Cotton strip assay detects soil microbial degradation differences among crop rotation and tillage experiments on Vertisols.

The cotton strip assay (CSA) is a simple and inexpensive method of evaluating management effects on soil microbial decomposition. The average loss of tensile strength of cotton strips buried 3 to 35 days in soils from two long-term tillage and crop-rotation experiments was of the order: cotton-wheat rotation > minimum-tillage cotton monoculture > maximum-tillage cotton monoculture. The study suggests CSA can be an effective indicator to delineate microbial activity, soil organic carbon or crop biomass as influenced by agricultural practices in cotton fields.

Tail-drain sediments are a potential hotspot for nitrous oxide emissions in furrow-irrigated Vertisols used to grow cotton: A laboratory incubation study.

The impacts of soil properties and urea fertigation on nitrous oxide (N2 O) emissions from uncropped areas of furrow-irrigated Vertisol paddocks are unknown. We sampled soils from the head-ditch end (upslope) and sediments from the tail-drain end (downslope) of 10 Vertisols under irrigated cotton (Gossypium hirsutum L.) production in northeastern Australia. Four replicates of each sample were incubated in open-top polyvinyl chloride (PVC) chambers at 25 ± 2°C for 25 d. Nitrous oxide emissions were measured periodically after simulated irrigations on Days 0 and 15 with either water or, for soils, urea solution. Compared with the soils, sediments were enriched in silt, total organic carbon (TOC), total nitrogen (TN), ammonium N, and dissolved organic C (DOC) but had lower pH and sand content. Sediments emitted more N2 O than soils from the same paddocks after water irrigations. Nitrous oxide fluxes varied by two orders of magnitude between paddocks, with most variation explained by baseline nitrate N, TOC, pH (inversely), and sand content. Urea solution applied to soils at 30 kg N ha-1 irrigation-1 increased N2 O emitted, but more so after the second irrigation. In irrigated cotton systems, tail-drain sediments are a potential hotspot for N2 O emissions that has not previously been documented.

Storage and initial processing of water samples for organic carbon analysis in runoff.

Runoff monitoring experiments are often conducted in remote sites. Sample collection and dispatch for analysis are often delayed due to sites' remoteness and limited local laboratory facilities. The standard method of dissolved organic carbon (DOC) and total organic carbon (TOC) analysis in water samples requires storage of the samples at 4 °C after collection and analysis with a minimum of delay. However, there is no freezing storage time limit specified to avoid degradation. To overcome the limitations of this approach, we investigated the method of storage, that is refrigeration (4 °C) versus freezing (-18 °C), and the effect of storage time on DOC and TOC fractions in runoff water. Storage of samples at 4 °C for more than a week resulted in a decline of TOC and DOC concentrations in runoff water.•Freezing unfiltered water samples immediately after collection minimized TOC losses during storage, however, it may lead to variable DOC results.•Filtering a subsample of runoff or irrigation water immediately after collection using a 0.45 µm filter and freezing both the filtered and unfiltered samples until analysis of DOC and TOC, respectively, can minimize losses during storage.

Coring lubricants can increase soil microbial activity in Vertisols.

It is essential that sampling procedures for biological measurements are done in a way that reflects the soil processes, whilst limiting sampling artefacts. In heavy clay Vertisol soils, coring lubricants are often considered necessary in order to extract and recover soil for quality and health assessments. Previous reports into the use of coring lubricants have found soil carbon measurements to be inflated but to date, a study to evaluate the effects of these lubricants on soil microbial activity, has not been forthcoming. We measured soil carbon dioxide (CO2) evolution in response to the addition of common coring lubricants, to determine the effects upon soil microbial activity to the depth of 100 cm. Application of coring lubricants to the surface soil layers of field collected cores did not significantly influence CO2 evolution however, microbial activity increased in deeper soil layers (30-100 cm) with the use of WD-40, mould stripper and silicone oil. When the ratio of coring lubricant to soil was increased to ~5 g coring lubricant to 100 g-1 soil, there was a significant (P = .001) effect on microbial activity, with silicone oil and mould stripper inflating measurements by at least 5%, whilst olive oil and WD-40 were similar to the control. The results imply that when using coring rigs to recover soil for microbial functional analysis in Vertisols, the use of coring lubricants is best avoided, with further research recommended.

Comparison of methods for measuring soil microbial activity using cotton strips and a respirometer.

In order to develop a method of measuring the level of microbial activity in soil that is suitable for use by farmers, land managers, and other non-scientists, a simple method for determining soil microbial activity was evaluated and compared with two standard techniques. Soils sampled from vegetable farms in south east Queensland were incubated in the laboratory under controlled moisture and temperature conditions. Three methods were used to measure soil microbial activity, a respirometry method and two methods using the cotton strip assay (CSA) technique (image analysis and tensometer). The standard CSA method measured loss of tensile strength over a 35 day incubation period of buried cotton strips using a tensometer. The new CSA technique measured the intensity of staining by microbes using a flatbed scanner to create an image of the cotton strip whose staining percentage was determined using Photoshop software. The respirometry method used the substrate induced respiration rate (SIR) to determine microbial biomass in the soil at day 12 of incubation. The strong correlation between the image analysis method and the tensometer method (r(2)=0.81), a technique used by scientific researchers, suggests that the image analysis method could be used to monitor aspects of soil biological health by general community land-care groups and farmers. The image analysis method uses equipment which is readily available and, while not strongly correlated with more precise measurements of soil biological activity such as microbial biomass (r(2)=0.26), it can detect gross trends in biological health in a soil monitoring program. The CSA method using image analysis was the cheapest technique to measure soil microbial activity. CSA using image analysis can be a valuable tool in conjunction with other simple indicators of soil physical and chemical health such as slaking and pH to monitor soil amelioration or rehabilitation programs.

Comparison of soil analytical methods for estimating wheat potassium fertilizer requirements in response to contrasting plant K demand in the glasshouse.

The traditional soil potassium (K) testing methods fail to accurately predict K requirement by plants. The Diffusive Gradients in Thin-films (DGT) method is promising, but the relationship between the DGT-measured K pool and plant available K is not clear. Wheat (Triticum aestivum L., cv. Frame) was grown in 9 Australian broad acre agricultural soils in a glasshouse trial until the end of tillering growth stage (GS30) with different plant K demands generated by varying plant numbers and pot sizes. Different K concentrations in soils were varied by 4 rates of K fertilizer application. The relative dry matter and K uptake were plotted against the soil K test value (CaCl2, Colwell and NH4OAc and DGT K measurements). To obtain 90% of maximum relative dry matter at low root density (closest to field conditions), the critical value of the NH4OAc K method was 91 (R2 = 0.56) mg kg-1. The DGT K method was not able to accurately predict relative dry matter or K uptake due to a weak extraction force for K from soils with high CEC values. Further endeavor on increasing K extraction force of the DGT method is warranted to obtain accurate plant available K results.

Effect of sugarcane cropping systems on herbicide losses in surface runoff.

Herbicide runoff from cropping fields has been identified as a threat to the Great Barrier Reef ecosystem. A field investigation was carried out to monitor the changes in runoff water quality resulting from four different sugarcane cropping systems that included different herbicides and contrasting tillage and trash management practices. These include (i) Conventional - Tillage (beds and inter-rows) with residual herbicides used; (ii) Improved - only the beds were tilled (zonal) with reduced residual herbicides used; (iii) Aspirational - minimum tillage (one pass of a single tine ripper before planting) with trash mulch, no residual herbicides and a legume intercrop after cane establishment; and (iv) New Farming System (NFS) - minimum tillage as in Aspirational practice with a grain legume rotation and a combination of residual and knockdown herbicides. Results suggest soil and trash management had a larger effect on the herbicide losses in runoff than the physico-chemical properties of herbicides. Improved practices with 30% lower atrazine application rates than used in conventional systems produced reduced runoff volumes by 40% and atrazine loss by 62%. There were a 2-fold variation in atrazine and >10-fold variation in metribuzin loads in runoff water between reduced tillage systems differing in soil disturbance and surface residue cover from the previous rotation crops, despite the same herbicide application rates. The elevated risk of offsite losses from herbicides was illustrated by the high concentrations of diuron (14µgL(-1)) recorded in runoff that occurred >2.5months after herbicide application in a 1(st) ratoon crop. A cropping system employing less persistent non-selective herbicides and an inter-row soybean mulch resulted in no residual herbicide contamination in runoff water, but recorded 12.3% lower yield compared to Conventional practice. These findings reveal a trade-off between achieving good water quality with minimal herbicide contamination and maintaining farm profitability with good weed control.