Mechanical properties and organic carbon of soil aggregates in the northern Appalachians

Aggregate properties determine the macroscale structural condition of the soil. Understanding of impacts of no-till and traditional agricultural practices on the mechanical properties of aggregates is fundamental to soil management. This study assessed the tensile strength (TS), bulk density (rho(agg)), soil moisture retention (SMR), and soil organic C (SOC) concentration of soil aggregates and determined the interrelationships among aggregate properties under long-term moldboard plow (MP), chisel plow (CP), disk with beef cattle manure (DM), no-till with beef cattle manure (NTM), no-till without beef cattle manure (NT), pasture, and forest systems in the North Appalachian region. Properties were determined on 1- to 8-mm aggregates from 0- to 30-cm soil depth. The TS and SMR (0 to -333 kPa) in NTM were higher than those in MP and CP (P < 0.01). The SOC concentration for NTM was higher than that for MP, CP, and NT (P < 0.01). The rho(agg) was 1.35 Mg m(-3) in NTM and approximately 1.61 Mg m(-3) in MP and CP (P < 0.01). Manuring had a positive and excessive tillage negative impact on aggregate properties. Aggregates from forest had the lowest TS (63 kPa) and rho(agg) (0.99 Mg m(-3)) and the highest SOC concentration (70 g kg(-1)), whereas the MP and CP had the highest TS (approximately 358 kPa) and the lowest SOC concentration (14 g kg(-1)) in 0- to 10-cm depth (P < 0.01). Mean rho(agg) was significantly higher than the density of bulk soil (Pb). The logtransformed TS (LogTS) increased with increasing p(agg) and decreased with increasing aggregate size and SOC. Size, SOC concentration, and p(agg) explained 84% of the variability of LogTS. Long-term (> 35 yr) no-till combined with manuring improved the aggregate properties contrasting with conventionally cultivated systems.