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Defining the quality of soil organic matter
Citation:
JOHNSON, M. G. Defining the quality of soil organic matter. Presented at Crop and Soil Science Department Seminar, Corvallis, OR, April 20, 2009.
Impact/Purpose:
Soils represent the largest terrestrial pool of carbon (C) and hold approximately two-thirds of all C held in these ecosystems.
Description:
Soils represent the largest terrestrial pool of carbon (C) and hold approximately two-thirds of all C held in these ecosystems. However, not all C in soils is of equal quality. Some fractions of the organic forms, i.e., soil organic carbon (SOC) have long residence times while other fractions may be quite labile. Depending on the relative proportioning of these fractions, soils can be a very stable global repository for C holding promise for managing soils to sequester C as a strategy to mitigate increasing concentrations of atmospheric CO2. However, it is important to recognize that the stability of C in soil generally increases the longer the soil remains relatively undisturbed. A dramatic loss of C can occur following disturbance and the extent of loss depends on the severity, frequency and duration of disturbance. For example, some portion of SOC is lost when native soils are converted to agriculture such as when forests are cleared or native prairies are plowed. Also, annual conventional tillage management, and frequency and severity of wildfires followed by erosion are disturbances that can substantially increase loss of SOC. Proper soil management can minimize such losses and in many cases re-sequester much of the SOC lost previously. Frequently, the variability in SOC quality is underappreciated. This has implications for knowing how to manage the soil resource properly to reduce the extent of SOC lost or to increase the ability of a soil to sequester and/or re-sequester C. In this presentation I will emphasize the importance of SOC and its quality not only to help mitigate atmospheric CO2 concentrations, but also to provide other essential ecosystem services. I will review key aspects of how SOC is stabilized, present several current definitions for SOC quality, describe methods used to quantify SOC quality, and suggest some new approaches. I will provide a brief analysis of SOC loss and recovery and the roles of various soil C quality fractions in each process. Finally, I will present results from two studies examining forest soils to demonstrate some of the differences in SOC that can be found in Oregon.