A TECHNIQUE FOR STUDYING RHIZOSPHERE PROCESSES IN TREE CROPS - SOIL-PHOSPHORUS DEPLETION AROUND CAMELLIA (CAMELLIA-JAPONICA L) ROOTS

Rhizosphere studies on tree crops have been hampered by the lack of a satisfactory method of sampling soils at various distances in the rhiz osphere. A modified root study container (RSC) technique developed for annual crops, grasses and legumes was used to study the mechanisms by which camellia plants (Camellia japonica L.) utilise soil P in the gl asshouse and field. Plants belonging to the Camellia family (e.g. tea) have the ability to utilise P from relatively unavailable native P so urces and for this reason camellia plants were selected for this study . In the glasshouse trial, the RSCs were filled with a Recent soil, tr eated with P fertilisers; North Carolina phosphate rock (NCPR), diammo nium phosphate (DAP), mono calcium phosphate (MCP) and single superpho sphate (SSP) at 200 mu g P g(-1) soil. A planar mat of roots was physi cally separated by a 24 mu m polyester mesh and the soil on the other side of this mesh was cut into thin slices parallel to the rhizoplane and analysed for pH, and different forms of P (organic, P-o and inorga nic, P-i) to understand P depletion at different distances from camell ia roots. Tn the field trial this technique was modified and used to s tudy the rhizosphere processes in mature camellia trees fertilised wit h only SSP and NCPR. In both field and glasshouse trials, all P fertil isers increased all the bulk soil P fractions except NaOH-P-o over unf ertilised soil with the greatest increases being in the H2SO4-P-i frac tion in the NCPR treatment and NaOH-P-i in the SSP treatment. Resin-P, NaOH-P-i and H2SO4-P-i were significantly lower in the rhizosphere so il compared to the bulk soil whereas NaOH-P-o was higher in the rhizos phere soil than in the bulk soil. Plant and microbial P uptake were th ought to be the major causes for the law resin-P rather than P fixatio n by Fe and Al because the NaOH-P-i fraction which is a measure of Fe- P and Al-P, also decreased in the rhizosphere soil. The rhizo-depositi on of NaOH-P-o suggests that labile inorganic P was immobilized by rhi zosphere microbes which were believed to have multiplied as a result o f carbon exudates from the roots. A marked reduction in pH (about 0.2- 0.4 in the glasshouse and 0.2 in the field trial) was observed near th e rhizoplane compared to that in the bulk soil in all treatments. The pH near the rhizoplane as well as in the bulk soil was highest for NCP R treated soil. The increase in pH in the NCPR treatment over the cont rol was consistent with the number of protons consumed during the diss olution of NCPR. In both trials, the dissolution of NCPR in the rhizos phere was higher than in the bulk soil due to lower pH and plant uptak e of solution P in the rhizosphere. The RSC technique proved to be a v iable aid to study the rhizosphere processes in tree crops.