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Impact of Biosolids and Tillage on Soil Organic Matter Fractions: Implications of Carbon Saturation for Conservation Management in the Virginia Coastal Plain

Stewart, Catherine E., Follett, Ronald F., Wallace, James, Pruessner, Elizabeth G.
Soil Science Society of America journal 2012 v.76 no.4 pp. 1257
agroecosystems, best management practices, biosolids, carbon, carbon sequestration, carbon sinks, coastal plains, disturbed soils, growers, no-tillage, organic matter, runoff, soil organic carbon, soil quality, statistical models, texture, Virginia
In the Virginia Coastal Plain, growers have practiced rotational no-tillage (RT) and continuous no-tillage (NT) for more than 30 yr to reduce runoff and improve soil quality. We sampled 48 grower’s fields representing three soil series varying in texture and managed under RT and NT. Half the fields received biosolids (S) application in 2001. We evaluated soil organic carbon (SOC) stocks, C distribution, and potential saturation limits in particulate organic matter (POM) and silt+clay (<53 μm) fractions in three soil series with different silt+clay protective capacities. Across the three soil series, NT increased SOC stocks compared to RT (33.2 ± 1.8 vs. 28.3 ± 0.9 Mg C ha) and S compared to no biosolids application (33.1 ± 1.8 vs. 28.4 ± 1.1 Mg C ha) for the 0- to 20-cm depth. Tillage and biosolids effects on SOM fractions were only significant for POM and showed no evidence of C saturation with statistical modeling. Silt+clay-associated C showed no tillage or S effects and in one soil were better fit with a C saturation compared to a linear model, indicating saturating dynamics. In two soils, measured silt+clay C content was significantly greater than the calculated 1:1 and 2:1 protective capacity, also suggesting silt+clay saturation. Since these soils are near or exceed their silt+clay protective capacity, additional C storage will primarily occur in aggregate and POM C pools that are sensitive to subsequent changes in management pra ctice. Best management practices that decrease soil disturbance and increase C input are crucial to these agroecosystems where even single tillage events can cumulatively cause significant C loss.