Evaluation of Fiber Content Relative to Other Measures of Compost Stability

Authors: HUTCHINSON, MARK - UNIV OF MAINE; Griffin, Timothy

Submitted to: Compost Science and Utilization
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2007
Publication Date: 3/14/2008
Citation: Hutchinson, M., Griffin, T.S. 2008. Evaluation of Fiber Content Relative to Other Measures of Compost Stability. Compost Science and Utilization. 16(1):6-11.

Interpretive Summary: Improved predictive relationships between compost maturity and nitrogen (N) availability would be helpful in managing these soil amendments as nutrient inputs. We collected thirteen separate compost samples from a single windrow over a 91 d period. Our initial assessment of compost maturity used standard analyses, including total C and N, C:N ratio, and inorganic N concentration. The degradability of compost C and N was assessed by measuring the amount of carbon dioxide released over 24 hr, and also using commercially available compost evaluation kits which include both carbon dioxide and ammonia release to establish a Maturity Index. We also estimated slowly degradable carbon fractions in the composts using neutral detergent fiber (NDF) and lignin methods that were developed for ruminant feed analysis. The relationship between maturity and each compost characteristic was evaluated using simple linear regression. Some widely-used parameters, like compost C:N ratio, changed very little during the sampling period. Compost carbon dioxide release showed a strong relationship with time in the compost process. Likewise, compost fiber and lignin concentration increased as compost matured, as more easily available C was used by microbes. The Maturity Index was also related to the duration of composting, but fluctuated due to temporary changes in windrow conditions. These results indicate that microbial activity and fiber analysis can be used to establish relative differences in compost maturity and stability.

Technical Abstract: Improved predictive relationships between compost maturity and nitrogen (N) availability would be helpful in managing these soil amendments as nutrient inputs. We collected thirteen separate compost samples from a single windrow over a 91 d period. Initial assessment of compost maturity used standard analyses, including total C and N, C:N ratio, and inorganic N concentration. Compost C and N lability were assessed by measuring CO2 evolution during a 24 hr period, and also using commercially available compost evaluation kits which include both CO2 and NH3 release to establish a Maturity Index. We also estimated slowly degradable C fractions in the composts using neutral detergent fiber (NDF) and lignin methods developed for ruminant feed analysis. The relationship between maturity and each compost maturity index was evaluated using simple linear regression. Some widely-used parameters, like compost C:N ratio, changed very little during the sampling period. Compost CO2 evolution showed a strong linear relationship with time in the compost process (r2 of 0.82). Likewise, fiber and lignin concentration increased during compost maturation as more easily available C was used by microbes (r2 = 0.70 to 0.80). The Solvita Maturity Index was also related to the duration of composting (r2=0.59), but fluctuated due to temporary changes in windrow conditions. These results indicate that microbial activity and fiber analysis can be used to establish relative differences in compost maturity and stability.