Hydrochemical determination of source water contributions to Lake Lungo and Lake Ripasottile (central Italy)

Lake Lungo and Lake Ripasottile are two shallow (4-5 m) lakes located in the Rieti Basin, central Italy, that have been described previously as surface outcroppings of the groundwater table. In this work, the two lakes as well as springs and rivers that represent their potential source waters are characterized physio-chemically and isotopically, using a combination of environmental tracers. Temperature and pH were measured and water samples were analyzed for alkalinity, major ion concentration, and stable isotope (δ2H, δ18O, δ13C of dissolved inorganic carbon, and δ34S and δ18O of sulfate) composition. Chemical data were also investigated in terms of local meteorological data (air temperature, precipitation) to determine the sensitivity of lake parameters to changes in the surrounding environment. Groundwater represented by samples taken from Santa Susanna Spring was shown to be distinct with SO4 2- and Mg2+ content of 270 and 29 mg/L, respectively, and heavy sulfate isotopic composition (δ34S=15.2‰ and δ18O=10‰). Outflow from the Santa Susanna Spring enters Lake Ripasottile via a canal and both spring and lake water exhibits the same chemical distinctions and comparatively low seasonal variability. Major ion concentrations in Lake Lungo are similar to the Vicenna Riara Spring and are interpreted to represent the groundwater locally recharged within the plain. The δ13CDIC exhibit the same groupings as the other chemical parameters, providing supporting evidence of the source relationships. Lake Lungo exhibited exceptional ranges of δ13CDIC (±5‰) and δ2H, δ18O (±5 ‰ and ±7 ‰, respectively), attributed to sensitivity to seasonal changes. The hydrochemistry results, particularly major ion data, highlight how the two lakes, though geographically and morphologically similar, represent distinct hydrochemical facies. These data also show a different response in each lake to temperature and precipitation patterns in the basin that may be attributed to lake water retention time. The sensitivity of each lake to meteorological patterns can be used to understand the potential effects from long-term climate variability

Biogeochemical and microbial seasonal dynamics between water column and sediment processes in a productive mountain lake: Georgetown Lake, MT, USA

This manuscript details investigations of a productive, mountain freshwater lake and examines the dynamic relationship between the chemical and stable isotopes and microbial composition of lake bed sediments with the geochemistry of the lake water column. A multidisciplinary approach was used in order to better understand the lake water- sediment interactions including quantification and sequencing of microbial 16S rRNA genes in a sediment core as well as stable isotope analysis of C, S, and N. One visit included the use of a pore water sampler to gain insight into the composition of dissolved solutes within the sediment matrix. Sediment cores showed a general decrease in total C with depth which included a decrease in the fraction of organic C combined with an increase in the fraction of inorganic C. One sediment core showed a maximum concentration of dissolved organic C, dissolved inorganic C, and dissolved methane in pore water at 4 cm depth which corresponded with a sharp increase in the abundance of 16S rRNA templates as a proxy for the microbial population size as well as the peak abundance of a sequence affiliated with a putative methanotroph. The isotopic separation between dissolved inorganic and dissolved organic carbon is consistent with largely aerobic microbial processes dominating the upper water column, while anaerobic microbial activity dominates the sediment bed. Using sediment core carbon concentrations, predictions were made regarding the breakdown and return of stored carbon per year from this temperate climate lake with as much as 1.3 Gg C yr(-1) being released in the form of CO2 and CH4

Fluvial organic carbon composition regulated by seasonal variability in lowland river migration and water discharge

Identifying drivers of seasonal variations in fluvial particulate organic carbon (POC) composition can aid sediment provenance and biogeochemical cycling studies. We evaluate seasonal changes in POC composition in the Río Bermejo, Argentina, a lowland river running ∼1,270 km without tributaries. Weekly POC concentration and isotopic composition from 2016 to 2018 show that during the wet season, increased lateral channel migration generates an influx of 13C-enriched and 14C-enriched floodplain-sourced material, overprinting the 13C-depleted and 14C-depleted headwater signature that is observed during the dry season. These findings demonstrate how channel morphodynamics can drive variability of POC composition in lowland rivers, and may modulate the composition of POC preserved in sedimentary archives

Stable isotope characterization of the ecohydrological cycle at a tropical treeline site.

We investigated the seasonal variation in pools of water available to mature trees growing at high elevation in a tropical environment. The study focused on the dominant tree species (Pinus hartwegii) at about 3800 m a.s.l. on Nevado de Colima, Mexico, where climate is typical of the North American Monsoon System. Stable isotope ratios of hydrogen and oxygen in water extracted from soil, xylem, and leaves were measured through a cycle of two dry and two wet seasons in 2003-2004. Isotopic ratios were also measured in accumulated precipitation, a few single precipitation events, and in spring water over the two-year period. Based on evidence from water, stable isotopes in soil, and xylem samples, trees utilized water from relatively shallow soil depths, which are representative of current conditions, rather than tapping groundwater, which is more representative of long-term trends. While the stable isotope signature in environmental waters showed a slightly different pattern before and during the monsoon, the more pronounced differences in leaf water isotopes between the two seasons, due to drought stress, will lead to a clear seasonal isotopic signal in tree ring cellulose. This study represents a unique snapshot of water cycling in a tropical treeline ecosystem, where our understanding of eco-hydrological pathways is limited. This type of analysis is also useful for proper calibration of stable isotopic signals in tree ring records

Conversion of a Semiarid Nevada Soil to Irrigated Agriculture Preferentially Removes Labile Carbon

Due to the scarcity of arable land, semiarid rangelands are often converted to irrigated croplands, which is likely to affect soil organic carbon (SOC) due to changes in C inputs into the soil and environmental factors regulating decomposition. In this study, soil density and particle size fractions as well as their C and N contents, stable isotopic composition, and chemical characterization by mid-infrared spectroscopy were measured in a native shrubland and an adjacent agricultural site under alfalfa cultivation for at least 50 years in western Nevada. Cultivation significantly reduced the amount of C and N in the surface soils and the proportion of C present in the labile fractions. The delta C-13 and delta N-15 values of the SOC reflected dominant vegetation types at each site, and suggested most SOC was root-derived. The potential decomposition rate of SOC was higher in the shrubland than in the alfalfa surface soil reflecting the larger amount of labile C present in the shrubland soils. Spectroscopy results suggested that the greater recalcitrance of the alfalfa soils was due to insoluble SOC moieties. Additional analyses of buried, SOC-rich, A horizons at both sites showed that slower decomposition of 'deep' SOC was due to lower substrate quality supported by fractionation and spectroscopy data. The results of this study showed that converting a semiarid shrubland into irrigated cropland significantly reduced SOC content but increased overall stability of residual SOC

Iron-bound organic carbon in forest soils: quantification and characterization

Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8% of total OC (TOC) in forest soils. Atomic ratios of OC: Fe ranged from 0.56 to 17.7, with values of 1-10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (f(Fe-OC)) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and f(Fe-OC) increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 degrees C. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in C-13 compared to the non-Fe-bound OC, but C/N ratios did not differ substantially. In summary, C-13-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of f(Fe-OC) being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC