Phosphorus Speciation in Soil and Plants.

To better understand the dynamics of P in soil and plants, chemical characterization and solution 31P nuclear magnetic resonance spectroscopy (NMR) were applied to a natural vegetation system dominated by bracken (Pteridium aquilinum (L.) Kuhn) and British bluebells (Hyacinthoides non-scripta (L.) Chouard ex Rothm.) and to different types of organically amended agricultural soils. Organic P (Po) was dominant in the natural system while the agricultural soil of the total P more than 80% was inorganic P (Pi) mainly in the form of orthophosphate. A detailed quantitative analysis of the P forms in three fields assigned codes (FWa, FWo and FWp) with contrasting coverage of bracken and bluebell, their original native vegetation was undertaken in 2013. Soils were collected in areas dominated by both plants, from April to September 2013 weeks (W1 – W20) in order to cover the main above-ground lifecycle stages. Chemical characterization of the soils showed differences in total P, total Po and plant available P (Mehlich-3 extraction). The total P content of the soils from the three fields showed a slight non-significant increase after bluebell flowering. Quantitative assessment using 31P NMR showed differences in the nature of P forms in the soil and this was reflected in the nature of the vegetation cover, and extent of plant litter deposition. The most dominant P form found in the NaOH-EDTA soil extracts of FWa and FWo were the organic P forms (68.1 – 84.3 %), (61.3 – 79.1 %) respectively, most especially orthophosphate monoesters (53.1 – 83.8 %), (50.3 – 79.4 %), mainly as myo-inositol hexakisphosphate (myo-IP6) or phytate, while the inorganic P form (32.8 – 58 %) was the most dominant on FWp mainly as orthophosphate (ortho-P) (30.7- 56.8 %). The increased myo-IP6 concentration in the soil was linked to the shedded old bluebell bulb below ground containing up to 40 % myo-IP6. Bluebell seeds, another potential route of P transfer into soil, also contained 60 % myo-IP6 of total P. 31P nuclear magnetic resonance (NMR) spectroscopy was also used in elucidating the speciation and distribution of P species in diverse plant seeds (cumin, fennel, flax, mustard, poppy and sesame seeds). Phosphorus speciation by 31P NMR showed that P was mainly present in organic forms such as phytate and α- and β-glycerophosphate in poppy, sesame, mustard, fennel and cumin seeds. The inorganic P forms detected included orthophosphate and pyrophosphate. In particular, the highest amount of orthophosphate was found in NaOH-EDTA extracts of fennel seeds (41.7 %) and the lowest in mustard seeds (9.3 %) and sesame seeds (6.9 %). For the organic P forms, the highest concentration of phytate was found in mustard xiv seeds (85.2 %) and the lowest in fennel seeds (43.3 %). This result implied that in most seed producing plants P, transferred from the plants vegetative parts to the developing seeds during seed maturation, is converted to phytate (organic P) in addition to being stored as orthophosphate (inorganic P). Phenologically either bracken or bluebells grow actively throughout the year. In a semi-natural ecosystem, competition between bluebell and bracken is highest on bracken crozier emergence, which dense bluebell coverage seem to delay. P speciation was identified as an underpinning driver: For bracken, P was present mainly in form of soluble inorganic orthophosphate (41- 96.1 %), while glycerophosphates were the main Po (2.4 – 58.9 %) detected in rhizome, pinnae or stipe. Contrarily bluebell bulbs contained mostly myo-IP6 (6.7 – 52.3 %) possibly aiding survival at low temperatures, because of bluebell’s active growth starting in early autumn. Within the whole plant, the bulb acts as a source and primary sink of P, mainly as myo-IP6. This might be a survival mechanism against P supply interruption during bluebell’s growth cycle while at the same time making P less available for others. The relatively higher total P content of bluebell bulbs (0.67 – 2.7 g kg-1) compared to bracken rhizomes (0.43 – 1.30 g kg-1) also supports this. Bracken’s competitive advantage relies on its dominance of the extensive rhizome system, for which this study showed its ability to redistribute nutrients. Specifically, there was very little differences in the P species between plant parts; instead orthophosphate was shuttled from rhizome to pinnae and returned. The effect of a variety of organic fertilizers additions (pig or cow slurry, farm yard manures, broiler litter, compost and paper sludge/waste) from 1990 to 2014 on the distribution and accumulation of soil Pi and Po forms in three different soil types Harper Adams (HAU, sandy loam), Terrington (TER, silty clay loam) and Gleadthorpe (GT, loamy sand) was investigated. A sequential fractionation scheme and 31P NMR of NaOH-EDTA soil extracts was used to speciate P. Total P concentration in all soils ranged from 0.76 g kg-1 – 1.49 g kg-1 and was predominantly inorganic P (51.2 – 90.8 %). The differences in pH suggests that P species in HAU and GT (pH 6.5) would likely be bound to Al/Fe oxides and hydroxides. At more alkaline pH for TER (pH 7.9) mainly Ca-P minerals would occur. Phosphorus speciation analysis supported this with orthophosphate (82.9 –95.5 %) as the most dominant P form detected. This high inorganic to organic P ratio in conjunction with a low C/P ratio (< 200) suggested that mineralization of organic P mainly occurred in these soils. Myo-IP6 was the most dominant organic P form (1.6 – 8.9 %) followed by scyllo-IP6 (0.7– 4.6 %). Orthophosphate diesters were detected in only one sample (GT) but in trace amounts (0 – 0.5 %). Polyphosphate and xv phosphonates were not detected in any sample. The similar composition of P species across the various treatments suggests that the additions of different manures to the soil only lead to an increase in inorganic P species mainly ortho-P, likely caused by the rapid mineralization of organic P forms in the manure-treated soils. The result also suggested that the abundance and accumulation (Legacy P) of the various P forms, as determined by sequential extraction, were dependent on the nature of manure treatment, soil type and pH of the soils