Atmospheric concentrations of CO2 have reached unprecedented levels, posing a threat to the health of the biosphere. Soils store the largest amount of terrestrial carbon (C) and can be used as a method for sequestering atmospheric C. The cycling of C between the atmosphere and terrestrial ecosystems is directly influenced by soil microbial activity and respiration. Compost amendments to soil systems have the potential to shift the composition of the microbial community and to increase C sequestration potential. Rangeland ecosystems represent a valuable opportunity for implementing climate mitigation practices with compost amendments. This study aims to answer the following questions: (1) Does a one-time compost addition to rangeland soils induce a shift in the microbial community one year post application? (2) Does compost addition increase or decrease the rate of CO2 efflux over the growing season? and (3) Does MRT of SOC differ between soils treated with compost and soils not treated with compost?
Three rangeland sites located in Gunnison, Colorado were manipulated with compost additions in June of 2019: Coldharbour (CH), Parker Pastures (PP) and Wiley Lane (WL). Soil moisture, temperature and respiration rates were measured throughout the 2020 growing season. Soil samples were collected and analyzed for bulk density, SOC pool, nitrogen (N) pool and microbial phospholipid fatty acids. A one-time compost application had no effect on the microbial community composition of rangeland soils one year post application. Daily CO2 efflux rates were not influenced by compost application, however, the cumulative release of C-CO2 2 over the growing season was higher in the non-composted soils. The largest difference of cumulative efflux between treatments was seen at the CH site where control soils had an average C-CO2 efflux of 811.1 g C m2-1d-1 and treatment soils had an average C-CO2 efflux of 652.8 g C m2-1d-1. Mean Residence Time (MRT) of SOC across sites ranged from 0.64 – 3.21 years and was significantly higher in the treatment soils than in the control soils. On average, compost application treatment increased MRT of SOC by 0.35 years across sites. As the C pools were not significantly different between treatments, the significantly lower efflux observed in the treatment soils was responsible for the increase in MRT of SOC found. These findings present a valuable representation of how improved land management practices on rangeland soils can positively contribute to soil C sequestration.
