Carbon sequestration in a temperate grassland; management and climatic controls

Soil management practices that result in increased soil carbon (C) sequestration can make a valuable contribution to reducing the increase in atmospheric CO2 concentrations. We studied the effect of poultry manure, cattle slurry, sewage sludge, NH4NO3 or urea on C cycling and sequestration in silage grass production. Soil respiration, net ecosystem exchange (NEE) and methane (CH4) fluxes were measured with chambers, and soil samples were analysed for total C and dissolved organic C (DOC). Treatments were applied over 2 years and measurements were carried out over 3 years to assess possible residual effects. Organic fertilizer applications increased CO2 loss through soil respiration but also enhanced soil C storage compared with mineral fertilizer. Cumulative soil respiration rates were highest in poultry manure treatments with 13.7 t C ha−1 in 2003, corresponding to 1.6 times the control value, but no residual effect was seen. Soil respiration showed an exponential increase with temperature, and a bimodal relationship with soil moisture. The greatest NEE was observed on urea treatments (with a CO2 uptake of −4.4 g CO2 m−2 h−1). Total C and DOC were significantly greater in manure treatments in the soil surface (0–10 cm). Of the C added in the manures, 27% of that in the sewage pellets, 32% of that in the cattle slurry and 39% of that in the poultry manure remained in the 0–10 cm soil layer at the end of the experiment. Mineral fertilizer treatments had only small C sequestration rates, although uncertainties were high. Expressed as global warming potentials, the benefits of increased C sequestration on poultry manure and sewage pellet treatments were outweighed by the additional losses of N2O, particularly in the wet year 2002. Methane was emitted only for 2–3 days on cattle slurry treatments, but the magnitudes of fluxes were negligible compared with C losses by soil respiration