Background Phosphorus (P) fertilizer is usually applied in excess of plant requirement and accumulates in soils due to its strong adsorption, rapid precipitation and immobilisation into unavailable forms including organic moieties. As soils are complex and diverse chemical, biochemical and biological systems, strategies to access recalcitrant soil P are often inefficient, case specific and inconsistently applicable in different soils. Finding a near-universal or at least widely applicable solution to the inefficiency in agricultural P use by plants is an important unsolved problem that has been under investigation for more than half a century. Scope In this paper we critically review the strategies proposed for the remobilization of recalcitrant soil phosphorus for crops and pastures worldwide. We have additionally performed a meta-analysis of available soil 31 P-NMR data to establish the potential agronomic value of different stored P forms in agricultural soils. Conclusions Soil inorganic P stocks accounted on average for 1006 ± 115 kg ha −1 (57 ± 7%), while the monoester P pool accounted for 587 ± 32 kg ha −1 (33 ± 2%), indicating the huge potential for the future agronomic use of the soil legacy P. New impact driven research is needed in order to create solutions for the sustainable management of soil P stocks.

Background: The dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (Po) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction. Scope: We asked a group of experts to consider the global issues associated with Po in the terrestrial environment, methodological strengths and weaknesses, benefits to be gained from understanding the Po cycle, and to set priorities for Po research. Conclusions: We identified seven key opportunities for Po research including: the need for integrated, quality controlled and functionally based methodologies; assessment of stoichiometry with other elements in organic matter; understanding the dynamics of Po in natural and managed systems; the role of microorganisms in controlling Po cycles; the implications of nanoparticles in the environment and the need for better modelling and communication of the research. Each priority is discussed and a statement of intent for the Po research community is made that highlights there are key contributions to be made toward understanding biogeochemical cycles, dynamics and function of natural ecosystems and the management of agricultural systems

Crop rotations with species able to solubilize soil P can result in improved P availability for subsequent crops. Ruzigrass [Urochloa ruziziensis (R. Germ. and C.M. Evrard)] has been shown to increase soil available P, but there are few studies on its effect on soil P forms and how growing ruzigrass interacts with P fertilization, which is important in defining fertilization strategies. This study aimed to investigate soil P fractions resulting from cropping ruzigrass in rotation with soybean [Glycine max (L.) Merr.] fertilized with broadcast triple superphosphate (TSP) or reactive rock phosphate (RRP) and TSP applied to soybean furrows under no‐till. Soil samples were taken after ruzigrass desiccation and before soybean planting from long‐term experimental plots in Botucatu, in southeastern Brazil. Phosphate management included RRP and TSP broadcast on the soil surface, and TSP applied to soybean furrows, in the presence or absence of ruzigrass. Broadcast TSP increased labile (anion exchange resin‐extractable P) and moderately labile P fractions extracted with 0.1 M NaOH, whereas RRP increased moderately labile P extracted with HCl. There was no interaction between phosphate management configurations. Regardless of phosphate management, the presence of ruzigrass resulted in a lower residual‐P concentration in deeper soil layers and higher concentrations of labile and moderately labile P in the uppermost soil layers. Ruzigrass introduction into the cropping system resulted in higher soil labile P content compared with fallow, probably because it can take up moderately labile soil P fractions that are recycled into the system, regardless of the P fertilization strategy.Core Ideas Ruzigrass grown in soybean off‐season affects soil fertility. Soil P cycling by ruzigrass may result in benefits to next crops. Agronomic efficiency of phosphate fertilizers may be improved by ruzigrass.

Growing ruzigrass (Urochloa ruziziensis) in crop rotation systems has been suggested as a strategy to increase soil phosphorus (P) cycling and P availability. However, despite increased P lability shown in routine soil analysis, decreased grain yields of crops grown after ruzigrass have been observed. The objective of this study was to evaluate soil P availability to maize (Zea mays) in low or high-P soil cropped to ruzigrass. Soil P lability was evaluated using Hedley fractionation and pearl resin extractions, and P desorption/adsorption was assessed by isothermal titration calorimetry (ITC). Phosphorus changes in soil-P fractions in the maize rhizosphere were studied in a greenhouse experiment. Growing ruzigrass resulted in higher resin-extractable P and soil organic matter (SOM) contents than fallow. However, in soil cropped with ruzigrass, maize P uptake and P desorption were lower, and P adsorption to soil was higher than soil under fallow. In general, organic P bound to Fe and Al was non-available. Phosphorus sorption as assessed with ITC was a better indicator of P bioavailability to maize than pearl resin and Hedley fractionation, and suggested that P was less bioavailable after ruzigrass due to increased SOM, which resulted in the formation of metal phytate and more effective organo-metal sites for ligand exchange. Greater P solubility and availability in fallowed soil appeared to be partly due to the dissolution of Carelated P, greater P desorption, and less potential for P adsorption. Isothermal titration calorimetry is a useful semi-quantitative tool for understanding P sorption behavior.

It has been suggested that some tropical grasses can acquire phosphorus (P) from hematite and gypsite by exuding organic acid anions (OAs). However, it remains to be determined exactly which OAs could be involved in each case. The objective of this study was to verify the exudation OAs by ruzigrass (Urochloa ruziziensis), palisade grass (U. brizantha), and Guinea grass (Megathyrsus maximus) as a response to P deficiency. The grasses were grown in leachate columns with adequate and deficient P nutrient solutions. The concentration of OAs in the leacheate and root surface, as well as shoot and root dry matter, and P uptake were determined. Citrate, isocitrate, and malate concentration in leachates and root surfaces increased with P starvation, mainly for the Urochloa grasses. Oxalate exudation was similar for the grasses under adequate P supply, but was lower in Guinea grass under P starvation. Palisade grass showed a higher concentration of total OAs in the root surface than the other species due to a great production of oxalate and isocitrate. Palisade grass showed greater dry matter yields regardless of P deficiency, and Guinea grass always had the higher shoot:root ratio. Urochloa grasses have a higher capacity to cope with low P availability by exuding OAs along with a lower shoot:root ratio than Guinea grass.

Core Ideas There have been suggestions that ruzigrass increases soil P availability.Ruzigrass was grown in rotation with soybean from 2012 to 2016.The observed effect was opposite from the expected under long‐term field conditions.Crop rotation with ruzigrass resulted in a lower soybean grain yield than fallow. Under no‐till farming systems, the use of crop rotations with species adapted to low P soils may enhance soil P availability through P cycling. Growing ruzigrass [Urochloa ruziziensis (R. Germ. and C.M. Evrard) Morrone and Zuloaga] as a cover crop has shown to increase resin extractable P in soils. However, it is not clear how the next crop responds to ruzigrass in the long term. The objective of this study was to evaluate the long‐term effect of growing ruzigrass on soil P availability to soybean [Glycine max (L.) Merr.]. The evaluations were performed over 5 yr on a ruzigrass–soybean crop rotation, in Botucatu, Brazil. The treatments were P rates (0, 13, and 26 kg ha−) applied to soybean seed furrows, and ruzigrass or fallow during the off‐season. Soil samples were taken after ruzigrass desiccation, and soil P was extracted with resin (Presin). The use of ruzigrass increased soil organic matter (SOM) by approximately 20% compared with fallow, regardless of P rates, and increased Presin concentration in the 0‐ to 10‐cm soil depth by approximately 10% with 26 kg ha− of P. Surprisingly, grain yield and soybean leaf P concentration were lower after ruzigrass compared with fallow. Resin seemed to be unsuitable to compare P availability in different cropping systems. In the long‐term, growing ruzigrass as a cover crop in the off‐season decreases P and N availability to soybean, eventually decreasing soybean grain yield. Further studies are needed to understand the mechanisms involved in this unexpected soybean response when cropped in rotation with ruzigrass.

Part of the nitrogen (N) fertilizer applied to crops is lost to the environment, contributing to global warming, eutrophication, and groundwater contamination. However, low N supply stimulates soil organic N turnover and carbon (C) loss, since the soil N/C ratio in soil is quasiconstant, ultimately resulting in land degradation. Grasses such as ruzigrass (Urochloa ruziziensis) grown as winter pasture or a cover crop in rotation with maize (Zea mays) can reduce N leaching, however, this may induce N deficiency and depress yields in the subsequent maize crop. Despite the potential to decrease N loss, this rotation may negatively affect the overall N balance of the cropping system. However, this remains poorly quantified. To test this hypothesis, maize, fertilized with zero to 210 kg N ha -1 , was grown after ruzigrass, palisade grass (Urochloa brizanta) and Guinea grass (Pannicum maximum), and the N inputs, outputs and partial N balance determined. Despite the intrinsically poor soil quality associated with the tropical Ultisol, maize grown after the grasses was efficient in acquiring N, resulting in a negative N balance even when 210 kg ha -1 of N was applied after Guinea grass. Losses by leaching, N2O emission and NH3 volatilization did not exceed 13.8 kg ha -1 irrespective of the grass type. Despite a similar N loss among grasses, Guinea grass resulted in a higher N export in the maize grain due to a higher yield, resulting in a more negative N balance. Soil N depletion can lead to C loss, which can result in land degradation.

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