Stabilization of chemical and biochemical characteristics of grass straw and leaf mix during in-vessel composting with and without seeding material

Abstract

Grass straw and leaf mix (GL) were composted in-vessel with and without seeding material. Changes in pH, CO₂-evolution rate (CER), and the microbial biomass in terms of total extractable lipid phosphate (TELP) contents were monitored until 150 d. The microbial biomass contents were nearly stabilized at about 0.11 mmol TELP kg^-1 DM from the 60th day onward. The CER decreased gradually with the progression of composting and became stable also from the 60th day onward at around 1 g CO₂-C kg^-1 DM d^-1. On the contrary, the pH increased gradually until the 40th day and then became stabilized at about 8.8 during the remaining period of composting. The characteristics of the GL composts (150 d) were compared with those from other ready-composts. The germination indices (GI) of Dia sorghum seeds showed that all the composts were biologically mature. Although the pH varied from compost to compost, microbial biomass of GL and incubated ready-composts was about 0.1 mmol TELP kg^-1 DM. Similarly, CER did not exceed a maximum value of 1.2g CO₂-C kg^-1 DM d^-1. Significant positive effects of seeding material addition were detected in total organic carbon and organic matter contents of GL composts. However, the effect of the addition of a seeding material was not obvious in the case of the microbial biomass and CER measurements. The results show that the germination index and CER together can give sufficiently reliable information about the maturity of GL composts, although the validity of our results for large-scale open field composting needs to be examined further.

Key words:

• CO₂ evolution rate
• composting
• plant residues
• seed germination index
• total extractable lipid phosphates