Long-term Impacts of Conservation Management Practices on Soil Carbon Storage, Stability, and Utilization under Cotton Production in West Tennessee

Biogeochemical cycling of soil carbon (C) is heavily influenced by conservation agricultural (CA) practices. This study examined SOC stability under three CA practices: reduced nitrogen (N) fertilizer application rate, cover cropping, and zero-tillage implemented for 31 years. Respiration rates measured from a 602-day incubation period were fitted to a double-pool first order exponential model of SOC decomposition. The active [respired] SOC pool showed distinct differences between applications of reduced (34N kg ha^-1 [-1]) and high fertilization rates (101N kg ha^-1) combined with tillage, and suggest that high fertilizer applications with conventional tillage allocated more C into a rapidly turning over C-pool. The slow SOC pool dynamics revealed that given more time, soil C losses would stabilize under reduced N-rates and no-tillage conservation practices.

More detailed biological controls on broad nutrient acquisition efforts were indicated by extracellular enzyme potential activities of whole soil, and reflected differences between N availability and N form under organic or inorganic additions to this cotton system. The effects on soil physical properties showed that less soil disturbance had a clear effect on the distribution of dry aggregate fractions, where no-tillage had greater abundance of large macroaggregates (> 2 mm) and persisted when combined with cover crop management.

Macro- (> 0.25 mm) and microaggregates (< 0.25 mm) within conventional tillage systems were distinct from those in no-tillage systems due to greater enzyme activity in total C, N, and phosphorus (P), and could indicate faster decomposition rates in conventional tillage compared to no-tillage management. To determine where soil C was most vulnerable to decomposition across the aggregate fractions, respired C was measured after a short-term incubation study and showed greater mean respired C under small macroaggregates (0.25-1 mm) and microaggregates compared to large and medium (1-2 mm) macroaggregates. However, future research must measure total C from each aggregate fraction to determine if soil aggregates can act as a physical protection barrier to sequester SOC.