A pH-based pedotransfer function for scaling saturated hydraulic conductivity reduction: improved estimation of hydraulic dynamics in HYDRUS

Hydraulic conductivity is a key soil property governing agricultural production and is thus an important parameter in hydrologic modeling. The pH scaling factor for saturated hydraulic conductivity (Ks) reduction in the HYDRUS model was reviewed and evaluated for its ability to simulate Ks reduction. A limitation of the model is the generalization of Ks reduction at various levels of electrolyte concentration for different soil types, i.e., it is not soil specific. In this study, a new generalized linear regression model was developed to estimate Ks reduction for a larger set of Australian soils compared with three American soils. A nonlinear pedotransfer function was also produced, using the Levenberg–Marquardt optimization algorithm, by considering the pH and electrolyte concentration of the applied solution as well as the soil clay content. This approach improved the estimation of the pH scaling factor relating to Ks reduction for individual soils. The functions were based on Ks reduction in nine contrasting Australian soils using two sets of treatment solutions with Na adsorption ratios of 20 and 40; total electrolyte concentrations of 8, 15, 25, 50, 100, 250, and 500 mmolc L−1; and pH values of 6, 7, 8, and 9. A comparison of the experimental data and model outputs indicates that the models performed objectively well and successfully described the Ks reduction due to the pH. Further, a nonlinear function provided greater accuracy than the generalized function for the individual soils of Australia and California. This indicates that the nonlinear model provides an improved estimation of the pH scaling factor for Ks reduction in specific soils in the HYDRUS model and should therefore be considered in future HYDRUS developments and applications

Land amendment for irrigation with coal seam gas water and subsequent rainfall

The demand for Coal Seam Gas (CSG) water as an irrigation resource is increasing in arid and semi-arid regions of Australia. This study investigated the magnitude of impact of rainfall on soil infiltration, dispersion and surface aggregate breakdown following irrigation with untreated CSG water. Additionally, it identifies management strategies for irrigation by untreated CSG water and subsequent rainwater via amending black Vertisol and red Ferrosol with gypsum, elemental sulphur, and Hydroguard-i™ polymer for the Darling Downs region, Australia. Two soils were repacked in soil columns passing 10 or 20 ML/ha of rainwater or untreated CSG water with 340, 220 Kg/ML gypsum and sulphur respectively, and 20 Kg/ha of Hydroguard-iTM to protect the soil surface aggregate stability, or without soil amendments (control). Saturated hydraulic conductivity (Ksat), dispersion, and surface aggregate breakdown observation post rainfall simulation were measured. Soils were severely dispersed with low Ksat for both soils where the untreated CSG water solution was applied to non-amended soils, as would be expected. Although, where soil was amended with gypsum and elemental sulphur, maintained Ksat and aggregate stability indicated that amendments had been successful. Hydroguard-i™ presented a visible protection of soil surface aggregates from mechanical force of raindrop impact, but was considered to be inappropriate for irrigated agriculture lands due to the rapid Ksat decrease in coarser textured soil (Ferrosol); there was no observable influence in fine textured soil (Vertisol). Subsequent leaching with rainwater caused a significant reduction in Ksat with increased leachate pore volumes, causing the soil solid phase to disperse as a result of rapid dilution of the soil solution. In this case the land amendment was exhausted, indicating amendment buffering needs to be considered on rainfall magnitude. Keywords: Soil amendment, rainwater, CSG water, hydraulic conductivity, clay dispersion

A comparison of rehabilitated coal mine soil and unmined soil supporting grazed pastures in south-east Queensland

Land that is disturbed by mining activities is required to undergo suitable rehabilitation. This study compared soils supporting grazed pasture on land that was rehabilitated after coal mining activity with that on unmined land. Pasture biomass, and soil physical and chemical properties important for pasture production and sustainability were intensively monitored on three sites that had completed rehabilitation at different times over the last 10 years, and one unmined control site. A further 18 unmined grazing sites were monitored for benchmarking purposes. Analysis of soil properties of plant available phosphorus and nitrogen, salinity and sodicity in the first year of the study suggested little difference in terms of benefits or constraints to pasture production between the rehabilitated and control sites. Plant-available phosphorus was sufficiently high in the two oldest rehabilitated sites that a fertiliser response would not be expected. Soil depth and the pasture rooting depth at the rehabilitated sites were at the shallow end of the wide range observed across the benchmark and control sites. Higher pasture biomass at the rehabilitated sites compared with the control at the initiation of the trial was attributed more to differences in grazing history than differences in soil attributes

Impact of irrigation wastewater pH on saturated hydraulic conductivity of acidic, neutral, and alkaline Kaolinitic soils

There is an increasing need to use marginal quality water, including industrial treated wastewater and saline and sodic water for irrigating land in arid and semi-arid regions globally. The use of marginal quality of water potentially increases soil structural degradation, decreasing permeability (Bennett 2012; Shainberg et al. 1981), whereby excess sodium (measured as the sodium adsorption ratio, SAR) can result in both intra- and inter-crystalline swelling leading to eventual dispersion (Dang et al. 2018a; Ezlit et al. 2013). Furthermore, this effect can be enhanced or reduced depending on electrical conductivity (EC) of the irrigation water (Dang et al. 2018b). The effect of SAR and EC on saturated hydraulic conductivity (Ks) was classically studied by Quirk and Schofield (1955), with the body of work since identifying that the Ks reduction due marginal quality water depends on soil clay content and mineralogy, soil organic matter, and the electrolyte composition and concentration (Bennett and Raine 2012). However, the effects of marginal water pH and alkalinity on the extent of Ks reduction are less well understood, especially in relation to a soils initial pH and alkalinity. Naturally formed soils usually have a pH ranging from 4 to 10 (Szabolcs 1989), and the soil pH in a specific soil is basically a function of the soil clay minerals, organic portion, associated ion exchange, and hydrolysis reactions (Sumner et al. 1991). The aim of this study was to investigate the impact of different pH and EC of treatment solutions at SAR 20 and 40 on Ks reduction for Kaolinitic soils with different original pH values, enhancing the current understanding of Australian soils

The potential for a rehabilitated coal mine soil to support livestock grazing in south-east Queensland

Land that is disturbed by mining activities is required to be suitably rehabilitated. A trial was initiated to compare the performance of livestock grazing pasture sown on land that was rehabilitated after coal mining activity with that of livestock grazing pasture on unmined land. Pasture biomass, and soil structural, nutritional and hydrological properties important for pasture production and sustainability were intensively monitored on three sites rehabilitated at different stages over the last 10 years, and one unmined Control site. A further 18 unmined grazing sites were monitored for benchmarking purposes. Preliminary results for soil ammonium, nitrate and potentially mineralisable nitrogen suggest little difference in terms of benefits or constraints to pasture production between the rehabilitated and Control sites. Plant-available phosphorus was sufficiently high in the two oldest rehabilitated sites that a fertiliser response would not be expected. Subsoil and rooting depth of the rehabilitated sites was within the range observed across the benchmark sites and shallower than in the Control site. Higher pasture biomass in the rehabilitated sites compared with the Control at the initiation of the trial was attributed more-so to differences in grazing history than differences in soil attributes. Analysis of year one monitoring data is ongoing

Growing-season soil microbial respiration response to long-term no tillage and spring ridge tillage

No tillage (NT) and spring ridge tillage (SRT) are two common applications of conservation tillage. Although conservation tillage is known to exert major control over soil microbial respiration (SMR), the growing-season SMR response to these two applications remains elusive. In order to better understand the influence of conservation tillage practices, this experiment was conducted in an experimental field using NT and SRT for 17 years. In situ measurements of SMR, soil temperature and soil water content (SWC) were performed. Soil samples were collected to analyze soil porosity, soil microbial biomass (SMB) and soil enzymatic activities. Results show that the two conservation tillage systems had a significant difference (p0.05). Despite SRT increasing the proportion of micro-porosities and meso-porosities, the soil macro-porosities for NT were 7.37% higher than that of SRT, which resulted in higher bacteria and fungi in NT. Owing to SRT damaged the hypha, which had disadvantage in soil microbe protection. Inversely, less soil disturbance was a unique advantage in NT, which was in favor of improving soil macro-pores and SWC. Redundancy analyses (RDA) showed SMR was positively correlated with soil macro-pores, SMB and SWC. Furthermore, the Pearson correlation test indicated that SMB and soil enzymatic activities did not have a significant correlation (p>0.05). This study results suggest that SRT is more conducive to carbon sequestration compared with NT in cropland

Sustainability of beef production from brigalow lands after cultivation and mining. 1. Sown pasture growth and carrying capacity

Context: New Acland coal mine in south-eastern Queensland is seeking to rehabilitate mined land to pastures that are safe, stable and sustainable for beef production. Little is known of the productivity and sustainability of grazing previously mined land in the Darling Downs study region. Additionally, information is required to specify management guidelines for sustainable grazing of regional land types retired from cultivation. Aims: Identify pasture growth characteristics, rainfall use efficiencies and long-term carrying capacities of subtropical sown pastures established on lands rehabilitated after open-cut coal mining in comparison to sown pastures established on un-mined but previously cultivated lands. Methods: Pasture growth and quality (% nitrogen) were observed using the Swiftsynd methodology in ungrazed exclosures with three sites on rehabilitated lands of the Acland Grazing Trial over a 5-year period (2014–2018), and 13 sites on unmined lands over periods of 2–5 years providing data for modelling pasture growth. Key results: Peak pasture yield (TSDM for autumn harvests) averaged for 2017 and 2018 was greater (P < 0.1) on rehabilitated sites than unmined Poplar Box land type sites (5957 and 2233 kg/ha respectively) but similar to Brigalow Uplands and Mountain Coolibah land type sites (3946 and 3413 kg/ha respectively). Pasture rundown was evident, with pasture N uptake decreasing over 5 years at some sites. Soil mineral N supply (potentially mineralisable N and mineral N) in spring was a useful indicator of N uptake over the following growing season. Simulations using the GRASP pasture growth model for the grazing trial period predicted rainfall use efficiencies of 12.0, 7.0, 9.1 and 4.8 kg/ha.mm rainfall for rehabilitated sites and unmined sites on Brigalow Uplands, Mountain Coolibah and Poplar Box land types respectively. Long-term carrying capacities based on estimates of long-term median pasture growth and 30% utilisation were 4.39, 3.58 and 5.92 ha/adult equivalent respectively for the unmined land types, and 2.45 ha/adult equivalent for the rehabilitated lands. Conclusions: Rehabilitated land can be as productive as unmined but previously cultivated land. Implications: Grazing management plans for sustainable management of mined and unmined lands can be developed using data from the present study. The plans will assist with the transition of rehabilitated lands to commercial agriculture

Sustainability of beef production from brigalow lands after cultivation and mining. 3. Pasture rundown, climate and grazing pressure effects

Context: The Acland Land System overlying the Walloon sandstone coal deposits in southern Queensland is generally marginal for cropping but well suited to grazing, and thus cultivated land is commonly returned to pasture. Rehabilitation of these lands after open-cut coal mining seeks to be safe, stable and self-sustaining to satisfy requirements for ecologically sustainable development. Aims: The present paper evaluates the sustainability and economic viability of beef production on (a) lands retired from cultivation and then rehabilitated with sown pastures after open-cut coal mining at the New Acland mine site, and (b) similar nearby pasture lands that were not mined but were also retired from cultivation. Methods: The GRASP grazing systems model was modified and calibrated with short-term (5-year) grazing trial data (soil, pasture and cattle observations), and then used with long-term (60-year) weather data to estimate effects of land type, pasture rundown, climate and grazing pressure on productivity and economic returns. The productivity of three rehabilitated sites and 15 unmined sites were evaluated, including pastures on six commercial properties. Key results: Estimates of long-term mean annual growth of pastures on unmined lands retired from cultivation on three land types (Mountain Coolibah, Brigalow Uplands and Poplar Box) were 3398, 2817 and 2325 kg/ha respectively. Pasture growth was greater on rehabilitated lands; 3736 kg/ha on the site most typical of rehabilitated lands and a mean of 4959 kg/ha across three sites. Seasonal conditions had large effects on cattle liveweight gain (133–213 kg/head per year during the trial); however, pasture growth was the main driver of beef production and economic returns per hectare. In GRASP, potential nitrogen uptake was used to influence key pasture growth processes and accounted for 64% of variation in observed annual growth. The short-term lift and subsequent rundown in productivity typically associated with sown pastures was estimated to have increased mean annual pasture and cattle productivity during the 2014–2018 trial period by up to 17% and 25% respectively. Estimates of long-term mean annual beef production and economic returns for the unmined lands were less than estimated for rehabilitated lands and were 139 kg/head.year (45 kg/ha.year) and AU$154/adult equivalent. Conclusions: Rehabilitated lands were found to be sustainable for beef production at grazing pressures up to 30% utilisation of annual pasture growth, and comparable with grazing systems on native and sown pastures in good condition. Pastures on unmined lands retired from cultivation had reduced productivity. Implications: Overgrazing is a significant and on-going residual risk to sustainable production. Grazing regimes need to continually adjust for changes in novel landscapes, pasture condition and climate. The methods used in the present study could be applied more generally

Efficacy of delaying cotton defoliation to mitigate compaction risk at wet harvest

A recent rapid change in the cotton harvesting system has increased the risk of soil compaction within the cotton industry with the inception of the John Deere 7760 cotton harvester, a round-bale module builder that weighs >36 Mg. This project involved a novel approach to reducing the risk of soil compaction, whereby cotton defoliation was delayed at times of high field moisture so that the evapotranspiration demands of the crop could be used to dry down the soil profile and consequently reduce the compaction risk at harvest. A field trial at Aubigny, Queensland, was used to evaluate the merit of the proposed management strategy in the 2014–15 growing season, in conjunction with a modelling approach to assess the long-term effectiveness of the strategy in several Australian cotton-growing regions. Although the proposed strategy did reduce the compaction risk, the risk reduction was insufficient for the strategy to be deemed effective. Nonetheless, a strong correlation was found between small changes in soil moisture and changes in observable compaction. An observed 10% increase in soil bulk density after traffic suggested damage to soil pore networks. Furthermore, the depth of compaction was observed well beyond the feasible cultivation depth (to 80 cm)