Physico-chemical, biochemical and microbial properties of the rhizospheric soils of tree species used as supports for black pepper cultivation in the humid tropics

Abstract

The most popular live stakes used as supports in black pepper (Piper nigrum L.) plantations of the humid tropics are Ailanthus triphysa (Dennst.) Alston., Erythrina variegata L., Gliricidia sepium (Jacq.) Steud. and Garuga pinnata Roxb. Studies on soil properties in the rhizosphere of these tree species are limited. We report here information on soil physico-chemical, biochemical and microbial properties in the rhizosphere of these tropical trees. A non-living support (granite pole) was also included in the study as control. Among the tree species, greater levels of soil organic C were registered in the rhizosphere of G. sepium (26.5 g kg−1), while the lowest level was registered by A. triphysa (21.6 g kg−1). Greater levels of dissolved organic-C, -N, and mineral N also corresponded to the rhizospheres of G. pinnata and G. sepium, while the lowest level was registered by E. variegata and A. triphysa. Bray P level was markedly greater in the G. pinnata rhizosphere (8.4 mg kg−1) and least in the E. variegata (5.2 mg kg−1) rhizosphere. The levels of exchangeable-K, -Ca and -Mg in the tree rhizospheres were, however, not always higher than the control. Greater accumulation of microbial biomass-C (CMIC), -N (NMIC) and -P (PMIC) was observed in the tree rhizospheres compared to the control. Among the tree species, CMIC levels were greatest in the G. sepium and G. pinnata rhizospheres (474.0 and 454.0 μg g−1 respectively) and least in the E. variegata rhizosphere (415.0 μg g−1). However, NMIC and PMIC levels did not vary markedly among the tree rhizospheres. The CMIC:SOC ratio was, in general, greatest for A. triphysa rhizosphere (1.96%) and least for the control (1.37%). In contrast, CMIC:NMIC ratio was greatest for control (18.6) compared to the tree species (14.3–15.1). The CMIC:PMIC ratio was greatest for G. sepium and G. pinnata (34.8 and 32.0 respectively) and almost identical for A. triphysa, E. variegata and control (28.8, 29.6 and 28.4 respectively). Soil respiration rates did not vary significantly among the tree species (22.0–25.0 μg CO2–C g−1 day−1) and the lowest soil respiration (17.0 μg CO2–C g−1 day−1) was registered in the control. Contrarily, metabolic quotient (qCO2) was greatest for control (83.0 mg CO2–C (g biomass C)−1 day−1) and least for the tree species (51.0–59.0 mg CO2–C (g biomass C)−1 day−1). The tree rhizospheres also positively affected the activities of enzymes like dehydrogenase, urease, acid phosphatase, aryl sulphatase and β- glucosidase. Principal component analysis (PCA) reflected the strong relationship between microbial activity and the availability of labile and easily mineralisable organic matter, the logical dependence of microbial biomass on soil nutrients and a decrease in substrate use efficiency in soils with low organic substrates. The results imply that among the tree species studied, G. sepium and to some extent G. pinnata can be used as live supports for the restoration of degraded black pepper plantations and overall improvement in soil quality in the plains of the tropics.