The function of the superficial root mat in the biogeochemical cycles of nutrients in congolese eucalyptus plantations.
OBJECTIVE
The importance of superficial root mats inside the forest floor for the nutrition of Amazonian rain forests has been extensively investigated. The present study was aimed at assessing the function of a root mat adherent to decomposing organic material observed in Eucalyptus plantations.


METHODS
The development of the root mat was studied through micromorphological observations of thin litter sections, and the influence of soil microtopography and soil water repellency on root mat biomass was assessed in situ on an area of 5 m2. In addition, input-output budgets of nutrients within the forest floor were established from measurements of litterfall, dissolved nutrients in gravitational solutions, and forest floor nutrient contents.


RESULTS
The amounts of nutrients released during litter decay in this ecosystem during the period of study were, on average, 46, 3, 4, 19 and 17 kg ha-1 year-1 for N, P, K, Ca and Mg, respectively. The simultaneous measurements of the chemical composition of throughfall solutions and leachates beneath the forest floor showed a very quick uptake of nutrients by the root mat during the decomposition processes. Indeed, the solutions did not become noticeably enriched in nutrients during their passage through the holorganic layer, despite large amounts of elements being released during litter decay. The root mat biomass decreased significantly during the dry season, and a preferential development in microdepressions at the soil surface was observed. A strong water repellency observed in these depressions might enhance the ability of the roots to take up water and nutrients during the dry periods.


CONCLUSIONS
The root mat was active throughout the year to catch the flux of nutrients from the biodegradation of the forest floor, preventing the transfer of dissolved nutrients toward deeper soil horizons. This mechanism is involved in the successful adaptation of this Eucalyptus hybrid in areas covered by 'climacic' savannas in Congo.
Laclau
Jean-Paul
JP
CIRAD/UR2PI, Département Forêt, Programme Arbres et Plantations, TA 10/C, 34398 Montpellier cedex 5, France. laclau@cirad.fr
Toutain
François
F
M'bou
Armel Thongo
AT
Arnaud
Michel
M
Joffre
Richard
R
Ranger
Jacques
J
D001673:Biodegradation, Environmental; D018533:Biomass; D003223:Congo; D003247:Conservation of Natural Resources; D005052:Eucalyptus / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism*; D008954:Models, Biological; D018517:Plant Roots / Q000737:chemistry / Q000502:physiology*; D012621:Seasons; D012987:Soil / Q000032:analysis; D014197:Trees / Q000502:physiology*; D014867:Water / Q000737:chemistry
D012987:Soil; D014867:Water