Effects of compost and manure additions on the greenhouse gas dynamics of managed grasslands

Grasslands cover approximately 30% of the terrestrial land surface, and have significant potential to increase soil C storage and thus lower atmospheric CO2 concentrations. Organic matter amendments (e.g., compost, manure) have been shown to be effective at increasing grassland soil C both through direct addition and by increasing net primary productivity. However, organic matter additions can also increase N2O and CH4 fluxes. The effects of organic matter amendments on both soil C and greenhouse gas emissions are dependent on their physical and chemical qualities. To explore the impacts of organic matter amendments of different chemical and physical qualities on soil C and greenhouse gas emissions we established research plots on three managed annual grasslands in California. Three replicate blocks were established at each site and included an untreated control, a manure treatment, and a compost treatment. At one site, an additional compost with a lower nitrogen content was also tested. In October 2011, a 1 cm layer of the designated amendment was added to each plot. All plots were sampled for soil (C and N, bulk density, temperature, moisture) and plant (community, aboveground biomass) properties, prior to and for two years following treatment. Plots were also sampled intensively for N2O, CH4, and CO2 fluxes using static chambers on over 35 days throughout the two rainy seasons, where sampling days were selected to target pulses following rain events. Results show that the amendments differentially affected soil C and greenhouse gases among the treatments. One year after treatment, C concentrations in the top 10 cm of soils had increased at all three sites by a mean of 0.5-1% on plots that received either compost treatment, but not on those that received manure. Lower in the profile (10-30 cm), C concentrations were increased by a smaller amount (<0.3%) and only in two of the sites. The untreated grassland soils were a small source of N2O during the first few days following the first rain of the second season (<25 ng N cm-2 h-1), but were negligible during the first post-treatment rain event (the second rain event of the season). Fluxes on all treated plots exceeded those on control plots for up to 7 days following early season rains, but were highly variable within and between treatments. Peak fluxes exceeding average rates of 50 ng N cm-2 h-1 occurred only during the first two days following the first rain of the season. After mid and late season rains, N2O emissions were negligible in all plots. Both the untreated and treated soils were typically a sink for CH4 (of <1.5 ng C cm-2 h-1) and highly variable, with rare exceptions during days with peak soil moistures when soils became a small source (<1.0 ng C cm-2 h-1). These results suggest that compost may contribute to climate change mitigation, but that C sequestration will be at least partially offset by increased N2O emissions.