Impact of the return to cultivation on carbon (C) sequestration

ABSTRACT:

A growing body of science indicates that carbon (C) can be sequestered in soil as a result of changes in land management. Generally, this requires land be taken out of cultivated agriculture; however, it has been postulated that gains in soil C can be quickly eliminated with return to cultivation. The objective of this study was to examine the impact of converting land back into cultivated agricultural management on C sequestration within two different soil types. Soil samples from nine depth increments (0–5, 5–10,10–15, 15–30, 30–45, 45–60, 60–75, 75–90, 90–105 cm) (0–2, 2–4, 4–6, 6–12, 12–18, 18–24, 24–30, 30–36, 36–42 in) were collected from a Blanton loamy sand (loamy, siliceous, semiactive, thermic Grossarenic Paleudults) and an Urbo clay loam (fine, mixed, active, acid, thermic Vertic Epiaquepts) in central Alabama, USA, that were under different land management systems. Management systems included forest, permanent pasture, and pasture converted to continuous cultivation for 1 and 2 years. Within the loamy sand soil, land management also included continuously cultivated (>40 yr), weedy-fallow for 5 years, and returned to cultivation after weedy-fallowed for 4 years. Soil samples were analyzed for total nitrogen (N), organic C, and soil C:N ratio. The clay loam soil had higher capacity to sequester C [147 Mg ha⁻¹ (66 tons ac⁻¹) for pasture], than the loamy sand soil [74 Mg ha⁻¹ (33 tons ac¹) for pasture]. Little difference was observed between the forested soil and the permanent pasture in the clay loam soil, with 139 Mg ha⁻¹ (62 tons ac¹) and 147 Mg ha⁻¹ (66 tons ac¹), respectively. In the loamy sand soil, large differences were observed for C between the forested and the permanent pasture sites, with 127 Mg ha⁻¹ (57 tons ac¹) and 74 Mg ha⁻¹ (33 tons ac¹), respectively. Results indicate that the vulnerability of soil to lose sequestered C will likely depend on soil type. The clay loam soils, although having higher levels of C, lost 55% of its C with 2 years of cultivation, while the loamy sand soil showed little significant loss of C content [below 0–5 cm (0–2 in)] within the same time frame.