In nutrient-limited conditions, plants rely on rhizosphere microbial members to facilitate nutrient acquisition, and in return, plants provide carbon resources to these root-associated microorganisms. However, atmospheric nutrient deposition can affect plant-microbe relationships by changing soil bacterial composition and by reducing cooperation between microbial taxa and plants. To examine how long-term nutrient addition shapes rhizosphere community composition, we compared traits associated with bacterial (fast-growing copiotrophs, slow-growing oligotrophs) and plant (C3 forb, C4 grass) communities residing in a nutrient-poor wetland ecosystem. Results revealed that oligotrophic taxa dominated soil bacterial communities and that fertilization increased the presence of oligotrophs in bulk and rhizosphere communities. Additionally, bacterial species diversity was greatest in fertilized soils, particularly in bulk soils. Nutrient enrichment (fertilized versus unfertilized) and plant association (bulk versus rhizosphere) determined bacterial community composition; bacterial community structure associated with plant functional group (grass versus forb) was similar within treatments but differed between fertilization treatments. The core forb microbiome consisted of 602 unique taxa, and the core grass microbiome consisted of 372 unique taxa. Forb rhizospheres were enriched in potentially disease-suppressive bacterial taxa, and grass rhizospheres were enriched in bacterial taxa associated with complex carbon decomposition. Results from this study demonstrate that fertilization serves as a strong environmental filter on the soil microbiome, which leads to distinct rhizosphere communities and can shift plant effects on the rhizosphere microbiome. These taxonomic shifts within plant rhizospheres could have implications for plant health and ecosystem functions associated with carbon and nitrogen cycling. IMPORTANCE Over the last century, humans have substantially altered nitrogen and phosphorus cycling. Use of synthetic fertilizer and burning of fossil fuels and biomass have increased nitrogen and phosphorus deposition, which results in unintended fertilization of historically low-nutrient ecosystems. With increased nutrient availability, plant biodiversity is expected to decline, and the abundance of copiotrophic taxa is anticipated to increase in bacterial communities. Here, we address how bacterial communities associated with different plant functional types (forb, grass) shift due to long-term nutrient enrichment. Unlike other studies, results revealed an increase in bacterial diversity, particularly of oligotrophic bacteria in fertilized plots. We observed that nutrient addition strongly determines forb and grass rhizosphere composition, which could indicate different metabolic preferences in the bacterial communities. This study highlights how long-term fertilization of oligotroph-dominated wetlands could alter diversity and metabolism of rhizosphere bacterial communities in unexpected ways.

Green stormwater infrastructure, such as constructed wetlands (CWs), is a type of stormwater control measure that can decrease nutrient and pollutant loads from urban stormwater runoff. Wetland soil microorganisms provide nutrient and pollutant removal benefits which can also result in ecosystem disservices such as greenhouse gas (GHG) emissions and can inadvertently exacerbate climate change. Microbial respiration by facultative anaerobes in anoxic conditions is the primary pathway for nitrogen removal (benefit). Similar anoxic conditions that support denitrifying microorganisms can also support obligate anaerobes that produce methane (CH 4 ) via methanogenesis (disservice). We examined nitrogen removal potential, GHG production, and microbial community structure within permanently flooded and shallow land or temporarily-flooded areas of a stormwater CW to identify zones for CW design optimization.Results indicate that permanently flooded zones compared to shallow land zones are greater sources of CH 4 emissions (80.80 ± 118.31, 2.32 ± 9.33 mg CH 4 -C m -2 hr -1 , respectively) and emit more carbon to the atmosphere (7161.27 kg CO 2 , 93.20 kg CO 2 equivalents, respectively). However, nitrogen removal potential rates were similar across both flooded and shallow land zones (24.45 ± 20.18, 20.29 ± 15.14 ng N 2 O-N hr -1 g -1 dry soil, respectively). At this particular CW, reduction of permanently flooded zones within the wetland could decrease GHG emissions (disservice) without limiting nitrogen removal (benefit) potential of the wetland. Holistic development and design of stormwater control measures, which account for microbial activity, provides the opportunity to maximize benefits (i.e., nutrient and pollutant removal) and reduce disservices (i.e., GHG emissions) of green stormwater infrastructure.

Abbreviations: corrected Akaike information criteria = AICc, NH 4 + = ammonium, C = carbon, 12 CO 2 = carbon dioxide, CW = constructed wetland, N 2 = dinitrogen, GWP = global warming 13 potential, GHG = greenhouse gas, CH 4 = methane, N= nitrogen, NO 3 -= nitrate, NO = nitric 14 oxide, NO 2 -= nitrite, N 2 O = nitrous oxide, OTU = operational taxonomic unit, PCR = 15 polymerase chain reaction, PERMANOVA = permutated analysis of variance, PCoA = principal 16 coordinates analysis, SCM = stormwater control measure 17Abstract 18Green stormwater infrastructure, such as constructed wetlands (CWs), is a type of 19 stormwater control measure that can reduce nutrient and pollutant loads in urban stormwater 20 runoff. Nutrient and pollutant removal processes within wetlands are primarily driven by 21 microbial functions that can also result in disservices such as greenhouse gas (GHG) emissions, 22 which can negate climate change mitigation. Specifically, microbial respiration by facultative 23 anaerobes in anoxic conditions is the primary pathway for nitrogen removal. Similar anoxic 24 conditions that support denitrifiers can also support obligate anaerobes that produce methane via 25 methanogenesis. In this study, we examined nitrogen removal potential, GHG production, and 26 microbial community structure within flooded and shallow land areas of a stormwater CW to 27 identify zones for CW design optimization. Our results indicate that permanently flooded zones 28 are sources of methane emissions and have the greatest contribution to climate change. However, 29 denitrification potential rates were similar across both flooded and shallow land zones. This 30 suggests that shallow land areas can provide nitrogen removal services with reduced GHG 31 emissions compared to flooded zones. In the case of this particular CW, a reduction of 32 permanently flooded zones within the wetland could decrease GHG emissions (i.e., disservice) 33 without limiting denitrification (i.e., benefit) potential of the wetland. We conclude that holistic 34 development and design of stormwater control measures, which account for microbial functions, 35 provides the opportunity to maximize benefits (i.e., nutrient and pollutant removal) and reduce 36 disservices (i.e., greenhouse gas emissions) of green stormwater infrastructure. 37

Human activities have led to increased deposition of nitrogen (N) and phosphorus (P) into soils. Nutrient enrichment of soils is known to increase plant biomass and rates of microbial litter decomposition. However, interacting effects of hydrologic position and associated changes to soil moisture can constrain microbial activity and lead to unexpected nutrient feedbacks on microbial community structurefunction relationships. Examining feedbacks of nutrient enrichment on decomposition rates is essential for predicting microbial contributions to carbon (C) cycling as atmospheric deposition of nutrients persists. This study explored how long-term nutrient addition and contrasting litter chemical composition influenced soil bacterial community structure and function. We hypothesized that long-term nutrient enrichment of low fertility soils alters bacterial community structure and leads to higher rates of litter decomposition especially for low C:N litter, but low-nutrient and dry conditions limit microbial decomposition of high C:N ratio litter. We leveraged a long-term fertilization experiment to test how nutrient enrichment and hydrologic manipulation (due to ditches) affected decomposition and soil bacterial community structure in a nutrient-poor coastal plain wetland. We conducted a litter bag experiment and characterized litter-associated and bulk soil microbiomes using 16S rRNA bacterial sequencing and quantified litter mass losses and soil physicochemical properties. Results revealed that distinct bacterial communities were involved in decomposing higher C:N ratio litter more quickly in fertilized compared to unfertilized soils especially under drier soil conditions, while decomposition rates of lower C:N ratio litter were similar between fertilized and unfertilized plots. Bacterial community structure in part explained litter decomposition rates, and long-term fertilization and drier hydrologic status affected bacterial diversity and increased decomposition rates. However, community composition associated with high C:N litter was similar in wetter plots with available nitrate detected, regardless of fertilization treatment. This study provides insight into long-term fertilization effects on soil bacterial diversity and composition, decomposition, and the increased potential for soil C loss as nutrient enrichment and hydrology interact to affect historically lownutrient ecosystems.

AbstractWhile wetlands represent a small fraction (~7%) of the world’s land surface, it is estimated that one third of wetlands have been lost due to human activities. Wetland habitat loss decreases ecosystem functions such as improving water quality and mitigating climate change. These microbially mediated functions are dependent on particular soil redox conditions, which are altered by soil hydrology and the presence of plants. Differences in microbial physiology allow certain taxa (aerobes and facultative anaerobes) to adapt to fluctuating (dry/wet) hydrologic conditions, while other taxa (obligate anaerobes) are better adapted to continually saturated conditions. Therefore, the duration of hydrologic periods can affect soil microbial community structure and function. Further, plant-derived carbon, nutrients, and air are released by diffusion belowground which also impacts microbial activity in soils. In this study, we hypothesized that redox status due to continuous flooding would support greater abundance of microbial taxa involved in methanogenesis (obligate anaerobes), but plant-mediated oxygen transport would decrease methane emissions. Using a mesocosm design, we manipulated duration of hydrologic condition (i.e., stable dry, stable flooding, and alternating wet/dry) and presence of plants to induce soil redox changes in wetland soils. We measured soil redox status, used targeted amplicon sequencing to characterize the bacterial and archaeal communities, and measured greenhouse gas production to assess microbial function. Hydrology and to a lesser degree plant presence influenced soil redox conditions. Hydrologic history strongly influenced microbial community composition, but plant presence and hydrologic treatment altered microbial function to a great degree. As predicted, plant presence decreased greenhouse gas production in the wetland mesocosms. While previous studies do not often include plants when assessing greenhouse gas emissions, this study highlights that plant-mediated decreases in greenhouse gas emissions are significant. If plant-mediated effects are not considered when estimating the carbon balance of ecosystems, then wetland carbon storage could be underestimated.