Crop residue from agricultural fields is often burned. This increases aerosols, impacts the environment and affects health. Moreover, it reduces soil health and productivity.
Crop residue can also be composted and used to improve soil fertility. But normally, it takes eight to twelve months to compost crop residue by conventional methods. The lignin and cellulose in the residue are difficult to decompose. They are broken down by thermophilic microbes that work best at more than 40 degrees celsius.
Can we leverage on thermophilic fungi to accelerate the process? What are the best microbes for the purpose?
A team of scientists from various ICAR institutions and the Global Centre for Environmental Remediation put their minds to the task. They collected fungal isolates from the Bhanpur dumping site at Bhopal. For years, the site, covering 36 acres, was used as landfill. The possibility of finding the right kind of organisms there is high. The team could isolate about a hundred microorganisms.
Based on their ability to break down lignin and cellulose and to perform well under higher temperatures, the team selected four fungi viz. Trichoderma viride, Rhizomucor pusillus, Aspergillus awamori and Aspergillus flavus. The scientists then tested these fungi on residue from locally-grown sorghum, soybean, maize, sugarcane, cotton, and pigeon pea crops. They also took sawdust since the lignin to cellulose ratio is higher than in agricultural residue.
The researchers set up two experimental conditions. One set contained two kilograms of sterilized crop residue and the usual cow dung slurry in plastic pots. Cow dung has bacteria that can break down cellulose. In the other set, they added the fungal consortia. Moisture content was maintained at 60% in both sets.
For the first week, when the microbes start breaking down the biomass, the temperature increases a little. At this time, the pots were maintained at 30-40 degrees centigrade. The temperature increased for about a month, decreased to 30-40 degrees in another month, cooled even further in the next month. And then the humification of compost started.
The scientists monitored the process over four months, sampling from the top, middle and bottom of the pots. They considered sixteen parameters of compost quality that are mentioned in scientific literature for measurement.
In the non-inoculated pots, cellulose breakdown started after the thermophilic stage, but in the inoculated ones, the process started much earlier. The cellulose remaining at the end of four months was much higher in non-inoculated pots. Cellulose from sorghum residue was the fastest to degrade. Compost made from cotton had the highest cellulose content.
While the bacterial population increased to maximum at the thermophilic stage, fungal populations increased in the first stage and decreased a little in the thermophilic stage to increase again later.
In the inoculated pots, nitrogen in nitrate form increased rapidly during decomposition. Ammoniacal nitrogen in the pots inoculated with the fungal species was half that in non-inoculated pots and well below safe limits by the end of the fourth month. The compost in non-inoculated pots did not reach maturity in four months.
Then they examined the sixteen parameters of compost quality that they were monitoring. Using Pearson’s correlation coefficients, the team found that the carbon to nitrogen ratio and the biodegradability index, calculated from water soluble carbohydrate and total organic carbon, can replace some of the other parameters. In fact, these parameters are adequate as measures of compost maturity.
Using principal component analysis, they reduced the number of parameters necessary to measure compost maturity and stability even further. The fluorescein diacetate assay which measures the breakdown of fluorescein by microbes is adequate to assess the compost’s stability, says Asha Sahu, ICAR-Indian Institute of Soil Science, Bhopal.
The finding that the consortium of four fungi can reduce composting time to four months is good news for farmers keen to adopt efficient bio-waste management and nutrient recycling. The development of indices to measure compost quality and stability will make life easier for researchers trying out new composting techniques.
J. Environ. Manage., 244: 144-153 (2019);
Central University of Kerala