Numerical analysis of nitrate leaching on pasture land in Southern Manitoba

Hartmut Holländer1, Simratpal Singh1, Zijian Wang1, Luca Coppi2, Mario Tenuta2
1 Department of Civil Engineering, University of Manitoba
2 Department of Soil Science, University of Manitoba

2.7 in Wasserquantitäts- und qualitätsmodellierung auf regionaler Skale - Herausforderungen und neue Ansätze

27.03.2020, 16:00-16:15, Weißer Saal

Nitrogen is a key agricultural input, which is crucial for crop growth, development, and yield. However, an excess application of nitrogen may result in the nitrate contamination of groundwater. This research focused on the estimation of nitrate leaching fluxes upon the application of pig slurry on pasture land in Southern Manitoba using physical based modeling and to further regionalise the point estimates of nitrate leaching fluxes at the field scale. The study site was the ‘Pasture and Swine Manure Management Site’ located in La Broquerie, Southern Manitoba. La Broquerie is one of the most concentrated areas of livestock production in Canada. The study site was divided into six different management types. Grazing livestock tended to concentrate near areas such as mineral feeders, water troughs and shelters causing higher deposition density of urine and feces, resulting in six bare earth areas in the study site. HYDRUS-1D was used to determine continuous recharge and nutrient leaching estimates. The simulated leaching estimates were regionalised using Cokriging. The difference in simulated and observed nitrate concentrations in groundwater was expressed in terms of RMSE between 0.023 and 5.12 mg NO3--N L-1, NSE between 0.66 and 0.96 and the ME between -1.03 mg NO3--N L-1 and 1.05 mg NO3--N L-1.

Simulation results suggested that two years of pig slurry application to the hay pasture on a coarse sandy soil did not cause significant accumulation of nitrate in the shallow groundwater. Even during the snowmelt and heavy precipitation events, the shallow groundwater nitrate concentrations never exceeded 10 mg NO3--N L-1. This was likely due to the uptake of nitrate by plant roots and low mobilisation of nitrate during snowmelt. Following the same slurry application treatments in the grazed pasture plots, the concentration of nitrate in groundwater was not an environmental concern even though it occasionally rose closer to the drinking water threshold. In contrast, nitrate concentrations above the drinking water threshold were simulated in BEAs throughout the vegetation period. Leaching of large amounts of nitrate to the groundwater occurred in these areas.

HYDRUS-1D simulated the soil moisture content and groundwater nitrate concentrations with a good agreement with the observed values. However, the standard HYDRUS-1D code was not able to simulate freezing and thawing of soil, which was the major drawback of this research. Cokriging allowed the cross-correlation of nitrate leaching with the application of slurry and improved the prediction of the resulting map. Regionalisation results suggested that bare earth areas (BEAs) and some areas around their periphery were under the concern of nitrate leaching.



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