Pastureland Soil Organic Carbon Storage Regulated by Pasture Species and Age Under Nitrogen and Water Addition in Northern China

Abstract

Soil organic carbon (SOC) is a key indicator of soil quality and an important component of the global carbon cycle. Enhancing SOC through crop rotation is a promising strategy; yet, the underlying mechanisms for SOC accumulation remain unclear. This study aimed to evaluate the effects of different pasture age, pasture species, irrigation, and nitrogen (N) fertilization treatments on SOC content and storage in pastureland, analyzing the SOC content and below-ground biomass (BGB) data of different soil layers (0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm) of each treatment under three factors (pasture species (Bromus inermis, Medicago varia, the 1:1 mixture), irrigation (CK, dry-season supplementation), and N fertilization (0 kg N hm−2 y−1, 75 kg N hm−2 y−1, and 150 kg N hm−2 y−1)), as well as the interaction effects of these factors. Pasture species, water and N addition levels, and pasture age all had significant (p < 0.05) effects on BGB. At the age of 1–3, the SOC content of monocultured Bromus inermis was slightly higher than the monocultured Medicago varia and the mixture, and at the age of 4–5, monocultured Medicago varia and the mixture were slightly higher than the monocultured Bromus inermis. Among them, the mixture was the highest. At the age of 2–5, the BGB of pastureland was significantly influenced by pasture species, N and water addition, and pasture age. Over a 5-year period, SOCs in the surface layer of the fallowed cropland accumulated 32.35 Mg ha−1, showing a very good carbon sequestration effect; especially the planting of a mixed pasture had a more significant positive effect on the accumulation of SOC. Therefore, for the low and medium yielding fields in China, according to the crop utilization target and production cycle, the purpose of improving soil quality can be effectively achieved through crop and grass rotation.