Priming of carbon decomposition in 27 dairy grazed soils after bovine urine additions

Abstract

Context: Soil organic matter (SOM) plays a vital role in carbon (C) storage and agricultural sustainability. Additions of bovine urine to soils can cause positive priming of soil C decomposition and represents a pathway for SOM loss. However, data is limited to a few soils. Aims: We investigated the priming response to bovine urine of 27 dairy grazed pasture soils from the North Island of New Zealand. Methods: Soils from Allophanic, Gley, Recent and Brown soil orders were collected. 14C-labelled dairy cow urine was applied (1000 kg N ha-1) to undisturbed soil cores and carbon dioxide (CO2) fluxes measured (25°C) for 21 days. Urine applications were repeated, and CO2 measured for a further 21 days (25°C). Water was the control treatment. Key results: CO2 fluxes rapidly increased after both urine additions by 86 ± 1% 24 h after the first urine addition, and 68 ± 4% after the second. Positive, negative and no priming were observed, and the mean absolute deviation of priming ranged between 200 and 1000 μg C g-1, and variability was greater after the second urine addition. Urine induced changes in pH and electrical conductivity (EC) had no effect on priming, and soil C contents were correlated to cumulative CO2, but not priming, and varied over time. Conclusions: Factors affecting soil priming remain elusive and priming was highly variable within and between soil types. Implications: The impacts of bovine urine on C pools requires further investigation to determine if, or when, urine patches are potential pathways for soil C loss.