Is partial substitution of animal manure for synthetic fertilizer a viable N2O mitigation option? An integrative global meta-analysis

Context or problem

Partially substituting synthetic fertilizer by animal manure has been proposed as a sustainable agricultural practice, from the perspectives of improving soil quality and mitigating climate warming.

Objective or research question

Previous studies showed that manure application combined with synthetic fertilizer (MACSF) also altered soil nitrous oxide (N₂O) emissions, while the magnitude varied substantially among experiments. We hypothesized that the difference in local conditions may be responsible for the contrasting impact of MACSF on N₂O emissions, while their significance and relative importance have not been explicitly assessed.

Methods

In order to quantify the response of N₂O emissions to MACSF and its geographic variability, we conducted a meta-analysis by combining 291 observations from 103 sites across the globe. Also, we evaluated the impact of local conditions on N₂O emissions and compared their relative importance using the gradient boosting decision tree (GBDT) algorithm.

Results

Overall, MACSF did not have significant effect on soil N₂O emissions, with a global average effect size (ES) of 1.07 ± 0.10 (95 % confidence interval, CI) relative to the same amount of synthetic nitrogen fertilizer. The corresponding N₂O emission factor (EF) was estimated as 0.93 ± 0.13 % (95 % CI). However, both ES and EF varied substantially with local conditions, of which the substitution ratio (SR) and soil pH were identified as the most important factors. The ES increased significantly (p < 0.01) with the increasing SR. Critical SR was approx. 50 %, above which MACSF may exacerbate N₂O emissions. Moreover, both response indices increased significantly with the decreasing soil pH. MACSF in acidic soils (pH < 6.5) tended to increase N₂O emissions, with ES significantly (p < 0.05) higher than 1.0.

Conclusion

The contribution of MACSF to N₂O mitigation varies subustantially with local conditions, of which SR and soil pH are the most important two. MACSF reduces N₂O emissions only in the cases with low SR and neutral to alkaline soils.

Implications or significance

This study offers an integrative global synthesis of the impact of MACSF on N₂O emissions. By identifying the key controlling factors, the findings serve to guide the development of region-specific tailored substitution strategies, considering its contribution to N₂O emission mitigation.