Mid-term tracing of 15N derived from urine and dung in soil microbial biomass

Large amounts of C and N are returned to pasture soils by grazing animals in the form of urine and dung. Therefore, a field trial was carried out to investigate the mid-term effects of N-15-labeled excrements, produced by feeding a cow with N-15-labeled grass silage, on the soil microbial biomass. Simulating the deposition of excrements, N-15-labeled urine and dung were applied to a 0.09-mA(2) area of a sandy pasture soil in October 2000 and 2001. Applied amounts of N were 1,030 and 1,052 kg ha(-1), respectively. Soil was sampled at 0-15 cm depth, three times over 7 months and analyzed for total C and N, and microbial biomass C and N. Recovery of urine and dung N in microbial biomass was determined by N-15 analysis of K2SO4 extracts of pre-extracted fumigated and unfumigated soils. Under dung patches, microbial biomass C was 16% and 45% higher, and microbial biomass N was 24% and 57% higher than under the untreated soil in 2001 and 2002, respectively. Under urine patches, microbial biomass C was increased after 12 weeks and decreased after 27 weeks. Microbial biomass assimilated 7% to 17% and 10% to 21% of the N-15 applied initially as urine and dung, respectively. These percentages were considerably higher than those for artificially with spiked N-15 urea-created and labeled manures reported in previous experiments. An important reason may be that the naturally N-15-labeled N components behave differently in soil than urea spikes.