Title

Soil organic carbon and aggregate dynamics in a long-term agroecosystem

Date of Completion

January 2005

Keywords

Agriculture, Agronomy|Biology, Ecology|Agriculture, Soil Science

Degree

Ph.D.

Abstract

The ability to increase pools of soil organic carbon (SOC) in agricultural ecosystems is of interest both for sequestering atmospheric CO2, and for restoring organic matter pools important to soil health. It has been well established that tillage and harvest management regimes can influence SOC in cropland, but long-term, continuous experiments are rare. We investigated the effects of tillage and residue application on SOC dynamics, turnover of relic forest-derived C and cycling of new C4 (corn)-derived C in whole soil and in water stable soil aggregates after 31 yr of continuous management at the University of Connecticut. The plots have been under no-till (NT) or conventional till (CT) management, with residues returned (+) or removed (-) annually within each tillage treatment. Chapter 1 focuses on soil C dynamics and evaluates relic and new carbon allocation after 28+ years of continuous management. Our results indicate that there may be a rapid cycling of the aboveground C4-C back to the atmosphere as CO2 or lost as dissolved organic C from the soil profile. Such a rapid cycling of returned C4-C suggests that the annual return of aboveground biomass may not increase soil C storage over the long term once soils have reached a steady-state SOC level. Chapter 2 provides an extensive review of the soil aggregate literature, focusing on comparisons between CT and NT management and including our site at the University of Connecticut. Chapter 3 explores soil aggregate distribution and C dynamics at our long-term agroecosystem. In soils from our experiment, strong treatment-induced patterns are seen in the >2000-μm macroaggregates, with NT and residue inputs significantly increasing SOC content and aggregate distribution. However, NT management did not increase SOC or aggregate distribution in the other aggregate size classes. Soils at our site have a relatively high organic C content and a high concentration of Fe+2 and illite clay possibly leading to strong clay-metal-organic matter bonds that protect organic materials from microbial decomposition and increase aggregate stability. It is possible that the combination of soil mineralogy, organic matter, and time has contributed to the buildup of SOC in soil aggregates. ^

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