SUMMARY AND RESEARCH NEEDS
Nitrogen mineralization and nitrate leaching remain important today for land application programs. In addition, new regulations and existing and emerging environmental issues warrant additional attention and research. Nutrient management legislation, eutrophication from both N and P, and NH3 and greenhouse gas emissions are relevant examples. Advances in research methods will allow us to further our knowledge in a number of areas and will increasingly influence our research approaches and productivity.
Research needs are either general in nature and apply to all plant nutrient issues, or are specific for N or P. A framework for determining plant nutrient issues that can be used for all land-applied materials is needed. Formal guidelines exist for manures and biosolids, but not for other materials or for mixtures of materials. This forces managers to conduct a complete evaluation for new materials or mixtures when this may not be necessary. Similarly, modeling exercises should supplement field or laboratory evaluations and investigators are encouraged to perform extensive characterizations of materials and conditions to support model calibration and validation. Best management practices for nutrients need to be integrative across other issues related to sustainable land application such as metals, pathogens, and aesthetics. The effects of BMPs at the watershed scale warrant additional investigation. Ancillary benefits from land application, such as C sequestration, plant disease suppression, and soil quality benefits, need to be further explored. Overall, a systems approach should be adopted in more studies. Relevant examples include the effect of animal diet modifications ranging from animal productivity to water quality benefits, or the influence of injection of manures and biosolids for decreasing NH3 emissions while increasing N2O emissions.
Quick tests for PMN offer the promise of improving our estimates of PAN, and possibly reducing environmental issues associated with N from land application of residuals, and warrant further investigation. It is possible, however, that variability in residuals and issues with precision and accuracy during application would overshadow small improvements in PAN estimates, and this should be considered as well. Basic research is needed on the mineralization process. This will likely involve the study of microbial and macrofaunal ecology of the process as affected by temperature, water content, and soil characteristics. Similarly, further research is needed to identify pools of mineralizable N and their relationship to easily measurable parameters in residuals. Interest in NH3 as an atmospheric pollutant will likely increase, and studies on amendments for reducing NH3 emissions is needed along with field-based techniques for estimating NH3 losses. Feedlots and waste lagoon remediation after facility closure will become an important topic. These sites have high NH4–N concentrations and represent significant and long-term sources of NO3–N for ground water when nitrification occurs.
Numerous research needs are evident for P. Work should continue on environmentally available P in residuals with some consideration of a national P index. In a related issue, much needs to be learned about interpretation of edge of field P losses and the fate of P between the edge of the field and the point of impact. Strategies for removing or reducing the solubility of P in residuals warrant further investigation. Regulations that prohibit residuals application, or restrict them to less than N-based applications, may discourage land application in the long term and encourage disposal strategies that are not based on beneficial reuse. In addition, the long-term efficacy of alum and other amendments used to reduce P bioavailability in soils and residuals is unknown. Finally, the continued use of new and improved spectroscopic techniques will move us toward a more mechanistic understanding of environmental P chemistry.
Contribution no. 04-308-J of the Kansas Agricultural Experiment Station, Manhattan.