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The series of papers in this issue of AMBIO represent technical presentations made at the 7th International Phosphorus Workshop (IPW7), held in September, 2013 in Uppsala, Sweden. At that meeting, the 150 delegates were involved in round table discussions on major, predetermined themes facing the management of agricultural phosphorus (P) for optimum production goals with minimal water quality impairment. The six themes were (1) P management in a changing world; (2) transport pathways of P from soil to water; (3) monitoring, modeling, and communication; (4) importance of manure and agricultural production systems for P management; (5) identification of appropriate mitigation measures for reduction of P loss; and (6) implementation of mitigation strategies to reduce P loss. This paper details the major challenges and research needs that were identified for each theme and identifies a future roadmap for catchment management that cost-effectively minimizes P loss from agricultural activities.

In the north-western European countries Norway, Sweden, United Kingdom (UK) and Ireland, variability in the forms, amounts and timing of phosphorus (P) loss from agricultural land is related to national differences in climate, soil, hydrological conditions and agricultural production. The dissolved form of P constitutes 9-93% of the total phosphorus (TP) in water, subsurface drainage can contribute 12-60% and surface erosion 40-88% of TP transfer. TP export in small agricultural streams is generally in the range 0.3-6 kg ha )1 year )1 , with the highest losses in Norway and UK. All four countries are complying with the EU Water Framework Directive and developing a range of measures based on P source with transport controls over P losses. A decreasing trend in TP losses has been detected in agricultural streams following the introduction of measures to reduce erosion in Norway. Average P concentrations in Swedish streams have shown a reduction of nearly 2% per year since 1993 as a result of measures introduced in southern Sweden. However, in two large rivers in agricultural regions of Sweden, the concentrations of suspended solids (SS) and TP were shown to increase by 0.4% and 0.7% per year, respectively, over the period 1975-2004, possibly as a result of climate change. It is too early to detect trends in agricultural contributions to P in surface waters as a result of catchment-sensitive farming (CSF) in the UK and Ireland.

This review summarizes current knowledge from the literature and experimental studies on the role of cover crops (CCs) in reducing nitrogen (N) leaching and phosphorus (P) losses to waters under the marine and humid continental climate conditions of southern Scandinavia and Finland. Field leaching studies from 11 sites indicate that undersown ryegrass (Lolium spp.) CCs are robust, with average N uptake in aboveground CC biomass of 7 to 38 kg N ha -1 (6.2 to 34 lb N ac -1). Use of CCs sown at harvest (e.g., crucifers) is restricted to southern Scandinavia for climate reasons. The mean relative reduction in N leaching reported for all CCs investigated was 43%, but it ranged between 62% increase instead of a reduction after a red clover (Trifolium pratense) CC on a clay soil to a reduction of 85% to 89% with a perennial ryegrass CC on sandy soils in Denmark (36 to 51 kg ha -1 [32 to 46 lb ac]). The data indicate that CCs do not substantially reduce total P losses by runoff and leaching. The effects of CCs on total P leaching varied between a relative increase of 86% and a decrease of 43%. Climate conditions involving freezing-thawing during winter increased the risk of losses of dissolved P from CC biomass. CCs have been implemented to varying degrees into agri-environmental programs. They are mandatory in Denmark and subsidized in Norway, Sweden, and Finland. CCs are grown on 8% of arable land in Denmark, 5% in Sweden, 1% in Finland, and 0.5% in Norway, but CC area is now increasing dramatically in Finland due to a new subsidy program. In all countries there is a need, and potential, for increased use of CCs, but there are several constraints, particularly reduced interest among farmers. There is a clear need to identify CC systems and develop implementation strategies for appropriate distribution of CCs on different soils and regions with respect to required reductions in N leaching and P losses. For this, more knowledge is required, especially about the effect of CCs on P losses (e.g., the effect of species with different partitioning between shoot and root biomass and the effects of CC systems with harvesting of biomass). There is also a need to devise balanced solutions for maintaining and increasing the frequency of CCs in crop rotations to exploit the possible benefits of CCs in reducing nutrient losses.Key words: cover crop species-nitrogen leaching-perennial ryegrass-phosphorus leaching-phosphorus surface runoff Helena Aronsson is a senior lecturer with extension duties and Barbro Ulén is a professor,

Fresh applications of phosphorus (P) may cause incidental' losses of dissolved and particulate P forms in land runoff when rainfall interacts directly with fertilizers and manures which are spread, or excreted, onto the soil surface. Research indicates that when incidental P losses (IPL) occur, they often make the dominant (50±98 %) contribution to measured P loads in surface and sub-surface runoff from field plots, with increased risk of eutrophication where P-enriched runoff is routed directly to the watercourse. Rates of P loss are temporally and spatially very variable (< 1 to 25 % of total P applied) depending on the amount of P applied; the P release properties of the materials applied (% P extractable in water), the timing of storm events after application and the amounts of runoff generated. Large P applications left on the surface of wet, frozen, compacted, and intensively underdrained soils in high rainfall areas are particularly vulnerable to IPL. Concentrations of P in runoff are often greatest during the first storm event following P application, but can remain high for several weeks, or even months after application. Catchment scale impacts are more difficult to quantify due to the effects of dilution with runoff from fields without IPL, and the contributions of P from other sources. Approaches to predicting IPL are discussed and need to be improved in order to assess the contribution of IPL to catchment P loads in river basin management planning. Strategies to control IPL should focus on adoption of more sensitive management practices in fields with a high risk of rapid runoff or preferential flow, and are likely to very effective.Ereignisbezogene Phosphorverluste ± Sind sie signifikant und können sie verhergesagt werden?Eine gerade erfolgte Ausbringung von Phosphor (P) kann Ursache für ereignisbezogene ªepisodischeº Austräge an gelöstem und partikulärem P mit dem Abfluss sein, wenn Niederschlag direkt auf mineralischen oder organischen Dünger an der Bodenoberfläche trifft. Untersuchungen zeigten, dass diese ereignisbezogenen P-Austräge bei Auftreten oft den Hauptbeitrag (50±98 %) zu gemessenen P-Frachten in oberflächlichem und unterirdischem Austrag aus Parzellen ausmachen. Sie bedeuten ein erhöhtes Eutrophierungsrisiko, wenn die P-angereicherten Austräge direkt in Gewässer gelangen. Die Raten der P-Austräge sind zeitlich und räumlich sehr variabel (< 1 bis 25 % des insgesamt zugeführten P) und hängen ab von der Höhe der P-Zufuhr, den P-Freisetzungseigenschaften des aufgebrachten Materials (% wasserextrahierbarer P), dem Zeitpunkt des Niederschlagsereignisses nach P-Applikation und der Abflussmenge. Groûe applizierte P-Mengen, die auf der Oberfläche nasser, gefrorener, verdichteter und intensiv drainierter Böden in niederschlagsreichen Regionen verbleiben, sind besonders von ereignisbezogenen P-Austrägen betroffen. Die Konzentrationen an P im Abfluss sind oft am grössten während des ersten Starkniederschlagsereignisses nach der P-Applikation; sie können jedoch über mehrere Wochen oder sogar Monate...