Under areas where rainfall exceeds evapotranspiration, soil acidification is an ongoing process that can be either accelerated by the activity of plants, animals, and
humans or slowed down by careful management practices [1,2]. In areas that remain unaffected by industrial pollution, soil acidification is mainly caused by the
release of protons (H) during the transformation and cycling of carbon (C), nitrogen (N), and sulfur (S) in the soil-plant-animal system [3,4]. Under managed
systems much of the accelerated soil acidification is caused by increasing N and
S inputs into the farming system (Table 1). For example, in areas of Australia
where legumes have been grown continuously for more than 30 years, the soil pH
has decreased by about one unit [5-10]. Similarly, in New Zealand, intensively
managed legume-based dairy pastures require applications of approximately 2.5
tons of lime per ha every 6 years [11,12] to neutralize acidity mostly generated
through loss of N from an accelerated N cycle.

Role of carbon, nitrogen, and sulfur cycles in soil acidification.

Summary


1. Tests of the effectiveness of rhizobia isolated from several common Acacia species showed significant variation among host populations in plant growth but no indication that plant performance depended on which Acacia species the rhizobial isolate was derived from. Variation in effectiveness was primarily between rhizobial isolates within a host species rather than across rhizobial isolates from different hosts. No interactions were found between host population, host origin of rhizobial strain and rhizobial isolate.



2. A study using a range of less common Acacia species showed significant variation in performance of several species (A. silvestris, A. nano-dealbata, A. trachyphloia, A. filicifolia, A. leucoclada) as a function of the inoculating rhizobial strain; two other species (A. cangaiensis, A. glaucocarpa) were marginally non-significant. For A. filicifolia and A. silvestris, growth was significantly better when plants were inoculated with their own isolates vs. those from other host species.



3. Comparisons of the performance of a third set of Acacia species found that several common species (A. melanoxylon, A. mearnsii, A. irrorata, A. binervata) varied little in their response to particular rhizobial isolates; for these species, most host–rhizobial combinations resulted in similar growth responses. Several less common species showed significant variation in performance across the rhizobial isolates tested.



4. Preliminary investigations of host specificity among legume genera showed significant variation with respect to host performance as a function of the rhizobial isolate with which plants were inoculated. Some species (e.g. A. mearnsii and Indigofera australis) were variable in their responses, while others (e.g. Hardenbergia violacea and A. melanoxylon) performed well with most isolates.



5. A comparison of the effectiveness of several Acacia-derived strains on their own hosts vs. their effectiveness on the other legume genera in the study showed that, in general, the Acacia strains performed well on other genera. However, on some host species (e.g. I. australis) the Acacia strains were less effective than the Indigofera strains, suggesting some host specificity.



6. While there was a general correlation at the among-species level in Acacia–rhizobia interactions (i.e. isolates that performed well on one host species often performed well on others), growth performance with any particular host–rhizobial combination was more difficult to predict than within-species comparisons. Our data also suggest that, at least for the genus Acacia, rare species are likely to exhibit a greater degree of host specificity than hosts that are widely distributed, and re-establishment may require special attention with respect to rhizobial strains that are used.



7. Overall, these results have clear implications for applied projects focused on the restoration of degraded habitats or abandoned farmland. Simply replanting native species into such areas may be unsuccessful unless appropriate rhizobial strains are also reintroduced. Our results suggest that when no strains for a particular host species are available, strains from its closest relative will have the highest probability of success.

Variation in the effectiveness of symbiotic associations between native rhizobia and temperate Australian legumes: interactions within and between genera

Improved nitrogen use efficiency would be beneficial to agroecosystem sustainability in the northern Great Plains of the USA. The most common rotation in the northern Great Plains is fallow–spring wheat. Tillage during fallow periods controls weeds, which otherwise would use substantial amounts of water and available nitrogen, decreasing the efficiency of fallow. Chemical fallow and zero tillage systems improve soil water conservation, and may improve nitrogen availability to subsequent crops. We conducted a field trial from 1998 through 2003 comparing nitrogen uptake and nitrogen use efficiency of crops in nine rotations under two tillage systems, conventional and no-till. All rotations included spring wheat, two rotations included field pea, while lentil, chickpea, yellow mustard, sunflower, and safflower were present in single rotations with wheat. Growing season precipitation was below average in 3 of 4 years, resulting in substantial drought stress to crops not following fallow. In general, rotation had a greater influence on spring wheat nitrogen accumulation and use efficiency than did tillage system. Spring wheat following fallow had substantially higher N accumulation in seed and biomass, N harvest index, and superior nitrogen use efficiency than wheat following pea, lentil, chickpea, yellow mustard, or wheat. Preplant nitrate-N varied widely among years and rotations, but overall, conventional tillage resulted in 9 kg ha −1 more nitrate-N (0–60 cm) for spring wheat than did zero tillage. However, zero tillage spring wheat averaged 11 kg ha −1 more N in biomass than wheat in conventional tillage. Nitrogen accumulation in pea seed, 45 kg ha −1 , was superior to that of all alternate crops and spring wheat, 17 and 23 kg ha −1 , respectively. Chickpea, lentil, yellow mustard, safflower, and sunflower did not perform well and were not adapted to this region during periods of below average precipitation. During periods of drought, field pea and wheat following fallow had greater nitrogen use efficiency than recropped wheat or other pulse and oilseed crops.

/pdf/diversified-cropping-systems-in-semiarid-montana-nitrogen-45g79w0i9p.pdf

Diversified cropping systems in semiarid Montana: Nitrogen use during drought

Elizabeth Drew, David Herridge, Ross Ballard, Graham O’Hara, Rosalind Deaker, Matthew Denton, Ron Yates, Greg Gemell, Elizabeth Hartley, Lori Phillips, Nikki Seymour, John Howieson and Neil Ballard

Inoculating legumes: a practical guide

The pea leaf weevil (Sitona lineatus L.) (Coleoptera: Curculionidae) is a serious pest of field pea (Pisum sativum L.) and faba bean (Vicia faba L.) (Fabales: Fabaceae) in Europe, Asia, Africa, and North America. Foliar feeding by adults and root nodule consumption by larvae reduces seed yield and N contributions to the soil. Foliar insecticides have limited efficacy against pea leaf weevil, prompting this study to investigate an integrated approach. Nitrogen fertilizer, Rhizobium inoculant, and thiamethoxam (systemic insecticide) were assessed alone and in combination for their effects on pea leaf weevil damage to field pea plants and seed yield. Nitrogen fertilizer did not reduce foliar feeding but did reduce root nodules by 35 to 43%. In 2008 only, N fertilizer improved yield by 0.48 t ha -1 relative to nonfertilized plots. Thiamethoxam seed treatment reduced foliar feeding for 40 to 50 d after planting in 2008 and 2009 but had no significant impact on seed yield. Rhizobium inoculation increased yield by 0.74 t ha -1 compared to noninoculated plots in 2008. No effects of N fertilizer, Rhizobium inoculant or thiamethoxam on yield were observed in 2009, probably because root nodule damage was not affected by any treatment. Results of this study suggest that the most efficient method of preventing yield loss due to S. lineatus infestation is to inoculate field peas. Further investigation of thiamethoxam, preferably in large-scale production systems, is required to better understand the efficacy of thiamethoxam against pea leaf weevil.

Integrated Management of Sitona lineatus with Nitrogen Fertilizer, Rhizobium , and Thiamethoxam Insecticide

Nodulation studies on legumes exotic to Australia: the influence of soil populations and inocula of Rhizobium leguminosarum bv. viciae on nodulation and nitrogen fixation by field peas

Grain legume-cereal rotations are unsustainable on acid soils because they promote acidification of surface soil through nitrate leaching. Two field experiments were conducted on red, clay-loams in the cropping zone of central western New South Wales to determine whether soil mineral N concentrations during crop growth are higher under pea than barley, and whether the nitrate concentration under pea crops can be decreased by ammending soil with cereal straw before sowing.Significantly higher mineral N, particularly nitrate, was found under pea than under barley, as early as 6 weeks following autumn sowing, and also in spring. The pea effect represented an increase of up to 23 kg N/ha of mineral N (0-30 cm). It is proposed that the source of higher nitrate concentration under pea may be residual soil nitrate not utilised by pea, or nitrate derived from the mineralisation of pea roots or exudate. The increase in soil nitrate during pea growth contributes to greater postharvest soil mineral N and higher wheat yields after pea, but also increases the risk of soil acidification. Soil ammendment with cereal straw was partially effective in reducing nitrate concentration under pea, but a more effective treatment is required.

Nitrate accumulation under pea cropping and the effects of crop establishment methods: a sustainability issue

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Nodulation studies on legumes exotic to Australia: the influence of soil populations and inocula of Rhizobium leguminosarum bv. viciae on nodulation and nitrogen fixation by field peas

Nitrate accumulation under pea cropping and the effects of crop establishment methods: a sustainability issue