Nitrous oxide (N 2 O) is the third important long-lived greenhouse gas next to carbon dioxide and methane and croplands are considered biogeochemical hotspots of soil N 2 O emissions. To reduce soil N 2 O and other greenhouse emissions, climate-smart agricultural practices including biochar application have been applied. Many studies have been conducted with biochar application but results from these studies are not conclusive. To address this issue, meta-analysis, a quantitative review that synthesizes results from multiple independent studies, has been widely used. The results from different meta-analyses also differ but are seldomly evaluated. In this study, we evaluated meta-analyses on the effects of biochar application on soil N 2 O emissions. A grand mean response ratio (RR) was further proposed to estimate an overall effect and the impacts of experiment setting, properties of biochar and soil, and agricultural practices. We found 18 metaanalysis papers were published between 2014 and 2022. Sample size (publications or experiments) varied from less than 30 to more than 1000, with a mean sample size of 275. RR was calculated in all studies except one. While four meta-analyses did not find a significant effect of biochar application on soil N 2 O emissions, all others reported reductions of soil N 2 O emissions, but the magnitude ranged from −10.5% to −54.8%. Synthesizing all results from these meta-analyses, we found that biochar application overall significantly reduced the soil N 2 O emissions by 38.8%. The impacts increased with experimental duration till one and half years and reduced after that. Biochar application rate and C:N ratio had large influence on the effects of biochar application on soil N 2 O emissions. This study demonstrated that while meta-analysis provides a more comprehensive and better estimation, the inconsistence among these studies may need to be further evaluated.A grand mean RR based on meta-analyses could be more accurate and representative than single meta-analysis.
Literature review is an important component in any scientific research. In ecological and agricultural sciences, many studies have been conducted over years. With accumulation of scientific studies and published papers, it is critical to summarize and evaluate these previous research findings. Different literature review methods have been applied, including traditional qualitative literature review, quantitative meta-analysis, and more recently, mega-analysis, or meta-meta-analysis. Here we briefly describe these different approaches and draw attention to the recent development of data synthesis. Several case studies were used to illustrate the application of these methods in the ecological and agricultural sciences.
Background Precipitation plays an important role in crop production and soil greenhouse gas emissions. However, how crop yield and soil nitrous oxide (N2O) emission respond to precipitation change, particularly with different background precipitations (dry, normal, and wet years), has not been well investigated. In this study, we examined the impacts of precipitation changes on corn yield and soil N2O emission using a long-term (1981–2020, 40 years) climate dataset as well as seven manipulated precipitation treatments with different background precipitations using the DeNitrification-DeComposition (DNDC) model. Results Results showed large variations of corn yield and precipitation but small variation of soil N2O emission among 40 years. Both corn yield and soil N2O emission showed near linear relationships with precipitation based on the long-term precipitation data, but with different response patters of corn yield and soil N2O emission to precipitation manipulations. Corn yield showed a positive linear response to precipitation manipulations in the dry year, but no response to increases in precipitation in the normal year, and a trend of decrease in the wet year. The extreme drought treatments reduced corn yield sharply in both normal and wet years. In contrast, soil N2O emission mostly responded linearly to precipitation manipulations. Decreases in precipitation in the dry year reduced more soil N2O emission than those in the normal and wet years, while increases in precipitation increased more soil N2O emission in the normal and wet years than in the dry year. Conclusions This study revealed different response patterns of corn yield and soil N2O emission to precipitation and highlights that mitigation strategy for soil N2O emission reduction should consider different background climate conditions.
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