Compost feedstock and maturity level affect soil response to amendment

Although composts are commonly used soil amendments in a variety of production systems, literature reports on the effects of compost on soil properties are inconsistent. This study examined three mature composts, two immature composts, and one commercial compost produced by combining highly decomposed material with fresher material; all were produced from materials acceptable to organic production systems. The composts were mixed with soils and incubated for 114 days, and microbial, chemical, and physical properties were examined over time. One immature compost sustained a high level of soluble C, C mineralization, and microbial biomass throughout the incubation and increased soil aggregation. The other immature compost appeared to release soluble C that was relatively resistant to microbial decomposition; it was less effective at stimulating microbial activity and increasing aggregation. The compost produced by combining highly decomposed and fresh feedstocks was chemically stable, but it sustained a high level of soluble C, C mineralization, and microbial biomass and did not cause N immobilization. Despite its stability, this compost was highly stimulatory to microbial populations, and this method of producing compost deserves further study. Fluorescence spectroscopy showed promise as a way to investigate the chemistry of the soluble C released during compost decomposition.