Aluminum toxicity to tropical montane forest tree seedlings in southern Ecuador: response of biomass and plant morphology to elevated Al concentrations

Aims: In acid tropical forest soils (pH <5.5) increased mobility of aluminum might limit aboveground productivity. Therefore, we evaluated Al phytotoxicity of three native tree species of tropical montane forests in southern Ecuador. Methods: An hydroponic dose-response experiment was conducted. Seedlings of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson were treated with 0, 300, 600, 1200, and 2400 μ M Al and an organic layer leachate. Dose-response curves were generated for root and shoot morphologic properties to determine effective concentrations (EC). Results: Shoot biomass and healthy leaf area decreased by 44 % to 83 % at 2400 μ M Al, root biomass did not respond (C. odorata), declined by 51 % (H. americanus), or was stimulated at low Al concentrations of 300 μ M (T. chrysantha). EC10 (i.e. reduction by 10 %) values of Al for total biomass were 315 μ M (C. odorata), 219 μ M (H. americanus), and 368 μ M (T. chrysantha). Helicarpus americanus, a fast growing pioneer tree species, was most sensitive to Al toxicity. Negative effects were strongest if plants grew in organic layer leachate, indicating limitation of plant growth by nutrient scarcity rather than Al toxicity. Conclusions: Al toxicity occurred at Al concentrations far above those in native organic layer leachate

Aluminum toxicity in a tropical montane forest ecosystem in southern Ecuador

Aluminum phytotoxicity frequently occurs in acid soils (pH < 5.5) and was therefore discussed to affect ecosystem functioning of tropical montane forests. The susceptibility to Al toxicity depends on the sensitivity of the plant species and the Al speciation in soil solution, which can vary highly depending e.g., on pH, ionic strength, and dissolved organic matter. An acidification of the ecosystem and periodic base metal deposition from Saharan dust may control plant available Al concentrations in the soil solutions of tropical montane rainforests in south Ecuador. The overall objective of my study was to assess a potential Al phytotoxicity in the tropical montane forests in south Ecuador. For this purpose, I exposed three native Al non-accumulating tree species (Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson) to increased Al concentrations (0 – 2400 μM Al) in a hydroponic experiment, I established dose-response curves to estimate the sensitivity of the tree species to increased Al concentrations, and I investigated the mechanisms behind the observed effects induced by elevated Al concentrations. Furthermore, the response of Al concentrations and the speciation in soil solution to Ca amendment in the study area were determined. In a final step, I assessed all major Al fluxes, drivers of Al concentrations in ecosystem solutions, and indicators of Al toxicity in the tropical montane rainforest in Ecuador in order to test for indications of Al toxicity. In the hydroponic experiment, a 10 % reduction in aboveground biomass production occurred at 126 to 376 μM Al (EC10 values), probably attributable to decreased Mg concentrations in leaves and reduced potosynthesis. At 300 μM Al, increased root biomass production of T. chrysantha was observed. Phosphorus concentrations in roots of C. odorata and T. chrysantha were significantly highest in the treatment with 300 μM Al and correlated significantly with root biomass, being a likely reason for stimulated root biomass production. The degree of organic complexation of Al in the organic layer leachate, which is central to plant nutrition because of the high root density, and soil solution from the study area was very high (mean > 99 %). The resulting low free Al concentrations are not likely to affect plant growth, although the concentrations of potentially toxic Al3+ increased with soil depth due to higher total Al and lower dissolved organic matter concentrations in soil solutions. The Ca additions caused an increase of Al in the organic layer leachate, probably because Al3+ was exchanged against the added Ca2+ ions while pH remained constant. The free ion molar ratios of Ca2+:Al3+ (mean ratio ca. 400) were far above the threshold (≤ 1) for Al toxicity, because of a much higher degree of organo-complexation of Al than Ca. High Al fluxes in litterfall (8.8 – 14.2 kg ha−1 yr−1) indicate a high Al circulation through the ecosystem. The Al concentrations in the organic layer leachate were driven by the acidification of the ecosystem and increased significantly between 1999 and 2008. However, the Ca:Al molar ratios in organic layer leachate and all aboveground ecosystem solutions were above the threshold for Al toxicity. Except for two Al accumulating and one non-accumulating tree species, the Ca:Al molar ratios in tree leaves from the study area were above the Al toxicity threshold of 12.5. I conclude that toxic effects in the hydroponic experiment occurred at Al concentrations far above those in native organic layer leachate, shoot biomass production was likely inhibited by reduced Mg uptake, impairing photosynthesis, and the stimulation of root growth at low Al concentrations can be possibly attributed to improved P uptake. Dissolved organic matter in soil solutions detoxifies Al in acidic tropical forest soils and a wide distribution of Al accumulating tree species and high Al fluxes in the ecosystem do not necessarily imply a general Al phytotoxicity.Aluminiumtoxizität tritt in sauren Böden bei pH-Werten < 5.5 auf und wurde als ein möglicher Grund für ein verkümmertes Baumwachstum in tropischen Bergregenwäldern diskutiert. Die Anfälligkeit für Al-Toxizität ist von der Pflanzenart abhängig sowie von der Al-Speziierung, die je nach pH-Wert, Ionenstärke und gelöster organischer Substanz (DOM) in der Bodenlösung stark variieren kann. Eine Versauerung des Ökosystems und periodische Baseneinträge mit Saharastäuben könnten pflanzenverfügbare Al-Konzentrationen in tropischen Bergregenwäldern in Südecuador beeinflussen. Das Ziel meiner Studie war es, eine potentielle Al-Phytotoxizität in tropischen Bergregenwäldern Südecuadors abzuschätzen. Zu diesem Zweck habe ich einen hydroponischen Versuch mit drei einheimischen, nicht Al-akkumulierenden Baumarten (Cedrela odorata L., Heliocarpus americanus L., und Tabebuia chrysantha (Jacq.) G. Nicholson) unter ansteigenden Al-Konzentrationen durchgeführt, Dosis-Wirkungskurven erstellt um die Al-Sensitivität der Baumarten zu ermitteln und die Mechanismen untersucht, die zu den im hydroponischen Versuch beobachteten Effekten geführt haben. Des Weiteren wurde der Einfluss von Ca-Einträgen in das Ökosystem auf die Al-Konzentrationen und die Speziierung in der Bodenlösung untersucht. Im letzten Schritt habe ich alle bedeutenden Al-Flüsse, Kontrollgrössen der Al-Konzentrationen in Bodenlösungen und Indikatoren der Al-Toxizität im Untersuchungsgebiet ermittelt. Im hydroponischen Versuch wurde die oberirdische Biomasseproduktion bei Al-Konzentrationen von 126 . 376 μM Al (EC10-Werte) um 10 % reduziert und ist auf eine gestörte Mg-Aufnahme und Verringerung der Photosynthese zurückzuführen. In der Behandlung mit 300 μM Al hat die Wurzelbiomasseproduktion von T. chrysantha signifikant zugenommen, ebenso die P-Gehalte in den Wurzeln. Aluminium in der Bodenlösung aus der organischen Auflage, welche aufgrund der hohen Wurzeldichte eine wichtige Rolle für die Nährstoffaufnahme spielt, war fast vollständig organisch komplexiert (Mittelwert > 99 %). Die Konzentrationen an freiem Al3+ stiegen mit der Bodentiefe, da die DOM Konzentration mit der Bodentiefe abnimmt. Jedoch ist eine toxische Wirkung durch die geringen Konzentrationen an freiem Al3+ unwahrscheinlich. Calciumeinträge bewirkten einen Anstieg der Al-Konzentration in der Bodenlösung aus der organischen Auflage, vermutlich durch den Austausch von Al3+ durch Ca2+-Ionen bei gleichbleibendem pH-Wert. Aufgrund der weitaus stärker ausgeprägten Komplexierung von Al gegenüber Ca waren die molaren Ca2+:Al3+-Ionen Verhältnisse (Mittelwert ca. 400) deutlich über dem Grenzwert (≤ 1) für Al-Toxizität. Hohe Al-Flüsse im Streufall (8.8 - 14.2 kg ha−1 yr−1) deuten auf einen ausgeprägten Al-Kreislauf im Ökosystem. Die Al-Konzentrationen in der Bodenlösung aus der organischen Auflage sind mit der Versauerung des Ökosystems zwischen 1999 und 2008 signifikant angestiegen. Mit Ausnahme von zwei Al-Akkumulatoren und einer nicht-akkumulierenden Spezies, lagen die molaren Ca:Al-Verhältnisse in Blättern von Waldbäumen aus dem Untersuchungsgebiet über dem Al-Toxizitätsgrenzwert von 12.5. Daraus schliesse ich, dass toxische Effekte im hydroponischen Versuch bei Al-Konzentrationen auftraten, die deutlich über den Konzentrationen in der Bodenlösung aus der organischen Auflage liegen. Der Rückgang der oberirdischen Biomasseproduktion mit steigender Al-Konzentration im hydroponischen Versuch wurde durch eine verringerte Mg-Aufnahme und damit reduzierte Photosynthese verursacht. Eine Stimulation der Wurzelbiomasseproduktion bei niedrigen Al-Konzentrationen ist vermutlich einer verbesserten P-Aufnahme zuzuschreiben. Die toxische Wirkung von Al in sauren tropischen Waldböden wird durch Komplexierung mit DOM reduziert. Eine weite Verbreitung von Al-akkumulierenden Pflanzen und hohe Al-Flüsse im Ökosystem implizieren nicht zwangsläufig eine Al-Phytotoxizität

Accumulation of cadmium and uranium in arable soils in Switzerland

Mineral phosphorus (P) fertilizers contain contaminants that are potentially hazardous to humans and the environment. Frequent mineral P fertilizer applications can cause heavy metals to accumulate and reach undesirable concentrations in agricultural soils. There is particular concern about Cadmium (Cd) and Uranium (U) accumulation because these metals are toxic and can endanger soil fertility, leach into groundwater, and be taken up by crops. We determined total Cd and U concentrations in more than 400 topsoil and subsoil samples obtained from 216 agricultural sites across Switzerland. We also investigated temporal changes in Cd and U concentrations since 1985 in soil at six selected Swiss national soil monitoring network sites. The mean U concentrations were 16% higher in arable topsoil than in grassland topsoil. The Cd concentrations in arable and grassland soils did not differ, which we attribute to soil management practices and Cd sources other than mineral P fertilizers masking Cd inputs from mineral P fertilizers. The mean Cd and U concentrations were 58% and 9% higher, respectively, in arable topsoil than in arable subsoil, indicating that significant Cd and U inputs to arable soils occurred in the past. Geochemical mass balances confirmed this, indicating an accumulation of 52% for Cd and 6% for U. Only minor temporal changes were found in the Cd concentrations in topsoil from the six soil-monitoring sites, but U concentrations in topsoil from three sites had significantly increased since 1985. Sewage sludge and atmospheric deposition were previously important sources of Cd to agricultural soils, but today mineral P fertilizers are the dominant sources of Cd and U. Future Cd and U inputs to agricultural soils may be reduced by using optimized management practices, establishing U threshold values for mineral P fertilizers and soils, effectively enforcing threshold values, and developing and using clean recycled P fertilizers

Uranium in agricultural soils and drinking water wells on the Swiss Plateau

Mineral phosphorus fertilizers are regularly applied to agricultural sites, but their uranium (U) content is potentially hazardous to humans and the environment. Fertilizer-derived U can accumulate in the soil, but might also leach to ground-, spring and surface waters. We sampled 19 mineral fertilizers from the canton of Bern and soils of three arable and one forest reference sites at each of four locations with elevated U concentrations (7–28 μg L−1) in nearby drinking water wells. The total U concentrations of the fertilizers were measured. The soils were analysed at three depth intervals down to 1 m for general soil parameters, total Cd, P, U and NaHCO3-extractable U concentrations, and 234/238U activity ratios (AR). The U concentrations and AR values of the drinking water samples were also measured. A theoretical assessment showed that fertilizer-derived U may cause high U concentrations in leaching waters (up to approx. 25 μg L−1), but normally contributes only a small amount (approx. 0–3 μg L−1). The arable soils investigated showed no significant U accumulation compared to the forest sites. The close positive correlation of AR with NaHCO3-extractable U (R = 0.7, p < 0.001) indicates that application of fertilizer can increase the extractable U pool. The lack of depth gradients in the soil U concentrations (1.5–2.7 mg kg−1) and AR (0.90–1.06) ratios are inconsistent with the accumulation of U in the surface soil, and might indicate some leaching of fertilizer-derived U. The AR values in the water samples were close to 1, possibly suggesting an influence of fertilizer-derived U. However, based on findings from the literature and considering the heterogeneity of the catchment area, the agricultural practices, and the comparatively long distance to the groundwater, we conclude that fertilizer-derived U makes only a minor contribution to the elevated U concentrations in the water samples

Aluminum cycling in a tropical montane forest ecosystem in southern Ecuador

Growth limitation induced by Al toxicity is believed to commonly occur in tropical forests, although a direct proof is frequently lacking. To test for the general assumption of Al toxicity, Al, Ca, and Mg concentrations in precipitation, throughfall, stemflow, organic layer leachate, mineral soil solutions, stream water, and the leaves of 17 native tree species were analyzed. We calculated Al fluxes and modeled Al speciation in the litter leachate and mineral soil solutions. We assessed potential Al toxicity based on soil base saturation, Al concentrations, Ca:Al and Mg:Al molar ratios and Al speciation in soil solution as well as Al concentrations and Ca:Al and Mg:Al molar ratios in tree leaves. High Al fluxes in litterfall (8.77 ± 1.3 to 14.2 ± 1.9 kg ha− 1 yr− 1, mean ± SE) indicated a high Al circulation through the ecosystem. The fraction of exchangeable and potentially plant-available Al in mineral soils was high, being a likely reason for a low root length density in the mineral soil. However, Al concentrations in all solutions were consistently below critical values and Ca:Al molar and the Ca2 +:Alinorganic molar ratios in the organic layer leachate and soil solutions were above 1, the suggested threshold for Al toxicity. Except for two Al-accumulating and one non-accumulating tree species, the Ca:Al molar ratios in tree leaves were above the Al toxicity threshold of 12.5. Our results demonstrate high Al cycling through the vegetation partly because of the presence of some Al accumulator plants. However, there was little indication of an Al toxicity risk in soil and of acute Al toxicity in plants likely reflecting that tree species are well adapted to the environmental conditions at our study site and thus hardly prone to Al toxicity

Aluminum toxicity to tropical montane forest tree seedlings in southern Ecuador: response of biomass and plant morphology to elevated Al concentrations

In acid tropical forest soils (pH < 5.5) increased mobility of aluminum might limit aboveground productivity. Therefore, we evaluated Al phytotoxicity of three native tree species of tropical montane forests in southern Ecuador. An hydroponic dose-response experiment was conducted. Seedlings of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson were treated with 0, 300, 600, 1200, and 2400 mu M Al and an organic layer leachate. Dose-response curves were generated for root and shoot morphologic properties to determine effective concentrations (EC). Shoot biomass and healthy leaf area decreased by 44 % to 83 % at 2400 mu M Al, root biomass did not respond (C. odorata), declined by 51 % (H. americanus), or was stimulated at low Al concentrations of 300 mu M (T. chrysantha). EC10 (i.e. reduction by 10 %) values of Al for total biomass were 315 mu M (C. odorata), 219 mu M (H. americanus), and 368 mu M (T. chrysantha). Helicarpus americanus, a fast growing pioneer tree species, was most sensitive to Al toxicity. Negative effects were strongest if plants grew in organic layer leachate, indicating limitation of plant growth by nutrient scarcity rather than Al toxicity. Al toxicity occurred at Al concentrations far above those in native organic layer leachate