Can organic matter additions to rangelands mitigate climate change?

Background/Question/Methods

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).

Results/Conclusions

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.