Plant Nutrient Issues for Sustainable Land Application
Gary Pierzynski, Tom Sims, Miguel Cabrera, Steve Hodges, Steve Smith, and Frank Humenik
In the past decade, interest in plant nutrient issues for sustainable land application has been driven by environmental concerns. Many of these environmental concerns have been known for some time while others have only recently come to our attention. The indicators of concern are phosphorus (P) and nitrogen (N) in surface waters, ammonia emissions, nitrate leaching, and emissions of greenhouse gases. There have been significant advancements in basic research in these areas and several pieces of federal legislation in the United States have stimulated a considerable amount of applied research.
The 40 CFR Part 503 biosolids regulations laid the foundation for federal regulations on plant nutrients and land application. The regulations transformed land application programs for biosolids into N management systems by limiting applications to the agronomic rate, defined as anticipated N uptake by the intended crop. Since then, the USEPA has acted to regulate concentrated animal feeding operations with the USDA providing guidance on nutrient management. The USDA Code 590 Practice Standards for nutrient management stipulate that manures be applied on a N basis when the risk of P loss to surface water is low and either not be applied or be applied on a P basis when the risk is high. States are given the choice of using soil test P thresholds or a P site index to determine the risk, or some combination of these approaches. The threshold approach offers simplicity while ignoring the possibility of high soil P levels with a low potential for transport off site. The P site index approach is more complicated but offers more flexibility. A detailed review of best management practices was presented.
Advances have been made in the area of N mineralization, which is an important component in the determination of potentially available N (PAN) in residuals. Modeling of mineralization, particularly that which takes into account climate and soil data, offers the potential for a more accurate determination of PAN. Quick tests, generally involving near or mid-infrared spectroscopy, may make it possible to more accurately determine PAN on residuals immediately prior to land application. Ammonia volatilization is also an important component of PAN but recent interest is because of potential environmental effects from ammonia in the atmosphere. Methods for reducing ammonia emissions from livestock operations are being explored as are new techniques for quantifying emissions under field conditions. The use of constructed wetlands to induce denitrification in liquid animal and municipal wastes holds promise as a treatment option that is an alternative to land application. Injection of liquid wastes reduces ammonia losses but increases energy requirements and emissions of nitrous oxide.
Considerable work has been done by individual states in developing P assessment tools for determining risk of P loss in surface runoff. Calibration and validation of these tools is an ongoing concern. Research is needed on the interpretation of edge of field P losses so that the appropriate risk can be assigned. A related issue is the relative availability of P in different residuals. The availability of P in biosolids can be influenced by the treatment process. Similarly, the availability of P in residuals and soils can be influenced by various amendments. Alum or water treatment residuals containing alum are two materials studied the most. These materials also act to reduce ammonia emissions. The amount and characteristics of P in animal manures can be influenced by the diet. The use of high available P grains or phytase enzyme supplementation can reduce total P in animal manures by reducing the need for diet supplementation with inorganic P. There are subsequent benefits in terms of reductions in runoff P losses when these manures are land applied. Several advances in spectroscopy have allowed for a more complete understanding of the chemical forms of P in residuals and soils. The use of synchrotron-based x-ray absorption spectroscopy has identified chemical forms of inorganic P while the use of modern NMR has allowed more specific identification of organic P species.
2. Future Directions:
The conference afforded numerous opportunities for discussion about sustainable land application in general and about plant nutrients in particular. Papers on plant nutrients were well attended. Each of the discussion sessions following the oral papers were used in their entirety. In addition, two separate small-group discussion sessions were well attended for the entire allotted time. The comments below are the products of those sessions and are divided into a general category for general issues or topics that cover both N and P and into specific sections for points on N and P.
A. General Comments:
1. Residuals should be characterized for all nutrients essential to plants, animals and humans, not just N and P.
2. There are significant technology transfer needs. Training materials for a number of the topics presented at the conference would be welcome (e.g., P assessment tools, N mineralization models, etc.). Specifically for biosolids, a revision of the Process Design Manual is in order. Non-EPA outlets should be considered for some of these products.
3. Some efforts should be made to develop uniform standards for all residuals and mixtures of residuals. Several participants commented on the difficulty in developing land application guidelines for materials or mixtures of materials other than manures or biosolids.
4. Since modeling efforts are progressing, current studies on land application should provide a better characterization of residual properties, soils, and climate so as to be able to support model validation efforts.
5. Considerable efforts should be made to investigate innovative uses and fates for plant nutrients in residuals, and for residuals in general. What realistic options are available besides land application and land filling?
6. Efforts need to continue to improve the public perception of land application. Use of the terms residuals, by-products, or biosolids instead of wastes or sewage sludge was provided as one example.
7. Research needs, regulatory concerns, and best management practices need to be integrative across the other major issues related to sustainable land application such as metals, pathogens, and aesthetics.
8. Other benefits of sustainable land application need to be studied including carbon sequestration, plant disease suppression, and soil quality.
9. Efforts need to continue to make residuals more attractive to end users, especially in the urban environment. Examples provided were increasing the N:P ratio, co-processing with other residuals, and other value-added contributions.
10. The issue of carbon and nutrient pollution credits needs further exploration in the regulatory arena.
11. Work is needed on quantifying the value of best management practices. This includes the reduced environmental impact as well as the economic value.
12. Decision support systems for sustainable land application need to be developed. Consideration should be given to providing incentives for adopting environmentally friendly practices.
13. More work needs to be done at the watershed scale. Two examples provided were for runoff modeling and for animal carrying capacity.
14. A systems approach should be adopted for many changes. Two examples were N and P budgets for entire livestock or wastewater treatment facilities and studies of animal diet manipulation.
15. Economic evaluations of our research and regulatory efforts are needed.
16. The EPA and USDA need data supporting the success of our current nutrient management practices.
17. Work should continue on establishing or illuminating the scientific bases for environmental regulations.
1. Work should continue on environmentally available P in biosolids with some consideration to a national P index for land applied biosolids.
2. Work is needed on P removal strategies from residuals. The precipitation of soluble P as highly insoluble P compounds was mentioned as one strategy. This P could be stored or immobilized to remove this P from the current waste stream and possibly be utilized at a later time.
3. Similar to animal diets, the role of food, home care products, and personal care products on P in biosolids should be investigated.
4. The potential negative impact of P-based applications on land application needs to be considered. Will P-based applications increase the amount of biosolids or other residuals that are placed in landfills, for example.
5. Remediation strategies for P-saturated soils should be investigated.
6. The use of P availability constants in P assessment tools is a continuing research need.
7. Two issues with respect to P losses in surface runoff were raised. The interpretation of edge of field losses needs additional work as does the fate of P from the edge of the field to the point of impact.
8. The long-term efficacy of alum, water treatment residuals, and other amendments for reducing P availability is a research need.
9. Continued use of new and improved spectroscopic techniques to study chemical forms of P in residuals and soils is warranted.
1. Work should continue on quick tests for potentially mineralizable N.
2. Work is needed on the closure of lagoons and other structures used to store residuals. One issue is that ammonium enriched materials may begin to nitrify and become long-term sources of nitrates.
3. More basic information on the mineralization process is needed. This will likely involve the study of the microbial and macrofaunal ecology of the process as affected by temperature, water content, and soil characteristics.
4. Additional work on amendments for reducing ammonia volatilization is warranted.
5. Research is needed on the interactions between various management practices such that the least harm is done to the environment. The issue of injecting liquid manures was used as an example. This process reduces ammonia emissions but increases energy use and nitrous oxide emissions.
6. Additional research is needed to better understand the effect of timing of rain and irrigation on ammonia volatilization losses from surface application of residuals.
7. Work should continue on simulation models to improve our capability to estimate N mineralization and immobilization under field conditions.
8. Further research is needed to identify pools of mineralizable N and their relationship to easily-measurable variables in residuals.
9. Composted residuals can be applied at higher application rates with less concern over nitrate leaching as compared to residuals that have not been through a composting process. The potential benefits of this strategy warrant investigation, particularly for remediation and for general improvement of soil quality.