David F. Herridge

Measurement of N2 fixation in maize (Zea mays L.)—ricebean (Vigna umbellata [Thunb.] Ohwi and Ohashi) intercrops

The yield of N in maize (Zea mays L.) and ricebean (Vigna umbellata [Thumb.] Ohwi and Ohashi) were compared on a Tropoqualf soil in North Thailand in 1984 and 1985. Both species were grown in field plots in monoculture or as intercrops at a constant planting density equivalent to 8 maize or 16 ricebean plants per m2. The contribution of symbiotic N2 fixation to ricebean growth was estimated from measurements of the natural abundance of15N (δ15N) in shoot nitrogen and from analysis of ureides in xylem sap vacuumextracted from detached stems.The natural abundance of15N in the intercropped ricebean was found to be considerably less than that in monoculture in both growing seasons. Using maize and a weed (Ageratum conyzoides L.) as non-fixing15N reference plants the proportions (P15N) of ricebean shoot N derived from N2 fixation ranged from 0.27 to 0.36 in monoculture ricebean up to 0.86 when grown in a 75% maize: 25% ricebean intercrop. When glasshouse-derived calibration curves were used to calculate plant proportional N2 fixation (Pur) from the relative ureide contents of field collected xylem exudates, the contribution of N2 fixation to ricebean N yields throughout the 1985 growing season were greater in intercrop than in monocrop even at the lowest maize:legume ratio (25∶75). Seasonal patterns of sap ureide abundance indicated that N2 fixation was greatest at the time of ricebean podset. The averagePur andP15N in ricebean during the first 90 days of growth showed identical rankings of monocrop and intercrop treatments in terms of N2 fixation, although the two sets ofP values were different. Nonetheless, seasonal estimates of N2 fixation during the entire 147 days of legume growth determined from ureide analyses indicated that equivalent amounts of N could be fixed by ricebean in a 75∶25 intercrop and in monoculture despite the former being planted at one-quarter the density.

Greenhouse gas emissions profile for 1 tonne of wheat produced in Central Zone (East) New South Wales: a life cycle assessment approach

Abstract. Life Cycle Assessment (LCA) has become an increasingly common approach across different industries, including agriculture, for environmental impact assessment. A single-issue LCA focusing on greenhouse gas emissions was conducted to determine the emissions profile and total carbon footprint of wheat produced in the Central Zone (East) of New South Wales. Greenhouse gas emissions (in carbon dioxide equivalents; CO2-e) from all stages of the production process, both pre-farm and on-farm, were included. Total emissions were found to be 200u2009kg CO2-e per t of wheat at the farm gate, based on a 3.5u2009t/ha grain yield. The relative contribution of greenhouse gas emissions from different components of the production system was determined, with most emissions (37%) coming from pre-farm production and transport of fertiliser (30%) and lime (7%) and from the nitrous oxide (N2O) emitted from the nitrogenous fertiliser applied to the crop (26%). Other important emissions included the CO2 emissions from the use of fertiliser and lime (15%) and the production, transport and use of diesel (16%). The relative importance of other minor emissions is also discussed. For a higher yielding crop (5.0u2009t/ha), total emissions were found to be 150u2009kg CO2-e per t of wheat. This paper considers the effect of different management scenarios on the emissions profile and the effect of adopting a N2O emissions factor, which is based on current New South Wales field research, rather than the current Australian National Greenhouse Accounts National Inventory Report default value. This LCA provides a template from which comparative farming systems LCA can be developed and provides data for the Australian Life Cycle Inventory.

Quantification of Biological Nitrogen Fixation in Agricultural Systems

15 N natural abundance method and the ureide assay have now been developed to the point that onfarm measures of legume N 2 fixation can be conducted with confidence. Data from diverse systems (pastures, pulses, or legume oilseeds in areas of Australia, Asia, the Middle East and Africa) show clearly that environmental and management constraints to legume growth (basic agronomy, nutrition, water supply, diseases and pests) are the major regulators of N 2 fixation, although practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), or enhance soil nitrate levels (excessive tillage, extended fallows, fertilizer N, rotations) can also be critical. In the past, the amounts of N 2 fixed by legumes have been derived from shoot-based determinations of plant N. However, recent applications of 15 N shoot-feeding techniques to crop and pasture legumes suggest that a major portion (30–50%) of the total plant N is associated with the nodulated roots. Therefore, it is critical that estimates of below-ground N are included when biological nitrogen fixation (BNF) is quantified. Otherwise, the contributions of legumes to the N economies of agricultural systems may be substantially underestimated.

Greenhouse gas (N2O and CH4) fluxes under nitrogen-fertilised dryland wheat and barley on subtropical Vertosols: Risk, rainfall and alternatives

The northern Australian grains industry relies on nitrogen (N) fertiliser to optimise yield and protein, but N fertiliser can increase soil fluxes of nitrous oxide (N2O) and methane (CH4). We measured soil N2O and CH4 fluxes associated with wheat (Triticum aestivum) and barley (Hordeum vulgare) using automated (Expts 1, 3) and manual chambers (Expts 2, 4, 5). Experiments were conducted on subtropical Vertosol soils fertilised with N rates of 0–160kgNha–1. In Expt 1 (2010), intense rainfall for a month before and after sowing elevated N2O emissions from N-fertilised (80kgNha–1) wheat, with 417gN2O-Nha–1 emitted compared with 80g N2O-Nha–1 for non-fertilised wheat. Once crop N uptake reduced soil mineral N, there was no further treatment difference in N2O. Expt 2 (2010) showed similar results, however, the reduced sampling frequency using manual chambers gave a lower cumulative N2O. By contrast, very low rainfall before and for several months after sowing Expt 3 (2011) resulted in no difference in N2O emissions between N-fertilised and non-fertilised barley. N2O emission factors were 0.42, 0.20 and –0.02 for Expts 1, 2 and 3, respectively. In Expts 4 and 5 (2011), N2O emissions increased with increasing rate of N fertiliser. Emissions were reduced by 45% when the N fertiliser was applied in a 50:50 split between sowing and mid-tillering, or by 70% when urea was applied with the nitrification inhibitor 3,4-dimethylpyrazole-phosphate. Methane fluxes were typically small and mostly negative in all experiments, especially in dry soils. Cumulative CH4 uptake ranged from 242 to 435g CH4-Cha–1year–1, with no effect of N fertiliser treatment. Considered in terms of CO2 equivalents, soil CH4 uptake offset 8–56% of soil N2O emissions, with larger offsets occurring in non-N-fertilised soils. The first few months from N fertiliser application to the period of rapid crop N uptake pose the main risk for N2O losses from rainfed cereal cropping on subtropical Vertosols, but the realisation of this risk is dependent on rainfall. Strategies that reduce the soil mineral N pool during this time can reduce the risk of N2O loss.

Crop-available water and agronomic management, rather than nitrogen supply, primarily determine grain yield of commercial chickpea in northern New South Wales

Abstract. Chickpea (Cicer arietinum L.) is considered an effective rotation crop in Australia’s northern grains region; however, concerns exist that grain yields of commercial crops are reduced because of nitrogen (N) deficiency related to inadequate nodulation and N2 fixation. As part of a program to address these issues, we report on the monitoring of 22 commercial fields around Moree, northern NSW, during 2005–07 that were designated for chickpea, and an associated farmer survey (81 respondents). Our objectives were to determine whether the monitored crops were limited by N and to develop recommendations that would optimise productivity for farmers growing chickpeas. In 2005, only soil water and nitrate data were collected from the six fields designated for chickpea. In 2006 and 2007, almost complete datasets were assembled from the 16 chickpea fields or crops, including soil water and nitrate at sowing, row spacing, plant density, plant height, stubble cover, weed density and composition, shoot biomass, grain yield, nodulation and N2 fixation (%N derived from the atmosphere (%Ndfa) and total crop N fixed). The associated survey provided insights into farmer knowledge of, and practices related to, inoculation. Field monitoring indicated moderate–high levels of soil nitrate at sowing (averages 114, 126 and 110u2009kg N ha–1 to 1.2u2009m depth for 2005, 2006 and 2007, respectively) and generally low plant nodulation (0.11–1.16u2009g fresh wt plant–1) and N2 fixation (0–62%Ndfa and 0–87u2009kg N ha–1). Grain yield varied between 0.53 and 2.91u2009tu2009ha–1 across the 14 monitored crops, with averages of 1.89u2009tu2009ha–1 in 2006 and 1.02u2009tu2009ha–1 in 2007. Although total crop N and grain yields were highly correlated with total (i.e. soilu2009+u2009fixed) N supply, there was no evidence that the monitored chickpea crops were N-limited. Rather, we conclude that soil N and biologically fixed N were complementary in supplying N to the crops, the grain yields of which were primarily determined by the supply of plant-available water (PAW) and water-use efficiency (WUE). Simple and multivariate regression analyses showed that stubble cover during the fallow (positively correlated with sowing PAW) and sowing date (positively correlated with crop WUE) were significant determinants of grain yield. We conclude that farmers could improve inoculation practice by ensuring the time between seed inoculation and sowing is always <24u2009h.

Soil mineral nitrogen benefits derived from legumes and comparisons of the apparent recovery of legume or fertiliser nitrogen by wheat

Nitrogen (N) contributed by legumes is an important component of N supply to subsequent cereal crops, yet few Australian grain-growers routinely monitor soil mineral N before applying N fertiliser. Soil and crop N data from 16 dryland experiments conducted in eastern Australia from 1989–2016 were examined to explore the possibility of developing simple predictive relationships to assist farmer decision-making. In each experiment, legume crops were harvested for grain or brown-manured (BM, terminated before maturity with herbicide), and wheat, barley or canola were grown. Soil mineral N measured immediately before sowing wheat in the following year was significantly higher (Pu2009<u20090.05) after 31 of the 33 legume pre-cropping treatments than adjacent non-legume controls. The average improvements in soil mineral N were greater for legume BM (60u2009±u200916u2009kgu2009N/ha; nu2009=u20095) than grain crops (35u2009±u200920u2009kgu2009N/ha; nu2009=u200926), but soil N benefits were similar when expressed on the basis of summer fallow rainfall (0.15u2009±u20090.09u2009kgu2009N/ha per mm), residual legume shoot dry matter (9u2009±u20095u2009kgu2009N/ha per t/ha), or total legume residue N (28u2009±u200911%). Legume grain crops increased soil mineral N by 18u2009±u20099u2009kgu2009N/ha per t/ha grain harvested. Apparent recovery of legume residue N by wheat averaged 30u2009±u200910% for 20 legume treatments in a subset of eight experiments. Apparent recovery of fertiliser N in the absence of legumes in two of these experiments was 64u2009±u200916% of the 51–75u2009kg fertiliser-N/ha supplied. The 25 year dataset provided new insights into the expected availability of soil mineral N after legumes and the relative value of legume N to a following wheat crop, which can guide farmer decisions regarding N fertiliser use.

Cradle-to-farmgate greenhouse gas emissions for 2-year wheat monoculture and break crop–wheat sequences in south-eastern Australia

Abstract. We used life cycle assessment methodology to determine the cradle-to-farmgate GHG emissions for rainfed wheat grown in monoculture or in sequence with the break crops canola (Brassica napus) and field peas (Pisum sativum), and for the break crops, in the south-eastern grains region of Australia. Total GHG emissions were 225u2009kg carbon dioxide equivalents (CO2-e)/t grain for a 3u2009t/ha wheat crop following wheat, compared with 199 and 172u2009kgu2009CO2-e/t for wheat following canola and field peas, respectively. On an area basis, calculated emissions were 676, 677 and 586u2009kgu2009CO2-e/ha for wheat following wheat, canola and field peas, respectively. Highest emissions were associated with the production and transport of fertilisers (23–28% of total GHG emissions) and their use in the field (16–23% of total GHG emissions). Production, transport and use of lime accounted for an additional 19–21% of total GHG emissions. The lower emissions for wheat after break crops were associated with higher yields, improved use of fertiliser nitrogen (N) and reduced fertiliser N inputs in the case of wheat after field peas. Emissions of GHG for the production and harvesting of canola were calculated at 841u2009kgu2009CO2-e/ha, equivalent to 420u2009kgu2009CO2-e/t grain. Those of field peas were 530u2009kgu2009CO2-e/ha, equivalent to 294u2009kgu2009CO2-e/t grain. When the gross margin returns for the crops were considered together with their GHG emissions, the field pea–wheat sequence had the highest value per unit emissions, at AU

Validation of NBudget for estimating soil N supply in Australia’s northern grains region in the absence of soil test data

787/tu2009CO2-e, followed by wheat–wheat (

Soil N2O emissions under N2-fixing legumes and N-fertilised canola: A reappraisal of emissions factor calculations

703/tu2009CO2-e) and canola–wheat (

Greenhouse gas emission reductions in subtropical cereal-based cropping sequences using legumes, DMPP-coated urea and split timings of urea application

696/tu2009CO2-e). Uncertainties associated with emissions factor values for fertiliser N, legume-fixed N and mineralised soil organic matter N are discussed, together with the potentially high C cost of legume N2 fixation and the impact of relatively small changes in soil C during grain cropping either to offset all or most pre- and on-farm GHG emissions or to add to them.