Herbicide residues in Australian grain cropping soils at sowing and their relevance to crop growth.

Herbicides are used extensively in Australian grain cropping systems. Despite occasional observations of herbicide-induced phytotoxicity, there is little information on the persistence and carryover of multiple herbicide classes in cropping soils and the risk to subsequent crops. Two soil surveys were conducted, in 2015 (n = 40) and 2016 (n = 42), across different Australian grain cropping fields prior to sowing of winter crops, and soil samples analysed for herbicide residues (16 analytes in 2015 and 22 analytes in 2016). Samples in 2015 were taken at two depths (0-10 cm and 10-30 cm), whilst samples in 2016 were taken in topsoil (0-10 cm) only, but from two discrete locations in each field. Our research in both years found at least one herbicide (or herbicide metabolite) residue at all sites, with a median of 6 analytes detected in 2015 and 7 analytes detected in 2016. The most frequently detected residues were glyphosate and its primary breakdown product aminomethylphosphonic acid (AMPA), in 87 and 100%, respectively, of topsoil (0-10 cm) samples in 2015, and 67 and 93% of samples in 2016. The median concentration of glyphosate in 2015 was 0.12 mg kg-1, while AMPA was 0.41 mg kg-1. In 2016, median concentrations of glyphosate and AMPA were 0.22 mg kg-1 and 0.31 mg kg-1. Residues of 2,4-dichlorophenoxyacetic acid, trifluralin and diflufenican were also detected in >40% of topsoil samples in both seasons, but with median concentrations of <0.05 mg kg-1. A literature review found limited availability of phytotoxicity thresholds for major grain crops exposed to soilborne herbicide residues. A risk assessment using available thresholds suggested that although up to 29% of fields contained trifluralin residues that could constrain cereal crop growth, and 24% of fields contained residues of phenoxy or sulfonylureas that could affect dicotyledonous crops, the majority of these fields when planted with tolerant crops would be unlikely to be affected by herbicide residues. More work is required to ascertain the spatial distribution, bioavailability and phytotoxicity of residues and residue mixtures to enable a more accurate agronomic risk assessment.

Data validating the use of rubidium as a non-radioactive tracer for the localised proliferation of wheat roots in acidic or limed subsoil.

Existing technologies for lime (CaCO3) incorporation into acidic field soils result in the heterogeneous distribution of limed and acidic soil sections. In a study characterising the response of wheat (Triticum aestivum L.) to the amendment of an acidic soil profile with vertically limed slots [1], elucidation of the dynamics of root proliferation within the acidic and limed soil sections was a prerequisite to understanding the mechanisms driving the above-ground responses. Rubidium (Rb) has been used widely as a non-radioactive tracer for root activity [2] in soil. However, the contrasting pH in a heterogeneously limed soil profile and related aluminium toxicity effects to roots can influence the availability and uptake of Rb, and quantitative data relating Rb uptake to root phenology in this scenario are lacking. To validate the use of Rb as a tracer for root activity within vertically limed slots in an acidic soil profile, its uptake by wheat roots from acidic or limed sections of subsoil, and its relation to root architecture was assessed. Wheat plants were grown in a glasshouse in 29 cm deep, vertically split soil columns with acidic (pH 3.9), Al-toxic subsoil on one side and the same soil amended with lime on the other side. Rubidium chloride was applied at 5, 10 or 20 mg Rb kg-1 to either limed or acidic soil sections. Wheat plants were grown for 28 days, after which the Rb content in shoots and the root length and diameter in each of the discrete soil sections was measured. Foremost, the Rb amendments (5, 10 or 20 mg Rb kg-1 soil) did not induce any toxic effects; shoot dry weight and root length in the limed and acidic sections of the subsoil were not statistically different among the rubidium-amended and non-amended treatments, regardless of its placement (limed vs. acidic sections). Average root lengths in the limed sections of the subsoil (69.5 m section-1) were approximately 10-fold greater than in the acidic sections (6.3 m section-1). Likewise, the concentration of Rb in shoots was, on average, 7-fold greater where Rb was applied to the limed (vs. acidic) subsoil section and was positively influenced by the rate of Rb amendment in the limed (p ≤ 0.05), but not the acidic section of the subsoil. Rubidium uptake into shoots was significantly correlated (p ≤ 0.05) with the length of roots within the Rb-amended subsoil section. The uptake of Rb from acidic or limed subsoil sections was determined by the root length in the Rb-amended subsoil section, regardless of the rate of Rb amendment. The uptake of Rb per unit of root length from acidic or limed sections of subsoil was not significantly different. The data validate the use of Rb as a tracer for the dynamics of root length proliferation in limed subsoil sections in a heterogeneously limed acidic soil profile.