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 ¹⁵N-labeled excrements, produced by feeding a cow with ¹⁵N-labeled grass silage, on the soil microbial biomass. Simulating the deposition of excrements, ¹⁵N-labeled urine and dung were applied to a 0.09-m2 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 ¹⁵N 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 ¹⁵N applied initially as urine and dung, respectively. These percentages were considerably higher than those for artificially with spiked ¹⁵N urea-created and labeled manures reported in previous experiments. An important reason may be that the naturally ¹⁵N-labeled N components behave differently in soil than urea spikes.