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.