Litter decomposition study in young and in old forest by chemical and structural analyses

The decomposition of litter and subsequent release of nutrients in plant-available forms is an essential process in the functioning of forest ecosystems. The rate of litter decomposition is mainly affected by several variables such as climatic condition, biomass activity and quality of decomposing material ( Swift et al., 1979). Although many studies have been carried out to investigate the effects of litter decomposition using different vegetal species, it is still not well known the relative importance of the chemical parameters, among whose cellulose and lignin are considered significant indicators of litter decomposition. Hence, it is important to provide more information from a wide range of environments to better understand the litter decomposition process. We have supposed that the age of the forest can influence the litter decomposition and consequently its transformation in soil organic material. The aim of the present study was to combine chemical parameters, diffuse reflectance Fourier transform (DRIFT), CF-IRMS(d13C, d15N) and thermal (TG-DTA) analyses in order to investigate the variables characterizing the litter decomposition into different forest stands: an old (270-year old hardwood) and a young forest (20 yr old) by using the litter-bag technique. Plant species composition of both stands was a mixture of oak (Quercus robur), ash (Fraxinus angustifolia), poplar (Populus alba) and billow (Salix alba). Fresh leaf litter was collected from the forest floor. Five grams of air-dried leaf litter were weighed into each litter-bag. Eighteen litter-bags per site were anchored in the forest floor. Three litter-bags per site were collected periodically over the 8-months period. Content of C, N, natural isotopic ratio of d13C and d15N, cellulose, lignin, aliphatic component, biomass N were determined. In addition a structural investigation were carried out by DRIFT and thermal analyses on litter samples. DRIFT spectra of litter showed a significant changes of the relative intensity of some marker bands assigned to vibrations of lignin (1515 cm-1), cellulose (895 cm-1), hemicellulose (1730 cm-1, a band for xylan) in both forest stands. A further structural modification appeared in the region at around 2940-2830 cm-1 assigned to CH stretching in aliphatic components. The decomposition of aliphatic component was higher in old forest than in the young one. Moreover, the results of TG-DTA analysis indicated that the cellulose content (first exothermic peak) decreased about 18% in young forest while was about of 12% in old forest in according to chemical analysis. On the contrast the lignin content (second and third exothermic peaks) increased, particularly in old forest. Statistical treatment of investigated variables showed that the relationship between each chemical parameter on degradation time was described by a linear model (P < 0.05). In particular, the coefficient regression values of following variables: C, cellulose, d13C, lignin and biomass N content observed in young forest were greater (P<0.05) than those observed in old forest. The multiple linear regression indicated that there was a statistically significant (P<0.05) relationship between the variables chosen as dependent (Y) and the remaining taken as independent (X’s). For instance the biomass N, which is considered a important degradation parameter, was explained for 80% (P<0.05) by N content and d15N in old forest while in young forest it was explained for 75% (P<0.05) by d15N and aliphatic content. On the basis of these results, it appears that the litter decomposition is qualitatively different when comparing young and old forest; this suggests that the degradation process as whole is a complex process that exhibits several phase in subsequent times