|dc.description.abstract||There have been reports of losses of soil carbon (C) in New Zealand pastures under dairy grazing. Acceleration of mineralisation or leaching of soil C following urine deposition may decrease soil C. However, little research of the effect of cow urine on soil C has been conducted. The overall objective of this thesis was to investigate the potential for dairy cow urine to solubilise soil C, which can then be lost through mineralisation or leaching. Soils from both grazed pasture and Pinus radiata plantations (termed pasture and pine soils) were investigated.
The concentration, composition, and bioavailability of urine-C were investigated. Cow urine contained 7.5±1.2 g C L–¹, with a C:N ratio of 2:1. About 45% of the C content of cow urine was attributed to hippuric acid, the other major contributors were urea, an unidentified amide and phenaceturic acid. On incubation at 25°C, up to 25% of urine-C was degraded in 28 days, demonstrating that cow urine is biodegradable and could potentially act to prime C mineralisation in soils.
Solubilisation of soil C (0-20 cm soil depth) following urine application was tested by measuring adsorption of urine-C and subsequent desorption of soil C in air-dried pine and pasture soils. While adsorption was low at 3% of urine-C, the solubilisation of soil C by urine ranged between 11-28% of soil C concentration for 5 different soils, however, solubilisation was likely overestimated due to the use of air-dried soils. Soil C solubilisation was also measured in field moist soils applied with artificial urine, and was less than that reported from air-dried soils.
Priming of soil C mineralisation, solubilisation, and extracellular enzyme activity were measured using moist repacked pine and pasture soil cores (0-5 cm soil depth) treated with cow urine or radio-labelled artificial urine. Positive priming of soil C mineralisation, where more carbon dioxide (CO₂) was produced than C added, was measured following application of cow urine in both pine and pasture soils. The pasture soils lost 5.1±0.9%, and the pine soils 4.0±0.1%, of soil C concentration as CO₂ during a 84 day incubation. Positive priming was attributed to increased microbial and urease activity and accelerated soil C mineralisation in urine treated soils. The remaining extracellular enzyme activities assayed were unlikely to have contributed to soil C priming. In contrast to the positive priming measured following cow urine application, treatment of soil with artificial urine resulted in less CO₂ produced than C added – or negative priming. Increased soil pH following urine application may have played an important role in increasing C mineralisation as water soluble C increased with increasing soil pH. Therefore, cow urine can cause priming of soil C mineralisation and lead to a loss of soil C. However, artificial urine may not adequately model cow urine with respect to C cycling.
Soil C solubilisation by urine and subsequent leaching from undisturbed pasture soil (0-5 cm soil depth) was assessed by applying δ¹³C enriched urine. Leaching resulted in a loss of 0.45±0.03% of soil C concentration, which was 10 times greater than the loss of soil C in the water control treatment (0.048±0.001%). The leaching of soil C was small compared to the 5% loss of soil C by priming in the repacked core experiment. Soil solubilisation in the undisturbed cores was less than both repacked cores and air-dried soils, demonstrating that soil C solubilisation increases with increasing soil disturbance. The acid neutralising capacity (ANC) forcing potential of cow urine was 11.8 meq L–¹, which was more than 20 times greater than sodium nitrate fertiliser application (30 kg N ha-¹ year-¹), although the nitrogen loading rates of the urine were higher than the fertiliser . ANC forcing has been linked to increasing soil pH and dissolved organic C leaching, and may have been a factor in soil C solubilisation under urine patches. Upon assessing water stable aggregates after urine application, disaggregation of soil was not a major factor in soil C solubilisation.
The key conclusions were that:
• there was strong evidence to support the hypothesis that urine deposition led to soil C solubilisation and priming of soil C mineralisation, that could have contributed to the reported declines in soil C in dairy pastures;
• priming of soil C decomposition was 10 times greater than leaching and may be the predominant mechanism of soil C loss following urine deposition; and
• urine deposition resulted in greater losses of soil C from pasture soils than pine soils, in contrast to expectations that soil C in pasture soils would have been acclimatised to urine application.
The work in this thesis was laboratory based and requires further testing under field conditions. Further work is also needed to establish the mechanisms of soil C solubilisation and the role of solubilised soil C in priming of soil C mineralisation.||