Evaluation of the mobility and discrimination of Ca, Sr and Ba in forest ecosystems: consequence on the use of alkaline-earth element ratios as tracers of Ca

Abstract

A comprehensive understanding of Ca cycling in an ecosystem is desirable because of the role of this element in tree mineral nutrition and its status as a major base cation on the soil exchange complex. The determination of the origin of Ca in forests is particularly indicated in regard of important changes linked to acid inputs and intensive logging. Natural strontium isotopes are increasingly used as tracers of Ca in forest ecosystems for qualitative and quantitative assessments. Nevertheless this method is limited to relatively simple systems with two sources of nutrients. Some recent studies coupled Sr/Ca or Sr/Ba ratios to Sr isotopic measurements in order to solve more complex systems. Such method has however associated with it some uncertainties: this approach assumed that Ca, Sr and Ba behave similarly throughout the ecosystem and does not take into account the Ca biopurification processes occurring in some tree's organs which can alter element ratio. The present work focuses on two deciduous species covering large areas in Europe: European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). In order to test the similarity of behaviour between Ca, Sr and Ba, their concentrations were measured extensively in the major compartments of two forest ecosystems. In parallel, the discrimination process inside tree organs was studied in 23 stands for beech and 10 stands for oak. We found that Sr and Ca behave similarly in all soil and tree compartments. By contrast, Ba and Ca appear to have contrasting behaviours, especially in streams, soil solution and soil exchange complex (no correlations between element concentrations). Sr/Ba and Ba/Ca ratios must therefore be used with care as tracer of Ca. The Ca biopurification is absent in roots and slight in bole wood but is large in bark, twigs and leaves. The discrimination factors (DF) between wood and leaves are characteristic of the two species studied and do not change significantly as a function of the soil Ca status (acidic or calcareous soils). Therefore, strontium-calcium DF can be used as a correction factor of the Sr/Ca ratio of leaves when this ratio is used in connection with Sr isotopic ratios. This correction allows to solve systems of tree nutrition with more than two sources of Ca.

Use of foliar Ca/Sr discrimination and 87Sr/86Sr ratios to determine soil Ca sources to sugar maple foliage in a northern hardwood forest

“…Until recently most studies assumed that the Ca/Sr ratio did not change appreciably upon plant uptake (Ba factor of 1.4) based on studies of nutrient uptake from solutions into crop plants (Runia 1987), but recently several studies have showed more widely variable Ca/Sr ratios across various tree tissues (Poszwa et al 2000;Watmough and Dillon 2003;Bullen and Bailey 2005;Drouet and Herbauts 2007). Studies in a wide range of locations have used 87 Sr/ 86 Sr ratios to determine Sr sources to foliage and wood and to then infer Ca sources based on the assumption that Ca/Sr discrimination was not large (Dambrine et al 1997;Poszwa et al 2002Poszwa et al , 2004Kennedy et al 2002;Blum et al 2002;Dijkstra et al 2003;Drouet et al 2005;Berger et al 2006).…”

Partitioning of the source of leaf calcium of American beech and sugar maple using leaf Ca/Sr ratios: a predominantly surficial but variable depth of Ca uptake

“…Large variation in leaf Ca/Sr ratios were observed between trees (results not shown) but leaf Ca/Sr ratios for the 3, 6 and 13 m heights within individual trees did not vary significantly ( Figure 2) with variation within a tree averaging less than +5%. The preferential uptake of Ca over Sr is therefore likely limited to the bole/canopy interface and independent of the length of the bole as suggested by Drouet and Herbauts (2008). With a discrimination factor of 1.78 ± 0.17 for beech as measured by Blum et al (2012), an overestimation or underestimation of a few percentage points would be of no consequence for the partitioning of leaf Ca/Sr.…”

“…Strontium closely matches Ca behavior in plant-soil systems because of its similar charge and ionic radius (Capo et al 1998). However, a preferential plant uptake and translocation of Ca over Sr is usually observed (Bailey et al 1996;Drouet and Herbauts 2008;Poszwa et al 2000).…”

“…Wood Ca/Sr along the bole of European beech (Fagus sylvatica L.) (Drouet and Herbauts 2008) and red pine (Picea rubens Sarg.) (Momoshima and Bondietti 1990) did not vary significantly, which suggests that the preferential uptake is at the soil/root and/or xylem/leaf interfaces.…”

“…(Momoshima and Bondietti 1990) did not vary significantly, which suggests that the preferential uptake is at the soil/root and/or xylem/leaf interfaces. Correction for the preferential uptake of Ca over Sr is therefore necessary to identify or quantify the sources of Ca (Drouet and Herbauts 2008;Poszwa et al 2000).…”