Dynamic responses of soil aggregate-associated organic carbon and nitrogen to different vegetation restoration patterns in an agro-pastoral ecotone in northern China

Abstract

Soil aggregates are the basic soil structure units and they have important effects on the accumulation and stability of soil organic carbon (OC) and nitrogen (N). However, the responses of OC and N in bulk soils and aggregates to the restoration plant type and time in ecologically fragile regions with low clay contents require elucidation. Thus, to determine the dynamics and distributions of soil OC and N under different vegetation restoration processes, we investigated four revegetation patterns (Caragana microphylla, grassland, apricot trees, and Chinese pines) within different recovery times (5–25 years) in an agro-pastoral ecotone in northern China, with adjacent cropland as the control. The OC and N contents were measured in the bulk soils and aggregate fractions, and the geometric mean diameter (GMD) and mean weight diameter (MWD) were calculated. The results indicated that the mass of the soil was dominated by microaggregates when averaged across the four revegetation patterns, followed by silt plus clay, and macroaggregates comprised the lowest mass. Microaggregates contributed significantly more OC and N to the bulk soil due to their large mass. Revegetation significantly increased the mass percentage of macroaggregates, MWD, and GMD (P < 0.005). The increases in the OC and N concentrations in bulk soil and aggregates induced by revegetation were related to the soil depth and vegetation type, where the highest increases occurred in each aggregate fraction with Caragana microphylla and grassland at a soil depth of 0–10 cm. These results demonstrate that revegetation of abandoned cropland has great potential for sequestering OC and N in the bulk soil and each aggregate fraction, especially with C. microphylla and grassland.