Organic production of herbs, ornamentals and greenhouse vegetables has developed during the last decades. Growing media for this production are generally based on peat or mixtures of peat and composted material such as animal manure (Eklind et al., 2001). However, as there is increasing focus on exploitation of the peat bogs, and organic animal manure can be sparse a more appropriate growing medium might be produced by composting plant residues. When using compost as growing medium for greenhouse production, the quality requirements are high (Ozores-Hampton et al., 1999).
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One main requirement is high nutrient content. High nutrient availability will in general support microbial growth resulting in a higher degree of degradation (Eiland et al., 2001). Nutritional quality of the compost is dependent on various parameters. Altering the initial C/N ratio or the type of material will influence the mineralisation rate and thus the availability of nutrients (Eiland et al., 2001). Affecting process parameters will lead to changes in microbial communities, which might result in an altered mineralisation pattern (Smars et al., 2002). Alternatively, recent findings showed that the N mineralisation rate could be managed by postponing the addition of some of the nutrient rich material (Dresboll and Thorup-Kristensen, submitted).
Splitting the addition of the nutrient rich material at the initiation of a composting process based on wheat straw as a structural element and clover-grass hay as nutritional component was shown to alter the mineralisation rate, generating twice as high mineralised nitrogen levels after 8 weeks of composting (Dresboll and Thorup-Kristensen, submitted). This could probably be explained by an alteration in the microbial communities during composting. A limited amount of N may be required during decomposition since much C from plant residues such as straw materials is only slowly available to microorganisms. In some cases recycling of N from the microbial biomass may be adequate to meet the N requirements (Bremer et al., 1991). Additionally, microorganisms, especially fungi, have a considerable capacity to adapt to N deficient conditions and a surplus of N initially might then result in increased gross immobilisation, probably due to changes in microbial communities. It has been suggested that there is a high N demand during the first stages of decomposition when soluble and easily degradable C compounds are mineralised while the N demand is lower when the more recalcitrant C compounds are decomposed (Recous et al., 1995). Thus, addition of sufficient N to initiate the decomposition, combined with delayed addition of nutrient rich material when more recalcitrant compounds are degraded in a less N demanding process, could lead to less immobilisation and consequently more mineralised nitrogen (Dresboll and Thorup-Kristensen, submitted).
Another main requirement of compost used as a growing medium for containerised plants is stability. Stability is mainly dependent on the choice of material but will also be influenced by the nutrient content. Various stability and maturity parameters have been suggested and distinction between the two measurements can be difficult. Maturity is associated with plant growth potential and phytotoxicity whereas stability is associated with the degree of microbial activity (Bernal et al., 1998; Eggen and Vethe, 2001). As phytotoxic compounds are produced by microorganisms in unstable compost the two terms are closely related. Amongst others, a low C/N ratio and a high NO3-/NH4+ ratio are often used as maturity parameters (Bernal et al., 1998) whereas stability in general is determined by respiration rate or heat development. When using compost as a growing medium for containerised plants, stability is furthermore critical as large volume losses should be avoided (Jensen et al., 2001; Gruda and Schnitzler, 2004). The structure of the material after composting will also affect the volume losses when used as growing medium, as the particle size distribution influences the way the media is compressed by gravity (Gruda and Schnitzler, 2004).
This study is a natural extension of the studies by Dresboll and Thorup-Kristensen (submitted) and is based on the compost produced with postponed addition of part of the nutrient rich material. The objective was to examine the nutritional quality and stability of plant-based compost to be used as growing medium. It was hypothesised that the postponed addition of part of the nutrient rich material would affect the N mineralisation leading to more available N. The postponed addition was expected to influence the decomposition of recalcitrant compounds in the compost and thereby alter the stability as growing medium. In a semi-natural environment we examined the changes in C/N, mineralised N, mass loss and volume loss during a long-term incubation experiment. Growth conditions were simulated by keeping the compost in tubes and leaching these regularly.
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