Duncan, D. S. 2016. Linking soil microbiology and environmental conditions to variability in nitrous oxide production in bioenergy cropping systems. Dissertation, University of Wisconsin- Madison, Madison, Wisconsin.
Agroecosystems may differ in multiple ecosystem properties, among them nitrous oxide (N2O) production and soil microbial community composition. We hypothesized that perenniality, plant species richness, and exogenous nitrogen inputs all influence N2O production directly through regulation of substrate concentrations and other environmental conditions and indirectly through changes to soil microbial functional characteristics. We studied the interplay among cropping systems, microbial communities, and N2O production in the context of an agronomic trial of potential bioenergy feedstock cropping systems.
We measured N2O production from 2009-2014 and collected accompanying data on soil temperature, water-filled pore space, and inorganic nitrogen concentrations. Individual N2O fluxes and aggregate annual N2O emissions were lower in perennial systems relative to annual ones, but were not consistently influenced by plant species richness in perennial systems. Environmental variables defined upper limits for N2O fluxes, but did little to explain cropping system effects or their lack.
We explored microbial community differences between continuous corn and prairie systems using membrane lipid profiling, amplicon sequencing, and functional gene annotations from shotgun metagenomic sequencing. The strength of cropping system effects differed among methods, with the strongest effects observed in lipid profiles. We used elastic net modeling to correlate community profiles to aggregate N2O emissions. Only the corn system could be effectively modeled, with the best models created from 16S rRNA amplicons and functional gene abundances.
We used bacterial functional gene abundance profiles to characterize microbial communities across a broader range of cropping systems. The strength of cropping system effects varied among site years. Ecological factors such as perenniality and species diversity did not determine abundance patterns for either the full set of genes explored or for groups of genes with similar functions. Similarly, individual denitrification pathway genes did not systematically differ among cropping systems.
Cropping system effects on N2O production and functional gene abundances were weaker than anticipated. Despite this, elastic net modeling linked gene abundance patterns to variation in N2O emissions with considerable accuracy. This indicates that within-cropping system variability in N2O production and functional genes are in some way connected.
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