Spatiotemporal dynamics of cropland soil nitrogen over the past four decades in China's Mollisols region: Implications for sustainable nitrogen management

Abstract The changes in nitrogen (N) content of fertile Mollisols because of unsustainable management practices and climate change pose serious threats to food security and environmental sustainability. However, the lack of spatiotemporal dynamic information on soil N content across different soil depths hinders sustainable N management. To address this gap, over 10,000 soil total nitrogen (TN) samples were collected based on soil types and land use types from Northeast China, one of the world's four major Mollisol regions. Subsequently, space–time automatic machine learning was developed to model the spatiotemporal dynamics of cropland soil TN content at multiple depths (0–30, 30–60 and 60–100 cm) from 1980 to 2023. The results showed that topsoil (0–30 cm) TN content declined from 1.96 g kg −1 in 1980 to 1.74 g kg −1 in 2023, with an average decline rate of 0.006 g kg −1 year −1 . The changes in topsoil TN content exhibited three distinct phases: a significant increase from 1980 to 1989, followed by a rapid decline from 1989 to 2008, and a slower decline from 2008 to 2023. Regarding different provinces, Heilongjiang and Jilin experienced a significant decline in topsoil TN content over the past four decades, while Liaoning exhibited TN accumulation. Despite a 3.15 Tg increase in external N inputs over the past four decades, topsoil N storage has decreased by 28.62 Tg. Portions of the combined surplus N from crop production and lost topsoil N leached into deeper soil layers, resulting in an increase of 7.88 Tg and 14.77 Tg in soil N storage at depths of 30–60 cm and 60–100 cm, respectively. This study emphasizes that, with the annual increase in exogenous N inputs, mitigating the loss of topsoil N and the accumulation of N in deeper soil layers is a key challenge for sustainable N management in China's Mollisols region, aiming to achieve the dual goals of increasing crop yields while enhancing environmental sustainability.