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«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»

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Unlike nitrogen, phosphorus (P) excreted by dairy cows is not lost via volatilization. Only small amounts of both nutrients are separated out into the solid fraction during solid-liquid separation (Meyer et al., 2004). Furthermore, wash water added to the liquid manure stream from the milking barn contributes relatively small amounts of either nutrient to the liquid manure stream and does not affect the N to P ratio. Hence, the only major process affecting the N to P ratio in the liquid manure stream between the time of excretion and the storage lagoon outlet is N volatilization. Also, where significant bedding materials (straw, composted manure, etc., see chapter 3) are imported to the dairy, the N to P ratio of the bedding material collected into the liquid manure stream may affect the overall N to P ratio in the liquid manure stream. In freestall dairies where bedding materials do not contribute significantly to the liquid manure stream nutrients, comparison of the N to P ratio in freshly excreted manure (i.e., urine and feces) with the N to P ratio in manure collected from the lagoon yields an estimate of the overall atmospheric N losses from the production area (freestall lanes, flush system, and lagoon). We measured and compared N to P ratios in excreta and lagoons on 20 dairies in the Merced area (Robinson and Mitloehner, unpublished data). At the investigated dairies, the range of estimated atmospheric N losses, based on the N to P ratio reduction, was found to be between 20 and 35%.

Readily hydrolyzable organic N Method: Following a recommendation by the NRC (2003, p.109), an upper limit for atmospheric N losses that occur before lagoon storage can also be obtained by using the following simplified approach: The N excretion estimates in Chapter 2 of this report indicated that the average N excretion of dairy cows in California is 169 kg/head/yr.

Approximately 50% (84 kg/head/yr) of total N is excreted in urine and 50% is excreted in feces.

Approximately 70% (~60 kg) of the N in urine is in the form of urea, which is the N form that is readily hydrolyzed to ammonia and which is subject to volatilization. While some of the organic N from feces may be mineralized and therefore also be subject to volatilization, this process predominantly occurs in dry lot corrals and/or within lagoons rather than on flushed surfaces because it requires a relatively long process time. Based on these California-specific approximations and because of the relatively short residence time of excreta on these surfaces, the biological ceiling for atmospheric N losses from freestalls and flushlanes is approximately 35% of N excreted. This number is consistent with the above results from the N to P ratio measurements, although the latter accounted for losses not only in the freestall and flushlanes but also in the lagoon.

4.3.2 Atmospheric N losses from liquid manure storage

A process-based mechanistic model, as recommended by NRC (2003), can be used to quantify the dynamic interaction between the various factors that contribute to volatilization of ammonia from stored liquid dairy manure (e.g., lagoons). We developed an ammonia mass transfer model (see Appendix B for details on computation and validation) and calculated N volatilization from liquid manure storage lagoons under different conditions of lagoon depth and pH. Sample computations of ammonia volatilization from the surface of a waste storage lagoon were obtained for a lagoon with a volume of 9 million gallons, which is assumed to hold the liquid manure from 1000 cows for 90 days, at a rate of 100 gallons per cow per day. Two different depths, three pH levels, and NH3-N concentrations were assumed for the lagoon based on the levels encountered on dairies. Ammonia emission rates are higher in warm weather than in cold weather. Over 65% of annual ammonia emissions occur in the five months from May through September. Model results for a dairy waste water lagoon under climate conditions representative

of Fresno County are summarized in Table 4-1:

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The calculations show a wide range of possible ammonia emissions from lagoons, ranging from

3.0 to 62.4 kg/cow/yr depending on lagoon depth and pH, for a specific location. The calculated average for ammonia losses from lagoons, based upon this research, is 19.4 kg/cow/yr. Using an average N excretion for dairy cows in California of 169 kg/head/yr (Chapter 2), we suggest that the range of atmospheric N losses from dairy storage lagoons is between 1.8 and 37%, with an average of 12%. Again, this range is consistent with the N to P ratio estimates and the maximum hydrolysable N estimates.

Table 4-2: Estimated atmospheric N Loss (kg N/head/yr) from dairy manure in the production area of a typical freestall dairy in the SJV assuming 169 kg N excreted/cow/yr.

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4.3.3 Atmospheric N losses: Dependency on frequency of freestall flushing, on use of lagoon recycled (vs. fresh) water for flushing and on time spent in retention ponds More frequent flushing of freestalls will certainly decrease atmospheric N losses from freestalls and flushlanes as it removes the highly reactive urine urea and fecal urease to the pond, where depth and pH conditions will sharply curtail urea N volatilization as ammonia. In addition, the incremental effect of more frequent flushing on suppressing ammonia volatilization will be less with each subsequent increase in flushing frequency.

Use of recycled lagoon water for flushing will expose urea from the lagoon to a shallow water depth, with agitation, containing urease from fresh feces, and this will almost certainly increase ammonia volatilization, although it may be quantitatively trivial.

Nitrogen emissions from storage ponds (lagoons) will increase with time spent in the lagoon.

However the amounts volatilized will vary dramatically due to factors such as lagoon depth (which impacts its surface area), pH, ambient temperature and wind speed over the surface.

Overall there are insufficient data to quantify atmospheric N losses associated with effects of frequency of freestall flushing, use of lagoon recycled (vs. fresh) water for flushing and time spent in retention ponds.

4.3.4 Total atmospheric N losses in the animal production area Considering the above estimates of potential atmospheric nitrogen losses from freestalls, flushlanes, and lagoons (Table 4-2), we suggest that the atmospheric N losses in liquid manure from typical dairies in the Central Valley ranges from approximately 20% to approximately 40% of excreted N. We emphasize that these are approximate average losses across multiple dairy farms and that these do not include atmospheric losses during the land application of manure.

Given the above estimates, actual losses on individual dairy farms may vary widely prior to the land application process, but are unlikely to exceed 40%. The wide range partly stems from differences in manure distribution patterns within and among dairies (Chapter 2) and from differences in residence times of the animals on flushed concrete floors. Also recall that on dairies with flush system, the fraction of manure collected into liquid storage may range anywhere from 21% to 100% (Table 3-1). Not all manure N reaches the liquid storage structure, particularly if solid-liquid separation is in use. Furthermore, highly variable amounts of excreted manure can be deposited on non-flushed surfaces.

4.4 Summary Use of a universal animal ‘emission factor’ for reactive N compounds (e.g., ammonia) from commercial dairies is not possible because of the limited number of field measurements on which they are based and the wide emission variability among and within dairies. We concur with the National Research Council (NRC, 2003), that there is no single emission factor that could possibly describe atmospheric N losses from dairies.

To determine atmospheric N losses from existing dairies, we concur with NRC (2003) in recommending the use of process-based models coupled with a whole-farm and farm component N balance approach that describes potential atmospheric N losses for the different farm component processes. The process-based model approach coupled with total N balance predicts atmospheric N losses between excretion and land application and helps manage nutrient application rates to crops at agronomic rates. The Committee emphasizes however, that while this approach is technically viable, it requires extensive data measurement, record keeping, and is associated with significant estimation errors.

There are insufficient data available to quantify atmospheric N losses associated with effects of frequency of flushing, use of recycled water for flushing and time spent in retention ponds.

However, in general, more frequent flushing, of fresh water for flushing and shorter residence times in lagoons will tend to decrease ammonia volatilization. However the quantitative impact of these strategies is unknown at this time.

In light of these findings and in light of California-specific conditions, we suggest that atmospheric N losses from liquid manure (i.e., freestalls and flush lanes and lagoons) used for dairy planning and permitting purposes, are considered to range between 20% and 40%. The use of a single number (“emission factor”) is strongly discouraged. Note, that these losses do not include atmospheric N losses in the land application (crop production) area, which are discussed in chapter 5.

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5.1 Dairy Farming and the Nitrogen Cycle in Crop Fields Green plants use nitrogen (N) to manufacture proteins, chlorophyll, and other essential plant biochemicals necessary for their growth. Plants acquire N primarily from soils within the rootzone. Most of the N in soil is a part of the soil organic matter. For prevention of a long-term decline in the soil organic matter, N must be added at least at rates that will replace the N removed in the harvested crop. But terrestrial ecosystems, agricultural or natural, are inherently inefficient, and leaching of N beyond the root extraction zone is unavoidable; N must also be added to the soil to compensate for non-harvest, non-leaching system losses (Fig. 5-1).

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Fig. 5-1: Major Components of the Nitrogen Cycle in a Forage Crop Fertilized with Dairy Manure and Commercial Fertilizer.

A thorough understanding of N sources (inputs) and sinks (output and losses) in the soil rootzone (“soil”3) and the dynamic interaction between N sources, the crop-soil system, and the N sinks will provide relevant information to assess how much fertilizer or manure N must be Note that this review does not address losses in the deep vadose zone above the water table but below the rootzone. For purposes of this document, the term “soil” refers only to the uppermost 3 ft – 6 ft (root zone) of the unsaturated zone.

applied to meet the crop N uptake demands and to compensate for harvest N removal and N losses from the crop-soil system. An assessment of the soil nitrogen cycle provides the basis for determining agronomic rates of N application.

The dynamics of nitrogen within the crop-soil system are more complex than those of other nutrients such as potassium (K). This is due to the fact that nitrogen within the crop-soil system occurs in three different forms -- organic nitrogen (Norg), ammonium (NH4+), and nitrate (NO3-).

Organic nitrogen from crop residue, in the soil organic matter pool, and in manure applications is essentially immobile within the soil, strongly sorbs to soil particles, and is not available for plant uptake. Mineralization of organic nitrogen to ammonium is a microbial process that depends on the availability of carbon as a microbial food source and favorable temperature, moisture and other growth conditions. Ammonium nitrogen is plant-available, but also sorbs to soil particles.

Under most California conditions, ammonium nitrogen is rapidly (days to weeks) converted to nitrate (nitrification). Nitrate is plant-available, is not adsorbed to soil particles, and readily moves with soil water.

Agronomic application rates must be defined within the constraints of the nitrogen cycle and its dynamics within the crop-soil system. The need to satisfy nitrogen and other nutrient uptake by crops is shared by all agronomic systems. Cropping systems on dairies in the Central Valley,

however, differ from most other agronomic systems in two significant ways:

The use of manure instead of or in addition to commercial fertilizer (similar only to organic farming systems).

High nitrogen throughput due to production of high-biomass crop species, and two crops per year on much of the acreage. (See for example Table 5-1).

Table 5-1: Dairy forage field N inputs and harvest removals.

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In considering the appropriate agronomic rate for nutrient management with manure, it is important to understand that manure-based nutrient management further differs from standard

commerical fertilizer-based nutrient management systems in three important aspects:

the application method is different manure is a composite (multi-nutrient) fertilizer a significant amount of the nitrogen applied is in the organic form The first issue (manure application method) is rapidly evolving as technologies are currently developed to properly deliver and mix liquid manure with irrigation water that is applied to the crop. This issue is beyond the scope of this report. The second issue, manure composition, is discussed in chapter 2 of this report. The third issue, application of significant amounts of organic N in manured systems, speaks to the fact that much of the applied nitrogen must be mineralized before it becomes plant-available. Knowledge of nitrogen mineralization rates is therefore critical to proper management of nutrients in a manure-based system.

In this chapter, we will review the main components of the N cycle relevant to N management at Central Valley dairies where manure is applied to irrigated forage crops. On the source (input) side, these components include commercial fertilizer, manure, atmospheric deposition, and irrigation water nitrogen (section 5.2). On the sink (output and loss) side of the N cycle, the key components are harvest removal (section 5.3), gaseous losses, and leaching losses (section 5.4).

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