XANES Demonstrates the Release of Calcium Phosphates from Alkaline Vertisols to Moderately Acidified Solution
Calcium phosphate (CaP) minerals may comprise the main phosphorus (P) reserve in alkaline soils, with solubility dependent on pH and the concentration of Ca and/or P in solution. Combining several techniques in a novel way, we studied these phenomena by progressively depleting P from suspensions of two soils (low P) using an anion-exchange membrane (AEM) and from a third soil (high P) with AEM together with a cation-exchange membrane. Depletions commenced on untreated soil, then continued as pH was manipulated and maintained at three constant pH levels: the initial pH (pHi) and pH 6.5 and 5.5. Bulk P K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the main forms of inorganic P in each soil were apatite, a second more soluble CaP mineral, and smectite-sorbed P. With moderate depletion of P at pHi or pH 6.5, CaP minerals became more prominent in the spectra compared to sorbed species. The more soluble CaP minerals were depleted at pH 6.5, and all CaP minerals were exhausted at pH 5.5, showing that the CaP species present in these alkaline soils are soluble with decreases of pH in the range achievable by rhizosphere acidification.
The release of phosphorus in alkaline vertic soils as influenced by pH and by anion and cation sinks
Alkaline Vertisols contain calcium phosphate (CaP) minerals that dissolve in response to both acidification and the depletion of concentration of phosphorus (P) or calcium (Ca) in the soil solution, conditions commonly observed within the rhizosphere. In these soils, reserve-P is defined as the difference between the concentrations of P extracted by 0.5M sodiumbicarbonate and 0.005M sulfuric acid. Tomimic rhizosphere modification we sequentially extracted P from three alkaline Vertisols that contained concentrations of reserve-P ranging from 300 to 6500 mg kg⁻¹ using an anion sink, and a combined anion and cation sink. The extractions commenced on untreated soil, and then three target pH regimes were imposed: 1) maintain the initial pH; 2) incrementally acidify to pH 6.5 then maintain; and 3) incrementally acidify to pH 5.5 then maintain. Extractable P increased with decreasing solution pH in all soils until the acid soluble P was depleted. In each soil and at each pH level,more P was extracted when the combined sink was used compared with the anion sink alone. The release of acid soluble-P in these soils was indicative of CaP minerals of varying thermodynamic stability. In addition to the relatively constant concentrations of P extracted at the initial pH with the anion sink, moderate acidification to pH 6.5 released 9% of the reserve-P in the high P soil, but this varied from 18 to 33% in the two other soils containing lower soil P. These findings show that the release of P in alkaline soils beyond that measured by a bicarbonate extractant is influenced by the modification of soil pH and by the removal of Ca from the solution, which has implications for plant availability and response to added fertiliser. Further research is needed to identify the individual species of soil P that are involved in buffering the soil solution, and what their potential availability is to plants via rhizosphere modification.
A Preliminary Investigation of the Potential of Sentinel-1 Radar to Estimate Pasture Biomass in a Grazed Pasture Landscape
Knowledge of the aboveground biomass (AGB) of large pasture fields is invaluable as it assists graziers to set stocking rate. In this preliminary evaluation, we investigated the response of Sentinel-1 (S1) Synthetic Aperture Radar (SAR) data to biophysical variables (leaf area index, height and AGB) for native pasture grasses on a hilly, pastoral farm. The S1 polarimetric parameters such as backscattering coefficients, scattering entropy, scattering anisotropy, and mean scattering angle were regressed against the widely used morphological parameters of leaf area index (LAI) and height, as well as AGB of pasture grasses. We found S1 data to be more responsive to the pasture parameters when using a 1 m digital elevation model (DEM) to orthorectify the SAR image than when we employed the often-used Shuttle Radar Topography 30 m and 90 m Missions. With the 1m DEM analysis, a significant quadratic relationship was observed between AGB and VH cross-polarisation (R2 = 0.71), and significant exponential relationships between polarimetric entropy and LAI and AGB (R2 = 0.53 and 0.45, respectively). Similarly, the mean scattering angle showed a significant exponential relationship with LAI and AGB (R2 = 0.58 and R2 = 0.83, respectively). The study also found a significant quadratic relationship between the mean scattering angle and pasture height (R2 = 0.72). Despite a relatively small dataset and single season, the mean scattering angle in conjunction with a generalised additive model (GAM) explained 73% of variance in the AGB estimates. The GAM model estimated AGB with a root mean square error of 392 kg/ha over a range in pasture AGB of 443 kg/ha to 2642 kg/ha with pasture LAI ranging from 0.27 to 1.87 and height 3.25 cm to 13.75 cm. These performance metrics, while indicative at best owing to the limited datasets used, are nonetheless encouraging in terms of the application of S1 data to evaluating pasture parameters under conditions which may preclude use of traditional optical remote sensing systems.
The influence of land use and management on soil carbon levels for crop-pasture systems in Central New South Wales, Australia
Changes in land use and management have been proposed as a way to increase soil organic carbon (SOC) in crop and pasture systems. Some of the proposed activities to improve SOC are the introduction of pasture phases in cropping systems, stubble retention, no-till cropping, improved fertilisation, introduction of more productive pasture species and grazing management. There is also growing interest in novel farming systems, such as pasture cropping (intercropping cereal crops with established perennial pastures), which may improve SOC. However, there have been few broad scale surveys to determine whether these land management changes have an impact on commercial farms. In this study, comparisons of land use were established for mixed farming and pasture cropping systems in the slopes region (average annual rainfall: 500-650mm) and for cropping and pasture in the plains region (average annual rainfall: 300-500mm) of Central West NSW, Australia. The survey aimed to determine the difference in SOC stocks (Mg C ha-1) and the composition of three soil organic carbon fractions (particulate - POC, humus - HUM and resistant - ROC). The influences of management actions and pasture composition were also assessed across pasture and cropping land uses. Cropping systems had lower SOC stocks in the soil than pasture systems in each region, but pasture cropping was not different from perennial pasture. Generally, there were larger differences in the POC due to land use and management than the other SOC fractions. Management practices in cropping systems explained greater variability in SOC than in pastures. For cropping systems, higher amounts of P fertiliser were associated with higher SOC, POC and ROC while higher amounts of N fertiliser were associated with lower SOC, POC and ROC. For pastures, the proportion of bare ground was associated with lower SOC and POC. These associations indicate there is an opportunity to increase SOC by converting cropping land to permanent pasture, increasing the frequency of pasture phases, changing crop fertiliser regimes and reducing bare ground in pastures, but further work is needed to verify the causality behind these associations.
Transformation of Calcium Phosphates in Alkaline Vertisols by Acidified Incubation
Acid-soluble soil phosphorus (P) is a potential resource in P-limited agricultural systems that may become critical as global P sources decrease in the future. The fate of P in three alkaline Vertisols, a major agricultural soil type, after acidic incubation was investigated using synchrotron-based K-edge X-ray absorption near-edge structure (XANES) spectroscopy, geochemical modeling, wet chemistry soil extraction, and a P sorption index. Increases in labile P generally coincided with decreased stability and dissolution of calcium phosphate (CaP) minerals. However, only a minor proportion of the CaP dissolved in each soil was labile. In two moderate-P soils (800 mg P kg–1), XANES indicated that approximately 160 mg kg–1 was repartitioned to sorbed phases at pH 5.1 of one soil and at pH 4.4 of the second; however, only 40 and 28% were labile, respectively. In a high-P soil (8900 mg P kg–1), XANES indicated a decrease in P of 1170 mg kg–1 from CaP minerals at pH 3.8, of which approximately only 33% was labile. Phosphorus mobilized by agricultural practices without concurrent uptake by plants may be repartitioned to sorbed forms that are not as plant-available as prior to acidification.
Estimating pasture biomass with active optical sensors
We investigated relationship between pasture biomass and measures of height and NDVI (normalised difference vegetation index). The pastures were tall fescue (Festuca arundinacea), perennial ryegrass (Lolium perenne), and phalaris (Phalaris aquatica) located in Tasmania, Victoria and in the Northern Tablelands of NSW, Australia. Using the Trimble® GreenSeeker® Handheld active optical sensor (AOS) to measure NDVI, and a rising plate meter, the optimal model to estimate green dry biomass (GDM) during two years was a combination of NDVI and falling plate height index. The combined index was significantly correlated with GDM in each region during winter and spring (r² = 0.62-0.77, P <0.001). Regional calibrations provided a smaller error in estimates of green biomass, required for potential application in the field, compared to a single overall calibration. Data collected in a third year will be used to test the accuracy of the models.
Crop choice and planting time for upland crops in Northwest Cambodia
Crop yields are declining in Northwest Cambodia and crop failure in the pre-monsoon season is commonplace with 69% of farmers surveyed stating that drought is a constraint to production. Farmers currently lack knowledge to adopt more sustainable farming practices. A trial was conducted in Samlout District, Battambang Province, Northwest Cambodia to investigate the feasibility of a sowing time two months later than typical local practices. The aim of the shift in sowing time was to increase crop yield and reduce crop failure due to heat and drought stress throughout the season. A secondary aim was to compare sequences of continuous maize ('Zea mays' L.), and maize in rotation with peanut ('Arachis hypogaea'), sun- flower ('Helianthus annus'), sorghum ('Sorghum bicolor'), cowpea ('Vigna unguiculata') or mungbean ('Vigna radiata'). Sunflower and sorghum would be new crop type introductions, whilst the other crops are part of the traditional farming system in this region. The trial was undertaken for four cropping seasons over two years, during which time crops produced successful yields from the new sowing time windows. However, it was the maize-sunflower sequence that produced the highest gross margins. Maize-sunflower returns were $514 per hectare per annum more than the typical planting of continuous maize, and over $1100 per hectare per year higher than the other maize-legume and maize-sorghum rotations. Continuous maize produced the most stable yields across the four seasons and maize-sunflower produced the second highest mean yield. Results from modelling of soil moisture suggest that a shift in sowing time may avoid the extreme heat of the pre-monsoon season, and align crop growth stages with periods of more reliable rainfall. Site specific surface soil moisture data and rainfall was entered into the APSIM model to predict the soil profile moisture throughout the growing season (r² = 0.73). Both the modelling and on-farm research resulted in higher crop yields compared with traditional practices and expectations, and a low probability of crop failure. Crops of maize, sunflower and sorghum grew well from an early October sowing date into the post monsoon season and produced good yields on stored soil water with low plant stress due to mild seasonal conditions. Delayed sowing may prove to be the best option for farmers in the Northwest upland, achieved by a simple shift of sowing dates.
Incremental acidification reveals phosphorus release dynamics in alkaline vertic soils
Phosphorus (P) available to plants in alkaline, vertic soils is thought to be buffered by the dissolution of various calcium phosphate minerals (CaP), driven by pH and the concentration of Ca and/or P in solution. To investigate this hypothesis we incrementally acidified 6 alkaline vertic soils of CaP 300-6000 mg kg⁻¹ in the presence or absence of an anion exchange P sink. Following the early removal of solution and sorbed P sources, P recovery remained low until soil pH passed key thresholds. These thresholds varied little between soils and with the sink (pH 6.0-6.3), and soil pH buffer capacity affected the amount of acid required to approach the thresholds. Dissolution of CaP species occurred 0.7-1.0 pH units higher where the solution P concentration was kept below 1 μM using the anion exchange membrane sink compared to acidification without the sink. The data support the hypothesis that rhizosphere acidification may increase the availability of CaP minerals to plants; however, the dependence of P release dynamics on pH buffering capacity may put P release beyond the reach of some plant species. Consequently, research is necessary to quantify both plant acidification potential in these soils and the effect of concomitant removal of Ca on release of CaP species into plant available forms.