Phosphorus enrichment in floodplain subsoils as a potential source of freshwater eutrophication

Authors

Christoph Weihrauch, Collin Joel Weber

Despite decades of management efforts, freshwater eutrophication has not been effectively mitigated in each affected ecosystem. This might be due to insufficient knowledge of the sources of phosphorus (P) inputs into surface waters. We sampled 2 m-deep soil profiles in four floodplain areas under differently managed grassland in Germany under dry and moist conditions regarding soil moisture and precipitation. Four soil P fractions of decreasing solubility were determined. We found systematic enrichment of easily soluble P forms in the floodplain subsoils (average: from 87.5 cm depth). Water-soluble P in these “deep P stocks” was positively correlated with total phosphorus concentrations in the adjacent surface waters. Our data cautiously suggest increased P mobilization from deep P stocks under moist conditions. Drier conditions coincided with increased P retention, resulting in relatively large amounts of easily soluble P which could readily be desorbed and lost at the next stronger precipitation event. We found no effects of grassland management on deep P stock features and dynamics. Deep P stocks might be considered a new source of diffuse P losses from soils. To effectively mitigate freshwater eutrophication, best management practices need to be developed to minimize P transfer from deep P stocks.

Nutrients and Heavy Metals in Legacy Sediments: Concentrations, Comparisons with Upland Soils, and Implications for Water Quality

Authors

Alyssa Lutgen, Grant Jiang, Nathan Sienkiewicz, Katie Mattern, JinJun Kan, Shreeram Inamdar

Concentrations of nutrients and heavy metals in streambank legacy sediments are needed to estimate watershed exports and to evaluate against upland inputs. Concentrations of nutrients and heavy metals were determined for legacy sediments in 15 streambanks across northeastern Maryland, southeastern Pennsylvania, and northern Delaware. Samples were collected from multiple bank depths from forested, agricultural, urban, and suburban sites. Analyses were performed for fine (<63 μm) and coarse sediment fractions. Nutrient and heavy metal concentrations were significantly higher in fine than coarse legacy sediments and water extractable nutrient concentrations were significantly greater for fine sediments. Nutrient and heavy metal concentrations were highest in streambank legacy sediments associated with urban land use, but few differences were found with bank depth. Total N (40–3,970 mg/kg) and P (25–1,293 mg/kg) and bioavailable P (0.25–48.8 mg/kg) concentrations for legacy sediments were lower than those for reported for upland soils. This suggests that legacy sediments could serve as sink or source of N and P depending on the redox conditions and stream water nutrient concentrations. However, despite low concentrations, caution should be exercised since streambank erosion and legacy sediment mass loadings could be high, these sediments are in immediate proximity of aquatic ecosystems, and biogeochemical transformations could result in release of the nutrients.

Temporal and spatial variability of terrestrial diatoms at the catchment scale: controls on communities

Authors

Jasper Foets​, Carlos E. Wetzel, Adriaan J. Teuling, Laurent Pfister

Diatoms are generally regarded as inhabitants of water bodies. However, numerous taxa are able to survive and reproduce in a variety of non-aquatic ecosystems. Although terrestrial diatoms are discussed extensively in the literature, most of those studies covered floristic aspects and few information exists on their ecology. This lack of knowledge thwarts their potential use as environmental markers in various applications. As a way forward, we investigated the seasonal patterns and the role of different disturbances on the community composition. We collected soil diatom samples in 16 sites across the Attert River basin (Luxembourg) every 4 weeks for a period of 14 months. Our results indicate that forests create a stable microhabitat for diatoms and that temporal variation of the diatom communities is mainly controlled by farming practices rather than seasonal changes in environmental variables. We also found out that communities need one to 2 months to reestablish a new, stable community after a significant change in the environment. We were able to confirm the applicability of the Pollution-Sensitivity Index (IPS) to identify anthropic disturbances.

Dynamic transfer of soil bacteria and dissolved organic carbon into small streams during hydrological events

Authors

Florian Caillon & Jakob Schelker

Small headwater streams interlink catchment soils with the river network. As water makes its way from the hillslopes to the stream, it may transport dissolved organic carbon (DOC) and potentially soil microbes into stream water. In this study, we aimed at quantifying the dynamic transfer of DOC and microbial life, namely bacteria from catchment soils into streams. We hypothesized that increased soil saturation enhances the lateral inflow of bacteria and DOC into streams. To address this hypothesis, we sampled six first order streams and three soil transects at two different depths located within the pre-alpine Oberer Seebach (OSB) catchment in Austria over a duration of 2 years. We found a strong variation in DOC concentrations (range 0.4–5.6 mg L⁻¹) and bacterial abundances (range < 500,000–3,863,000 cells mL⁻¹) measured by flow-cytometry. The highest values of DOC and bacterial cells occurred during high flow events. DOC concentration and bacterial abundance were correlated across all streams and seasons. In soils, DOC ranges were higher and were also correlated with bacterial abundance, while DOC concentrations were ∼ 10 times higher per bacterial cell than in streams. Overall we show that soils provide a dynamic inflow of bacteria and DOC to first order streams. Most probably, this results in a dynamic and reoccurring inoculation of small streams from catchment soils during runoff events. We propose that this dynamic microbial inoculation of small streams is potentially relevant for microbial community dynamics of downstream receiving waters.

 

The role of catchment soils and land cover on dissolved organic matter (DOM) properties in temperate lakes

Authors

Margot Sepp, Toomas Kõiv, Peeter Nõges, Tiina Nõges

Dissolved organic matter (DOM) is a critical component in freshwater ecosystem functioning. The main sources of DOM in lakes are allochthonous inputs from the catchment and autochthonous in-lake production. This study focused on the role of catchment characteristics on the qualitative and quantitative properties of DOM in small temperate lakes along a gradient of alkalinity. We examined DOM properties based on the optical absorbance and fluorescence measurements of water from 34 Estonian lakes. The content and composition of DOM were highly diverse in the lakes studied, e.g. the dissolved organic carbon (DOC) concentrations varied from 3.2 to 53.0 mg L⁻¹. Land cover, soil, and catchment hydrology and geology had substantial effects on DOM in lakes. Stock of soil organic carbon (SOC) in the catchment and water exchange rate (a descriptor of catchment hydrology, reciprocal of water residence time) had major positive effects on DOC concentrations. The aromaticity and molecular weight of DOM, i.e. the relative abundance of humic substances, and the dominance of allochthonous DOM increased with the drainage ratio (catchment area/lake area) and the percentages of bogs, and Dystric and Fibric Histosols (peat soils in transitional mires and bogs, respectively) in the catchments. Dominance of non-humic over humic substances and autochthonous over allochthonous DOM in lakes corresponded to calcareous catchments and higher percentages of Gleyic Rendzinas (thin soils on calcareous rock), Sapric Histosols (peat soils in mires) and open spaces (areas with little vegetation). Our results showed that soil variables had in general a greater effect than land cover and were more informative for describing the role of catchment characteristics on DOM in lakes. Patterns in DOM quantity and quality found in our study were similar to patterns found in other temperate lakes; therefore, our results have important implications for understanding catchment-lake interactions across the temperate region.

Spatial-temporal variability of soil moisture: Addressing the monitoring at the catchment scale

Authors

Jacopo Dari, Renato Morbidelli, Carla Saltalippi, Christian Massari, Luca Brocca

Soil moisture plays a fundamental role in the mass and energy balance between the land surface and the atmosphere, making its knowledge essential for several hydrological and climatic applications. The aim of this study is to extend the current knowledge of soil moisture spatial-temporal variability at the catchment scale (up to 500 km²). The main implication is to provide guidelines to obtain soil moisture values representative of the mean behaviour at the medium-sized river basin scale, which is useful for remote sensing validation analysis and rainfall-runoff modeling. To this end, 23 measurements campaigns were carried out during a time span of 14 months at 20 sites located within the Upper Chiascio River Basin, a catchment with a drainage area of about 460 km² in the Umbria Region (central Italy). The data set allowed the analysis of both soil moisture temporal stability and its dynamics. On the basis of statistical and temporal stability approaches, it was investigated how factors such as climatic regime and geomorphology influence soil moisture behaviour. For the investigated area, the spatial variability of soil moisture was higher in dry periods with respect to wet periods, mainly due to the rainfall pattern characteristics during different periods of the year. Soil moisture values recorded during wet periods showed a better correlation than those recorded during dry periods. The maximum number of required samples, to obtain the mean areal soil moisture with an absolute error of 3% vol/vol, was found equal to 12. The temporal stability analysis showed that during wet periods just one “optimal” measurement point can provide values of soil moisture representative of the catchment-mean behaviour, while during dry periods the number of “optimal” measurement points became equal to two. Therefore, at the adopted spatial scale the use of a single measurement point can lead to significant errors. From the perspective of soil moisture dynamics, the decomposition of the spatial variance showed that the contribution of the time-invariant component (temporal mean of each site) was predominant on respect to the total spatial variance of absolute soil moisture data, for almost the whole observation period. Results provided guidance to optimize soil moisture sampling by performing targeted measurements at a few selected points representative of the catchment-mean behaviour.

Carbon storage dynamics of temperate freshwater wetlands in Pennsylvania

Authors

Tara Mazurczyk, Robert P. Brooks

Healthy wetlands play a significant role in climate change mitigation by storing carbon that would otherwise contribute to global warming, leading to the reduction of water and food resources as well as more extreme weather phenomena. Investigating the magnitude of carbon storage potential of different freshwater wetland systems using multiple ecological indicators at varying spatial scales provides insight and justification for selective wetland restoration and conservation initiatives. We provide a holistic accounting of total carbon values for 193 wetland sites, integrating existing carbon algorithms to rapidly assess each of the following carbon pools: above-ground, below-ground, soil, woody debris, shrub cover, and herbaceous cover. Aspects of soil, vegetation, and ecosystem characteristics and stressors were measured to obtain an overall understanding of the ecosystems ability to store carbon (long-term) along a gradient of human disturbance. Based on a review of the literature, methods were prioritized based on the initial data available from field measurements as well as their practicality and ease in replicating the process in the future. Lacustrine human impounded (88.7 ± 18.0 tC/ha), riverine beaver impounded (116.2 ± 29.4 tC/ha), riverine upper perennial (163.3 ± 11.8 tC/ha), riverine lower perennial (199.2 ± 24.7 tC/ha), riverine headwater complex (159.5 ± 22.2 tC/ha), perennial/seasonal depression (269.6 ± 42.4 tC/ha), and slope (162.2 ± 14.6 tC/ha) wetland types were compared. Overall results showed moderate variability (9.33–835.95 tC/ha) for total carbon storage values across the wetland types, with an average total carbon storage of 174.6 ± 8.8 tC/ha for all wetlands. Results show that carbon storage was significantly higher (p = 0.002) in least disturbed wetland sites. Apart from perennial/seasonal depression wetlands, all reference standard wetlands had greater carbon storage, less disturbance impact, and a greater extent of forest cover than non-reference wetlands. Carbon storage values calculated were comparable to published literature.

Can denitrification explain coastal wetland loss: A review of case studies in the Mississippi Delta and New England

Authors

John W. Day, Ronald D. DeLaune, John R. White, Robert R. Lane, Rachael G. Hunter, Gary P. Shaffer

There has been considerable discussion over the past several years about the potential negative effects of nutrient loading on coastal wetland stability. In particular, there have been concerns that high nitrate concentrations can fuel denitrification that can lead to soil organic matter loss and wetland deterioration. Here we review these issues for three case studies where there have been elevated levels of dissolved inorganic nitrogen, especially nitrate, coincident with wetland deterioration. These case studies include the Breton Sound estuary that receives diverted Mississippi River water at Caernarvon LA, a freshwater assimilation wetland that receives treated municipal effluent from a wastewater treatment plant at Hammond, LA, and a tidal creek in the Plum Island Sound estuary of northeastern Massachusetts where nitrate was introduced on each flood tide during the growing season for nearly ten years. We review the physical setting, ecology and biogeochemistry of these sites and use stoichiometric calculations to estimate how much soil organic matter decomposition could be accounted for by denitrification. Results of these calculations show that denitrification rates could not have caused the observed marsh deterioration at the two LA marshes. Denitrification may play a significant role in marsh loss at the MA site, however due to the unique hydrology of this site, denitrification rates induced by very high nitrogen loading rates in concert with the hydrology of the site may play a more significant role in marsh deterioration.

Wetland Soil Properties and Resident Bacterial Communities Are Influenced by Changes in Elevation

Authors

Philip O. Lee, Cory Shoemaker, Julie B. Olson

Soils from four sites distributed along an elevation gradient from marsh to coastal forest in a wetland bordering the Gulf of Mexico were sampled over a 16-months period. In addition to measuring a suite of environmental conditions, terminal restriction fragment length polymorphism analyses of the resident bacterial communities were performed. Wetland soil bacterial communities varied across both space and time, with all measured variables (temperature, pH, percentage of soil organic matter, salinity, and concentrations of sulfide, NH₄⁺, NO₃⁻, and soluble reactive phosphorus) showing significant site by time interactions. Analyses of bacterial communities showed both marsh zones (Spartina and Cladium) supported similar communities, as did the ecotone and coastal forest. Bacterial communities within coastal forest soils were significantly different than those within marsh soils, and the ecotone communities were significantly different from the Spartina marsh soil. Temperature and pH were the most influential environmental factors impacting bacterial community composition but no predictable patterns were identified, suggesting that community changes are likely the result of intrinsic factors that are affected by local-scale processes. The dynamic nature of the physiochemical variables within wetlands suggests that more work is needed to determine potential interactive effects on bacterial community structure.

Development of a nitrogen-fixing cyanobacterial consortium for surface stabilization of agricultural soils

Authors

Xin Peng, Mary Ann Bruns

Cyanobacteria are recognized as important colonizers and protectors of soil surfaces, particularly in biological soil crusts (BSCs) of arid and semiarid regions. Comparatively, little attention has been paid to the growth of cyanobacteria, algae, and moss on agricultural soils in more humid regions like eastern North America. Growth of soil surface consortia (SSCs) in agricultural fields is typically rapid and ephemeral yet recurrent, thereby differing from classical BSCs of semiarid regions and algal mats of aquatic systems. Naturally occurring or intentionally applied cyanobacteria to agricultural soils could thus provide renewable sources of carbon and nitrogen (N) and a means to improve soil resilience. Here, we describe a soil microcosm-based protocol to assess cyanobacteria for their ability to form SSCs using three criteria: reliability of serial transfers in N-free culture media, robust growth in soil microcosms, and resistance to detachment from soil particles subjected to water flushing. Screening of 100 enrichment cultures from local SSCs yielded three that exhibited robust growth on N-free solid media and consistent microscopic appearance as filamentous, heterocystous cyanobacteria. One enrichment (DG1) grew reliably in liquid N-free media and was selected for comparison with pure cultures of commercial strains of other heterocystous cyanobacteria. Growth and biomass density of DG1 and commercial strains on moist, N-limited soils were tracked using chlorophyll a measurements and water flush tests. Anabaena spp. grew faster on soil surfaces, but their 80-day SSCs did not adhere to soil as well as DG1 or Nostoc spp. in water flush tests. The ability of DG1 and Nostoc spp. to produce flocculated growth in liquid culture appeared to be associated with greater soil adherence. While Nostoc spp. formed stable SSCs in soil microcosms, they exhibited lower growth rates and biomass densities than DG1. Attempts to purify the cyanobacterial strain(s) from other bacteria in the DG1 enrichment were unsuccessful. Based on initial metagenomics analysis, the DG1 enrichment was a consortium containing at least six other bacterial genotypes but dominated by one or more closely related strains of Cylindrospermum spp. (Nostocaceae). The presence of bacterial associates did not interfere with rapid growth and high biomass density in soil microcosms, as well as SSC stability in water flush tests. The artificial SSCs formed by DG1 showed good potential for use as a renewable N source for agroecosystems.