Does 46 years of conservation tillage and crop rotations change soil carbon and nitrogen distribution and storage?

Abstract

Soil conservation practices, including no-tillage and crop rotation, are widely promoted to enhance soil quality and sequester carbon (C), thereby contributing to climate change mitigation. This study investigates the long-term impacts of tillage intensity and crop rotation on soil organic carbon (SOC) and total nitrogen (TN) distribution and storage to a depth of 1 m in a U.S. Midwest Mollisol. Tillage treatments included no-tillage (NT), intermediate or reduced tillage (chisel plow: CP), and intensive tillage (moldboard plow: MBP), with three crop rotations: continuous corn (CC), corn-soybean (CB), and continuous soybean (BB). After 46 years of continuous management, NT systems stored, on average, 15 Mg ha⁻¹ more SOC and 2.0 Mg ha⁻¹ more TN than tilled systems when the full depth profile was considered. The SOC and TN storage were similar between the CP and MBP treatments, suggesting limited benefit of CP over MBP in long-term C and N retention. Crop rotation had minimal effects on SOC storage; however, CC systems accumulated significantly more TN than CS and BB rotations, particularly when assessed on an equivalent soil mass basis. Additionally, tillage practices had a pronounced effect on the vertical distribution of SOC and TN, especially within the upper 75 cm of the soil profile. These findings highlight the importance of deep soil sampling for accurately capturing the full impact of management practices and suggest that long-term NT is more effective at promoting SOC and TN storage than conventional tillage, regardless of intensity.