Abstract
Recent publication attested that in the urban forest of Brazzaville, litter decomposition is faster with almost 45% of initial weight loss than in the dry season, where an average loss of 26% in initial litter weight is noted (Ifo et al., 2018). This study was carried out in the urban forest of Brazzaville to follow the decomposition of some organic compounds/secondary metabolites (reducing Sugars, total Flavonoid and Polyphenols) of the leaves litters of two tropical species Antiaris toxicaria Lesch and Millettia laurentii De Wild. Thin-layer chromatography and spectrophotometric assay of these metabolites were used on the samples of litters collected in the field on various dates of follow-up of the decomposition (0 d, 14 d, 28 d, 42 d, 56 d, 72 d and 84 d). The chromatographic profile of initial litters shows a series of spots on yellow florescence materializing presence of flavonoids, green fluorescence revealing the presence of the acids phenols derived from the cinnamic acid. But the chromatographic profile of the two litters in decomposition after two weeks remains without structural information, being able to characterize the decomposition of the chemical families highlighted in the initial litters. Also, the analysis of quantitative total reducing sugar in the initial litters, gives average concentrations of 64.4, 58.6, 57.5 g EG/kg Ms respectively for the litters of Millettia laurentii De Wild, Antiaris toxicaria Lesch and the mixed litters (Millettia laurentii and Antiaris toxicaria). Comparatively with the other types of litters, the initial average concentrations in phenolic compounds (polyphenols and flavonoids totals) were the highest for the litters of Antiaris toxicaria Lesch (27.3 g EAG/kg Ms and 13.07 g EC/kg Ms) (P = 0.001). The losses of organic chemical compounds are more significant during the first two weeks of experiment than after this period. Antiaris toxicaria Lesch loses on average 43.8 g EG/kg Ms of reducing Sugars, 12.21 g EC/kg Ms of totals flavonoids and 26.4 g EAG/kg Ms of total polyphenols, equivalent to 30% of loss of the initial weight. Average losses of 45.7 g EG/kg Ms were obtained for reducing sugars, 1.5 g EC/kg Ms for totals flavonoids and 8.72 g EAG/kg Ms for totally phenols in for the litters of Millettia laurentii, comparable to 24% in initial weight loss. This study showed on the one hand, the direct link between rainfall and litters decomposition and the losses in weight of the litters resulted in dissolution in the water of the studied compounds.
Highlights
Tropical forests play many functions, including attenuation of global warning effects through the sequestration of carbon in the atmosphere
This study was carried out in the urban forest of Brazzaville to follow the decomposition of some organic compounds/secondary metabolites of the leaves litters of two tropical species Antiaris toxicaria Lesch and Millettia laurentii De Wild
We noted that decomposition remains very high for the litter of Antiaris toxicaria (k' = 0.73) in comparison with the litter of Millettia laurentii (k' = 0.26) one month after the beginning of experimental design (Table 1). k' values were 3 times less important after three months of experimentation for Antiaris toxicaria litters, while k value was two times less important for the litters of Millettia laurentii
Summary
Tropical forests play many functions, including attenuation of global warning effects through the sequestration of carbon in the atmosphere They contain an important aboveground biomass stock of carbon (IPCC, 2006; Lewis et al, 2009) and are teeming with very large plant biodiversity (de Wasseige et al, 2015; Ifo et al, 2016). Apart from these large forest areas which extend over millions of square kilometers, there are in many places in the tropics, and within cities, urban forests. Urban areas are often hot spots for air pollution from the automotive industry (Garty et al, 1996)
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