Priming of soil decomposition leads to losses of carbon in soil treated with cow urine

The extent to which priming of soil carbon (C) decomposition following treatment with cow urine leads to losses of soil C has not been fully investigated. However, this may be an important component of the carbon (C) cycle in intensively grazed pastures. Our objective was to determine soil C losses via priming in soil treated with cow urine and artificial urine. Cow urine, water, 14C-urea artificial urine, and 14C-glucose artificial urine were applied to repacked soil cores and incubated at 25°C for 84 days. We used radio-labelled artificial urine to determine the extent to which urea hydrolysis contributed to elevated carbon dioxide (CO2) emissions in urine-treated soil and as a comparison to the priming effects of cow urine. Water-soluble C, pH, dehydrogenase activity, urease activity, and CO2 evolution were monitored during the incubation. Priming of soil C decomposition (more CO2-C evolved than was added as a C source) in the cow urine treatment was 4.2 ± 0.7 mg C g–1 (5.2 ± 0.9% of soil C concentration, corrected for water control). In the cow urine treatment, ~54% of retained urea was hydrolysed and it contributed 0.4 ± 0.1 mg CO2-C g–1 to total CO2 fluxes. Low urea hydrolysis may have been due to decreased urease activity in the cow urine treatment due to the large amounts of urea present and the increased pH. Dehydrogenase activity was elevated immediately after cow urine application, and indicates that priming was likely due to heightened microbial activity. Negative priming (less CO2-C evolved than was added as a C source) was measured in the artificial urine treatments and this may reflect the differences in composition between the cow and artificial urines. Solubilisation of soil C was also found in the artificial urine treatments, but it did not appear to be correlated with increased pH or periods of greater urea hydrolysis. While cow urine decreased soil C by positively priming soil C decomposition, our artificial urine did not. Therefore, caution is recommended when using artificial urine for C-cycling research. The mechanisms by which both increased soil pH and priming occurs in urine-treated soils require further investigation.