Soil organic carbon in playas and adjacent prairies, cropland, and Conservation Reserve Program land of the High Plains, USA

Abstract

Soil organic carbon (SOC) is an important reservoir for atmospheric CO2 associated with climate warming. The High Plains, USA, lacks region-wide SOC estimates within playa wetlands and their adjacent watershed. Croplands often have less SOC than grasslands, and the Conservation Reserve Program (CRP; former croplands planted to grass) may return SOC. Our goals were to estimate SOC within playa wetlands and investigate small scale differences within wetland catchments across a broad agriculturally modified landscape. We estimated SOC (kgm−2) to 50cm depth from 4 soil cores/catchment (in playas and 10, 40 and 100m into uplands) at 56, 52, and 54 sites in native grassland, CRP and cropland, respectively. At a subset of sample locations within each land use type, we estimated SOC to 1m depth to characterize SOC missed by shallow sampling. In playa wetlands, CRP SOC from 0 to 50cm was 18% greater than croplands, but native grassland playa SOC did not differ from other land-uses. From 0 to 1m, SOC in native grassland wetlands and uplands was 20% greater than the same habitats within croplands, while CRP lands were intermediate. Native grassland playa SOC also was 16% greater than in surrounding native short grass prairie. Playas therefore represent an important SOC repository in the High Plains ecoregion. CRP playas and uplands may require an additional 10–30 years to resemble native grassland SOC. SOC increased with playa area throughout CRP and native grassland catchments, suggesting playa hydrogeomorphology influences adjacent upland SOC. High Plains playas store 20.8Tg C and cropland conversion caused a cumulative loss of 2.0Tg C from 82,000ha of playas. Currently, CRP enrollment on over 25,000ha of playas has returned 0.2Tg C (95% CI: 0.1–0.3), only half the historic SOC lost by cropland conversion within CRP playas. To promote SOC storage, native grasslands and large playas should be preserved and CRP enrollments should be maintained over long timescales.