Abstract
Forest water reclamation is a cost-effective wastewater management strategy where treated municipal wastewater is applied to forested land to leverage natural filtration processes. This method has also long been thought to be environmentally friendly due to the ability of forests to filter out contaminants from partially reclaimed wastewater. However, long-term application of reclaimed wastewater may be harmful to forest ecosystem health. The research featured in this thesis uses a four-decade time-series of forest water reclamation facilities in North Idaho to investigate the long-term impacts of forest water reclamation on forest nitrogen (N) cycling. First, the rates of NO₃⁻ and NH₄⁺ mineralization and immobilization were investigated using soil and litter samples. We found that the rate of NO₃⁻ mineralization in soils and NO₃⁻ immobilization in litter were higher at long-established facilities, where reclaimed wastewater was applied for more than 25 years. Second, we measured the rate of N loss through gaseous efflux by using gradient method. We analyzed nitric oxide (NO) and nitrous oxide (N₂O) abundance in gas samples using gas chromatography and mass spectrometry (GCMS). We did not find evidence that N₂O or NO emissions were affected by wastewater application. Finally, we measured the abundance of a variety of N-cycling functional genes to assess microorganism response to long-term reclaimed wastewater addition. We extracted DNA from soil samples and analyzed gene abundance using real-time quantitative polymerase chain reaction (qPCR) tests. We quantified the abundance of ammonia-oxidizing archaeal genes (amoA-AOA), ammonia-oxidizing bacteria genes (amoA-AOB), NO-reducing genes (nirS and nirK), and N₂O-reducing genes (nosZ). We found that amoA-AOA and amoA-AOB, nirS, and nosZ were more abundant in effluent-treated soils at long-established facilities. nirK was more abundant in effluent-treated soils but did not differ based on facility establishment date. The increased abundance of functional genes at long-established facilities indicates a response by nitrogen-cycling microorganisms to prolonged wastewater application. Our observations demonstrate that prolonged reclaimed wastewater application to nutrient-limited coniferous forests leads to a cumulative effect on nitrogen-cycling activity and nitrogen-cycling microbial communities within the soil profile. While we could not find evidence that N gas efflux increases in response to forest water reclamation, future research may utilize refined methodology to yield stronger conclusions.