No author has a financial or proprietary interest in any material or method mentioned.

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.

Findings concerning P removal in buffer zones (BZs), constructed wetlands (CWs), and ponds in Finland, Norway, Sweden, and Denmark are presented in this paper because most such studies have been published only in Nordic languages. Retention of P was tested in 11 BZs, four CWs (less than 0.5‐m deep and vegetated with macrophytes), and seven ponds (deeper than 0.5 m). The grass buffer zone (GBZ) and vegetated buffer zone (VBZ) plots were compared with plots without a BZ; and P retention in CWs, ponds, and some BZs was estimated by subtracting total phosphorus (TP) mass in the outlet from TP mass in the inlet. Buffer zones decreased loads of TP from agricultural runoff water by 27 to 97% (0.24–0.67 kg ha−1 yr−1). The retention as a percentage increased with increasing BZ width. The BZ's upper part was, however, most effective in mitigating TP mass loads (1.6–4.4 g m−2), due to the importance of sedimentation as a retention process. The ponds and CWs reduced TP loads by 17 and 41%, respectively (2–116 g m−2 yr−1). The retention increased with the surface‐area/watershed‐area ratio. CWs were more effective in retaining TP than were ponds, possibly due to shallower depths and dense vegetation. The retention of dissolved reactive phosphorus (DRP) was inconsistent, both in BZs and in CWs. Vegetation should be harvested in BZs to decrease the DRP losses. Harvesting of vegetation is not recommended in CWs.

Phosphorus (P)-rich by-products, such as meat and bone meal (MBM) and fur animal manures, are potential P sources in plant production systems. However, the solubility of P and its availability to plants in these forms has not been evaluated. We characterized P solubility in MBM, fox manures (FoxM) and dairy manure (DairyM) by Hedley fractionation and assessed P availability for ryegrass in a pot experiment. Up to 81% of P was water-soluble in DairyM, but only about 3 and 5-28% was soluble in MBM and FoxM products, respectively. Of the P in MBM and FoxM, 90 and 65-89%, respectively, was soluble only in 1 M HCl. Most of the P was inorganic; DairyM contained the highest share (14%) of organic P. Based on ryegrass yields and P uptake in a 3-year pot experiment with three P levels (25, 50 and 100 mg kg -1 ), P availability was equal in the DairyM and superphosphate (SP) treatments. Compared with the availability of P in DairyM and SP, 19 and 35-54% of the P in MBM and FoxM, respectively, was immediately available to the plant; for the 3-year period with ten ryegrass cuts, the respective P availabilities increased to 63 and 69-87%. Additions of the sparingly soluble P sources MBM and FoxM increased the acid-soluble P concentrations in the experimental soil, with MBM having the strongest effect. However, the acid-soluble P fraction decreased with time. Although the immediate bioavailability of P in sparingly soluble P sources was lower than that in DairyM and SP, our results suggest that their use as a long-term P supply for perennial plants in noncalcareous soils should be encouraged.

Recent work has shown that a significant portion of the total loss of phosphorus (P) from agricultural soils may occur via subsurface drainflow. The aim of this study was to compare the concentrations of different P forms in surface and subsurface runoff, and to assess the potential algal availability of particulate phosphorus (PP) in runoff waters. The material consisted of 91 water-sample pairs (surface runoff vs. subsurface drainage waters) from two artificially drained clayey soils (a Typic Cryaquept and an Aeric Cryaquept) and was analyzed for total suspended solids (TSS), total phosphorus (TP), dissolved molybdate-reactive phosphorus (DRP), and anion exchange resin-extractable phosphorus (AER-P). On the basis of these determinations, we calculated the concentrations of PP, desorbable particulate phosphorus (PPi), and particulate unavailable (nondesorbable) phosphorus (PUP). Some water samples and the soils were also analyzed for 137Cs activity and particle-size distribution. The major P fraction in the waters studied was PP and, on average, only 7% of it was desorbable by AER. However, a mean of 47% of potentially bioavailable P (AER-P) consisted of PPi. The suspended soil material carried by drainflow contained as much PPi (47-79 mg kg-1) as did the surface runoff sediment (45-82 mg kg-1). The runoff sediments were enriched in clay-sized particles and 137Cs by a factor of about two relative to the surface soils. Our results show that desorbable PP derived from topsoil may be as important a contributor to potentially algal-available P as DRP in both surface and subsurface runoff from clayey soils.