Ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) co-occupy every agricultural soil that has been examined to date; however, we know little about AOA and AOB population dynamics and activity under field conditions. A case can be made that the relative contributions of AOA and AOB to soil nitrification might shift in different phases of crop rotation and during different seasons of the year. We have chosen a simple two-year cropping cycle of winter wheat/fallow to test our hypothesis that environmental conditions combined with shifts in NH4+ availability will influence the dynamics of AOA and AOB contributions to nitrification. We hypothesized that the relative success of AOA and AOB through the two-year cropped/fallowed cycle will depend upon a combination of differential growth responses to the application of fertilizer NH4+-N and differential abilities to survive the NH4+ limiting conditions that exist over the majority of the two-year cropping cycle.
Results/Conclusions
By sampling soil monthly from cropped-to-fallowed (CF) and fallowed-to-cropped (FC) phases of a two year wheat/fallow cycle, and adjacent uncultivated long term fallowed land (LTF), evidence was obtained for effects of cropping phase and season on soil nitrification potentials (NPs), the contributions of AOA and AOB to recovered NPs (RNP), AOA and AOB amoA abundance, and AOB community composition. In both CF and FC, NP and RNP were greatest in spring (Apr-May) and lowest in winter (Jan), but did not change significantly in LTF. Whereas there were no significant differences in AOA amoA abundance in 2010 among the three treatments, AOB amoA abundance was greatest in Apr 2010 and declined significantly by Oct in FC and LTF, and by Dec in CF. In both CF and FC, NP increased significantly between Jan and Apr 2011 regardless of fertilizer N application to FC in Mar. During this period, neither RNPAOA or RNPAOB rates increased significantly in CF (no fertilizer N applied 2011), and their relative contributions to RNP did not change despite a significant increase in AOA amoA abundance. In FC (fertilizer N applied 2011) there were significant increases in both RNPAOA and RNPAOB rates, in the relative contribution of AOB to RNP, and in AOB amoA abundance, but no significant change occurred in AOA amoA abundance. There were no significant effects of LTF, cropping phase or season on AOA community composition; however, significant effects of LTF, CF, and FC were detected on AOB community composition.