Title: Predictors of nitrogen-fixing activity across a local gradient in fire history for a temperate semiarid grassland Authors
Submitted to: Soil Science Society of America Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: December 15, 2013
Publication Date: March 8, 2014
Citation: Rout, M., Vermeire, L.T., Reinhart, K.O. 2014. Predictors of nitrogen-fixing activity across a local gradient in fire history for a temperate semiarid grassland. Soil Science Society of America Annual Meeting. Online paper 84584. Technical Abstract: Since nitrogen is the primary limiting nutrient in terrestrial ecosystems, understanding the regulators of nitrogen (N2) fixation is critical. Our aim was to identify predictors of free-living N2-fixer activity across a fire history gradient in a temperate semiarid grassland. We predicted that recent fire would increase soil nitrogen and phosphorus, and the activity of a functional gene (nifH) for N2 fixation would be positively correlated with phosphorus and negatively correlated with nitrogen. We sampled 18 plots from a mixed-grass prairie site in the Northern Great Plains. Plots had variable prescribed fire histories (e.g. time since fire) and were sampled five times from September 2011 - August 2012. For each, we quantified 21 soil properties and the activity of the nifH gene (DNA and mRNA) using quantitative PCR. The seasonal sampling tracked the progression of a regional drought. Greater levels of nifH DNA and mRNA were observed in early (moist) than late (drought stage) sampling. We then analyzed these two distinct periods separately using multiple linear regression. For the moist period, we determined the greatest amount of variation in nifH mRNA was explained by a model with 4 variables (adjR2= 0.36). The most influential variables were manganese followed by iron and then temperature. The nifH mRNA was positively correlated with manganese and negatively correlated with iron and temperature. For the dry period, the greatest amount of variation in nifH mRNA was explained by a complex model with 11 variables (adjR2= 0.54). The most influential variables were calcium followed by manganese and organic matter. During this period, nifH mRNA was positively correlated with organic matter and negatively correlated with calcium and manganese. Soil moisture was a main factor limiting nifH activity. Some soil properties were also useful predictors of nifH mRNA. However, we failed to detect correlations between nifH activity and either fire history variables, nitrogen, or phosphorus. A small number of other soil properties, that often varied depending on whether analyzes were for the moist vs. dry periods, were useful predictors of variation in nifH activity.