Mark B. Peoples

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N2 via biological N2 fixation (BNF).

Biological nitrogen fixation: Investments, expectations and actual contributions to agriculture

Inputs of biologically fixed N into agricultural systems may be derived from symbiotic relationships involving legumes and Rhizobium spp., partnerships between plants and Frankia spp. or cyanobacteria, or from non-symbiotic associations between free-living diazotrophs and plant roots. It is assumed that these N2-fixing systems will satisfy a large portion of their own N requirements from atmospheric N2, and that additional fixed N will be contributed to soil reserves for the benefit of other crops or forage species. This paper reviews the actual levels of N2 fixation attained by legume and non-legume associations and assesses their role as a source of N in tropical and sub-tropical agriculture. We discuss factors influencing N2 fixation and identify possible strategies for improving the amount of N2 fixed.

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems

Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30–40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legumegrain may be relatively small.Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability of N in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogenemitted from nodules as a by-product of N2, fixation.

Can the Synchrony of Nitrogen Supply and Crop Demand be Improved in Legume and Fertilizer-based Agroecosystems? A Review

Asynchrony between nitrogen (N) supply and crop demand is the source of many environmental hazards associated with excess N in the biosphere. In this review, we explore some of the complexity of the synchrony issue in agroecosystems that obtain N via legume rotations or synthetic fertilizers. Studies that have simultaneously compared the fate of both sources of N suggest that in rainfed agricultures, crops recover more N from fertilizer, but a higher proportion of the legume N is retained in the soil and N losses tend not to differ greatly from either source. However, investigations from irrigated cropping systems indicate that legume N is generally less susceptible to loss processes than fertilizers. Such general conclusions need to be qualified by acknowledging that not all comparative studies have used ȁ8best management practices’ when applying the fertilizer or legume residues. When information-intensive management approaches are used, fertilizer-based systems can potentially out-perform the synchrony achieved by legume-based rotations. We suggest that the inclusion of perennials in cropping systems may hold the greatest promise for decreasing the risk of N losses in future farming systems.

Enhancing legume N2 fixation through plant and soil management

Atmospheric N2 fixed symbiotically by associations between Rhizobium spp. and legumes represents a renewable source of N for agriculture. Contribution of legume N2 fixation to the N-economy of any ecosystem is mediated by: (i) legume reliance upon N2 fixation for growth, and (ii) the total amount of legume-N accumulated. Strategies that change the numbers of effective rhizobia present in soil, reduce the inhibitory effects of soil nitrate, or influence legume biomass all have potential to alter net inputs of fixed N. A range of management options can be applied to legumes growing in farming systems to manipulate N2 fixation and improve the N benefits to agriculture and agroforestry.

Factors regulating the contributions of fixed nitrogen by pasture and crop legumes to different farming systems of eastern Australia

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N2 fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N2 fixed ranged from 30 to 160 kg shoot N fixed ha−1 yr−1 for annual pasture species, 37–128 kg N ha−1 yr−1 for lucerne, and 14 to 160 kg N ha−1 yr−1 by pulses. These data have provided new insights into differences in factors controlling N2 fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales. Consequently N2 fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced. Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N2 fixation for growth (19–74%) and more variable relationships between N2 fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots. The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included. This has important implications for the interpretation of the results of previous N2 fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations.

Chickpea increases soil-N fertility in cereal systems through nitrate sparing and N2 fixation

There is a need to introduce cropping practices in the northern N.S.W. cereal belt that increase N supply for cereal, and in particular wheat, production. Annual crop legumes, grown in rotation with cereal crops, can contribute to the total pool of N in the soil and improve yields of the cereals. Experiments, in 1989 and 1990 at two sites near North Star, N.S.W., are described which aim to (i) assess the effects of chickpea on concentrations of soil nitrate, both in terms of “N sparing” during legume growth and release of N bound in the crop residues; (ii) quantify N2 fixation by chickpea using the natural15N abundance and modified N different techniques; and (iii) explore the concept of water use efficiency (WUE) of N2 fixation for use in the management of N in cropping systems. The two sites and three rates of fertilizer N (0, 50 and 100 kg ha−1) provided six combinations of site, season and nitrate-N fertility. Nitrogen fixed ranged from 29 to 85 kg ha−1, with amounts influenced by method of assessment and fertilizer N treatment. There was general agreement between the natural 15N abundance and modified N difference methods. Soil nitrate spared by chickpea ranged from 6 to 31 kg N ha−1, over sites and treatments, and averaged 11 kg N ha−1 at Windridge and 26 kg N ha−1 at Glenhoma. In the May sampling in the following year, i.e. after the summer and autumn fallow, differences in soil nitrate-N between the chickpea and wheat plots ranged from 29 to 51 kg ha−1, and averaged 44 kg ha−1 (Windridge) and 43 kg ha−1 (Glenhoma). Water use efficiency of chickpea N2 fixation varied between 0.14 and 0.24 kg ha−1 mm−1, with the higher values associated with higher N2 fixation.

Break-crop benefits to wheat in Western Australia – insights from over three decades of research

Abstract. Broadleaf break crops improve cereal yield through disease and weed control, increased nitrogen (N) availability and other mechanisms. In the rainfed farming systems of Australia the magnitude of the yield benefit is highly variable, yet is a major driver for adoption of break crops which are often less profitable and more risky than cereals. Declining area of break crops throughout Australia has re-ignited interest in better understanding the circumstances in which break-crop benefits can be maximised from a farming systems perspective. We compiled and analysed a database of 167 crop sequence experiments conducted throughout Western Australia in the period 1974–2007 to evaluate the impact on wheat (Triticum aestivum L.) grain yield from the use of narrow-leafed lupin (Lupinus angustifolius L.), field pea (Pisum sativum L.), canola (Brassica napus L.) or oats (Avena sativa L.), or following a long fallow where no crop had been sown the previous year. Adjusted for the years in which each was represented the average yield benefit to wheat compared with wheat after wheat was 0.60, 0.45, 0.40, 0.35 and 0.30 t/ha following lupin, field pea, canola, oats or fallow, though direct comparisons between break crops could not be made as few experiments (3) included all species. For all break crops, the mean wheat yield increase was independent of the level of wheat yield, representing a step-change rather than a proportional improvement in yield. Analysis of the larger number and spread of lupin experiments revealed that break-crop benefits increased in higher rainfall areas, following higher yielding lupin crops (>1.5 t/ha), and that the break-crop benefit in terms of yield and water-use efficiency increased significantly after 1991. These observations were often related to the level and/or effectiveness of diseases or grass weed control in the break crop; however, increased contribution of fixed N was also likely with better legume crops. For both lupin and field pea, the magnitude of the break-crop response declined as rate of N fertiliser applied to subsequent wheat crop increased, although non-N related benefits (disease and weed control) tended to dominate wheat response to lupin after 1989. Significant break-crop benefits from lupins (+0.40 t/ha) persisted to a third wheat crop (n = 29) but effects were inconsistent beyond that point. The magnitude, persistence and reliability of the break-crop benefits revealed in this study provide a more accurate framework to assess their likely benefit within the farming system. Further information is required to define the key ‘trigger points’ for the major drivers of the response – water, N, weeds and disease – at which the benefits outweigh the higher risk of these crops and would influence the decision to include them within the system.

The effect of timing and severity of water deficit on growth, development, yield accumulation and nitrogen fixation of mungbean

Mungbean (Vigna radiata L.), as a dryland grain legume, is exposed to varying timing and severity of water deficit, which results in variability in grain yield, nitrogen accumulation and grain quality. In this field study, mungbean crops were exposed to varying timing and severity of water deficit in order to examine: (1) contribution of the second flush of pods to final grain yield with variable timing of relief from water deficit, (2) the sensitivity to water deficit of the accumulation of biomass and nitrogen (N) and its partitioning to grain, and (3) how the timing of water deficit affects the pattern of harvest index (HI) increase through pod filling. The results showed that the contribution of the second flush to final yield is highly variable (1-56%) and can be considerable, especially where mid-season stress is relieved at early pod filling. The capacity to produce a second flush of pods did not compensate fully for yield reduction due to water stress. Relief from mid-season stress also resulted in continued leaf production, N-2 fixation and vegetative biomass accumulation during pod filling. Despite the wide variation in the degree of change in vegetative biomass and N during pod filling, there were strong relationships between grain yield and net-above-ground biomass at maturity, and grain N and above-ground N at maturity. Only in the extreme situations were HI and nitrogen HI affected noticeably. In those treatments where there was a large second flush of pods, there was a pronounced biphasic pattern to pod number production, with HI also progressing through two distinct phases of increase separated by a plateau. The proportion of grain yield contributed to by biomass produced before pod filling varied from 0 to 61% with the contribution greatest under terminal water deficit. There was a larger effect of water deficit on N accumulation, and hence N-2 fixation, than on biomass accumulation. The study confirmed the applicability of a number of long-standing physiological concepts to the analysis of the effect of water deficit on mungbean, but also highlighted the difficulty of accounting for timing effects of water deficit where second flushes of pods alter canopy development, biomass and yield accumulation, and N dynamics. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.

Improved subsoil macroporosity following perennial pastures

Biopores left in the soil by perennial and annual pastures and their effects on macroporosity, water infiltration and the water use and productivity of subsequent wheat and canola crops were investigated in a field experiment on a Sodosol in southern New South Wales. Phases of both lucerne (4 years) and phalaris (10 years) improved the macroporosity and water infiltration into the dense B horizon compared with continuous annual crops and pastures. After removal of lucerne and phalaris, the subsoil (> 12 cm depth) contained similar numbers of pores > 2 mm diameter (228 and 190/m2, respectively) compared with a mean of 68/m2 after annual crops. However water infiltration rate after lucerne was greater than after phalaris, apparently because of more numerous pores > 4 mm, rather than a change in total porosity. The subsoil after phalaris on the other hand contained more pores 0.3 mm in diameter and a higher total porosity, possibly because of more roots around this diameter, and a longer period without traffic or cultivation. The number of lucerne biopores in the subsoil remained unchanged (170–180/m2) for at least 2 crops after the lucerne was removed although the average size decreased. The volume of water extracted from the subsoil by crops following lucerne was similar to that following annual crop/pasture for 10 of the 12 crop comparisons made. For 2 of the crops, more subsoil water (22 and 24 mm) was used after lucerne than after annuals, and in 1 season this was associated with higher yield of canola. During the 3-year study there was no winter waterlogging or post-anthesis water stress, so there was little opportunity for yield responses to improved subsoil structure. The results confirm speculation that the unfavourable structure of dense subsoils can be improved by the biological action of perennial pasture roots, although reduced wheel traffic and cultivation during the pasture phases may also play a role. Further studies will be necessary to demonstrate associated yield improvements.