Response of Greenhouse Gas Emissions to Varying Compost Rates and Soil Moisture Under Laboratory Conditions.

Seasonal variability of greenhouse gas (GHG) fluxes in croplands is controlled by moisture, temperature and amendments. Soil accumulation of labile carbon (C) and nitrogen (N) during the autumn serves as substrates for microbial mineralization and potential losses to leaching and GHG emissions during reoccurring freeze-thaw cycles between winter and summer. In this study, we investigated the effect of the interaction between four rates of composted manure (0, 15, 30 and 45 Mg ha-1), or inorganic fertilizer (89 kg ha-1 ammonium phosphate + 119 kg ha-1 ammonium sulphate) and three levels of soil moisture on carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes during laboratory incubation simulating the transition between winter (5oC), early spring (10oC), spring (15oC) to early summer (25oC). Soils were incubated for a period of eight weeks (two weeks for each temperature). GHG fluxes were measured at 1, 4, 7 and 14 days of every temperature-time period. Inorganic fertilizer (IF) had the highest CO2 emissions at 5 and 10% moisture. Decomposition rates increased with compost applications at 5% moisture and 15oC (spring temperature). Compost generally kept CO2 emissions at minimal compared to IF. The addition of water to the soils receiving higher levels of compost led to the assimilation of more CH4 as the soil warmed up to 25oC (summer temperature) . Going into the summer (25oC), soil organic matter mineralization accelerated at high levels of compost and moisture (10% moisture) which led to more N2O emission, however, all N2O emissions from compost rates were still 30% lower than IF. Compost treatments were observed to conserve more N, making N more available to plants. CO2 emissions increased with increasing soil moisture and temperature; the highest from IF which rose from 1000 g ha-1day-1 at 25oC when soil had ambient moisture to about 8000 g ha-1 day-1 at 25oC when soil moisture increased to 10% ( 700% increase). The addition of water to the soil receiving 30 and 45 t ha-1 compost at 25oC caused the soil to assimilate more CH4 compared to when they were at ambient moisture. The small magnitude of CH4 fluxes (-2 to 1 g ha-1day-1) indicated that dryland soils were not important CH4 emitters. Increasing soil moisture enhanced N2O emissions. N2O emissions increased from a maximum of 650 g ha-1day-1 at ambient moisture to 8500 g ha-1day-1 at 10% moisture. At 5% and 10% moisture, the highest N2O emissions occurred in the control and IF compared to compost at all temperature suggesting that soil microbes immobilize soil N when an organic nutrient source is applied, thus reducing the amount lost to the atmosphere.