Agroecosystem Effects on Carbon Sequestration and Soil Function in Tennessee Valley (Alabama) Paleudults

Abstract

Agronomic management affects soil organic matter (SOM) pools that impact chemical and physical properties, soil carbon (C) sequestration, soil quality, and ultimately, soil function. Pools of SOM and management-dependent soil chemical and physical properties were measured on Decatur (Fine, kaolinitic, thermic Rhodic Paleudult) map units, a benchmark soil in the Tennessee Valley region of Alabama. These soil systems provide pedological environments for investigating the interaction of increasing SOM with relatively high near-surface quantities of sesquioxides and phyllosilicate clays. Agroecosystems investigated included long-term (≥ 15 years) pasture, conservation (reduced tillage) cotton (Gossypium hirsutum) with and without grain row-cropping, and conventional cotton row-cropping systems. The objectives of the study were to: 1) quantify and relate soil organic C (SOC) pools [SOC, particulate organic C (POC), and active organic C (AC)], 2) calculate soil C sequestration rates in row crop agroecosystems over a ten-year duration, 3) quantify select soil chemical and physical properties and relate these properties to management practices and SOM, and 4) relate these soil chemical and physical properties to soil quality and soil function. Soils were sampled and characterized for taxonomic placement, and sites were sampled to a 50-cm depth in four depth increments. Significant (α=0.05) differences were observed for SOC pools near surface, but results were mixed with depth. Strong correlations existed between all SOC pools both near surface (0-5 cm) and when pooled across all depths. To a depth of 50 cm, the pasture system (73.1 Mg C ha-1) and conservation row crop systems (51.3 Mg C ha-1) sequestered 36 and 94% more SOC than conventional row crop systems (37.7 Mg C ha-1), respectively. Soil C sequestration rates for conservation systems were on average 0.6 Mg ha-1 yr-1, while conventional row crop systems were relatively static. Carbon sequestration rates and amounts were commensurate with other studies within the southeastern U.S. region. Several soil chemical (e.g., ion exchange capacity and extractable nutrients) and physical properties (e.g., water stable aggregates, water dispersible clay and Atterberg limits) were significantly correlated with SOC pools. Cation exchange capacity (CEC) increased 2.6 cmolc kg-1 per every 10 g kg-1 increase in SOC. The pasture system had lower anion exchange capacity than row crop systems, which suggests that SOM masks (+) charged sites on iron oxides. Plastic and liquid limits increased approximately 5% per 10 g kg-1 increase in SOC in the surface (0-5 cm). Near-surface (0-5 cm) aggregation was improved by increasing SOC, as water dispersible clay was decreased approximately 2% and water stable aggregates were increased approximately 2% per every 10 g kg-1 SOC. Similarly, differences in SOC between stable and non-stable aggregate fractions suggest aggregation in these surface horizons is more strongly related to SOM than iron oxide content. Similar to past studies, systems with decreased surface disturbance resulted in improved soil quality. This study provides quantifiable relationships among SOM and soil properties essential to soil function (e.g., nutrient and water retention and trafficability) in the Tennessee Valley region.