Correction to: Are soils beneath coniferous tree stands more acidic than soils beneath deciduous tree stands?

Fig. 1. was amended to reduce the size of the map and improve formatting of the manuscript. The authors claim this amendment does not affect the information being conveyed.

Are soils beneath coniferous tree stands more acidic than soils beneath deciduous tree stands?

In 2008, the Mulberry River, a National Wild and Scenic River, was listed as impaired due to low pH (below pH 6.0). Over the last 50 years, the volume of conifers in the Ozark region has increased 115% since 1978 which may result in the acidification of nearby aquatic ecosystems. The objective of this study was to determine if differences exist in soil and litter chemical properties between deciduous and coniferous tree stands. Aboveground litter (n = 200) and soil (n = 400) at 0- to 5- and 5- to 15-cm depths were collected at paired deciduous and coniferous stands at 10 locations within the Mulberry River watershed and analyzed for a suite of chemical parameters. There were no differences (P > 0.05) in several measures of soil acidity between deciduous and coniferous stands. Litter collected from the coniferous stands was more acidic than deciduous litter (4.4 vs 4.7; P < 0.05). Cation exchange capacity, exchangeable Ca and Mg, and water-soluble P and Mg contents differed (P < 0.05) by stand and depth. Cation exchange capacity and exchangeable Ca and Mg were greatest in the 0- to 5-cm depth interval of the coniferous stands. Water-soluble P and Mg contents were greatest within the 0- to 5-cm depth interval which did not differ (P > 0.05) between stand but were greater than the 5- to 15-cm depth interval. Although limited to the top 15-cm of soil, the similarity in soil acidity between stands suggests that conifer growth may not be a substantial source of acidity to the Mulberry River.

Coupling High-Frequency Stream Metabolism and Nutrient Monitoring to Explore Biogeochemical Controls on Downstream Nitrate Delivery.

Instream biogeochemical process measurements are often short-term and localized. Here we use in situ sensors to quantify the net effects of biogeochemical processes on seasonal patterns in baseflow nitrate retention at the river-reach scale. Dual-station high-frequency in situ nitrate measurements, were coupled with high-frequency measurements of stream metabolism and dissolved inorganic carbon, in a tributary of the Buffalo National River, Arkansas. Nitrate assimilation was calculated from net primary production, and combined with mass-balance measurements, to estimate net nitrification and denitrification. The combined net effects of these instream processes (assimilation, denitrification, and nitrification) removed >30-90% of the baseflow nitrate load along a 6.5 km reach. Assimilation of nitrate by photoautotrophs during spring and early summer was buffered by net nitrification. Net nitrification peaked during the spring. After midsummer, there was a pronounced switch from assimilatory nitrate uptake to denitrification. There was clear synchronicity between the switch from nitrate assimilation to denitrification, a reduction in river baseflows, and a shift in stream metabolism from autotrophy to heterotrophy. The results show how instream nitrate retention and downstream delivery is driven by seasonal shifts in metabolic pathways; and how continuous in situ stream sensor networks offer new opportunities for quantifying the role of stream biota in the dynamics, fate, and transport of nitrogen in fluvial systems.

Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis.

The magnesium (Mg) isotopic compositions of 40 seawater samples from the Gulf of Mexico and of one seawater sample from the southwest Hawaii area were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to investigate the homogeneity of Mg isotopes in seawater. The results indicate that the Mg isotopic composition of seawater from the Gulf of Mexico is homogeneous, both vertically and horizontally, with average values for δ(26)Mg = -0.832 ± 0.068 and δ(25)Mg = -0.432 ± 0.053 (n = 40, 2SD)--identical to those of seawater from Hawaii (average δ(26)Mg = -0.829 ± 0.037 and δ(25)Mg = -0.427 ± 0.033) and to the average literature values of seawater worldwide (δ(26)Mg = -0.83 ± 0.11 and δ(25)Mg = -0.43 ± 0.06, n = 49, 2SD). Collectively, global seawater has a homogeneous Mg isotopic composition with δ(26)Mg = -0.83 ± 0.09 and δ(25) Mg = -0.43 ± 0.06 (2SD, n = 90). The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precipitation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Considering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excellent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis.