Materials and Methods

2.1. Compost

Three different types of compost, based on clover-grass hay as a nutrient rich component and wheat, Mischanthus or hemp straw as structural components, were compared. The wheat compost was a mixture of clover-grass hay (30 kg) and wheat straw (50 kg) giving an initial C/N ratio of 26. The Mischanthus and hemp composts contained the same materials but half of the wheat straw was replaced by 20 kg Mischanthus respectively hemp straw. The C/N ratio of the Mischanthus compost was 26 whereas the C/N ratio in the hemp compost was only 16, as the hemp material used were harvested fresh and had a high N content. The main objective was to examine physical properties of the composts, and the amount of the different materials added were prioritised over C and N content, when determining the proportions of the materials. Shredded (<20 mm length), oven dried clover-grass hay was used, whereas the straw materials were air dried and shredded to pieces of <50 mm length. The plant materials were mixed and watered to a water content of 60-65%, and placed in 800 L wooden compost boxes measuring 0.7 x 1.0 x 1.2m (height x width x length). Additional 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. After 3 weeks of composting the compost was turned, and the total composting time was 10 weeks.

2.2. Sampling

Samples were collected at the initiation of the composting process and after 3, 7% and 10 weeks of composting. At initiation, after 3 weeks and after 10 weeks samples were collected after the compost was mixed thoroughly, approximately 250 g compost was collected and subsamples for further analysis were obtained from this. After 7% weeks 5 samples of approximately 50 g was collected randomly in the compost pile and pooled. Subsamples of the pooled mass were collected for analysis. Temperature was measured continuously in the centre of the composting boxes using standard acid proof stainless steel Pt-100 probes connected to a data logger (Datataker DT500).

2.3. Weight losses

Compost boxes were weighed at initiation of the composting process, when turned after three weeks and finally after 10 weeks and weight losses were determined. In addition, nylon mesh litterbags (mesh size 1x1.2mm) were placed in the compost boxes of the Mischanthus and hemp composts. In the Mischanthus compost litterbags containing wheat straw (3 g), mischanthus straw (3 g) or a mixture of the two straw types (1.5 g of each) was placed whereas the hemp compost had litterbags containing wheat straw, hemp straw or a mixture of these materials in the same amounts as in the Mischanthus litterbags. This made it possible to follow the actual weight losses of the structural components and the effect of mixing them.

2.4. Physical and chemical analysis

Water content was determined by weight loss of compost samples, which were oven dried at 80°C for 24 hours. 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). Samples were acidified by pouring 60 ml 0.1M HCl over the entire sample before drying to avoid NH4+ losses.

The NH4+ and NO3- content was determined in a 2M KCl extract (compost (20 g fresh weight): solution ratio 1:10) followed by shaking for 45 minutes and centrifugation. The supernatant was filtered through Advantec 6 (Frisenette Aps.) filters and stored frozen. As the extracts were strongly coloured by organic compounds, they were cleared by shaking the extract with active carbon for 15 minutes and filtered (0.45^m pore size) to avoid interference when measured spectrophotometrically. The analysis for NH4+ and NO3- content was conducted by standard colorimetric methods using flow-injection analysis (FIA) (Keeney and Nelson, 1982).

2.5. Particle size distribution

After the composting period the particle size distribution was measured for the three compost types. Approximately 1g of compost was placed in water for 24 hours, distributed evenly on a perspex tray and scanned on a flat bed scanner. The scanned image was analysed by the software program WinCam (Regent Instruments Inc.) measuring particles at 600 dpi corresponding to a pixel size of 0.042 mm in length. Length, width and amount of particles are given by the software and particle size distribution was determined. An estimate of the particle volume was produced by a simplified assumption that height and width of particles were identical, and was calculated as length*width2. This is of course a rough estimate, however, volume of particles provide more information on the physical properties than particle area.

2.6. Water retention

Water retention capacity and air-filled porosity was measured on a sandbox. Samples were prepared and measured according to the European standard (PrEN 13041, 1999; EN 13040, 2000). Compost was filled in containers, saturated in water and equilibrated on a sand box at a water tension of 5 kPa. The compost was then transferred into double ring sample cylinders, rewetted and equilibrated at 1 kPa water tension. The rings were separated, and the compost of the upper ring was discarded. Hereafter, the lower ring contained a known volume of compost. Water retention was measured at tensions of 1,2,3,5 and 10 kPa. Samples were then dried at 103°C and water retention and air-filled porosity was determined from the obtained results.

2.7. Statistics

The results were calculated as an average of three replicates of each treatment (five replicates for determination of particle size distribution and water retention) and analysed with the GLM procedure of the SAS statistical package (SAS Institute Inc., Cary, NC, USA)

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