«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
Several researchers have observed significant NH3 loss directly from plants. Francis et al. (1993) analyzed the N fertilizer recovery by irrigated corn in which 15N isotopic techniques were used to track the N distribution. The study was conducted at different sites, in different years, with fertilizer rates ranging from 45 to 270 lbs N ac-1. The post-flowering N losses from the aboveground biomass of corn were quantified under the different N input regimes. When the isotopic dilutions were factored in, the N losses from aboveground biomass ranged from 40 to 72 lbs N ac-1, which amounted to 52 to 73% of the N unaccounted for in 15N balance calculations. While the mechanisms for the absorbed N to be lost from plant tissue are unknown, Francis et al.
(1993) concluded that the observations were convincing and that failure to include direct plant N losses when calculating an N budget leads to overestimation of losses from the soil by denitrification, leaching, and ammonia volatilization directly from the soil.
At present, however, little data is available in the published literature to substantiate the reported observations. As the N balance is customarily conducted at harvest to account for the applied N, the N loss through the plant tissue becomes part of the overall losses (volatilization, denitrification, leaching) and does not change the perspective with respect to the maximum N requirements for plant growth.
Seasonal losses of N due to root zone denitrification are difficult to predict, as the rate of the process may vary dramatically over time and space. Ryden and Lund (1980) reported that total N losses due to denitrification in irrigated soils planted with vegetables ranged from 85 to 208 lbs N ac-1 yr-1. In another study, when liquid dairy manure was applied by sprinkler to a loamy sand soil for production of silage corn and bermudagrass/ryegrass hay, the estimated denitrification loss ranged from 11% to 37% of the total manure N applied (Lowrance et al., 1998).
Many irrigated dairy forage fields in the San Joaquin Valley are low in soil organic matter (2%), are classified as excessively to moderately well drained, and receive regular applications of dairy manure. For that combination of characteristics, Meisinger and Randall (1991) in a review of the topic estimate that denitrification losses will vary from 8% to 28% of inorganic N in soils. In a typical double-cropped forage system in the Central Valley, the annual N inputs range from 350 to 675 lbs of N ac-1. Based on the values of Meisinger and Randall (1991), the annual denitrification N loss from fields receiving dairy manure would range from 30 to 190 lbs N ac-1.
On the other hand, for manure systems in a loamy sand soil in Merced County, Harter et al.
(2001, 2002) and Campbell-Mathews (2004) concluded that the combined denitrification and volatilization losses were negligible (or within measurement error of leaching losses) compared to leaching losses to groundwater (Appendices G, H). Campbell-Mathews (2004), for example, reported total nitrogen losses (leaching, volatilization, and denitrification) of 159 lbs ac-1 yr-1 for the last five years. These losses resulted in recharge water nitrate-nitrogen (NO3-N) concentrations of 25-50 ppm (mg L-1), a concentration that is only possible if nearly all losses are attributed to leaching.
5.4.4 Leaching of Nitrate from the Root Zone
Leaching of water and solutes below the root zone can be minimized but not entirely eliminated, especially with the furrow and border check irrigation systems used by most dairy farmers in the Central Valley. In these gravity flow systems, the water distribution is susceptible to the variations of topography, soil texture, surface conditions, and soil moisture content across the field. The irrigation schedule typically is a compromise to balance between irrigating the heavier, clay-rich portions of the field and the lighter, sandier portions of the field. Irrigating the heavier portions of the field too early would cause yield loss due to saturated soil conditions, while irrigating the more sandy portions of the field too late would cause crop water stress. Application amounts are usually determined not by how much water is needed to refill the soil profile in the root zone, but by how much water is needed to reach the end of the field. This may result in excess total water being applied and/or more water infiltrating the soil at the head of the field than in other parts. The non-uniformity in water application invariably leads to nitrate leaching past the root zone in some parts of the field if nitrate is available in the root zone. Furthermore, salts introduced through applications of dairy manure, fertilizer, and irrigation water will accumulate and concentrate in the root zone. They must be leached to sustain healthy plant growth. In the Central Valley, rainfall coupled with water losses due to non-uniform irrigation is often adequate to remove the excessive salts and maintain an annual balance. If leaching due to inefficient irrigation were entirely eliminated, salts would accumulate in the root zone to harmful levels during low-rainfall years.
In the Central Valley, the practically achievable irrigation application efficiency (IAE) for gravity flow irrigation systems is 70 to 85%, depending on the soil texture, size of the fields, and other field-specific factors (Hanson et al., 1999). This is equivalent to a leaching fraction (LF) of
0.15 to 0.30 in which LF denotes the fraction of total water input that is leached beyond the root zone. Under those conditions, nitrogen leaching is minimized by using split applications of fertilizer or manure (e.g., Harter et al., 2001; Campbell-Mathews, 2004; Nakamura et al., 2004).
Simulations of a typically irrigated dairy forage crop system in the San Joaquin Valley indicate that N losses can be managed to be as small as 10% - 15% of the applied and available N, after subtracting denitrification and volatilization losses (Appendix C).
5.5 Mineralization of Organic Nitrogen The rate of mineralization of organic nitrogen is dependent on microbial processes acting upon various organic compounds under favorable temperature, moisture, and other conditions.
Mineralization of organic nitrogen in manure added to soil generally follows an exponential pattern, i.e., a very rapid initial rate followed by an increasingly slower rate as the more labile organic compounds are decomposed. The mineralization rate is often expressed in the form of a half-life of (labile) organic nitrogen (Appendix D). Half-life refers to the time need to mineralize half of the organic nitrogen applied. A longer half-life of the organic nitrogen corresponds to a slower mineralization rate. A review of field studies (Appendix D) indicates that typical nitrogen half-lives for organic N under California conditions likely range from a few tens of days to a few hundred days.
The associated dynamics of the organic nitrogen subject to various mineralization rates that vary seasonally according to a typical California climate were investigated by Guanglon et al.
(Appendix E). They show that essentially complete mineralization of manure organic nitrogen (more than 95%) may be achieved within a 1 to 7 year time period depending on the mineralization rate, with organic N half-lives ranging from 50 to 280 days. Mineralization rates are higher during the summer (2-4 times faster than in winter and early spring), hence organic N applied in spring and early summer will be mineralized much faster than organic N applied in the fall or winter. For mineralization rates corresponding to a half-life of 90 days or less, a long-term quasi-steady-state in the organic nitrogen pool is achieved within the second year after beginning manure applications, whereas it takes 5 years to achieve quasi-steady-state for a half-life of 280 days. Modeling analysis of soil water and nitrogen dynamics using data from a Merced County dairy field study indicate that, in sandy soils, mineralization rates appear to be relatively rapid (half-life of 90 days or less, Appendix H).
Across most of the likely range of mineralization rates, organic N can be shown to be a steady, relatively slow-release source of plant-available and leachable nitrogen. The steady release of mineral N from the organic N source is in contrast to the strong seasonal variability in plant uptake needs (Fig. 5-2). The mismatch between the rate at which organic nitrogen becomes plant-available nearly year-round and the rate at which nitrogen is seasonally needed by the crop leads to an inherent limitation in manure nutrient management: Applying manure with high organic nitrogen content may not provide sufficient nitrogen to meet the crop N demand during the most rapid growth stage, yet far exceed crop N uptake during the remainder of the cropping season (and during fallow periods), when it is potentially subject to leaching.
5.6 Long-Term Dynamics of the Soil Organic Nitrogen Pool Where farmers rely heavily on organic N sources (as in organic farming or conventional farming with livestock manure), the amount of organic matter in the soil increases and with it the amount of soil organic N. If the cropping practices are constant over long periods of time (years), the soil organic matter (SOM) level will reach some steady state, that is, SOM no longer increases.
Given the fact that many fields have routinely received dairy manure for ten or more years, it is reasonable to assume that long-term forage production fields are in a “quasi” steady-state condition, that is, the annual amount of organic N that is mineralized is approximately equal to the average annual amount of organic N applied or generated as biomass (but not removed at harvest).
While at steady state over the long-term, the soil organic nitrogen pool is potentially very dynamic over the short-term with diurnal (daily), weekly, and seasonal changes in SOM and soil organic N content. Organic matter and organic N dynamics, primarily the mineralization rates at these shorter time-scales control the production of plant-available and leachable forms of nitrogen.
In California, few long-term field studies have been conducted in which soil organic matter and organic N levels have been monitored. One such study currently in progress at UC Davis has shown that high inputs of organic matter (from compost and cover crop residues) in an organic cropping system have increased total N content of the top 12 inches of a loam soil over a 10-year period by 804 lbs N per acre, compared to only 70 lbs N per acre in a four-year conventionally fertilized crop rotation (Poudel et al., 2001). The annual change in organic N storage in the organic cropping system was therefore less than 100 lbs N per acre-yr.
5.7 Nitrogen Rate Guideline: Putting It All Together5.7.1. Agronomic Rate
The previous sections discussed the various components of the nitrogen cycle in manured field soils assuming good agricultural practices. From an agricultural planning and management perspective this information provides the basis for determining the agronomic rate of N applications (from application of nitrogen fertilizer, manure N, and other N sources).
“Agronomic rate” refers to the minimum N input needed to sustain a successful and profitable crop production. In considering land application of organic manure, it is meant to stand in contrast to a “(waste) disposal rate.” The agronomic rate represents the supplements that make up the total N nutritional needs of plants and should take into account N from all sources (including inputs of commercial fertilizers, irrigation water N, fixation of atmospheric N and indigenous soil N) and sinks (including losses of nitrogen through denitrification and ammonia volatilization). The agronomic rate may also include an allowance (additional N) to compensate for field non-uniformity and uncertainty about the crop response.
In commercial production, agronomic rates are not a generalized and fixed N input; instead, they are site- and case-specific quantities depending on production goals, expected N sources and sinks of the soil, and crop N requirements. In the spirit of environmental protection, the off-site and downstream environmental impacts of land application should also be factored into the agronomic rate. The agronomic rate represents the N input for the best biomass production without causing undo groundwater pollution and/or degradation of downstream water quality.
In well-managed fields, application of N according to the agronomic rate would achieve a satisfactory production goal while minimizing the potential for nitrate leaching. In this context, the agronomic rate may be generalized. Broadbent and Carlton (1979) reported that with increasing N fertilizer rate, there was only a slight increase of nitrate in the soil profile available for leaching when fertilizer N input was kept at or below the agronomic rate. When the N fertilizer inputs exceeded the requirement for maximum crop yield, the residual nitrate concentration in the soil profile increased sharply.
If we use the best estimates summarized in the previous sections to account for all the N sinks in the type of crop treatments discussed above, the total N loss in terms of the N applied may be tallied to derive an achievable apparent nitrogen recovery (ANR) and in turn, a nitrogen input requirement (NIR) which is then the agronomic rate for dairy manure applications (Table 5-4).
In this assessment, we assume that dairy wastewater from storage lagoons is applied on a low organic matter sandy or loamy sand soil along with irrigation water for corn-winter forage double cropping.
Based on the estimates summarized in Table 5-4, 36 to 62% of the applied N may be lost through the N removal pathways and is not available to the plants. Therefore, the apparent nitrogen recovery (ANR), which represents the percentage of applied N taken up by plants, will be the remainder and equals 38 to 64% of the applied N. The nitrogen input requirement (NIR), which indicates the agronomic rate plus other inputs, will then be the reciprocal of ANR and is equal to
1.56 to 2.78 times the crop N harvest removal.