Decoupling of unpolluted temperate forests from rock nutrient sources revealed by natural ⁸⁷ Sr/ ⁸⁶ Sr and ⁸⁴ Sr tracer addition

Abstract

An experimental tracer addition of 84 Sr to an unpolluted temperate forest site in southern Chile, as well as the natural variation of 87 Sr͞ 86 Sr within plants and soils, indicates that mechanisms in shallow soil organic horizons are of key importance for retaining and recycling atmospheric cation inputs at scales of decades or less. The dominant tree species Nothofagus nitida feeds nearly exclusively (>90%) on cations of atmospheric origin, despite strong variations in tree size and location in the forest landscape. Our results illustrate that (i) unpolluted temperate forests can become nutritionally decoupled from deeper weathering processes, virtually functioning as atmospherically fed ecosystems, and (ii) base cation turnover times are considerably more rapid than previously recognized in the plant available pool of soil. These results challenge the prevalent paradigm that plants largely feed on rockderived cations and have important implications for understanding sensitivity of forests to air pollution. , and Na ϩ ) in sensitive soils have raised concerns over whether acid rain (1-6), reductions in atmospheric base cations (7-8), and forest cutting (9) are degrading forest ecosystems in temperate regions of Europe and North America (10, 11). Studies thus far have focused on regions where biogeochemical cycles are already strongly disturbed by human activities (1-14). To understand natural biogeochemical cycles, we consider here temperate rainforests in southern Chile that have not been exposed to acid rain or other significant anthropogenic influences (15).Forests of the Cordillera Piuchué Ecosystem Study (CPES; 42°22Ј S, 74°03Ј W) are in contact with among the least polluted atmospheres in the world (15, 16). Precipitation chemistry is nearly exclusively dominated by dilute sea-salt aerosols delivered by persistent westerly winds from the southern Pacific Ocean (15). The broad-leaved CPES rainforests are diverse in tree species composition, have high basal areas (approximately 50-90 m 2 ͞ha) and canopy heights of Ϸ30 m, receive 500-700 cm annual precipitation [of which approximately 17-28% falls in summer (December-March n ϭ 3 yr)], and are underlain by thin unglaciated soils (Ͻ50-60 cm of mineral soil on top of saprolite) on Precambrian mica schist bedrock (15,17). Soils are acidic and contain a thick organic horizon (Ϸ0-25 cm) with high fine root biomass.To determine the source and cycling of plant-available base cation nutrients in CPES soils and plants, we conducted Sr isotopic studies. Although Sr 2ϩ is not a plant nutrient or important weathering product, it closely tracks Ca 2ϩ chemistry because of its similar charge and ionic radius (Fig. 1). It serves as a useful proxy for Ca 2ϩ (and to a lesser degree other base cations) for tracing weathering processes because it has several isotopes that can be used to determine the rock weathering versus atmospheric contributions to base cation pools within soil-plant systems (4, 12-14, 18, 19). We adopted two independent strategies in our Sr isotopic study...

Factors affecting arbuscular mycorrhizal fungi of Chilean temperate rainforests

“…The Coastal Mountains, which are considered a refugium for biodiversity (Armesto et al, 2010), have higher plant taxonomic and phylogenetic diversity than the Andes Mountains (Villagrán and Armesto, 2005). The bedrock of the Coastal Mountains is highly weathered (100 -120 kyrs stand age); thus, nutrient inputs from the atmosphere are important (Kennedy et al, 2002), particularly at south of 37°S latitude were the Pacific Ocean plays a relevant subsidiary function in the nutrient dynamics of these forests (Kennedy et al, 2002). Meanwhile, most of the Andes Mountains were covered by glaciers during the Last Glacial Maximum (LGM); thus, young volcanic soils (<5 kyrs stand age; Villagrán and Armesto, 2005) that are richer in nutrients are found in this mountain system.…”

Status of soil acidification in North America

“…Studies that have attempted to quantify Ca weathering rates in acidic forest soils in the eastern US generally have found that rates are low (TURNER et al 1990;JOHNSON, LINDBERG 1992;KENNEDY et al 2002;DIJKSTRA et al 2003) in comparison with typical combined rates of tree uptake and soil leaching . A recent study using strontium isotopes to investigate base cation nutrition in acid deposition impacted forests raises new concerns about the capacity of mineral weathering to replenish exchangeable base cations.…”

“…A recent study using strontium isotopes to investigate base cation nutrition in acid deposition impacted forests raises new concerns about the capacity of mineral weathering to replenish exchangeable base cations. Unpolluted temperate forests can become nutritionally decoupled from deeper weathering processes, virtually functioning as atmospherically fed ecosystems (KENNEDY et al 2002). KENNEDY et al (2002) showed that base cation turnover times are considerably more rapid than previously recognized in the plant available pool of soil.…”

“…Unpolluted temperate forests can become nutritionally decoupled from deeper weathering processes, virtually functioning as atmospherically fed ecosystems (KENNEDY et al 2002). KENNEDY et al (2002) showed that base cation turnover times are considerably more rapid than previously recognized in the plant available pool of soil. These results challenge the prevalent paradigm that plants largely feed on rock-derived base cations and further strengthen the hypothesis that mineral weathering will not be able to replenish cation pools depleted by atmospheric deposition and vegetative uptake.…”

Constraining soil mineral weathering 87Sr/86Sr for calcium apportionment studies of a deciduous forest growing on soils developed from granitoid igneous rocks

“…Trees in that downstream-downslope zone may therefore rely more heavily on soil mineral weathering because of faster weathering. As suggested by Kennedy et al (2002), species and rooting depth could be important factors in determining the contributions of atmospheric deposition to the Sr and Ca pool in trees.…”