Subramanian, S. K. 2008. Effect of topography and soil properties on spatial variability of soil carbon (C) sequestration in different crop management systems of a long-term experiment. Dissertation, Michigan State University, East Lansing, MI, USA.

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Accurate quantification of total soil C is needed to assess the sequestration potential of different management systems; however the accuracy in estimating soil C to determine best management practices is affected by the inherent variability of soil properties. I hypothesized that spatial variability present in soil C distribution across landscape to a large extent is related to topographical gradients; and that tillage and management practices might further buffer or intensify the influence of topography on C variability. The study was carried out at the Long Term Ecological Research site at Kellogg Biological Station, Michigan. In this work, the effects of topographical features, soil properties and different management practices on soil C dynamics and characterized spatial variability pattern in soil C distribution in each management system were analyzed. Treatments studied were short rotation woody perennial system (poplar) and agronomic systems including chemical input based chisel-plow (CT), no-till (NT) system and organic based chisel-plow system with cover crops (CT-cover). As hypothesized, spatial variability characteristics of the soil C in the agronomic treatments, but not in the poplar systems, were strongly affected by topographical gradients. Within agronomic system, topography effect was stronger in organic based chisel-plow system with cover crops (CT-cover) compared to chemical input based chisel-plow (CT) and no-till (NT) system. Alter controlling the topographical effect, the greatest difference in terms of spatial variability characteristics between poplar and agronomic systems was observed in variogram values near the origin. Results indicated that in poplar, C is much more variable at very short distances as compared to agronomic systems. Total C and overall variability in C observed at poplar was found to be similar to that of NT, and slightly greater than CT-cover and differs substantially only with CT. Among the agronomic systems, CT-cover was as efficient as the NT system in restoring C and N and the largest C and N benefits of CT-cover in the landscape were observed in valleys as compared to upper slope positions. The net changes in soil C content that occurred in the past 15-20 years under different agricultural management practices and in never tilled soils were estimated after accounting for the variations in baseline C, soil texture and topographical features among the studied treatments. Results indicated that soil C has decreased by about 10 percent at conventional chisel-plow management soil since 1986. No-till and organic based chisel-plow system neither gained nor lost C as compared to the baseline values (1986-88). Baseline C was found to be negatively related with changes in C content and C loss was found to increase with increase in baseline values. Over all results demonstrated that the conservational management practices have prevented total C loss as compared to conventional management but there was no net gain in C over the past two decades in the studied Michigan soils.

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