Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters.

Organic matter (OM) composition, microbial biomass and microbial activity in a planted (Schoenoplectus tabernaemontani), gravel-bed wetland receiving cumulative OM (determined as volatile suspended solids) loadings over 5 years from farm dairy wastewater (8.2 kg OM m−2) and in situ plant residues (8.4 kg OM m−2) were investigated. Organic deposits above and within the gravel stratum (0–100- and 100–400 mm depths) were collected from six sites (with three transverse points per site) along the wetland channel. They were sequentially extracted for labile and stable OM fractions and determined for total carbon (C), total nitrogen (N), biomass C, biomass N, and microbial activity (respiration rate). Over 90% of the OM accumulated in the wetland was present as stable OM fractions. Humic acid, fulvic acid and humin were the predominant stable OM fractions, accounting for ∼63–96% of total C in surface deposits and the gravel substratum. The predominance of stable OM fractions in the wetland was attributed to the refractory nature of OM inputs (lignocellulose and humic compounds) from wetland plant litter and the applied dairy wastewater. Clogging of the gravel pore spaces over a 5-year wetland operation was therefore a result of the accumulation of refractory organic solids, originating from plant litter and applied dairy wastewater. Mineral aluminium (Al) and iron (Fe) compounds did not play a major role in OM stabilisation and accumulation in the wetland since organic C associated with Al and Fe accounted for <2% of the total C in surface deposits and the gravel substratum. Humic compounds were at least 2-fold higher in surface deposits and the top 100 mm of the gravel-bed than the lower gravel substratum, suggesting that pore clogging by these compounds was more prominent in the top layer of the gravel-bed. Microbial respiration rate and microbial biomass were significantly (P≤0.001) correlated with sediment OM fractions, suggesting that these microbial parameters may be used to predict changes in the labile and stable fractions of OM accumulation in the wetland. Both quantitative assessment of OM content and qualitative determination of labile and stable OM fractions in surface deposits and the gravel substratum are equally important in the understanding of OM accumulation-decomposition and pore clogging in gravel-bed wetlands.

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