Management of the composting process

The initial content of mineralised N in compost used as growing media is important, as availability of nutrients is essential for seedling growth. Management of composting processes in order to control the mineralisation patterns can thus be of great importance.

Control of a composting process and the properties of the end product can be achieved by at least two different strategies. One strategy is to adjust process parameters, such as moisture level, temperature or oxygen content (Beck-Friis et al., 2001; Smars et al., 2002). Another is to alter the starting conditions by changing the composition or type of material used so that C/N ratio or fibre composition is changed (Eklind and Kirchmann, 2000a; Eklind and Kirchmann, 2000b; Eiland et al., 2001a). A third strategy is to influence the composting process by delaying the addition of part of the material as opposed to including all the material from the start (Dresboll and Thorup-Kristensen, submitted A; Dresboll et al., in prep). Thus, without changing material or composting parameters, the properties of the end product can be affected by postponing some of nutrient rich material, probably due to changes in the microbial communities.

The strategy to control the composting processes by postponing the addition of some of the nutrient rich material was based on the hypothesis that efficient compost with high content of available N could be prepared by splitting the addition of the nutrient rich material during the composting process (Dresboll and Thorup-Kristensen, submitted A). The first addition at the initiation of the composting process should supply sufficient N to support the microbial turnover of the readily available carbohydrates. The remaining nutrient rich material should be added later in the process when the turnover of the wheat straw would already be proceeding. Decomposition of the newly added material would then result in less N immobilisation compared to the compost produced by a single addition at the beginning of the process (Dresboll and Thorup-Kristensen, submitted A).

In materials with a high C/N ratio, such as wheat straw, nitrogen has often been recognised as a limiting factor for microbial growth and activity during the decomposition of plant material in soil (Recous et al., 1995). However, experiments on the effect of additional N supply to decomposing plant residues in soil showed different results, ranging from increases to decreases of the decomposition rate (Fog, 1988). When adding more N-rich material to compost based on clover-grass and wheat straw, the mineralisation rate was found to increase (Dresboll and Thorup-Kristensen, submitted A). The effect of added N on decomposition may depend on the plant material, as degradation is influenced by nutrient content and anatomical structure of the material. Parameters such as N source and the time scale of the decomposition process are also influencing the effect of added N. If decomposition is N limited, the effect of added N was shown to have an increasing effect on the decomposition rate although delayed (Dresboll et al., in prep).

There is a high N demand during the initial stages of decomposition of plant residues in soil when soluble and easily degradable C compounds are mineralised, while the N demand is lower when the more recalcitrant C compounds are decomposed (Recous et al., 1995). Since much C from plant residues such as straw materials is only slowly available to microorganisms, leading to low C use efficiency for growth, a limited amount of N may be required during decomposition, and recycling of N may then be adequate to meet the N requirements (Bremer et al., 1991). Microorganisms, especially fungi, have a considerable capacity to adapt to N deficient conditions. Consequently, an initially large amount of N could lead to gross immobilisation. This could be a result of synthesis of a microbial biomass with a lower C/N ratio such as a change from 'low-N-fungi' to 'high-N-bacteria' or of higher N losses (Bremer et al., 1991).

Dresboll and Thorup-Kristensen (submitted A) showed that when postponing the addition of some of the nutrient rich material, the mineralisation patterns were altered significantly (Fig. 4). These results supported the hypothesis that a limited amount of N is needed in the initial decomposition of the readily available carbohydrates of the straw material (Bremer et al., 1991). The soluble compounds during the initial phases of decomposition are generally degraded by bacteria, leading to increased incorporation of N into organic compounds, and thus increased immobilisation compared to conditions with low N availability, where fungi with a higher C/N ratio decompose more recalcitrant compounds (Recous et al., 1995; Klamer and Baath, 1998). The fungal/bacteria index increases during the decomposition of material with a high initial C/N ratio in soil. The same phenomenon occurs during the initial phases of composting (Eiland et al., 2001a) confirming the fungal dominance in degrading recalcitrant compounds. When the additional N was added in the study by Dresboll and Thorup-Kristensen (submitted A), the readily available carbohydrates were presumably already degraded, and less N demanding fungi dominated the decomposition. Thus, when the N was mineralised from the supplemental clover-grass hay it was not re-immobilised by the microbial population to the same degree as when all clover-grass hay was added initially. Therefore the delayed addition of clover-grass hay resulted in a higher total release of inorganic N during the experimental period (Dresboll and Thorup-Kristensen, submitted A).

The compost from the experiments by Dresboll and Thorup-Kristensen (submitted A) was used as a growing medium for greenhouse grown lettuce. This revealed that the initial N content of the compost with the postponed addition was too high for lettuce requirements and inhibited root growth. Thus, in the following composting experiment conducted, the initial C/N ratio was increased based on calculations of an expected more balanced nutrient release. Conversely, the changes resulted in a too low initial N input leading to inhibition of the mineralisation processes and almost no net mineralisation was observed during the composting period.

Composting Process

Time (weeks)

Figure 4. Total mineralised nitrogen in plant based compost with all material initially (full-mix), 1/3 of the nutrient rich matrial initially and 1/6 of the nutrient rich material initially. The remaining nutrient rich material was added after 3 weeks. (Dresboll and Thorup-Kristensen, submitted A).

Time (weeks)

Figure 4. Total mineralised nitrogen in plant based compost with all material initially (full-mix), 1/3 of the nutrient rich matrial initially and 1/6 of the nutrient rich material initially. The remaining nutrient rich material was added after 3 weeks. (Dresboll and Thorup-Kristensen, submitted A).

Hence, no effect of the postponed addition was observed in this experiment (Dresboll and Thorup-Kristensen, submitted A). The compost was subsequently used in a leaching tube experiment to follow the further mineralisation pattern for six month (Dresboll et al., in prep). Regardless of the lack of net mineralisation during the composting time, a significant higher mineralisation was seen in the treatment with postponed addition after 12 weeks of incubation in the leaching tubes (Dresboll et al., in prep). In conclusion, postponing the addition of some of the material, lead to differences in the mineralisation pattern, independent of the initial nitrogen input. This was probably due to changes in the microbial community structure during composting (Dresboll and Thorup-Kristensen, submitted A).

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