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 C anadian provinces of Quebec and Ontario, and most of it is spread on a gricultural fields. Quantitative information on the impact of manure m anagement practices on levels of soluble organic carbon (SOC) and emis sions of CO2 is important for assessing whether this management signif icantly contributes to increasing atmospheric CO2 concentrations. The objective of this study was to measure the effects of dairy rattle man ure (applied at 0, 56, and 112 Mg ha(-1)) on SOC levels in, and soil s urface CO2 fluxes from, a typical maize (Zea mays L.) field in central Canada, from April to October. The higher rate of manure increased bo th the CO2 emissions and the SOC levels by a factor of two to three co mpared with the control. Fluxes of CO2 were very low immediately after thaw, increased sharply following manure application and increased ag ain in mid-June at the time when temperature and soil moisture increas ed; thereafter, fluxes declined throughout the rest of the season. Ove r the season, which was drier than normal, SOC was not a good predicto r of CO2 flux. Carbon dioxide flux increased proportionately less for the second 56 Mg ha(-1) increment of manure added than for the first i ncrement. Factors other than the quantity of SOC limited soil respirat ion at the highest manure application rate. Carbon dioxide is contribu ted to the atmosphere at a lower rate, and proportionately more manure C is retained in soil with increasing levels of manure applied.