Dissolved organic carbon replenishment enhances organic matter mineralization under extremely saline-sodic conditions following straw incorporation

Abstract

Soil sodicity, in combination with salinity, enhances organic matter solubility and impacts organic carbon dynamics. However, the mechanisms of organic carbon mineralization in saline-sodic soils, especially in extremely sodic environments, remain unclear. We conducted a three-month incubation experiment using a randomized block design to clarify this mechanism. With ¹³C-labelled maize straw as the substrate, we compared organic matter mineralization and the priming effect (PE) in two saline-sodic soils (exchangeable sodium percentage 50.1 % and 36.5 %, respectively) with that in a nonsaline soil (exchangeable sodium percentage 4.3 %). Soil samples were collected at 4, 14, 59, and 103 days to analyze dissolved organic carbon (DOC), microbial biomass, and bacterial community composition. Notably, despite similar cumulative CO₂ emissions across all soils over 103 days, the saline-sodic soils displayed distinct temporal dynamics in organic carbon mineralization. In the initial four days, they had reduced primed CO₂ and straw-derived CO₂, but these disparities narrowed by the end of the incubation. Additionally, in saline-sodic soils, despite lower microbial biomass and diversity, higher DOC levels were observed. DOC concentrations significantly increased over time after straw addition and showed a significant positive correlation with total CO₂ emissions (including native soil CO₂, primed C, and straw-derived CO₂). Moreover, DOC concentration was positively correlated with genera whose abundance surged following straw addition under extremely sodic conditions. Partial least squares path modeling illustrated that DOC concentration had a more significant direct impact on total CO₂, straw-derived CO₂, and PE in saline-sodic soils compared to the nonsaline soil, and indirectly affected them by modulating microbial co-occurrence network modules. Our study thus emphasizes the pivotal role of DOC in facilitating mineralization and priming in saline-sodic soils, offering energy and substrates to microbial communities.