Crop Species Influence Abundance of Denitrifier Communities in an Agricultural Field.

Crop species are known to influence soil organic carbon either by increasing its availability or by changing its quality i.e. the type of organic carbon compounds. Plant roots can change soil conditions through nitrate uptake, carbohydrate exudation and respiration which influence oxygen supply level, potentially affecting denitrifier community, and nitrous oxide (N₂O) emissions.  The objective of this study was to evaluate the effect of seven crop species (red clover, soybean, Italian ryegrass, barley, canola, corn and potato) on the abundance of denitrifier communities, nosZ transcript levels and N₂O emissions and respiration in 2007 and 2009 in a two-year potato crop rotation system.  Abundance of denitrifier communities had a 1.1- to 3.6-fold change between soils planted to different crop species. The nirS group abundance from soil planted to Italian ryegrass was significantly greater compared to soils planted to other crop species in both years.   The abundance of the nirK denitrifiers was similar between soils under the different crop species with the exception of a significantly lower abundance in soil planted to corn (2007) and potato (2009) compared to soils from four other crop species suggesting that conditions were unfavorable for this community.  The nosZ denitrifier abundance was relatively similar between crop species in both years. Respiration, which reflects availability of the organic carbon, was lower in potato (2007) and corn (2009) compared to other crop species. The number of nosZ transcripts per cell was greater in soils cultivated to Italian ryegrass and barley compared with soybean and red clover in 2007 while in 2009, the number of nosZ transcripts per cell was greater in soil cultivated under Italian ryegrass compared with all other crop species.  This result suggests that oxygen supply level and/or nitrate and reduced intermediate concentrations were more conducive for the up-regulation of denitrification genes in soils under Italian ryegrass.  There was no significant difference in N₂O emissions between crop species in 2007 probably due to high variability in the data, however N₂O emissions were greater from soil planted with potato compared to all other crop species in 2009.  The results demonstrate that crop species influenced N₂O emissions and denitrifier community abundances but not all denitrifier communities responded similarly.  The crop species stimulated or reduced denitrifier community abundances perhaps through a change in oxygen supply level or organic carbon quality but not availability since there was no relationship between change in respiration and denitrifier community abundances.