In New Zealand, agriculture is predominantly based on pastoral grazing systems and animal excreta deposited on soil during grazing have been identified as a major source of nitrous oxide (N 2 O) emissions. Forage brassicas (Brassica spp.) have been increasingly used to improve lamb performance. Compared with conventional forage perennial ryegrass (Lolium perenne L.), a common forage in New Zealand, forage brassicas have faster growth rates, higher dry matter production and higher nutritive value. The aim of this study was to determine the partitioning of dietary nitrogen (N) between urine and dung in the excreta from sheep fed forage brassica rape (B. napus subsp. oleifera L.) or ryegrass, and then to measure N 2 O emissions when the excreta from the two different feed sources were applied to a pasture soil. A sheep metabolism study was conducted to determine urine and dung-N outputs from sheep fed forage rape or ryegrass, and N partitioning between urine and dung. Urine and dung were collected and then used in a field plot experiment for measuring N 2 O emissions. The experimental site contained a perennial ryegrass/white clover pasture on a poorly drained silt-loam soil. The treatments included urine from sheep fed forage rape or ryegrass, dung from sheep fed forage rape or ryegrass, and a control without dung or urine applied. N 2 O emission measurements were carried out using a static chamber technique. For each excreta type, the total N 2 O emissions and emission factor (EF3; N 2 O-N emitted during the 3-or 8-month measurement period as a per cent of animal urine or dung-N applied, respectively) were calculated. Our results indicate that, in terms of per unit of N intake, a similar amount of N was excreted in urine from sheep fed either forage rape or ryegrass, but less dung N was excreted from sheep fed forage rape than ryegrass. The EF3 for urine from sheep fed forage rape was lower compared with urine from sheep fed ryegrass. This may have been because of plant secondary metabolites, such as glucosinolates in forage rape and their degradation products, are transferred to urine and affect soil N transformation processes. However, the difference in the EF3 for dung from sheep fed ryegrass and forage rape was not significant.Keywords: Excreta, N partitioning, sheep, brassicas, nitrous oxide ImplicationsNitrous oxide (N 2 O) emissions from soil are one of the main components of greenhouse gases in the agricultural sector and animal excreta have been identified as a major source of these emissions. Our study showed that feeding forage rape reduced the N 2 O-N emission factor (EF3; N 2 O-N emitted during the 3-month measurement period as a per cent of animal excreta N applied) for sheep urine by about 60%, compared with feeding perennial ryegrass. This finding is significant for developing mitigation measures to reduce EF3 for animal urine. Further study needs to be conducted under different soil and climate conditions to confirm this finding. IntroductionIn New Zealand, agriculture is predominantly based on g...
OVERSEER® Nutrient Budgets (Overseer) is an agricultural management support tool that examines the flow of nutrients in a farming system. There is increasing pressure from a range of users for transparency of the way Overseer functions, particularly the modelling of nitrogen (N) and phosphorus (P) loss to water. The aim of this paper is to provide a conceptual description of the way Overseer models the distribution and fate of N and P in a pastoral system, to support user understanding and correct model use. The core of Overseer is a nutrient budget, which accounts for the flow of nutrient into, around and off the farm. The key strength of Overseer is its ability to model these nutrient transfers around the farm, identifying how much, where and when nutrients move. Other parts of the model then estimate the fate of these nutrients. Nitrogen and P cycle differently around the farm, which is reflected in the way they are modelled. This paper is intended to be a support document for understanding the way Overseer models N and P, and where more detailed information is required, it may be found on the Overseer website (www.overseer.org.nz). Keywords: Nutrient budget, model, leaching, run-off
The effectiveness of multiple small doses of the nitrification inhibitor dicyandiamide (DCD) to decrease denitrification under warm moist conditions was tested in a 1-year field trial on a grazed dairy pasture. DCD was applied approximately every 4 weeks as an aqueous spray onto ten replicate plots 3 days after rotational grazing by dairy cows. Each application was at the rate of 3 kg DCD ha(1 , with a total annual application of 33 kg ha (1 . Denitrification was assessed 5 days after each DCD application using the acetylene block method. At the end of the trial, the rate of degradation of DCD under summer conditions was measured. DCD significantly decreased the mean annual nitrate concentration by about 17%. Denitrification and denitrification enzyme activity were highly variable and no significant effect of DCD in decreasing denitrification was detected. In the summer month of December, DCD degraded rapidly with an estimated half-life of 593 days (mean and standard deviation).
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