In organic matter (SOM) levels have also been positively

In the regions with low rainfall rate, Cl? expected to be accumulated in the soil due to high evaporation/rainfall ratio. In contrast, in the tropical regions, high rainfall often results in thorough leaching of Cl? from soils and may cause deficiency of Cl? for plants. Luckily, the internal requirement of Cl? for most plants is small. For example, the Cl requirement for optimal plant growth is in the range 0.2?0.4 g kg?1 dry matter, but the average contents of Cl in plants were found in the range 2?20 g kg?1 dry matter (Marschner and Rimmington, 1988), which is about 10-50 times more than the prerequisite. Cl seems to be in excess of biochemical needs in most locations, for example (Redon et al., 2011) found total chlorine (TX) in forest soils from France in the range (0.45 ?1.0 g kg?1). Therefore, the available Cl for the plants could be more than they need via either selective uptake (active) or water uptake (passive). From the plants, Cl returns to soils, where extensive chlorination of soil organic matter takes place (Gustavsson et al., 2012) and the processes retaining Cl in the soil resulting in accumulation, large soil chlorine storage, allowing extensive Cl recycling by plant transport. The soils for which this have been observed are northern soils rich in organic matter (Lovett et al. 2005; Johansson et al. 2003; Bastviken et al. 2006; Clarke et al. 2009; Svensson et al. 2012). Moreover, the soil organic matter (SOM) levels have also been positively correlated with chlorination and soil Cl storage (Gustavsson et al., 2012). On the other hand many highly productive environments are found far from the coast (McGroddy and Silver, 2011) with little mineral Cl in soils, e.g., with little input from the ultimate Cl sources, and also lower OM in the soils (Batjes, 2002; De Moraes et al., 1996) which could assume low potential for Cl accumulation in the soils, examples of such environments are inland tropical rainforests. In spite of the very different settings from the Northern ecosystems with high soil organic matter (SOM), the few available data from tropical inland forests in Amazon indicate extensive plant uptake and release as throughfall (Cornu et al., 1998), i.e., similar extensive Cl cycling as in the Northern forests, but the nature of the Cl cycling and especially the potential for Cl storage in such soils are unclear.


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