The single heterogeneous paddock approach to modelling the effects of urine patches on production and leaching in grazed pastures
V. O. Snow A C , I. R. Johnson B and A. J. Parsons AA AgResearch – Grasslands, PB 11008, Palmerston North 4442, New Zealand.
B IMJ Consultants, PO Box 1590, Armidale, NSW 2350, Australia.
C Corresponding author. Email: Val.Snow@agresearch.co.nz
Crop and Pasture Science 60(7) 691-696 https://doi.org/10.1071/CP08390
Submitted: 3 November 2008 Accepted: 22 April 2009 Published: 14 July 2009
Abstract
Despite the fact that urine patches within grazed paddocks are the primary source of N leaching, virtually all pastoral simulation models assume a uniform spatial return of urinary-N to the soil. This simple spatial averaging might not be appropriate if the aim of the modelling is to explore leaching losses because of the non-linearity caused by the high N concentration in urine patches. Here we describe the single heterogeneous paddock (SHP) approach to modelling the dynamics of N in pastoral systems. We also examine the potential for manipulating rate parameters in a simpler uniform-return model (URM) to compensate for the lack of explicit description of urine patches.
Comparison of simulation results from the URM and SHP showed some differences in the patterns of production and a substantial difference in leaching. Depending on soil and climate simulated, there was 5–30% higher pasture production in the URM because simulated leaching in the URM was 5–85% of that simulated by the SHP. Examination of the ratio of the outputs from the two models revealed that the differences in pasture production and N fixation in the URM could probably be corrected with a change in parameter values. This was not true of leaching where there was considerable variation and skew in the ratios, so at the very least, any correction factor would be highly soil and climate specific.
We suggest that models of grazed grass–legume systems can probably adequately simulate production with a simple URM but that the simulation of leaching requires an explicit representation of the heterogeneous urine return. The SHP approach is one methodology for this but this has implications for model and software complexity and for model run-time duration.
Additional keywords: EcoMod, simulation model, pastoral systems, spatial heterogeneity, grassland.
Acknowledgments
The authors thank other members of the Whole Farm Systems Analysis and Tools (WFSAT; www.wfsat.org) team for valuable discussion and contributions to this work and also thank an anonymous reviewer for constructive and interesting comments that added to this study. This work was funded by the Foundation for Research, Science and Technology through contract C10X0809 ‘Rural Futures’.
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