Understanding Of Biogeochemical Cycles Research Articles

Realistic measurement uncertainties for marine macronutrient measurements conducted using gas segmented flow and Lab-on-Chip techniques

Accurate and precise measurements of marine macronutrient concentrations are fundamental to our understanding of biogeochemical cycles in the ocean. Quantifying the measurement uncertainty associated with macronutrient measurements remains a challenge. Large systematic biases (up to 10%) have been identified between datasets, restricting the ability of marine biogeochemists to distinguish between the effects of environmental processes and analytical uncertainty. In this study we combine the routine analyses of certified reference materials (CRMs) with the application of a simple statistical technique to quantify the combined (random + systematic) measurement uncertainty associated with marine macronutrient measurements using gas segmented flow techniques. We demonstrate that it is realistic to achieve combined uncertainties of ~1–4% for nitrate + nitrite (ΣNOx), phosphate (PO43-) and silicic acid (Si(OH)4) measurements. This approach requires only the routine analyses of CRMs (i.e. it does not require inter-comparison exercises). As CRMs for marine macronutrients are now commercially available, it is advocated that this simple approach can improve the comparability of marine macronutrient datasets and therefore should be adopted as ‘best practice’.Novel autonomous Lab-on-Chip (LoC) technology is currently maturing to a point where it will soon become part of the marine chemist’s standard analytical toolkit used to determine marine macronutrient concentrations. Therefore, it is critical that a complete understanding of the measurement uncertainty of data produced by LoC analysers is achieved. In this study we analysed CRMs using 7 different LoC ΣNOx analysers to estimate a combined measurement uncertainty of < 5%. This demonstrates that with high quality manufacturing and laboratory practices, LoC analysers routinely produce high quality measurements of marine macronutrient concentrations.

Redox Processes Impacting the Flux of Iron(II) from Shelf Sediments to the OMZ along the Peruvian Shelf

Iron (Fe) is a limiting nutrient in many regions of the open ocean and can also play a key role in controlling primary productivity in Eastern Boundary Upwelling Systems (EBUS). In EBUS regions, where intense oxygen minimum zones (OMZs) contact the continental shelf, significant iron inputs can result from the supply of Fe(II) from reducing sediments. How much of this iron makes it to the photic zone depends on physical processes mixing over different time scales (minutes to decades) and the kinetics of redox and complexation processes impacting the biogeochemical cycling of iron. In this work we examine the controls on Fe(II) release from shelf sediments across the Peruvian OMZ by measuring Fe(II) and hydrogen peroxide (H2O2) in the water column and benthic boundary layer (BBL) and applying a simple 1D mixing model, with either 1 or 2 layers, where the flux of Fe(II) to the water column is treated as analogous to radon, that the decay rate is constant within the mixing layer. Our modeling approach then allows us to compare our estimated decay rate against published oxidation rates for specific oxidants of Fe(II) in OMZ waters and check the validity of our approach. Our data indicate that throughout the OMZ, Fe(II) decay rates may be partially influenced by H2O2, but it is most likely that nitrate-dependent anaerobic Fe(II) oxidizing (NDFO) bacteria are the main oxidizers. In the secondary nitrite maxima (SNM), abiotic NO2– or biotic-mediated processes may also be important. This work highlights the importance and uses of redox species in understanding biogeochemical cycles in the ocean.

Evaluating the fidelity of the cerium paleoredox tracer during variable carbonate diagenesis on the Great Bahamas Bank

Abstract Inferring redox conditions for ancient marine environments is critical to our understanding of biogeochemical cycles over Earth history. Because of the redox sensitivity of cerium (Ce) relative to other rare earth elements (REEs) and its uptake in marine carbonates, the Ce anomaly (Ce/Ce*) is widely applied to ancient carbonates as a proxy for local redox conditions in the water column. However, carbonate sediments and rocks are particularly vulnerable to multiple stages and styles of post-depositional diagenetic alteration where the diagenetic redox conditions and fluid compositions can vary widely from overlying seawater. Evaluations of the effects of this post-depositional alteration for the Ce anomaly have mostly been limited to ancient carbonate rocks rather than recent, well-characterized analog facies. Here, we report on analyses of REE plus yttrium concentrations (REY) and Ce anomalies in bulk carbonate samples from drill cores collected in the Bahamas (Clino and Unda) that allow us to track loss or retention of primary signals of initial oxic deposition through a range of subsequent alteration scenarios mostly under anoxic conditions. Specifically, these materials have experienced well-constrained overprints linked to meteoric processes and marine burial diagenesis, including dolomitization. Our results show that, regardless of mineralogy, diagenetic fluid composition, and redox state, the REY patterns in these carbonates, including the Ce anomaly, are similar to those of modern oxic seawater, indicating that they likely record the seawater signatures of primary deposition. As such, the Ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even when subject to multiple stages and styles of diagenetic alteration, confirming its utility in studies of ancient marine redox.

Benthic fluxes of trace metals in the Chukchi Sea and their transport into the Arctic Ocean

The Chukchi Sea is a primary site for shelf-ocean exchange in the Arctic region and modifies Pacific-sourced water masses as they transit via the Bering Strait into the Arctic Ocean. The aim of this study was to use radium and trace metal distributions to improve our understanding of biogeochemical cycles in the Bering and Chukchi Seas, and evaluate their potential response to future changes in the Arctic. We investigated the distributions of dissolved and total dissolvable trace metals (Cd, Fe, Ni, Cu, Zn, Mn, Co, and Pb) in the Bering and Chukchi Seas during spring. In addition, the long-lived radium isotopes (226Ra and 228Ra) were measured as tracers of benthic trace metal inputs. Trace metal concentrations, especially Fe and Mn, were highly elevated in Chukchi shelf waters compared with the open Arctic Ocean and Bering Strait. Trace metal, nutrient, and Ra patterns suggested that Fe, Mn, and Co concentrations were predominantly controlled by reductive benthic inputs, whereas the other trace metals were influenced by biological uptake and release processes. We propose that Fe, Mn, and Co in the Chukchi Sea are supplied from shelf sediments during winter overturning, and we combine the 228Ra fluxes with the distributions of Fe, Mn, and Co to provide a first estimate of their benthic fluxes in the region. The average benthic flux of 228Ra was 1.49 × 108 atoms m−2 d−1, which is among the highest rates reported globally. Estimated dissolved Fe (D-Fe) flux from the sediments was 2.5 μmol m−2 d−1, whereas D-Mn and D-Co fluxes were 8.0 μmol m−2 d−1 and 0.2 μmol m−2 d−1, respectively. The off-shelf transport of D-Fe to the Arctic Ocean is estimated to be about 10–25% of the benthic Fe flux, with the remainder retained on the shelf due to scavenging and/or phytoplankton uptake. Our results highlight the importance of the Chukchi Sea as a major source of the micro-nutrients to the Arctic Ocean, thereby supporting primary production. Long-term changes in factors that affect cross-shelf mixing, such as the observed reduction in ice cover, may therefore enhance shelf nutrient inputs and primary productivity in the Arctic.

A pulse of summer precipitation after the dry season triggers changes in ectomycorrhizal formation, diversity, and community composition in a Mediterranean forest in California, USA

Rapid responses of microbial biomass and community composition following a precipitation event have been reported for soil bacteria and fungi, but measurements characterizing ectomycorrhizal fungi remain limited. The response of ectomycorrhizal fungi after a precipitation event is crucial to understanding biogeochemical cycles and plant nutrition. Here, we examined changes in ectomycorrhizal formation, diversity, and community composition at the end of a summer drought and following precipitation events in a conifer–oak mixed forest under a semiarid, Mediterranean-type climate in CA, USA. To study the effects of different amounts of precipitation, a water addition treatment was also undertaken. Ectomycorrhizal fungal diversity and community composition changed within 6 days following precipitation, with increased simultaneous mortality and re-growth. Ectomycorrhizal diversity increased and community composition changed both in the natural rainfall (less than 10 mm) and water addition (50 mm) treatments, but larger decreases in ectomycorrhizal diversity were observed from 9 to 16 days after precipitation in the water addition treatment. The changes were primarily a shift in richness and abundance of Basidiomycota species, indicating higher drought sensitivity of Basidiomycota species compared with Ascomycota species. Our results indicate that ectomycorrhizal formation, diversity, and community composition rapidly respond to both precipitation events and to the amount of precipitation. These changes affect ecosystem functions, such as nutrient cycling, decomposition, and plant nutrient uptake, in semiarid regions.

Nitrate drawdown during a shelf sea spring bloom revealed using a novel microfluidic in situ chemical sensor deployed within an autonomous underwater glider

Here we describe, for the first time, the use of a miniaturized Lab-on-Chip (LoC) nutrient sensor deployed within an autonomous underwater vehicle (AUV; Kongsberg Seaglider) to collect high-resolution nitrate (nitrate + nitrite) data in a highly dynamic shelf sea environment. Seasonally stratified temperate shelf seas act as important carbon sinks, where primary production is controlled by the availability of nutrients such as nitrate. Spring phytoplankton blooms and sporadic mixing events can drastically modify the availability of nitrate on temporal scales from hours to days. Traditional sampling methods are unable to capture high frequency events that can be clearly observed using a wet-chemical microfluidic system deployed within a glider. We highlight firstly, an excellent agreement between the LoC and shipboard nitrate + nitrite measurements (r2 = 0.98 n = 11). Secondly, the LoC was able to observe a decrease in nitrate within the surface mixed layer from 5.74 μM (4th) to 1.42 μM (25th) during the onset of the spring bloom, whilst bottom layer concentrations remained constant (6.86 ± 0.16 μM), with an estimated analytical uncertaintly of <0.2 μM. Thirdly, the ability of an LoC sensor deployed within an AUV to accurately capture simultaneous biogeochemical and physical parameters at an enhanced resolution, on both spatial and temporal scales, improves our understanding of biogeochemical cycles within the dynamic temperate shelf sea environments.

Early Cambrian origin of the shelf sediment mixed layer

The mixed layer of modern oceans is a zone of fully homogenized sediment resulting from bioturbation. The mixed layer is host to complex biogeochemical cycles that directly impact ecosystem functioning, affecting ocean productivity and marine biodiversity. The timing of origin of the mixed layer has been controversial, with estimates ranging from Cambrian to Silurian, hindering our understanding of biogeochemical cycling and ecosystem dynamics in deep time. Here we report evidence from the Global Stratotype Section and Point (GSSP) of the basal Cambrian in the Burin Peninsula of Newfoundland, Canada, showing that a well-developed mixed layer of similar structure to that of modern marine sediments was established in shallow marine settings by the early Cambrian (approximately 529 million years ago). These findings imply that the benthos significantly contributed to establishing new biogeochemical cycles during the Cambrian explosion.

Explaining Leaf Nitrogen Distribution in a Semi-Arid Environment Predicted on Sentinel-2 Imagery Using a Field Spectroscopy Derived Model

Leaf nitrogen concentration (leaf N, %) is an essential component for understanding biogeochemical cycling. Leaf N is a good indicator of grass or forage quality, which is important for understanding the movements and feeding patterns of herbivores. Leaf N can be used as input for rangeland carrying capacity and stocking rate models. The estimation of leaf N has been successful using hyperspectral and commercial high spatial resolution satellite data such as WorldView-2 and RapidEye. Empirical methods have been used successfully to estimate leaf N, on the basis that it correlates with leaf chlorophyll. As such, leaf N was estimated using red edge based indices. The new Sentinel-2 sensor has two red edge bands, is freely available, and could further improve the estimation of leaf N at a regional scale. The objective of this study is to develop red edge based Sentinel-2 models derived from an analytical spectral device (ASD) spectrometer to map and monitor leaf N using Sentinel-2 images. Field work for leaf N and ASD data were collected in 2014 (December) in and around Kruger National Park, South Africa. ASD data were resampled to the Sentinel-2 spectral configuration using the spectral response function. The Sentinel-2 data for various dates were acquired from the European Space Agency (ESA) portal. The Sentinel-2 atmospheric correction (Sen2Cor) process was implemented. Simple empirical regression was used to estimate leaf N. High leaf N prediction accuracy was achieved at the ASD level and the best model was inverted on Sentinel-2 images to explain leaf N distribution at a regional scale over time. The spatial distribution of leaf N is influenced by the underlying geological substrate, fire frequency and other environmental variables. This study is a demonstration of how ASD data can be used to calibrate Sentinel-2 for leaf N estimation and mapping.

Understanding the Compositional Variability of the Major Components of Hydrothermal Plumes in the Okinawa Trough

Studies of the major components of hydrothermal plumes in seafloor hydrothermal fields are critical for an improved understanding of biogeochemical cycles and the large-scale distribution of elements in the submarine environment. The composition of major components in hydrothermal plume water column samples from 25 stations has been investigated in the middle and southern Okinawa Trough. The physical and chemical properties of hydrothermal plume water in the Okinawa Trough have been affected by input of the Kuroshio current, and its influence on hydrothermal plume water from the southern Okinawa Trough to the middle Okinawa Trough is reduced. The anomalous layers of seawater in the hydrothermal plume water columns have higher K+, Ca2+, Mn2+, B3+, Ca2+/SO42-, and Mn2+/Mg2+ ratios and higher optical anomalies than other layers. The Mg2+, SO42-, Mg2+/Ca2+, and SO42-/Mn2+ ratios of the anomalous layers are lower than other layers in the hydrothermal plume water columns and are consistent with concentrations in hydrothermal vent fluids in the Okinawa Trough. This suggests that the chemical variations of hydrothermal plumes in the Tangyin hydrothermal field, like other hydrothermal fields, result in the discharge of high K+, Ca2+, and B3+ and low Mg2+ and SO42- fluid. Furthermore, element ratios (e.g., Sr2+/Ca2+, Ca2+/Cl−) in hydrothermal plume water columns were found to be similar to those in average seawater, indicating that Sr2+/Ca2+ and Ca2+/Cl− ratios of hydrothermal plumes might be useful proxies for chemical properties of seawater. The hydrothermal K+, Ca2+, Mn2+, and B3+ flux to seawater in the Okinawa Trough is about 2.62–873, 1.04–326, 1.30–76.4, and 0.293–34.7 × 106 kg per year, respectively. The heat flux is about 0.159–1,973 × 105 W, which means that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Biogeochemical cycling at the aquatic–terrestrial interface is linked to parafluvial hyporheic zone inundation history

Abstract. The parafluvial hyporheic zone combines the heightened biogeochemical and microbial interactions indicative of a hyporheic region with direct atmospheric/terrestrial inputs and the effects of wet–dry cycles. Therefore, understanding biogeochemical cycling and microbial interactions in this ecotone is fundamental to understanding biogeochemical cycling at the aquatic–terrestrial interface and to creating robust hydrobiogeochemical models of dynamic river corridors. We aimed to (i) characterize biogeochemical and microbial differences in the parafluvial hyporheic zone across a small spatial domain (6 lateral meters) that spans a breadth of inundation histories and (ii) examine how parafluvial hyporheic sediments respond to laboratory-simulated re-inundation. Surface sediment was collected at four elevations along transects perpendicular to flow of the Columbia River, eastern WA, USA. The sediments were inundated by the river 0, 13, 127, and 398 days prior to sampling. Spatial variation in environmental variables (organic matter, moisture, nitrate, glucose, % C, % N) and microbial communities (16S and internal transcribed spacer (ITS) rRNA gene sequencing, qPCR) were driven by differences in inundation history. Microbial respiration did not differ significantly across inundation histories prior to forced inundation in laboratory incubations. Forced inundation suppressed microbial respiration across all histories, but the degree of suppression was dramatically different between the sediments saturated and unsaturated at the time of sample collection, indicating a binary threshold response to re-inundation. We present a conceptual model in which irregular hydrologic fluctuations facilitate microbial communities adapted to local conditions and a relatively high flux of CO2. Upon rewetting, microbial communities are initially suppressed metabolically, which results in lower CO2 flux rates primarily due to suppression of fungal respiration. Following prolonged inundation, the microbial community adapts to saturation by shifting composition, and the CO2 flux rebounds to prior levels due to the subsequent change in respiration. Our results indicate that the time between inundation events can push the system into alternate states: we suggest (i) that, above some threshold of inundation interval, re-inundation suppresses respiration to a consistent, low rate and (ii) that, below some inundation interval, re-inundation has a minor effect on respiration. Extending reactive transport models to capture processes that govern such dynamics will provide more robust predictions of river corridor biogeochemical function under altered surface water flow regimes in both managed and natural watersheds.

The detritus-based microbial-invertebrate food web contributes disproportionately to carbon and nitrogen cycling in the Arctic

The Arctic is the world’s largest reservoir of soil organic carbon and understanding biogeochemical cycling in this region is critical due to the potential feedbacks on climate. However, our knowledge of carbon (C) and nitrogen (N) cycling in the Arctic is incomplete, as studies have focused on plants, detritus, and microbes but largely ignored their consumers. Here we construct a comprehensive Arctic food web based on functional groups of microbes (e.g., bacteria and fungi), protozoa, and invertebrates (community hereafter referred to as the invertebrate food web) residing in the soil, on the soil surface and within the plant canopy from an area of moist acidic tundra in northern Alaska. We used an energetic food web modeling framework to estimate C flow through the food web and group-specific rates of C and N cycling. We found that 99.6% of C processed by the invertebrate food web is derived from detrital resources (aka ‘brown’ energy channel), while 0.06% comes from the consumption of live plants (aka ‘green’ energy channel). This pattern is primarily driven by fungi, fungivorous invertebrates, and their predators within the soil and surface-dwelling communities (aka the fungal energy channel). Similarly, >99% of direct invertebrate contributions to C and N cycling originate from soil- and surface-dwelling microbes and their immediate consumers. Our findings demonstrate that invertebrates from within the fungal energy channel are major drivers of C and N cycling and that changes to their structure and composition are likely to impact nutrient dynamics within tundra ecosystems.

Effects of land-use change and fertilization on N2O and NO fluxes, the abundance of nitrifying and denitrifying microbial communities in a hilly red soil region of southern China

Nitrous oxide (N2O) and nitric oxide (NO) are generally interrelated in soil nitrogen biogeochemical cycles. However, the effects of land-use change and fertilization on N2O and NO fluxes and the underlying mechanisms are not well understood, especially in fields converted from rice paddies (RPs) to upland cultivation. This study investigated the effects of land-use change from RPs to a citrus orchard (OR) as well as fertilization on N2O and NO fluxes, and the gene abundance of nitrifying and denitrifying microbial communities by using static chamber-gas chromatography and quantitative real-time PCR (qPCR). Structural equation modeling was performed to determine whether soil properties and the abundances of nitrifiers and denitrifiers influenced in situ N2O and NO fluxes. Land-use change from RPs to an OR and fertilization significantly increased N2O and NO fluxes. Land-use change increased AOA abundance, did not affect AOB abundance, and decreased nirK, nirS, and nosZ abundances. The growth of AOA and AOB abundance in response to fertilization was generally stimulated by providing more NH4+, but sometimes was inhibited by lower pH. Fertilization did not significantly affect the abundance of denitrifiers. Soil properties and functional microbial groups were explanatory variables, while land-use change altered their role in predicting in situ N2O and NO fluxes. For N2O fluxes, dissolved organic carbon was the strongest predictor in the RPs, whereas the primary explanatory factor was nirK abundance in the OR. For NO fluxes, soil temperature was the most important explanatory variable. Our results demonstrate the need for comprehensive approaches incorporating nitrifying and denitrifying processes to improve our understanding of biogeochemical cycles.

Hydrodynamic characteristics of Wujiangdu Reservoir during the dry season—a case study of a canyon reservoir

With the development of hydropower in the karst area of Southwest China, a series of cascade canyon reservoirs have been formed through the construction of dams. Given that hydrodynamic conditions in canyon reservoirs play a pivotal role in controlling the spatiotemporal distribution of physical and chemical properties of the stored water, hydrodynamic characteristics are of great importance in understanding biogeochemical cycles in those reservoirs. To further this understanding, a field campaign was conducted in the Wujiangdu Reservoir of Guizhou Province. It was found that from the reservoir inlet to the front of the dam, velocity (v) was negatively correlated and had a logarithmic relationship with distance along the ship track (s) under dry-season flow conditions [v = −0.104ln (s) + 0.4756]. Analysis showed that dry-season flow velocity had no significant correlation with water temperature, pH, or dissolved oxygen (DO). However, when velocity decreased to 0.061 m/s, water depth increased abruptly. In addition, DO displayed a sudden drop and the trend in pH changed from increasing to decreasing, while water temperature showed an opposite trend, indicating the existence of a transition zone from the river to the reservoir.

Response of sedimentary organic matter source to rainfall events using stable carbon and nitrogen isotopes in a typical loess hilly-gully catchment of China

Understanding the information on sedimentary organic matter (SOM) source is of great importance for better understanding biogeochemical cycling and terrestrial carbon sequestration on the Earth’s surface. Such information, especially during field rainfall process, however, is unavailable or difficult to assemble. In order to further study the dynamic variation of sedimentary organic matter source during rainfall process, suspended sediment were sampled manually at the outlet of two paired typical hilly-gully watersheds of Loess Plateau in China [Qiaozi East watershed (QEW) and Qiaozi West watershed (QWW)], and source materials included subsoil (i.e., soils from channel bank and gully bank) and topsoil (i.e., surface soils from different slope positions in land uses), along with C3 and C4 plants. In this study reported, natural stable isotopes (13C and 15N) and elemental compositions (TOC and TN), combined with a isotope mixing model (stable isotope analysis in R, SIAR) were successfully used to assemble this information. The results showed that the response of source contributions to SOM displayed difference significantly during different rainfall processes, showing fluctuation with flow. Subsoil was the main source of SOM pool for two similar small watersheds (accounting for 38.50% for QWW and 35.63% for QEW). Minor contribution was sourced from the C3 and C4 plants (< 25%), but only 10% for the C4 plants, indicating that the C3 plants dominated the input of organic matter in superficial soils of this study region. The source of sedimentary organic matter transported by erosion showed different contribution proportions from potential sources in two similar small watersheds with different management practices, indicating that management practice could decrease the soil organic matter loss by reducing the sediment transportation and flow during rainfall process in the Loess Plateau of China.

Autonomous Silicate Electrochemical Sensor in Seawater

Long term monitoring of dissolved inorganic nutrient concentrations such as silicate is the key to improve our understanding of biogeochemical cycles in the ocean. Electrochemical methods are proposed to measure in situ silicate concentration as they allow to develop miniaturized, reagentless sensor. We report here on the development and the optimization of the in situversion of our silicate sensor. Silicate is a non-electroactive specie and its determination is performed by measuring its corresponding silicomolybdic complex formed in situ at acidic pH after the oxidation of a molybdenum electrode while the counter electrode is isolated behind a Nafion® membrane [1]. The mechanical design of the sensor has been adapted from the laboratory prototype containing two connected cells: one for the complexation and one for the detection on a gold electrode (Æ=2mm) by cyclic voltammetry. All the electrochemical and chemical parameters, such as the Mo oxidation, the complexation time, the silicomolybdic complex detection as well as the complex transfer from one cell to the other and the cleaning protocol have been optimized to detect the smallest silicate concentration possible with the shortest time. Reproducible calibrations performed with this sensor provided linear regressions between 2.5 µmol L-1 and 70 µmol L-1 and a limit of detection of 1 µmol L-1. The 1st version of the sensor has been validated so far down to 50 meters using a pressure column and deployed in the Kiel fjord (Baltic Sea, Germany) down to 25 meters. The electronics (including datalogger) was also validated in situ as well as the solenoid pump placed into an equi-pressure compartment filled with dielectric oil. Some mechanical improvements have been done and the 2ndversion of the silicate sensor is currently in fabrication. This new version will be deployed off Chile (South of Coquimbo) in April 2017 and results obtained will be discussed. [1] Lacombe et al., Talanta 77 (2008) 744-750.

A simple and rapid method for isotopic analysis of nickel, copper, and zinc in seawater using chelating extraction and anion exchange

Stable isotope ratios of nickel, copper, and zinc are powerful tools for elucidating the biogeochemical cycling of trace metals in the ocean. However, analytical difficulties have impeded isotopic studies of these metals. We present a simple and rapid method for simultaneous analysis of Ni, Cu, and Zn isotope ratios in seawater using NOBIAS Chelate-PA1 resin and anion exchange resin. A NOBIAS Chelate-PA1 resin column was used to quantitatively collect Ni, Cu, and Zn from seawater and thoroughly remove the seawater matrix. Subsequent anion exchange purified and separated the Ni, Cu, and Zn from each other. The blanks used in this method (0.22 ng for Ni, 0.29 ng for Cu, and 0.53 ng for Zn) were sufficiently low to determine the isotope ratios of Ni, Cu, and Zn in surface seawater. Using this method, we analyzed GEOTRACES reference seawater samples (i.e., SAFe D1 and SAFe D2), National Research Council Canada certified materials (i.e., CASS-5 and NASS-6), and seawater samples collected from different depths in the subarctic South Pacific. The results were consistent with previously reported values. This method is expected to accelerate isotopic research and contribute to our understanding of biogeochemical cycling in the ocean.

Environmental controls on canopy foliar nitrogen distributions in a Neotropical lowland forest.

Distributions of foliar nutrients across forest canopies can give insight into their plant functional diversity and improve our understanding of biogeochemical cycling. We used airborne remote sensing and partial least squares regression to quantify canopy foliar nitrogen (foliar N) across ~164km2 of wet lowland tropical forest in the Osa Peninsula, Costa Rica. We determined the relative influence of climate and topography on the observed patterns of foliar N using a gradient boosting model technique. At a local scale, where climate and substrate were constant, we explored the influence of slope position on foliar N by quantifying foliar N on remnant terraces, their adjacent slopes, and knife-edged ridges. In addition, we climbed and sampled 540 trees and analyzed foliar N in order to quantify the role of species identity (phylogeny) and environmental factors in predicting foliar N. Observed foliar N heterogeneity reflected environmental factors working at multiple spatial scales. Across the larger landscape, elevation and precipitation had the highest relative influence on predicting foliar N (30% and 24%), followed by soils (15%), site exposure (9%), compound topographic index (8%), substrate (6%), and landscape dissection (6%). Phylogeny explained ~75% of the variation in the field collected foliar N data, suggesting that phylogeny largely underpins the response to the environmental factors. Taken together, these data suggest that a large fraction of the variance in foliar N across the landscape is proximately driven by species composition, though ultimately this is likely a response to abiotic factors such as climate and topography. Future work should focus on the mechanisms and feedbacks involved, and how shifts in climate may translate to changes in forest function.

Evidence of multiple thermokarst lake generations from an 11 800‐year‐old permafrost core on the northern Seward Peninsula, Alaska

Permafrost degradation influences the morphology, biogeochemical cycling and hydrology of Arctic landscapes over a range of time scales. To reconstruct temporal patterns of early to late Holocene permafrost and thermokarst dynamics, site‐specific palaeo‐records are needed. Here we present a multi‐proxy study of a 350‐cm‐long permafrost core from a drained lake basin on the northern Seward Peninsula, Alaska, revealing Lateglacial to Holocene thermokarst lake dynamics in a central location of Beringia. Use of radiocarbon dating, micropalaeontology (ostracods and testaceans), sedimentology (grain‐size analyses, magnetic susceptibility, tephra analyses), geochemistry (total nitrogen and carbon, total organic carbon, δ13Corg) and stable water isotopes (δ18O, δD, d excess) of ground ice allowed the reconstruction of several distinct thermokarst lake phases. These include a pre‐lacustrine environment at the base of the core characterized by the Devil Mountain Maar tephra (22 800±280 cal. a BP, Unit A), which has vertically subsided in places due to subsequent development of a deep thermokarst lake that initiated around 11 800 cal. a BP (Unit B). At about 9000 cal. a BP this lake transitioned from a stable depositional environment to a very dynamic lake system (Unit C) characterized by fluctuating lake levels, potentially intermediate wetland development, and expansion and erosion of shore deposits. Complete drainage of this lake occurred at 1060 cal. a BP, including post‐drainage sediment freezing from the top down to 154 cm and gradual accumulation of terrestrial peat (Unit D), as well as uniform upward talik refreezing. This core‐based reconstruction of multiple thermokarst lake generations since 11 800 cal. a BP improves our understanding of the temporal scales of thermokarst lake development from initiation to drainage, demonstrates complex landscape evolution in the ice‐rich permafrost regions of Central Beringia during the Lateglacial and Holocene, and enhances our understanding of biogeochemical cycles in thermokarst‐affected regions of the Arctic.

Uncertainty in the net hydrologic flux of calcium in a paired‐watershed harvesting study

AbstractMonitoring solutes in precipitation inputs and stream water exports at small watersheds has greatly advanced our understanding of biogeochemical cycling. Surprisingly, although inputs to and outputs from ecosystems are instrumental to understanding sources and sinks of nutrients and other elements, uncertainty in these fluxes is rarely reported in ecosystem budgets. We illustrate error propagation in input–output budgets by comparing the net hydrologic flux of Ca in a harvested and reference watershed at the Hubbard Brook Experimental Forest, New Hampshire. We identify sources of uncertainty and use a Monte Carlo approach to combine many sources of uncertainty to produce an estimate of overall uncertainty. Sources of uncertainty in precipitation inputs included in this study were: rain gage efficiency (undercatch or overcatch), gaps in measurements of precipitation volume, selection of a model for interpolating among rain gages, unusable precipitation chemistry, and chemical analysis. Sources of uncertainty in stream water outputs were: stage height–discharge relationship, watershed area, gaps in the stream flow record, chemical analysis, and the selection of a method for flux calculation. The annual net hydrologic flux of Ca in the harvested and reference watersheds was calculated from 1973 through 2009. Relative to the reference watershed, the harvested watershed showed a marked increase in Ca flux after it was cut in 1983–1984, and slowly declined toward pretreatment levels thereafter. In 2009, the last year evaluated, the 95% confidence intervals for the annual estimates approach the 95% confidence intervals of the pretreatment regression line, suggesting that the increased net loss of Ca in the harvested watershed may soon be indistinguishable from the reference. Identifying the greatest sources of uncertainty can be used to guide improvements, for example in reducing instances of unusable precipitation chemistry and gaps in stream runoff. Our results highlight the value of estimating uncertainty in watershed studies, including those in which replication is impractical.

Modelling interception loss using the revised Gash model: a case study in a mixed evergreen and deciduous broadleaved forest in China

AbstractInterception loss accounts for a substantial portion of incident precipitation and evapotranspiration in forest ecosystems. Hence, identifying its magnitude is crucial for our understanding of biogeochemical cycling and related hydrological processes. In this study, gross rainfall partitioning into interception loss, throughfall and stemflow were measured and modelled using the revised Gash model for a mixed evergreen and deciduous broadleaved forest over the 2014 growing season. Field survey results revealed that interception loss accounted for 14.3% of gross rainfall, while understory rainfall was 84.8% throughfall and 0.9% stemflow. The revised Gash model produced a fairly good agreement between observed and estimated rainfall partitioning. The model underestimated interception loss by only 6.6%, while throughfall and stemflow were also slightly misestimated. Hence the interception loss predictions from the model were robust and reliable for this mixed evergreen and deciduous broadleaved forest. As quantified by the model, the vast majority of interception loss occurred as evaporation from the canopy under saturated conditions: 54.9% evaporated during rainfall events, and 38.3% after rainfall ceased. The sensitivity analysis indicated that predictions from the revised Gash model were most affected by changes in canopy storage capacity (S), followed by the mean evaporation rate (Ē) during rainfall events, the mean rainfall rate ( ) and last canopy cover (c). Model predictions were least sensitive to trunk parameters (St and pt). Copyright © 2016 John Wiley &amp; Sons, Ltd.