Particle size fractionation as a method for characterizing the nutrient content of municipal green waste used for composting

In order to better characterize mechanically shredded municipal green waste used for composting, five samples from different origins were separated into seven particle size fractions (>20 mm, 10-20 mm, 5-10 mm, 2-5 mm, 1-2 mm, 0.5-1.0 mm and <0.5 mm diameter) and analyzed for organic C and nutrient content. With decreasing particle size there was a decrease in organic C content and an increase in macronutrient, micronutrient and ash content. This reflected a concentration of lignified woody material in the larger particle fractions and of green stems and leaves and soil in the smaller particle sizes. The accumulation of nutrients in the smaller sized fractions means the practice of using large particle sizes for green fuel and/or mulch does not greatly affect nutrient cycling via green waste composting. During a 100-day incubation experiment, using different particle size fractions of green waste, there was a marked increase in both cumulative CO2 evolution and mineral N accumulation with decreasing particle size. Results suggested that during composting of bulk green waste (with a high initial C/N ratio such as 50:1), mineral N accumulates because decomposition and net N immobilization in larger particles is slow while net N mineralization proceeds rapidly in the smaller (<1 mm dia.) fractions. Initially, mineral N accumulated in green waste as NH4+-N, but over time, nitrification proceeded resulting in accumulation of NO3−-N. It was concluded that the nutrient content, N mineralization potential and decomposition rate of green waste differs greatly among particle size fractions and that chemical analysis of particle size fractions provides important additional information over that of a bulk sample.