Western Australian Soils Research Articles

Biogeographic distribution and molecular epidemiology of Clostridioides (Clostridium) difficile in Western Australian soils.

Clostridioides (Clostridium) difficile is a leading cause of infectious diarrhea in humans and production animals and can be found in a variety of environmental sources. The prevalence and diversity of multi-locus sequence type clade 5 strains of C. difficile in Australian production animals suggest Australia might be the ancestral home of this lineage of One Health importance. To better understand the role of the environment in the colonization of humans and animals in Australia, it is important to investigate these endemic sources. This study describes the prevalence, molecular epidemiology, and biogeographic distribution of C. difficile in soils of Western Australia. A total of 321 soil samples from remote geographical locations across the eight health regions of Western Australia were screened for C. difficile and isolates characterized by PCR ribotyping and toxin gene profiling. C. difficile was isolated from 31.15% of samples, with the highest prevalence in the Perth Metropolitan Health Region (49.25%, n = 33/67). Overall, 52 different strains [PCR ribotypes (RTs)] were identified, with 14 being novel, and 38% (38/100) of isolates being toxigenic, the most common of which was RT014/020. Five unique novel isolates showed characteristics similar to C. difficile clade 5. This is the first study of C. difficile isolated from soils in Australia. The high prevalence and heterogeneity of C. difficile strains recovered suggest that soils play a role in the survival and environmental dissemination of this organism, and potentially its transmission among native wildlife and production animals, and in community and hospital settings.IMPORTANCEClostridium difficile is a pathogen of One Health importance. To better understand the role of the environment in human and animal colonization/infection, it is critical that autochthonous reservoirs/sources of C. difficile be investigated. This is the first study of C. difficile isolated from soils of Western Australia (WA). Here, the ecology of C. difficile in WA is described by examining the geographic distribution, molecular epidemiology, and diversity of C. difficile isolated from soils across WA.

Bradyrhizobium agreste sp. nov., Bradyrhizobium glycinis sp. nov. and Bradyrhizobium diversitatis sp. nov., isolated from a biodiversity hotspot of the genus Glycine in Western Australia.

Strains of the genus Bradyrhizobium associated with agronomically important crops such as soybean (Glycine max) are increasingly studied; however, information about symbionts of wild Glycine species is scarce. Australia is a genetic centre of wild Glycine species and we performed a polyphasic analysis of three Bradyrhizobium strains—CNPSo 4010T, CNPSo 4016T, and CNPSo 4019T—trapped from Western Australian soils with Glycine clandestina, Glycine tabacina and Glycine max, respectively. The phylogenetic tree of the 16S rRNA gene clustered all strains into the Bradyrhizobium japonicum superclade; strains CNPSo 4010T and CNPSo 4016T had Bradyrhizobium yuanmingense CCBAU 10071T as the closest species, whereas strain CNPSo 4019T was closer to Bradyrhizobium liaoningense LMG 18230T. The multilocus sequence analysis (MLSA) with five housekeeping genes—dnaK, glnII, gyrB, recA and rpoB—confirmed the same clusters as the 16S rRNA phylogeny, but indicated low similarity to described species, with nucleotide identities ranging from 93.6 to 97.6% of similarity. Considering the genomes of the three strains, the average nucleotide identity and digital DNA–DNA hybridization values were lower than 94.97 and 59.80 %, respectively, with the closest species. In the nodC phylogeny, strains CNPSo 4010T and CNPSo 4019T grouped with Bradyrhizobium zhanjiangense and Bradyrhizobium ganzhouense, respectively, while strain CNPSo 4016T was positioned separately from the all symbiotic Bradyrhizobium species. Other genomic (BOX-PCR), phenotypic and symbiotic properties were evaluated and corroborated with the description of three new lineages of Bradyrhizobium. We propose the names of Bradyrhizobium agreste sp. nov. for CNPSo 4010T (=WSM 4802T=LMG 31645T) isolated from Glycine clandestina, Bradyrhizobium glycinis sp. nov. for CNPSo 4016T (=WSM 4801T=LMG 31649T) isolated from Glycine tabacina and Bradyrhizobium diversitatis sp. nov. for CNPSo 4019T (=WSM 4799T=LMG 31650T) isolated from G. max.

Evaluation of the dynamic cone penetrometer to detect compaction in ripped soils

Land degradation due to compaction is a critical issue facing 21st century agriculture. Deep ripping is a popular solution to remediate compacted Western Australian soils. However, these soils are particularly susceptible to recompaction under vehicle traffic: reliable methods to detect and monitor compaction are therefore needed to inform remediation strategies.Cone penetrometer testing (CPT) is a popular method to detect compaction under vehicle traffic in a range of soil conditions. However, traditional CPT equipment is unsuitable for large-scale use due to its expense and bulk. Dynamic penetrometers circumvent this issue by being inexpensive and man-portable. Such devices have seen recent success in determining properties of soft geotechnical materials but little is known of their performance in ripped soils. This study evaluated the ability of the “PANDA 2” dynamic penetrometer to detect compaction in ripped soils after the passage of aMassey Ferguson four-tonne tractor, which was typical of vehicles used at the test site. Two test sites of contrasting soil types were identified which had previously been ripped and left fallow and untrafficked for several years. Penetration resistance was measured along a high-resolution grid prior to trafficking and after one and five vehicle passes and compared to results from trial pits. Laboratory testing also examined the device's accuracy at shallow depths under controlled conditions. Results showed that the PANDA 2 was able to detect significant changes in penetration resistance after trafficking. However, several limitations on the device's use when interpreting field data were identified. Based on the findings of this study, dynamic penetrometers are not recommended to monitor compaction in ripped soilsfor the weight of vehicle used here. However, the devices may be of use when examining the passage of heavier vehicles.

Assembly and transfer of tripartite integrative and conjugative genetic elements

Integrative and conjugative elements (ICEs) are ubiquitous mobile genetic elements present as "genomic islands" within bacterial chromosomes. Symbiosis islands are ICEs that convert nonsymbiotic mesorhizobia into symbionts of legumes. Here we report the discovery of symbiosis ICEs that exist as three separate chromosomal regions when integrated in their hosts, but through recombination assemble as a single circular ICE for conjugative transfer. Whole-genome comparisons revealed exconjugants derived from nonsymbiotic mesorhizobia received three separate chromosomal regions from the donor Mesorhizobium ciceri WSM1271. The three regions were each bordered by two nonhomologous integrase attachment (att) sites, which together comprised three homologous pairs of attL and attR sites. Sequential recombination between each attL and attR pair produced corresponding attP and attB sites and joined the three fragments to produce a single circular ICE, ICEMcSym1271 A plasmid carrying the three attP sites was used to recreate the process of tripartite ICE integration and to confirm the role of integrase genes intS, intM, and intG in this process. Nine additional tripartite ICEs were identified in diverse mesorhizobia and transfer was demonstrated for three of them. The transfer of tripartite ICEs to nonsymbiotic mesorhizobia explains the evolution of competitive but suboptimal N2-fixing strains found in Western Australian soils. The unheralded existence of tripartite ICEs raises the possibility that multipartite elements reside in other organisms, but have been overlooked because of their unusual biology. These discoveries reveal mechanisms by which integrases dramatically manipulate bacterial genomes to allow cotransfer of disparate chromosomal regions.

Ancient landscapes and the relationship with microbial nitrification.

Ammonia oxidizing archaea (AOA) and bacteria (AOB) drive nitrification and their population dynamics impact directly on the global nitrogen cycle. AOA predominate in the majority of soils but an increasing number of studies have found that nitrification is largely attributed to AOB. The reasons for this remain poorly understood. Here, amoA gene abundance was used to study the distribution of AOA and AOB in agricultural soils on different parent materials and in contrasting geologic landscapes across Australia (n = 135 sites). AOA and AOB abundances separated according to the geologic age of the parent rock with AOB higher in the more weathered, semi-arid soils of Western Australia. AOA dominated the younger, higher pH soils of Eastern Australia, independent of any effect of land management and fertilization. This differentiation reflects the age of the underlying parent material and has implications for our understanding of global patterns of nitrification and soil microbial diversity. Western Australian soils are derived from weathered archaean laterite and are acidic and copper deficient. Copper is a co-factor in the oxidation of ammonia by AOA but not AOB. Thus, copper deficiency could explain the unexpectedly low populations of AOA in Western Australian soils.

Relationship between Metals Leached and Soil Type from Potential Acid Sulphate Soils under Acidic and Neutral Conditions in Western Australia

Metal contaminants are likely to be mobilised from soil when in contact with acidic drainage. Soils containing sulphide are often associated with significant quantities of trace metals. Understanding the source of metal pollution is of significant concern for management because sulphide-containing soils are found in close proximity to estuaries, lowland rivers and lakes. This study focuses on Western Australian soils, which are typically sandy and well weathered. Two leachate trials, a batch and a column method, investigated the possibility of characteristic leachate signatures with respect to future traceability of metal contamination from soil drainage. Leaching signatures were assessed for four soil types (grey sand (GS), iron-rich sand (FeRS), silty sand (SS) and peat) found coastally between Perth and Albany, in Western Australia. Trace metals leached from the four soil types showed significant differences for both the metals leached and the concentrations for the column leaching trial. No nickel (<1.0 µgL−1) but relatively high concentrations of arsenic were leached from the peat site. The FeRS leachates contained chromium, copper, high aluminium and relatively high iron. The GS leached extremely high iron and relatively high manganese concentrations. In comparison, only small concentrations of nickel, iron, aluminium, arsenic and manganese leached from the SS columns. Selenium did not leach above reporting limits (<1.0 µg L−1) from any of the soil types. This work suggests that leachate signatures from different soil types exist and that soil type should be considered when determining the risk of aquatic impact associated with acidic drainage.

서부 호주 밀 재배지역 내 자연보호구역에 서식하는 흰개미 Drepanotermes tamminensis (Hill) 집에 저장된 식물유체의 영양물질 함량

본 연구는 서부 호주의 Durokoppin 자연 보호 구역에 서식하고 있는 흰개미에 의해 수확되어 개미집에 저장되어 있는 식물유체와 영양물질 함량을 조사하기 위해 수행되었다. 개미집에 저장된 식물유체의 경우 두 조사지역간의 유의적 차이는 없었으며, 개미집에 저장된 식물유체를 분류한 결과 잎과 가지 등 7가지 종류의 식물 유체로 확인되었다. 개미집 내 저장된 식물유체의 영양물질 함량은 유의적 차이는 없었지만 shrubland 보다 Woodland가 높았으며, 영양물질 총량 역시 shrubland보다 Woodland가 많았다. 연구결과를 종합해 보면 흰개미는 조사지역에서 다양한 종류의 식물유체를 수확하여 이용하고 있으며, 조사지역내 토양 유기물 함량이 적은 점을 고려할 때 생태계 영양물질 순환에 있어서 중요한 역할을 하고 있는 것으로 사료된다. This study estimated the mound litter biomass and the nutrient content of mound litter in the two study plots in Durokoppin Nature Reserve, Western Australia between 2004 and 2005. There were no significant differences in biomass of litter in individual mounds between the two study plots. Seven components of litter were found in the mounds. The nutrient concentrations were higher in the woodland than in the shrubland plot, although the differences were not statistically tested, and the total amount of each nutrient measured was generally greater in the woodland than in the shrubland plot. The aforementioned results show that D. tamminensis harvests various plant material according to biomass availability. The role of this termite takes on particular importance in view of the fact that Western Australian soils are notoriously impoverished in nutrients.

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl2 solution) varied ten-fold (averaging 2.3 µmol g −1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl2 solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.

Cadmium accumulation in agricultural soils in Western Australia

Phosphatic fertilizers contain appreciable concentrations of cadmium (Cd) which may elevate the Cd concentrations of soils and crops, yet the impact of Cd on poorly buffered Western Australian soils is not known. We measured soil Cd, P and Zn concentrations at different depths and with total (mixed acid) and partial extractants (CaCl2, EDTA and HCl) at sites with a known phosphate fertilizer history. Accumulation of Cd in soils where P fertilizers had been applied varied greatly, with the Cd content of fertilized soils generally having significantly higher Cd contents than for unfertilized soils. In a ferruginous sandy loam, the highest concentrations of Cd were in the top 2 cm and these concentrations increased with increasing amounts of fertilizer application. A gravelly lateritic soil in an orchard contained 3.6 mg Cd kg-1, which was 20-fold higher than the equivalent unfertilized soil. Very sandy horticultural soils which are characteristic of Western Australia did not have higher exchangeable Cd, and showed small increases in total Cd, despite the high amounts of phosphatic fertilizers applied. The largest proportional increases in soil Cd were generally for analyses using partial extractants rather than for total Cd.

Improving soil physical fertility and crop yield on a clay soil in Western Australia

In the low rainfall area of Western Australia, clay soils with massive soil structure form a major part of the area sown to wheat. Yield increases on such soils have been poor in the last decade compared with those on other soil types. An experiment was conducted over 4 years (1997–2000) using a factorial combination of soil ripping to 0.4 m, application of commercial grade gypsum at 2.5 t/ha, and addition of complete nutrients based on soil test each year. All crop residues were retained after harvest and returned to the soil. The experiment was conducted in a wheat–field pea rotation at Merredin, WA. Soil water infiltration rate, soil strength, bulk density, water-stable aggregates, cation exchange capacity, and wheat yields were measured. Grain yields of wheat and field peas were increased by deep ripping, the addition of gypsum, or the addition of complete nutrients in some years. The main treatment effects on yield were additive, as significant interactions between the treatments on yield were seldom found. However, all the main treatments also significantly improved many of the soil physical properties related to crop growth. In 2000, 4 years after the treatments were applied, soil water infiltration rate was increased by more than 200%, strength of the topsoil decreased by around 1600 kPa, and soil bulk density decreased by 20%. Gypsum application increased water-stable aggregates, but soil mixing caused by deep ripping reduced them. The combination of soil ripping and gypsum application in the presence of complete nutrients and annual return of crop residues to the soil is suggested to improve crop grain yield and soil physical fertility on a range of Western Australian soils.

Mineralogy and potassium release from some Western Australian soils and their size fractions

Seven surface horizon soils and their sand, silt, and clay fractions were characterised for mineralogy, and K release by extracting samples with 1 mM CaCl 2 solution daily for 10 days. The low silt content is characteristic of many Western Australian soils, which may provide a partial explanation of the paucity of available K in soils that contain little silt-size primary minerals. The sand and silt fractions were dominated by quartz and contained significant amounts of K-feldspars. The clay fraction was dominated by kaolinite, and some samples contained illite/mica, inhibited vermiculite, and gibbsite. On a per unit weight basis the clay-fraction released the largest amount of K followed in sequence by the silt and sand. The contribution of size fractions to total K release by the soil ranged from 50 to 87%, 2 to 7%, and 10 to 44% for the clay, silt, and sand, respectively. Linear plots of K release versus time 1/2 for the soils, and the sand and silt fractions, indicated that a parabolic diffusion equation adequately describes the K release process. For some clay samples this diffusion controlled kinetic is not strictly obeyed during the initial period of K desorption due to rapid exchange of adsorbed K at sites on external surfaces. The Elovich equation plots show a discontinuity in slope and support the hypothesis of the multireactive nature of K exchange sites for these soils. The parabolic diffusion rate constant closely predicted K supply to plants as it has a close positive relationship (r = 0.99) with total K uptake by ryegrass for 260 days of growth.

Soil factors affecting the availability of potassium to plants for Western Australian soils: a glasshouse study

A glasshouse experiment was conducted with 41 surface and 8 subsurface soils to measure their potassium (K) supply capacities and K depletion of soils by ryegrass growth for 260 days and harvesting at ~40-day intervals. Dry matter yield ranged from 0.22 g to 25.4 g/kg soil, cumulative K uptake ranged from 0.006 to 1.49 cmol/kg soil, and values of K concentration (%) in the first cut herbage ranged from 0.40% to 5.97%. Some of the light-textured soils were so impoverished in K that symptoms of K deficiency appeared during the first growth period. Water-soluble K + exchangeable K accounted for 43–100% of cumulative K uptake by the ryegrass. Multiple regression analysis indicated that 68% of the variation in dry matter yield and 90% of the variation in K uptake may be predicted by the exchangeable K content of these soils. The 6 harvests of ryegrass extracted only 0.21–12.07% of total K from these soils, which was not sufficient to cause discernible mineralogical changes in most soils. For some soils vermiculite was formed at the expense of illite/mica by K release to plants. For soils containing vermiculite but no other K-bearing clay minerals, vermiculite peaks broadened on K depletion by plants. Major proportions of total K in these soils are present in silicate minerals, yet only minor amounts are released to plants by very slow weathering processes. For soils that do not contain any K bearing clay minerals, very minor amounts of feldspar may have dissolved to release K.

Comparison of seven phosphorus sorption indices

Seven methods of estimating or predicting phosphorus (P) sorption capacity were compared for 47 neutral to acid Western Australian soils. Two methods, the P buffer capacity (PBC) and the Fox and Kamprath procedure, provided reliable indices of P sorption from well defined P sorption isotherms, but they are not quick routine methods because several levels of P addition are required. The other five routine procedures included two versions of the P retention index (PRI), determined by adding one level of P, and three soil properties, oxalate extractable iron (oxalate Fe), oxalate extractable aluminium (oxalate Al), and pH measured in sodium fluoride [pH (F)], that are known to indicate P sorption capacity. All the indices were well related to one another. The oxalate Fe index was the least well related to PBC whereas oxalate Al, one of the PRI indices, and pH (F) were closely related to PBC and could be used as quick, economical procedures to assess the P sorption capacity of soils.

The relation between soil water retention and particle size distribution parameters for some predominantly sandy Western Australian soils

The soil water retention curve (WRC) may be estimated from soil texture if there is shape similarity between the normalised cumulative particle mass size distribution curve (PSD) and the WRC. For similar shaped curves, parameters describing the shape of the PSD may also describe the shape of the corresponding WRC. We studied the relation between PSDs and WRCs at 4 sites in Western Australia with predominantly sandy soils and fitted a sigmoidal function to all normalised PSDs and WRCs. The model gave an excellent description of all PSD and WRC data sets. The parameter describing the slope of the WRC was correlated with the slope of the PSD. However, the asymptotic minima of the PSDs differed from the WRCs leading to poor estimates of the WRC residual water contents, θr. This difference was most marked in 2 clay subsoils and resulted in progressively greater errors in prediction of water content with increasing negative pressure head.

Effect of legume-wheat rotation on the water balance, nitrogen dynamics, and soil acidification in Western Australian soils

No abstract available.

Phosphorus adsorption by a range of western Australian soils related to soil properties

Abstract The capacity of 36 Western Australian soils to adsorb phosphorus (P) was measured by three different methods: P retention index (PRI), P buffering capacity (PBC), and P adsorption (PA). The P adsorption values measured by all three methods varied markedly with soil type. When the P adsorption values were correlated with several soil properties, using simple and multiple linear regressions, PRI, PBC, or PA values were found to be significantly correlated with the aluminium oxide content of the soils. In addition, PBC and PRI was correlated with organic carbon content. The role of aluminium oxide (Al2O3) in the soil was apparently more important in determining the P adsorption capacity of the soils than that of iron (Fe), even though the iron oxide (Fe2O3)content of all the soils studied was consistently higher than the aluminium oxide content. The relationship between P adsorption and the selected soil properties, as determined by multiple linear regression, explained 45–59% of the variation:arabicPRI = ‐10.87 + 9.94 organic C (%) + 160.02 Al2O3 (%), r2 = 0.45.arabicPBC = ‐0.004 + 1.532 organic C (%) + 22.26 Al2O3 (%), r2 = 0.57.arabicPA = 3.52 + 248.75 Al2O3 (%), r2 = 0.59.

Changes in the forms of cadmium with time in some Western Australian soils

Changes in the forms of Cd with time could affect its uptake by plants and hence potential toxicity to animals and humans. The effect of time on the forms of native and added Cd was studied in four West Australian soils which differed in their clay, hydrous oxide and organic matter content. Sequential extraction of soluble (KCl), exchangeable (BaCl2), bound to organic matter (NaOCl), bound to oxides/clays (ammonium oxalate) and residual (concentrated acids) forms of Cd was carried out at different time intervals after the addition of Cd. The Cd that was added to the soils transformed with time to less soluble forms; the extent depending upon the type of soil. In addition, the rate of transformation in a particular type of soil was affected by both pH and rate of Cd addition. Soluble cadmium in the sandy soil decreased with time whereas the exchangeable form increased. The extent of the changes increased with increase in pH. In the peaty sand at pH &lt; 5, exchangeable Cd increased apparently at the expense of soluble Cd. At higher pH values, however, Cd bound to organic matter increased with time as exchangeable Cd decreased. In the lateritic podzolic soil (dominated by kaolinite), there was no effect of time on the forms of Cd at pH values &lt;. At pH &gt; 6, the exchangeable form of Cd decreased whereas Cd bound to organic matter and residual Cd increased with time. In the yellow earth (dominated mainly by goethite), soluble Cd decreased with time at pH values &lt; 5 and became a negligible fraction at pH 6. Exchangeable Cd decreased with time at pH values &gt; 5 whereas Cd bound to oxides and residual Cd increased with time at all the pH values.

Sodium-fluoride pH of South-Western Australian soils as an indicator of P-sorption

Phosphate sorption by the surface horizon of 228 acid to neutral Western Australian (W.A.) soils is more closely related (r(2) = 0.76) to the content of oxalate-extractable aluminium than to any other soil constituent. This fraction corresponds to poorly ordered inorganic and organic Al compounds that release considerable amounts of OH- to NaF solution. Thus the abundance of these compounds in soil may be estimated by measurement of the pH of a NaF extract (pH((NaF)) This association enables the rapid and moderately accurate prediction in the field of the P-sorption capacity of soils (r(2) = 0.72) by measuring pH(NaF) With a. simple, portable pH meter. For many W.A. soils, it is probable that well crystalline aluminium and iron oxides, clay minerals and other soil constituents are of secondary importance in determining P-sorption and that most P-sorption is due to poorly ordered and organically complexed forms of Al.

Evaluation of the 137Cs techniques for estimating wind erosion losses for some sandy Western Australian soils

The utility of the caesium-137 (137Cs) technique, for estimating the effects of wind erosion, was evaluated on the soils of a semi-arid agricultural area near Jerramungup, Western Australia. The past incidence of wind erosion was estimated from field observations of soil profile morphology and an existing remote sensing study. Erosion was limited to sandy surfaced soils (0-4% clay), with a highly significant difference (P &lt; 0.0001) in 137Cs values between eroded and non-eroded sandy soils, with mean values of 243 � 17 and 386 � 13 Bq m-2 respectively. Non-eroded soils, with larger clay contents, had a mean 137Cs scontent of 421 � 26 Bq m-2, however, due to considerable variation between replicate samples, this value was not significantly different from that of the non-eroded sands. Hence, although the technique discriminates between eroded and non-eroded areas, the large variation in 137Cs values means that from 27 to 96 replicate samples are required to provide statistically valid estimates of 137Cs loss. The occurrence of around 18% of the total 137Cs between 10 and 20 cm depth in these soils, despite cultivation being confined to the surface 9 cm, suggests that leaching of 137Cs occurs in the sandy soils, although there was no relationship between clay content and 137Cs value for either eroded or non-eroded soils. In a multiple linear regression, organic carbon content and the mean grain size of the eroded soils explained 35% of the variation in 137Cs content. This relationship suggests that both organic carbon and 137Cs are removed by erosion, with erosion being more prevalent on soils with a finer sand fraction. Clay and silt contents do not vary with depth in the near-surface horizons of the eroded sandy soils, hence it is likely that wind erosion strips the entire surface horizon with its 137Cscontent, rather than selectively winnowing fine material.

Effect of P-buffer capacity and P-retention index of soils on soil test-P, soil test P-calibrations and yield response curvature

Twenty-three virgin Western Australian soils of different buffer capacities (BC) for phosphorus (P) were collected. The effects of BC on the relationships between Colwell soil test P and the level of P applied, yield and soil test P, and yield and the level of P applied were studied. Wheat (Triticum aestivum cv. Reeves), grown for 27 days in a glasshouse, was used. Two methods of measuring P sorption of soils, P buffer capacity (PBC) and P retention index (PRI), were used. The PBC is determined from a multi-point sorption curve. The PRI is a new, diagnostic, one-point, sorption method now widely used for commercial soil P testing in Western Australia. Both PBC and PRI produced similar results. The relationship between soil test P and the level of P applied was adequately described by a linear equation. When the slope coefficient of the linear equations was related to PBC or PRI, there was no relationship. The other two relationships were adequately described by a Mitscherlich equation. When the curvature coefficient of the Mitscherlich equation was related to PBC or PRI, the trend was for the value of the coefficient to decrease with increasing PBC or PRI. Consequently, as the capacity of the soil to sorb P increased the trend was for larger soil test P or higher levels of P application to produce the same yield.