Discussion

4.1. Process parameters

In general, maximal mass reduction is desirable during composting of waste products. However, the objective of these experiments was to produce a growing medium while mass reduction was not a goal. Mass losses during composting of different materials such as deep litter or household waste are found in a wide range from about 40 to 80% (Sommer and Dahl, 1999; Eklind and Kirchmann, 2000b). The mass losses in these experiments were found within the lower end of this range. Mass losses are most importantly dependent on composting time and type of material (Eklind and Kirchmann, 2000b). To obtain even lower mass losses more stable structural materials could be considered.

Total C losses resembled losses normally seen during decomposition of plant material (Bremer et al., 1991). It could have been expected that the mass and C losses would be lower in treatment 2 and 3 in experiment I with the delayed addition of nutrient rich material, since the decomposition was expected to proceed more slowly but steadily after the initial decomposition of the readily available carbohydrates. However, no decrease or even an increase of the mass and C loss was seen in treatment 2 and 3 compared to treatment 1. This could be explained by the higher microbial activity in treatment 2 and 3 when the supplementary clover-grass hay was added. The second peak of microbial activity in treatment 2 and 3 was detectable on the temperature curves, as temperature exceeded 70°C again when the extra clover-grass was added. Normally a small increase in temperature occurs when the compost is turned due to better aeration and humidity conditions. This was not observed in these experiments probably due to sufficient aeration in the compost as a result of the experimental set-up and the relatively small size of the composting boxes.

pH of both experiments tentatively followed the same pattern. During the first 3-4 weeks pH increased. This is normally observed during the initial part of the composting process when organic N is mineralised, as this is a proton assimilating process (Beck-Friis et al., 2003). The following decrease in pH coincided with NO3- production as the nitrification process releases protons.

4.2. Maturity parameters

During composting the C/N ratio decreased to around 10 in all three treatments of experiment I, which indicated the biological stability of the composts (Bernal et al., 1996). The C/N ratio can be used as a compost maturity parameter implying a stable organic matter content and absence of phytotoxic compounds (Bernal et al., 1998). Another maturity parameter is the ratio between NH4+-N and NO3- -N as decreasing amounts of NH4+-N

combined with increases in NÜ3"-N concentrations towards the end of composting suggests that intensive biological decomposition has been completed (Paré et al., 1998). This shift in inorganic N was seen in experiment I. In experiment II on the other hand the C/N ratio never declined to less than 20 and no NO3- was produced confirming that decomposition became N limited.

4.3. Nitrogen mineralisation

The mineralisation in treatment 1 in experiment I followed a pattern often observed during decomposition in compost (Eklind and Kirchmann, 2000b). Initially the NH4+ content increased due to the microbial decomposition of readily available N rich compounds. After the first three weeks the NH4+ content declined, most likely due to oxidation of NH4+ to NO3-. During the microbially very active initial phase NH4+ can be accumulated, which can result in an elevation of the pH. This combination of high pH, high NH4+ concentrations and high temperatures promote NH3 volatilisation and the highest ammonia losses occur during this phase (Witter and Lopez-Real, 1987; Martins and Dewes, 1992, Beck-Friis et al., 2003). Despite the presence of conditions permitting ammonia losses the total N losses were low, 429% of initial N content with no significant difference between the two experiments. Variation between replicates was high, which probably was due to uncertain determination of the dry matter content of the total amount of compost in the boxes. Compared to what is normally seen during composting these losses were relatively small. During composting of animal manure, household waste, and other waste products losses of at least 50% may be observed (Witter and Lopez-Real, 1988, Martins and Dewes, 1992; Eklind and Kirchmann, 2000b). The low N losses in these experiments could partly be explained by the experimental set-up as the compost was not rotated and aerated as much as compost in reactors (Eklind and Kirchmann, 2000a; Beck-Friis et al., 2003) or in heaps (Martins and Dewes, 1992; Sommer, 2001). Additionally, composting time was short compared to other experiments, although this might have a minor effect on the N losses, as the main part is lost during the first weeks of composting (Witter and Lopez-Real, 1987; Sommer, 2001).

Nitrate contents were not measurable until after the initial three weeks of composting. The nitrifying bacteria oxidizing NH4+ to NO3- show an optimal temperature of about 40°C (Prescott et al., 1990), thus the high temperatures during the initial phase probably inhibited nitrification as these bacteria were killed. Nitrifying bacteria probably survived in the peripheral zone of the composting boxes where the temperature was not as high as the centre of the compost pile. Nitrate content in the compost increased after the temperature decreased and the compost was turned, suggesting that nitrifying bacteria were mixed into the compost. In treatment 1 of experiment I the NO3- content continued to increase steadily throughout the remaining composting time.

Postponing the addition of some of the nutrient rich material altered the mineralisation patterns significantly in experiment I. During the first three weeks the NH4+ content was low as no net mineralisation occurred, probably because inorganic N was immobilised during the degradation of soluble and easily degradable carbohydrates. Recous et al. (1995) observed a decrease in the ratio of N immobilised to C mineralised with time confirming the initial high

N demand. When the supplemental clover-grass hay was added after three weeks an increase in NH4+ content was observed, indicating that the organic N from the clover-grass hay amendment was mineralised and N immobilisation was lower than in treatment 1. These results support the hypothesis that a limited amount of N is needed initially in the decomposition of the readily available carbohydrates of the straw material (Bremer et al., 1991). Usually bacteria degrade the soluble compounds during the initial phases of decomposition whereas 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. The same phenomenon occurs during the initial phases of composting (Eiland et al., 2001) confirming the fungal dominance in degrading recalcitrant compounds. When the additional N was added 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. The NO3-content remained, however, low for three weeks more than in treatment 1. This can be explained by the lower NH4+ production during the initial three weeks compared to treatment 1 but also by the fact that the extra addition increased temperatures to above 70°C again and thereby postponed the growth of nitrifying bacteria. The steep increase in NO3- after six weeks resulted in the NO3- content being twice as high as in treatment 1 at the end of the composting process.

When used as a growing media for lettuce (Lactuca sativa L.) root growth was inhibited during the first few weeks in the treatments with postponed nutrient application in experiment I probably due to the high NH4+ content (unpublished results). Due to these results the initial N level was reduced in experiment II resulting in a N level, which probably was so low that almost no net mineralisation occurred. Only a small net production of NH4+ occurred during the first three weeks in treatment 1 where after immobilisation was detected (Fig. 5a and b). During the first three weeks treatment 1 of experiment II was comparable to treatment 2 of experiment I, having similar initial C/N ratios. Hence, presumably sufficient N was available for the initial bacterial decomposition of soluble compounds in treatment 1 of experiment II. The increase in temperature as well as the C and weight losses indicated considerable microbial activity. During decomposition of plant material in soil Recous et al. (1995) observed that if mineral N was not available the C decomposition decreased but it did not stop. Despite the low N content in treatment 1 of experiment II the C mineralisation proceeded throughout the experimental period, suggesting that the initial low N content could have altered the microbial succession. Decomposition might have been dominated by fungi which may have the ability of effective remobilisation and transfer of N to actively growing parts (Cowling and Merrill, 1966). As no supplemental clover-grass hay was added in treatment 1 of experiment II no considerable net N mineralisation was observed after the initial three weeks.

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Organic Gardeners Composting

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