- Agronomic: corn, hay
- Animal Products: dairy
- Animal Production: manure management, grazing - rotational, feed/forage
- Crop Production: continuous cropping
- Education and Training: on-farm/ranch research
- Soil Management: nutrient mineralization, organic matter, soil quality/health
Groundwater quality and gases were evaluated at four grazing paddocks and one conventionally cropped field. These results support the idea that denitrification actively mitigates groundwater nitrate contamination beneath management intensive grazing paddocks compared to conventional cropping. It is clear from the solute and dissolved gas composition of groundwater beneath the pasture that conditions were highly favorable for denitrification in groundwater under grazing and that a large portion of the nitrate that leached to groundwater was transformed to harmless N2 gas. Similar patterns were not evident at the corn study site due to the absence of a sufficient supply of dissolved organic carbon (DOC) to fuel the denitrification reaction.
By definition, sustainable agricultural practices have positive or very little negative impact on the environment. A lesson learned from “conventional” agriculture is that we cannot assume that all practices are benign. Whenever the natural holding capacity for a parcel of land is exceeded, we create the potential for negative consequences to the natural system. These negative impacts can degrade ecosystem health and function, reducing both the quality of rural lifestyle and the ability of the system to sustain agricultural activities over time. For example, concerns about non-point source water quality problems have been increasing nationwide. Although point sources are largely controlled, groundwater quality in many areas continues to deteriorate, particularly where aquifers are shallow or underlie sandy soils and conventional, high input agriculture is practiced. This is the case for much of the North Central Region, including Central Wisconsin, an area experiencing agriculture-related degradation of groundwater quality. In Wisconsin, federal drinking water standards for nitrate-N are exceeded in about 10% of the private wells and in more than 40% of the private wells in some areas (Wisconsin Groundwater Coord. Council, 2000; UWSP Groundwater Center Database, Unpub. Data). Studies indicate that 90% of groundwater contaminants in Wisconsin result from agriculture (Shaw, 1994; Mechenich and Kraft, 1997). This contamination of groundwater affects rural private wells and small municipal wells with a cost to human and livestock health, and to the economic health of small communities for construction of replacement wells or nitrate removal facilities.
In an attempt to minimize agricultural impacts on the environment, much effort and money are invested nationally to develop and implement agricultural Best Management Practices (BMPs). Some BMPs successfully achieve environmental goals, whereas others do not. Studies conducted to evaluate BMPs recommended to protect groundwater in Central Wisconsin (e.g., fertilizer rate and timing, tillage management, etc.) indicate that current BMPs are not reducing contaminants in groundwater to levels consistent with federal drinking water standards (Saffigna et al., 1997; Osborn et al., 1990; Shaw and Turyk, 1992; Mechenich and Kraft, 1997; Stites and Kraft, 1997). Close hydrologic connection between surface and groundwater in the area promotes the exchange and movement of contaminants.
The role of Management Intensive Grazing (MIG)
It is desirable to find sustainable and profitable agricultural practices that can reduce impacts to groundwater in environmentally sensitive areas. Our contention is that annual cropping in many of these landscapes has exacerbated leaching, because annual crops cannot capture water or nutrients over as long a period nor in as great a quantity as many perennial crops (Bergstrom, 1987; Peterson and Russelle, 1991; Randall et al., 1997; Kelley and Russelle, 2000). MIG can be an economically viable form of dairy farming (Kriegl, 2000) that uses perennial plants within much of a farming system. One must recognize, however, that the intensity of nitrogen cycling is generally greater in grazed than non-grazed systems, thereby increasing the risk of nitrogen losses.
The concept of using MIG to reduce nutrient impacts to water quality is not a new one. It appears to be an agricultural option which has several components that may protect groundwater. Manure is distributed over the land, which if properly managed is always vegetated, allowing plants to quickly utilize nutrients in the dung and urine during the growing season. Grasses and legumes are the primary food source for the livestock throughout the grazing season (May – October), so fewer annual crops need to be grown as feed (Bobbe, 1994). This reduces nutrient inputs and eliminates pesticide inputs that make their way to groundwater. Many studies have shown that rotational grazing can reduce input costs (feed, fuel, machinery), improve animal health, and if done properly improve forage quality (Jackson-Smith, 1996; Tranel and Frank, 1991; Bender, 1997; Mann, 1997; UW-Madison, 1993). However, in MIG systems, manure is not always distributed evenly, and therefore nutrients contained in the manure and urine can be concentrated in areas within the paddocks (Mathews et al., 1996; Peterson and Gerrish, 1996; Russelle, 1996; Cropper, 1997). These areas of nutrient accumulation typically relate to location of water, lanes, and shade trees. As dairy farmers expand their use of MIG, many are interested in applying fertilizer to improve pasture production, thus creating the potential for similar environmental consequences as from conventional farming practices.
One way that natural or managed land can moderate nitrate accumulation in ground and surface water is through denitrification. Denitrification is the naturally occurring process by which nitrate is reduced by microorganisms to gases, primarily nitrous oxide (N2O) and dinitrogen (N2). Of these two gases, nitrous oxide is formed first and is an undesirable end product, because it acts as a ‘greenhouse’ gas and destroys stratospheric ozone. The final step in denitrification yields dinitrogen gas, which makes up about 78% of our atmosphere and is a benign end product.
The substrates for denitrification are nitrate and available energy compounds [usually carbon, although methane and reduced sulfur and iron compounds can play a role (Pedersen et al., 1991; Korom, 1992)], and the organisms are active only when oxygen pressures are low. Nitrate disappearance from groundwater may be due to denitrification, but also occurs by immobilization (plant and microbial uptake) and a biotic fixation by condensation reactions (Groffman et al., 1996). Grassed and forested riparian zones are often recommended to reduce nitrate loading to surface water, because both can enhance denitrification. For example, Verchot et. al., (1997) measured significant reductions of nitrate concentrations in groundwater as it moved from below a cropped field to below a grass buffer.
Denitrification rates can be quite high in groundwater, but can also be low or nonexistent (see Groffman et al., 1996, for several references). Many studies have found insufficient dissolved organic carbon concentrations to support denitrification (e.g., Hiscock et al., 1991), even when nitrate supply was adequate and oxygen levels were low. Denitrification rates from pastures are often higher than annual cropland (e.g., Bijay-Singh et al., 1989; Sotomeyer and Rice, 1996), but results depend on experimental conditions. Significant losses typically are confined to situations where water filled pore space in the soil is greater than 80% (Rudaz et al., 1999), which is rare in coarse-textured soils. Thus, one would not expect to find high denitrification rates in pastures on sandy soils, except in zones in or near the water table, under temporary wet conditions, and in areas affected by dung or hoof compaction.
Several investigators have found that ‘patchiness’ in energy substrate is typical under forests and riparian zones, resulting in very low denitrification rates in much of the matrix, but very high rates in small volumes or microsites where labile carbon is present (Parkin, 1987; Parkin et al., 1987; Nelson et al., 1995). These patches support microbial growth, which reduce oxygen concentrations, thereby facilitating anaerobic processes like denitrification. The distribution of fresh plant residues is often positively correlated with denitrification intensity in the field (e.g., Aulakh et al., 1984; de Cantazaro and Beauchamp, 1985; Parkin, 1987). Although denitrification rate and potential typically decline exponentially with depth into the soil (e.g., Weier et al., 1993; Luo et al., 1998), Jarvis and Hatch (1994) found greatly increased denitrification potential in subsoils under pasture compared to annual cropland. Under the conditions of their experiment, potential denitrification rate could be as high as 200 kg nitrogen per hectare, considering the entire 6-m-deep soil profile.
Jacinthe et al. (1998) found that these patches represented less than 1% of the aquifer weight. The most active patches in their study site were comprised of decomposing roots. Unlike pastures in humid environments, where shallow rooted (ca. 45 cm) forages like white clover and perennial ryegrass dominate, pastures in MIG are comprised of more deeply rooted grasses (orchard grass, tall fescue, reed canary grass, timothy, and smooth brome grass), with a small percentage of legumes (red clover, birdsfoot trefoil, alfalfa, and Kura clover). Root systems of these grasses can penetrate to 2.5 m or more, and alfalfa is well known for its extremely deep root system. In preliminary sampling, we found roots to 1 m in paddocks with the shallow groundwater table (Bestul farm) and to 1.8 m at another collaborator’s farm (Onan farm). More complete reduction of nitrate to dinitrogen gas may be promoted at depth in the soil, in part because nitrous oxide cannot escape the soil rapidly and can be reduced further, making such losses of nitrogen more environmentally benign (Rolston et al., 1976).
In addition, the patchiness of excreta addition is a factor in pastures. Denitrification rates are typically increased by fertilizer application (Scholefield et al., 1991; Colbourne, 1992). Similarly, urine ‘hotspots’ [Freifelder et al. (1998)] on 10 to 15% of the pasture can contribute more to total denitrification than the remaining pasture area (Ruz-Jerez et al., 1994). Denitrification rate may also be affected by pasture management as indicated in a watershed study conducted by Valiela et al. (1997), in which they estimated that denitrification rates were 10 times greater in heavily used than in infrequently grazed pastures. Conversion of forests to pastures greatly increased nitrous oxide loss (Matson and Vitousek, 1990), but losses were higher in young pastures (less than 10-years-old) than in older pastures (Keller et al., 1993).
The primary objective is to determine whether denitrification is higher in soil and groundwater under MIG than annual cropping. We are focusing on coarse and medium-textured soils, where nitrate loading potential is higher than for fine-textured soils.