Effects of tree species on root exudation and mineralization of organic acids in a tropical forest

Kazumichi Fujii,Chie Hayakawa,Tininsih Sukar

doi:10.5194/egusphere-egu21-2131

Abstract

&lt;p&gt;Tropical forests can develop by roots foraging nutrients in the highly weathered soils. In rhizosphere, soil volume affected by roots, tree species modify carbon (C) and nutrient cycles directly through root exudation and indirectly through increased microbial activity. We test whether root exudation and rhizosphere C fluxes of organic acids and sugars differ between dominant dipterocarp trees and pioneer trees (Macaranga gigantea). To quantify the C fluxes of organic acids in the rhizosphere soils, we measured in situ root exudation from mature trees, concentrations of monosaccharides and organic acids (acetate, oxalate, malate, and citrate) in the rhizosphere and bulk soil fractions, and mineralization kinetics of &lt;sup&gt;14&lt;/sup&gt;C-radiolabelled substrates. Organic acid exudation increases with increasing root surface area. Dipterocarp roots release greater amounts of malate, while monosaccharides are dominant exudates of pioneer trees. Microbial activities of malate mineralization increase in the rhizosphere soil both under dipterocarp and pioneer trees. The greater C fluxes of malate mineralization, compared to root exudation, suggests rhizosphere microbes are another malate producer under dipterocarp trees. Both root exudation composition and rhizosphere microbes increase malate production with increasing phosphorus demands and with increasing soil acidity.&lt;/p&gt;

Highlights

In the tropical forests, long-term weathering and acidification of soils generally result in a decrease in available phosphorus (P) (Walker and Syers, 1976)
Malate was a dominant organic acid exuded from Dipterocarp roots, while monosaccharides were dominant exudates of pioneer Macaranga trees
Organic acid exudation increased with increasing root surface area and with decreasing soil pH

Summary

Introduction

In the tropical forests, long-term weathering and acidification of soils generally result in a decrease in available phosphorus (P) (Walker and Syers, 1976). This is hypothesized to cause a decline in biomass productivity (Wardle et al, 2004), but high productivity could be maintained by plant species diversity and habitat differentiation (Fujii et al, 2018). Root exudation of labile substrates increases the biomass and/or activity of rhizosphere microorganisms and promotes nutrient cycles through priming effects (Dijkstra et al, 2013). There was a paradox that root exudation increases microbial activity to consume exuded LMWOAs in the rhizosphere (Fujii et al, 2013). Mineralization and sorption risk under/overestimating the efficacy of the exuded LMWOAs in rhizosphere processes (Jones et al, 2003)

Results

Discussion

Conclusion