Optimizing nodulation in chickpea for nitrogen fixation and yield in the northern grains belt of NSW

Abstract

Sub-optimal nodulation of chickpea (Cicer arietinum L.) has previously been identified as potentially limiting N2 fixation and yield in the northern grains belt of NSW, one of the largest chickpea producing areas in Australia. The aim of this thesis was to identify the primary factors and processes inhibiting nodulation and quantify their relative importance to farming systems within this region. The further aim was to use this information to formulate management guidelines to assist farmers to optimise nodulation and N2 fixation. Ultimately this will improve productivity of their chickpea crops and the farming systems which potentially benefit from them. The research undertaken to address the issue of sub-optimal nodulation took a systemic approach. This involved a comprehensive evaluation of the farmer and their management practices at the higher level of the systems hierarchy down to soil microbiological processes and interactions existing at the lower level. The methodology used to identify potentially limiting factors included surveying 61 chickpea farmers on various aspects of chickpea nodulation and N2 fixation, the benchmarking of 15 commercial chickpea crops (2006 and 2007 seasons) and the characterisation of the size (MPN plant infection test), gross N2 fixing capacity (WSIT), genetic composition (RAPD-PCR) (26 soils) and competitive ability of naturalised populations (15 soils) of chickpea rhizobia present in these northern grains belt soils. Twenty-nine individual rhizobial strains were selected based on either their ability to out-compete the inoculant strain CC1192 for nodule occupancy or their origin from populations of high symbiotic capacity. These strains were evaluated for their N2-fixing capacity and as a potential replacement for CC1192. Yields of benchmarked crop ranged from 0.49 to 2.91 t ha-1 which is consistent with the variation in plant available water in each of the study years but generally lower than the best yields recorded in the region (up to 3 t ha-1). For these crops variation in nodulation explained little of the variation in yield. No strong link was established between management practices employed by chickpea farmers and inhibition of nodulation, N2-fxation and/or yield. This lack of response to nodulation was likely due to adequate levels of soil N at sowing meeting the N requirements of the crop. Of the 21 paddocks earmarked for chickpea production in 2005-7 levels of soil nitrate at sowing averaged 116 kg N ha-1 with 67% of paddocks having more than 100 kg nitrate-N ha-1. Whilst the benchmarking study did not confirm previous findings implicating poor nodulation in low crop yields, it did confirm that many crops are sown into high N soils. This reduces dependency on N2 fixation, thus limiting the N benefit received from including a leguminous crop in a rotation. The survey of farmers, and the benchmarking study, identified areas for potential improvement that may assist in optimising nodulation and N2 fixation. Firstly, the farmer survey revealed that 82% of farmers were not using soil tests to determine N status of paddocks selected for chickpea production, which is of concern considering the known inhibitive effects of high soil N on nodulation. Farmers largely remain uninformed of the N status of their paddocks and are consequently selecting paddocks with inappropriate levels of soil nitrate. Whilst not significant there was a decline in nodulation with increasing starting soil nitrate (r=-0.44; P>0.05). Secondly, some farmers are employing poor inoculation practices such as delaying the sowing of inoculated seed for >24 h, which conflicts with current recommendations. Private agronomists were identified by farmers as their primary source of information regarding inoculation. They could, therefore, be utilised to extend to farmers ways in which their current inoculation practices can be improved e.g. minimising the time between inoculation and sowing. Thirdly, inadequate management of broadleaf weeds was the most significant agronomic issue affecting productivity and indirectly affecting potential N2 fixation. High weed density significantly reduced chickpea plant height (r=-0.53; P=0.05). Whilst no single agronomic management practice accounted for a large portion of variability in nodulation or N2 fixation in the benchmarked crops, the cumulative effect of addressing each of these individual issues may help to optimise nodulation, N2 fixation and ultimately chickpea productivity. Despite the potential importance of the soil and agronomic factors discussed above, microbiological factors emerged as an issue of greater significance. Microbiological studies identified the evolution of novel rhizobial strains capable of infecting chickpea. They comprised 86% of naturalised populations of chickpea rhizobia in the evaluated soils. Genetic sequencing of a selection of these novel strains revealed that they belonged to the genus Mesorhizobium. It is hypothesised that these novel strains have evolved through horizontal transfer of symbiotic genes from the inoculant strain CC1192 to native mesorhizobia in the soil. Some of these novel strains appear to be highly competitive, successfully out competing CC1192 to occupy 47% of nodules harvested from the commercially grown chickpea plants in the benchmarked crops. This was a surprising result considering the apparent high competitive capacity of CC1192. Whilst not inhibiting nodulation per se these novel strains may be compromising N2 fixation, with 58% of evaluated rhizobial populations having a reduced N2-fixing capacity relative to CC1192. In this study, however, there was no significant association (P>0.05) between nodule occupancy and N2-fixation (r=0.09) or yield (r=0.07) of the chickpea crop, presumably because reduced effectiveness was masked by the high soil N levels that met the N requirements of the chickpea crops. Further evaluation in low N soils is recommended to confirm the extent to which naturalised strains of reduced effectiveness may be compromising N2-fixation and productivity of chickpea crops in this region. Spatial distribution of nodules (i.e. proximally or distally located to seed) had no significant bearing on N2 fixation (r=0.21; P=0.51) and/or final grain yield (r=0.39; P=0.19). Nodules located further from the seed were, however, are more likely to be occupied by strains other than CC1192 (χ2=36.9; P is less than 0.001).Evaluation of the N2 fixing effectiveness of 29 individual strains present within these naturalised populations further confirmed that the evolutionary process in some instances has resulted in a loss in symbiotic capacity, with 41% of strains having reduced N2 fixing capacity relative to CC1192. The remaining 59% of strains were of equal effectiveness to CC1192 with one having superior N2-fixing ability. Strains that have potential as replacement inoculant strains were identified, but they require further testing to confirm their suitability. Replacing the current inoculant strain with one of superior competitive and symbiotic capacity may be the most effective means of overcoming competitive naturalised populations. The initial aim of this thesis was to identify the primary factors inhibiting nodulation. However, the systems approach taken to researching this issue has revealed that occupancy of nodules by strains of reduced effectiveness is relatively more important than farmer management practices that might suppress nodulation. Whilst these strains with reduced symbiotic capacity may not suppress nodulation, they potentially compromise N2 fixation. It follows that failure to address the problem of competitive but less effective strains of rhizobia will mean that agronomic management to improve the performance of poorly nodulated crops will have limited success. Despite effects at the microbiological level being identified as fundamentally more important than broader scale issues such as farmer practices, it is still recommended that good agronomic practice be adhered to e.g. paddock selection, inoculation procedures and weed management. By adopting this multi-faceted approach it is anticipated that nodulation, N2-fixation and productivity of chickpea crops will be optimised and ultimately the full potential of chickpea as a component of farming systems in the northern grains belt of NSW realised.

Date of Award	2009
Original language	English