Soluble organic carbon and carbon dioxide fluxes in maize fields receiving spring-applied manure

More than 19 million Mg of dairy manure are produced annually in the Canadian provinces of Quebec and Ontario, and most of it is spread on agricultural fields. Quantitative information on the impact of manure management practices on levels of soluble organic carbon (SOC) and emissions of CO2 is important for assessing whether this management significantly contributes to increasing atmospheric CO2 concentrations. The objective of this study was to measure the effects of dairy rattle manure (applied at 0, 56, and 112 Mg ha(-1)) on SOC levels in, and soil surface CO2 fluxes from, a typical maize (Zea mays L.) field in central Canada, from April to October. The higher rate of manure increased both the CO2 emissions and the SOC levels by a factor of two to three compared with the control. Fluxes of CO2 were very low immediately after thaw, increased sharply following manure application and increased again in mid-June at the time when temperature and soil moisture increased; thereafter, fluxes declined throughout the rest of the season. Over the season, which was drier than normal, SOC was not a good predictor of CO2 flux. Carbon dioxide flux increased proportionately less for the second 56 Mg ha(-1) increment of manure added than for the first increment. Factors other than the quantity of SOC limited soil respiration at the highest manure application rate. Carbon dioxide is contributed to the atmosphere at a lower rate, and proportionately more manure C is retained in soil with increasing levels of manure applied.