Factors affecting the potential contributions of N2 fixation by legumes in Australian pasture systems

Abstract

The amounts of foliage nitrogen (N) fixed by various annual and perennial legumes growing in Australian pastures range from <10 to >250 kg N/ha.year. Differences in N2 fixation result from variations in the proportion of the legume-N derived from atmospheric N2 (%Ndfa) and/or the amount of legume-N accumulated during growth. On-farm surveys of %Ndfa achieved by legumes growing in farmers’ paddocks in Australia indicated that N2 fixation contributed >65% of the legume’s N requirements in three-quarters of the annual legumes examined, but this decreased to two-thirds of lucerne (Medicago sativa; also known as alfalfa), and half of white clover (Trifolium repens) samples. Factors such as low numbers or the poor effectiveness of rhizobial strains in the soil, water stress, high soil concentrations of N, and nutrient disorders contribute to poor nodulation and %Ndfa values <65%, but there is also evidence that the observed %Ndfa can be dependent on the legume species present, and whether the legume is grown in a pure stand or in a mixed sward. The accumulation of legume-N relates primarily to the legume content and net productivity of the pasture. For many legume species, ~20 kg of shoot-N is fixed on average for every tonne of herbage dry matter produced. Legume productivity can be influenced by (i) sowing and establishment techniques and other strategies that enhance the legume content in pasture swards; (ii) the amelioration of soil constraints; (iii) the use of new legume species (and host–rhizobial strain combinations) that are more tolerant of hostile soil environments than subterranean clover (T. subterraneum) or annual medics (Medicago spp); and (iv) the inclusion of perennials such as lucerne to offset the year-to-year variability in productivity and N2 fixation that is a common occurrence with annual legumes.

Diversifying crop rotations with pulses enhances system productivity

“…We found that the process of crop residue decomposition added additional mineral N to the soil N pools. By contrast, the summerfallow system lost soil N during the postharvest period, whereas the cereal monoculture systems had little or no change in postharvest soil N. The soil N dynamics are complicated 40 , and the quantity of soil N can change with many factors 41,42 . In the present study, the largest change in soil N over the postharvest period occurred in the fall 2007 to spring 2008 period when the summerfallow fields lost 63.9 N kg ha −1 , whereas the fields after pulses increased soil N by 25.3 kg ha −1 , and the fields after wheat increased by 18.5 kg ha −1 .…”

A review of organic carbon accumulation in soils within the agricultural context of southern New South Wales, Australia

“…During the KohnRot trial from 1963 to 1981 the soil N accumulation rate of 45-65 kg N/ha/year was associated with N fixation rates of 95-113 kg N/ha/year (Helyar et al, 1997). In general, modern pasture systems have the potential to fix ∼250 kg N/ha/year due to improved varieties and inoculants, and better management of soil constraints (Peoples et al, 2012), hence the potential for soil N accumulation has improved.…”

Can differences in 15N natural abundance be used to quantify the transfer of nitrogen from legumes to neighbouring non-legume plant species?

“…Therefore, the use of the d 15 N of N 2 -fixing legume shoots for the term d 15 N leg (eN) in Equation (4) actually implies that of all of the possible sources of legume N and pathways that can contribute to N transfer to a non-legume (Table 1), only aboveground fixed N was transferred. Given the potential size of below-ground pool of legume N and rhizosphere deposition of N (Fustec et al, 2010;Peoples et al, 2012), and reports that intercropping enhances root production and turnover of fine roots (Lehmann and Zech, 1998), a first approximation for N sources for transfer might more plausibly equate to a below-ground pathway of transfer of fixed N from the roots and/or nodules rather than above-ground sources of fixed N. This would particularly the case for annuals where opportunities for N contributions from foliage will be limited during a single growing season. Below-ground Bergersen et al (1988), Boddey et al (2000), Unkovich et al (2008Unkovich et al ( , 2000, and Unkovich ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 sources of N are also likely to be important in forage and agroforestry systems where above-ground parts are regularly harvested or pruned as this can induce senescence of nodules and roots (Vance et al, 1979;Sanginga et al, 1995;Jalonen and Sierra, 2012).…”