Nathalie Mahieu

The phosphorus composition of temperate pasture soils determined by NaOH-EDTA extraction and solution 31P NMR spectroscopy

Information on the composition and dynamics of soil phosphorus (P) remains limited, but is integral to understanding soil biogeochemical cycles. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterise NaOH–EDTA extractable P in 29 permanent pasture soils from England and Wales (total carbon 29–80 g kg−1 soil, clay 219–681 g kg−1 soil, pH 4.4–6.8). Total P ranged between 376 and 1981 mg P kg−1 soil, of which between 45 and 88% was extracted with NaOH–EDTA. The extracts were dominated by orthophosphate monoesters (29–60% extracted P) and inorganic orthophosphate (21–55% extracted P), with smaller concentrations of orthophosphate diesters (2–10% extracted P), pyrophosphate (1–7% extracted P), phosphonates (0–3% extracted P), and traces of polyphosphates. Orthophosphate diesters were subclassified into phospholipids (1–7% extracted P) and DNA (1–6% extracted P). Signals slightly downfield of inorganic orthophosphate were tentatively assigned to aromatic orthophosphate diesters similar in structure to R-(−)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate. Such signals are rarely detected in soil extracts, but were present in relatively large concentrations in the samples analysed here (2–5% extracted P). Relationships between functional P groups and soil properties suggested that the various functional groups are involved in the soil P cycle to different extents. In particular, concentrations of orthophosphate monoesters appeared to be controlled by the potential for chemical stabilisation in soil, whereas DNA and pyrophosphate were strongly correlated with the microbial biomass, suggesting an active involvement in biological nutrient turnover.

Decomposition and nutrient release from radiata pine (Pinus radiata) coarse woody debris

Abstract The dynamics of decomposition of thinning slash and nutrient release were studied in a radiata pine (Pinus radiata D Don) plantation forest in New Zealand. This study examined decomposition of coarse woody debris (CWD) components (log-wood, log-bark, and side branches) originating from stands thinned between 1 and 13 years previously. Changes in component density were used to estimate the decay rates. Both chemical analyses and 13 C nuclear magnetic resonance (NMR) spectroscopy were conducted to investigate relationships between decomposition and chemical composition. The rate of decomposition was the fastest for log-wood followed by log-bark, which in turn decomposed faster than side-branch material. After 13 years, log-wood, log-bark and side branches lost 59, 55 and 24% of their initial mass, respectively. Single exponential model analysis indicated that the half-life of total thinning slash (sum of log-wood, log-bark and side branches) was 13.25 years. Proximate analyses showed that the faster rate of decomposition of log-wood was mainly due to greater carbohydrate concentration, while greater concentrations of polyphenol and lignin were responsible for the slower decomposition rate of log-bark. The slow rate of decomposition of side branches was due to unfavorable micro-climate (most of the side branches were not in contact with soil even after 9 years of decomposition) as well as greater lignin and polyphenol concentrations. Carbon-13 NMR analysis revealed that during decomposition the relative proportions of O-alkyl and acetal C, which represent carbohydrates, decreased while N-alkyl, aromatic, and phenolic C, which represent tannins and acid insoluble compounds including lignin, increased in all thinning slash components. Net release of nutrients (N, P, K, Ca and Mg) occurred during thinning slash decomposition, in contrast to earlier studies, although the concentrations of most nutrients increased with time. Nutrient release was attributed to the nature of the thinning slash materials and the high proportion of bark material in particular. Although there was a net release, the rate of release of C and the majority of nutrients from thinning slash was slow making it an important C sink and long-term source of nutrients.

Quantification of myo-inositol hexakisphosphate in alkaline soil extracts by solution 31P NMR spectroscopy and spectral deconvolution

Inositol phosphates are the dominant class of organic phosphorus (P) compounds in most soils, but are poorly understood because they are not easily identified in soil extracts. This study reports a relatively simple technique using solution 31P NMR spectroscopy and spectral deconvolution for the quantification of myo-inositol hexakisphosphate (phytic acid), the most abundant soil inositol phosphate, in alkaline soil extracts. An authentic myo-inositol hexakisphosphate standard added to a re-dissolved soil extract gave signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1. Spectral deconvolution quantified these signals accurately (102 ± 4%) in solutions containing a mixture of model P compounds by resolving the envelope of signals in the orthophosphate monoester region. In NaOH-EDTA extracts from a range of lowland permanent pasture soils in England and Wales, concentrations of myo-inositol hexakisphosphate determined by spectral deconvolution ranged between 26 and 189 mg P kg−1 soil, equivalent to between 11 and 35% of the extracted organic P. Concentrations were positively correlated with oxalate-extractable aluminum and iron but were not correlated with total carbon, total nitrogen, clay, or the microbial biomass. This suggests that myo-inositol hexakisphosphate accumulates in soils by mechanisms at least partially independent of those controlling organic matter stabilization and dynamics. Furthermore, myo-inositol hexakisphosphate concentrations were positively correlated with plant-available inorganic P and negatively correlated with the carbon-to-organic P ratio, suggesting that biological P availability may, in part, regulate myo-inositol hexakisphosphate concentrations in soils, perhaps because organisms capable of degrading this compound are favored in more P-limited environments. Solution 31P NMR spectroscopy and spectral deconvolution offers a relatively simple method of quantifying myo-inositol hexakisphosphate in soil extracts.

Mineralisation of soil orthophosphate monoesters under pine seedlings and ryegrass

The effects of radiata pine (Pinus radiata D. Don) seedlings and ryegrass (Lolium perenne L.) on the mineralisation of orthophosphate monoesters in 7 grassland soils were assessed in a 10-month pot trial using NaOH–EDTA extraction and solution 31P NMR spectroscopy. Extraction with NaOH–EDTA recovered 46–86% of the total soil P, and NaOH–EDTA-extractable organic P determined by molybdate colourimetry ranged between 194 and 715 mg/kg soil, representing 34–85% of the total soil organic P. Orthophosphate monoesters were the predominant species of the extracted organic P in all soils, with much smaller concentrations of orthophosphate diesters, and traces of phosphonates. Concentrations of orthophosphate monoesters were consistently lower in soils under pine (103–480 mg P/kg soil) compared with the initial soils (142–598 mg P/kg soil) and most soils under grass (122–679 mg/kg soil). Mineralisation of myo-inositol hexakisphosphate accounted for 18–100% of the total mineralisation of orthophosphate monoesters in most soils under radiata pine. This suggests that supposedly recalcitrant inositol phosphates are available for uptake by radiata pine, although the extent of this varies among soils.

Mechanisms of phosphorus solubilisation in a limed soil as a function of pH

Phosphorus (P) quantity-intensity relationships are central to the solubility and release of P from soil to water. Relationships between P extractable by 0.5 M NaHCO extractable P (Olsen P; quantity, Q) and P extractable by 0.01 M CaCl(2) (CaCl(2)-P; possible predictor of soil solution or drainage water P; intensity, I) are curvilinear: above a certain Olsen P concentration, CaCl(2)-P becomes much more soluble than when below it. Aluminium-, Fe- and Ca-P forms (extractable by Olsens reagent) are thought to control P solubility. Thus, our objectives were to identify P forms in equilibrium with CaCl(2)-P via solubility equilibrium experiments, and the behaviour of CaCl(2)-P in relation to Al, Fe and Ca associated P, determined with 31P high power decoupling magic angle spinning nuclear magnetic resonance spectroscopy (31P HPDec/MAS NMR). Results indicated that two Q-I relationships occurred, one for soils above pH 5.8, and the other for soils below pH 5.8. Above pH 5.8, soils were saturated with respect to hydroxyapatite (Ca(5)(PO(4))(3)OH) and undersaturated with respect to beta-tricalcium phosphate (beta-Ca(3)(PO(4))(2)), while log ion-activity products showed that all soils and pHs were either saturated or in equilibrium with variscite (AlPO(4).2H(2)O) or its amorphous analogue. Using 31P HPDec/MAS NMR, Ca-P was best correlated with CaCl(2)-P in soils above pH 5.8, and with Al-P in soils below this pH. This study demonstrates the value of solid-state NMR in conjunction with wet chemical techniques for the study of labile P and P loss from pasture soils with a wide range of managements.

The effect of soil acidity on potentially mobile phosphorus in a grassland soil

SUMMARY This study compared phosphorus (P) speciation and the relationship between bicarbonate extractable (Olsen) P and 001  CaCl extractable P (a measure of potentially mobile P) in soils from plots of the Park Grass experiment started in 1856 at IACR-Rothamsted, UK and with and without nitrogen as (NH ) SO and with and without calcium carbonate (CaCO , lime). A point, termed the change point, was noted in Olsen P, above which 001  CaCl -P increased at a greater rate per unit increase in Olsen P than below this point. Previous findings have shown a change point for soils with a pH 58 at 56 mg Olsen Pkg and at 120 mg Olsen Pkg for soils below this pH. Soils given (NH ) SO annually since 1856 and with lime periodically since 1903 mostly had a pH between 37 to 57, some of these (NH ) SO treated soils were limed to pH 65 and above from 1965. Irrespective of their pH in 199192 all the soils had a similar change point (120 mg Olsen Pkg) to that found for other soils with pH 58 (112 mg Olsen Pkg). In a laboratory study lasting 30 days, the addition of CaCO to acid soils from the field experiment that had received (NH ) SO had a similar change point to soils with pH 58 irrespective of pH, suggesting soil P chemistry was controlled by the long period of soil acidity and this was not reversed by a short period at a higher pH. The effect of pH was attributed to the creation of P sorptive surfaces on aluminium precipitates compared with less acidic soils (pH 58) where there was less exchangeable Al to be precipitated. This was confirmed with solid-state P nuclear magnetic resonance, which indicated that for soils of similar total P concentration and pH, there was twice as much amorphous Al-P in soils given (NH ) SO compared with those without. Changes in pH as a result of applications of (NH ) SO or lime can greatly change the concentration of potentially mobile P due to the effects on Al solubility. Although there was less potentially mobile P in soils with pH 58 than in soils above this pH, it is usually advised in temperate regions to maintain soils about pH 65 for arable crops.

Analysis of Phosphorus in Sequentially Extracted Grassland Soils Using Solid State NMR

Sequential extraction and solid state 31P NMR spectroscopy were used to investigate the relative solubility of phosphorus (P) in a series of soils under permanent grassland in New Zealand. This involved comparing spectra of unextracted soil with residual soil after extraction with 1 M NH4Cl–0.1 M NaOH and 1 M NH4Cl–0.1 M NaOH–0.5 M H2SO4. Results confirmed that alkali extraction selectively removed P associated with aluminum (Al) and iron (Fe), while subsequent extraction with acid removed calcium (Ca)-P. This indicates that fractionation schemes can be used to isolate categories of P species involved in soil P cycling and the added value of solid state NMR for the analysis of soil P species in situ.

Second-order paramagnetic rhenium(III) complexes: solid-state structure and assignment of the carbon-13 magnetic resonance spectra in solution

Single-crystal X-ray diffraction of trans,mer-[ReCl3(PEt2Ph)3] has confirmed the structure derived from 1H and 13C NMR spectroscopy: space group Pbca, a= 15.122(3), b= 20.019(4), c= 21.689(4), A, z= 8. The carbon-13 NMR spectra of [ReCl3(PEt2Ph)3] and [ReCl3(PPrn2Ph)3] have been assigned on the basis of the known proton-assignments by 13C–1H correlation spectroscopy experiments. The spectra show no couplings between 13C and 13P. The C1 carbon resonances for the aryl groups, unlike the Cα resonances of the alkyl groups, are too broad for detection. The proton relaxation rates T1–1 and T2–1 as well as the linewidths in three different magnetic fields, corresponding to proton frequencies of 250, 400 and 600 MHz, show an increase with field strength.