Fine Root Biomass and Soil N and P in North Queensland Rain Forests1

Abstract

[Extract] The development of an extensive surface root mat is one of the major mechanisms that enhances nutrient conservation in tropical rain forest, particularly in ecosystems growing on nutrient-poor, heavily leached soils (Jordan & Herrera 1981, Cuevas & Medina 1988). In these root mats, it is the fine unsuberized roots that are primarily responsible for the retention of nutrients (Edwards & Grubb 1982, Gower 1987, Berish & Ewe1 1988). The level of development of these fine root mats has been found to be very variable within the tropics, with the biomass of roots <5-mm diameter in the top 40 cm of soil ranging from 1.1 t/ha to 123.4 t/ha (Sanford 1985, Gower 1987, Cavelier 1992). According to the source-sink theory of resource allocation, based on theorem three of Bloom et al. (1985), trees growing on infertile sites should allocate a greater proportion of their resources into fine root production than those growing on fertile sites, as this “investment” in nutrient acquisition should increase growth and/or reproduction. Vitousek and Sanford (1 986) found that the available data suggested that the biomass of functionally active roots was substantially greater on infertile sites. Soil nitrogen has been suggested as the major factor governing belowground carbon allocation in temperate forest ecosystems (Grier et al. 1981, Aber et al. 1985, Nadelhoffer et al. 1985); however, there is conflicting evidence of the role nitrogen plays in influencing fine root biomass in tropical rain forest. A nutrient amendment study in a tropical rain forest in Hawaii found that nitrogen addition resulted in a significant reduction in fine root biomass (Gower & Vitousek 1989). In contrast, the availability of nitrogen was not a significant factor influencing fine root biomass at two sites in Costa Rica (Gower 1987). At these sites, the availabilities of phosphorus and calcium were suggested to be the major factors influencing fine root biomass (Gower 1987). The aims of this study were to document the variation in fine root biomass in soils of differing fertility and to investigate the relationship between fine root biomass and soil nitrogen and available phosphorus in north Queensland tropical rain forests.