Phosphorus sequestration in soil aggregates after long-term tillage and cropping

Cultivated soils in the Everglades are being converted to their historic use as pastures or seasonally flooded prairies as parts of restoration efforts, but long-term cultivation may have altered soil P distribution and availability which may pose eutrophication hazards upon change in land use. The objectives of this study were to determine the distribution of P in soil chemical and physical fractions for contrasting long-term land management practices. The distribution of P in labile, Fe–Al bound, Ca bound, humic–fulvic acid, and residual pools in five aggregate-size fractions were measured for fields under sugarcane (Saccharum sp.) cropping for 50 years and perennial pasture for 100 years. Both land uses were characterized by a high degree of macroaggregation, as aggregates >0.25 mm contained 76 and 83% of the total soil under cultivation and pasture, respectively. Soils under sugarcane sequestered a total of 77 kg ha⁻¹ more P than pasture at 0–15 cm. The distribution of P in chemical fractions significantly varied between land uses as cultivation increased P sequestration in Ca-bound fractions more for sugarcane (244 kg P ha⁻¹) than pasture (65 kg P ha⁻¹). Pasture sequestered more P in organic pools, as storage in humic–fulvic acid and residual fractions were 26 and 25%, respectively, higher than sugarcane. Labile P was 100% higher for pasture than sugarcane, but Fe–Al bound P storage did not differ between land uses. Aggregation increased P sequestration in humic–fulvic acid and residual fractions, and P storage in organic pools increased with increasing aggregate size. In contrast, cultivation decreased aggregation and increased P accumulation in inorganic fractions. Long-term cultivation altered the distribution of soil P from organic to inorganic pools. The P stored in inorganic pools is stable under current land use, but may be unstable and pose eutrophication hazards upon onset of future land use change to the seasonally flooded prairie ecosystem.     

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.     

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.     

A comparison of soil and microbial carbon,nitrogen, and phosphorus contents,and macro-aggregate stability of a soil under native forest and after clearance for pastures and plantation forest

Total, extractable, and microbial C, N, and P, soil respiration, and the water stability of soil aggregates in the F-H layer and top 20 cm of soil of a New Zealand yellow-brown earth (Typic Dystrochrept) were compared under long-term indigenous native forest (Nothofagus truncata), exotic forest (Pinus radiata), unfertilized and fertilized grass/clover pastures, and gorse scrub (Ulex europaeus). Microbial biomass C ranged from 1100 kg ha^-1 (exotic forest) to 1310kg ha^-1 (gorse scrub), and comprised 1–2% of the organic C. Microbial N and P comprised 138–282 and 69–119 kg ha^-1 respectively, with the highest values found under pasture. Microbial N and P comprised 1.8–7.0 and 4.9–18% of total N and P in the topsoils, and 1.8–4.4 and 23–32%, respectively, in the F-H material. Organic C and N were higher under gorse scrub than other vegetation. Total and extractable P were highest under fertilized pasture. Annual fluxes through the soil microbial biomass were estimated to be 36–85 kg N ha^-1 and 18–36 kg P ha^-1, sufficiently large to make a substantial contribution to plant requirements. Differences in macro-aggregate stability were generally small. The current status of this soil several years after the establishment of exotic forestry, pastoral farming, or subsequent reversion to scrubland is that, compared to levels under native forest, there has been no decline in soil and microbial C, N, and P contents or macro-aggregate stability.     

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.     

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.     

Distribution and Sorption of Phosphate in a Savanna Soil Under Improved Pastures in Northern Nigeria

Abstract

Land use patterns affect soil nutrient transformation and availability. The study determined the distribution of phosphorus (P) fractions and sorption in five pasture fields composed of Andropogon gayanus, Brachiaria decumbens, Chloris gayana, Digitaria smutsii, and Stylosanthes guianensis. The objectives were to characterize P fractions in improved pastures and to determine the effect of forage species on soil P lability. Total P (P_t) across the pastures was not significantly different. Organic P (P_o) accounted, on the average, for 64% of P_t. Resin‐P, considered the plant‐available P, ranged from 4 to 10 mg kg^?1, suggesting acute P deficiency in the pastures. The sum of P fractions extracted by 0.5 M NaHCO₃, 0.1 M NaOH, and 1.0 M HCl, together with the resin‐P, accounted for less than 35% of P_t. Factor analysis indicated that plant‐available P approximated by resin‐P was furnished by ?HCO₃‐P_o mineralization and HCl‐P. The highest concentrations of ?HCO₃‐P_o and ?OH‐P_o were maintained by Brachiaria decumbens. Grouping P_i and P_o fractions into labile and nonlabile fraction showed that Brachiaria decumbens maintained the greatest concentration of labile P as a proportion of its P_t. The pasture soils sorbed between 31 and 65% of added P from a standard concentration of 50 mmol kg^?1. Phosphorus sorbed by soils from the pasture fields was in the order: Digitaria smutsii=Stylosanthes guianensis>Brachiaria decumbens=Chloris gayana>Andropogon gayanus, whereas resin recovery of sorbed P was greater in Brachiaria decumbens than other pastures. Between 82 and 92% of sorbed P was bound irreversibly. It was concluded that the relatively high concentration of labile P maintained by soil under Brachiaria decumbens was probably related to its capacity to sequester more carbon than the other pastures.     

The chemical nature of organic phosphorus that accumulates in fertilized soils of a temperate pasture as determined by solution 31P NMR spectroscopy

Inefficiency of fertilizer phosphorus (P) use in grazing systems is often associated with the accumulation of inorganic and organic P in fertilized soil. However, the chemical nature of the accumulated organic P remains poorly understood. The aim of this study was to use solution ³¹P nuclear magnetic resonance (NMR) spectroscopy on sodium hydroxide–ethylenediaminetetraacetic acid (NaOH‐EDTA) extracts to identify the chemical nature of organic P in soils from a medium‐term (13 years) permanent pasture field experiment. This included an unfertilized pasture (P0), and treatments designed to maintain soil P fertility at near ‘optimum' (P1) and ‘supra‐optimum' (P2) levels for pasture growth; pastures at all levels of soil P fertility were continuously grazed with either a moderate or high stocking rate (SR09 and SR18). Approximately 20% of the fertilizer P added to pastures was recovered as organic P in NaOH‐EDTA extracts at the P1 level of soil P fertility in the 0–10 cm soil layer, and the majority (≈ 65%) of this was detected as the broad phosphomonoester signal. In addition, several specific forms of phosphomonoesters (myo‐ and scyllo‐inositol hexakisphosphate, α‐ and β‐glycerophosphate, and RNA mononucleotides) and phosphodiesters were detected across all soils but at low concentrations. This study shows that phosphate fertilization of pastures primarily results in the accumulation of complex forms of phosphomonoesters rather than that of specific forms of recognizable biomolecules (e.g ., myo‐inositol hexakisphosphate).     

Composition of the organic phosphorus fraction in basidiocarps of saprotrophic and mycorrhizal fungi

In our screening, we aimed to detect phosphonates and other forms of organic phosphorus in basidiocarps and vegetative mycelia of six common basidiomycetes. Organic phosphorus-containing compounds were extracted in alkali and analysed using ³¹P NMR. Monoesters, diesters, pyrophosphates and polyphosphates detected in high amounts reflected the high metabolic activity in basidiocarps (growth, production of basidiospores). Phosphonates were present in all samples, in concentrations ranging from 14 mg kg⁻¹ of the extracted phosphorus in Boletus badius basidiocarp to 140 mg kg⁻¹ in Amanita muscaria vegetative mycelium. Detection of phosphonates in basidiocarps together with our previous evidence from laboratory experiments support the fungal production of natural phosphonates in forest ecosystems.     

Extractability and Transformation of Copper and Zinc Added to Tropical Savanna Soil under Long-Term Pasture

The extractability and slow reactions of copper (Cu) and zinc (Zn) in a weathered savanna soil under Brachiaria decumbens, Digitaria smutsii, and Stylosanthes guianensis pastures were determined in a laboratory incubation study using a sequential extraction to remove operationally defined fractions of the metals, consisting of exchangeable, organically bound, precipitate [occluded in aluminum (Al) and iron (Fe) oxides], and residual metal fractions. The soils from the pasture fields were spiked with 100 mg Cu kg^–1 soil and 200 mg Zn kg^–1 soil for 24 weeks. Copper and Zn extractable with 1 N potassium nitrate (KNO₃) solution decreased exponentially with time but reached a steady state after 2–3 weeks. The concentrations of Cu and Zn exchangeable with potassium (K) were greater in the Digitaria smutsii field soil than Brachiaria decumbens and Stylosanthes guianensis field soils. The exchangeability of added Cu and Zn (indexed Mⁿ⁺ _(exch)) with time was described by a simple exponential decay equation: Mⁿ⁺ _(exch) = αe^βt, where α is a constant, β is a coefficient that defines the rate of transformation of added Cu and Zn from the exchangeable to nonexchangeable pools, and t is time. The β values for Cu (0.040–0.076 mg kg^–1 d^–1) were almost 10 times greater than those of Zn (0.005–0.007 mg kg^–1 d^–1). Sequential extraction of added Cu and Zn indicated that between 26 and 30% of the total Cu and between 19 and 30% of the total Zn were associated with organic matter. Similarly, between 35 and 38% of total Cu and between 47 and 60% of total Zn were associated with Fe, Al, and manganese (Mn) oxides. The differential capacity of the pasture fields to transform added Cu and Zn from exchangeable and labile form to nonlabile and nonexchangeable form appears to be governed by organic matter (OM), pH, and active Fe ratio in the pasture field soils.     

Land-use and fertilization effects on P forms in two European soils: resin extraction and 31P-NMR analysis

Macroporous anion-exchange resin extraction and ³¹P-NMR spectroscopy of dialysed NaOH extracts were used to investigate the effects of land use (Taubenberg, Bavaria: spruce forest, deciduous forest, permanent grass, arable) and fertilization (Askov, Denmark: unmanured, mineral fertilizer, animal manure) on forms of phosphorus in soil with emphasis on the potentially labile organic (P_o) and inorganic (P_i pools. Carbon content ranged from 12.5–118.1 g kg^?1 and total P (P_i) content from 511 to 2063 mg kg^?1. For all soils, the C:P_o ratios of SOM decreased in the order: whole soil, 150: 1–44:1; alkali extract, 57:1–41: 1; resin extract, 20:1–9:1; suggesting an increasing P functionality of the OM with increasing P_o lability. Analysis of functional relation showed a close relation between resin P_o and ³¹P-NMR estimates for diester-P including teichoic acid-P, indicating that these species contributed significantly to the labile P_o pool as determined by the resin method. The most marked effects of land-use were an increase in P_i under grass and arable, a concurrent sequestration of P_o and SOM under grass, and a depletion of P_o under arable. The amount of resin P_j appeared to be a function primarily of fertilization, and amounted to around 100 mg kg^?1 in the fertilized soils irrespective of the SOM content and P source. The forest soil and the unfertilized agricultural soil had much smaller resin P_j values. The soil under grass had the largest amounts in resin P_o and diester-P including teichoic acid-P, indicating a rapid turnover of P_o with build-up of a large potentially labile, microbially derived P_o pool. ³¹P NMR also showed large proportions of labile P_o species in soils where microbial activity is restrained by acidity (Taubenberg spruce forest, phosphonates) or where highly microbially altered OM is relatively enriched (Taubenberg arable, diester-P including teichoic acid-P). We conclude that the resin used in this study isolates a structurally and functionally reasonably uniform pool of potentially labile soil P_o.     

Combined application of inoculant,phosphorus and organic manure improves grain yield of cowpea

ABSTRACT

Low concentrations of P and organic manure in savanna soils limit cowpea response to rhizobia. The study was conducted to determine the combined effect of P and organic manure on cowpea response to rhizobia in a factorial experiment arranged in randomized complete block design with three replications on smallholder farmers’ fields in northern Ghana in 2015. The factors were two levels of Bradyrhizobium inoculant, two levels of P fertilizer, three treatments of manure (fertisoil, cattle manure, and no manure). Addition of Bradyrhizobium inoculant to P and fertisoil significantly increased shoot biomass yield from 1677 kg ha^?1 in the plots without Bradyrhizobium inoculation to 1913 kg ha^?1. Likewise, the addition of Bradyrhizobium inoculant to P and cattle manure significantly increased shoot biomass from 1437 kg ha^?1 to 1813 kg ha^?1. Grain yield increases of 1427 and 1278 kg ha^?1 were obtained over the control when either fertisoil or cattle manure and P, respectively, were added to Bradyrhizobium inoculant. The value cost ratio for adding Bradyrhizobium inoculant to phosphorus and fertisoil was two indicating that it could be attractive to risk-averse smallholder farmers. The study demonstrated the potential of the combined application of organic matter and P to improve cowpea response to Bradyrhizobium inoculation.     

Nitrate leaching loss under annual and perennial pastures with and without lime on a duplex (texture contrast) soil in humid southeastern Australia

Mineral N accumulates in autumn under pastures in southeastern Australia and is at risk of leaching as nitrate during winter. Nitrate leaching loss and soil mineral N concentrations were measured under pastures grazed by sheep on a duplex (texture contrast) soil in southern New South Wales from 1994 to 1996. Legume (Trifolium subterraneum)‐based pastures contained either annual grass (Lolium rigidum) or perennial grasses (Phalaris aquatica and Dactylis glomerata), and had a control (soil pH 4.1 in 0.01 m CaCl₂) or lime treatment (pH 5.5). One of the four replicates was monitored for surface runoff and subsurface flow (the top of the B horizon), and solution NO₃^– concentrations. The soil contained more mineral N in autumn (64–133 kg N ha^?1 to 120 cm) than in spring (51–96 kg N ha^?1), with NO₃^– comprising 70–77%. No NO₃^– leached in 1994 (475 mm rainfall). In 1995 (697 mm rainfall) and 1996 (666 mm rainfall), the solution at 20 cm depth and subsurface flow contained 20–50 mg N l^?1 as NO₃^– initially but < 1 mg N l^?1 by spring. Nitrate‐N concentrations at 120 cm ranged between 2 and 22 mg N l^?1 during winter. Losses of NO₃^– were small in surface runoff (0–2 kg N ha^?1 year^?1). In 1995, 9–19 kg N ha^?1 was lost in subsurface flow. Deep drainage losses were 3–12 kg N ha^?1 in 1995 and 4–10 kg N ha^?1 in 1996, with the most loss occurring under limed annual pasture. Averaged over 3 years, N losses were 9 and 15 kg N ha^?1 year^?1 under control and limed annual pastures, respectively, and 6 and 8 kg N ha^?1 year^?1 under control and limed perennial pastures. Nitrate losses in the wet year of 1995 were 22, 33, 13 and 19 kg N ha^?1 under the four respective pastures. The increased loss of N caused by liming was of a similar amount to the decreased N loss by maintaining perennial pasture as distinct from an annual pasture.     

Phosphorus fractions and dynamics in surface earthworm casts under native and improved grasslands in a Colombian savanna Oxisol

The objective of this study was to assess the effect of a native anecic species on phosphorus availability in an Oxisol characterised by a low chemical fertility. Experiments were carried out at Carimagua research station in a representative site of the isohyperthermic savannas on the Colombian Orinoco basin. One field study and two laboratory/incubation studies were performed in a natural herbaceous savanna and a Brachiaria decumbens and Pueraria phaseoloides pasture. In the laboratory, experiment pots were prepared containing soil collected from the respective field paddock's topsoil. Total P content was higher in earthworm casts than in the surrounding soil in field samples, 50% in native savanna soil and more than 100% in pasture soil. In casts produced under laboratory conditions this increase was relatively low (10-20%). Under field conditions, almost without exception, all P fractions were increased in casts relative to the original soil (corresponding to the increase in total P content), being relatively greater in the labile inorganic P fractions. In addition, samples from the natural savanna showed that pH of casts was higher (5.2) than that of soil (4.6) in both field and laboratory samples. Except in the native savanna under field conditions, the phosphatase activity was reduced in casts by 16.7 to 44%. From our results we conclude that earthworms in the field incorporate P from litter or other organic sources (i.e. undecomposed plant and root material, earthworm faeces) which is not normally measured in the analysis of bulk soil.     

Effect of continuous compost application on humus composition and nitrogen fertility of soils in a field subjected to double cropping

We investigated the effect of continuous compost application on humus composition and N fertility of soils in a field subjected to double cropping (paddy rice and barley) for 25 years. Soil samples were collected from three different plots: (a) No-NF, fertilizer containing P and K but no N; (b) F, fertilizer containing N, P, and K; and (c) F+C, fertilizer plus compost. The amounts of total humus, extracted humus, and humic and fulvic acids increased in the order No-NF<F≪F+C. The amounts of humic and fulvic acids were 2.7 and 1.7 times larger in the F+C plot than in the F plot, respectively. The degree of humification of the humic acids decreased in the order No-NF<F<F+C. The absorption curves and ¹³C-NMR spectra (TOSS method) of the humic acids indicated the presence of lignin-like structure, and its degree was the strongest in the F+C plot. The ¹³C-NMR spectra showed distinct differences in the distribution of carbon species between humic and fulvic acids. In humic acids, the content of aromatic-C, ranging from 37 to 44%, was the highest among carbon species. In fulvic acids, the content of O-alkyl-C, ranging from 45 to 51%, was the highest. The amounts of phosphate buffer-extractable N (PEON) and total N (TN) increased in the order No-NF<F<F+C. The amounts of PEON and TN were 1.2 and 1.7 times larger in the F+C plot than in the F plot, respectively. Present and previous findings indicated that continuous compost application could improve various properties of soils in a field subjected to long-term double cropping.     

Phosphorus speciation and colloidal phosphorus responses to short-term cessation of fertilization in a lime concretion black soil

Long-term excessive application of mineral fertilizer has led to soil acidification and phosphorus(P) accumulation, increasing the risk of P loss and environmental pollution, and cessation of fertilization is widely considered as a cost-effective management strategy to relieve this situation; however, how such cessation influences P speciation and concentrations in a bulk soil and colloidal fractions and whether decreasing P concentration might maintain soil fertility remain unclear. In this stu...     

Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador

Tropical regions are currently undergoing remarkable rates of land use change accompanied by altered litter inputs to soil. In vast areas of Southern Ecuador forests are clear cut and converted for use as cattle pastures. Frequently these pasture sites are invaded by bracken fern, when bracken becomes dominant pasture productivity decreases and the sites are abandoned. In the present study implications of invasive bracken on soil biogeochemical properties were investigated. Soil samples (0-5 cm) were taken from an active pasture with Setaria sphacelata as predominant grass and from an abandoned pasture overgrown by bracken. Grass (C₄ plant) and bracken (C₃ plant) litter, differing in C:N ratio (33 and 77, respectively) and lignin content (Klason-lignin: 18% and 45%, respectively), were incubated in soils of their corresponding sites and vice versa for 28 days at 22 °C. Unamended microcosms containing only the respective soil or litter were taken as controls. During incubation the amount of CO₂ and its δ¹³C-signature were determined at different time intervals. Additionally, the soil microbial community structure (PLFA-analysis) as well as the concentrations of KCl-extractable C and N were monitored. The comparison between the control soils of active and abandoned pasture sites showed that the massive displacement of Setaria-grass by bracken after pasture abandonment was characterized by decreased pH values accompanied by decreased amounts of readily available organic carbon and nitrogen, a lower microbial biomass and decreased activity as well as a higher relative abundance of actinomycetes. The δ¹³C-signature of CO₂ indicated a preferential mineralization of grass-derived organic carbon in pasture control soils. In soils amended with grass litter the mineralization of soil organic matter was retarded (negative priming effect) and also a preferential utilization of easily available organic substances derived from the grass litter was evident. Compared to the other treatments, the pasture soil amended with grass litter showed an opposite shift in the microbial community structure towards a lower relative abundance of fungi. After addition of bracken litter to the abandoned pasture soil a positive priming effect seemed to be supported by an N limitation at the end of incubation. This was accompanied by an increase in the ratio of Gram-positive to Gram-negative bacterial PLFA marker. The differences in litter quality between grass and bracken are important triggers of changes in soil biogeochemical and soil microbial properties after land use conversion.     

Biomass,nodulation and N2 fixing response by subclover and pink serradela to phosphorus fertilization

In Mediterranean areas, sustainable production of rainfed pastures is associated with plant species which withstand low soil fertility and variable annual drought, phosphorus (P) being the primary limiting nutrient for legume growth and N₂ fixation. A study was undertaken under environmentally controlled conditions to compare the response of subclover and pink serradela to P supply using a moderately acid soil, very poor in available P, in terms of biomass, nodulation pattern, N₂ fixation and P use efficiency. A P dose in the range of 30–60 kg P ha^?1 is recommended as a starter for pastures under the present conditions. A higher P rate reduced the fixing nodule number and biomass, and the N₂ fixation rate. Fixed N₂ per unit of nodule P by pink serradela as monocrop was about 80% greater than the sole subclover, demonstrating its higher efficiency on the use of nodule P. Thus, pink serradela needs a lower P rate to produce the same amount of biomass per unit area as sole subclover. Long-term studies in field conditions, using these and other new pasture legume species are needed to consolidate the present findings.     

The ability of the DGT soil phosphorus test to predict pasture response in Australian pasture soils – a preliminary assessment

Diffusive gradients in thin‐films (DGT) technology provides an alternative assessment of available phosphorus (P) for a range of crops, suggesting a preliminary examination of the performance of the new DGT‐P test, compared to existing bicarbonate extractable Olsen and Colwell P tests, for pastures is justified. This study utilized historic data from the Australian National Reactive Phosphate Rock (NRPR) study (1992–1994) that included 25 experimental sites representing a wide range of soil types and climates used for pasture production. Stored (~19 yr) soil samples were analysed for DGT‐P, Olsen P and a single point P buffering index (PBI) and re‐analysed for Colwell P. Results showed the traditional bicarbonate extractable Colwell (r² = 0.45, P < 0.001) and Olsen P (r² = 0.27, P < 0.001) methods predicted relative pasture P response more accurately, compared to the novel DGT‐P test (r² = 0.09, P = 0.03) when all 3 yr of data were examined. We hypothesize that the harsher bicarbonate extraction used for the Olsen and Colwell methods more accurately reflects the ability of perennial pasture roots to access less labile forms of P, in contrast to the DGT‐P test, which does not change the soil pH or dilute the soil and appears unable to fully account for a plants ability to solubilize P. Further studies are needed to compare the capacity of DGT‐P to measure P availability in perennial pasture systems and to better understand the soil chemical differences between pasture and cropping systems.     

Transformations of chemically fixed liquid anhydrous ammonia by soil microorganisms

Summary The application of liquid anhydrous NH₃ to soil leads to chemical fixation of NH₃ by organic matter and of NH _inf4 ^sup+ by clay minerals. A laboratory study was conducted to ascertain the biological transformations of newly fixed liquid anhydrous ¹⁵NH₃ in a Drummer silty clay loam by incubation of the ¹⁵N-labelled soil with glucose for 0, 7, 30, and 90 days and by sequential extraction of organic-matter-fixed ¹⁵NH₃ with 0.15 M Na₄P₂O₇, 0.15 M KOH, 0.1 M NaOH, and acidified dimethyl sulfoxide. About 16% of the ¹⁵NH₃ injected was fixed, of which 52% was accounted for by clay fixation. The various humic fractions (fulvic acid, humic acid, and humin) were obtained, and the distribution patterns of the fixed ¹⁵NH₃-N in these fractions were compared. The potential availability of the fixed ¹⁵NH₃-N was also estimated. The percentage of the ¹⁵NH₃ recovered as organic-matter-fixed ¹⁵NH₃ decreased as the length of incubation increased (to 28% after 90 days); the decrease was attributed in part to an increase in the amount recovered as clay-fixed NH _inf4 ^sup+ (from 52 to 64%). Changes in the distribution of the organic-matter-fixed ¹⁵NH₃-N in the humic fractions included: (1) an increase in the relative amount of the fixed ¹⁵NH₃ as humic acid in both the Na₄P₂O₇ and KOH extracts, (2) an increase in the percentage of organic-matter-fixed ¹⁵NH₃-N in the fulvic acid fractions as high-molecular-weight components (determined by dialysis) or as generic fulvic acid (determined by sorption-desorption from XAD-8 resin), and (3) an increase in the percentage of the organic-matter-fixed ¹⁵NH₃ as humin. The potential availability of the organic-matter-fixed ¹⁵NH₃-N decreased as the length of the incubation increased, from 22 to 4% over the 90-day incubation period, and was correlated significantly (0.05 level) with Na₄P₂O₇-extractable N. These results suggest that organic-matter-fixed liquid anhydrous NH₃ is initially more labile than the native soil N but becomes less labile with time.