What microorganisms need to be able to do all the work?

3.5.1

Nutrients

Above we mentioned that most microorganisms need organic carbon compounds (C) but they cannot live only on carbon. Like every other living organism, they need to be supplied with macronutrients (such as N, P, K, Mg, S) and microelements.

Probably one of the most important elements that microorganisms need, besides carbon, is nitrogen (N). It is needed for the synthesis of proteins, amino acids and DNA. Microorganisms need significantly more N than plants, on a weight for weight basis. The average C:N ratio of bacteria is about 10, while in some plant tissues it can be as high as 1001_{. When N supply is limited, the composting process slows down. Therefore, in composting}

practice, it is very important to create a good feedstock mix with an optimal C:N ratio. It has been reported that the optimum C:N ratio for composting is varies between 25 and 3510_{. If starting material has a C:N ratio of less}

than 25 it can be degraded too quickly, hence producing too much heat causing ashing. N will be then lost to the air in the form of ammonia (NH₃) gas. If the C:N ratio is above 35, then degradation of organic matter might slow down significantly. If the value is higher, the rate of decomposition will be slow and the resulting compost may not be properly stabilised. High temperatures cannot be reached under these conditions and seeds, plant and human pathogens can survive. The C:N ratio decreases during the process itself because part of the carbon is lost as CO₂ gas from microbial respiration, while most of N is retained in the system5_.

The desired C:N ratio can be achieved by analysing the feedstocks and correcting the C or N contents. Green waste may typically have a C:N ratio of 45-50 and would need to be combined with a material with a high N content such as food waste or poultry litter in order to produce a satisfactory compost. The N content is measured as total Kjeldahl N while the C content can be estimated from the ash content11 _by:

It should be noted that while the C:N ratio is routinely used as an indicator of “compostability”, there are significant differences in the biodegradability of various organic materials that are unrelated to the C:N ratio10_.

As mentioned above, the C:N ratio falls during composting as CO₂ is formed, and a final value of around 15 is generally aimed for, as one of the indicators for compost maturity.

N contained in composting feedstocks is predominantly in organic form12_{. During the first steps of composting,}

decomposition of amino acids leads to the formation of ammonium (NH4). Due to its relatively low solubility,

ammonia gas (NH₃) may soon be lost to the air. The water solubility of NH₃ at regular pH levels is low. However, NH3 solubility increases however, when the pH is lowered to 6 so more N can be retained. To conserve N in the

system, a special group of bacteria, called nitrifying bacteria, mineralises NH₃ gas via nitrite (NO₂) to nitrate (NO3). These bacteria need oxygen to be able to complete this transformation. Unfortunately, these bacteria

are purely mesophilic so NH₃ is bound to be lost during the thermophilic stage. Nitrification starts once the composting process is generating less heat. Initially it is slow and it only reaches full capacity during the curing stage. NO₃ can later be used by plants as an N source. The ratio between NO₃-N and NH₄-N is often used as an indicator of compost maturity (see chapter 4). In mature compost the majority of the soluble inorganic nitrogen will be present in nitrate form, which cannot be further oxidised. The transformation routes of nitrogen during composting and compost storage are shown in Figure 3.3.

Figure 3.3 The nitrogen cycle during composting (modified after Maeda at al., 2011).

3.5.2

Oxygen

Composting would not take place without a sufficient supply of oxygen (O2). If insufficient oxygen is present,

then aerobic microorganisms are replaced by microorganisms which are able to survive without oxygen. These anaerobic microorganisms begin degrading organic matter in a process called fermentation. This type of transformation leads to incomplete oxidation of organic compounds to CO₂, water and heat. As a result, some undesired organic compounds such as volatile fatty acids are produced, causing bad odours and turning the compost phytotoxic. Good aeration is also necessary for the complete nitrification process (oxidation of NH3/ NH4 to NO3) mentioned in Chapter 3.4.1. Nitrite (NO2) from the first step of nitrification may accumulate without

sufficient oxygen6_{. Nitrite is toxic to many microorganisms and plants. NO}

2 may also accumulate during the

storage of insufficiently aerated compost. NO3 is then reduced via NO2 to nitrogen gas (N2) in the process called

denitrification carried out by anaerobic bacteria.

Although a compost pile is mostly an aerobic environment, zones with little or no oxygen can also exist5_.

3.5.3

Water

Microorganisms also need water (H₂O) in addition to nutrients and oxygen. They cannot function in dry environments. Fungi tend to be less sensitive to a lack of water. Bacteria however, need water to survive. They can only survive in water film on surfaces. Therefore, moisture content is a very important factor in composting. If the pile is too dry, composting will not occur. A moisture content of 30% is often proposed as a minimum and 45-55% as the optimum during the thermophilic phase. It is important, however, not to overwater the compost pile. A high moisture content makes diffusion of oxygen to deeper parts of the pile difficult. This results in oxygen depletion, and microorganisms begin to ferment the organic matter instead of composting. Thus, the key is to keep the composting pile moist but not too wet. Overwatering may also cause leaching of nutrients and environmental pollution. Optimum moisture content is also essential for proper cooling of the compost to prevent overheating and ashing of the organic matter.