Rory O. Maguire(1), Gitte H. Rubæk(2), Bob. H. Foy(3), and Brian Haggard(4) (1) Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061, (2) Department of Agroecology and Environment, Faculty of Agricultural Sciences, University of Aarhus, Blichers Allé 20, P.O. Box 50, DK-8830 Tjele, Denmark, (3) Agri-Food and Biosciences Institute, Newforge Lane, Belfast BT9 5PX, (4) Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, AR 72701
rmaguire@vt.edu Introduction
Since recognizing P as a major pollutant of surface waters, much research has been done on a wide variety of mitigation options. Using this research as a basis, many regions of the world have now started to implement strategies at the field and catchment scales to minimize P losses. However, approaches used to address P losses vary greatly by country and catchment, and in the US by state. Many of the regulations target animal producers due to manure management issues. Water quality is regulated in the US under the Clean Water Act, and much of the
implementation of strategies to improve water quality has been the result of lawsuits citing failure to meet Clean Water Act standards. In Europe, the EU Water
Framework Directive now sets criteria for “good water quality” in rivers, lakes and coastal waters. Plans to fulfill these criteria are presently being prepared in EU countries. Exactly how effective the mitigation options will be remains to be seen, as changes in practices take many years to yield results in terms of water quality improvements.
Examples of mitigation options that have been implemented a) Denmark
Agricultural P loss has become the major contributor to the eutrophication of many Danish lakes and estuaries, as the loss of P from point sources has been
considerably reduced due to the environmental regulations initiated in the 1980’s (Poulsen og Rubæk, 2005). At that time little was known about the agricultural contribution of P to surface waters and the regulations addressing agriculture therefore focused on e.g. nitrogen and handling of manure. However, knowing what we know today, many of these regulations of agriculture also had significant impacts on P. Law-enforced regulations like restrictions on animal density on agricultural land, norms for how much N (in manure and fertilizer) can be given to each crop, rules for calculation of available N in manures and mandatory nutrient budgets for control improved the distribution and utilization of N as well as P in manures and fertilizers. Even though a surplus of P still can be added in areas with the highest animal densities, an upper limit of the P surplus was indirectly introduced with these
regulations. Furthermore, the demand for 9 months storage capacity for animal manure made it possible to shift from autumn application towards spring application.
This resulted in further improvements of nutrient utilization and most probably also a reduction in the manure-P losses through surface runoff and runoff though macro-pores to tile drains in the wet winter season. Manure application methods introduced to reduce ammonia volatilization (application with trail hoses and direct injection into the soil and demands that manure applied on bare soil has to be incorporated within 6 hours after application) probably decreased these losses even further.
A combination of increased awareness of the potential environmental risks and the fear of future regulation inspired pork producers to begin implementing new feeding strategies, with less mineral P supply, even before this was encouraged though a new tax on feed phosphates implemented in 2004. The P excretion from pigs for slaughtering declined from 1.05 kg P per animal in 1985 to 0.72 kg P per animal in 2000, due to new feeding recommendations and phasing out feed phosphates with low P availability. It declined further to 0.62 kg P/animal in 2002 due to the
substitution of inorganic feed phosphates with phytase to increase phythate digestibility (Poulsen and Rubæk, 2005).
All in all, 20 years of environmental regulation of Danish agriculture have resulted in a decline in the national P surplus from 23 kg P ha-1 in 1985 to 13 kg ha-1 in 2002, and an increase in the utilization P from 32% in 1985 to 52 % in 2002. In spite of this, the agricultural contribution of P to surface waters has remained constant (~0.5 kg P ha-1). Reasons for this might be: (1) the considerable delays in many of the expected effects, (2) it is a too short time period to track temporal trends in agricultural
contribution to the P load in surface waters or (3) that the negative effect of the P surplus at the moment overwhelms the positive effect of improved nutrient
management. New initiatives are therefore looked for to reduce P loss from Danish agriculture. The most recent and the probable future initiatives include: (1)
introduction of 10 m buffer zones along streams and lakes, (2) research and
development regarding low-P feeding strategies and regarding targeted mitigation in areas having high risk for P loss (3) improved management of streams and
extensification of the cultivation in areas neighboring surface water, (4) processing of animal manures, such as slurry separation, utilization for energy production in biogas or combustion plants.
b) Chesapeake Bay
The Chesapeake Bay is a huge estuary on the eastern sea board of the US, with a watershed of 64,000 square miles (166,000 km2) stretching across 6 states (New York, Pennsylvania, Maryland, Delaware, Virginia and West Virginia). Due to concerns about rapidly decreasing water quality in the Chesapeake Bay, the Chesapeake Bay Commission was formed and set nutrient and sediment reduction
targets to the Chesapeake Bay to meet Clean Water Act standards. The Commission recently published a report which outlined six cost effective strategies to reduce nutrient inputs to the Bay, four of which related to agricultural practices aimed at P reductions (CBC, 2004):
1) Diet and feed adjustments to decrease dietary and hence manure P. This has been mandated for poultry in Maryland, which drove implementation both there and in neighboring states due to the integrated nature of the poultry industry. Data available in Delaware shows that the P concentration in poultry litter has decreased 30-40% as a result of improved diets. Full implementation could prevent 100 Mg P yr-1 entering the Bay with no anticipated costs.
2) Nutrient management plans (NMPs), which represent the most common approach to control P in the watershed. The NMPs prescribe the rate and timing for fertilizer and manure applications to eradicate excess applications.
Full implementation of NMPs could prevent 363 Mg P yr-1 from entering the Bay at a cost of $62 per kg.
3) Enhanced nutrient management, where farmers apply 15% less nutrients than recommended for full yield and receive compensation for yield reductions. Full implementation could prevent 363 Mg P yr-1 from entering the Bay at a cost of
$210 per kg.
4) Conservation tillage that minimizes soil disturbance and maintains cover crops and crop residues to minimize soil erosion. This was estimated to be the most attractive option, with the potential to prevent 1,177 Mg P yr-1 entering the Bay at no additional cost.
Implementation of these strategies has varied by state, but between 1985 to 2002 it is estimated that P loads to the Bay decreased from 12,318 Mg P yr-1 to 8,863 Mg P yr-1, with further reductions (to 5,818 Mg P yr-1) needed to meet Clean Water Act standards. Additional efforts are underway to implement other agricultural best management practices, with much regional variability. These include fencing animals out of streams to avoid direct deposition of manure and stream bank erosion, dietary P reductions in non-poultry such as swine and dairy, and alternate uses of manures such as burning for energy production.
References
CBC (Chesapeake Bay Commission), 2004. Cost effective strategies for the bay: 6 smart investments for nutrient and sediment reductions.
www.chesbay.state.va.us/Cost_Reports.htm
Poulsen, H.D. & Rubæk, G.H., 2005. Fosfor i dansk landbrug. Omsætning, tab og virkemidler mod tab. DJF rapport - Husdyrbrug 68, 163-182.
http://web.agrsci.dk/djfpublikation/djfpdf/djfhu68.pdf