Influence of different tree species on the chemical properties in rhizosphere and bulk soils.
Pura Marcet Sr., J. Carlos Souto, Saleta Gonzalez, and Dolores Baamonde. Universidad de Vigo. Escuela de Ingeniería Técnica Forestal., Escuela de Ingeniería Técnica Forestal. Campus a Xunqueira s/n, 36005 Pontevedra - SPAIN, Pontevedra, 36005, Spain
The rhizosphere can be functionally defined as a highly dynamic, solar/plant-driven micro-environment that is characterized by feedback loops of interactions between root processes, soil characteristics, and the dynamics of the associated microbial population (Wenzel et al 1999) Soil characteristics such as pH and redox potential, nutrient status, the presence of contaminants, and physical properties all influence root growth and the dynamics of microbial populations. On the other hand, soil characteristics are modified by plants by means of root exudation, uptake of nutrients and contaminants and root growth. In conclusion, the rhizosphere shows a very dynamic microenvironment governed by interactive reaction between its major components: soil, plant, and microorganisms (Lombi et al. 2001). Consequently, the objective of this study were to investigate selected chemical properties and availability of metals in rhizosphere and bulk soils, under different tree species. Soil samples were taken under Acacia dealbata (Link) , Quercus robur (L) and Eucaliptus globulus (Labrill) , of similar ages (five years) growing in acid forest soils, derived from granite. These tree species are characteristics of the forest of Galicia (NW Spain). The main environmental factors (e.g. climate, parent material, etc) were similar at each sampling site. At each of the sampling site, three trees, of similar specie, were carefully uprooted. An initial separation between rhizosphere and bulk soil was performed at the sampling site. The roots sampled were hand shaken and the soil adhering to the roots was considered as rhizosphere material. The soil falling from the roots and the remainder of the soil collected were regarded as bulk soil (Rollwagen and Zasoski, 1988). The pH of the water extract (soil to solution=1:2,5) was measured. Concentration of organic carbon and nitrogen of the soil were determined by a CHN analyzer. The NO·-3 ,NH+4 and PO43- bioavailable content was determinated with a segmented-flow auto analyzer. The bioavailable elements analysed were Al, B, Cu, Fe, K, Mn, Na and Zn, in CaCl2 and H2O extracts, the analysis was carried out by inductively coupled plasma spectrometry. These elements are of interest both in term of plant nutrition and from an ecotoxicological perspective (McBride, 1994). All statistical analyses were carried on the software SPSS for Windows. The results indicate that the pH in Acacia, Quercus and Eucaliptus rhizosphere soils are lower than in bulk soil. There are several sources of H+ that contribute to the lowering of soil pH, perhaps the most important source in soils results from the resiration of plant roots and soil microorganisms. Much stronger organic acids are produced as by products of the microbial decomposition of plant tissues. The levels of organic matter, total nitrogen and available NH+4 in rhizosphere soils are significantly greater than the bulk soil. The value of NO·-3 content in Quercus and Eucaliptus rhizosphere soils are lower than bulk soils. In most studies, the rhizosphere appears to be impoverished in metals (Lorenz etal 1997) and ther studies suggest that the rhizosphere is enriched in metals (Wang et al 2002). Hinsinger (1998) explained the depletion or the enrichment of the rhizosphere by the capacity of the soil to replenish the soluble or exchangeable forms of metals. In this study the availability of Al, B, Cu, Fe, K, Mn, Na and Zn was higher in rhizosphere soil than the bulk soil in all cases. And water extractable Mg and Ca concentration under Eucaliptus and Al concentration under Acacia,were higher in bulk soil. The effects of tree species on the rhizosphere chemistry are not very pronounced. However, the results suggest that the rizosphere soil under Acacia present highest content of organic matter and nitrogen total, and under Quercus highest NH+4 and NO·+-3 contents. In conclusion, the rhizosphere is the most acidic soil component, has a higher content in organic matter and nitrogen total, as well as a greater ammonium content and availability of Al, B, Cu, Fe, K, Mn, Na and Zn. These results indicate that the rhizosphere possesses different properties compared to the bulk soils.