Discussion Mei
Mineralisation
Postponing the addition of some of the nutrient rich material in the short-delay treatment during the composting process did not show the expected effect on the mineralisation rate as no net mineralisation was observed. Despite that, large differences in the mineralisation pattern were observed between the four treatments during the subsequent incubation. The short-delay treatment had a higher content of accumulated mineralised N, whereas postponing the addition for 8 weeks did not alter the mineralisation pattern, compared to the full-mix treatment receiving all the material from the start. However, even though net mineralisation patterns are comparable, this does not confirm that the processes behind are similar as the mineralisation and immobilisation processes take place simultaneously (Amlinger et al., 2003) and has not been examined here. Although delayed due to the low nutrient content, the mineralisation pattern in the short-delay treatment corresponded to the findings of Dresboll and Thorup-Kristensen (submitted), that postponed addition of some of the nutrient rich material can increase the mineralisation of nitrogen. This has been explained by the changes in microflora occurring when N accessibility is altered.
Despite the similarities in postponing the addition of some of the nutrient rich material in the short-delay and the long-delay treatments, the delay affected the two treatments very differently. In the short-delay treatment the extra addition resulted in an additional temperature increase to thermophilic conditions, whereas the addition in the long-delay treatment did not affect the temperature due to the setup, and thus the following degradation could not be ascribed to composting but to decomposition at a constant low temperature of 20°C. Composting is generally defined as an aerobic degradation of organic matter under development of heat. Temperature is a significant factor in determining the relative advantage of some population over another and is thus, the dominant physiochemical parameter controlling microbial activity during composting (Ishii and Takii, 2003). Hence, the main part of the decomposition in the long-delay treatment has been dominated by different microorganisms than in the short-delay treatment, where part of the decomposition was controlled by thermophilic microorganisms. This might be part of the explanation for the different patterns observed.
The C/N ratio decreased only to 13-15 during incubation, in the full-mix, short-delay and long-delay treatments, as N continuously was removed by leaching. If the leached accumulated N was included in the determination of final C/N ratio, estimates of a C/N ratio of 9-10 was obtained. During the incubation period, the most profound changes in C/N ratio (Fig. 1a) and mass (Fig. 1b) occurred in the long-delay treatment, as almost no decomposition had occurred before the incubation period. Thus, when clover-grass hay was added to the compost just before incubation in the tubes microbial activity increased and further decomposition occurred. Even in the treatments which received the nutrient rich material during the previous composting process high mass losses were observed. The losses during the leaching tube incubation confirmed that decomposition during the 8 weeks of composting was nutrient limited and the decomposition rate thus inhibited. However, some decomposition had occurred, confirming that despite the N deficiency during decomposition, C mineralisation occurred although the decomposition rate was much lower (Recous et al., 1995). The nitrogen limitation in the 25%-treatment was clear even after the incubation period, as weight losses during the period were low and the C/N ratio still above 30. During a composting process the highest mass losses occur during the initial mesophilic phase and the shift from the mesophilic to the thermophilic phase as these are the periods of highest microbial activity due to the availability of carbohydrates (Bernal et al., 1998; Klamer and Baath, 1998). Initially, easily available carbohydrates are decomposed followed by breakdown of proteins, lipids, and complex carbohydrates which is accelerated by the high temperatures of the thermophilic phase, hence leading to high mass losses. During composting and incubation the total mass losses were largest in the short-delay treatment which could be explained by the extra rise in temperature when the supplementary clover-grass hay was added reflecting the increased microbial activity. As the supplementary clover-grass hay was added right before initiation of the incubation period in the long-delay treatment no similar temperature increment was observed in this treatment due to the small amount of material in the tubes and no insulation.
Stability
Despite having received the same amount of material although at different times, the full-mix, the short-delay and the long-delay treatments differed in the way the material were lost during the incubation time. As the long-delay treatment received the clover-grass hay right before incubation, a higher loss was expected. During the previous composting process the compost of the 25% and the long-delay treatments only lost a small amount of the mass as degradation was nutrient limited. This tendency continued in the incubation experiment for the 25%-treatment whereas the long-delay treatment had high losses after the application of the clover-grass hay. Losses during the incubation period could be ascribed to CO2 release and inorganic N in the leachate. In addition some organic N could have been lost in the leachate, as small particles of organic matter.
The degree of volume loss in the full-mix, the short-delay and the long-delay treatments revealed visually what was measured in mass loss and initial C/N ratio that is the instability of the composts. Used as a growing medium for containerised plants this would not be satisfactory (Gruda and Schnitzler, 2004) as the growing medium would almost have disappeared during the production time. However, the presence of roots in the growing medium would have stabilised it somewhat more than the growing medium alone. In soil, roots and microorganisms compete for nutrients (Jingguo and Bakken, 1997), why the presence of roots could have inhibited further C mineralisation in the compost as plant roots would have inhibited microbial decomposition by competing for the available nutrients. In general, roots have the competitive advantage compared to microorganisms as roots grow and is distributed in the medium and thereby can exploit a larger volume for available N. Furthermore, the mere presence of roots would also stabilise the media and compaction would be reduced. Still, some volume loss is expected ranging from 15-50 % when using compost based growing media for plants with long production time, although compression of the growing medium in the containers before planting could reduce the volume losses (Jensen et al., 2001; Gruda and Schnitzler, 2004).
The highest volume and mass losses were found in the long-delay treatment followed by the full-mix and short-delay treatments. The 25%-treatment had significantly lower volume and mass losses. Interestingly, the losses of volume and the losses of mass differed considerably in all four treatments. In the long-delay treatment a volume loss of 75% corresponded to a mass loss of 45%, revealing the structural breakdown of the material. Although only half of the material was lost due to microbial respiration the structure of the material was altered so much during decomposition that compaction increased due to smaller particle sizes and thereby less air filled porosity. Such a volume loss caused by reduction in particle size and compression due to gravity results in a reduction of macropores together with an increase in micropores (Gruda and Schnitzler, 2004). This will lead to higher water retention but less aeration. As the air-filled porosity decrease with time and aeration properties cannot easily be changed during plant growth, high initial aeration levels are essential (Caron and Nkongolo, 1999).
Fibre content
No significant differences were found between the hot water solubles and ADF, cellulose and lignin content of the four treatments at the start of the incubation period indicating a similar degradation pattern independent of the clover-grass amendment. Lignin degradation did not occur during incubation with exception of the short-delay treatment where a decrease in lignin content was observed. This might be explained by the altered microflora caused by the 3 weeks delay in addition of the nutrient rich material allowing the presence of slow growing lignin degraders.
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