A technique for studying rhizosphere processes in tree crops : soil phosphorus depletion around camellia (Camellia japonica L.) roots

Abstract

Rhizosphere studies on tree crops have been hampered by the lack of a satisfactory method of sampling soils at various distances in the rhizosphere. 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 glasshouse and field. Plants belonging to the Camellia family (e.g. tea) have the ability to utilise P from relatively unavailable native P sources and for this reason camellia plants were selected for this study. In the glasshouse trial, the RSCs were filled with a Recent soil, treated with P fertilisers; North Carolina phosphate rock (NCPR), diammonium phosphate (DAP), mono calcium phosphate (MCP) and single superphosphate (SSP) at 200 μg P g-1 soil. A planar mat of roots was physically separated by a 24 μm 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, Po and inorganic, Pi) to understand P depletion at different distances from camellia roots. In the field trial this technique was modified and used to study the rhizosphere processes in mature camellia trees fertilised with only SSP and NCPR. In both field and glasshouse trials, all P fertilisers increased all the bulk soil P fractions except NaOH-Po over unfertilised soil with the greatest increases being in the H2SO4-Pi fraction in the NCPR treatment and NaOH-Pi in the SSP treatment. Resin-P, NaOH-Pi and H2SO4-Pi were significantly lower in the rhizosphere soil compared to the bulk soil whereas NaOH-Po was higher in the rhizosphere soil than in the bulk soil. Plant and microbial P uptake were thought to be the major causes for the low resin-P rather than P fixation by Fe and Al because the NaOH-Pi fraction which is a measure of Fe-P and Al-P, also decreased in the rhizosphere soil. The rhizo-deposition of NaOH-Po suggests that labile inorganic P was immobilized by rhizosphere microbes which were believed to have multiplied as a result of 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 the 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 NCPR treated soil. The increase in pH in the NCPR treatment over the control was consistent with the number of protons consumed during the dissolution of NCPR. In both trials, the dissolution of NCPR in the rhizosphere was higher than in the bulk soil due to lower pH and plant uptake of solution P in the rhizosphere. The RSC technique proved to be a viable aid to study the rhizosphere processes in tree crops.