The activity of glyphosate and other herbicides in soil

The effects of herbicides on a legume Rhizobium symbiosis were studied in laboratory experiments. Root applications of all herbicides examined reduced nodulation of legumes grown in aqueous nutrient media. The growth of Rhizobium trifolii TA1 was reduced by 2 - 20 mg ai 1-1 of diquat, 2 mg ai 1-1 of paraquat, 10 mg ai 14 of glyphosate and 2 mg ail-1 of chlorsulfuron. No other herbicide affected rhizobia growth when applied at 2 - 20 mg ai l-1 of nutrient medium. Inoculation with TA1 pre-treated with amitrole, atrazine or glyphosate decreased nodulation of sub-clover plants indicating that these herbicides may affect the nodulation potential of certain strains of Rhizobium. Residues of 2,4-D, amitrole, diquat, trifluralin and glyphosate persisted in a Walpeup sandy loam in sufficient concentration for four months after application to soil to affect growth and symbiotic activity of sub-clover. The behaviour of glyphosate in soil, under various conditions was studied in the laboratory. Adsorption of glyphosate as depicted by Freundlich K constant was greater in an acid soil than in three alkaline soils and values for this constant ranged from 8 - 67.8 at 23.5C and 4.3 - 57.8 at 10C. Rate of decomposition of 14C-glyphosate at 25C decreased slowly over the experimental period in all soils. Two compartments of adsorbed glyphosate in soil were identified as labile glyphosate and non-labile glyphosate and these reflected the strength of adsorption of the chemical. The amount of glyphosate in the labile firm for the soils ranged from 24 - 34.5% of the total and half-life ranged from 6 - 9 days. The amount of glyphosate in the non-labile form for soils ranged from 67 . 75% of the total and half-life ranged from 222 to 835 days. At 10C, the two compartments of glyphosate adsorption were identified for the Walpeup and Rutherglen soils but only one compartment could be identified in the Wimmera and Culgoa soils. Methodology was developed to permit extraction and analysis of glyphosate and AMPA in soil. Recovery of glyphosate from soils where time between fortification and extraction was only 30 sec. was 84.6 - 104%. However where extraction was delayed 13 hours, recoveries were 47.6 - 66.8%. The extractant (0.1 M triethylamine) was shown to be unable to desorb adsorbed glyphosate. Studies revealed that at 25C, the pool of extractable glyphosate was rapidly depleted by decomposition. At this temperature, the pool of extractable glyphosate was supplemented by slow desorption of non-labile glyphosate for each soil. At 10C, depletion of the pool of extractable glyphosate was considerably slower. For the Walpeup and Rutherglen soils, the rate of desorption of glyphosate from the non-labile pool was less than the rate of loss by decomposition of the herbicide. Rate of desorption of non-labile glyphosate in the Wimmera soil was shown to be the same as the rate of loss by decomposition of the herbicide. Loss of extractable glyphosate in the Culgoa soil occurred by decomposition and by slow adsorption of extractable herbicide from the labile to the non-labile form. The effects of residues of glyphosate in the field following an autumn and a summer application were investigated at selected field sites. Following the autumn application, phytotoxic activity of glyphosate was observed in sub-clover plants growing at the Walpeup and Culgoa sites but not at the Wimmera site. Growth and nodulation of plants sown up to 4 weeks after herbicide treatment were reduced at the Walpeup site. Only nodulation of plants sown up to 4 weeks after treatment was reduced at the Culgoa site. Results suggest that residues of glyphosate are only likely to significantly affect the growth of susceptible plants during winter on sandy soils. Following summer application of glyphosate, no phytotoxic activity of the herbicide was observed for sub-clover plants grown in the Walpeup sandy loam. Results suggest that in a hot summer, it is unlikely that residues of glyphosate in any soil would cause significant damage to plant growth

Rhizo-lysimetry: facilities for the simultaneous study of root behaviour and resource use by agricultural crop and pasture systems

Background: Rhizo-lysimeters offer unique advantages for the study of plants and their interactions with soils. In this paper, an existing facility at Charles Sturt University in Wagga Wagga Australia is described in detail and its potential to conduct both ecophysiological and ecohydrological research in the study of root interactions of agricultural crops and pastures is quantitatively assessed. This is of significance to future crop research efforts in southern Australia, in light of recent significant long-term drought events, as well as potential impacts of climate change as predicted for the region. The rhizo-lysimeter root research facility has recently been expanded to accommodate larger research projects over multiple years and cropping rotations.Results: Lucerne, a widely-grown perennial pasture in southern Australia, developed an expansive root system to a depth of 0.9 m over a twelve month period. Its deeper roots particularly at 2.05 m continued to expand for the duration of the experiment. In succeeding experiments, canola, a commonly grown annual crop, developed a more extensive (approximately 300%) root system than wheat, but exhibited a slower rate of root elongation at rates of 7.47 x 10(-3) m day(-1) for canola and 1.04 x 10(-2) m day(-1) for wheat. A time domain reflectometry (TDR) network was designed to accurately assess changes in soil water content, and could assess water content change to within 5% of the amount of water applied.Conclusions: The rhizo-lysimetry system provided robust estimates of root growth and soil water change under conditions representative of a field setting. This is currently one of a very limited number of global research facilities able to perform experimentation under field conditions and is the largest root research experimental laboratory in the southern hemisphere

Trends and perceptions of rural household groundwater use and the implications for smallholder agriculture in rain-fed Southern Laos

In Laos, the extent of rural groundwater use is largely unknown. This paper presents results of a study conducted in Champasak province, Southern Laos, to examine rural household attitudes to groundwater use and management; and to gauge farmer perceptions of the opportunities/constraints of increased groundwater use for smallholder agriculture. Significant differences in groundwater use, total cash income and perceived groundwater quality were found. Further agricultural expansion was constrained by insufficient labour and the limited area of individual landholdings, not by the limited groundwater supply, which was the preferred source to fulfil increased demands for domestic/agricultural use. There were no groundwater regulations

Quantification of an overlooked water resource in the tropical rainfed lowlands using RapidEye satellite data: a case of farm ponds and the potential gross value for smallholder production in southern Laos

In southern Laos, water stored in farm ponds is largely underutilised as it is perceived to be unfit for human consumption; subsequently, groundwater is the preferred source for domestic and agricultural consumption. For the first time, this paper presents the results of a study designed to quantify the total pond water volume within the landscape via remote-sensing methods in two districts in Champasak province that could be used to improve rural household cash income through the expansion of market-oriented dry season crop production. Water bodies were delineated via simple classification of RapidEye data using the Normalised Difference Water Index and a sub-classification was performed to distinguish between ponds and the streamflow network. Final estimates of total pond volume in Sukhuma and Phonthong districts were ∼2.30 × 10 m and 3.55 × 10 m, respectively; and the average pond volume across both districts was ∼1987 m. Sensitivity analysis of the potential gross value of farm ponds for irrigation of dry season, vegetable production typical of market-oriented smallholder activities in the area indicated that substantial gross economic gains could be made from better use and management of these resources