Spatial and temporal variations of soil respiration in a Eucalyptus plantation in Congo
Introduction
The evaluation of biospheric fluxes and stocks of carbon is of major importance in the context of increasing CO2 concentration in the atmosphere and the related potential change in climate. Carbon sequestration in forested ecosystems often results from a small difference between photosynthetic carbon fixation (gross primary production) and ecosystem respiration (Granier et al., 2000, Valentini et al., 2000). Soil is the biggest carbon pool of the continental biosphere (Schimel, 1995) and requires a particular attention, especially for short rotation plantation because it is a major compartment for durable carbon sequestration, the aboveground biomass being frequently removed and transformed into wood products with short life-times. While conversion of forest to pasture (Fearnside and Barbosa, 1998) or forest to tree plantation (Smith et al., 2002) often led to a net loss of soil carbon, more variable results have been collected for afforestation depending on previous land used, climate, soil type and planted species (Paul et al., 2002). For example, growing an Eucalyptus plantation (E. camaldulensis) over tropical savanna on sandy Entisol in Brazil led to a 17% decrease in soil organic carbon after one cycle whereas there was a slight increase observed on loamy Oxysol (Zinn et al., 2002). The soil texture and the ability of clay minerals to protect organic matter from microbial mineralisation are thought to affect carbon dynamics in afforested areas (Paul et al., 2002).
Soil respiration is one of the main components of ecosystem respiration (Granier et al., 2000, Janssens et al., 2001), and small changes in soil respiration may strongly affect soil carbon sequestration (Raich and Schlesinger, 1992). Therefore, it is important to obtain good estimates of soil respiration and to understand environmental controls on the underlying processes. Soil respiration is the sum of an autotrophic component by roots and the associated rhizosphere and a heterotrophic component by soil micro-organisms that decompose the organic materials from both aboveground and belowground litter (Bowden et al., 1993, Boone et al., 1998, Epron et al., 1999b, Epron et al., 2001). Several factors may affect these two processes. Soil respiration exhibits a high spatial and temporal variability. Spatial heterogeneity of soil respiration has been related to either root biomass, microbial biomass, litter amount, soil organic carbon, soil nitrogen, cation exchange capacity, soil bulk density, soil porosity, soil pH, or site topography (Hanson et al., 1993, Fang et al., 1998, La Scala et al., 2000, Xu and Qi, 2001). Seasonal variations of soil respiration have often been associated with either changes in soil temperature (Anderson, 1973, Edwards, 1975, Ewel et al., 1987a; Fang et al., 1998, Longdoz et al., 2000) or changes in both soil temperature and soil water content (Garret and Cox, 1973, Hanson et al., 1993, Davidson et al., 1998, Epron et al., 1999a; Qi and Xu, 2001, Xu and Qi, 2001).
Up to now, only few studies have dealt with soil respiration in tropical plantations (Ewel et al., 1987a, Ewel et al., 1987b; Lamade et al., 1996, Fang et al., 1998) despite their relevance to the “Clean Development Mechanism”. Eucalyptus plantations account for 25% of tropical plantations and cover about 1.5 × 105 km2. In the past 25 years, 430 km2 of clonal Eucalyptus plantations have been established in the littoral savannas of Congo and intensively managed for pulpwood production. The present study was done in the framework of an integrated program on carbon fluxes and sequestration in perennial tropical plantations (ATP CIRAD). Specifically, our objective was first to quantify the annual soil carbon efflux in a 3-year-old Eucalyptus plantation in coastal Congo. We investigated the effects of seasonal changes in soil temperature and soil water content on soil respiration. We further analysed the spatial variation of soil respiration within the stand and we attempted to relate these variations to the plantation structure and to local soil characteristics.
Section snippets
Site description
The study site is located in the Eucalyptus plantation zone, which covers about 430 km2 along the Atlantic coast in the Pointe Noire region (Congo, 4° S, 12° E, 100 m elevation). The mean annual air humidity and air temperature are high (85% and 25 °C) with low seasonal variations (about 2% and 5 °C, respectively). Mean annual precipitation is 1200 mm with a dry season between May and September. The soil is an arenosol according to the F.A.O. classification. The pH of the topsoil (0–20 cm) is about
Temporal trend in soil respiration
Soil respiration exhibited pronounced seasonal variations with minimum values below 1.6 μmol m−2 s−1 at end of the dry season in September and a maximum value of 5.6 μmol m−2 s−1 after re-wetting in December (Fig. 1). This pattern clearly reflected those of soil water content, which decreased from 10.8% in January to 3.1% in September. Soil temperature decreased from March (30 °C around day 70) to September (25 °C around day 260). There was a rather poor correlation between soil respiration and soil
Discussion
Seasonal variation in soil respiration is thought to be largely explained by either soil temperature alone (Anderson, 1973, Edwards, 1975, Longdoz et al., 2000) or soil temperature and water content in sites exhibiting a dry season as in some temperate areas or under Mediterranean climate (Garret and Cox, 1973, Hanson et al., 1993, Keith et al., 1997, Davidson et al., 1998, Epron et al., 1999a; Qi and Xu, 2001, Rey et al., 2002). Soil temperature exerted a strong influence on soil respiration
Acknowledgements
This research activity was carried out in the framework of the CIRAD funded ‘ATP Carbon’ project. UR2PI and ECO-SA (Eucalyptus Du Congo, SA) have provided additional funding and research facilities.
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