New methods for mapping of urine patches in pastures
By Søren O. Petersen, René Larsen, Anton Thomsen and Henning Hougaard, Danish Institute of Agricultural Sciences, Department of Agroecology
Traditionally, dairy cattle spend a substantial part of the year on pastures. For organic farming within EU it is specified that ”all mammals must have access to pasturage or an open-air exercise area” which they must be able to use whenever ”weather conditions and the state of the ground permits” (EEC, 1991).
Dairy production systems are characterized by a considerable nitrogen surplus. Grazing cattle excrete surplus nitrogen mainly in the urine (Figure 1), and resulting concentrations in urine patches can range from 20 to 80 g N m-2 or even more. In the soil, urinary nitrogen is rapidly degraded to ammonia. In this extreme environment, the risk for leaching or atmospheric losses of nitrogen is particularly high, and precise estimates of nitrogen balances for grazed pastures must therefore be able to properly account for transformations and losses associated with urine depositions.
It is very difficult to characterize the distribution of excretal returns within the pasture. Urine patches will show up in the pasture as darker green areas, but only if the nitrogen status of the pasture is low. Models have assumed a negative binomial (Richards and Wolton, 1976) or random distribution (Hutchings and Kristensen, 1995), but at the field level there can be some additional heterogeneity. This is due to the fact that animals spend more time in some parts of the pasture than in others, for example around the drinking trough.
Urine distribution described by image analysis
In connection with a project funded by DARCOF (acronym: DINOG), a new approach was tested for mapping the distribution of nitrogen deposited during grazing. An instrument designated as the Foulum Image Capture Facility (see photo gallery) was mounted on a lift and taken to 13 m height, where pictures were taken around the drinking trough. The instrument contains four digital cameras, each with a resolution of 1300 by 1025 pixels and equipped with, respectively, blue, green, red and near infrared filters of 40 nm bandwidth.
A campaign in June 2002 revealed a very patchy distribution in the absorption of photosynthetic light (Figure 2, red colour). The red colour reflects the amount of chlorophyll in the pasture and is closely related to the nitrogen content of the plant material. We therefore believe that this method offers a possibility for describing nitrogen deposition by grazing cattle, for example after a limited period of grazing. Figure 2 also clearly shows that the concentration of patches is elevated around the drinking trough, confirming that there is macro-scale heterogeneity in grazed pastures. Such heterogeneity should be accounted for in models of nitrogen turnover, since there can be interactions between overlapping urine depositions, as well as between urine deposition and soil physical conditions (Oenema et al., 1997).
Urine distribution described by electrical conductivity
Experimental studies of, for example, nitrous oxide (N2O) emissions from pastures need to ensure that urine patches are properly represented in a measurement program. Nitrous oxide is a product of microbial processes, which convert ammonia into nitrate and gaseous forms of nitrogen. However, because of the patchy distribution of urine, it can be very difficult to make a representative selection of sampling points. It is not possible to use the information derived from image analysis, because this represents nitrogen that has already been removed from the soil solution.
Within the DINOG project, an alternative approach was introduced in 2003 that is based on measurement of soil ionic strength. In agricultural soils, ionic strength (or electrical conductivity) is typically dominated by inorganic nitrogen (Smith and Doran, 1996), and we therefore expected that measurements of electrical conductivity could reveal the distribution of inorganic nitrogen derived from urine.
Normally, measurement of electrical conductivity requires soil sampling and laboratory analysis, which is both destructive and time-consuming. We adopted a novel approach based on impedance measurements in the field, which are obtained by the use of time domain reflectrometry (TDR). Briefly, an electrical signal is sent via a cable tester to metal probes inserted into the soil, from where the signal is returned to the instrument (Figure 3) . The time course and shape of the signal can be used to determine volumetric soil water content, but also to determine the impedance, which is inversely related to electrical conductivity in the soil water. Impedance measurements can therefore be used to quantify the ionic strength in the soil solution around the probe. TDR measurements are largely non-destructive and take <1 min. The method is therefore suitable for fast and detailed mapping of soil conditions. The data can be processed and sorted within an hour, and the information used for selection of sampling points for nitrogen process studies which reflect the distribution of nitrogen more precisely than a random selection of sampling points.
Figure 4 exemplifies the use of TDR measurements for selection of sampling points for N2O emission measurements. Impedance was measured at 1-meter intervals along three different transects away from the drinking trough (left). The measurement results were sorted (right), and a subset of sampling points selected for N2O measurements (shown in blue). Figure 5 illustrates N2O measurements in the field.
Ionic strength includes both ammonium and nitrate and, therefore, will not reveal whether a urine patch is new or several days old. However, with a suitable number of sampling points, urine patches of different age should be represented.
References
EEC. 1991. Council Regulation [EEC] No 2092/91. http://europa.eu.int/eur-lex/en/consleg/main/1991/en_1991R2092_index.html
Hutchings N.J. and Kristensen, I.S. 1995. Modelling mineral nitrogen accumulation in grazed pasture: Will more nitrogen leach from fertilized grass than unfertilized grass clover? Grass Forage Sci. 50: 300-313.
Oenema, O., Velthof, G.L., Yamulki, S. and Jarvis, S.C. 1997. Nitrous oxide emissions from grazed grassland. Soil Use Manage. 13: 288-295.
Richards, I.R. and Wolton, K.M. 1976. The spatial distribution of excreta under intensive cattle grazing. J. Brit. Grassland Soc. 31: 89-92.
Smith, J.L. and Doran, J.W. 1996. Measurement and use of pH and electrical conductivity for soil quality analysis. In: J. W. Doran and A.J. Jones (eds.) Methods for Assessing Soil Quality. SSSA Special Publ. No. 49, Soil Sci. Soc. Am., Madison, WI, pp. 169-185.
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