Spatiotemporal heterogeneity of soil available nitrogen during freeze-thaw cycles in a watershed: A 3-year investigation

The spatial pattern of soil available nitrogen (AN) is the key reference for farmland fertilization however, it is still unclear how freeze-thaw cycles (FTC) altered the spatiotemporal heterogeneity of AN at the watershed scale. In this study, the geostatistical, traditional analysis and structural equation models were used to detect how topography, land management, and soil properties influence the spatiotemporal heterogeneity of AN in both before- and after FTCs of Mollisols of a watershed in Northeast China for three consecutive years. The results showed that AN decreased in >88% of areas of the watershed after FTCs, and the mean value significantly decreased by >7.6% (>10.4 mg kg(-1)) in 0-20 cm soil depth during 2016-2017 years and 2017-2018 years. The AN distribution, especially in the surface soil layer, tended to be aggregated after FTCs. The AN decreased in the areas near the areas of top slope position, while it tended to increase in areas of downslope position covered by forest although the total N decreased here after FTCs. The AN-change (AN(before FTCs) minus AN(after FTCs)) influenced by land use types was greater than slope steepness. The slope aspect as the key factor combined with slope steepness, slope position, and previous crops influenced the spatiotemporal heterogeneity of AN-change after FTCs. The higher content of AN before FTCs accelerated more N loss during FTCs, and this process was mainly influenced by soil pH, nitrogen (N), and phosphorus. The N loss (5.7%) did not significantly alter the AN-change after FTCs in the watershed. Generally, the spatial heterogeneity of AN-change was mainly influenced by topography, land management, and soil properties (before FTCs) during FTCs in the watershed. In order to sustain a high level of AN, the mediation of N transformation and both vertical- and horizontal migration, caused by the dynamics of soil water potential and snowmelt erosion should be considered during FTCs.