Changes in soil carbon dynamics during juniper expansion into tallgrass prairie

The Flint Hills of Kansas, a grassland region dominated by C₄ tallgrasses, was the setting for this study of the effects of a dramatic vegetation change on soil carbon dynamics. Tallgrass prairie is a mesic type of grassland with precipitation sufficient to support either the native C₄ grasses or trees. In eastern and midwestern United States, Juniperus virginiana L., also called redcedar, has encroached on grasslands, prairies, and abandoned pastures. The purpose of this study was to quantify changes in belowground C cycling due to a shift in vegetation from tallgrass prairie to juniper forest.; Modern techniques for determining plant origins of soil organic matter have utilized naturally contrasting 13C:12C abundance ratios of plants using the C₃ and C₄ photosynthetic systems. We used this technique to characterize changes in soil carbon dynamics accompanying the conversion of prairie to forest. Due to a large consistent background of soil organic carbon with the isotope ratio of native C ₄ plant litter, we can reliably identify the contrasting isotope ratio of net C₃ inputs to the soil, and C₃-CO₂ respired by the soil. Differences in isotope abundance ratios allowed us to quantify net accumulation of new C input in forests from C₃ trees and the turnover of residual prairie C. Approximately 42% of the SOC at the mineral soil surface (0–2.5 cm) was derived from new juniper inputs, while only ∼6% was juniper-derived at 25 cm depth. We also investigated the vegetation origin of soil size-fractions, and determined that the new C was prominent from 0–10 cm and distributed primarily in the largest soil particle sizes. We observed a 38% decrease in the rate of total soil respiration in juniper forests. Consequently, this study suggests that soil carbon turnover in juniper forests will take approximately 9 years longer than C turnover in grassland soils. We also learned that the substrate for microbial respiration under controlled conditions was new C₃ juniper-derived carbon. Compared to heterotrophic respiration rates at 0–10 cm soil depth, rates decreased by ∼50% from 10–20 cm. If aggrading juniper forests mature to closed canopy forest, they have the potential to alter soil C cycling across thousands of hectares in the central and eastern United States.