Organic carbon dynamics and soil stability in five semiarid agroecosystems

Interest in integrated crop-livestock agroecosystems (ICL) has increased due to their versatility in management options, potential to offset increasing levels of atmospheric C and enhanced agronomic and ecosystem sustainability. Identifying agroecosystems that have the greatest potential for C sequestration requires an understanding of soil organic C distribution within aggregate fractions. Six soil aggregate C pools were physically isolated to evaluate the C sequestration potential of three ICLs and two continuous cotton (CTN) agroecosystems in the Texas High Plains. The proportions of the water stable aggregate fractions were used to calculate mean weight diameter, an indicator of soil stability. The first ICL (FRG_CTN) included paddocks of dryland perennial native grasses, a foxtail millet-cotton (Setaria italica [L.] P. Beauv. and FiberMax 9058F, respectively) rotation, and WW B-Dahl – Old World Bluestem [bluestem; Bothriochloa bladhii (Retz) S.T. Blake] under deficit irrigation (replacement of approximately 30% evapotranspiration). The second ICL (OWB_BER) included paddocks of deficit irrigated bermudagrass [bermuda; Cynodon dactylon (L.) Pers.] and bluestem and the third ICL (FRG_RC) included paddocks of irrigated bluestem and row crop production. Soil samples (0–5 and 5–20 cm) were collected in July 2010. In general, ICLs increased water stable macroaggregates providing a physical protective shell for SOC and increasing C sequestration potential. A strong correlation between SOC and mean weight diameter identified a critical SOC level of 5.5 Mg ha−1 for the greatest increases in mean weight diameter. Of the five agroecosystems evaluated, FRG_RC and OWB_BER ranked the highest in terms of mean weight diameter (130% larger), whole SOC (up to 45% more), and intra-aggregate microaggregate SOC (157% greater) relative to CTN production. Increased stability and reduced exposure of intra-aggregate fractions resulted in relatively greater intra-aggregate microaggregate SOC under perennial vegetation. The potential to serve as significant SOC accumulators may aid in offsetting increasing atmospheric C levels, and while specific to these semiarid soils, the identified critical SOC level can act as a target for producers to minimize and ultimately reverse soil degradation.