SPATIAL VARIATION AND MANAGEMENT IMPACTS ON SOIL ORGANIC CARBON IN A NORTHEASTERN US WATERSHED

Authors: Dell, Curtis; Sharpley, Andrew

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2005
Publication Date: 6/20/2006
Citation: Dell, C.J., Sharpley, A.N. 2006. Spatial variation and management impacts on soil organic carbon in a northeastern us watershed. Journal of Soil and Water Conservation. 61:129-136.

Interpretive Summary: Increasing the quantity of carbon stored in soil in the form of organic matter is one means to lower atmospheric CO2 concentrations and reduce the potential for global climate change. Carbon credit programs, which pay landowners to build levels of soil organic carbon (SOC), are being considered. Reducing tillage intensity, adding organic materials like manure, switching to no-till planting, or conversion of cropland to pasture or forest are some of the options land owners have to increase SOC. However, detection of increases in SOC in response to changing agricultural management is complicated by a high degree of spatial variation within the landscape. In the current study, SOC variation was assessed at various scales within a small (39.5 ha), mixed-use watershed in central Pennsylvania. The watershed was managed by three farmers each using different tillage methods and manure application frequencies. Soil samples were collected from the upper 5 cm of soil at 30-m grid intervals across the entire watershed and from smaller grids with 10- or 0.6-m sampling intervals. Geo-statistics were used to access spatial relationships and indicate the number of samples needed to accurately estimate SOC levels. Differences in SOC among the three farms were also assessed to detect if differences in management impacted SOC in rowcrop fields. Additionally, 3-m2 micro-plots were sampled (0-5 and 5-20 cm depths) from a common soil series in cultivated fields of each of the three farms. Geo-statistics indicated that sampling on a 30-m grid probably was not sufficient to address spatial variation, but sampling at 10-m intervals adequately accounted for spatial variation. As expected, SOC concentrations in pasture (47 g C kg-1) and forest soils (44 g C kg-1) were greater than cultivated fields (20 g C kg-1). However, differing tillage and manure usage among the three farms had little impact on organic C storage. SOC was slightly greater in the upper 5 cm of soil with conservation tillage or chisel plowing and manure application (20 g C kg-1) compared to moldboard plowing (17 g C kg-1), but increases were offset by greater SOC in the 5-20 cm depth of moldboard plowed soils.

Technical Abstract: Increasing the accumulation of organic carbon in agricultural soils provides one means to reduce atmospheric CO2 concentrations and the potential for global climate change. However, detection of changes in soil organic C (SOC) in response to changing agricultural management is complicated by spatial variation in highly variable landscapes. SOC variation was assessed at various scales within a small (39.5 ha), mixed-use watershed in central Pennsylvania. Surface soil samples (0 to 5 cm) were collected over 30-, 10-, and 0.6-m grid intervals within the watershed and the variation in measured SOC concentrations assessed by geo-statistical analysis. Additionally, 3-m2 micro-plots were sampled (0-5 and 5-20 cm depths) in cultivated fields managed differently by three separate farmers to further assess SOC accumulations in response to varying management practices. Geo-statistics indicated that sampling on a 30-m grid probably was not sufficient to address spatial variation, but sampling at 10-m intervals minimized the spatial variation. As expected, SOC concentrations in pasture (47 g C kg-1) and forest soils (44 g C kg-1) were greater than cultivated fields (20 g C kg-1). However, differing tillage and manure usage among the three farms had little impact on organic C storage. SOC was slightly greater in the upper 5 cm of soil with mulch tillage or chisel plowing and manure application (20 g C kg-1) compared to moldboard plowing (17 g C kg-1), but increases were offset by greater SOC in the 5-20 cm depth of moldboard plowed soils.