The Impact of Carbon Amendments on the Carbon Budget of Soils on a Farm in Central Washington

Increasing atmospheric CO2 concentration over the last 200 years has necessitated new strategies for sequestering carbon (C). Agricultural soil C, depleted by centuries of land use conversion and agricultural practices, is an excellent potential C sink. Conservational land management practices, including zero tillage, cover cropping, crop rotation, and C amendments such as biochar and compost, are used to increase C in soils. In 2016, Spoon Full Farm, in Thorp, WA, converted from traditional hay farming to diverse organic products and adopted these strategies to sequester C in their soil. To determine the impact of their C sequestration strategies, several measurement techniques were employed. CO2 flux was measured by continuous chamber and survey methods in 2018 and 2020. Soil C and N concentration and isotope ratios were analyzed at depths ranging from 0 to 60 cm in summer 2017, and 0 to 35 cm in summer 2019, and fall 2020. Between 2016 and 2018, compost was applied in a mixed vegetable garden, and significant C gains were demonstrated, as well as increased CO2 efflux. However, these C gains did not persist. By fall 2019, C concentrations in the garden soil were only moderately elevated compared to C concentrations before compost application, suggesting that much of the added C was lost. In fall 2019, a relatively thin layer of mint compost was applied to a timothy hay field, with the intent of similarly increasing C concentration. However, in contrast to the observations from the vegetable garden at the same farm, there was no distinguishable change in soil C concentration in the hay field eight months later. CO2 flux also was not statistically different between composted and uncomposted areas in the same field in summer or fall 2020. There was only minor change in N concentration and isotope ratios. These findings, resulting from measurements taken over 4 years, underscore the complexity of C dynamics, and the challenges in sequestering C in agricultural soils.