Soil biogeochemical responses to multiple co-occurring forms of human-induced environmental change

Human activities cause a multitude of environmental issues, including increased temperatures and altered precipitation patterns associated with climate change, air pollution, and other impacts of urbanization. One area highly affected by these issues is the Sonoran Desert, specifically the Phoenix metropolitan area where urbanization is among the most rapid in the United States. Most studies investigate these multiple environmental change factors independently or sometimes in pairs, but rarely all together as co-occurring forms of change. We examined how the simultaneous manipulation of increasing temperatures, altered precipitation patterns, nitrogen deposition, and urbanization influenced soil respiration and mineral N pools in the Sonoran Desert. Soil was collected from urban and exurban sites, from both nitrogen-fertilized and control plots. To simulate projected climate change, the soils were incubated in microcosm at the annual average Phoenix temperature as well a 2 ℃ increase under a factorial precipitation treatment of decreased frequency and increased pulse size. Our results show that C and N dynamics were altered by all four forms of environmental change. However, the dominance of significant 3- and 4-way interactions among the four environmental factors for both respiration and mineral N pools demonstrates that the impact of any given form of environmental change will depend on the levels of the other environmental factors. In other words, the cumulative effect of altered precipitation, fertilization, temperature, and urbanization on soil biogeochemical processes is not necessarily predictable from their individual impact.

Suppression of methane uptake by precipitation pulses and long-term nitrogen addition in a semi-arid meadow steppe in northeast China

In the context of global change, the frequency of precipitation pulses is expected to decrease while nitrogen (N) addition is expected to increase, which will have a crucial effect on soil C cycling processes as well as methane (CH₄) fluxes. The interactive effects of precipitation pulses and N addition on ecosystem CH₄ fluxes, however, remain largely unknown in grassland. In this study, a series of precipitation pulses (0, 5, 10, 20, and 50 mm) and long-term N addition (0 and 10 g N m^-2 yr^-1, 10 years) was simulated to investigate their effects on CH₄ fluxes in a semi-arid grassland. The results showed that large precipitation pulses (10 mm, 20 mm, and 50 mm) had a negative pulsing effect on CH₄ fluxes and relatively decreased the peak CH₄ fluxes by 203-362% compared with 0 mm precipitation pulse. The large precipitation pulses significantly inhibited CH₄ absorption and decreased the cumulative CH₄ fluxes by 68-88%, but small precipitation pulses (5 mm) did not significantly alter it. For the first time, we found that precipitation pulse size increased cumulative CH₄ fluxes quadratically in both control and N addition treatments. The increased soil moisture caused by precipitation pulses inhibited CH₄ absorption by suppressing CH₄ uptake and promoting CH₄ release. Nitrogen addition significantly decreased the absorption of CH₄ by increasing NH₄ ⁺-N content and NO₃ ^--N content and increased the production of CH₄ by increasing aboveground biomass, ultimately suppressing CH₄ uptake. Surprisingly, precipitation pulses and N addition did not interact to affect CH₄ uptake because precipitation pulses and N addition had an offset effect on pH and affected CH₄ fluxes through different pathways. In summary, precipitation pulses and N addition were able to suppress the absorption of CH₄ from the atmosphere by soil, reducing the CH₄ sink capacity of grassland ecosystems.

Multi-model assessment of hydrological and environmental impacts on streambed microbes in Mediterranean catchments

Microbes inhabiting intermittent streambeds are responsible for controlling and developing many biogeochemical processes essential for the ecosystem functions. Although streambed microbiota is adapted to intermittency the intensification of water scarcity and prolonged dry periods may jeopardise their capacity to cope with hydrological changes. This study aims to evaluate whether, and to what extent, the duration of dry periods affects streambed microbial density, diversity, composition (16S rRNA gene diversity) and functions (extracellular enzyme activities and respiration). Our results highlight the fact that hydrology modulates the community composition and, to some extent, the functions carried out under different environmental conditions. The relative abundance of certain taxa inhabiting the driest intermittent communities differs significantly from those found at sites with continuous flow. Microbial functional metrics revealed a progressive increase in recalcitrant carbon degradation activity at sites with an extended dry phase. In contrast, bacterial density and diversity were mainly influenced by the catchment land use, agriculture enhanced density but reduced diversity, and the presence of riparian vegetation supported greater streambed bacterial diversity. From this perspective, a combination of prolonged dryness with reduced riparian vegetation and increased agricultural land cover could compromise the ecosystem functioning by threaten microbially mediated processes linked to the carbon cycle.

Changes in riparian hydrology and biogeochemistry following storm events at a restored agricultural stream

Quantifying changes in riparian biogeochemistry following rainfall events is critical for watershed management. Following storms, changes in riparian hydrology can lead to high rates of nutrient processing and export and greenhouse gas (GHG) release. We assessed shifts in hydrology and biogeochemistry 24 and 72 hours post-rainfall following storms of three different magnitudes in an agricultural riparian zone influenced by stream restoration in the Piedmont region of North Carolina, USA. Post-storm changes in water table height, soil moisture, groundwater flow, and lateral hydraulic gradient were related to biogeochemical processing. Though near-field nitrate (NO3-) concentrations were elevated (median: 13 mg nitrogen (N) L-1 across storms), substantial riparian NO3- removal occurred (89-96%). High N removal throughout the study occurred concurrently with release of dissolved solutes (e.g., soluble reactive phosphorus [SRP]) and fluxes of gases (carbon dioxide [CO2], nitrous oxide [N2O], and methane [CH4]), based on storm timing, magnitude, and intensity. A high intensity, short duration storm of low magnitude lead to release of CO2 across the riparian zone and low SRP removal. A storm of intermediate duration/magnitude towards the beginning of the summer lead to mobilization of near-field NO3- and release of N2O in the upper riparian zone and SRP in the lower riparian zone. Finally, a larger storm of longer duration lead to pronounced near-stream release of CH4. Therefore, it is important to expand research of biogeochemical response to different types of storm events in restored riparian zones to better balance water quality goals with potential greenhouse gas emissions.

Twenty Years of Riparian Zone Research (1997-2017): Where to Next?

Riparian zones have been used for water quality management with respect to NO in subsurface flow and total P (TP), sediments, and pesticides in overland flow for decades. Only recently has the fate and transport of soluble reactive P (SRP), Hg, emerging contaminants, and greenhouse gas (GHG) fluxes (NO, CO, and CH) been examined in riparian zones. Overall, riparian zones are efficient at reducing emerging contaminants in subsurface flow and only function as hot spots of methylmercury production in the landscape when dominated by Hg-rich wet organic soils. However, riparian zones do not provide consistent benefits with respect to SRP removal or GHG emissions. Although most existing riparian models almost exclusively focus on NO removal, recent developments in riparian models demonstrate the potential for using easily accessible digital environmental datasets to simulate and scale up riparian functions beyond NO removal to include SRP, TP, and GHG dynamics. To further inform integrated watershed management efforts, more research should be conducted on how various practices, including stream restoration, subsurface drainage, two-stage ditches, beaver dam analogues, denitrification bioreactors and permeable reactive barriers, artificial wetlands, and short-rotation forestry crops affect riparian water and air quality functions. Riparian zone benefits should be discussed not only with respect to water and air quality, but also in terms of recreation, habitat for wildlife, and other ecosystem services. More research is needed to fully address potential water quality or air quality tradeoffs associated with riparian zone management in a multicontaminant-multiuse landscape context.

The impact of flooding on aquatic ecosystem services

Flooding is a major disturbance that impacts aquatic ecosystems and the ecosystem services that they provide. Predicted increases in global flood risk due to land use change and water cycle intensification will likely only increase the frequency and severity of these impacts. Extreme flooding events can cause loss of life and significant destruction to property and infrastructure, effects that are easily recognized and frequently reported in the media. However, flooding also has many other effects on people through freshwater aquatic ecosystem services, which often go unrecognized because they are less evident and can be difficult to evaluate. Here, we identify the effects that small magnitude frequently occurring floods (< 10-year recurrence interval) and extreme floods (> 100-year recurrence interval) have on ten aquatic ecosystem services through a systematic literature review. We focused on ecosystem services considered by the Millennium Ecosystem Assessment including: (1) supporting services (primary production, soil formation), (2) regulating services (water regulation, water quality, disease regulation, climate regulation), (3) provisioning services (drinking water, food supply), and (4) cultural services (aesthetic value, recreation and tourism). The literature search resulted in 117 studies and each of the ten ecosystem services was represented by an average of 12 ± 4 studies. Extreme floods resulted in losses in almost every ecosystem service considered in this study. However, small floods had neutral or positive effects on half of the ecosystem services we considered. For example, small floods led to increases in primary production, water regulation, and recreation and tourism. Decision-making that preserves small floods while reducing the impacts of extreme floods can increase ecosystem service provision and minimize losses.

Acidity and organic matter promote abiotic nitric oxide production in drying soils

Soils are an important source of NO, particularly in dry lands because of trade-offs that develop between biotic and abiotic NO-producing processes when soils dry out. Understanding how drier climates may offset the balance of these trade-offs as soils transition toward more arid states is, therefore, critical to estimating global NO budgets, especially because drylands are expected to increase in size. We measured NO emission pulses after wetting soils from similar lithologies along an altitudinal gradient in the Sierra Nevada, CA, where mean annual precipitation varied from 670 to 1500 mm. Along the gradient, we measured field NO emissions, and used chloroform in the laboratory to reduce microbial activity and partition between biotic and abiotic NO-producing processes (i.e., chemodenitrification). Field NO emission pulses were lowest in the acidic and SOM-rich soils (4-72 ng NO-N m^-2 s^-1 ), but were highest in the high-elevation barren site (~560 ng NO-N m^-2 s^-1 ). In the laboratory, NO emission pulses were up to 19× greater in chloroform-treated soils than in the controls, and these abiotic pulses increased with elevation as pH decreased (6.2-4.4) and soil organic matter (SOM) increased (18-157 mg C g^-1 ). Drought can shift the balance between the biotic and abiotic processes that produce NO, favoring chemodenitrification during periods when biological processes become stressed. Acidic and SOM-rich soils, which typically develop under mesic conditions, are most vulnerable to N loss via NO as interactions between pH, SOM, and drought stimulate chemodenitrification.

Aridity and plant uptake interact to make dryland soils hotspots for nitric oxide (NO) emissions

Nitric oxide (NO) is an important trace gas and regulator of atmospheric photochemistry. Theory suggests moist soils optimize NO emissions, whereas wet or dry soils constrain them. In drylands, however, NO emissions can be greatest in dry soils and when dry soils are rewet. To understand how aridity and vegetation interact to generate this pattern, we measured NO fluxes in a California grassland, where we manipulated vegetation cover and the length of the dry season and measured [δ(15)-N]NO and [δ(18)-O]NO following rewetting with (15)N-labeled substrates. Plant N uptake reduced NO emissions by limiting N availability. In the absence of plants, soil N pools increased and NO emissions more than doubled. In dry soils, NO-producing substrates concentrated in hydrologically disconnected microsites. Upon rewetting, these concentrated N pools underwent rapid abiotic reaction, producing large NO pulses. Biological processes did not substantially contribute to the initial NO pulse but governed NO emissions within 24 h postwetting. Plants acted as an N sink, limiting NO emissions under optimal soil moisture. When soils were dry, however, the shutdown in plant N uptake, along with the activation of chemical mechanisms and the resuscitation of soil microbial processes upon rewetting, governed N loss. Aridity and vegetation interact to maintain a leaky N cycle during periods when plant N uptake is low, and hydrologically disconnected soils favor both microbial and abiotic NO-producing mechanisms. Under increasing rates of atmospheric N deposition and intensifying droughts, NO gas evasion may become an increasingly important pathway for ecosystem N loss in drylands.

Investigating the influence of long-range transport on surface O3 in Nevada, USA, using observations from multiple measurement platforms

The current United States (US) National Ambient Air Quality Standard (NAAQS) for O3 (75 ppb) is expected to be revised to between 60 and 70 ppb. As the NAAQS becomes more stringent, characterizing the extent of O3 and precursors transported into the US is increasingly important. Given the high elevation, complex terrain, and location in the Intermountain West, the State of Nevada is ideally situated to intercept air transported into the US. Until recently, measurements of O3 and associated pollutants were limited to areas in and around the cities of Las Vegas and Reno. In 2011, the Nevada Rural Ozone Initiative began and through this project 13 surface monitoring sites were established. Also in 2011, the NASA Ames Alpha Jet Atmospheric eXperiment (AJAX) began making routine aircraft measurements of O3 and other greenhouse gases in Nevada. The availability of aircraft and surface measurements in a relatively rural, remote setting in the Intermountain West presented a unique opportunity to investigate sources contributing to the O3 observed in Nevada. Our analyses indicate that stratosphere to troposphere transport, long-range transport of Asian pollution, and regional emissions from urban areas and wildfires influence surface observations. The complexity of sources identified here along with the fact that O3 frequently approaches the threshold being considered for a revised NAAQS indicate that interstate and international cooperation will be necessary to achieve compliance with a more stringent regulatory standard. Further, on a seasonal basis we found no significant difference between daily 1-h maximum O3 at surface sites, which ranged in elevation from 888 to 2307 m, and aircraft measurements of O3 <2500 m which suggests that similar processes influence daytime O3 across rural Nevada and indicates that column measurements from Railroad Valley, NV are useful in understanding these processes.