Soil phosphorus (P) cycling in agroecosystems is highly complex, with many chemical, physical, and biological processes affecting the availability of P to plants. Traditionally, P fertilizer recommendations have been made using an insurance‐based approach, which has resulted in the accumulation of P in many intensively managed agricultural soils worldwide and contributed to the widespread water quality issue of eutrophication. To mitigate further environmental degradation and because future P fertilizer supplies are threatened due to finite phosphate rock resources and associated geopolitical and quality issues, there is an immediate need to increase P use efficiency (PUE) in agroecosystems. Through cultivar selection and improved cropping system design, contemporary research suggests that sufficient crop yields could be maintained at reduced soil test P (STP) concentrations. In addition, more efficient P cycling at the field scale can be achieved through agroecosystem management that increases soil organic matter and organic P mineralization and optimizes arbuscular mycorrhizal fungi (AMF) symbioses. This review paper provides a perspective on how agriculture has the potential to utilize plant and microbial traits to improve PUE at the field scale and accordingly, maintain crop yields at lower STP concentrations. It also links with the need to tighten the P cycle at the regional scale, including a discussion of P recovery and recycling technologies, with a particular focus on the use of struvite as a recycled P fertilizer. Guidance on directions for future research is provided. Core Ideas There is an urgent need to increase P use efficiency in agroecosystems. Crop yields could be maintained at lower than recommended soil test P concentrations. Both the quantity and quality of organic matter influence P availability. Further research on ability of organic P to supply P to crops is needed. Struvite has the potential to fill an important niche in P recycling.

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Thiessen Martens, J. R. and Entz, M. H. 2011. Integrating green manure and grazing systems: A review. Can. J. Plant Sci. 91: 811–824. Green manuring, also referred to as cover cropping, is an ancient practice that is gaining popularity, especially in ecologically integrated farming systems. Much green manure research in Canada has focused on legumes, where green manure plant material is incorporated into soil. This review focuses on the role of livestock in utilizing traditional and novel green manure crops adapted to the Canadian prairies. Legume and non-legume green manure plant species are discussed in terms of suitability to grazing management by different livestock species. Integrating grazing livestock into green manure systems affects nutrient cycling and potential nitrogen (N) loss pathways. However, losses may not be substantially different from other production systems, especially when loss mitigation practices are employed. Grazing green manures may also affect soil biological and physical properties. We conclude that grazing green manures may provide economic as well as biological advantages over the traditional approach of soil incorporation. For example, a green manure biomass yield of 5000 kg ha−1 is sufficient to produce 175 kg ha−1 of animal live weight gain, providing a gross revenue of $385 to $770 ha−1 at April 2011 prices, while returning at least 75% of N and other nutrients to the field. Barriers to farmer adoption of grazed green manure systems include a lack of livestock management knowledge and infrastructure.

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There is a need to design intensive cropping systems that can reap multiple benefits from annual forages including animal feed, soil fertility and weed control. Considering pea/oat (Pisum sativumcv. 40–10/Avena sativacv. Legget) as a standard green manure, this study investigated the productivity, weed competitiveness, utilization and nitrogen (N) benefit from grazed and ungrazed green manures to spring wheat (Triticum aestivumcv. Waskada) and fall rye (Secale cerealecv. Hazlet). A set of 3-year experiments was carried out in Carman, Manitoba, Canada in 2009, and was repeated in 2010 and 2011. Green manures were grazed by 2–3 ewes and 2–5 lambs for 24 h (1111–1667 sheep days per ha). Averaged over experiments pea/oat mix, hairy vetch (Vicia villosaL.) and sweetclover (Melilotus officinaliscv. Norgold) above-ground dry matter (DM) production were 5036, 5032 and 4064 kg ha−1, respectively. Lentil (Lens culinariscv. Indianhead), a mixture of seven species and soybean (Glycine maxcv. Prudence) produced the least amount of DM over 3 years; 3589, 3551, 3174 kg ha−1, respectively. Pea/oat and hairy vetch were the most weed-competitive species and, averaged over 3 years, contained less than 15% weed DM. Utilization of green manures by grazing animals varied little among species across years and ranged from 28 to 86% for individual species and years. When combined across experiments grazing increased N availability to the wheat crop. The grazing effect was significant for wheat DM production, N uptake and grain N, but not significant for yield across experiments. Averaged over 3 years, wheat took up 107 kg N ha−1from grazed plots versus 98 kg N ha−1from ungrazed plots. A significant species×management interaction for total (wheat+fall rye) N uptake in 2009 indicated that increasing the proportion of legumes in the green manure increased N benefit from grazing. Fall rye productivity was not affected by grazing. We recommend pea/oat and hairy vetch as two green manure species to enhance the overall system performance to achieve high level of DM production, good weed competition, utilization by sheep and provision of N benefit to the following wheat and fall rye crops.