The fate of nitrogen in an animal urine patch as affected by urine nitrogen loading rate and the nitrification inhibitor dicyandiamide

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.