Risk of phosphorus losses from Andosols under fertilized pasture

Fertilization of grassland pastures may be a non-point pollution source in the Azores archipelago, despite the high phosphorus (P) retention of Andosols. To evaluate the risk of P desorption, representative Andosols samples (0-15 cm) were subdivided in five layers and different P pools were measured. The risk of P unloading into waters was assessed by the degree of phosphorus saturation (DPS), and by the P concentration in equilibrium solutions (0.01 M CaCl₂). The higher contents in the superficial layers suggest P accumulation due to pasture overfertilization. The organic P represented about 54% of the total P, with an overall average of 2.66 g P_t/kg. Despite being above the agronomic threshold, the soil with the highest average mean values of extractable inorganic P, 77 mg P_Olsen/kg and and 73.7 mg P_AL/kg, is still below environmental thresholds and none of the soils had DPS values above 25%, which is the critical value associated with eutrophication of surface waters. Similarly, all the P concentrations in the equilibrium CaCl₂ solutions were below the critical limits. Therefore, P desorption from these soils did not seem to be the main process responsible for effective waterbodies eutrophication in the Azores. Since mineral fertilizers are applied superficially, the hypothesis of their direct runoff during rainfall events, even before their complete dissolution and interaction with the soil matrix, must be considered. Consequently, P fertilization with deep-banding systems may be the alternative to the interdiction of fertilizers in the most sensitive and hilly areas of the watersheds.

Opportunities for mobilizing recalcitrant phosphorus from agricultural soils: a review

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 31P-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.

Comprehensive study of excess phosphate response reveals ethylene mediated signaling that negatively regulates plant growth and development

Excess Phosphorus (P) in agriculture is causing serious environmental problems like eutrophication of lakes and rivers. Unlike the enormous information available for phosphate starvation response (P₀), very few information is available for the effect of excess phosphate P_i on plants. Characterization of Excess Phosphate Response (EP_iR) is essential for designing strategies to increase phosphate accumulation and tolerance. We show a significant modulation in the root developmental plasticity under the increasing supply of excess P_i. An excess supply of 20 mM P_i (P₂₀) produces a shallow root system architecture (RSA), reduces primary root growth, root apical meristem size, and meristematic activity in Arabidopsis. The inhibition of primary root growth and development is indeterminate in nature and caused by the decrease in number of meristematic cortical cells due to EP_iR. Significant changes occurred in metal nutrients level due to excess P_i supply. A comparative microarray investigation of the EP_iR response reveals a modulation in ethylene biosynthesis and signaling, metal ions deficiency response, and root development related genes. We used ethylene-insensitive or sensitive mutants to provide more evidence for ethylene-mediated signaling. A new role of EP_iR in regulating the developmental responses of plants mediated by ethylene has been demonstrated.