The low water solubility of struvite is thought to limit its agronomic utility as a phosphorus (P) fertilizer compared with highly soluble P fertilizers. Furthermore, struvite's fertilizer potential is complicated by its hypothesized soil pH‐dependent solubility, crop‐specific interactions, and limited availability of struvite‐derived N, which may explain conflicting reports of crop responses to struvite compared with conventional P fertilizers. A systematic literature review and meta‐analysis was conducted to evaluate the effects of soil pH, soil test P (STP), P rate, struvite particle size, and struvite‐derived N on crop aboveground biomass, P concentration, P uptake, and N uptake. Struvite‐fertilized plants yielded higher biomass, P concentration, and P uptake compared with ammonium phosphates, and superphosphates in soils with pH < 6 and crop responses decreased with increasing pH. Crop responses to struvite were inversely related to experiment duration to soil mass ratios (d kg−1) used in greenhouse studies, opposite to the hypothesized benefit of more roots per unit soil on struvite dissolution. The proportion of total N applied derived from struvite increased with increasing struvite‐P application rate and was inversely related to total N uptake, likely due to the increased crop reliance on slowly available struvite‐N. Crop responses were potentially overestimated by high STP and/or P rates and underestimated due to N limitation from large proportions of total N applied derived from struvite. Evaluations of struvite collectively indicate its efficacy as a P fertilizer is affected by soil pH and its contribution to total N application.
Redirecting anthropogenic waste phosphorus (P) flows from receiving water bodies to high P demand agricultural fields requires a resource management approach that integrates biogeochemistry, agronomy, engineering, and economics. In the US Midwest, agricultural reuse of P recovered from spatially colocated waste streams stands to reduce point‐source P discharges, meet agricultural P needs, and—depending on the speciation of recovered P—mitigate P losses from agriculture. However, the speciation of P recovered from waste streams via its chemical transformation—referred to here as recovered P (rP) differs markedly based on waste stream composition and recovery method, which can further interact with soil and crop characteristics of agricultural sinks. The solubility of rP presents key tensions between engineered P recovery and agronomic reuse because it defines both the ability to remove organic and inorganic P from aqueous streams and the crop availability of rP. The potential of rP generation and composition differs greatly among animal, municipal, and grain milling waste streams due to the aqueous speciation of P and presence of coprecipitants. Two example rP forms, phytin and struvite, engage in distinct biogeochemical processes on addition to soils that ultimately influence crop uptake and potential losses of rP. These processes also influence the fate of nitrogen (N) embodied in rP. The economics of rP generation and reuse will determine if and which rP are produced. Matching rP species to appropriate agricultural systems is critical to develop sustainable and financially viable regional exchanges of rP from wastewater treatment to agricultural end users.
Core Ideas
There is high potential for recovering P (rP) from point sources for agricultural reuse.
rP speciation depends on recovery source and method, interacts with soils and crops.
Engineering, agronomic, and economic considerations of rP are context‐specific.
Fall application of anhydrous ammonia (NH3) is a common practice for corn (Zea mays L.) production in the midwestern United States, but evaluations to date have relied entirely on yield comparisons that provide no means of distinguishing fertilizer from soil N uptake. To quantify fertilizer N uptake efficiency (FNUE) when using this practice, field trials using 15NH3 were conducted between 2016 and 2018 at four sites in a corn–soybean (Glycine max L. Merr.) rotation and at two sites under continuous corn. At each site, 224 kg N ha−1 was applied with and without the use of nitrapyrin (NP) to inhibit nitrification. Relative to grain yields without fertilizer N, response to fall N fertilization occurred at four of the six sites studied, averaging 53% (4.0 Mg ha−1) in two growing seasons with below‐normal rainfall. The use of NP was beneficial at only one site for increasing total N uptake, but resulted in a decrease at another, along with a significant reduction in grain yield. Isotopic estimates of FNUE (i.e., F15NUE) ranged from 12 to 34% (21% on average) for grain and from 16 to 42% (28% on average) for total aboveground biomass, and N derived from fertilizer ranged from 20 to 46% (32% on average). Both isotopic parameters were highest for the site lowest in native N availability, demonstrating the potential of site‐specific N management to improve fall NH3 fertilization by accounting for soil N mineralization.
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