Recent U.S. federal legislation has addressed various aspects of nutrient management, including land application of by-products. These include the promulgation of 40 CFR Part 503 regulations for land-applied biosolids (USEPA, 1993), the USDA-NRCS Conservation Practice Standards for Nutrient Management Code 590 (USDA, 1999), the USEPA concentrated animal feeding operation (CAFO) regulations (USEPA, 2003), and the USDA National Organic Standards (USDA, 2000). The influence of these regulations on scientific literature has been considerable as they have stimulated a great deal of research.
The plant nutrient aspects of the Part 503 regulations were based on concerns regarding nitrate leaching, and required estimates of PAN, including potentially mineralizable nitrogen (PMN), to determine appropriate application rates. The 503 regulations raised the issue of N need versus N removal as a basis for determining application rates. For example, biosolids could be applied to legume crops, whose relatively high N removal rates permit greater application rates than those made to some nonleguminous crops. Because the 503 regulations did not address P, the greater application rates were viewed positively in the overall efforts of a land application program to manage N.
At the national level, the Code 590 standards provide guidelines for nutrient management to be developed further and followed by each state. The Code 590 standards require that animal wastes be applied on a N basis if risk of P loss is low and on a P basis if P loss risk is high. The Code 590 standards offer three possible means for identifying areas with high risk of P loss. There are two threshold approaches, one based on environmental criteria and the other on agronomic criteria, and a P index rating. Combinations of these approaches are permitted. Soil test P levels are used as criteria to limit or prohibit P applications when the P threshold is reached. Agronomic thresholds, which are based on potential crop response to P applications, are typically the most restrictive approach. Environmental thresholds are designed to prevent unacceptable P losses via surface or subsurface flow, and are based on the relationship between soil test phosphorus (STP) and runoff or tile drainage composition.
Adoption of a P index rating system by a state agency necessitates development based on local climatic conditions, soils, and management practices. The complexity of systems varies widely among states (Sharpley et al., 2003). Factors typically used in P index systems include STP, P additions from inorganic and organic sources, method of application for P additions, soil erosion, irrigation erosion, and proximity to bodies of water. These source and transport factors interact to influence the potential for P impairment of surface waters. Many P index systems are constructed with a great deal of professional judgment as to how the site characteristics will collectively influence P loss and, therefore, require validation and/or calibration. However, relatively few states have correlated measured versus predicted P losses in some fashion.
Threshold approaches have the advantages of being simple and related to water quality or agronomic P needs. Disadvantages include the lack of consideration of P transport, the implication that widespread distribution of P is better than limited distribution, and lack of inclusion of best management practices. The P index systems provide a more comprehensive view of P fate and transport, which implies the necessity of best management practices. Use of a P index system introduces flexibility to allow greater STP levels when P transport potential is low. This can be a critical factor in areas where conservative thresholds may severely restrict land application of by-products. One significant disadvantage to the P index system is the requirement of complex inputs and the need of a trained professional to complete the assessment.
The CAFO regulations, officially published in the Federal Register early in 2003, are in the early stages of implementation. Animal feeding operations are defined as systems that confine animals for 45 d or more per year where little or no vegetation exists. An animal feeding operation can be designated a CAFO if the number of animals exceeds certain limits or if the operation is deemed to have significant negative effects on the environment. A permit that includes a nutrient management plan must be obtained to begin or continue a CAFO. Each state will determine how it handles the nutrient management plan requirements. Most states will likely simply adopt the Code 590 standards to satisfy this need.
The USDA national organic standards stipulate that foods that are labeled "organic" must be produced without the use of synthetic substances, with some exceptions, and without certain nonsynthetic substances. The standards also stipulate that application of plant or animal materials must be managed ... in a manner that does not contribute to the contamination of crops, soil, or water by plant nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances. Raw or composted animal manures are allowed, whereas biosolids are prohibited. Harvest restrictions are required following the use of raw manures. One of the key challenges to organic production is providing adequate N while minimizing the accumulation of P and heavy metals in soils (Mikkelsen, 2000).