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Backgrounds and aims: N rhizodeposition by legumes leads to enrichment of N in soils and in companion plants. N rhizodeposition can be divided into two major components, root exudation and root senescence. Our aim was to quantify N root exudation in white clover ( Trifolium repens L.) through an estimation of short-term N rhizodeposition and to assess its impact on N transfer to companion perennial ryegrass ( Lolium perenne L.) grown in mixture with clover. Method: N provided in the root atmosphere for 3 days was used to estimate transfer of symbiotically fixed nitrogen (SFN) to the growing medium by clover grown in pure stand and to ryegrass by clover grown in mixture for 2 months. Results: The proportion of N rhizodeposited over the 3 days increased from 3.5 % of SFN in pure stand to 5.3 % in mixture. The N-enrichment of ammonium from the adhering substrate shows that a part of the rhizodeposited N was released in the form of ammonium. 4 % of the rhizodeposited N was taken up by ryegrass during the labelling period. Conclusions: This study showed a significant contribution of root N exudation to the total N rhizodeposition of legumes and in the transfer of N to grasses. [ABSTRACT FROM AUTHOR]

Background and aims: Aluminium (Al) toxicity limits pasture production in acidic soils, and there is evidence that antioxidant enzymes such as superoxide dismutase (SOD) play a key role against Al-induced oxidative stress. We have investigated (i) the relationship between Al-toxicity and oxidative damage as well as the time-course of SOD activity and (ii) the expression profiles of two SOD isoforms in the roots of two ryegrass cultivars. Methods: Jumbo (Al-sensitive) and Nui (Al-semi-tolerant) ryegrass cultivars were cultured hydroponically with 0 or 0.2 mM Al. Roots were colleted during 48 h to determine Al uptake, lipid peroxidation, SOD activity and the expression of LpCu/Zn-SOD and LpFe-SOD isolated genes. Results: LpCu/Zn-SOD and LpFe-SOD were classified phylogenetically as putative mitochondrial and plastidial SODs, respectively. Although Al accumulation did not differ between the two cultivars, lipid peroxidation increased 2.2-fold in Jumbo but only scarcely in Nui by the end of the time-course. An early response in terms of total SOD activity and differential expression of LpCu/Zn-SOD and LpFe-SOD genes occurred in both cultivars after Al treatment. Conclusions: SOD isoforms were up-regulated over a longer period in Nui compared with Jumbo, leading finally to a greater protection in roots against the short term Al-toxicity. [ABSTRACT FROM AUTHOR]

Grazing ruminants urinate and deposit N onto pastoral soils at rates up to 1,000 kg ha, with most of this deposited N present as urea. In urine patches, nitrous oxide (NO) emissions can increase markedly. Soil derived CO fluxes can also increase due to priming effects.While NO fluxes are affected by temperature, no studies have examined the interaction of pasture plants, urine and temperature on NO fluxes and the associated CO fluxes. We postulated the response of NO emissions to bovine urine application would be affected by plants and temperature. Dairy cattle urine was collected, labelled with N, and applied at 590 kg N ha to a sub-tropical soil,with and without pasture plants at 11°, 19°, and 23°C. Over the experimental period (28 days), 0.2% (11°C with plants) to 2.2% (23°C with plants) of the applied N was emitted as NO. At 11°C, plants had no effect on cumulative NO-N fluxes, whereas at 23°C, the presence of plants significantly increased the flux, suggesting plant-derived C supply affected the NO producing microbes. In contrast, a significant urine application effect on the cumulative CO flux was not affected by varying temperature from 11-23°C or by growing plants in the soil. This study has shown that plants and their responses to temperature affect NO emissions from ruminant urine deposition. The results have significant implications for forecasting and understanding the effect of elevated soil temperatures on NO emissions and CO fluxes from grazed pasture systems. [ABSTRACT FROM AUTHOR]

Plant species differ in their capacity to use nonexchangeable potassium (NEK) in soils. In this study two typical plants with high K use efficiency, ryegrass and grain amaranth, were compared with regard to their capacity to use K from five K-bearing minerals. Biomass relative yield and K uptake data indicated that ryegrass was much more efficient than grain amaranth at using NEK in minerals. Root exudates of grain amaranth collected under hydroponic culture contained considerable amounts of oxalic and citric acids, while these acids were not detected in ryegrass root exudates. Compared with grain amaranth, the kinetic parameters of K uptake by ryegrass roots were characterized by a significantly higher K uptake rate (V) and a significantly lower C, the minimum external K concentration at which K is taken up. The dynamic release of NEK from minerals in various solutions showed that the release rate of NEK was largely K-concentration dependent and some thresholds of K concentration prevented further NEK release from minerals. The K thresholds were related to mineral type and increased in the presence of Ca or Na in solutions. The positive effect of H (20 mmol L) on NEK release was also mainly attributed to elevating the thresholds of K concentration, rather than to the effects of weathering. The results indicated that the main mechanism by which plant species efficiently use NEK in minerals was to the capacity of plants to absorb K at low concentrations. The lower the C for the root K uptake, the higher the expected NEK use efficiency of the plant. [ABSTRACT FROM AUTHOR]

Soil moisture content (SMC) and throughfall were monitored along transects between rows of radiata pine ( Pinus radiata D. Don) planted at 7 m row spacing and grown with ryegrass/clover or lucerne pastures or with no understorey. SMC was recorded to 1 or 2 m depths for years 2–6 or 4–6, respectively. The predominant rainfall from the south–south–west (SSW) direction created rain shadows (60% of open pasture) on the northern sunny side of the trees. The rain shadows increased as the trees grew but their position were modified by stand pruning and thinning in years 4 and 5. SMC in the top meter showed strong positional effects with the lowest SMC under the tree crowns and on the northern side. There were strong seasonal patterns driven by high spring and summer evapotranspiration, modified by rain shadows and stand age. For the no-understorey treatment, SMC remained high midway between tree rows until the trees were 4 years old and started utilizing this moisture. For the two pasture understoreys, the difference in SMC between the north and south sides decreased with time after trees were thinned and pruned. In the no-understorey there was little difference between the two aspects. Understorey evapotranspiration and fine rooting patterns also helped explain soil moisture use patterns. No difference was detected between two contrasting radiata pine tree types. The effective rooting depth of the ryegrass/clover understorey was <0.7 m while that of lucerne and trees were deeper. Alternative tree planting practices may reduce some of the negative effects of tree-understory moisture competition. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential micronutrient for animal and human nutrition, but whether it is essential to plants remains controversial. However, there are increasing experimental evidences that indicate a protective role of Se against the oxidative stress in higher plants through Se-dependent glutathione peroxidase (GSH-Px) activity. The effects of the Se chemical forms, selenite and selenate, the rate of their application on shoot Se concentration and their influence on the antioxidative system of ryegrass ( Lolium perenne cv. Aries), through the measurement of GSH-Px activity and lipid peroxidation, were evaluated in an Andisol of Southern Chile. Moreover, a soil–plant relationship for Se was determined and a simple method to extract available Se from acid soils is proposed. In a 55-day experiment ryegrass seeds were sown in pots and soil was treated with sodium selenite or sodium selenate (0–10 mg Se kg−1). The results showed that the Se concentration in shoots increased with the application of both selenite and selenate. However, the highest shoot Se concentrations were obtained in selenate-treated plants. For both sources of Se, there was a significant positive correlation between the shoot Se concentration and the GSH-Px activity; and the Se-dependence of this enzymatic activity was related especially with the chemical form of applied Se rather than the Se concentration in plant tissues. Furthermore, the lipid peroxidation, as measured by Thiobarbituric Acid Reactive Substances (TBARS), decreased at low levels of shoot Se concentration, reaching the lowest level at approximately 20 mg Se kg−1 in plants and then increased steadily above this level. In addition, the acid extraction method used to evaluate available Se in soil showed a positive good correlation between soil Se and shoot Se concentrations irrespective of chemical form of Se applied. [ABSTRACT FROM AUTHOR]