Conserving N from high n crop residues under field conditions by using on- and off-farm organic biological waste materials

The objective of this study was to test organic biological waste (OBW) materials for their potential to immobilize N under field conditions. Two field experiments (randomised block design with 3 replicates) were set up: one on a heavy sandy loam and one on a sandy soil. Each plot received cauliflower residues and an OBW material (green waste compost (GWC) or saw dust), which were incorporated into the soil with a rotavator. At regular times soil samples were taken to a depth of 0.90 m in 4 layers. The mineral N and microbial N content of the soil samples were determined. In both soils, the mineral N content in OBW treatments was more or less similar to that in the cauliflower treatment, and no significant differences could be determined. The microbial biomass analysis showed that no extra N was immobilized in the OBW treatments compared to the cauliflower treatment. So, neither the GWC nor the saw dust immobilized N released from the cauliflower residues despite their high C:N ratio. Apparently, some factors other than the OBW composition had an effect on the N immobilization. Some of these factors might be the structure of the OBW materials, the degree of mixing between crop residues and OBW materials, a good incorporation into the soil, weather conditions, soil texture and soil compaction.