How much is soil nitrous oxide emission reduced with biochar application? An evaluation of meta‐analyses

Abstract

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, Meta-Analysis, and Mega-Analysis in Ecological and Agricultural Sciences

“…However, as the number of meta-analyses in literature increases, results from different meta-analyses on same scientific issues are found to be inconsistent. The overall effective size of treatment generated in meta-analyses is highly dependent on the data selection and methodology used [4] [33]. Recently, mega-analysis, also called meta-meta-analysis or second order meta-analysis, has been applied to provide a systematic review in which the results from multiple meta-analyses are combined and synthesized.…”

“…As such, mega-analysis can only be used to address questions that have been investigated in multiple meta-analyses, and to resolve inconsistencies across meta-analyses. Like meta-analysis, mega-analysis can identify sources of variability and heterogeneity in the results and provide a more precise estimate of the overall effect size [4].…”

“…Mega-analysis provides a better estimation and resolves the inconsistence among different meta-analyses. Inconsistence among different meta-analyses has been recognized in previous studies [4] [34]. Synthesizing the results from metaanalyses could identify potential causes of inconsistence and generate more reliable treatment effects.…”

“…Agroecological mitigation practices such as organic fertilizer and reduced tillage may enhance N 2 O emissions. More recently, Kaur et al [4] conducted a mega-analysis and evaluated 18 meta-analyses of the effects of biochar application on soil N 2 O emissions (Figure 2). The magnitudes of the effects varied among different meta-analyses and biochar application significantly reduced soil N 2 O emissions by 38.8% (95% confidence interval from 32.4% to 44.8%).…”

“…Traditional literature review, currently widely used meta-analysis, and more recently mega-analyses have been applied in ecological and agricultural sciences to summarize and synthesize the findings from multiple studies [1] [2] [3] [4]. One common purpose of these reviews is to summarize the current research status on a specific research topic, identify knowledge gaps, and provide future research direction [2] [3].…”