Anette Giesemann

Interpretation of sulfur cycling in two catchments in the Black Forest (Germany) using stable sulfur and oxygen isotope data

The isotopic composition of SO42- in bulk precipitation, canopy throughfall, seepage water at three different soil depths, stream water, and groundwater was monitored in two forested catchments in the Black Forest (Germany) between November 1989 and February 1992. Isotope measurements on aqueous sulfate were complemented by δ34S-analyses on SO2 in the air, total sulfur and inorganic sulfate in the soil, and bedrock sulfur, in order to identify sources and biogeochemical processes affecting S cycling in catchments with base poor, siliceous bedrock. Stable S isotope data indicated that atmospheric deposition and not mineral weathering is the major source of S in both catchments since δ34S-values for sulfate in the soil, in seepage water, and in stream water were generally found to be similar to the mean δ34S-values of precipitation SO42- (+2.1. However, δ18O-values of seepage water SO42- at 30 cm and especially at 80 cm depth were depleted by several per mil with respect to those of the atmospheric deposition (+7.5 to +13.5. This indicates that in both catchments a considerable proportion of the seepage water SO42- is derived from mineralization of carbon-bonded soil S and must therefore have cycled through the organic soil S pool. δ34S-values for different S compounds in the solid soil were found to differ markedly depending on S fraction and soil depth. Since atmospheric S deposition with rather constant δ34S-values was identified as the dominant S source in both catchments, this is interpreted as a result ofin situ isotope fractionation rather than admixture of isotopically different S. The differences between the δ34S-values of seepage water and soil sulfate and those of organic soil S compounds are consistent with a model in which SO42- uptake by vegetation and soil microorganisms favours34SO42- slightly, whereas during mineralization of organic soil S to aqueous SOSO42-,32S reacts preferentially. However, the data provide evidence for negligible isotope fractionation during physico-chemical S transformations such as adsorption/desorption in aerated forest soils.

Isotope ratios and concentrations of sulfur and nitrogen in needles and soils of Picea abies stands as influenced by atmospheric deposition of sulfur and nitrogen compounds

Concentrations and natural isotope abundance of total sulfur and nitrogen as well as sulfate and nitrate concentrations were measured in needles of different age classes and in soil samples of different horizons from a healthy and a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), in order to study the fate of atmospheric depositions of sulfur and nitrogen compounds.The mean δ15N of the needles ranged between −3.7 and −2.1 ‰ and for δ34S a range between −0.4 and +0.9 ‰ was observed. δ34S and sulfur concentrations in the needles of both stands increased continuously with needle age and thus, were closely correlated. The δ15N values of the needles showed an initial decrease followed by an increase with needle age. The healthy stand showed more negative δ15N values in old needles than the declining stand. Nitrogen concentrations decreased with needle age.For soil samples at both sites the mean δ15N and δ34S values increased from −3 ‰ (δ15N) or +0.9 ‰ (δ34S) in the uppermost organic layer to about +4 ‰ (δ15N) or +4.5 ‰ (δ34S) in the mineral soil. This depth-dependent increase in abundance of 15N and 34S was accompanied by a decrease in total nitrogen and sulfur concentrations in the soil. δ15N values and nitrogen concentrations were closely correlated (slope −0.0061 ‰ δ15N per μmol eq N gdw−1), and δ34S values were linearly correlated with sulfur concentrations (slope −0.0576 ‰ δ34S per μmol eq S gdw−1). It follows that in the same soil samples sulfur concentrations were linearly correlated with the nitrogen concentrations (slope 0.0527), and δ34S values were linearly correlated with δ15N values (slope 0.459). A correlation of the sulfur and nitrogen isotope abundances on a Δ basis (which considers the different relative frequencies of 15N and 34S), however, revealed an isotope fractionation that was higher by a factor of 5 for sulfur than for nitrogen (slope 5.292). These correlations indicate a long term synchronous mineralization of organic nitrogen and sulfur compounds in the soil accompanied by element-specific isotope fractionations.Based on different sulfur isotope abundance of the soil (δ34S=0.9 ‰ for total sulfur of the organic layer was assumed to be equivalent to about −1.0 ‰ for soil sulfate) and of the atmospheric SO2 deposition (δ34S=2.0 ‰ at the healthy site and 2.3 ‰ at the declining site) the contribution of atmospheric SO2 to total sulfur of the needles was estimated. This contribution increased from about 20 % in current-year needles to more than 50 % in 3-year-old needles. The proportion of sulfur from atmospheric deposition was equivalent to the age dependent sulfate accumulation in the needles. In contrast to the accumulation of atmospheric sulfur compounds nitrogen compounds from atmospheric deposition were metabolized and were used for growth. The implications of both responses to atmospheric deposition are discussed.

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N2O gas is involved in global warming and ozone depletion. The major sources of N2O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N2O produced by soil AOA and associated N2O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N2O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ15Nbulk and δ18O -N2O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A 15N-NH4+-labeling experiment indicated that N2O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N2O from AOA may be attributable to the relative contributions of these two processes. The highest N2O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N2O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N2O emissions from soil nitrification may be attributable to AOA.

Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification – a pure culture study

RATIONALE The contribution of fungal denitrification to the emission of the greenhouse gas nitrous oxide (N2O) from soil has not yet been sufficiently investigated. The intramolecular (15)N site preference (SP) of N2O could provide a tool to distinguish between N2O produced by bacteria or fungi, since in previous studies fungi exhibited much higher SP values than bacteria. METHODS To further constrain isotopic evidence of fungal denitrification, we incubated six soil fungal strains under denitrifying conditions, with either NO3(-) or NO2(-) as the electron acceptor, and measured the isotopic signature (δ(18)O, δ(15)Nbulk and SP values) of the N2O produced. The nitrogen isotopic fractionation was calculated and the oxygen isotope exchange associated with particular fungal enzymes was estimated. RESULTS Five fungi of the order Hypocreales produced N2O with a SP of 35.1 ± 1.7 ‰ after 7 days of anaerobic incubation independent of the electron acceptor, whereas one Sordariales species produced N2O from NO2(-) only, with a SP value of 21.9 ± 1.4 ‰. Smaller isotope effects of (15)Nbulk were associated with larger N2O production. The δ(18)O values were influenced by oxygen exchange between water and denitrification intermediates, which occurred primarily at the nitrite reduction step. CONCLUSIONS Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2(-) and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and δ(18)O values of the N2O produced.

Sulfate reduction in a forested catchment as indicated by δ34S values of sulfate in soil solutions and runoff

Abstract In a forested catchment in the Fichtelgebirge mountains (NE-Bavaria, Germany) the long term SO(4) (2-) budget (average 1988-1994) indicated that about 40% of the input with throughfall (16.8 kg SO(4) (2-) S·ha(-1)·yr(-1)) was retained in the catchment. In order to identify processes acting as potential SO(4) (2-) sinks, δ(34)S values of SO(4) (2-) in soil solutions and runoff were measured between May and November 1994. δ(34)S values of the runoff and the fen were higher (5.8‰) than the δ(34)S values of the soil solution of the oxic soils in the terrestrial area (3.9‰). Because there is no lithogenic S source within the catchment, it can be assumed that SO(4) (2-) deposition is the only S source in the catchment. Thus the results were interpreted as a result of SO(4) (2-) reduction within the catchment, because the uptake of (32)S is favoured during the dissimilatory SO(4) (2-) reduction and (34)S is consequently enriched in the soil solution. To estimate the amount of SO(4) (2-) reduced isotopic fractionation factors between - 9‰ and -46‰ were considered, resulting in SO(4) (2-) reduction rates of 1.8-9.3 kg SO(4) (2)-S·ha(-1)yr(-1). It was concluded that besides dissimilatory SO(4) (2-) reduction another sink exists in the catchment (e.g. SO(4) (2-) sorption in deep soil layers).

Sulphur isotope fractionation during sulphur mineralization: results of an incubation–extraction experiment with a Black Forest soil

Laboratory incubation–extraction experiments were used to study sulphur isotope fractionation during sulphur mineralization in Oh and Ah horizons of a Black Forest soil. Changes in δ34S values for sulphate extracted every three days with deionized-distilled water over a three week incubation period were small (<1.5‰). A second experiment used the addition of dilute ammonium sulphate solution, enriched in 34S relative to soil sulphur, to demonstrate unequivocally that sulphate adsorption and desorption during the incubation were negligible. Sulphur isotope fractionation during mineralization of carbon-bonded sulphur was shown to be a two-step process. The first and slower step was the formation of soluble organic sulphate from carbon-bonded sulphur, accompanied by a kinetic isotope effect, k32/k34=1.0040±0.0008. The faster step, identified with the hydrolysis of organic sulphate favored 34S in the product with a k32/k34=0.9967±0.0003. Leaching the soils led to a loss of isotopically light organic sulphate from the organic sulphur pool and is likely the process responsible for progressively heavier δ34S values for organic sulphur with depth in undisturbed forest soils in the Black Forest region.

Evaluation of sulphur cycling in managed forest stands by means of stable S-isotope analysis

Sulphur cycling was evaluated in a 20 to 60 year old Norway spruce (Picea abies L. Karst) ecosystem in the Black Forest near Schluchsee, SW Germany, by means of stable sulphur isotope analysis.Soil and plant material were analysed for S-content and S-isotopic composition to gather information on the S-distribution in the ecosystem. Two out of three adjacent watershed areas, highly comparable to each other were fertilized with MgSO4 and (NH4)2SO4 respectively, where sulphate was enriched in the 34S-isotope compared to the sulphur present in the ecosystem. As the fertilizer S served as a tracer, comparison of the S-isotopic composition of total and inorganic S in the soil and S in spruce needles from both the treated and the control sites led to new information of S-turnover processes.The S-isotopic composition of spruce needles changed markedly after the fertilizer application. Within half a year a shift towards the S-isotopic composition of the fertilizers sulphate indicated uptake of the sulphate by the trees, although this uptake did not become visible with the S content of the needles.Regarding the soil, a shift in the S-isotopic composition of the total sulphur was not that striking as with the needles, although the phosphate extractable sulphate showed a clear shift towards the S-isotopic composition of the fertilizer sulphate.

Fungal oxygen exchange between denitrification intermediates and water

RATIONALE Fungi can contribute greatly to N2O production from denitrification. Therefore, it is important to quantify the isotopic signature of fungal N2O. The isotopic composition of N2O can be used to identify and analyze the processes of N2O production and N2O reduction. In contrast to bacteria, information about the oxygen exchange between denitrification intermediates and water during fungal denitrification is lacking, impeding the explanatory power of stable isotope methods. METHODS Six fungal species were anaerobically incubated with the electron acceptors nitrate or nitrite and (18)O-labeled water to determine the oxygen exchange between denitrification intermediates and water. After seven days of incubation, gas samples were analyzed for N2O isotopologues by isotope ratio mass spectrometry. RESULTS All the fungal species produced N2O. N2O production was greater when nitrite was the sole electron acceptor (129 to 6558 nmol N2O g dw(-1)  h(-1)) than when nitrate was the electron acceptor (6 to 47 nmol N2O g dw(-1)  h(-1)). Oxygen exchange was complete with nitrate as electron acceptor in one of five fungi and with nitrite in two of six fungi. Oxygen exchange of the other fungi varied (41 to 89% with nitrite and 11 to 61% with nitrate). CONCLUSIONS This is the first report on oxygen exchange with water during fungal denitrification. The exchange appears to be within the range previously reported for bacterial denitrification. This adds to the difficulty of differentiating N2O producing processes based on the origin of N2O-O. However, the large oxygen exchange repeatedly observed for bacteria and now also fungi could lead to less variability in the δ(18)O values of N2O from soils, which could facilitate the assessment of the extent of N2O reduction.

Soil carbon isotopic composition and soil carbon content in an agroecosystem during six years of Free Air Carbon dioxide Enrichment (FACE)

The Free Air Carbon dioxide Enrichment (FACE) experiment conducted at the Federal Agricultural Research Centre (FAL) in Braunschweig in an arable crop rotation (total duration six years) allowed us to trace carbon (C) input in the soil C pool, as the CO2, used in the experiment to increase the atmospheric CO2 concentration, was depleted in 13C. Accurate assessment of the C input by means of stable C isotope analysis requires detailed knowledge on the spatial distribution of both the C isotopic composition and the C content in the soil C. Assumed changes in these parameters were examined. CO2 enrichment treatment over a six year period resulted in a clear trend towards an increase of soil C content in the uppermost 10 cm of soil. About 4.9% of the soil C present under ambient air conditions, and 10.7% present under elevated CO2 conditions were determined as new input. However, the results are not statistically significant yet. †Revised version of a paper presented at the 30th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 8–10 October 2007, Bayreuth, Germany.

Influence of Earthworms on the Sulfur Turnover in the Soil

Abstract The effects of earthworm activity on the concentration and isotopic composition of total sulfur in soils was investigated using batch experiments. Two ecologically different lumbricid species, the anecic Lumbricus terrestris and the endogeic Aporrectodea caliginosa, were used. The earthworms were fed birch leaves, beech leaves, cattle manure or mixed plant litter. All food sources differed isotopically (δ(34)S) from the soil (Parabraunerde). As a reference, one experiment was carried out without additional food. The experimental results show, that both earthworm species influence the total S-content and the δ(34)S-values in the soil by digestion of the different food sources. The differences in the total S-content of the earthworm tissues and in the S-isotopic composition of the casts can be attributed to the ecological differences between the earthworm species.