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<jats:title>Abstract</jats:title><jats:sec> <jats:title>Purpose</jats:title> <jats:p>Alfalfa is one of the most valuable forage crops in temperate climate zones. <jats:italic>Ensifer meliloti</jats:italic>, the endosymbiont of alfalfa, contains all the denitrification genes but the capacity of alfalfa root nodules to produce nitrous oxide (N<jats:sub>2</jats:sub>O) is not known. In this work, N<jats:sub>2</jats:sub>O emissions as well as the influence of bacteroidal denitrification on nodulation competitiveness and N<jats:sub>2</jats:sub>O release from alfalfa nodules has been investigated.</jats:p> </jats:sec><jats:sec> <jats:title>Methods</jats:title> <jats:p><jats:italic>Medicago sativa</jats:italic> cv. Victoria plants were inoculated with <jats:italic>E. meliloti</jats:italic> 1021, a periplasmic nitrate reductase (Nap) defective mutant, a Nap overexpressing strain and a nitrous oxide reductase defective mutant. Plants were grown in the presence of different nitrate and copper treatments and subjected to flooding during one week before harvesting. N<jats:sub>2</jats:sub>O production by the nodules was analysed by using gas chromatography. Methyl viologen-dependent nitrate reductase (MV<jats:sup>+</jats:sup>-NR), nitrite reductase (MV<jats:sup>+</jats:sup>-NIR) and nitrous oxide reductase (N<jats:sub>2</jats:sub>OR) enzymatic activities were measured in isolated bacteroids.</jats:p> </jats:sec><jats:sec> <jats:title>Results</jats:title> <jats:p>Alfalfa root nodules produce N<jats:sub>2</jats:sub>O in response to nitrate and flooding. Overexpression of Nap improved nodulation competitiveness and induced N<jats:sub>2</jats:sub>O emissions from nodules. Copper is required for an effective symbiosis as well as triggered a reduction of N<jats:sub>2</jats:sub>O production due to the induction of the N<jats:sub>2</jats:sub>OR and a reduction of NIR activities in the bacteroids.</jats:p> </jats:sec><jats:sec> <jats:title>Conclusion</jats:title> <jats:p>Alfalfa root nodules emit N<jats:sub>2</jats:sub>O. Nap is involved in nodulation competitiveness and in N<jats:sub>2</jats:sub>O emissions by the nodules. Bacteroidal N<jats:sub>2</jats:sub>OR and NIR activities are modulated by Cu and may be considered as effective targets for the mitigation strategies of N<jats:sub>2</jats:sub>O emissions derived from alfalfa crops.</jats:p> </jats:sec>

<jats:title>Abstract </jats:title><jats:p>The effects of pH on nutrient availability are not solely caused by to the effects on reaction with soils but are an interaction between these effects and the effects on rate of uptake by plants. Some effects are specific to particular ions, but an important aspect is that plant roots and soil particles both have variable charge surfaces. This influences availability, but in opposite directions. Sulfate is an example of this interplay. Its sorption by soil decreases markedly with increasing pH and thus “soil availability” increases. However, plant uptake also decreases with increasing pH thus “plant availability” decreases. For phosphate, the plant effect is stronger than the soil effect and uptake decreases with increasing pH. In contrast, effects of increasing pH on molybdate adsorption are so large that they dominate the overall effect. Sorption of cations, such as zinc or copper, increases with increasing pH but uptake rate also increases. The net effect is a small decrease in availability with increasing pH. Boron is an exception; there are small effects of pH on sorption; and it is the uncharged boric acid molecules that are taken up by plant roots. Their uptake is not affected by charge and uptake is proportional to the concentration of uncharged boric acid molecules. We argue that emphasis on the effects of pH on reactions with soil has led to a distorted picture of the effects of pH on nutrient availability.</jats:p>