Modified biochar affects CO2 and N2O emissions from coastal saline soil by altering soil pH and elemental stoichiometry

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The application of biochar in degraded farmland improves soil productivity while achieving the recycling of agricultural waste. The collapse of the physical structure of coastal saline soils will greatly reduce the carbon sequestration potential of biochar. Phosphorus- and magnesium-modified biochar greatly improve the stability of biochar, which endows them with the potential to greatly improve the organic carbon pool of coastal saline soil. However, changes in the properties of modified biochar increase the uncertainty of microbial driven CO2 and N2O release by affecting soil chemistry properties. In this study, through laboratory soil microcosmic experiment, we investigated the effects of magnesium-modified biochar (BCMg) and phosphorus-modified biochar (BCP) on CO2 and N2O releases from coastal saline soils, and further uncovered their potential mechanisms. Compared with unapplied biochar (CK) and unmodified biochar (BC) treatment, BCMg reduced both the releases of CO2 and N2O, and BCP decreased N2O release but enhanced CO2 release. pH is the medium through which BCMg affects the release of CO2 and N2O. Specifically, BCMg increased soil pH above 8.5, which reduced the metabolic activity of the microbial community, and the abundance of bacteria directly or indirectly involved in N2O production, thereby decreasing the releases of CO2 and N2O. The amendment of BCP changed soil elemental stoichiometry causing microbial N-limitation. Increasing CO2 release and decreasing N2O release were strategies for microorganisms to cope with N-limitation. These findings suggested that BCMg is superior to BCP in mitigating greenhouse gas emissions, providing a basis for the application of modified biochar to improve the carbon pool and reduce greenhouse gas emissions of coastal saline soil.