Soil Structure and Organic Matter Dynamics: Influence of Tillage and Soil Physical Properties

Abstract

Understanding structural controls over soil organic carbon (SOC) dynamics might improve our ability to predict inconsistent tillage influences on SOC sequestration. This research investigated the influence of soil structure on SOC mineralization and SOC fraction dynamics. A laboratory incubation was conducted using two soils with different levels of organic matter collected in Urbana, IL, (OM-HIGH and OM-LOW). A series of field studies were conducted using a pair of tillage trials in DeKalb, IL on a Drummer silty clay loam soil and in Monmouth, IL on a Muscatine silty loam soil. While the C mineralization and the labile SOC were greater in the OM-HIGH than in the OM-LOW treatment, higher specific C mineralization rates in the OM-LOW soil indicate that SOC mineralization was not determined solely by labile SOC size. Field studies showed that mean and specific SOC mineralization rates from no tilled (NT) and conventionally tilled (CT) soils did not differ in DeKalb, whereas in Monmouth, those were higher from CT than from NT soil. Soils were fractionated into loose- and aggregate occluded particulate (LPOM and OPOM) and humified (HF) organic matter fractions. In DeKalb, NT practices increased HF C, but not OPOM C contents relative to CT soils. However, in Monmouth, only the OPOM C content was higher in soils under NT than CT management. The least limiting water range (LLWR), which quantifies structural controls over moisture limitations at wet and extremes, successfully explained patterns in SOC mineralization rates at these two sites. Aggregate dynamics, expressed in terms of dry mean weight diameter (DMWD) and aggregate turnover rates, was used to develop a conceptual model to understand SOC fraction dynamics. Slow aggregate turnover in DeKalb limited the amount of POM incorporated into aggregates. Accumulation of HF under NT at DeKalb was explained by the protective effect of larger DMWD. More POM was occluded by aggregates in Monmouth where relatively rapid aggregate turnover prevented accumulation of HE. Collectively, these studies explain how soil structure provides protection of SOC. Variable tillage influences on SOC dynamics in these two sites were explained by different soil structural qualities and aggregate dynamics.