Abstract

Soil available nitrogen (AN) is highly related to crop growth and N leaching in the agricultural environment, but the heterogeneity of AN and its driving mechanisms have not been clarified in the Mollisols region. In this study, a total of 610 soil samples were collected from 0‒60 cm soil depths in a small watershed (1.86 km²) of northeastern China, and both traditional statistics and geostatistical methods were used to analyze how topographic factors and land managements influence the spatial heterogeneity of soil AN. The results showed that spatial autocorrelation (C₀/[C + C₀]) of AN has a moderate level in 0‒20 cm (34.1%) and 20‒30 cm (37.3%), and a strong level in 40‒50 cm (15.8%) soil depths. AN was highest on the bottom, followed by the summit and the middle slope across all aspects in 0‒20 cm soil depths, but not consistent at 20‒60 cm soil depths. The slope steepness of 6‒8% was an inflection point (increase/decrease) of AN dynamic under different tillage systems. The horizontal and vertical distribution of AN may be influenced by the plough pan (near 30 cm soil depths). Forestland tends to reduce N loss to the lower slope position in watershed scale. Generally, to increase crop yield, reasonable fertilization and conservation tillage should be applied on the middle slope to counteract the low AN content. On the other hand, to reduce N loss, management of areas with high AN should be a priority, such as the bottom of slopes, places where land uses intersect, and watershed outlets.