Ammonia-oxidizer and denitrifier populations in boreal forest soils of differing nitrogen availability Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qz20sw58m

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  • Nitrification and denitrification are major biological processes transforming nitrogen (N) in soils to plant available N, highly leachable nitrate (NO₃⁻) and gaseous N oxides. Although many studies in the past have studied N cycling communities, the effect of increased N inputs on ammonia-oxidizer and denitrifier population dynamics is still under active investigation. The purpose of this study was to examine the variation of ammonia-oxidizer and denitrifier populations in boreal forest soils providing markedly different localized sources of N availability. Here, we sampled along a natural N-availability gradient, characteristic of the inherent variation in N supply and plant productivity in a boreal forest, and in a long-term fertilized site receiving three levels of N fertilizer, of which the highest was terminated over 10 years ago. Quantitative PCR of archaeal (AOA) and bacterial (AOB) ammonia-oxidizer genes (amoA) and denitrifier genes (nitrite reductase, nirS and nirK; nitrous oxide reductase, nosZ) was used to quantify the populations. In these boreal soils the abundance of AOB amoA dominated over AOA amoA. The size of the AOB population was significantly affected by fertilization application, whereas AOA was not; however, AOA amoA gene copies did increase in response to the high mineralization rates at the high N site along the natural gradient. Our findings indicate that AOB dominate ammonia oxidation in heavily fertilized soils and AOA may contribute significantly to ammonia oxidation in low N environments controlled by N mineralization. The abundance of nirK and nosZ responded most to N additions, but nirS did not, suggesting nirK-bearing denitrifiers dominate these high organic matter containing, well-drained soils. The size of the nirK and nosZ functional groups increased with increasing N availability at all sites. It was found that differences in gene copies per gram of soil could not be explained directly by increased NO₃⁻ concentrations but, rather indirectly, through an increase in organic matter substrate quality. In fact, the increased organic matter quality explained the high denitrifier population after fertilization cessation suggesting a resilient effect of fertilization lasting over a decade; whereas, the ammonia-oxidizer community returned to baseline population size after N fertilization was stopped. The community size results along the natural gradient were combined with previously published data on potential nitrification and denitrification rates. It was found that the difference in ammonia-oxidizer and denitrifier populations was positively related to differences in process rates. The study of these functional genes provides insight into the variability of the microbial community populations responsible for regulating important steps in the N cycle.
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