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Aquatic macrophytes' decomposition is a source of recalcitrant carbon in the long term contributing to humic substances (HS) formation. Understanding the influence of plant detritus quality and oxygen availability over molecular changes of these compounds provides ecological information related to their cycling. This study described the molecular variation of dissolved HS from Eichhornia azurea, Egeria najas, Oxycaryum cubense and Salvinia molesta decomposition under aerobic and anaerobic conditions. The aquatic HS formed from the four aquatic macrophytes showed similar features (e.g. molecular weight and aromaticity). This fact indicates little influence of the detritus quality or availability of oxygen on the fulvic acids (FA) and humic acid characteristics. Under aerobic condition a decrease in the polysaccharides content in FA occurred. HS from E. najas were related to less-recalcitrant features, while HS from S. molesta were related to recalcitrant.

Characterization of humic substances from five macrophyte species decomposed under different nutrient conditionsMacrophyte detritus contribute up to 50 % of carbon available for humic substances (HS) synthesis in aquatic ecosystems which influence the carbon supply for many ecological processes. This study reports on a qualitative characterization of the HS from five aquatic macrophytes species with different traits (free-floating: Eichhornia crassipes, Pistia stratiotes, and Salvinia auriculata; emerged: Sagittaria montevidensis; amphibian: Urochloa arrecta) decomposed under oligotrophic and eutrophic conditions. To do this, we performed a decomposition experiment using the litter bag technique in two tropical reservoirs (Rio de Janeiro State, Brazil). The nutrient and fiber content in the plant tissues used in the decomposition assays were analyzed. From the remaining detritus in the litter bags, the HS were extracted using analytical procedures based on their solubility in different pH ranges. The HS ratios were calculated considering the ratio of non-humified and humified materials (Humification Index -HI), the percentage of humic acid (HA) and the relationship between humic acid and fulvic acid (Polymerization Degree -PD). The amphibian and emerged species showed the highest cell wall content and the lowest and the highest C:N:P ratios, respectively. Eutrophication increased the HI values of all plant detritus. The detritus from the amphibian macrophyte showed the highest HA proportion (oligotrophic: 25.2 %; eutrophic: 21 %) and E. crassipes detritus the lowest HA yields (oligotrophic: 13 %; eutrophic: 11.4 %). The PD values were directly related to the HA percentage and the detritus from E. crassipes and P. stratiotes showed the lowest values. The C:N and C:P ratios and the macrophyte life form had minor importance in the HS genesis. The fiber content in the plant tissues predicted the HA yields and therefore the PD that increased in the eutrophic reservoir. The non-humic material was the main fraction of detritus in both reservoirs suggesting carbon accumulation mainly in the eutrophic reservoir with high macrophyte biomass.