Carbon and greenhouse gas dynamics with different organic amendments to grassland soils

With the growing need to slow global climate change, efforts to understand and use carbon (C) drawdown pathways to reduce atmospheric carbon dioxide (CO₂ ) have become increasingly important. Compost application to grasslands has been proposed as a tool to sequester C in soils, with co-benefits to soil fertility, water holding capacity, and decreased erosion. However organic matter amendments may also stimulate greenhouse gas (GHG) emissions. The short term stimulation of emissions are not well represented in most biogeochemical models. Our study elucidates the early phase greenhouse gas dynamics of a range of organic amendments across different soil types.

We conducted laboratory incubations on replicate samples to test the effect of seven different organic amendments on net GHG fluxes from California grassland soil. Amendments ranged from raw feedstocks to composted green waste, manure, and food waste. A second incubation experiment tested the net GHG effect of the most common composted manure amendment on five different soil types representing a range of climate regimes within the region.

All organic amendments stimulated heterotrophic respiration compared to the control, but emissions from composted organic matter were significantly lower than emissions from raw manure feedstock amendments. Nitrogen-rich amendments yielded higher respiration rates and also higher stocks of aggregate-protected soil C. Initial soil organic C concentrations were a significant determinant of respiration rates in compost-treated soils. Nitrous oxide and methane emissions across sites were highly variable, but were not significantly affected by compost application in any soil. Results were consistent with DayCent model simulations of compost treatments at the same sites. The combined results suggest that composting amendments lowers their C footprint, and that over both short and long timespans, compost-amended grassland soils sequester more C than they release through GHG emissions.