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Insights into ³³phosphorus utilisation from Fe- and A-hydroxides in Luvisol and Ferralsol subsoils
(2019)
There is currently relatively little available information on subsoil phosphorus (P) use for crop production as a function of soil order. In this study, a rhizobox experiment was performed using subsoils of two reference soil groups, an
Orthic Ferralsol and a Haplic Luvisol. To evaluate the immediate P uptake by wheat (Triticum aestivum L.) from different subsoil P pools during 14 days of growth, subsoil bands were spiked with KH₂PO₄ solution associated to Fe-hydroxide
(³³P-Fe), to Al-hydroxide (³³P-Al), in free form (³³P-OrthoP), or in trace amounts without any additional ³¹P (³³P-NoP). At the beginning of the experiment, the soil water content was set at 75% of water-holding capacity, corresponding to an initial soil matric potential of -12 ± 1 kPa. During plant growth, soil moisture decreased in both soils, but soil matric potentials in both soils did not drop below field capacity (-33 kPa; pF 2.5). The shoot dry weights of the Ferralsol were 1.2 to 1.8 times those of the Luvisol. Despite elevated soil P availability in the Luvisol, shoot P concentrations did not differ between the two soils. The amount of ³³P taken up by the shoots from the oxide phases was 15% to 40% greater in the Ferralsol treatments than in those in the Luvisol treatments. It was concluded that the more favourable physical soil conditions facilitated ³³P uptake from both oxidic phases from the Ferralsol subsoil relative to the Luvisol subsoil, despite better P phytoavailability in the latter.
Global agricultural productivity depends on the use of finite phosphorus (P) resources of which not only the topsoil, but also subsoil, can hold immense reserves. To assess potential soil contribution to plant nutrition, we compared the P status of Stagnic Cambisol profiles in experimental plots that received different P fertilizer applications (control, triple superphosphate (TSP), compost, compost+TSP) for 16 years. Sequential fractionation was combined with P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify the chemical P speciation. Fertilized topsoils (21 to 69 kg P ha-1 a-1) showed P reserves larger by a factor of 1.2 to 1.4, and subsoil P reserves larger by a factor of 1.3 to 1.5 than those of the control. P-XANES revealed the predominance of inorganic P species such as moderately labile Fe- (46 to 92%), Al- (0 to 40%), and Ca- (0 to 15%) P compounds besides organic P (0 to 13%) in all treatments. The fertilizer application slightly altered P speciation throughout the profiles, but the type of fertilizer had no significant effect on it. Optimal plant growth requirements are restricted by the exchangeable P from the solid phase within the soil solution. Therefore, ongoing research focuses on the accessibility of P from P loaded amorphous Fe- and Al-hydroxides, previously identified as the predominant abiotic P forms. To assess their P desorption potential, P-33 rhizotron experiments combined with P-33 isotopic exchange kinetics (IEK) are underway. Preliminary results indicated that besides differences in P binding capacity of soil hydroxides, physical soil parameters, such as the matric potential, strongly control soil P availability, thus plant P acquisition rates can vary among different soil types. Our results gained new detailed information about P bioavailability under agricultural practice. The investigations towards P bioaccessibility may contribute to improved interpretation of soil P tests and reduced fertilizer recommendations.