A TECHNIQUE FOR STUDYING NUTRIENT DYNAMICS IN THE SOIL-ROOT INTERFACE OF FOREST TREE SPECIES

Abstract

Numerous development programmes are in operation for safeguarding theexisting natural forests and replanting of man-made forest plantations in thecountry. In this context, detailed research studies on forest plant species and'oils play a dominant role. However little or no published information isivailable on the studies of nutrient needs and their behavior in Sri Lankanforest soils. Unlike natural forest eco-systems, commercial felling of treespecies in man-made forests could remove considerable amount of nutrientsfrom the soil and if logging is continued without replenishing the depletednutrients to the soil, it may cause severe consequences on the growth of forestspecies in the future.

The knowledge on nutrient dynamics in the soil-root interface is important inunderstanding and estimating nutrient demands of plant species because theconditions at the soil-root interface (rhizosphere) are considerably differentfrom, and influence plant growth more, than those at a distance from theroots. For this reason, many researchers have been interested in studying thecharacteristics of the rhizosphere, relative to those of the bulk soil. Therhizosphere is a narrow soil cylinder (about 0-2 mm radius) surrounding theroot and therefore, it is technically difficult to study the root inducedchemical changes in this zone. One problem is the small amount ofrhizosphere soil available for chemical analysis and another is thedetermination of the line of demarcation between rhizosphere and the bulksoil. Nevertheless much less is known about the rhizosphere process in treecrops especially in the field due to absences of a dependable method forsampling thin sections of rhizosphere soil.

The Rhizosphere Study Container (RSC) technique developed for annualcrops was modified to study the rhizosphere processes in Camellias and teaplants under glasshouse and field conditions. The modified RSC is a twocomponentdevice, made-up of two (PYC) cylinders, the upper compartmenthaving an internal diameter of 82 mm and 25 mm depth and the lowercompartment having an internal diameter of 74 mm and 50 mm depth. Thetwo compartments were separated by a 24 /lm pore diameter polyester mesh.Both compartments are filled with soil and plant roots were allowed to grow in the upper compartment and the roots striking the polyester mesh wereunable to penetrate the mesh and therefore grew horizontally along the meshforming a root mat. The soil below the mesh therefore represents therhizosphere and the zone of transition demarcating the bulk soil. Thinsections of rhizosphere soil at various distances from the mesh (rhizoplane)could be sliced using a piston microtome and chemically analyze todetermine root induced chemical changes. Studies showed markeddifferences in soil pH, Phosphorous depletion patterns and differences inphosphate rock dissolution around fine roots of Camellias and tea. Thispaper makes an attempt to emphasize the use of modified RSC technique instudying rhizosphere process in forest trees.