Can organic matter additions to rangelands mitigate climate change?
Rangelands are ubiquitous, covering over a quarter of the terrestrial land surface, but have been largely overlooked for their potential to mitigate climate change. These systems store significant carbon (C) belowground and can increase soil C storage, lowering atmospheric CO2 concentrations. However, many rangeland soils are currently degraded and are losing carbon. Organic matter additions can increase soil C, but may also stimulate N2O and CH4emissions. The effects of organic matter additions to rangelands likely depend on their physical and chemical qualities.
We conducted a 3-year experiment on grazed annual grasslands to assess the response of soils to organic matter amendments with a range of nitrogen (N) contents, but with similar C:N. Three replicate blocks were established per site and included a control, manure treatment, and plant-waste compost treatment. At one site, a compost with lower N content was also tested on replicate plots. Plots were sampled for soil (C, N, temperature, moisture) and plant (community, biomass) properties prior to and for two years following treatment. Soil N2O, CH4, and CO2fluxes were sampled using static chambers on dates selected to target pulses following rain events (n = 35 d).
Compost enhanced soil C pools but dairy manure did not. Compost-amended plots had greater soil C compared to all other plots at all sites and during both years, amounting to 5-15 Mg C ha-1 (0-30 cm). Plots treated with lower-N compost had slightly greater but more variable C stocks. Untreated soils were a small N2O source, but only during the first few rainy days of the second season (<25 ng N cm-2 h-1). Fluxes from treated plots exceeded controls for up to 7 days following early rains, but were variable within and between treatments. Fluxes exceeding 50 ng N cm-2 h-1 were observed on treated plots during only two days following the first rain. Low N compost amendments exhibited slightly lower N2O emissions and shorter periods of positive fluxes. All plots were a sink for CH4 (< 2.0 ng C cm-2 h-1), except isolated hotspots at peak soil moistures.
This study suggest that compost additions to grasslands may contribute to climate change mitigation, but C sequestration will be partially offset by N2O emissions. Furthermore, outcomes will depend on amendment physical and chemical qualities. Additional climate benefits may be obtained when compost is produced from materials diverted from high-emission wastestreams.