A 31P nuclear magnetic resonance study of the phosphorus species in alkali extracts of soils from long-term field experiments

The different forms of phosphorus (P) in 0.5 m sodium hydroxide extracts of soils from long-term field experiments at Rothamsted were characterized by ³¹P-nuclear magnetic resonance spectroscopy (NMR). The extract from an old grassland soil (pH 4.6) from a plot of the Park Grass Continuous Hay Experiment that had received no fertilizer or lime for at least 125 years contained the following forms of P: inorganic orthophosphate (22% of the extracted P), orthophosphate monoesters (49%), orthophosphate diesters (14%), phosphonates (3%), pyrophosphate (4%) and two unidentified forms of P (7%). The soil extract from a Park Grass plot given inorganic phosphate fertilizer (35 kg P ha^?1) annually for 121 years contained the same forms of P and, in addition, a small amount of polyphosphate. There was also evidence of an increase in the orthophosphate monoester fraction. Another old grassland soil, of pH 6.1, contained more total and organic P than Park Grass but the extract contained fewer forms of P: inorganic orthophosphate (14% of the extracted P), orthophosphate monoesters (39%), orthophosphate diesters (34%) and an unidentified form (13%). An area of this grassland that had been ploughed up 20 years previously, and kept bare since, contained less organic P. The extract contained less of the phosphate diesters but the more stable monoesters remained relatively unchanged.     

Characterization of phosphorus in sub-alpine forest and adjacent grassland soils by chemical extraction and phosphorus-31 nuclear magnetic resonance spectroscopy

We used chemical extraction methods and ³¹P-nuclear magnetic resonance (NMR) to investigate the effects of vegetation on the amount and structural composition of phosphorous (P) in the sub-alpine soils of central Taiwan. Chemical extraction methods were used to measure inorganic P (Pi) and organic P (Po) in main soil horizons. The soil P composition was assessed by ³¹P-NMR spectroscopy on alkaline EDTA–NaOH extracts. According to the results of chemical extractions, the forest soil had a higher amount of Pi than the grassland soil, which might be a result of the mineralization of Po. ³¹P-NMR spectra showed inorganic orthophosphate (up to 67%) and orthophosphate monoesters (up to 75%) as the major forms of P extracted in forest and grassland soils, respectively. Smaller proportions of orthophosphate diesters and trace amounts of phosphonates and pyrophosphate were found. With possible hydrolysis of P compounds during chemical extraction and slight systemic error in the processes of extraction with NMR, the results from NMR analysis are, in general, consistent with those of chemical extraction.     

Phosphorus compounds in subarctic Fennoscandian soils at the mountain birch (Betula pubescens)—tundra ecotone

Nutrient availability will partly regulate the response of high latitude ecosystems to climate warming, but phosphorus biogeochemistry is poorly understood in Arctic soils. We used NaOH-EDTA extraction and solution ³¹P nuclear magnetic resonance (NMR) spectroscopy to determine phosphorus compounds in subarctic soils from three locations in the Fennoscandian mountains contrasting in latitude and continentality. Soils were taken from open tundra and mountain birch (Betula pubescens Ehrh.) forest at each location. Between 87 and 95% of the total phosphorus was extracted from the surface 2 cm of the organic soil horizons. Most of the extracted phosphorus was orthophosphate monoesters (44-55%), with smaller concentrations of inorganic orthophosphate (15-24%), orthophosphate diesters (12-16%), pyrophosphate (3-18%), inorganic polyphosphate (0-15%) and phosphonates (0-4%). The orthophosphate diesters were further subclassified into DNA (9-13% extracted phosphorus) and phospholipids (1-6% extracted phosphorus), although strong signals in the orthophosphate monoester region of the spectra, consistent with the degradation of phosphatidyl choline in alkaline solution, suggested that phospholipid concentrations were substantially underestimated. The phosphorus composition was broadly similar among soils from the three locations, although no phosphonates were detected in tundra soils from the most southerly site. Deeper organic horizons tended to contain a greater proportion of orthophosphate monoesters than at the surface. The abundance of phosphorus compounds that would be considered readily degradable in temperate environments probably reflects the slow organic matter decomposition in these cold, acidic soils, and suggests that phosphorus availability is unlikely to limit ecosystem productivity on mesic soils at the birch-tundra ecotone during changes induced by climate warming.     

Soil phosphorus characterisation by 31p nuclear magnetic resonance

Abstract

Five New Zealand topsoils, including three sampled under introduced pasture and two in native tussock grasslands, were extracted with 0.5 M NaOH, and the different classes of phosphorus compounds in the extracts distinguished by ³¹p nuclear magnetic resonance (n.m.r.).

Inorganic orthophosphate and orthophosphate monoesters were the major forms of phosphorus in all soil extracts. The tussock‐grassland soil extracts showed the greatest diversity of phosphorus forms, and included compounds with n.m.r. signals ascribed to phosphonates, a previously unreported form of soil phosphorus.     

Organic phosphorus species in humic acids of mountain soils along a toposequence in the Northern Caucasus

Organic P was investigated in humic acids extracted from mountain soils developed in the subalpine, upper subalpine and alpine zones of the Northern Caucasus. P contents of humic acids varied between 3.4 and 14.2 g P kg^?1, depending on P contents of the parent vegetation and on site conditions. Organic P was accumulated at sites where microbial activity is restrained due to soil acidity, low soil temperature and hydromorphy.³¹ P NMR spectroscopy revealed that orthophosphate monoesters were the dominent P species (72–85% of extract- able P), orthophosphate diesters amounted to 12–21%, and phospho- nates ranged between 0 and 9%. Humic acids of soils under cold and wet climatic conditions showed highest concentrations in phospho- nates and orthophosphate diesters. Hence, the accumulation of organo-P in the Caucasian mountain soils was partly due to increasing proportions of potentially available organic P species.     

Organophosphates in agrogray soils with periodic water logging according to the data of <Superscript>31</Superscript>P NMR spectroscopy

The composition of organic phosphorus compounds was studied using the ³¹P NMR spectroscopy method in agrogray soils with periodic water logging. The phosphorus content was determined by the specific difference between the hydrological and the redox regimes of these soils. The phosphorus of the organic compounds in the agrogray soils with contrasting water regimes is composed mostly of phosphoric monoesters and diesters, including nucleic and teichoic acids. The relative distribution of the monoesters and inorganic orthophosphates is shown depending on the climate and the soil’s position in the relief. The area of the monoester peaks increases by two times and that of the mineral orthophosphate decreases by six times in the agro-gray soils of Bryansk opolie with an optimal regime of moistening and evaporation in comparison with the agro-gray gleyed soils of Kolomna opolie. As the degree of the soil hydromorphism in the sequence of deeply gleyed soils and gleyic soils increased, the portion of monoesters decreased. Favorable conditions for the microbial activity are formed in soils with a contrasting redox regime, and this is expressed in the accumulation of labile diesters. Inverse relationships were found between the distributions of the mono- and diesters in iron-manganic nodules and in the soils enclosing them; this was caused by the different mechanisms of the stabilization of the stable and labile phosphorus containing compounds. A high percentage of mineral orthophosphate in the nodules allows assuming the presence of chemisorbed orthophosphate ions in organomineral phosphate-metal-humus complexes. The transformation of iron-manganic nodules under the influence of drying demonstrates the more direct participation of microorganisms in the nodules’ formation than the contribution of the physicochemical processes.     

Influence of aggregate size on phosphorus changes in a soil cultivated intermittently: analysis by <Superscript>31</Superscript>P nuclear magnetic resonance

A pot trial using wet-sieved soil aggregates (>4, 4–2, 2–1, 1–0.5, 0.5–25, and remaining soil <0.25 mm) from a soil that had been cultivated out of permanent pasture and used for winter forage crops for 2 years examined changes in P forms before and after 35 weeks when resown with perennial ryegrass. Soil analyses showed that P was depleted after 35 weeks growth. Changes in P forms were analyzed by ³¹P nuclear magnetic resonance of soil NaOH-EDTA extracts, which removed 98–96% of total P (about 1,080 mg kg⁻¹ in unsieved soil before pasture growth). This indicated that aggregate size influenced the concentrations and forms of P probably via a combination of physical protection and moisture status: orthophosphate, monoesters, diesters and pyrophosphate increased with decreasing size, while phosphonates and polyphosphates were unaffected. The increase in pyrophosphate was attributed to fungal growth, while decreases in orthophosphate and labile organic P (diesters) decreased due to either leaching or mineralisation and plant uptake. The largest decrease was associated with orthophosphate, which could be replenished by fertiliser. However, given the soil’s high potential for P loss, this should only be done to meet conditions for optimal plant growth as any excess would increase the risk of loss. To further minimise P loss without affecting pasture yield, management should maintain or improve soil structure.     

The phosphorus composition of contrasting soils in pastoral,native and forest management in Otago,New Zealand: Sequential extraction and 31P NMR

The distribution and form of P in soil is central to the sustainability of agricultural practice. This study used sequential fractionation and ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) of NaOH–EDTA extracts to examine the influence of pastoral, native (undisturbed) and forest land use on soil P forms in 5 contrasting soils ranging from a Regosol to a Rendzina in Otago, New Zealand. Climatic factors likely to influence soil P distribution were negated by careful site selection. Together with a decrease in soil organic C (31%), total P decreased in forested soils (mean=674 mg kg⁻¹) compared to native soils (mean=784 mg kg⁻¹). In contrast, the ratio of inorganic to organic P increased (10%) probably due to mineralization of organic P in forest soils, while for pasture soils, accumulation of P in inorganic forms due to P inputs via fertilisers and animal dung was to blame. Investigation of the organic P forms in NaOH–EDTA extracts of each land use by ³¹P NMR indicated that diesters were greatest in the native soil (4–12% of total P in spectra), and declined as a proportion of total P in pasture soils and more so in forest soils. This was reflected in a decline of the diester to monoester ratio. However, the ratio was generally greater in forest than pasture soils and attributed to the labile nature of diesters, mineralization of monoesters in forest soils, and an increase in monoesters in pasture soils from inositol phosphates in plant debris. This effect was pronounced in the Regosol due to sandy texture and the preferential accumulation of plant debris in coarse particle size fractions. Due to the depletion of soil P reserves, forest soils in the area should be followed by pasture and well managed fertiliser additions before replanting.     

Sequential extractions and 31P-NMR spectroscopy of phosphorus forms in animal manures, whole soils and particle-size separates from a densely populated livestock area in northwest Germany

The solubility and forms of phosphorus (P) were investigated in manures from chicken and pigs, eight whole soil samples and clay-, silt-, and sand-size separates from an arable and a grassland soil. Total P (P_t) in liquid pig manure (16.2 g kg^–1) and dry chicken manure (26.2 g kg^–1) was distributed between residual P (39–41% P_t), H₂SO₄–P (17–27% P_t), labile resin- and NaHCO₃–P (24–39% P_t), and NaOH-P (3–10% P_t). Most soils had larger proportions of NaOH-P and residual P, indicating reactions of manure-derived P compounds with pedogenic oxides and humic substances. Clay-size separates had the highest P-concentrations in all fractions and were particularly enriched in exchangeable and labile P forms. Solution ³¹P-nuclear magnetic resonance (NMR) spectra of 0.5 M NaOH extracts from manures and some soil samples showed greater signal intensities for orthophosphate and monoester P than 0.1 M NaOH extracts. This can be explained by alkaline hydrolysis phosphate diesters at higher NaOH concentrations and/or by preferential extraction of diesters at lower concentrations. The ³¹P-NMR spectra showed differences between the two manures and confirmed that increasing proportions of ester-P can be expected if they are spread to soils. The NaOH extracts of soil samples were characterized by large proportions of orthophosphate-P (mean 77% of assigned P compounds), which seemed to be slightly enriched in clay fractions whereas the extracts from silt contained more ester-P. Sequential extractions and ³¹P-NMR spectroscopy both showed that these excessively manured soils are likely to lose large amounts of P. Received: 15 July 1996     

Organic phosphorus speciation and pedogenesis: analysis by solution 31P nuclear magnetic resonance spectroscopy

Changes in phosphorus (P) during soil development are central to the understanding of labile P for plant productivity and soil P management. We used NaOH‐EDTA extraction with ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR), sequential P fractionation, and general soil chemical characterization to better our understanding of P dynamics within two chronosequences (Manawatu and Reefton) and one Basalt maturity sequence under original native vegetation. With time, orthophosphate and orthophosphate monoesters tended to increase with organic C to a maximum of about two‐thirds of NaOH‐EDTA‐extractable P in young soils (16 000 years in the Reefton chronosequence), but gradually declined thereafter to about one‐third of NaOH‐EDTA‐extractable P in the oldest soils (130 000 years old). This coincided with a depletion of P from primary minerals (e.g. apatite) and readily available P for plant production. This depletion of inorganic P resulted in a greater reliance on organic P cycling via mineralization, hence the depletion of the normally recalcitrant monoester‐P pool. Concomitantly, the build‐up of labile P species (diesters and pyrophosphate) and scyllo‐ over myo‐inositol hexakisphosphate occurred as soils developed, and might be attributed to microbial activity, including scavenging for P. This work highlights the importance of organic P cycling during pedogenesis.     

Can P NMR spectroscopy be used to indicate the origins of soil organic phosphates?

The relationship between organic P status of 4 soils, 20 microorganisms isolated from these soils (2 bacteria and 3 fungi for each soil) and 13 dominant plant species of typical natural ecosystems of these soils was evaluated. The soils used were represented by two pairs with different ratios of monoester and diester P, and of DNA and other diester P. A Dystric Podzoluvisol and an alpine Umbric Leptosol were characterized by a relatively high proportion of diester P including much DNA P, while a Calcic Chernozem and subalpine Umbric Leptosol had lower proportion of diesters containing relatively less DNA P. The proportions of P compounds in bacteria and plants were very similar on average, based on the monoester to diester P ratio and on the proportions of different diesters in alkaline extract, whereas fungi contained considerably higher proportions of monoesters and polyphosphates, and a higher proportion of phospholipids in the diester fraction. The results showed that the P_org composition of NaOH extracts from different soils was more similar to the composition of extracts from different groups of microorganisms. There was no clear correspondence between soil and microbial diester P proportion and composition. A high proportion of polyphosphate P including pyrophosphate P in soil extracts indicates a significant contribution of fungal P compounds in the soil while the monoester to diester P ratio, and DNA to non-DNA P ratio should be used with caution to interpret the origins of soil P_org. The relative contributions of microorganisms and plants to monoester and diester P in soils is only partially understood.     

31Phosphorus‐Nuclear magnetic resonance analysis in extracts of a phosphorus‐enriched volcanic soil of Chile

Abstract

The nature of organic and inorganic phosphorus (P) in fertilized and unfertilized samples of Vilcun soil, a Chilean medial mesic typic Dystrandept which was studied through ³¹phosphorus‐nuclear magnetic resonance (³¹P‐NMR) analysis carried out on a single alkaline extraction. The total P contents ranged from 1,506 mg P kg^‐1 (B horizon, unfertilized soil) to 7,541 mg P kg^‐1 (A horizon, fertilized soil). The magnitude of the P signal (SEM‐EDAX results) appears to be related to that of the iron (Fe) signal. Signals of ³¹P‐NMR spectra are attributable to inorganic orthophosphate, and orthophosphate monoesters and diesters. No important differences between horizons were observed. Apparently in this soil, P are mainly associated with iron oxides and organic matter. The organic P forms were not readily subjected to mineralization.     

The nature and origins of diester phosphates in soils: a 31P-NMR study

Soils of two climosequences in Russia were investigated by ³¹P-NMR spectroscopy. They comprised Dystric Podzoluvisols, Haplic Greyzems, Calcic Chernozems, and Gypsic Kastanozems, which are located along temperature and precipitation gradients of the Russian Plain. Another sequence of soils included forest Humic Cambisols and Umbric Leptosols of subalpine and alpine meadows, which are formed in different climatic conditions along a climosequence of the Mt. Malaya Khatipara (northern Caucasus). The results showed that accumulation of DNA was high in the cold, wet, and acid soils (Dystric Podzoluvisol, alpine Umbric Leptosol), while phospholipids and teichoic acids mainly accumulated in the more microbially active soils. We performed a laboratory incubation experiment to test the relationship between microbial biomass P and P species identified in soil extracts. The proportions of P compounds resonating at 0.5-3.0 ppm in the NaHCO₃ and H₂SO₄ extracts from the incubated Humic Cambisol increased. The amounts of phosphate diesters resonating at 0 ppm in the same extracts and in the subsequent NaOH extracts decreased after incubation. Based on the results of ³¹P-NMR spectroscopy of native soils and of the laboratory incubation experiment we concluded that signals at 0 ppm in spectra of soil alkaline extracts belong to DNA P which is mainly stabilised in soil organic matter outside microbial cells (at least in soils with relatively low microbial activity). Phospholipids-teichoic acids P extracted with 0.5 M NaHCO₃ seems to be derived from soil microbial biomass, and its proportion can reflect the microbial activity in the soil.     

Phosphorus forms in forest soil colloids as revealed by liquid‐state 31P‐NMR

Nanoparticles and colloids affect the storage and hence the availability of P in forest ecosystems. We investigated the fine colloids present in forest soils and their association with inorganic and organic P. To differentiate between the different P forms, we performed liquid‐state ³¹P‐nucelar magnetic resonance (³¹P‐NMR) measurements on forest bulk soil extracts, on colloid extracts and on the electrolyte phase of their soil suspensions. The ³¹P‐NMR spectra indicated that soil nanoparticles and colloids were more enriched with organic than with inorganic P forms compared to the electrolyte phase. The P concentration was enriched in the colloidal fraction in comparison to the bulk soil and the phosphate diesters were more dominant in the colloidal fraction when compared to the bulk soil. The colloidal P‐diester to P‐monoester ratios were 2 to 3 times higher in the colloidal fraction than in the bulk soil. In contrast, relatively large percentages of inorganic P were found in the electrolyte phase. Supplementary (not shown) Data are available at the JuSER Server (juser.fz‐juelich.de, reference number: FZJ‐2016‐01739) https://juser.fz‐juelich.de/record/283057 .     

Characterisation of organic phosphorus in overland flow from grassland plots using 31P nuclear magnetic resonance spectroscopy

Grazing animals are known to change the characteristics of agricultural grasslands as a source of and pathway for phosphorus (P) loss to water. Previous work, using physico-chemical analysis of the overland flow revealed that the presence of grazing animals increased the overall quantity of P being lost, in particular the unreactive and particulate P fractions. The aim of this study was to characterise the organic P (Po) fraction in overland flow from grazed and non-grazed grassland small plots using phosphorus-31 nuclear magnetic resonance (³¹P NMR) spectroscopy to give greater insight into P loss to water under simulated rainfall. The effect of the grazing animal was most pronounced in the dissolved unreactive P (DUP) and particulate unreactive P (PUP) fractions measured in overland flow from the grazed plots, over four times higher than from the non-grazed plots. Five distinct classes of P compounds were detected in the ³¹P NMR spectra, inorganic orthophosphate (δ = 6.83 ppm), orthophosphate monoesters (δ = 4.95–5.69 ppm), orthophosphate diesters (δ = 1.89 ppm), phosphonates (δ = 19.38 ppm), and pyrophosphates (δ = −3.26 ppm). Distinct signals at 5.69, 5.37, 5.10, and 4.95 ppm in the overland flow extracts from the plots indicated significant concentrations of myo -inositol hexakisphosphate in the orthophosphate monoester region. Orthophosphate diesters (assigned to phospholipids) and phosphonates were also only detected in overland flow collected from the grazed plot. These results indicate that normal grazing management practices may not only affect the concentrations of Po but also the forms of Po being transferred from grassland systems to water.     

Assessing the organic phosphorus status of an Oxisol under tropical pastures following native savanna using 31P NMR spectroscopy

³¹P nuclear magnetic resonance (NMR) spectroscopy, P fractionation, and a P sorption experiment were used to follow the changes in P in the A horizons (0–10 cm) of acid savanna soils, Colombia, after little P fertilization and 15 years' continuous growth of a grass (Brachiaria decumbens) and a grass/legume (B. decumbens+Pueraria phaseoloides) pasture. Ready P supply as analyzed by Bray P was low under native savanna (1.3 mg kg^-1 soil) and responded moderately on pasture establishment. Concurrently, the affinity of the soil for inorganic P declined slightly after pasture establishment. ³¹P NMR spectroscopy revealed that P associated with humic acids was dominated by monoester P followed by diester P. Smaller proportions were observed for phosphonates, teichoic acid P, orthophosphate, and pyrophosphate. P associated with fulvic acids had lower proportions of diester P and higher contents of orthophosphate. Under native savanna the reserves of labile organic P species (phosphonates and diester P including teichoic acid P) associated with humic and fulvic acids were 12.4 and 1.1 kg ha^-1, respectively, and increased to 18.1 and 1.8 kg ha^-1 under grass pasture, and to 19.5 and 2.3 kg ha^-1 under grass/legume pasture. These data emphasize the importance of labile organic P species in the P supply for plants in improved tropical pastures, and further indicate that humic acid P in particular responds to land-use changes within a relatively short time-scale. Earthworm casts were highly abundant in the B. decumbens+P. phaseoloides plot and were enriched in labile organic P species. We conclude that earthworm activity improves the P supply in soil under tropical pastures by creating an easily available organic P pool.Dedicated to Professor J.C.G. Ottow on the occasion of his 60th birthday     

Isolating the influence of pH on the amounts and forms of soil organic phosphorus

Soil pH influences the chemistry, dynamics and biological availability of phosphorus (P), but few studies have isolated the effect of pH from other soil properties. We studied phosphorus chemistry in soils along the Hoosfield acid strip (Rothamsted, UK), where a pH gradient from 3.7 to 7.8 occurs in a single soil with little variation in total phosphorus (mean ± standard deviation 399 ± 27 mg P kg^?1). Soil organic phosphorus represented a consistent proportion of the total soil phosphorus (36 ± 2%) irrespective of soil pH. However, organic phosphorus concentrations increased by about 20% in the most acidic soils (pH < 4.0), through an accumulation of inositol hexakisphosphate, DNA and phosphonates. The increase in organic phosphorus in the most acidic soils was not related to organic carbon, because organic carbon concentrations declined at pH < 4.0. Thus, the organic carbon to organic phosphorus ratio declined from about 70 in neutral soils to about 50 in strongly acidic soils. In contrast to organic phosphorus, inorganic phosphorus was affected strongly by soil pH, because readily‐exchangeable phosphate extracted with anion‐exchange membranes and a more stable inorganic phosphorus pool extracted in NaOH–EDTA both increased markedly as soil pH declined. Inorganic orthophosphate concentrations were correlated negatively with amorphous manganese and positively with amorphous aluminium oxides, suggesting that soil pH influences orthophosphate stabilization via metal oxides. We conclude that pH has a relatively minor influence on the amount of organic phosphorus in soil, although some forms of organic phosphorus accumulate preferentially under strongly acidic conditions.     

Phosphorus forms and dynamics as influenced by land use changes in the sub-humid Ethiopian highlands

In undisturbed tropical forest ecosystems, the phosphorus (P) cycle is essentially “closed” with minimal short-term losses or gains of P. The forms and dynamics of soil P, however, can be greatly affected by land use changes, which often involve changes in vegetation cover, biomass production and nutrient cycling in the ecosystem. Sequential extraction and ³¹P nuclear magnetic resonance (NMR) spectroscopy were used to investigate the influence of land use changes on the amount and structural composition of P in the sub-humid highlands of southern Ethiopia. Samples were collected from surface soils (0–10 cm) of natural forest, tea plantations and cultivated fields (25 years) at Wushwush and from Podocarpus dominated natural forest, Cupressus plantations and cultivated fields (30 years) at Munesa sites. Significantly lower (P<0.05) amounts of total P were found following clear-cutting and long-term cultivation (31% and 39%), and establishment of plantations (21% and 22%) at Wushwush and Munesa, respectively. The largest depletion of sodium bicarbonate-extractable organic P (NaHCO₃–P_o) (74% and 77%) and sodium hydroxide-extractable P_o (NaOH–P_o) (67% and 67%) due to cultivation occurred in the sand, followed by the silt (52% and 56%, NaHCO₃–P_o and 32% and 53%, NaOH–P_o) and the clay (33% and 42%, NaHCO₃–P_o and 28% and 35%, NaOH–P_o) size separates from the two sites, respectively. ³¹P NMR spectroscopy revealed that orthophosphate monoesters were the major organic P compounds (27–66%) followed by orthophosphate diesters (9–27%) and teichoic acids (7–11%). Unknown organic P species accounted for 3–8% (unknown A) and 3–5% (unknown B), whereas phosphonates made up 2–3%. The proportion of diester-P decreased in the order: natural forests (24% and 27%)>plantations (15% and 13%)>cultivated fields (10% and 9%) at Wushwush and Munesa, respectively. The percentages of teichoic acid, unknown A, unknown B and phosphonates also decreased, whereas the proportions of orthophosphate monoesters increased following land use changes. Greater decline in proportions of diester-P and teichoic acids were found in the silt than in the clay size separates, which may be attributed to stabilization of microbially derived organic P structures by closer association with clay minerals and/or sesquioxides. The results of sequential extraction and ³¹P NMR spectroscopy indicate that continuous cultivation with little or no P input not only decreased the amount, but also influenced the structural composition and bioavailability of P in these tropical soils. Thus measures have to be designed for replenishment and subsequent maintenance of soil P stocks, to ensure sustainable crop production in sub-humid highland agroecosystems of southern Ethiopia.     

13C NMR characterization of humic acids from soils of a development sequence

Humic acids were isolated from nine topsoils in New Zealand tussock grasslands. Cross-polarization ¹³C NMR spectra of solid samples were used to estimate fractions of carbon contained in different types of chemical functional groups. The degree of oxygen substitution of aromatic rings showed a strong negative correlation with soil development. Aromaticities greater than 0.25 were found in humic acids from only the two least-developed soils.     

Assessing the bioavailability of dissolved organic phosphorus in pasture and cultivated soils treated with different rates of nitrogen fertiliser

A proportion of dissolved organic phosphorus (DOP) in soil leachates is readily available for uptake by aquatic organisms and, therefore, can represent a hazard to surface water quality. A study was conducted to characterise DOP in water extracts and soil P fractions of lysimeter soils (pasture before and after, and cultivated soil after leaching to simulate a wet winter-autumn) from a field trial. Data on DOP in drainage waters from the field trial were also generated. In water extracts, used as a surrogate for soil solution and drainage water, 70-90% of the total dissolved P (TDP) concentration was made up of DOP, of which 40% was hydrolysable by phosphatase enzymes. Proportions of hydrolysable DOP to TDP in drainage waters of the field trial were less than in water extracts due to enhanced DRP loss via dung inputs, but still large at 35% of DOP. Analysis of lysimeter soils by sequential fractionation indicated that several organic P fractions changed with land use and due to leaching. Further investigation using NaOH-EDTA extracts and ³¹P nuclear magnetic resonance spectroscopy indicated that the greatest changes were a decrease in the concentrations of orthophosphate diester P and an increase in orthophosphate monoester P. This was attributed to mineralization by cultivation and plant roots and also to the leaching of mobile diester P. This study suggests that in such soils with a dynamic soil organic P pool, the concentration of readily bioavailable P in soil solution and drainage waters and the potential to impair surface water quality cannot be determined from the DRP concentration alone.