Impact of premature harvest of Miscanthus x giganteus for biogas production on organic residues, microbial parameters and earthworm community in soil

- Autumn harvest of Miscanthus reduced recycling of organic residues compared to spring harvest.
- Soil microbial parameters were negatively influenced by autumn harvest of Miscanthus.
- Earthworm community tended to be reduced by earlier harvest of Miscanthus.

Using Miscanthus for anaerobic digestion is of increasing interest due to low management efforts combined with relatively high methane yields. For this purpose, regular harvest in late winter or spring has to be shifted to autumn (September to November). Miscanthus is characterised by intense recycling of organic substances, mainly from litterfall during winter, which promotes beneficial conditions for soil organisms. Very little is currently known about the potential effects of premature harvest dates on habitat conditions for soil organisms, particularly earthworms.Accordingly, the present study involved a field trial in two established Miscanthus stands of different age. Parts of both stands were harvested in autumn in two subsequent years. At the end of the experimental period, sampling of earthworms and soil was done to observe the effects of autumn harvest dates on the earthworm community. Additionally to determination of earthworm species, abundance, and biomass, soil microbial parameters and amounts of organic residues were determined.Our results showed a significant reduction in the recycling of organic substances associated with premature harvest dates. The soil microbial biomass quantity was also reduced. Initial trends towards a C-depletion of the highly C-enriched soils of Miscanthus stands could also be determined after 2 years. The earthworm community was dominated by one anecic species (Aporrectodea longa) and one endogeic species (Aporrectodea caliginosa) and their numbers and biomass were correspondingly reduced in the autumn harvest treatments. Generally, the adverse effects on soil microbial parameters, as well as earthworms, were within acceptable limits. Nonetheless, we expect that the effects will become more significant with extended autumn harvesting. Therefore, we suggest yearly alternating harvest dates (autumn and spring) will maintain beneficial soil conditions, as well as economic efficiency.