|dc.description.abstract||The animal urine patch is the main source of nitrogen (N) loss from agricultural grazed pasture systems. Losses include emissions of nitrous oxide (N₂O), a potent greenhouse gas, and leaching of nitrate (NO3-) into waterways, both of which contribute to environmental degradation. Urine patch N loss also represents an economic loss of N from a farm. The loading rate of N in the urine patch is primarily determined by the animal’s dietary N intake and the subsequent excretion of N in the urine. Improving N use efficiency in the urine patch is therefore of critical importance, both environmentally and economically. There has been a considerable amount of research on the fate of N in a urine patch at a single N loading rate, as well as the fate of N from multiple urine rates on a single N pathway of loss or transformation, however there is a gap in current knowledge of the fate of N in multiple loss pathways from urine applied at varying N loading rates. The application of the nitrification inhibitor dicyandiamide (DCD) has been shown to reduce urine patch N losses and increase pasture N uptake however there has been little investigation into the effect of DCD at varying urine N loading rates. The objective of the project was to determine the effect of urine N loading rate, and the effect of DCD at varying urine N loading rates, on the fate of N in grassland soils.
Two experiments were carried out in 2009-2010 (year one) and 2010-2011 (year two) using soil monolith lysimeters collected from a free-draining sandy loam soil under pastoral dairy grazing in south-east Ireland. Dairy cow urine was diluted with water or fortified with urea to produce a range of total N concentrations which corresponded to urine N loading rate treatments of 0, 300, 500, 700 and 1000 kg N ha-1. Two litres of urine was applied in late autumn to the 0.2 m2 surface area of each lysimeter to mimic a dairy cow urine patch deposited in the field. DCD was applied twice, to lysimeters receiving urine at 500 and 1000 kg N ha-1, the day after urine and again in early spring. The DCD was applied in solution form at a rate of 15 kg DCD ha-1 per application. Nitrous oxide emissions, N leaching in drainage water and pasture N uptake were measured periodically following urine application, using standard methods. A mass balance determined the apparent recovery of N from each urine N loading rate. In year two, urine in the 1000 kg N ha-1 treatments (with and without DCD) received urea labelled with the isotope 15N which produced a mix containing 45 atom% 15N. Additional measurement of di-nitrogen gas (N₂) was carried out and lysimeters were destructively sampled at the end of year two to measure 15N recovery in the soil. A 15N balance was determined using the recovery of 15N in gaseous, drainage water, pasture and soil fractions.
Increasing the urine N loading rate resulted in an increase in the cumulative N₂O emissions, N leaching and pasture N uptake in both experiments. In all cases, highly statistically significant curvilinear relationships were found, with the amount of N recovered diminishing at the higher N rates, except for pasture N uptake, where the curvilinear relationship was exponential. The reason for the diminishing curvature was hypothesised as extra N at the higher N rates being recovered in pathways other than N2O emissions, N leaching and pasture N uptake. This was confirmed in the 15N balance study carried out in year two, by the recovery of 23% and 26% of urine N applied in soil N immobilisation and N₂ emissions, respectively. The large recovery of N₂ emissions from the 1000 kg N ha-1 urine treatment, was almost entirely derived from the process of co-denitrification, whereby the N in N₂ is derived from both urine N and native soil N sources. This finding is important both for recognising the contribution of a relatively unrecognised process to denitrification in grazed grassland, and at a broader level, to closing the gap of ‘missing N’ in the grassland N budget. The application of DCD reduced N₂O emissions, N leaching and increased pasture N uptake and dry matter yield; however, the responses were variable. There was no consistent interaction found between urine N loading rate and the application of DCD on N₂O emissions, N leaching or pasture N uptake. The most likely reason for the variable DCD response was the removal of the DCD by leaching or decomposition. DCD may be used as a mitigation strategy to reduce urine patch N loss in Irish grazed pastures, providing it remains in the soil at an effective concentration.
This work has clearly shown that an increase in the urine N loading rate applied to grassland soils increases the amount of N lost in N₂O emissions, lost in N leaching and taken up by pasture plants.||en