Tillage intensity and plant rhizosphere selection shape bacterial-archaeal assemblage diversity and nitrogen cycling genes

In agriculture, adoption of reduced tillage practices is a widespread adaptation to global change. The cessation of plowing reduces erosion, slows soil organic matter oxidation, and promotes soil carbon accrual, but it can also result in the development of potential N2O spots from denitrification activity. In this study, we hypothesized that 16S rRNA-based composition of bacterial-archaeal assemblages would differ in agricultural soils subjected for forty years to a range of disturbance intensities, with annual moldboard plowing (MP) being the most intensive. No-till planting (NT) represented tillage management with the least amount of disturbance, while chisel-disking (CD), a type of conservation tillage, was intermediate. All long-term tillage plots had been planted with the same crops grown in a three-year crop rotation (corn-soybean-small grain+cover crop), and both bulk and rhizosphere soils were analyzed from the corn and soybean years. We also evaluated denitrification gene markers by quantitative PCR at multiple points (three growth stages of corn and soybean). Tillage intensity, soil compartment (bulk or rhizosphere), crop year, growth stage, and interactions all exerted effects on community diversity and composition. Compared to MP and CD, NT soils had lower abundances of denitrification genes, higher abundances of nitrate ammonification genes, and higher abundances of taxa at the family level associated with the inorganic N cycle processes of archaeal nitrification and anammox. Soybean rhizospheres exerted stronger selection on community composition and diversity relative to corn rhizospheres. Interactions between crop year, management, and soil compartment had differential impacts on N gene abundances related to denitrification and nitrate ammonification. Opportunities for managing hot spots or hot moments for N losses from agricultural soils may be discernible through improved understanding of tillage intensity effects, although weather and crop type are also important factors influencing how tillage influences microbial assemblages and N use.

Growth Yields in Bacterial Denitrification and Nitrate Ammonification

ABSTRACT Denitrification and nitrate ammonification are considered the highest-energy-yielding respiration systems in anoxic environments after oxygen has been consumed. The corresponding free energy changes are 7 and 35% lower than that of aerobic respiration, respectively. Growth yield determinations with pure cultures of Paracoccus denitrificans and Pseudomonas stutzeri revealed that far less energy is converted via ATP into cell mass than expected from the above calculations. Denitrification with formate or hydrogen as electron donor yielded about 2.4 to 3.0 g dry matter per mol formate or hydrogen and 15 to 18 g dry matter per mol acetate. Similar yields with acetate were obtained with Pseudomonas stutzeri. Wolinella succinogenes and Sulfurospirillum deleyianum, which reduce nitrate to ammonia, both exhibited similar yield values with formate or H2 plus nitrate. The results indicate that ATP synthesis in denitrification is far lower than expected from the free energy changes and even lower than in nitrate ammonification. The results are discussed against the background of our present understanding of electron flow in denitrification and with respect to the importance of denitrification and nitrate ammonification in the environment.

Respiratory Nitrate Ammonification by Shewanella oneidensis MR-1

ABSTRACT Anaerobic cultures of Shewanella oneidensis MR-1 grown with nitrate as the sole electron acceptor exhibited sequential reduction of nitrate to nitrite and then to ammonium. Little dinitrogen and nitrous oxide were detected, and no growth occurred on nitrous oxide. A mutant with the napA gene encoding periplasmic nitrate reductase deleted could not respire or assimilate nitrate and did not express nitrate reductase activity, confirming that the NapA enzyme is the sole nitrate reductase. Hence, S. oneidensis MR-1 conducts respiratory nitrate ammonification, also termed dissimilatory nitrate reduction to ammonium, but not respiratory denitrification.