Although I am not completly sure, I believe that at least under Brazilian no-till systems (which are now most of our soybean growing), there is no accumulation of soil nitrate. Properly conducted no-till under tropical conditions depends strongly on high biomass production, and the crop before soybean is usually corn, pearl millet, sorghum or a grass forage, which would mean stubbles with high C/N ratios, and most probably a low decomposition rate. This should lead to low nitrate accumulation. You may be interested in papers by Mariangela Hungria on this subject.

In our research, we found a solution. Intercropping can alleviate the inhibitory effect of nitrate on nodulation and N2 fixation of legumes if the legume is intercropped with non-legumes. The mechanism behind the improvement of N2 fixation under high soil nitrate by intercropping is that non-legumes exhausted nitrate and benefited to nodulation and N2 fixation of legumes. The paper was published in Plant and Soil (Li et al., 2011).

Mario and Long: Thanks for the response. Here in Canada, especially in the black soil zones with sufficient moisture, soil N mineralization during the growing season can lower N2 fixation: https://www.agronomy.org/publications/aj/articles/104/6/1501?highlight=

Intercropping would be a great model without issues in harvesting in large scale farms of north america.

Just as prof. Long Li says, intercroppig may be a solution to lower inorganic (both nitrate and ammonium) N concentration in soil. However in many systems, intercropping is not easily feasable because of many factors. Lentil, in particular, is not a good competitor against other species due to its low growth rate, due to the little dimension of its canopy and to its reduced root growth. Thus I do not suggest to intercrop it.

There are a number of other agronomic solution to increase N2 fixation (BNF) (even if an increase in BNF does not imply an increase in biomass production, grain yield or harvest index). One solution is a correct management of the soil organic matter and the crop residue. The addition to soil of crop residues with high C/N ratio (higher than 40 or 50) will imply a sequestration of inorganic N by the mineralizer bacteria, but this may reduce N availability for the following crop.

Another strategy is sowing N catch crop, but pay attention that buring plant residues with high N content (as those of the catch crop during their vegetative growth, especially during anthesis) can result in a strong increase in N mineralization and thus in the inorganic N concentration of soil. N mineralization, in Canada, mainly occurs from May to September, I presume. If lentil is during its vegetative growth or its grain filling stage during this period, you could also achieve a reduction in BNF due to an increase in mineral N in soil. however, if you can use a forage crop (a grass or a asteracea as chicory, not legumes) as a catch crop and you can remove the biomass from the soil, it will surely result in a reduction of the inorganic N pool of the soil, but beware that continuesly removing the plant biomass during the years surely imply a reduction in the organic C pool of the soil.

Another strategy is the inoculation of soil with arbuscular mycorrhizal fungi (AMF) and other plant promothing fungi (as Trichoderma, etc) , plant growth promoting rhizobacteria (PGPR), phosphorus solubilizing bacteria (PSB) and high efficient rhizobia. The increase in soil microbic biomass can consist in many effects: first of all, the inoculation of rhizobia can directly increase BNF . Secondly, the inoculation with PSB and AMF can indirectly increase BNF through a increased plant P uptake given that rhizobia have a high P demand. In addition, consider that AMF and the other fungi have a high N demand, that they take up from soil as inorganic N (mainly ammonium), but pay attention that some Trichoderma species are plant pathogens.

In my experience at the University of Palermo (Italy) we (Giambalvo D., Amato G., Ruisi P., Di Miceli G., Frenda A.S. and I, of course) , we are very interested in studying the effects of many agronomic factors on BNF. In a study aimed to evaluate the long-term (18-year) impact of different tillage systems on faba bean (Vicia faba L.) productivity and BNF we observed that BNF was strongly influenced by tillage, being higher under NT than under CT (Giambalvo et al., Plant Soil (2012) 360:215–227).

In other studies we observed a increase in BNF of Trifolium alexandrinum when AMF were inoculated to soil in comparison to uninoculated control (Saia S.et al. (2010) Effects of arbuscular mycorrhizal symbiosis on growth and N2 fixation of Trifolium alexandrinum under late drought-stress conditions. Proceedings of Grassland Science in Europe 15 (H. Schnyder et al. Eds), Kiel. pp. 842-844), whereas in lentil we also observed an increase in plant biomass in AMf incoculated than uninoculated control, and this could be due to an increased N2 fixation rate. (Saia, S., et al. 2012. Effects of soil inoculation with arbuscular mycorrhizal fungi on plant growth and nutrient uptake of some Mediterranean species grown under rainfed field conditions. In 17th International Nitrogen Workshop (Ireland) - (Eds: Richards, K; Fenton, O; Watson, CJ.) Innovation For Sustainable Use Of Nitrogen Resources (pp.90-91). Wexford (Ireland).

In conclusion: a number of strategies can be adopted to increase BNF of lentil in long term NT systems and these strategies imply a

- management of the rotation (with speacies with high N demand);

- management of soil organic matter pool (fostering the stabilization of the soil organic matter) ;

- management of the microbiological fertility (increase in bacterial and fungal plant partners)

You can find the links to our papers in my linkedin account. (http://www.linkedin.com/in/sergiosaia)

Cheers, Sergio.