The role of the organic layer for phosphorus nutrition of young beech trees (Fagus sylvatica L.) revealed by multi-isotopic labelling (P-33; H2O-18) at two sites differing in soil phosphorus availability

The accumulation of an organic layer in forests is linked to the ratio between litterfall and decomposition rates, the latter being decelerated due to acidification and associated nutrient depletion with proceeding ecosystem development. Nevertheless, the nutrient pool in the organic layer might still represent an important interim storage and source for phosphorus (P) nutrition of forests on nutrient-poor soils. Due to the retention of P in soil e.g., by sorption to sesquioxides, P-poor ecosystems tend to show P recycling by organic matter decomposition. Our objective was to assess the importance of the organic layer to P nutrition of young beech trees. We established a mesocosm experiment including plants and soil from two forest sites differing in P availability. In half of all pots comprising both sites, the organic layer was present while the organic layer was lacking in the other half. We applied P-33 and H2O-18 to the pots. After 0h, 24h, 48h, 96h and 192h we destructively harvested the young beech trees, sampled the organic layer and mineral soil. P-33 activity was measured for every compartment in soil and plant (xylem, leaves, branches, stems) whereas δO-18 values in phosphate(δO-18P) were assessed for soil only. For both sites, δO-18P values in resin-extractable P in soil were close to those expected if isotope fractionation during intracellular pyrophosphate storage and subsequent release takes place. Therefore, δO-18P values indicate that bioavailable P in both soils has been cycled through microorganisms. However, the absence of an organic layer at the P-poor site resulted in a considerable shift of δO-18P values from those to be expected if P has been cycled through microorganisms. For both sites, the presence of the organic layer increased P-33 activity in xylem sap compared to the treatment without (104% P-poor site, 700% P-rich site). The total P-33 activity in plant tissue in pots from the P-rich site was not affected by the presence or absence of an organic layer after 192h, whereas a strong increase of 155 kBq/g DM was recorded for the P-poor site if an organic layer was present. Therefore, the key role of the organic layer for plant P nutrition on a P-depleted site was highlighted by our multi-isotopic labelling approach. In conclusion, our results suggest that P mobilization strategies differ among sites i.e., a P recycling vs. a P acquiring strategy

Soil Contamination due to Arsenic-Enriched Irrigation Water - Impact of Irrigation Practices

Increasing irrigation with arsenic (As) contaminated groundwater represents a growing problem in the densely populated delta and floodplain regions of SE Asia. The overarching aim of this study was to evaluate retention and mobility of As in soils in dependence of the irrigation practice. Irrigation of calcareous agricultural soils was simulated in a green-house experiment, in which artificial anoxic groundwater enriched in dissolved AsIII (10 mg/L) was applied regularly. We compared the following three different irrigation scenarios: permanently flooded, promoting reducing conditions (R); alternating flood irrigation, characterized by frequent changes in water saturation (RO); and sprinkler irrigation, maintaining permanently oxic conditions (O). Several wet chemical extraction procedures were carried out to characterize soil As storage pools at the end of the experiment. Pore water analysis reflected strongly reducing redox conditions (up to 42.9 mg/L dissolved Fe) for the R treatment, while less reducing conditions developed in the RO scenario (Fe max. 0.14 mg/L). Furthermore, As concentrations in pore water increased steadily to 1.34 (R) and 0.39 mg/L (O), respectively, with 20% (R) and 80% (RO) being present in the oxidized form AsV. The addition of As by irrigation water resulted in surprisingly similar depth distributions being independent of the irrigation treatment. Highest As contents (R: 52.2, RO: 49.6 and O: 43.9 mg/kg) occurred within the top 0-2 cm and decreased rapidly to values close the initial content (11.5 mg/kg) below 4 cm depth. This reflects a generally high sorption capacity of the soil for As. Even reductive dissolution of Fe-phases and the accompanying loss of sorption sites (R treatment) did not affect the As sorption behavior in general. However, pore water As concentrations and sequential extraction results point at a higher As mobility in case of the R treatment. This can be explained by the higher proportion of AsIII in the pore water, which is more mobile than AsV at the prevailing conditions. In sum, the three irrigation practices did not result in differences regarding the vertical distribution of As, but permanent flooding clearly increased the mobility of As as compared to the other treatments. The comparison of different wet chemical extraction procedures further emphasizes that protocol and sample treatment should be selected with caution, especially when redox conditions in the soil vary

Short- and long-term effects of biodiversity on soil nutrient concentrations in a semi-natural grassland: results from a 14-year experiment

Global biodiversity is declining at an alarming rate, which is likely to have important consequences on ecosystem functioning. Previous studies have shown that in the short term, higher plant biodiversity in grasslands is linked to lower soil nitrogen concentrations, particularly of nitrate, probably due to higher plant uptake. It is unknown, however, how this trend will develop in the long term. To establish long-term responses to experimental changes in biodiversity, long-term data in adequately high resolution is required to separate the long-term trend from seasonal variation in the data, and such data sets are still exceedingly rare. We present a data set of soil solution nitrogen and phosphorus concentrations collected every two weeks over 14 years after the establishment of an experimental grassland with varying levels of biodiversity. Analysis of this data allows us to determine a) whether the system has reached a new steady-state in soil nutrients after conversion from cropland soils to semi-natural grassland 15 years ago, and b) whether these steady-states are different for different levels of plant biodiversity. Furthermore, we expect to be able to detect c) the effects of extreme events (drought, flood) and d) temporal trends under different levels of plant biodiversity before the establishment of steady state. This will have important implications for our understanding of both the biodiversity-ecosystem functioning relationship and the nutrient dynamics of soils changing from previously fertilized systems to semi-natural grasslands. Our results might additionally have practical implications for the establishment and management of hay meadows

Stable hydrogen isotope ratios in crystal water of clay minerals

Hydrogen is the most abundant element in the Universe. But the utilization of the H isotopic composition (δH-2 value) of soil to elucidate biogeochemical processes or to serve as a palaeo climate proxy is still in its infancy. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. The δH-2 value of structural H in clay minerals – mainly from C-poor subsoils - has been studied since the 1970s. The δH-2 value of clay minerals mainly depends on (a) the average δH-2 value of ambient water at the site and time of formation, and on (b) the size of the equilibrium isotopic fractionation factor between water and clay mineral at the temperature of formation. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. Only nonexchangeable H in in structural water of minerals preserves its inherited δH-2 value and does not exchange with water at temperatures usually occurring in soil environments at the Earth’s surface. Nonexchangeable H is bound in crystal water, which integrates the δH-2 value of soil water over several millennia. This is in turn determined by palaeoclimatic variations of the precipitation’s δH-2 signal with distinguishable shifts e.g., from Pleistocene to Holocene. For a global data set, Ruppenthal (2014) reported a close correlation of bulk soil δH-2 values with those of the mean local precipitation and confirmed this for organic matter, while the clay fraction of soils was up to now not studied. We will adapt a steam equilibration method with water vapor of known H isotopic composition – formerly applied by Ruppenthal (2014) on SOM and bulk soil – to clay fractions and compare our results to the hitherto used heating treatments (200-250°C) under vacuum. We expect that the δH-2 signal of the clay fraction of Bt horizons will serve to differentiate soils developed in different climatic epochs (e.g., Holocene, last interstadial, last interglacial) by analyzing dated palaeo soil samples. To test the hypothesis that there is a similar global regression line of the δH-2 values in structural water of clay as up to now reported for bulk soils and soil organic matter, we will analyze the clay fraction in a global set of soil samples

Stable hydrogen isotope ratios in soil organic matter

Stable H isotope ratios are a promising indicator of OM transformation processes (Schimmelmann et al., 2006). δ2H values of bulk organic matter and of specific organic compounds can be used as ecological tracer and forensic tool if the proportion of H that readily exchanges with ambient moisture is accounted for (Wassenaar & Hobson, 1998). There are a few reports about the H isotope ratios in plant-soil systems illustrating that there is little knowledge of the controls of the isotopic composition of the non-exchangeable H fraction of bulk OM (Schimmelmann et al., 2006; Ruppenthal et al., 2015). The increasingly closer relationship between δ2H values of rainfall and of non-exchangeable H in OM (δ2Hn) in the order, plant – plant litter (above- and belowground) – soil along a climatic gradient (Ruppenthal et al., 2015) suggests that decomposition influences δ2Hn values in OM in a systematic way. However, there are knowledge gaps concerning the fractionation factors and the extent of incorporation of ambient water-H into the nonexchangeable fraction of H in OM during decomposition. Our research will focus on the mechanisms responsible for the strong correlation between δ2H values in rainfall and δ2Hn values of SOM. Therefore, our study aims to investigate (1) the incorporation of ambient water-H into the nonexchangeable H fraction in OM during decomposition by heterotrophic bacteria as model organisms and quantify apparent fractionation factors, (2) the extent of incorporation of ambient water into the nonexchangeable H fraction of OM by the soil microbial community under laboratory conditions, and (3) the extent to which H is incorporated into nonexchangeable OM pool from ambient water during decomposition of aboveground litter under field conditions. We will work with microcosms using two bacteria species and determine decomposition rates of litter. Steam equilibration (Ruppenthal et al., 2015) and TC/EA-IRMS are used as analytical tools. We expect that different decomposition rates because of differences in litter quality will be reflected by the extent of H incorporation from ambient water into the nonexchangeable H fraction of the products. Additionally, different litter types enriched in 2H will be buried in soil of forest stands. We hypothesize that the incorporation of 2H-depleted ambient water into 2H-enriched nonexchangeable H fraction of OM will depend on litter type, soil moisture/ temperature, and the heterotrophic activity during the experiment

Umsatz von organischer Bodensubstanz mittels stabiler Kohlenstoffisotope: eine Metaanalyse

Vergangene Arbeiten zeigten Änderungen des über δ13C Tiefenprofile abgeleiteten Umsatzes von organischer Bodensubstanz (OBS) mit der Jahresmitteltemperatur (JMT), Jahresniederschlagssummen (JNS) und der Zeit. Weniger stark ausgeprägte δ13C Tiefenprofile unter höherer JMT deuten - entgegen der generellen Annahme von steigendem Umsatz unter steigenden Temperaturen - auf sinkenden Umsatz von OBS hin. Reduzierte Bodenfeuchtigkeit könnte die Substratverfügbarkeit eingeschränkt haben, während die Dominanz von Verlagerungsprozessen die Tiefenverteilung von δ13C unter hohen JNS dominiert haben könnte. Auf einer Zeitskala von Jahrtausenden wird angenommen, dass Böden große Mengen von Kohlenstoff speichern und eine Reduzierung des Umsatzes von OBS stattfindet. Die δ13C Tiefenverteilungen entlang einer neuseeländischen Chronosequenz allerdings suggerieren, dass Umsatz der OBS mit der Zeit nicht kontinuierlich abnimmt. Ziel dieser Studie ist die Erweiterung bisheriger Ergebnisse zur Nutzung stabiler Kohlenstoffisotope als Proxy für Umsatz von OBS und die Auswertung innerhalb einer Metaanalyse. Hierzu wurden δ13C Tiefenprofile aus 22 global verteilten Studien hinsichtlich ihrer Änderung mit JMT und JNS ausgewertet. Zur Untersuchung der Zeit dienten Chronosequenz-Studien aus Deutschland, Kanada und Neuseeland. Entsprechend der kinetischen Theorie sind δ13C Tiefenprofile ausgeprägter unter steigender JMT mit einem linearen Anstieg des abgeschätzten Umsatzes (R2 = 0,52; P < 0,001). Im Gegensatz dazu gibt es keine linearen Zusammenhänge zwischen JNS und der Entwicklung von δ13C Tiefenprofilen, vermutlich durch Texturheterogenität der untersuchten Studien. Die Einflüsse von Zeit auf δ13C Tiefenprofile scheinen ökosystemspezifisch zu sein, mit einem Anstieg des abgeschätzten Umsatzes von OBS während der frühen Ökosystementwicklung und erkennbaren δ13C Tiefenprofile nach drei Dekaden. Die gewonnen Daten zeigen globale Trends der Nutzung von δ13C Tiefenprofilen zur Abschätzung des Umsatzes und liefern damit einen wichtigen Beitrag zum Verständnis der C Sequestrierung im Boden

Himalayan treeline soil and foliar C:N:P stoichiometry indicate nutrient shortage with elevation

Only a few studies have addressed the soil and foliar carbon:nitrogen:phosphorus (C:N:P) stoichiometry in alpine treeline ecotones. Moreover, information on the soil nutrient availability (primarily N, P) in such ecosystems is rare. To fill these gaps, we performed a multiple data sampling in a near-natural alpine treeline ecotone in Rolwaling Himal, Nepal. Our results show strongly varying soil C:N:P ratios and nutrient availabilitywith soil depth. Caused by high monsoon precipitation and coarse-grained soils with low water-holding capacities, a vertical transport of nutrients and potentially mineralizable soil organic matter (SOM) in soils occurs, which is a general problem in the study area impeding growing conditions for trees. Soil N and P availability, and soil C:P and N:P ratios decrease significantly as elevation increases, especially at the transition from krummholz (dominated by rhododendron campanulatum) to the alpine tundra (dwarf scrub heath). Soil C:N ratios increase significantly with elevation, most notably from the subalpine forest to krummholz and the alpine tundra. These altitudinal trends indicate increasing nutrient (N, P) shortage especially in the alpine tundra. LowNand P availability in alpine tundra soils are likely caused by a lower litter input from dwarf shrub vegetation, and a decline in litter mineralization in this altitudinal zone resulting in small accumulation of SOM. Nutrient availability in the entire study area is generally limited by low soil pH (from 2.5 to 4). In total six investigated tree species show diverse relationships between foliar and soil stoichiometric ratios, and soil nutrient availability. Significantly increasing foliar C:N and C:P ratios with elevation due to significantly decreasing foliar N and P concentrations suggest a limitation in N and P. Foliar N:P ratios indicate N rather than P limitation. Contrary to previous studies from different alpine treeline ecotones, we do not consider the Rolwaling treeline ecotone as an area of nutrient accumulation. We conclude that altitudinal variations in stand structures themselves govern nutrient cycling through the input of C, N, and P into soils by differences in leaf fall

Phosphordünger aus Klärschlamm: Evaluierung von Pyrolysekohle und Alkalipyrolyse

Phosphorus (P) in sewage sludge holds the main proportion of P currently lost in waste streams, and options to recycle it for agriculture are needed. Pyrolysis is a promising local alternative to centralized sewage sludge incineration. Pathogens and organic pollutants are destroyed and a carbon-rich substrate known as biochar is formed. We evaluated pyrolysis of sewage sludge as well as sintering of pyrolysis char with alkali salts. The products were applied in a pot experiment with calcareous and acidic soils to determine P uptake by ryegrass (Lolium multiflorum) and changes in soil pH, available P and cation exchange capacity. Two products were also tested in a field experiment with maize (Zea mays) on a neutral soil. Cadmium and mercury were partly lost during pyrolysis. The pot experiment revealed interactions between soil pH and P use efficiency of the various products, including reference fertilizers permitted in organic farming. Pyrolysis changed plant P availability only slightly compared to the parent sewage sludge, but sintering with alkali salts increased plant P availability to levels comparable to triple superphosphate. Potential implications for organic agriculture are discussed

15N tracing to elucidate links between biodiversity and nitrogen cycling in a grassland experiment

Nitrogen (N) cycling is a fundamental ecosystem function of high complexity because N undergoes many transformations in soil and vegetation. The effect of biodiversity loss on ecosystem functions in general, and on N cycling in particular, was studied in several manipulative field experiments. To generate a comprehensive view of the influence of species richness on all major N transformations, we conducted laboratory incubations, in which we added 15N-labeled ammonium and nitrate to soil samples of the “Jena Experiment”, a manipulative large scale, long-term biodiversity experiment in grassland. The experimental site is located in Jena, Germany. The design consists of 4 blocks and 82 plots with 1-60 species and 1-4 functional groups (grasses, legumes, small herbs, tall herbs). Approx. 400 g of field-fresh soil was sampled from each plot of one of the 4 blocks and divided into three aliquots of 100 g each. In order to trace N turnover, we amended the incubations (in triplicate) either with 15N-labelled (98 at%) ammonium, nitrate, or with a mixture of both. The samples were incubated for two months at 20°C. Soil solution was extracted 1, 2, 4, 9 and 16 days after 15N application by percolating 100 mL of nutrient solution through each vessel. Concentrations of NH4-N, NO3-N and total N in the extracts were determined with colorimetric methods. The N-isotopic composition in nitrate was analyzed by isotope ratio mass spectrometry (IRMS) using the denitrifier method. Ammonium N isotope ratios were determined using the “hypobromite oxidation” method, in which ammonium-N is converted to nitrite followed by azide reaction to nitrous oxide and IRMS analysis. The results will be comprehensively evaluated in a quantitative context using the modelling approach of Müller et al. (2007) to determine the size of six N pools and the rates of nine N transformations. Links between N transformation rates, N-pool size and plant species richness will be verified with the help of ANOVA

Carbon cycling and translocation processes in Baltic Sea coastal dune topsoils

Chronosequenzen bieten ideale Voraussetzungen zur Analyse des Einflusses pedo­genetischer Prozesse auf die Kohlenstoff- und Stickstoffdynamik im Boden. Die sich ändernden Verfügbarkeiten von Kohlenstoff (C) und Stickstoff (N) während der Pedogenese beeinflussen den Umsatz von organischer Bodensubstanz (OBS). Vergangene Arbeiten konnten zeigen, dass die Verteilung stabiler C Isotope im Oberboden zur Abschätzung des Umsatzes von OBS herangezogen werden kann. Entlang einer Dünenchronosequenz in Haast (Neuseeland) stieg der Umsatz von OBS zu Beginn der Pedogenese und erreichte einen Tiefpunkt nach dem Zerfall der Bäume aus erster Generation. Zudem zeigten C-13 Tiefenprofile Parallelen zur vertikalen Verteilung stabiler N Isotope im Oberboden. Ziele dieser Studie sind die Untersuchung von C-13 und 15-N Tiefenprofilen entlang einer Dünenchronosequenz der Ostsee. Hierzu werden entlang einer 60 bis 5500 Jahre alten Dünensequenz bei Swinemünde (Polen) Bodenproben entnommen und anhand ihrer C und N Isotopenverteilungen analysiert. Es wird ein Anstieg des Umsatzes durch ansteigende C Gehalte in frühen Phasen der Pedogenese erwartet. Atmosphärische N Deposition kann einen erheblichen Effekt auf die Vegetations- und mikrobielle Zusammensetzung sowie Umsatzprozesse während der Boden- und Ökosystementwicklung haben. Große Unterschiede der N Depositionen zwischen Dünengebieten der südlichen Ostsee und Neuseeland könnten daher 13-C sowie 15-N Tiefenprofile beeinflussen. Die gewonnen Daten liefern einen wichtigen Beitrag zum Verständnis der C Sequestrierung und der Stickstoffdynamik während der Pedogenese