Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ18 O signature.

The oxygen isotope composition (δ18 O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways. We assessed the relative contribution of seasonal needle and xylem water δ18 O variations to the intra-annual tree-ring cellulose δ18 O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ18 O measurements of soil water, needle water, and twig xylem water with intra-annual δ18 O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling. Intra-annual cellulose δ18 O values resembled source water δ18 O mean levels better than needle water δ18 O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (pex ). Maximum pex values were achieved in August and imprinted on sections at 50-75% of the ring. High pex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ18 O variability, we estimated a limited Péclet effect at both sites. Due to a variable pex , source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ18 O variations, potentially masking signals coming from needle-level processes.

A simple method for the removal of dissolved organic matter and δ15N analysis of NO3(-) from freshwater.

RATIONALE: Stable isotopes of nitrogen in nitrate (NO(3)(-)) are frequently used to identify nitrate sources and to study nitrogen (N) transformation processes, but the measurement methods available are generally rather labor intensive and/or costly, and dissolved organic matter (DOM) can interfere with the δ(15)N signature of nitrate. We therefore have developed a simple cleanup procedure for freshwater samples with low nitrate and high DOM concentrations. METHODS: Nitrate and DOM are extracted from a freeze-dried water sample by using a concentrated sodium hydroxide solution. By the subsequent addition of acetone, two liquid layers are formed, and nitrate migrates into the acetone while DOM remains in the concentrated NaOH solution, thus separating the nitrate from the DOM. For nitrogen isotope analysis, purified nitrate salts are combusted at 1030 °C to produce N(2) gas in an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS). RESULTS: With this novel technique up to 99% of DOM could be removed from river water and soil solutions. The method has been tested for sample amounts as small as 4 µmol NO(3)(-) with a precision of <0.1‰ (1SD). Nitrate standards are reproduced accurately without any blank correction. CONCLUSIONS: The benefits of this method are the lack of interferences derived from DOM on the δ(15)N signature and the ease of sample preparation.

A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis.

The nitrogen (δ(15)N) and oxygen isotope (δ(18)O) analysis of nitrate (NO(3)(-)) from aqueous samples can be used to determine nitrate sources and to study N transformation processes. For these purposes, several methods have been developed; however, none of them allows an accurate, fast and inexpensive analysis. Here, we present a new simple method for the isolation of nitrate, which is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture. In this solvent, all major nitrate salts are soluble, whereas all other oxygen-bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. Nitrate is first concentrated by freeze-drying, dissolved in the ternary solvent and separated from insoluble compounds by centrifugation. Anhydrous barium nitrate is then precipitated in the supernatant solution by adding barium iodide. For δ(18)O analysis, dried Ba(NO(3))(2) samples are directly reduced in a high-temperature conversion system to CO and measured on-line using isotope ratio mass spectrometry (IRMS). For δ(15)N analysis, samples are combusted in an elemental analyzer (EA) coupled to an IRMS system. The method has been tested down to 20 µmol NO(3)(-) with a reproducibility (1SD) of 0.1‰ for nitrogen and 0.2-0.4‰ for oxygen isotopes. For nitrogen we observed a small consistent (15) N enrichment of +0.2‰, probably due to an incomplete precipitation process and, for oxygen, a correction for the incorporation of water in the precipitated Ba(NO(3))(2) has to be applied. Apart from being robust, this method is highly efficient and low in cost.

A decade of monitoring at Swiss Long-Term Forest Ecosystem Research (LWF) sites: can we observe trends in atmospheric acid deposition and in soil solution acidity?

Trends in atmospheric acid deposition and in soil solution acidity from 1995 or later until 2007 were investigated at several forest sites throughout Switzerland to assess the effects of air pollution abatements on deposition and the response of the soil solution chemistry. Deposition of the major elements was estimated from throughfall and bulk deposition measurements at nine sites of the Swiss Long-Term Forest Ecosystem Research network (LWF) since 1995 or later. Soil solution was measured at seven plots at four soil depths since 1998 or later. Trends in the molar ratio of base cations to aluminum (BC/Al) in soil solutions and in concentrations and fluxes of inorganic N (NO(3)-N + NH(4)-N), sulfate (SO(4)-S), and base cations (BC) were used to detect changes in soil solution chemistry. Acid deposition significantly decreased at three out of the nine study sites due to a decrease in total N deposition. Total SO(4)-S deposition decreased at the nine sites, but due to the relatively low amount of SO(4)-S load compared to N deposition, it did not contribute to decrease acid deposition significantly. No trend in total BC deposition was detected. In the soil solution, no trend in concentrations and fluxes of BC, SO(4)-S, and inorganic N were found at most soil depths at five out of the seven sites. This suggests that the soil solution reacted very little to the changes in atmospheric deposition. A stronger reduction in base cations compared to aluminum was detected at two sites, which might indicate that acidification of the soil solution was proceeding faster at these sites.

Morphological and physiological responses of Scots pine fine roots to water supply in a dry climatic region in Switzerland.

In recent decades, Scots pine (Pinus sylvestris L.) forests in inner-Alpine dry valleys of Switzerland have suffered from drought and elevated temperatures, resulting in a higher mortality rate of trees than the mean mortality rate in Switzerland. We investigated the responses of fine roots (standing crop, morphological and physiological features) to water supply in a Scots pine forest in the Rhone valley. Before irrigation started in 2003, low- and high-productivity Scots pine trees were selected based on their crown transparency. The fine root standing crop measured in spring from 2003 to 2005 was unaffected by the irrigation treatment. However, irrigation significantly enhanced the fine root standing crop during the vegetation period when values from spring were compared with values from fall in 2005. Irrigation slightly increased specific root length but decreased root tissue density. Fine root O2-consumption capacity decreased slightly in response to the irrigation treatment. Using ingrowth cores to observe the responses of newly produced fine roots, irrigation had a significantly positive effect on the length of fine roots, but there were no differences between the low- and high-productivity trees. In contrast to the weak response of fine roots to irrigation, the aboveground parts responded positively to irrigation with more dense crowns. The lack of a marked response of the fine root biomass to irrigation in the low- and high-productivity trees suggests that fine roots have a high priority for within-tree carbon allocation.

Induction of callose in roots of Norway spruce seedlings after short-term exposure to aluminum.

Callose (1,3-beta-glucan) is a suggested physiological indicator of aluminum (Al) toxicity in crop plants. It is not known if callose serves a similar function in forest trees, because quantitative data on callose formation in tree roots are limited, particularly under controlled conditions. To evaluate callose as a physiological indicator of Al toxicity in tree roots, we quantified callose formation in roots of Norway spruce (Picea abies (L.) Karst.) seedlings. Seedlings were grown in simulated soil solutions in the presence or absence (control) of Al under controlled conditions. In seedlings grown in solutions containing 280 microM Al, callose concentrations in roots were twice as high as control values after 6 h of Al treatment and 5 times higher than control values after 1 day. Thereafter, root callose concentrations gradually decreased and were only twice as high as control values after 7 days. The presence of various Al concentrations in the simulated soil solutions indicated that callose was induced by a relatively low Al concentration (84 microM). We conclude that callose in tree roots is an indicator of Al toxicity.

Acidification of soil solution in a chestnut forest stand in southern Switzerland: are there signs of recovery?

In a previous study, a rapid acidification of soil solution was observed between 1987 and 1997 in a cryptopodzolic soil in southern Switzerland despite a reduction in acidic deposition. The molar ratio of base nutrient cations to aluminum (BC/Al) in the soil solution was used to assess acidification. The monitoring of the soil solution chemistry was continued at the same site between 1998 and 2003 to find out how long the delay in reaction to reduced deposition would last and whether the BC/Al ratios would recover. The reevaluation of all data collected during the 16-year observation period showed no clear improvement in the BC/Al ratios, except below the litter layer where the ratios greatly increased after 1998. Initial signs of recovery were also detected in the mineral horizons, the ratios stabilizing in the second part of the observation period. Sulfate concentrations decreased significantly below the litter mat in response to decreased S deposition. BC concentrations markedly declined below the litter layer and in the mineral horizons, which was attributed to the depletion of the BC exchangeable pool as a result of continued acidic deposition.