DYNAMICS OF SOIL MICROBIAL BIOMASS C, SOLUBLE ORGANIC C AND CO2 EVOLUTION AFTER 3 YEARS OF MANURE APPLICATION

Application of manure and fertilizer affects the rate and extent of mi neralization and sequestration of C in soil. The objective of this stu dy was to determine the effects of 3 yr of application of N fertilizer and different manure amendments on CO2 evolution and the dynamics of soil microbial biomass and soluble C in the field. Soil respiration, s oluble organic C and microbial biomass C were measured at intervals ov er the growing season in maize soils amended with stockpiled or rotted manure, N fertilizer (200 kg N ha(-1)) and with no amendments (contro l). Manure amendments increased soil respiration and levels of soluble organic C and microbial biomass C by a factor of 2 to 3 compared with the control, whereas the N fertilizer had little effect on any parame ter. Soil temperature explained most of the variations in CO2 flux (78 to 95%) in each treatment, but data from all treatments could not be fitted to a unique relationship. Increases in CO2 emission and soluble C resulting from manure amendments were strongly correlated (r(2) = 0 .75) with soil temperature. This observation confirms that soluble C i s an active C pool affected by biological activity. The positive corre lation between soluble organic C and soil temperature also suggests th at production of soluble C increases more than mineralization of solub le C as temperature increases. The total manure-derived CO2-C was equi valent to 52% of the applied stockpiled-manure C and 67% of the applie d rotted-manure C. Estimates of average turnover rates of microbial bi omass ranged between 0.72 and 1.22 yr(-1) and were lowest in manured s oils. Manured soils also had large quantities of soluble C with a slow er turnover rate than that in either fertilized or unamended soils.