Rhizosphers Effect Of Soil Nutrient Availability In Navel Orange Orchards Of South Jiangxi

Soils provide necessary nutrients for crop growth, and soil nutrient availability affects the growth, fruit production and quality of navel oranges tree. Rhizosphere effect of soil nutrient availability is recognized as plant roots influence on nutrient through their secretion and litters, and their interactions with microorganisms, soil enzyme and theirs environment. Therefore, three navel orange orchards, which varied with stand age of 9,20 and 31 years old, but similar in fertilizatiing practices in south Jiangxi were slected to determine and analyze rhizosphere and non-rhizosphere soil pH, the soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) at 0-15 cm,15-30 cm,30-45 cm depths. Additionally, soil sucrase, urease, acid phosphatase activities and leaf nutrient concentrations were measured during four seasons across a whole growth year. The main conclusions drawn from study are as follows:(1) Roots biomass gradually increased with the increasing stand age. Roots biomass decreased with increasing soil depth in the 9 yr-old orchards, but was higher in 20 yr-old orchards than in 9 and 31 yr-old orchards. Meanwhile, the annual average roots biomass across three stand ages showed the thick roots (diameter>2 mm)> medium roots (diameter 0.5-2 mm)> fine roots (diameter<0.5 mm)> dead roots. However, dead roots rapidly increased in 31 yr-old orahards, compared to 9 and 20 yr-old orchards.(2) The pH in rhizosphere soil, SOC, TN and TP in rhizosphere and non-rhizosphere soils increases with increasing orchard age. Soil pH, TN and TP were higher in rhizosphere soils than in non-rhizosphere soils, and all these rhizosphere effects were positive. Soil nutrients showed significant vertically spatial variations under the navel orange orchards of different ages. The pH in rhizosphere soil, SOC and TN in rhizosphere and non-rhizosphere soils decreased with increasing soil depth, but the change tendency of TP with soil depth was different with SOC and TN. The rhizosphere effects of all soil variables mentioned above showed "V" shape in their vertical distributions.(3) The ratios of C/N, N/P and C/P in the rhizosphere and non-rhizosphere soils showed not significant difference under each navel orange orchards of different stand ages. The C/N, N/P and C/P ratios in the rhizosphere and non-rhizosphere soils were 7.9,2.5 and 19.5 respectively. We preliminary inferred that soil N availability were more limiting factors than P supply to plant growth. (4) Soil sucrase, urease and acid phosphatase activities declined with the increasing stand age. Three kinds of enzyme acticities were higher in rhizosphere soils than in non-rhizosphere soils, and all these rhizosphere effects (R/S> 1) were positive. Soil sucrase and urease activities decreased, while soil acid phosphatase activity slightly increased with increasing soil depth. The rhizosphere effect of the three enzymes had inconsistency change tendencies in soil vertically spatial vatiations.(5) SOC, TN, TP concentrations of navel orange leaves had no significant differences among orchards of different stand ages. While there were significantly seasonal variations in leaf OC, TN, TP. The C/N, N/P and C/P stoichiometric ratios in navel orange leaves were 16,17 and 273 respectively. Thus we preliminarily inferred that relatively high P accumulation could easily lead to the potential risks of the environmental pollution.In sum, SOC, TN, TP concentrations increased with different planting age under the navel orange orchards. However the activities of these three enzymes, which are closed related with soil C, N and P transformation, showed decreasing tendency. Theirs rhizosphere effect also performed a decreasing tendency, and the supply of C, N and P showed more and more imbalanced with orchard development. Our results showed that the soil nutrient transformation rate and soil nutrient availability capacity performed a decline tendency. From a long-time perspective, the present fertilization management mode would easily lead to the decline of soil quality and the potential risks of the environmental pollution. Therfore, soil nutrient supply would not meet the demand of navel orange tree's growth under the orchards of south Jiangxi.