Soil organic carbon, its fractions and soil organic carbon stocks under different land use systems in Typic Ustrochrepts of northwest India

A study was conducted to assess the impact of different land use systems on soil organic carbon (SOC), its fractions and soil organic carbon stocks under different land use systems in a mixed watershed in the submontane region of north-west India. Soil samples were collected up to a depth of 120 cm from three locations each from maize-wheat, agro-horticulture and agroforestry (3 and 6-year plantation) land use systems. The results revealed that the surface soils (0–15 cm) under agroforestry had significantly higher SOC (5.3 mg g− 1) than in soils under other cropping systems and varied from 4.3 to 5.3 mg g− 1. The labile carbon (LC), aggregate associated carbon (AAC), particulate organic carbon (POC) and mineralizable carbon (MC) were also significantly higher in surface soils under agroforestry 6-year plantation than other three cropping systems and thereafter exhibited significantly lower content of SOC and its fractions in each of the depth as compared to the surface soil in all the cropping systems except maize-wheat system where LC increased significantly in the second depth and decreased gradually in the subsequent lower depths. The proportion of these fractions in relation to the soil organic carbon was lowest in soils under maize-wheat (10–70%) than the tree-based cropping systems (20–100%). The typical decrease of SOC content in the 3rd depth (30–60 cm) was the sharpest and thereafter it stabilized in lower depths in maize-wheat system. The SOC stock followed the trend: agroforestry 6-year (38 Mg ha− 1) > agroforestry 3-year (30.5 Mg ha− 1) > agrohorticulture or maize-wheat system (27.5 Mg ha− 1). Up to 33% lower SOC stocks were recorded in maize-wheat or agrohorticulture cropping systems than under the agroforestry land use system, thereby suggesting that conversion of forest land to agroecosystems can contribute to losses of up to 10.5 Mg ha− 1 SOC over time. Overall conclusion from this investigation is that SOC levels are strongly influenced by the prevailing land use systems.