J. Mark Powell

Nitrogen budget and soil N dynamics after multiple applications of unlabeled or ^ Nitrogen-enriched dairy manure

cially if manure is surface-applied (Thompson et al., 1987). Conversely, denitrification losses of manure N Repeated N applications to field crops, either as inorganic fertilizers are increased by incorporation or injection (Comfort et or animal manures, can lead to N buildup in soils with potential longterm environmental hazards. The objective of this 3-yr field study al., 1988), and because organic matter in manure serves was to monitor totaland mineral-N levels in soil after repeated as a substrate for denitrifier microorganisms, tend to be fertilizer or single or repeated dairy manure applications, and to comhigher than denitrification losses in fertilized soils pute an N balance for the soil-crop system. Unlabeled and 15N-enriched (Cates and Keeney, 1987). Immobilization of inorganic dairy manure were used. The experiment was conducted on a Plano N in manure, plus the greater volatilization and denitrificorn silt loam continuously cropped to corn (Zea mays L.) Manure cation losses, cause inorganic N in manure to be less increased totaland NO3–N levels in soil, especially in the 0to 30-cm plant-available than equal rates of inorganic fertilizer depth and in plots receiving frequent and recent manure applications. Manure increased NO3–N in the 0to 30-cm soil layer more than (Paul and Beauchamp, 1993). Because of its lower N fertilizer N, whereas the opposite was true in the 30to 60and 60availability, greater amounts of manure than fertilizer to 90-cm layers. There was a clear NO3–N buildup with repeated N are applied to crops. This can result in a steady accumanure treatments. Unlabeled N measurements were not accurate mulation of soil N. Long-term soil N accumulation on enough to track trends in soil total N levels, hampering the calculation dairy farms poses a serious environmental risk (Bouldin of an N balance. 15Nitrogen-labeled manure allowed for direct meaet al., 1984). Heavy and/or repeated manure applications surement and provided more accurate estimates of N recovery in soils and crops. During the 3-yr study period, an average of 18% of applied can lead to NO3–N buildup in soil and losses through manure 15N was recovered in corn silage and 46% remained in the leaching (Adriano et al., 1971; Mathers and Stewart, soil. Unaccounted-for 15N (36%) was assumed to be lost mainly by NH3 1974; Smith et al., 1980; Cooper et al., 1984). Dairy farms volatilization and denitrification. Most (82%) of the 15N remaining in in the Midwest are considered significant contributors of soil was present in the top 30 cm, irrespective of frequency of manure N to the hypoxic zone in the Gulf of Mexico (Burkart application. Although costly and time-consuming, the use of 15Nand James, 1999). labeled manure provided a much better approach to study the fate Excessive soil nutrient accumulation and losses to of manure N within the soil-crop system, compared with unlabeled manure. surface and ground water are pressing environmental challenges facing the dairy and other animal industries. As dairy herds expand to remain economically viable, I most agricultural soils, N is the most limiting a larger percentage of the available cropland is devoted nutrient, and it has to be supplied to cereal crops, to corn silage. The noted expansion of corn silage proparticularly in high productivity systems (Meisinger, duction (Battaglia, 1999; Shaver, 2000) is due to this 1984). Fertilizers, manure, and in some cases, legumes, crop’s ability to feed more cows (Bos taurus) than other are the principal N sources for crop production in mixed, forages per unit of cultivated area (Seglar, 1998), as well dairy-crop production systems. Whereas fertilizer N is as favorable economics (Klemme, 1998) to the farmer. readily soluble in soils and becomes immediately availHowever, the effects of shifting more land to corn silage able for crop uptake, it can also be highly susceptible on other system components, such as N use, buildup to leaching losses. Comfort et al. (1987) found that soil and loss remains to be determined. Since only a relainorganic N and downward movement were increased tively small amount of applied N is ultimately taken up to a greater extent by fertilizer than by manure N. On by the crop, we wanted to track the fate of the unused the other hand, only about half of manure N is inorganic, portion to see whether it was lost or remained in the soil. with the rest being present in organic forms. Organic N The objective of this study was to determine total and must be mineralized before it can be used by plants or inorganic soil N and the N balance of a continuous corn it becomes susceptible to losses. However, when fresh silage cropping system receiving two fertilizer or dairy manure or slurry contain appreciable amounts of urea manure N rates of different application frequency across or NH4, N can be easily lost via NH3 volatilization, espe3 yr. Unlabeled and 15N-enriched dairy manure were used, and the ability of each manure type to detect G.R. Muñoz and K.A. Kelling, Dep. of Soil Science, Univ. of Wiscontrends in soil N levels and account for applied N was sin, 1525 Observatory Dr., Madison, WI 53706; and J. Mark Powell, USDA-ARS Dairy Forage Research Ctr., 1925 Linden Dr. West, compared. The use of 15N-labeled manure was an essenMadison, WI 53706. Received 22 Feb. 2002. *Corresponding author tial part of the study because it allowed direct N tracking (grmunoz@uwalumni.com). in the cropping system, and provided more accurate measurements than unlabeled manure. Published in Soil Sci. Soc. Am. J. 67:817–825 (2003).

Links among Nitrification, Nitrifier Communities, and Edaphic Properties in Contrasting Soils Receiving Dairy Slurry

Soil biotic and abiotic factors strongly influence nitrogen (N) availability and increases in nitrification rates associated with the application of manure. In this study, we examine the effects of edaphic properties and a dairy (Bos taurus) slurry amendment on N availability, nitrification rates and nitrifier communities. Soils of variable texture and clay mineralogy were collected from six USDA-ARS research sites and incubated for 28 d with and without dairy slurry applied at a rate of ~300 kg N ha(-1). Periodically, subsamples were removed for analyses of 2 M KCl extractable N and nitrification potential, as well as gene copy numbers of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Spearman coefficients for nitrification potentials and AOB copy number were positively correlated with total soil C, total soil N, cation exchange capacity, and clay mineralogy in treatments with and without slurry application. Our data show that the quantity and type of clay minerals present in a soil affect nitrifier populations, nitrification rates, and the release of inorganic N. Nitrogen mineralization, nitrification potentials, and edaphic properties were positively correlated with AOB gene copy numbers. On average, AOA gene copy numbers were an order of magnitude lower than those of AOB across the six soils and did not increase with slurry application. Our research suggests that the two nitrifier communities overlap but have different optimum environmental conditions for growth and activity that are partly determined by the interaction of manure-derived ammonium with soil properties.

Nitrogen performance indicators for dairy production systems

Nitrogen (N) is invaluable for maintaining agricultural production, but its use, and particularly inefficient use, can lead to environmental losses. This paper reviews N use efficiency (NUE) and N surplus indicators for dairy production systems to assess their utility for optimising N use outcomes and minimising environmental N losses. Using case-study examples, we also assess realistic goals for these indicators and discuss key issues associated with their use. Published whole-farm NUE and whole-farm N surplus values ranged within 10–65% and 40–700 kg N ha–1 year–1 respectively. In a study of five catchments across New Zealand, whole-farm NUE was more strongly affected by catchment differences in soil and climatic conditions than by differences in management. In contrast, whole-farm N surplus differed both between- and within-catchments and was a good indicator of N losses to water. Realistic goals for both NUE and N surplus thus depend on the agro-climatic context of the dairy system and on its economic and environmental goals. Crop and animal NUE values can be valuable indicators for optimising fertiliser and feed use and minimising N losses. However, global or national whole-farm NUE values appear of limited value if the ultimate goal for setting targets is to reduce the environmental impact of N use; whole-farm level targets based on N surplus would be a more useful indicator for this purpose. Our review also reinforces the importance of standardising the variables that should be used to estimate NUE and N surplus values, to ensure equitable comparisons between different systems. Finally, NUE and N surplus targets should also be set in the context of other agro-environmental considerations.

Estimating nitrogen excretion and deposition by lactating cows in grazed dairy systems

Large N surpluses are a feature of most dairy farms worldwide. Despite the predominance of grazing systems in Australia, increasing animal densities and greater reliance on purchased feed mean that feed nutrient inputs and the role of grazing animals in N flows and deposited loads have grown. However, quantifying nutrient intakes and N excretion is difficult on commercial farms due to challenges in estimating pasture dry matter (DM) intake by grazing cattle. The aim of the present study was to quantify for commercial dairy farms, N excreted by lactating herds, the relative amounts of excreta N versus farm N inputs, and N loads deposited to different within-farm locations. Data were collected on at least five occasions from 43 diverse grazing system farms located in different climatic zones. An animal performance method for estimating annual DM intake was modified to calculate daily N intake and excretion and animal feed N use efficiency (NUE; milk N secreted/feed N intake). On average, these herds received 52% of their energy requirements from supplementary feeds despite the grazing base. Mean N intake (545 g cow–1 day–1), which greatly exceeded recommended dietary intakes, resulted in mean excretion of 433 g N cow–1 day–1 and low mean NUE (21%; range 11–39%). Calculated annual N flows through the lactating herds were equivalent to 69% of total N inputs onto these farms. When excreted N was apportioned to the locations visited by the lactating herds, deposition to pasture paddocks was not uniform. Almost 50% more excreted N was deposited to paddocks that were closer to the dairy shed, and approximately twice as much N was returned to feedpads and holding areas as deposited in dairy sheds and yards, with the potential for N accumulation and loss in these places. On average, 20% more N was deposited as excreta on pasture paddocks by the lactating herd than was applied as commercial fertiliser (168 vs 141 kg N ha–1 respectively). These results demonstrate that quantifying excreta N loads and spatial nutrient distribution by lactating cows can assist in improving N management in grazing system dairy farms.

Computational model of methane and ammonia emissions from dairy barns: Development and validation

Abstract The increased global demand for milk and other dairy products over the past decades is a cause for concern due to the potential for environmental impact. Ammonia produced by housed dairy cows can contribute to the formation of particulate matter and nitrous oxide which both contribute to the greenhouse effect. The methane produced by these cows also contributes to the greenhouse effect. Scientists and engineers face the challenge of developing methods to reduce the environmental impact of dairy production while not inhibiting the ability of producers to keep up with demand. Emission of methane and ammonia are highly dependent on feed composition, barn design and operation, manure management making this a challenging topic to study experimentally. Using computational models to simulate the generation and dispersion of gaseous species within dairy housing can facilitate the exploration of cost-effective gas mitigation strategies. Thus a steady-state computational fluid dynamics (CFD) model capable of simulating biologically based generation of methane, ammonia, and heat and their transport within the domain was developed and validated. The effect of buoyancy forces on the accuracy and stability of the solutions was explored. The model was validated with experimental data collected from emission chambers located at USDA-ARS Dairy Forage Research Center in Wisconsin, USA. Concentration of ammonia and methane, due to controlled injections from cylinders and biological generations from a dairy cow, were measured in the chambers using a FTIR gas analyzer. Results of the validated CFD model could be used to predict gaseous emissions under a range of environmental, design, and experimental treatment parameters.