Advancing field-based techniques to identify microbial allies in nitrogen retention in agricultural soils

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Kinsey M Reed, Plant and Soil Science, West Virginia University, Morgantown, WV and Ember Morrissey, Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV

Background/Question/Methods
The use of synthetic fertilizer in agriculture has become indispensable to feeding the global population. Unfortunately, a significant portion of the synthetic nitrogen applied to soil is lost through leaching, resulting in serious pollution. Therefore, finding ways to limit nitrogen losses while maintaining the global food supply is essential. When soil microbes assimilate nitrogen into their biomass, it is retained in soil and is slowly released to crops over time. Using quantitative stable isotope probing (qSIP), a method that tracks 15N into the DNA of microorganisms, it is possible to determine which microbial taxa are adept at N assimilation in agricultural soils. However, this method has yet to be applied in situ. The aim of this work was to 1) develop methods for 15N field qSIP in rhizosphere soils 2) determine if rhizosphere N assimilation processes differ between laboratory-based and field incubations and 3) identify soil prokaryotes important for N assimilation in the rhizosphere of maize under organic and conventional management. Rates of gross nitrogen immobilization and mineralization were measured using the 15N pool dilution in conventional and organic farm soil. Then, qSIP was used to peer into the demographics of which microbial taxa were assimilating the most 15N.
Results/Conclusions
The field-based measurements resulted in lower N assimilation rates than laboratory incubations. Additionally, the differences in rates between management strategies were less significant in the field-based incubations possibly due to root uptake, leaching, and the lack of soil disturbance. The preservation of soil peds in the field incubations means that less surface area is exposed, thus “protecting” the nitrogen inside from microbial use. Microbial nitrogen assimilation as determined by qSIP was somewhat lower and more variable under field conditions, possibly due to root uptake of the added nitrogen. Yet, under both field and laboratory conditions the prokaryotes assimilating N in the rhizosphere of organically grown maize were more diverse and abundant than conventionally grown maize. Because the laboratory rates were exaggerated, the use of in situ methods to determine rates of N transformation is crucial in order to understand how microbiome composition influences soil nitrogen cycling. These in situ measures can be used to optimize nitrogen fertilization and reduce air and water pollution. Using field-based qSIP, we were able to identify key microbial taxonomic groups that assimilate nitrogen. Management strategies that favor these taxa may mitigate nitrogen pollution without sacrificing crop yields.