Organic Vegetable Farming Promotes Physically Protected Carbon Accrual in Soil.

Agricultural management practices can affect soil organic carbon (SOC) stocks, and its aggregate distribution. Improving our understanding of management effects on SOC stocks and aggregation will support management decisions that ultimately enhance soil health. In this study, the quantity and stability of soil aggregates and SOC under conventional and organic vegetable farming systems were evaluated. Soil samples were collected from conventional and organic broccoli fields at the Colorado State University Horticulture Field Research Center at different soil depths (0-10, 10-20, and 20-30 cm). Soil samples were separated into three aggregate sizes including macroaggregates, microaggregates, and silt and clay by water-stable aggregate distribution. Soil aggregation based on mean weight diameter of aggregates was calculated and SOC was analyzed.  The results showed that i) soil aggregation was higher under organic than under conventional farming systems, ii) within both farming systems, aggregation in the top 0-10 cm soil depth was lower than the other depths, iii) SOC of bulk soil in the top 0-10 cm depth was higher under organic than under conventional farming systems, and iv) SOC in macroaggregates was also higher in organic systems in the 0-10 and 10-20 cm depths. Therefore, organic farming practices should be considered as a means to promote physically protected soil C accrual in vegetable cropping systems.

Keywords: Soil organic carbon, Soil aggregation, Organic farming practices