Transformation and loss of excretal nitrogen under winter management systems : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University, Palmerston North, New Zealand
Excreta from cattle animals contain large amounts of nutrients, particularly nitrogen, which could lead to substantial gaseous losses of ammonia and nitrous oxide to the atmosphere, as well as nitrogen leaching. These losses are greatest during wet conditions in winter. However, the situation could be improved through moving cows off grazing paddocks to a stand-off pad or housing system. Therefore, it is necessary to quantify ammonia emissions and evaluate denitrification potential (which leads to emission of N₂O) through various winter management systems in order to determine methods and technologies for efficient and effective mitigation of gaseous emissions. To understand the mechanism of nitrogen transformation and of reduction on gaseous emission from excreta in various winter management systems, a series of incubation studies and a field study were carried out investigating the suitability of several natural materials with absorbent properties, as media to reduce gaseous emission of ammonia and nitrous oxide. The incubation studies were undertaken using cow excreta that consisted of a 1:1 (v:w) mixture of fresh urine and dung collected from a dairy farm. A lab incubation study was conducted using excreta, and excreta amended with soil and sawdust treatments. A further lab incubation study was carried out using different levels of natural materials. The field study consisted of two stand-off pads in which crushed pine bark or sawdust were used as bedding materials. In the incubation study, ammonification was rapid in the case of excreta, compared to excreta amended with addition of natural materials. Whereas nitrification was very slow in the all treatments, only a small amount of nitrate ions could be detected till the end of incubation study. In the incubation study, both soil and sawdust appeared to significantly reduce ammonia emission. In comparison to excreta, amendment with soil (excreta: soil=1:2, w:w) and sawdust (excreta: sawdust=1:2, w:v) reduced ammonia loss by 32.9% and 19.5%, respectively. Excreta amended with a combination of soil and sawdust (1:1:1, w:w:v) was most effective, reducing ammonia emission by 34% under aerobic conditions. Nitrate concentration was found to be the crucial limiting factor affecting the denitrification rate in the incubation studies. When KNO₃ was added to the excreta, the denitrification rate was 43.8µg N₂O-N/g excreta/hour. However, the denitrification rate of the excreta amended with both glucose-C and KNO₃ was 114.4µg N₂O-N/g excreta/hour. Denitrification potential followed: excreta> excreta with sawdust> excreta with soil. On a field-scale stand-off pad, the carbon-rich natural materials pine bark and sawdust were shown to retain nitrogen effectively. After nine months of use, the bark retained 78% of the deposited excreta-N, while the sawdust pad retained 51%. Therefore, it can be concluded that reduction of nitrogen losses can be achieved by using stand-off pad or housing systems (herd homes) which incorporate the use of a carbon rich natural material or soil in winter.