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In headwater catchments, surface groundwater discharge areas have unique soil biogeochemistry and can be hot spots for solute contribution to streams. Across the northeastern United States, headwater hillslopes with surface groundwater discharge were enriched in soil Mn, including Watershed 3 of Hubbard Brook Experimental Forest, New Hampshire. Soils of this site were investigated along a grid to determine extent of Mn‐rich zone(s) and relationships to explanatory variables using ordinary kriging. The O and B horizons were analyzed for total secondary Mn and Fe, Cr oxidation potential, total organic C, moisture content, wetness ratio, and pH. Two Mn hot spots were found: a poorly drained, flowing spring (Location A); and a moderately well‐drained swale (Location B). Both had ∼6,000–9,000 mg Mn kg–1 soil. However, Location A had high Cr oxidation potential (a measure of Mn reactivity), whereas Location B did not. Location C, a poorly drained seep with slow‐moving water, had lower Mn content and Cr oxidation potential. Manganese‐rich soil particles were analyzed using X‐ray absorption near‐edge structure and micro‐X‐ray diffraction; the dominant oxidation state was Mn(IV), and the dominant Mn oxide species was a layer‐type Mn oxide (L‐MnO2). We propose input of Mn(II) with groundwater, which is oxidized by soil microbes. Studies of catchment structure and response could benefit from identifying hot spots of trace metals, sourced mainly from parent material but which accumulate according to hydropedologic conditions. Small‐scale variation in Mn enrichment due to groundwater and microtopography appears to be more important than regional‐scale variation due to air pollution. Core Ideas: Hot spots of soil Mn accumulate according to hydropedologic conditions and may influence surface water chemistry.Small‐scale variation in soil Mn is more important than regional‐scale variation from air pollution.Variation in Mn oxide reactivity may occur over short distances due to changes in hydropedologic conditions. [ABSTRACT FROM AUTHOR]

Stable isotope signatures in soil-derived N03 provide an opportunity for environmental source tracing, but rapid changes in N transformation rates caused by sampling disturbance may create an artifact in the signature of extracted N03. To study this, we measured net nitrification and ammonification rates and the ~`5N and ~18Q of N03 in soils from a water- shed in Camels Hump State Forest, Vermont, known to be sensitive to sampling disturbance. Eleven Oa and one A horizon samples (C/N ratio 15-25, pH 3.3-4.2) had N03 extracted for isotope analysis approximately 30 mm after sampling and again after a 2- to 3-d incuba- tion period at 10°C. Net nitrification rates during incubation were rapid and linear (0.9- 15.3 pmol L1 soil h'), with increases in N03 detectable within 1 h after sampling. The ~15N and ~18O of soil N03 changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in 615N03 became more depleted after incubation, as much as l3.5%o lighter. Soils that were initially relatively depleted in 6'5N03 became more enriched after incubation, as much as 16.5%o heavier. The latter had high net nitrification rates and low final NH4~ concentrations, probably caus- ing the enrichment of 15N in N03 due to a diminishing substrate pool. The 6180 of soil N03 showed little change during incubation and changes were not significantly related to changes in the 15N of N03, suggesting that denitrification was not a primary mechanism. Added enriched 15NH4CI (11 l.9%o) was rapidly incorporated into the soil N03 pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil N03, and the isotopic signature of extracted N03 may not be a reliable environmental tracer. [ABSTRACT FROM AUTHOR]

Trace metal accumulation by Mn in synthetic oxides and soil nodules has been attributed to specific adsorption and oxidation at Mn oxide surfaces, yet little is known about trace metal interactions with Mn in bulk soil. We investigated competitive effects of trace metal pretreatment on Cr oxidation in well-aerated, high-Mn soils, as well as accumulation of added Pb, Co, and Cu by soil Mn using microfocused synchrotron x-ray fluorescence (µSXRF). Short-term equilibrations of divalent Mn, Co, Pb, Cu, and Ni with soil samples weighed to contain equivalent amounts of NH2OH·HCl-extractable Mn resulted in substantial interference in Cr oxidation, confirming metal interactions with Cr-oxidizing sites on Mn oxide surfaces. Interference by Cu and Pb was greatest in samples likely to be low in competing sorbents, that is, low-pH soils and smaller samples of higher Mn soils, respectively. Strongest interference in Cr oxidation resulted from Mn and Co addition, suggesting specific affinity for soil Mn oxide surfaces, and was greatest in high-pH, high-Mn-valence soils previously shown to have the greatest Cr oxidation capacity. Nickel showed the weakest effect. Micro-SXRF scans revealed substantial spatial correlation of soil Mn with added Co and Pb, but Pb microdistribution was equally correlated with soil Fe. Only modest overlap of soil Mn with added Cu was observed. Our data suggest that specific affinity of Pb, Cu, and Ni for soil Mn oxides was weaker than that of Mn and Co. Higher Mn oxide valence may enhance sorption and subsequent oxidation of these oxidizable metals. [ABSTRACT FROM AUTHOR]

Nitrogen transformation rates in eight northeastern US research sites were measured in soil samples taken in the early season of 2000 and the late season of 2001. Net mineralization and nitrification rates were determined on Oa or A horizon samples by two different sampling methods—intact cores and repeated measurements on composite samples taken from around the cores. Net rates in the composite samples (n=30) showed three different temporal patterns: high net nitrification with minimal NH4+ accumulation, high net nitrification and high NH4+ accumulation, and minimal net nitrification and moderate NH4+ accumulation. The 4-week net rates in intact cores were about half that of the rates from the composite samples but were well related (R2>0.70). Composite samples from sites that exhibited high net nitrification were incubated with acetylene and net nitrification was completely stopped, suggesting an autotrophic pathway. Gross mineralization and nitrification (2000 only) rates were estimated using the isotope dilution technique. Gross rates of nitrification and consumption in intact cores were relatively low. Gross rates of mineralization and net rates of nitrification were both related to the soil C/N ratio, with higher rates generally occurring in sites containing Acer saccharum as a dominant or co-dominant species. The comparison of methods suggests that all provide a similar hierarchy of potential rates but that the degree of net nitrification is strongly influenced by the degree of sample disturbance. Differences between sites appear to be related to an interaction of soil (C/N) and vegetation (A. saccharum contribution) characteristics. [Copyright &y& Elsevier]

Phosphorus derived from agricultural practices has been targeted as a leading cause of water quality degradation in Lake Champlain. Mobilization of P from seasonally flooded agricultural soils is a concern. Using 14 soils from a research farm in New York's Champlain Valley, we characterized the available P status, extractable Fe and Al, P sorption capacities, and soluble phosphate release in flooded laboratory microcosms. Quantities of NH4–acetate available P ranged from 3 to 100 mg kg−1 and fluoride-extractable P from 10 to 211 mg kg−1 Flooding soils induced significant release of phosphate to the porewater over a 60- to 90-d period in 13 of the 14 soils studied. Porewater phosphate increases ranged from 2.2 to 27.0 times the initial phosphate concentrations. However, floodwater phosphate increases were much lower, with a maximum of 3.6 times the initial concentration. Average porewater phosphate concentrations over the flooding period ranged from 0.046 to 7.0 mg L−1 and average floodwater P from 0.032 to 3.70 mg L−1 Ammonium-acetate P and the degree of phosphorus saturation (DPS) were highly correlated with the average porewater and floodwater phosphate concentration. Average ratio of porewater to floodwater phosphate concentrations ranged from 1.0 to 3.3. Five soils that were lower in fluoride-extractable P had increasing porewater phosphate accompanied by increasing porewater Fe2+ and decreasing floodwater phosphate. Results suggest that P solubility and mobility were a function of both the available P status and redox cycling. [ABSTRACT FROM AUTHOR]