Composting and vermicomposting of biomass obtained from aquatic weeds are the best known approaches for biological stabilization of green waste by transforming them into a safer and more stabilized material – compost, which can be used as a soil conditioner in agricultural practices. Composting is defined as biological decomposition of organic matter to form stable humus like end product under controlled aerobic conditions. During composting, the organic components undergo several transformations producing various metabolites that may inhibit or stimulate the growth of the plants before they mature into compost, which is biologically safe and stable and contains newly formed organic and inorganic humic like substances. Quality of the compost depends on various factors such as the composting facility design, feedstock source and proportions, procedures used and length of maturation. Heavy metals do not degrade and tend to concentrate during composting due to microbial degradation of the organic matters and loss of carbon and water, posing a threat if the compost is to be applied in agricultural systems for growing food crops.
A recent study (Singh et al. 2014) evaluated the availability of heavy metals and nutrients during the agitated pile composting of phoomdi, one of the first reports of its kind from Loktak. Metal concentrations ranged from 13023-16563 (Fe), 667.5-852.5 (Pb), 587.6-692.4 (Mn), 187.3-230.8 (Zn), 143-222.2 (Ni), 117.3-232.3 (Cr), 59-68 (Cd), and 40-60.3 (Cu) mg kg-1. Concentrations increased during composting due to weight loss of the matter caused by decomposition of organic matter resulting in the release of CO2 and subsequent mineralization. Metal content of phoomdi compost followed the order Fe>Pb>Mn>Zn>Ni>Cr>Cd>Cu. Water soluble fraction of metals may be readily bioavailable due to decomposition of organic matter and formation of complex compounds with newly formed humic substances. It represents the most toxic fraction of the metals to plants. Water soluble metal fractions decreased in the range of 12.6-38.3% for Zn, 12.1-56% for Cu, 16.4-58.3% for Mn, 6.8-28.3% for Fe, 11.7-47.7% for Pb and 31.6-46.3% for Cr after composting. Concentrations of DTPA extractable metals ranged from 24.8-66.7% for Zn, 29-49% for Cu, 22.1-45.4% for Mn, 42.4-50% for Fe, 42.4-53% for Ni, 32.3-50.3% for Pb, 4.2-37% for Cd and 17-52.4% for Cr, respectively, during composting. Concentrations of leachable metal fractions ranged from 39-89 (Zn), 12-16 (Cu), 214.5-371 (Mn), 138-347.5 (Fe), 3.3-6 (Ni), 18.2-42.8 (Pb), 0.81-2.4(Cd) and 34.3-57.8 (Cr) mg kg-1.
Further, it was concluded that composting of phoomdi biomass followed by land application can be an economical option for the disposal of the available green biomass, as enormous growth of phoomdi has severe impact on the wetland ecosystem processes and functions. Transformation of phoomdi into compost may help protect the wetland and reduce the application of chemical fertilizers for agricultural practices. However, phoomdi is reported to act as a filter for the purification of the lake water and accumulate large amounts of hazardous wastes. From literature, it is observed that phoomdi species, viz. Alternanthera philoxeroides, Alisma plantago aquatica, Eichhornia crassipes, Nelumbo nucifera, Lemna sp., Polygonum sp., Spirodela polyrhiza, Salvinia cucullata, Pistia stratiotes, Ipomoea aquatica, Colocasia esculenta, Trapa natans, Zizania latifolia, Oenanthe javanica and Phragmites karka etc. are either accumulators or hyperaccumulators of various metals. For example, a recent report (Meitei and Prasad, 2016) showed high Fe accumulation in underground and aboveground parts of S. cucullata (9461.4 ± 1.09 and 6131.5 ± 1.21 mg kg-1 dw) followed by E. crassipes (2229.3 ± 0.63 and 1899.5 ± 0.72 mg kg-1 dw) > S. polyrhiza (2262.1 ± 0.89 and 1267.8 ± 0.48 mg kg-1 dw) > I. aquatica (2193.7 ± 0.60 and 1008.2 ± 0.26 mg kg-1 dw). Likewise, S. cucullata (3565.1 and 2383.4 mg kg-1 dw), E. crassipes (2673.3 and 1030.7 mg kg-1 dw) and A. philoxeroides (1858.2 and 1546.3 mg kg-1 dw) accumulates high amount of Mn in underground and aboveground parts from the water column of Loktak. Also, it is observed that the ability of phoomdi to concentrate metals increases in the compost; creating a possible threat or making the compost unfit for applications in agricultural soils without proper long term field studies on metal bioavailability.
Compost reported in the study crossed the threshold limits of various European, American and Australasian standards for metals allowed in compost for agricultural applications (WRAP, 2002). Cadmium, lead, nickel and chromium content were significantly higher in the compost due to weight loss in the course of composting. These results showed a possible risk to the health of humans and ecosystems because of metal leaching from phoomdi compost. Lead poisoning causes damage to the nervous system, reproductive system, kidney and brain. Similarly, cadmium toxicity leads to hypertension, lung damage and dysfunction of the kidney. Copper toxicity causes skin irritation, stomach disorder, kidney damage, anemia, hepatic damage, and gastrointestinal irritation. Manganese poisoning leads to neurological disorders and brain damage and zinc causes muscular stiffness, loss of appetite, nausea and irritation. Also, amount of metals in the phoomdi – compost will differ based on the species selected for composting, as phoomdi is composed of nearly 128 plants and different plants will have different accumulation capabilities for various metals.
In addition, phoomdi sequesters nearly 50% of the mineral nutrients present in the wetland system and helps in reducing their concentration within water, thereby suppressing algal growth. Conversion of phoomdi biomass into compost ignores the role of phoomdi as a biological sink and the negative consequences that may occur to the wetland ecosystem if harvesting is unsustainable. Phoomdi also play an important role in the socioeconomic and traditional life of the locals, with the presence of edible, medicinal, fodder, fuel, and house making materials, and plants useful in making handicrafts reported. During the process of composting, the edible plants such as O. javanica, Neptunia oleracea, Centella asiatica, and C. esculenta or medicinal plants, Hedychium coronarium, Cynodon dactylon, Argyreia nervosa, and Z. latifolia will be converted into compost. Further, Loktak represents the largest fishery resource of Manipur accounting for more than 50% of its fish production. More than 19,000 tons of fish was harvested form Loktak and adjoining lakes of Manipur during 2010-11, with 39% of the harvest from athaphoom fishing, contributing significantly to the socio economy (LDA, 2011). Keibul Lamjao National Park (KLNP), a unique floating wildlife reserve in the world is a continuous mass of floating phoomdi biomass of near to 40 sq. km. It is the only natural habitat of the endangered species of Elds deer, Rucervus eldi eldi McClelland, with a population of near to < 200 heads in 2000. Loktak with its several islands located inside the wetland and surrounded by floating phoomdi of different geometrical shapes makes it a unique destination for tourism.
Thus, the aspects mentioned should be considered before proposing the conversion of the floating phoomdi of Loktak into compost otherwise it may lead to disturbance of the socio economy and ecology of the wetland and surrounding environment. As there is an inherent risk of metal bioavailability to humans and other organisms if phoomdi – compost is applied to agricultural areas as an alternative for artificial fertilizers, long term field analysis is needed to investigate the bioavailability and leaching of metals into the environment. Therefore, application of phoomdi – compost and its use as a means to control phoomdi proliferation in Loktak is a promising hypothesis that needs further in-depth scientific study before use in the field for tomorrow but not quite an appropriate phoomdi managing plan for today.
The article was written by Dr. Maibam Dhanaraj Meitei, a resident of Kyamgei Maning Leikai, Imphal – 795003, Manipur