Evaluating the optimal land use pattern for saline-sodic soils from the perspective of nitrogen metabolism

Enhancing soil nitrogen storage is a global concern, particularly in soils affected by salinization. Land use changes significantly affect soil nitrogen cycle and its metabolic processes; however, their impacts on nitrogen availability and microbial nitrogen transformation in saline-sodic soils remain unclear. To address this knowledge gap, soils of six land use types – paddy field (PF), dryland (DL), converted paddy field to dryland (SGH), forestland (FL), grassland (GL), and wasteland (WL) – were collected to investigate the underlying mechanism of nitrogen transformations. Compared to WL, agricultural land use systems (PF, DL, SGH) significantly decreased (p < 0.05) soil pH (10.65–8.38 units), electrical conductivity (EC) (1.51–0.19 dS m ⁻¹), exchangeable sodium percentage (ESP) (86–8 %), sodium adsorption ratio (SAR) (203 to 13), and water-soluble salt ions. Moreover, agricultural land use systems significantly increased soil organic matter (SOM), available phosphorus (AP), available potassium (AK), and nitrogen fraction contents relative to WL and enriched nitrogen-metabolizing microorganisms. Furthermore, agricultural land use systems were more advantageous than non-agricultural land use systems in improving soil nitrogen availability, through affecting N fixation, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA). In addition, network analysis revealed that soil physicochemical properties shaped soil nitrogen-metabolizing microbial communities. Crucially, ammonium nitrogen (NH ₄ ⁺-N) and nitrite nitrogen (NO ₂ ^--N) were critical determinants of soil nitrogen metabolism dynamics. Therefore, agricultural land use systems, especially PF and DL, were conducive to the improvement of soil salinization and the promotion of soil nitrogen metabolism and storage in saline-sodic soils.