Soil Moisture Regime and Land Use History Drive Regional Differences in Soil Carbon and Nitrogen Storage Across Southern Wisconsin

Abstract

Agricultural land management can decrease soil organic carbon (SOC) and nitrogen (N) storage and adversely affect soil structure, but the actual impact can be confounded by the soil moisture regime. Wet or poorly drained soils tend to slow the process of organic matter oxidation and promote C storage, whereas comparatively drier or well-drained soils tend to promote organic matter turnover and C release. Therefore, the effects of land management, time since last disturbance, and soil moisture regime on near-surface soil bulk density (BD), SOC and total N (TN), and soil C:N ratios were evaluated across southern Wisconsin using a database generated during a 7-year period. Soil samples from the top 25 cm were collected from 169 sites consisting of prairie restorations and remnant prairies, Conservation Reserve Program land, agriculture (row crops and pasture), wetlands, and forests. Across all sites, soils with a udic soil moisture regime had lower (35%) native SOC, lower (31%) TN, greater (47%) BD, and lower (5.4%) soil C:N ratio than those with an aquic moisture regime. Agricultural (6.0 kg m⁻²) and restoration (5.8 kg m⁻²) sites had 30 and 32.3%, respectively, less SOC than native sites on udic soils, but agricultural (9.0 kg m⁻²) and restoration (8.8 kg m⁻²) sites with an aquic moisture regime shared similar SOC values with native sites under both udic (8.6 kg m⁻²) and aquic (10.6 kg m⁻²) moisture regimes. A 24.8% loss of native SOC attributed to agricultural land management occurred on aquic soils compared with a 33.6% loss on udic soils, and those effects were still present in prairie restorations. An apparent buffering capacity of wetter aquic soils on losses of C and N needs to be considered in quantifying soil C and N sequestration potential.