochar with high pH (> 8) and C/N (> 100) were recommended for upland applications, whereas medium doses of straw (20-40 t ha-1) with high C/N (> 50) were recommended for paddy soils, except for soils with high SOC, TN and C/N. Overall, this study contributes to propose optimal management strategies to benefit agricultural N use while minimizing climate change.

Keywords: Straw, Straw-derived biochar, N transformation, N2O emissions, Meta-analysis