«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
Mineralization of the organic N (section 5.5) exerts significant control on the soil N dynamics and on the sink pathways (section 5.6) and is therefore given attention here. We use the information to discuss appropriate agronomic rates for California dairy farms (section 5.7).
5.2 Sources of Nitrogen (Nitrogen Inputs) The primary sources of N for non-leguminous crops grown on California dairies are manufactured fertilizers and animal manure. Soil amendments may also contain significant amounts of N. Additional N is present in some irrigation waters as nitrate, and this may be a significant source of nitrogen (Table 5-2).
Atmospheric deposition of nitrogen oxides and ammonia are another source of N to cropland soils although the total annual deposition is relatively small. Total wet and dry N deposition in the San Joaquin Valley is approximately 14 lbs/A/year (16 kg N/ha/year) (Blanchard and Tonnessen, 1993; Mutters, 1995).
Atmospheric nitrogen also enters the crop-soil system via biological nitrogen fixation (BNF). In Central Valley dairies, the forage crop rotations commonly include legumes, mostly alfalfa and some clovers. Bacteria living in nodules in association with roots of these crops fix atmospheric di-nitrogen (N2), and therefore leguminous crops do not depend on other external sources for their N requirements. However, if N is available in the root zone, these crops will preferentially utilize the available soil N, and the microbes will only fix the atmospheric N to supplement their needs. Alfalfa is well known as an efficient scavenger of N and other nutrients from the soil. If it is well managed, alfalfa during the second and third years of production typically removes 400 to 500 lb N per acre per year in the harvested crop (Table 4-1, CPHA, 2002). Alfalfa N uptake from soil proportionally reduces the amount of biologically fixed N.
What is the net gain in soil N resulting from a typical three-year stand of alfalfa? It is well established that non-leguminous crops following alfalfa invariably need less N fertilizer than when following a non-legume crop. This reduction in N fertilizer need is referred to as a “legume credit.” The legume credit does not refer to the amount of biologically fixed N, but rather the net reduction in fertilizer N requirement to the following crop that is due to the legume in the crop rotation. The legume credit is estimated to be 50 to 80 lbs of N per acre, depending on how strong the alfalfa production was in its final year4.
5.3 Crop N Uptake and Harvest Removal5.3.1 Seasonal N Uptake Total
Plants can take up only inorganic forms of nitrogen, the main forms being NO3- and NH4+. These
forms are referred to as available nitrogen. Plant nitrogen uptake has two important components:
(1) The total nitrogen uptake of the crop and (2) the portion of the uptake that is eventually removed from the field at harvest. The total plant nitrogen uptake provides a target to assess the nutrient status in the soil relative to the crop demand. Plant nitrogen uptake needs must be available in the soil to ensure a crop that produces an economically acceptable yield and quality.
The second component, crop removal, is important since that is the minimum amount that must be replaced with fertilizer (chemical or manure or from other external sources) if the soil organic N reservoir is to be maintained. The portion of the plant that is not removed by harvest (roots, stubbles, etc.) will decay in the field and its nitrogen is returned to the soil nitrogen pool. That portion of the total plant nitrogen uptake therefore does not need to be replaced with external N inputs.
For long-term fertilizer management, the second component provides the application targets.
Approximate N contents per unit of crop yield are published in Table 4-1 of the Western Fertilizer Handbook (WFH), 9th edition (CPHA, 2002). That table presents crop N removal in E-mail survey of UC Cooperative Extension farm advisors and specialists conducted January 2005.
amounts per acre per unit of crop yield. If certain precautions are kept in mind, one may use these data (both 9th and 8th edition) to estimate the crop removal of N from the soil.
Some important forage species produced by Central Valley dairies are missing from Table 4-1 of the WFH, notably cool-season grass forages such as wheat, triticale, oats, and annual ryegrass, which are most commonly harvested by dairy producers for silage or green chop, rather than for grain or hay. For convenience, these are referred to here as “winter forages”.
Nitrogen uptake by winter forage crops (here reported in lbs N per ton at 70% moisture) varies by large amounts due to varying cutting stage and due to varying soil nitrogen conditions between individual fields, between individual farms, and from year-to-year. In California, under reasonably fertilized conditions, the average nitrogen removal has been found to be 9.1 lbs N per ton to 10.5 lbs N per ton, typically harvested at the later milk to soft dough stage (Roland D.
Meyer, personal communication; Peter Robinson, personal communication). In the Northern San Joquin Valley, field research on winter forage nutrient uptake has been and is currently being
conducted by UC Cooperative Extension (Campbell-Mathews, personal communication):
Among 56 individual variety-location-years, representing a range of fertility conditions, nitrogen removal averaged 13.8 lbs of nitrogen per ton of forage when harvested at the boot or early heading stage, and 11.3 lbs N per ton when harvested at the milk to soft dough stage.
Overfertilization may increase the nitrogen uptake of winter forages. However, when nitrogen concentrations are much higher than the 10.5 lbs N per ton at the milk to soft dough stage, an unknown but potentially significant fraction of the N can be in form of nitrate, which may cause nitrate poisoning in cattle (particularly, if the cattle have not been adjusted to high nitrate feeds).
Optimal fertilization balances the need to maximize crop growth and nutrient uptake with the need to limit the risk of nitrate toxicity in winter forages and the risk of nitrate leaching to groundwater. For planning winter forage plant N uptake, we recommend to use 10 lbs N per ton of forage at 70% moisture, harvested at the later milk to soft dough stage, as a preliminary guideline. Forages harvested at earlier growth stage (boot and flower) will have a higher per-ton requirement (but their harvested weigth per acre is lower). Further research is needed to clarify optimal nitrogen removal in winter forages in California with respect to nutrient management (to ensure sufficient yield while avoiding excessive fertilization) and with respect to animal toxicity (high levels of nitrate in forage).
Multiplying these removal rates (lbs per ton) by the yield allows the removal to be calculated for a range of yields using the approach taken in Table 4-1 of the WFH (9th ed.). Additionally, 8.7 lbs K2O and 3.6 lbs P2O5 per ton of forage (70% moisture content) are removed. Actual K and P uptake may vary greatly. In winter forages, K2O removal of 18 lbs per ton or more are not uncommon and considerably higher P2O5 removal has been documented as well. Note, that levels above 18 lbs K / ton of forage may lead to severe animal health problems including death.
Some additional precautions in the use of Table 4-1 of the WFH are noted here:
As stated in a footnote accompanying the table, “Actual nutrient removal may vary by 30% or more.” Moisture contents of the harvested materials are not given in the table, and the reader is left to assume that the nutrient concentrations displayed are at “standard” moisture contents i.e. grain and hay moisture contents are typically 10-12% while forages harvested for ensiling typically have a 70% moisture content.
To summarize, where actual data are not available, the WFH values (together with consideration of N system losses and all N inputs) can be used for assessing adequacy of crop land area for receiving manure. Where at all possible, such an assessment should rely on historical yields and plant N content representative of specific site conditions.
5.3.2 Timing of Nitrogen Uptake by Forage Crops Grown in the Central Valley Dairy operators in the Central Valley typically grow silage corn during the summer and cereal forages during the winter.
The cumulative corn nitrogen uptake forms an S shaped pattern, with low uptake during the first 30 days of growth, then rapid uptake until silking. Uptake after silking is slower. Classic work conducted by Hanway (1962) indicated that 75-80% of nitrogen in corn is taken up by the time of silking. More recent research (Karlen et al. 1988, Schepers, unpublished data) indicated that modern silage hybrids with a “staygreen” trait continue to take up significant amounts of N after silking. Those results are in agreement with the results of on-farm studies over six location-years near Hilmar, California, in which only 64% of nitrogen (range 52-79%) was taken up by corn at the silking stage (Campbell Mathews, 2001).
The cereal forages (also referred to as “winter forages”) are usually planted in mid- to late fall.
Multiple-year field trials on N uptake by winter cereals have been conducted by CampbellMathews (2003 and 2004). The trials show that N uptake of winter cereals grown in the Central Valley also follow an “S” shaped cumulative N uptake curve, with relatively low rates of nitrogen uptake during December and January. By mid-January, only about 50 lbs N/acre has typically been taken up in the aerial plant portion from a mid- to late fall planting. The exponential growth phase starts in mid-February to mid-March, depending on the species and variety of cereal. For late fall plantings of winter forages, as much as 85% of the nitrogen uptake takes place during late February, March and April. During that growth stage, the rates of nitrogen uptake can be dramatic - as much as 100 lbs/acre of nitrogen in a 15-day period (Table 5-3).
TABLE 5-3: Percent of total mid-April nitrogen uptake in N. San Joaquin Valley by period
This period of rapid uptake can be exploited to allow application of higher rates of nitrogen in the usual single water-run application of dairy nutrient water than would be applied to a summer corn crop, where 40-60 lbs of N per acre is a typical target for a single application.
Sometimes, winter forages are planted earlier in the fall, from mid-September to mid-October.
Since growing conditions are usually favorable during this period, nitrogen uptake will be considerably higher in the early fall than with later planting dates, with as much as 30% of the total nitrogen uptake occurring before January (Campbell Mathews, 2003).
When the N uptake of summer silage and wheat are combined, the annual N uptake pattern by a corn silage and late-fall-planted winter forage rotation in the Central Valley follows the pattern shown in Figure 5-2. Most of the nitrogen uptake in this system occurs during relatively short periods in the spring and in mid-summer. The actual uptake timing depends on planting date, weather, crop variety/species, nutrient availability, and for the winter forage, whether there are multiple cuttings.
Fig. 5-2: Potential N uptake rate of for silage-corn/winter forage double crop in the Central Valley of California.
5.4 Nitrogen Losses In addition to the N uptake of the crop, there are five other pathways through which N may be
removed from soils, namely:
The last pathway, surface runoff losses, will not be further discussed here. In California, by regulation, farmers must in as much as possible, prevent runoff from fields to which animal manure has been applied. The following review discusses the role of the remaining four pathways within the context of the N inventory in forage production soils amended with dairy manure.
5.4.1 Ammonia Volatilization from Soil
As manure is excreted from the animal, it is primarily organic nitrogen, but a portion of it is quickly converted to ammonium (NH4+). Under alkaline conditions, a portion of the ammonium N is in the gaseous ammonia (NH3) form and subject to volatilization loss to the atmosphere (Bouldin et al., 1984).
We are not aware of research in which NH3 losses have been measured during application of diluted lagoon water via surface gravity irrigation systems, by which method nearly all lagoon water is applied in California. In those systems, incorporation occurs through infiltration except that larger solids containing some of the organic N remain unincorporated on the soil surface.
A nearly 40-year-old fertilizer industry advisory states that up to 100 ppm (mg L-1) NH3-N (22 lbs NH3-N per acre inch) may be safely applied in surface irrigation water before ammonia losses become significant (Warnock, 1966). It has therefore been suggested that the NH4-N content in irrigation water that is blended with manure water be kept near or below this level to avoid significant N volatilization. Manure water in the storage lagoon contains from 50 to 1,000 ppm (mg L-1) NH4-N (Campbell-Mathews et al., 2001b), with typical concentrations of 200-500 ppm (mg L-1). During irrigations, farmers commonly dilute lagoon water with 5 to 10 parts of fresh source water. This would result in 20 to 100 ppm (mg L-1) NH4-N in the irrigation water.
When the crop has full canopy coverage, the leaves will likely reabsorb some of the volatile NH3 in the surface irrigation. Furthermore, the presence of the crop canopy reduces wind speed at the soil surface, reducing the upward flux of NH3. Temperature and irrigation water pH are also important controlling factors. Maximum NH3 loss rates will occur with high wind, high soil pH, negligible crop canopy, high temperatures, and high NH4+ concentration in the irrigation water and in the soil. In our judgment, there is a potential for significant N loss only in undiluted or slightly diluted manure water applications (applications at the upper end of the NH4-N concentration range) particularly if applied during the early growth of the crop. In systems with frequent, but well diluted manure water applications, ammonia losses from the ground surface will commonly be minimal during the irrigation (10% or less).
5.4.2 Ammonia Losses from Plants