Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters
Introduction
Organic matter (OM) accumulation is a typical feature in both natural and constructed wetlands (Albuquerque and Mozeto, 1997, Tanner et al., 1998a). In constructed wetlands treating organic wastes (e.g. domestic sewage and farm dairy wastes), the rate of OM accumulation is the net balance between inputs from external (organic wastewaters) and in situ sources (plant and microbial detritus), and outputs from decomposition and wetland outflows (Reddy and D'Angelo, 1996, Tanner et al., 1998a).
The function of constructed wetlands in treating wastewaters (Burgoon et al., 1995, Tanner et al., 1998a) is largely dependent on OM accumulation and turn-over rate (decomposition and cycling). Organic matter accumulation provides long-term storage of carbon (C) and nutrients (e.g. nitrogen and phosphorus) and a sustainable supply of C for microbial denitrification (Mitsch and Gosselink, 1993, Kadlec and Knight, 1996, Reddy and D'Angelo, 1996). However, the accumulated OM potentially contributes to the clogging of pore space in subsurface-flow constructed wetlands (particularly at the wastewater inlet zone) and may lead to a decline in wastewater retention time, reducing the efficiency of N and P removal from wastewaters (Burgoon et al., 1995, Tanner et al., 1998b).
The turn-over rate of OM and hence the overall functioning of wetlands in treating wastewaters is probably dependent on the size of microbial biomass, microbial activity and the decomposition potential of plant litter and applied organic wastewater (Mitsch and Gosselink, 1993, Reddy and D'Angelo, 1994, Reddy and D'Angelo, 1996). Indices based on microbial activity (e.g. ratios of microbial biomass C to soil total C) and labile soil OM fractions (e.g. proportions of OM as a water-extractable C fraction) have been suggested as sensitive indicators of changes in soil OM composition and may be used to evaluate the efficiency of constructed wetlands in processing and accumulating OM (Murata et al., 1995, Reddy and D'Angelo, 1996).
Although quantitative assessment of OM accumulation in constructed wetlands treating organic wastewaters has been investigated (Tanner et al., 1998a), there appears to be no published information on the qualitative nature of the accumulated OM fractions (Kadlec and Knight, 1996). Since the long-term sustainability of constructed wetlands in treating wastewaters is greatly dependent on OM turn-over, it is important to investigate OM composition and soil biomass activity in wetlands, particularly those receiving high internal and external OM loadings.
The main objectives of this study were:
- 1.
to characterise both labile and stable OM fractions in surface deposits and gravel substratum of a gravel-bed wetland, where significant OM accumulation (14.9 kg m−2 over a 5-year operation) has resulted in pore clogging and short-circuiting of wastewaters (Tanner et al., 1998a), and
- 2.
to relate wetland OM fractions to plant chemical composition, soil microbial biomass and soil microbial activity at different depths of the studied gravel-bed wetland.
Information obtained could help to explain OM decomposition-accumulation processes and clogging problems in a gravel-bed wetland.
Section snippets
Study site
Organic deposits (sludge-like deposits) were collected from above and within a gravel substratum (hereafter referred as surface deposits and gravel substratum, respectively) of one of the four pilot scale subsurface flow constructed wetlands which had been in operation for 5 years treating farm dairy wastewaters (pre-treated in two-stage anaerobic and facultative waste stabilisation ponds). Details of the design and operation of these wetlands and wastewater characteristics are given in Tanner
General trends of results obtained
Total C, TN, OM fractions (except humin C and humin N), BC, BN, and microbial respiration rate significantly decreased with depth (P≤0.0001) and with distance (at least at P≤0.05) along the wetland channel (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10).
Humin N showed no significant decline with distance along the wetland channel, while humin C in the top 100 mm of the gravel-bed significantly (P≤0.05) increased with wetland gradient (Fig. 9, Fig. 10).
Highly
Organic matter content, microbial biomass and microbial respiration
The 2-fold higher accumulation of TC and TN in surface deposits and the top 100 mm of the gravel-bed compared with the lower gravel substratum is attributed to the return of above-ground plant residues on the wetland surface and the regular input of dairy wastewater.
The death of plant roots and rhizomes, exudation of organic compounds from roots and high microbial biomass and activity (Fig. 3A,C) also probably contributed to a higher accumulated OM in the top 100 mm of the gravel-bed than in
Conclusions
Most (>90%) of the OM accumulated in a gravel-bed constructed wetland receiving farm dairy wastewater over a 5-year period was composed of stable (recalcitrant) OM fractions, while water-soluble C fractions accounted for <10% of the sediment OM. Humic acid, fulvic acid and humin were the predominant stable OM fractions, accounting for 63–96% of the OM in surface deposits and the gravel substratum, probably due to the recalcitrant nature of the OM inputs from wetland plant litter and the applied
Acknowledgements
Financial funding for this study was provided by the New Zealand Foundation for Research, Science and Technology. The author is grateful to Dr Chris Tanner for kindly providing his study site for this study and the wetland substratum samples used, Graham McBride for statistical advice, Stu Pickmere, Sharon Nguyen, and particularly James Sukias and Kerry Costley for excellent technical assistance. Valuable comments from Malcolm Downes, Graham McBride, Dr Chris Tanner and Prof. Kuan Goh on this
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