No abstract

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.

2Keywords:organic phosphorus mineralization, isotopic dilution method, 33 P, available 24 phosphorus, perennial forage, microbial P turnover 25Abstract 26Soil phosphorus (P) tests are designed to indicate plant-available inorganic orthophosphate (P i ), 27 but fail to account for P i that may become available through organic phosphorus (P o ) 28 mineralization. This P source may be especially important in soils with low concentrations of 29 solution and labile P i . We assessed gross P o mineralization and immobilizationusing labeling with 30 33 P in four calcareous Alfisolswithvarying concentrations of Olsen soil test Pthat were collected 31 from forage fields of dairy farmsin Ontario, Canada. Rapid microbial 33 P uptake during 32 incubation was found for the soils with the lowest available P i as indicated by both Olsen soil test 33 P and water-extractable P i . The tracer incorporation into microbial P after 8 days ranged from 7 34 to 44% of applied 33 P and was negatively related to water-extractable P i following a power-type 35 relationship. As concentrations of microbial P were similar in all soils, this suggests faster 36 turnover of P in the microbial biomass at water-extractable P i below 0.1 mg P kg -1 soil. Daily 37 gross P o mineralization rates ranged from 0.2 to 2.8 mg P kg -1 soil d -1 and contributed 7 to 56% 38 of the isotopically-exchangeable P in 8 days. Based on these findings, microbial processes have 39 the potential to make a significant contribution to forage P nutrition. 40 Introduction 41A thorough understanding of processes that govern phosphorus (P) availability in soils is 42 important as too little available orthophosphate (P i ) can limit crop yields, while excess P i can 43 result in off-site environmental degradation (Condron, 2004). P availabilityhas generally been 44 considered to be controlledby physical and chemical processes including sorption-desorption and 45 precipitation-dissolution; however, biologically driven processes including organic P (P o ) 46 mineralizationand microbial P i immobilization can also have a significant effect onP i availability 47 (Frossard et al., 2000; Bünemann, 2015). The ability of the microbial biomass to contribute to 48 3 plant available P depends on the quantity of the microbial P pool and its turnover time (time for a 49 nutrient pool to completely renew itself) (Oberson and Joner, 2005). 50Determination of the contribution of P o mineralization to plant-available P would be useful, 51 especially in systems with low concentrations of available P i .For example, organic agricultural 52 production systems often have low indices of plant-available P (Entz et al., 2001; Gosling and 53 Shepherd, 2005;Knight et al., 2010). At the same time, the ability 54 of commonly used soil tests to accurately predict plant-available P has been questioned because 55 they do not account for biological contributions to plant-available P i (Condron, 2004; Fortune et 56 al., 2005;Roberts et al., 2008; Nash et al., 2014). It can be assumed thatrelative to ph...

Adequate phosphorus (P) is needed for crop production, but excessive P poses a potential risk to water quality. Results from the cumulative P balance calculations within the indicator of risk of water contamination by phosphorus (IROWC‐P) developed in Canada were assessed to determine the spatial and temporal trends in P accumulation at a regional scale and to consider the implications of these trends. Regional cumulative P balances were calculated from census data as a proxy for soil test P (STP) values, including the contribution of fertilizer or manure P to these balances. Ideally, over time we would see a convergence of soil test values at the low end of the critical response range for crop growth, where agronomic and environmental considerations are balanced, but this does not appear to be the case for many regions in Canada. Nationally, about 61% of agricultural land was predicted to be low in STP, and over half of this land is failing to replace the P that is removed each year. While only about 10% of the agricultural land has accumulated significantly more P than is needed for crop growth, almost all of this land is continuing to accumulate P rather than drawing it down. Manure is the dominant P source for continuing accumulation in regions with high or very high estimated STP; reducing this input will be difficult because of the nature of manure and the investment in buildings and infrastructure tied to specific locations, but it is clear that current Canadian policies need strengthened. Core Ideas Neither deficiency nor excess of P in soil is desirable. Imbalanced P distribution across Canada shows significant areas of deficiency and excess. Regions with high P soils continue to be enriched while regions with low P soils are depleted. In most high P regions, continued P buildup is dominated by livestock manure. Options to rebalance P inputs in these regions include dispersing livestock operations.

No abstract