Hydrolysable carbohydrate in tropical soils under adjacent forest and savanna vegetation in Lamto, Côte d'Ivoire

Abstract

Carbohydrates represent 5–25% of the organic matter in soils. They constrain microbial activities and mineral nutrient production in soil and also reflect the whole microorganism community dynamic. The objective of this study was to determine the contents and composition of hydrolysable carbohydrates in soils collected in a forest–savanna mosaic landscape in the region of Lamto (Côte d’Ivoire). Capillary gas chromatography was used to identify and determine carbohydrates in soil profile under 4 tropical ecosystems: gallery and plateau forests, and grass and shrub tree savannas. Forest soils were higher in organic matter than savanna soils (0.50–2.96% C v. 0.53–1.22% C). The carbohydrate-C content of soils, expressed as percentage of total soil organic C, was low, a likely consequence of the tropical climate that promotes a rapid decomposition of surface plant debris. The carbohydrate-C content was higher under savanna soils (5–7%) than under forest soils (3–4%). Glucose, ribose, mannose, xylose, and galactose were the 5 most abundant extractable monosaccharides in all soils. Between them, only xylose and ribose are controlled by the vegetation type. The [(galactose + mannose) : (arabinose + xylose)] and [mannose : xylose] ratios suggested that most soil sugars derive from microbial biomass. The large abundance of microbial carbohydrates indicates intense microbial activities in the soil, and then rapid decomposition of soil organic matter favoured by the long wet season, with high temperatures and soil water availability at the site study. Results suggest clearly that the climate likely controls the amount and composition of carbohydrates in Lamto soils.

Cropping System Effects on Soil Monosaccharides in Western Burkina Faso

“…The high values of R2 clearly show that the soil carbohydrates were derived largely from microbial activities both under fallow lands and cultivated soil. This assertion is consistent with that of Nacro et al (2005) who conducted a similar study under tropical forest and savannah conditions. Indeed, the rapid decomposition and turnover of organic matter which occur under tropical climate, resulted in the formation of microbial products (Larré -Larrouy et al, 2003;Nacro et al, 2005).…”

“…The results of this study are in agreement with Nacro et al (2005) that high rainfall in wetter regions could induce deep leaching of carbohydrates. That explains the lower carbohydrates contents recorded in the top soils from Congo, Côte d'Ivoire and Brazil, which are located in wetter ecologies.…”

“…The contribution of total carbohydrate to total SOC under fallow practice (5-15%) was nevertheless higher than the range of 5 -7% recorded under savannah soils from Côte d'Ivoire (Nacro et al, 2005) and 4 -10% in shrub savannah soils from Congo (Kouakoua et al, 1999), and forested soils from Brazil (8 %) (Aminiyan et al, 2015). The results of this study are in agreement with Nacro et al (2005) that high rainfall in wetter regions could induce deep leaching of carbohydrates.…”

“…Many authors reported similar results in in tropical and temperate soils conditions (Baldock et al 1987;Kouakoua et al 1999;Rumpel and Dignac, 2006). However, other authors suggested that the ratio R2 [mannose / xylose] would be a more accurate indicator of the origin of soil carbohydrates (Murayama, 1977;Nacro et al, 2005). They argued that substantial amount of galactose and xylose can be derived from soil microorganisms and plant materials can synthetize arabinose.…”

“…In this study, the highest ratio R1 was lower (< 2) than that of Oades (1984), due to the tropical climate that promotes a rapid turn-over of labile pools of soil organic matter (Nacro et al 2005;Ouattara et al, 2006). But this ratio is higher than 1 under fallow lands and lower than 0.5 in cultivated soil.…”