Search

Nitrogen and phosphorous support the ecosystem by supplying nutrients to algae and aquatic plants. Having them in excess results in the eutrophication of waters creating quality problems. In the past, nitrogen has been widely investigated for wells in the context of groundwater flow. However, a national-scale nitrogen assessment in rivers and streams has not received enough attention. In this study, the Wilcoxon rank-sum test, a non-parametric hypothesis testing method, has been applied to nitrogen concentration in the form of nitrate-nitrogen and nitrite-nitrogen in rivers and streams of the Contiguous United States. This approach was particularly chosen because of the non-normal and positively skewed nitrogen levels occurring in the surface flow. This method was able to identify the impaired body of waters as well as quantify the confidence, significance, and errors involved. The Northern Appalachians (NAP), Northern Plains (NPL), and Xeric (XER) ecoregions were identified as the ecoregions worsening in the nitrogen-nitrate condition with NAP, and XER needed immediate action. The nitrite-nitrogen condition did not pose an immediate threat, so mitigation efforts should focus more on nitrate-nitrogen remediation. It was shown that the method was superior to the two-sample t-test by yielding lower type II errors. [ABSTRACT FROM AUTHOR]

Trait-based approaches are increasingly used to understand the responses of phytoplankton communities to changing environmental conditions. This paper analyses phytoplankton morphological traits (i.e. volume, surface to volume ratio) in a highly productive subtropical estuary (Río de la Plata, South America). The goal was to investigate whether morphological variability, weighted by biovolume, supports the hypothesis that light is a key limiting factor for primary producers in this turbid and nutrient-rich estuary. A transect from nearly freshwater to marine stations was sampled with increasing mixing depth, decreasing turbidity and dissolved inorganic nitrogen concentration. Primary production (PP) and total biovolume were highest on the marine side of the turbidity front. In this area, morphologies of different high-order taxa converged towards smaller individual size and higher surface to volume ratio ( S/ V). Phytoplankton weighted average S/ V increased with turbidity along the gradient, while individual size did so with salinity. High S/ V and low morphological variability supported higher PP, appearing as responses to cope with light limitation in this environment. Multiple linear regressions indicated that morphological variability alone explained 72 % of PP, reinforcing the utility of trait-based approaches when targeting relations between phytoplankton communities and ecosystem functioning. [ABSTRACT FROM AUTHOR]

Diet is a leading modifiable risk factor for chronic disease, but it remains difficult to measure accurately due to the error and bias inherent in self-reported methods of diet assessment. Consequently, there is a pressing need for more objective biomarkers of diet for use in health research. The stable isotope ratios of light elements are a promising set of candidate biomarkers because they vary naturally and reproducibly among foods, and those variations are captured in molecules and tissues with high fidelity. Recent studies have identified valid isotopic measures of short- and long-term sugar intake, meat intake, and fish intake in specific populations. These studies provide a strong foundation for validating stable isotopic biomarkers in the general US population. Approaches to improve specificity for specific foods are needed; for example, by modeling intake using multiple stable isotope ratios or by isolating and measuring specific molecules linked to foods of interest. [ABSTRACT FROM AUTHOR]

Abstract: Pasture management can have important implications on the amounts and long-term stability of soil organic carbon (SOC). We investigated the short-term impacts of grazing intensity and nitrogen (N) fertilization levels on C dynamics into the various SOC pools in rotationally stocked ‘Tifton 85’ bermudagrass (Cynodon spp.) pastures. Treatments consisted of three grazing intensities (based on target postgraze stubble heights of 8, 16, and 24 cm) and three levels of N fertilization (50, 150, and 250 kg N ha−1 yr−1). Soil samples (0- to 20-cm depth) were taken prior to imposing treatments and at the end of the 2-yr study. Grazing intensity and N levels had no effect on bulk SOC and N concentration and content. Greater stubble height and rate of N fertilizer resulted in a linear increase in particulate organic carbon (POC) and C and N concentration in the <53-μm particle-size fraction. Grazing intensity and N fertilization showed significant effects on δ13C values and the relative amounts of C3-C vs. C4-C. The δ13C data indicated that readily decomposable C components (C4-derived C) were lost in response to more intensive grazing (stubble height of 8 cm). Results demonstrated that POC and C associated with <53 μm particle-size fraction were sensitive indicators of short-term impacts of pasture management strategies on SOC. Further studies are warranted to examine the long-term impacts of grassland management on SOC pools in sandy soils of subtropical regions. [Copyright &y& Elsevier]