Methods 1
2.1. Experimental design and materials
Decomposition studies were conducted on air-dried straw of Mischanthus (Mischanthus oogiformis L.), hemp (Cannabis sativa L.) and wheat (Triticum aestivum L.). After harvest the hemp straw was left in the field and an initial decomposition process occurred. During this degradation the external bast fibres are released from the stem, this process is called retting. Materials were selected in order to produce compost of easily available plant residues; wheat straw was selected as it is cheap and especially easily available, Mischanthus straw as it previously has been used as structural element in compost, and hemp straw was selected due to the high fibre content. The materials were cut into pieces of 25 mm length. Further the materials were split vertically, the hemp and mischanthus straw in three partitions and the wheat straw in two partitions. Degradation of the wheat straw was examined at the basal and apical part of the stem, whereas the Mischanthus and hemp straw only was examined at the upper part of the stem. A minimum of 2 g of straw material was deposited in nylon mesh bags (mesh size 1.4 x 1.2 mm). The material was moistened and the litterbags were placed at 0.4m depths in 800 L wooden boxes measuring 0.7 x 1.0 x 1.2m (height x width x length) at the initiation of composting of a mixture of wheat straw and clover-grass hay.
The compost was made of wheat straw as structural component and clover-grass hay as a nutrient rich component. After harvest the wheat straw was air-dried and had a C/N ratio of approximately 100, whereas the clover-grass hay was oven dried after harvest and shredded into pieces of <20 mm. The C/N ratio of the clover-grass hay was 15. The wheat straw was roughly shredded; containing parts of up to 0.1 m in length and was mixed with clover-grass hay giving a C/N-ratio of 35.
The compost was mixed and watered to a water content of approximately 60%. Leaching losses during mixing were avoided by placing some of the straw material beneath the mixer to absorb runoff. After mixing this straw material was added to the compost in the box. Supplementary water was added whenever necessary during the composting time to keep the water contents above 50%. Heat loss was minimised by insulation with glass-wool mats and the boxes were passively aerated by heat convection. Total composting time was 8 weeks. After three weeks litterbags were carefully removed and the compost was turned and watered in a mixer. After the turning of the compost the litterbags were placed in the compost again.
2.2. Sampling
Litterbags (four replicates) were sampled after one, three and eight weeks of composting. The material in the litterbags was divided, a part of it was used for histological analyses and a part was oven dried at 70°C for 24 hours and used for chemical analyses. As a reference fresh green material and un-decomposed air-dried material of the three straw types were collected, fixed and analysed histological.
2.3. Physical and chemical analysis
Material for histological analysis was fixed in FAA (formaldehyde, acetic acid, alcohol, 1:1:18). For scanning electron microscopy studies the material was dehydrated in a series of ethanol and acetone, critical point dried (EMS critical point drier), mounted on stubs and coated with gold in a sputter coater (Polaron SC7640). Specimens were examined in a scanning electron microscope (CamScan MaXim 2040S).
Remaining litterbag content was ground after drying and hot water-solubles, ADF, cellulose and lignin content was measured by C fractionation using a modified Van Soest method in a fibre analyser (ANKOM220 fiber analyzer). Total N and C were measured by dry combustion of dried and finely ground samples with an automated N-C analyser interfaced with an isotope mass spectrometer (Carlo Erba, EA 1108).
2.4. Statistics
The results were calculated as an average of four replicates of each treatment; however the fibre content was calculated as the average of duplicates. The measurements from litterbags were truly independent over time and data were analysed using the GLM procedure of the SAS statistical package (SAS Institute Inc., Cary, NC, USA).
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