| Agricultural management practices impact the sources and sinks of carbon dioxide (CO2) and other greenhouse gases attributed to global warming. Application of long-term no-till (NT) practices to sequester soil C has been recognized in C trading and agroecosystem sustainability. Contributions by conservation management practices (e.g. no-till, cover cropping, and organic amendments) in sequestering C can be estimated at about one-fourth to one-third of annual atmospheric CO2 enrichment; whereas conventional tillage (CT) and management practices such as crop residue removal and low-input agriculture contribute to soil organic carbon (SOC) and nitrogen (SON) loss. This research examines the impacts of tillage and management induced changes on SOC and SON dynamics in diverse agroecosystems. The objectives of the research were to investigate tillage effects on water stable aggregation (WSA) and C and N distribution in bulk soil, aggregates, and particle-size fractions; and to evaluate various soil factors controlling WSA and SOC using a principal component analysis (PCA) method. Soil samples used in the investigation were from the surface 0- to 10-cm depth of NT and CT systems in a Wickham sandy loam (fine-loamy, mixed, semi active, thermic Typic Hapludult), Delanco fine-sandy loam (fine-loamy, mixed, mesic, Aquic Hapludult), and Wedowee sandy clay loam (fine, kaolinitic, thermic Typic Kanhapludult); representing North Carolina Coastal Plain, Mountain, and Piedmont locations, respectively. Water stable aggregation was determined on wet-sieved aggregates. Silt+clay fractions were fractionated according to particle size. Carbon, nitrogen, and free and non-crystalline aluminum (Al) and iron (Fe) (hydr)oxides were determined and enrichment factors for C (EC) and N (EN) (mass C or N per mass fraction/mass C or N per mass bulk soil) calculated. Relative to CT, NT increased WSA and SOC/N and MBC/N. In the Mountain, C and N pools in bulk soil and aggregates were 1.6-2.6 times greater under organic-amended NT (NTO) than CT. In the Piedmont, NT increased WSA by 1.4-2.0 times, SOC and N by 1.5-2.0 times, and aggregate associated C and N by 1.4-3 times. Average SOC in equivalent soil mass was 1.5 and 1.8 times greater under NT in the Mountain and Piedmont, respectively; and 1.1 times greater in the fallow system in the Coastal Plain. Under NT and CT, silt+coarse clay proportion was 3-12 times greater than fine clay but C and N concentrations and enrichment factors were 1.1-3 times greater in fine clay than silt+coarse clay across locations. Silt+coarse clay and fine clay C and N concentrations were 1.1 and 1.6 times greater under NT than CT, while EC and EN were 1.3 and 1.1-1.2 times greater under CT than NT, in the Mountain and Piedmont, respectively. The EC was 1.1 times greater under CT than NT in the Coastal Plain. Significant relationships existed between MWD and non-crystalline Fe, hot water extractable C (HWEC; which measures potentially bioavailable C), and SOC and bulk density in the Coastal Plain, Mountain, and Piedmont, respectively; whereas SOC related with clay in the Coastal Plain, humic matter in the Piedmont, and HWEC in all locations. These results illustrate greater enhancement of WSA and sequestration of C and N in bulk soil, aggregates, and the fine clay fraction under NT in the soils studied. The results further demonstrate the usefulness of the physical fractionation procedure used to differentiate SOC and N distribution in bulk soil, aggregates, and particle-size fractions, and of principal components in estimating WSA and SOC, under different tillage and management practices in diverse agroecosystems. |
