Measuring plant-associated nitrogen fixation in agricultural systems

An appraisal of recent field measurements of symbiotic N2 fixation by annual legumes

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

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

Symbiotic dinitrogen (N2) fixation of crop and pasture legumes is a critical component of agricultural systems, but its measurement is expensive and labour intensive. Simple models which can provide approximations based on crop or pasture dry matter production would be useful for agrononomists and those interested in regional nitrogen (N) cycle fluxes. We investigate meta analysis of published data on legume shoot dry matter production, shoot %N and legume %N fixed (%Ndfa) and look for relationships among these, as a possible way of providing useful approximations of N2 fixation. We restricted our analysis to Australian studies where we have ready access to the primary data and where cultivars, management and climate are more constrained compared to a universal dataset. Regression analysis between shoot dry matter and amounts of shoot N2 fixed were strong for all crop and pasture legumes with significant differences in slope and intercept values being obtained between pastures and crops, and between chickpea (Cicer arietinium) and all other crop and pasture legumes. Annual pasture legumes showed the strongest linear relationship between N2 fixation and shoot dry matter and had the greatest slope (20.2–24.3 kg N2 fixed/t), compared to 18.7 kg N2 fixed/t for the perennial pasture legume lucerne (alfalfa, Medicago sativa), and between 10.7 to 23.0 kg N2/t for crop legumes, depending upon species. It was recognised that the use of such shoot-based relationships would underestimate the total amounts of N2 fixed since the contributions of fixed N present in, or derived from, roots and nodules are not included. Furthermore there needs to be careful consideration of the validity of an intercept term, which might reflect suppression of N2 fixation at low dry matter and high soil mineral N availability, or possibly the use of non-linear regression. For chickpea crops grown in north-eastern Australia, multiple regression indicated that N2 fixation was much more closely correlated with %Ndfa than dry matter production. Evidence presented also indicated that %Ndfa of other crops and lucerne in this region may similarly be influenced by soil mineral N. The regression approach presented provides a statistical basis to approximate N2 fixation in the first instance. This work highlights some of the dangers of fitting single regressions to aggregated datasets and using these to approximate symbiotic N2 fixation. The analysis indicates that where pasture legumes are grown in mixtures with non-legumes, and driven to high dependence on N2 fixation, simple linear regressions may be quite useful, provided that possible differences between species are investigated as the slopes of the regressions between these can be quite different. For crop legumes, where low dependence on N2 fixation can occur at higher mineral N availability, there is a need to carefully consider the intercept term, obtain estimates of mineral N availability, and/or resort to non-linear models. The gross generalisations presented in scatter plots cannot be reliably applied any more specifically, even within the datasets from which they were generated, and in some cases even within legume species between regions. They cannot substitute for direct measurement where any certainty is required under a particular set of defined conditions.

/pdf/prospects-and-problems-of-simple-linear-models-for-1ffvpv2vjo.pdf

Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes

Agricultural systems are nitrogen (N) deficient throughout the world and N is the most important nutrient in realizing the maximum potential of the crop and sustainability of the environment. The production and utilization of the chemical N fertilizer (CNF) have played a great role in increasing crop yield and meeting the demand of population growth. However, with excessive input of chemical N fertilizers, nitrate N leaching and surface runoff seriously threat people health and pollute biological environments. Based on a large number of publications, this paper has comprehensively reviewed the importance of application of CNF, the problems caused by the excessive chemical N input, with attention mainly to the seriousness of nitrate N loss by leaching and runoff, factors affecting nitrate N leaching and runoff, methods for controlling nitrate leaching and runoff, and research needs in the future globally in details.

Nitrate N loss by leaching and surface runoff in agricultural land: A global issue (a review)

Functions quantifying relationships between N2 flxation by legumes and other factors would be useful to farmers in the management of legumes and nitrogen in their production systems. The 2 most critical factors regulating N2 flxation are legume yield and soil nitrate and both should be included as independent variables in the functions. Data from 9 experiments in northern New South Wales on soil nitrate at sowing, yields of shoot and grain dry matter (DM) and N of chickpea, and - 15 N of shoots of chickpea and non N2-flxing reference crops (wheat, barley, and uninoculated chickpea) were used to determine the percentage of chickpea N derived from N2 flxation (Pflx), total N2 flxed, and N balance (flxed N2 minus grain N). Data were then subjected to simple and multivariate regression analyses with Pflx and total N2 flxed as the dependent variables and soil nitrate, shoot N, and grain yield as the independent variables. Simple regression coe-cients (r 2 ) for Pflx were 0¢26 with shoot N as the independent variable, 0¢59 with soil nitrate, and 0¢62 with grain yield. Coe-cient values were increased in the 2-factor (multiple) regressions to 0¢74 (P<0¢001) for soil nitrate plus shoot N, and 0¢82 ( P< 0 ¢001) for soil nitrate plus grain yield. For total N2 flxed, the regression coe-cient was lower at 0¢68 ( P< 0 ¢001), using soil nitrate plus grain yield. We tested the functions against an independent data set with best prediction of Pflx involving the nitrate and grain yield equation (r 2 =0 ¢83; P< 0 ¢001). Total N2 flxed was not well predicted. We concluded that further development of such functions is warranted to reflne both accuracy and precision for chickpea and to extend the approach to other species. Additional keywords: N2 flxation regressions, natural 15 N abundance.

Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N2 fixation and N balance using soil nitrate at sowing and chickpea yield

Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N

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Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N2 fixation and N balance using soil nitrate at sowing and chickpea yield

Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N