Postharvest Organic Matter Retention as a Potential Mechanism for Soil Carbon Sequestration.

Research into postharvest management of forests often focuses on balancing the need for increased biomass yield against factors that directly impact the productivity of the subsequent stand (e.g. nutrient and water availability, soil microclimate, etc.). However, postharvest organic matter management also exerts a strong influence over the translocation of carbon (C) into and through the soil profile and may provide a mechanism to increase soil C content. The effects of contrasting postharvest organic matter retention treatments (bole-only removal, BO; whole-tree removal, WT) on soil-solution C concentration and quality were observed at the Fall River and Matlock Long-term Soil Productivity (LTSP) studies in Washington State. Solutions were collected monthly at depths of 20 and 100 cm and analyzed for total organic C (TOC), total organic nitrogen (TON) and TOC:TON ratio. Comparisons of TOC concentrations with depth illustrate divergent trends between the two treatments, with an overall decrease in TOC with depth in the BO treatment and an increase with depth in the WT treatment. Trends in TON concentrations with depth were less clear, although the relationship of the TOC:TON with depth shows a decrease in the BO treatment and little to no change in TOC quality in the WT treatment. This illustrates that the more recalcitrant organic matter (higher TOC:TON) is being removed from solution as it moves through the soil profile, suggesting that it is sorbing or precipitating onto the solid matrix rather than acting as a substrate for microbial utilization. Only 35-40% of the TOC moving past 20 cm in the BO treatment is present at 100 cm. Conversely, 98-117% of the TOC at 20 cm in the WT treatment is present at 100 cm. Thus, 11 and 30 kg C ha^-1 yr^-1 are lost from solution in the BO treatment at the Matlock and Fall River LTSP studies, respectively. Although much of this C is often assumed to be utilized for microbial respiration, the unique mineralogy of the soils of this region suggest that a significant portion may in fact be incorporated into the more recalcitrant soil C pool. Thus, postharvest organic matter retention may provide a mechanism to increase soil C sequestration.