«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
Simulations using ENVIRO-GRO (Appendix E) resulted in a range of 1.2 to 1.4 times plant N removal, considerably less than the N input requirement of 1.56-2.78 times crop harvest removal estimated from Table 5-4. However, the ENVIRO-GRO simulation did not include any NH3 losses from soils or plants. If the plant and soil NH3 losses from Table 5-4 are added to the ENVIRO-GRO simulation, the calculated range for nitrogen input requirement would be 1.4 to 2.1 (i.e., 140 – 210% of crop harvest N removal), much closer to the NIR derived from Table 5Table 5-4: Range of estimated losses of N inputs to forage crops in the Central Valley based on literature values.
We suggest that ammonia losses from plants should not be included. Plants can be both a source and a sink for ammonia. When background atmospheric ammonia levels are elevated above the compensation point, as would often be the case in the vicinity of a dairy, plants are more likely to be a sink rather than a source of ammonia. Furthermore, denitrification has often been estimated by researchers without considering plant losses of ammonia, and in some studies, researchers may unwittingly have included the latter. When plant ammonia losses are not included in Table 5-4, the resulting calculated NIR is 1.39 to 1.92 (139 to 192%) times the crop N harvest removal.
5.7.2 Field Examples of NIR and ANR
How do the above estimates of “practically achievable ANR and NIR” compare to field studies and simulations reported in the literature? There are many reports of the crop recovery of a single application of N fertilizer; considerably fewer such studies with a single application of animal manure; and very few studies in which N use efficiency over several years has been calculated.
Some investigators have used modeling or regional crop yield and fertilizer usage figures to estimate ANR. Most field studies indicate that NIRs of 140% - 150% are achievable in wellmanaged agricultural systems (Appendix F), well within the range derived above.
5.7.3 Field Nitrogen Balance
Figure 5-3 shows the result of a hypothetical field N budget, based on realistic transfer coefficients for the major N transformations and loss pathways. In the exercise, the crop harvest N removal, manure characteristics (not shown), and commercial fertilizer rate, along with the rate coefficients, were arbitrarily chosen but are within the typical ranges in the published literature. It was assumed that the soil organic N content was in a steady state. The budget balances N by calculating the rate of manure N that must be applied to achieve the desired crop N uptake and harvest removal.
Fig. 5-3: Hypothetical N balance for a forage crop fertilized with dairy manure and commercial fertilizer. Based on 100 lbs N removed in harvested crop. N losses by denitrification, leaching, and manure ammonia volatilization are set at 10%, 15%, and 15%, respectively. N in harvested crop equals 85% of total plant N uptake. Direct loss of ammonia from plants is assumed to be zero.
In this example, the 100 lbs crop harvest N removal is achieved by applying a total of 153 lbs N from external N inputs (i.e., NIR = 153% or 1.53). This is equivalent to an apparent nitrogen recovery (ANR) of 65% (1/1.53 x 100), which is within the range of ANR reported in the scientific literature for various cropping systems. In this scenario, loss of N by leaching is restricted to 15% of N inputs. The results presented here undoubtedly are subject to a great number of uncertainties. It nevertheless demonstrates that the upper end of computer-simulated optimal N loading rates of 1.4 to 1.65 times the crop N harvest removal are practical and, based on field observations, achievable if the production field is properly managed.
5.8 Needs for Further Study The ANR (Apparent Nitrogen Recovery) and the corresponding NIR (Nitrogen Input Requirement) for dairy manure applications are dependent on the specific agronomic and environmental circumstances at a particular location. While it is not practical to evaluate the ANR and NIR case by case, it may be appropriate that ANR and NIR targets be established for sub-regions, rather than generalizing over the entire Central Valley. Through this approach, the Central Valley may be subdivided into operationally homogeneous regions according to potentials for denitrification, ammonia volatilization, crop N uptake, and nitrate leaching. In Shaffer and Delgado (2002), the merits of this approach were elaborated. Pratt (1979) and Coppock and Meyer (1980) proposed an index for judging the “relative sensitivity” of irrigated croplands to leaching of nitrate. The index included factors relating to soils, crops, irrigation and climate. The UC Center for Water Resources has recently completed development of a regional nitrate leaching hazard index that classifies soil series in irrigated lands in the southwestern US according to the potential for nitrate leaching and denitrification (www.waterresources.ucr.edu).
For improvements, additional and more concise information is needed in the following areas:
Accurate estimates on the extent of N losses through the various pathways:
o Denitrification in soil profile in relation to soil texture and irrigation water management o Denitrification potential in the deeper unsaturated zone and in groundwater o Ammonia volatilization when dairy manure is applied on cropland o Nitrate leaching in relation to the irrigation and leaching fractions o Potential direct N losses through plants Role of alfalfa in the overall N budget of the dairy manure-irrigated forage production system;
Long-term organic N accumulation and mineralization in dairy manure-receiving fields;
Fate and transformation of N in solids separated from the liquid manure stream.
5.9 Summary Both field and modeling studies reviewed and implemented for this report consistently show that the N input requirements for forage crops will generally be in the range of 140% to 165% of the crop N harvest removal, assuming that the manure application would consist of lagoon water which is approximately 75% NH4-N. As stated above, inputs include not only manure and fertilizer N but also atmospheric N sources and nitrate present in irrigation water. Investigations of the crop N recovery in several field experiments showed that the appropriate N loading rate that minimizes N leaching and maximizes N harvest is between 140 to 150% of the N harvested.
Computer models indicated a somewhat larger range of 140% to 165%. While field studies provided important feedback on loss pathways and loss rates as well as mineralization rates, model simulations were well suited to study the dynamic behavior of the soil nitrogen pool and its interaction with the crop N uptake. Simulations are particularly valuable to understand the role of various loss pathways. Field mineralization, volatilization, and denitrification rates for specific field conditions can be obtained from detailed field and laboratory studies using standard model calibration and validation approaches.
The combined evidence from laboratory, field, and modeling studies indicates that precise nutrient management, while plausible in principle, may be problematic when implemented in full-scale production systems, as it requires careful timing of the N applications, close monitoring of the amount of N and water inputs, and best management of crop production. More importantly, the growers must show flexibility to make necessary adjustments on N inputs during the course of a growing season to achieve satisfactory results.
With respect to the potential for groundwater degradation, all of the computations and field observations point to a fundamentally critical issue: Given that practically achievable leaching fractions in border check and furrow systems are 15 to 30%, nitrate leaching is at best in the range of 10% to 15% of the N applied. Based on the above-described NIR of 140 to 165% of N removal at harvest, at annual crop yields that typically remove 400 – 600 lbs N ac-1 yr-1, input requirements will be in the range of 560-990 lbs N ac-1 yr-1. Hence, nitrate-nitrogen leaching losses – under optimal irrigation and nutrient management – will be in the range of 55 to 150 lbs N ac-1 yr-1. Assuming recharge rates in irrigated systems of 1 – 2 acre-feet per acre per year (300 – 600 mm per year), the nitrate concentration in the leachate is in the range of 10 to 55 ppm (mg L-1) NO3-N, which is at or above the regulatory limit for drinking water quality (10 mg L-1) and at or significantly above the average measured leachate value for other California farming systems (15 mg L-1, Rible et al. 1979). The potential for denitrification in the unsaturated zone below the root zone (not considered in this report) and within the Central Valley aquifers therefore becomes a key factor in determining, whether such (optimal) leaching water quality conditions will still cause groundwater degradation or whether denitrification naturally attenuates nitrate levels to non-degrading levels.
Chapter 6 - Phosphorus and Potassium in Manure Applications
6.1 Introduction With respect to land application, the relatively fixed composite chemical composition of manure has critical implications for nutrient management that are unlike those in commercial fertilizer systems. The fact that manure is a composite fertilizer, containing nitrogen (N), phosphorus (P), and potassium (K), means that the N:P:K ratios of the fertilizer application is set by the wastewater composition in the storage lagoon (which, in turn, is controlled by feed rations and water handling within the animal systems of the dairy). The control of these nutrient ratios is therefore outside the control of the nutrient manager. Hence, nutrient management that maximizes the benefit of one nutrient, customarily N, may mean that other constituents may be over-or-under-applied. In this respect, the most concern has been drawn to overloading of P and K caused by optimizing the N inputs. Losses of the beneficial P and K as plant nutrients may be minor issues. These nutrients (P and K) continuously build up in the soil under continued applications. Where surface runoff or tile-drain discharge occurs to nearby surface waters, phosphorus can be of concern, since it leads to the eutrophication of surface water, and some states are setting limits on how much may be applied. Hence the question, whether applications of P and K should be limited. If so, what should the limit be and under what circumstances?
6.2 Overview It has been shown in many locations throughout the United States and elsewhere that when livestock manure is applied at rates needed to meet the crop nitrogen requirement, phosphorus is often applied at higher rates than needed by the crop and may build up in the soil (see for example, Eghball and Power, 1999). In some cases this will lead to transfer of P to surface waters in runoff.
A cow’s diet consists of plant material of various forms and mineral supplements. Rarely would a mineral supplement contain nitrogen, but to supplement with materials that contain phosphorus is common. Therefore, the total amount of phosphorus inputs into the diet in relationship to the nitrogen can be controlled to be in the same proportion or higher than that taken off in plant material. If nitrogen and phosphorus excretion were in a similar proportion, then overall, phosphorus would be excessive for plant removal because 1) non-plant sources of phosphorus are added to the diet and 2) nitrogen is subject to losses primarily due to volatilization while phosphorus is not. Potassium is similarly conserved in relationship to nitrogen. Potassium, unlike phosphorus, however, can be taken up in the crops, especially winter cereals, at rates that far exceed crop requirements and these excessively high concentrations often result in undesirable animal health problems.
Few California studies are available that include measurements of the balance of P and K on a whole-farm basis. The few data sets that exist address only lagoon water and do not consider the additional P that may be present in dry manure and in the solids collecting in the bottom of the lagoon. If these solid materials are applied to land receiving lagoon water, P and K (in addition to N) must be taken into consideration to evaluate the nutrient status.
A data set was compiled as part of a University of California dairy BIFS (Biologically Integrated Farming Systems) study conducted on individual forage fields on nine dairies in the San Joaquin Valley. The data may be used to illustrate the mass balance of P and K in the forage production fields that received the dairy wastewater. (Tables 6-1 and 6-2).
In most dairies, P and K inputs to the forage production system from dairy manure were comparable to the amounts removed in the crop harvest (Dairies 8 and 10 were notable exceptions). As N inputs of the BIFS dairies were significantly higher than the N removal through the harvest of forage biomass, there is significant incentive to lower the N inputs by reducing the amount of manure applied. However, lower manure applications would significantly decrease P and K inputs.
In another study, Campbell Mathews followed nutrient application and removal on the same field near Hilmar, Merced County, for three years. In this study, manure N application was closely matched to crop N uptake (Table 6-3). In this case, the P applied was only slightly less than the P removed, while the potassium applied was generally well above the amount of the potassium removed in the harvest.
Table 6-3: Nutrient application and removal of nitrogen, phosphorus and potassium by corn silage and cereal forage on a dairy near Hilmar, CA.
Neither the BIFS data nor the Hilmar single field data are representative of the large variability in liquid manure composition observed on the respective dairies over the course of a year.