Difference in total N and its associated-aggregate N following cropland  restoration in a karst region, Southwest China

Abstract Cropland restoration has been reported as one of the most effective measures for soil total nitrogen (N) accumulation in karst degraded regions. However, the patterns of N associated-aggregate and its contribution to net soil N accumulation following cropland conversion remain poorly understood. The experiment included four treatments with one control and three restoration strategies, that is: maize-soybean rotation cultivation (control), sugarcane, mulberry and forage grass cultivation. Soil samples were selected to determine the soil aggregate amount and its associated N content and stock across 0–30 cm soil layer. Macro-aggregate (>2 mm) was the predominant aggregate fraction in all cropland use types and had the highest N stock. Forage grass cultivation substantially enhanced total N stock in bulk soil and aggregate fractions. Bulk soil N stocks were positively correlated with N content and stock aggregate-associated. The increase in N stock in forage grass soil was primarily attributed to the increased N stock within macro-aggregates, which is further attributed to the increased N content within macro-aggregates. Soil C:N ratio in macro-aggregate, 2–1 mm and micro-aggregate (<0.25 mm) but not in bulk soil was significantly changed following cropland conversion. Overall, forage grass cultivation replaced maize-soybean cultivation was suggested as an ecological restoration model to enhance soil N sequestration potential, owing to its role in increasing N stock of aggregate in the karst degraded region of Southwest China.