COS 142-4 - CANCELLED - Climate and soil health benefits of compost application to rangelands with steep slopes

Background/Question/Methods

Rangelands can be managed for carbon sequestration by the addition of compost. High upfront cost of compost is a major barrier to widespread adoption of this practice. Existing financial incentives to improve soil health through the application of compost require relatively flat topography, excluding rangelands with slopes exceeding 8%, due to concerns over nutrient runoff. Our team sought to generate new insight into how compost can be safely and effectively applied to steep slopes. We conducted a three-year field experiment on rangelands with steep topography (15-30% slope) to determine the efficacy of compost application as a carbon farming strategy. We measured soil organic carbon stocks to one meter annually and soil greenhouse gas fluxes at daily for two weeks after application and biweekly (wet season) to monthly (dry season) thereafter. Aboveground net primary production and plant community composition was measured annually during peak standing biomass. Runoff was collected at the bottom of each one acre plot (n=5) and analyzed for nitrate, ammonium, and phosphate concentrations.

Results/Conclusions

During the first two years of the experiment (2019 – 2021), soil organic carbon declined in control plots by 4.4 ± 1.7 Mg C/ha. In contrast, soil organic carbon increased by 0.83 ± 1.6 Mg C/ha in the composted plots. There were no significant treatment differences in soil fluxes of carbon dioxide, methane, or nitrous oxide. The effect of compost on plant biomass was influenced by aspect, with an increase occurring on south-facing slopes and no change on north-facing slopes. Interannual variability was very high and overwhelmed treatment differences. Mean aboveground net primary productivity was 414 ± 97 kg/ha (control) and 438 ± 80 kg/ha (compost) in 2020 and 58 ± 9 kg/ha (control) and 74 ± 3 kg/ha (compost) in 2021, reflecting two consecutive drought years. Volume of runoff was low across all plots. Our results suggest that planners and growers may consider compost application on rangelands with steeper topography, expanding the total acreage upon which improved soil management is considered feasible. This potential expansion of the compost addition practice to steeper slopes will greatly enhance the ability of rangelands in California to provide carbon sequestration benefits to help meet the state’s ambitious greenhouse gas reduction goals.