Converting croplands into perennials may contribute to the deep soil nitrogen (N) accumulation in N-limited region: Evidence from the arid and semi-arid Chinese Loess Plateau

Converting degraded croplands into perennials has been proposed as an effective method of soil N sequestration, however, the dynamics of deep soil N (>100 cm) following cropland conversion are not well understood. In this study, we synthesized 3049 observations to detect the changes in deep soil N content following cropland conversion on the arid and semiarid Loess Plateau. Our results showed that converting croplands into perennials significantly increased the soil N content by an average of 57.4%, 23.1%, and 29.5% in the surface (0-20 cm), subsurface (20-100 cm), and deep (100-200 cm) layers, respectively. The extent of the increase was influenced significantly by the land-use conversion types and tree species. Specifically, the conversion of croplands into deep-rooted forests or shrubs, particularly Robinia pseudoacacia and Caragana microphylla, exhibited higher advantages in deep soil N sequestration. Moreover, deep soil N sequestration increased significantly with time since cropland conversion (p < 0.001), and the rates in deep soils were approximately 26.1% and 66.7% of that in 0-20 and 20-100 cm soils, respectively. In the long term, converting croplands into forests and shrubs showed higher potential for deep soil N sequestration. Linear regression analysis showed that the changes of deep soil N sequestration were influenced significantly by initial soil N content (p < 0.001) and humidity index (p < 0.001), with the slopes in >100 cm layers being 2 to 3 times than that in the top meter, indicating higher sensitivity in deep soils. Overall, this study provides evidence that converting degraded croplands into perennials may contribute to deep soil N accumulation in N-limited regions, which could potentially alleviate N limitation and sustain long-term ecosystem carbon sequestration.