Carbon sequestration and soil health in conservation reserve program grasslands in the southern high plains



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Initiated in 1985, the Conservation Reserve Program (CRP) is one of the largest private-land conservation programs in the USA. The aim of CRP is to encourage landowners to convert highly erodible cropland or other environmentally sensitive acreage to vegetative covers, such as perennial grasslands, to improve environmental health. Numerous ecological benefits have been achieved as a result of the CRP program including soil erosion control, improvements in water quality, increased wildlife habitat, and carbon (C) sequestration. Soil microorganisms are involved in the majority of soil functions, such as nutrient provisioning and cycling, pest and pathogen protection, symbiotic and compensatory associations with plants, as well as the formation of stable aggregates to reduce soil erosion and build soil organic matter. Texas, and specifically the Southern High Plains region (SHP), leads the nation in acres enrolled in the CRP program. The three objectives of my research were: 1) to estimate a regionally specific C sequestration rate of the SHP region; 2) investigate the role of soil microbial processes that are important in C sequestration; and 3) to examine the influences of the long-term CRP grassland on soil biological/microbial health indicators in the SHP. A CRP chronosequence study (0-28 y as of 2014) was designed to address these objectives. Soil samples (0-10 cm and 10-30 cm) were collected in 2012 and 2014 from 26 fields across seven counties within the SHP and included seven croplands (representing 0 y in CRP), 16 CRP fields of varying ages (6-28 y), plus three rangelands sampled selected to serve as the reference system assumed that had never been plowed or cropped. The soils were dominated by either Amarillo or Patricia series, which are fine sandy loams and are highly susceptible to wind erosion. Relative to cropland, CRP grasslands with year-round plant cover provided supplementary plant organic residues into the soil and reduced soil diurnal temperature range, which supported greater microbial biomass and assisted C sequestration. Soil organic C (SOC) increased at a rate of 69.8 and 132.9 kg C ha-1 y-1 at depths of 0-10 cm and 0-30 cm, respectively. Scaling these results to the 0.92 million ha of CRP lands in the SHP resulted in an average C sequestration potential of 64.1 and 122 Gg C y-1 and would take approximately 74 and 77 y to reach the rangeland levels at depths of 0-10 cm and 0-30 cm, respectively. These results highlight the need for longer restoration periods and enhanced management efforts such as the proper integration of livestock into CRP lands to facilitate SOC accrual rates and associated ecosystem benefits, especially in the SHP region with extreme weather (e.g. frequent drought) and fragile, sandy soils. Soil microbial community size and metabolic activities increased and soil microbial community structure shifted in a direction of decreased bacterial abundance and increased fungal abundance particularly arbuscular mycorrhizal fungi (AMF) with the increasing CRP restoration years. The AMF provide numerous soil functions such as assisting plants in absorbing water and nutrients, especially phosphorus, increasing C sequestration, alleviating salinity, improving soil structure, sequestering potential toxic elements in soil, and biocontrol of plant pathogen. Rangelands that have not been disturbed by agricultural management practices and the old CRP fields were proposed to have highest AMF, but my data did not support this hypothesis. My results indicated that CRP restoration has a significant C sequestration potential and a great capability to recover soil health. However, there are still improvements that could be done if the management practices target enhancements of soil microbial community health, such as increased native grasses in the seeding mix or increased diversity with the addition of properly managed livestock to stimulate microbial processing important for ecosystem services within CRP land.



Conservation Reserve Program (CRP), Carbon (C) sequestration, Soil health