Organic phosphorus speciation in Australian Red Chromosols: stoichiometric control

Abstract

Organic phosphorus (P) plays an important role in the soil P cycle. It is present in various chemical forms, the relative amounts of which vary among soils, due to factors including climate, land use, and soil type. Few studies have investigated co-variation between P types or stoichiometric correlation with the key elemental components of organic matter– carbon (C) and nitrogen (N), both of which may influence P pool structure and dynamics in agricultural soils. In this study we determined the organic P speciation of twenty Australian Red Chromosols soils, a soil type widely used for cropping in Australia. Eight different chemical forms of P were quantified by 31P NMR spectroscopy, with a large majority (>90%) in all soils identified as orthophosphate and humic P. The strongest correlations (r2 = 0.77–0.85, P < 0.001) between P types were found among minor components: (i) between two inositol hexakisphosphate isomers (myo and scyllo) and (ii) between phospholipids and RNA (both detected as their alkaline hydrolysis products). Total soil C and N were correlated with phospholipid and RNA P, but not the most abundant P forms of orthophosphate and humic P. This suggests an influence of organic matter content on the organic P pool consisting of phospholipid and RNA, but not on inositol P or the largest organic P pool in these soils – humic P.

The composition of organic phosphorus in soils of the Snowy Mountains region of south-eastern Australia

“…1 contains prominent IP 6 peaks, especially for the two soils collected from above 1800 m above sea level (A1053 and A1054). This is in contrast to our previous studies on Australian soils where we noted the absence of, or relatively low intensity of, IP 6 peaks, which account for a relatively minor component (typically <12%) of the organic P in most Australian soils (Doolette et al 2011;McLaren et al 2014;Moata et al 2016). In previous studies, we reported that peaks due to a-and b-glycerophosphate have usually been larger than those of the IP 6 ; this is especially evident in the 31 P NMR spectra of Vertosols, for which IP 6 peaks were barely discernible (McLaren et al 2014).…”

“…By targeting soils from within the Australia-wide ASRIS database, samples from the highest altitudes of Australia were chosen to represent a series of typical alpine soils, which we hypothesised would exhibit spectral features in their 31 P NMR spectra more akin to those of soils from other relatively cold and wet locations. The composition of organic P in these alpine soils appears to differ to those previously reported for Australian soils (Doolette et al 2011;McLaren et al 2014;Moata et al 2016). In particular, the presence of phosphonates, which we identified in two of the soils at the coldest and wettest locations, have not been reported before in Australian soils but have been identified in cold, wet and acidic soils elsewhere where microbial activity is limited (Tate and Newman 1982;Zech et al 1987;Gil-Sotres et al 1990).…”

“…Management is another point of difference between the five soils analysed in this study and the majority of Australian soils we have previously reported on (Doolette et al 2011;McLaren et al 2014McLaren et al , 2015bMoata et al 2016). In particular, the alpine soils reported here are all unmanaged and under native vegetation, whereas the majority of soils we investigated previously are agricultural soils.…”

“…This study also serves to shed some light on a point of contention among researchers who use 31 P NMR spectroscopy to determine the speciation of organic P. It has been suggested that the broad feature we have consistently found to dominate 31 P NMR spectra of NaOH-EDTA extracts of Australian soils (Doolette et al 2011;McLaren et al 2014McLaren et al , 2015bMoata et al 2016) and identify as humic-P, but which is less obvious in many other 31 P NMR studies, may result from slight differences in methodology, in particular that we redissolved our freezedried extracts in water rather than NaOH-EDTA (Cade-Menun and Liu 2014; Kruse et al 2015). However, here we have shown that we can obtain 31 P NMR spectra very similar to those reported in other studies.…”

“…Recently, we hypothesised that the generally low concentrations of inositol phosphates reported for most Australian soils could be related to the generally warm conditions across most of Australia, and in particular, the warm environments from which the soils analysed to date have been sourced (McLaren et al 2015a;Moata et al 2016).…”