Long-term stability and mineralisation rate of compost is influenced by timing of nutrient application during composting of plant residues

Summary

The long-term nitrogen mineralization and stability of compost based on easily available plant residues was examined. Four different composts were prepared from wheat straw and clover-grass hay, one treatment receiving all material initially (100/0), two treatments where addition of 75% of the clover-grass was postponed for 3 weeks (75/25,3w) respectively 8 weeks (75/25,8w) and one treatment receiving only 25% of the clover-grass added to the other treatments (25/0). Mineralization and stability of the compost was followed for 24 weeks in leaching tubes, leached regularly by 0.01 M CaCl2. Postponing the addition of the clover-grass hay for 3 weeks (75/25,3w) during composting had a significant effect on the N mineralization rate leading to more mineralised N during the leaching period. Postponing the addition for 8 weeks (75/25,8w) did not affect mineralization pattern. Most of the decomposition in this treatment occurred in the leaching tubes after the composting step and no temperature increase (thermophilic phase) was observed here. Thus, the decomposition was seemingly dominated by different microorganisms compared to the thermophilic phases during composting observed in i.e. 75/25,3w and 100/0. When only 25% of the clover-grass hay was added (25/0) decomposition was so N limited that almost no net mineralization occurred.
Postponing the addition for 3 weeks (75/25,3w) increased the stability of the compost during the initial 14 weeks of incubation probably because more slowly decomposable compounds were degraded; however, after 24 weeks of incubation losses were similar to losses in the treatment receiving all the material from the start. Thus, the postponed addition was shown to alter the mineralization pattern and stability of the compost in a long-term scale. This has implications for production of growing media: A disadvantage of the postponed addition is the higher mass loss during composting. However, the 75/25,3w treatment with the supplementary addition postponed for 3 weeks remained stable for a longer period than the other treatments and might be a functional improvement as growing medium for plants with a shorter growth period.