Abstract:
Compost maturity is an important determinant of end use for
composted municipal yard debris, and generally refers to the effect the
compost has on plants. The rate of microbial respiration is an indicator of
compost stability. The objectives of this research were to: i) determine
whether continuous aeration resulted in more rapid maturity of composted
yard debris than windrow turning; ii) determine which maturity indicators
distinguish between mature and immature compost; iii) measure rates of
CO₂ evolution during active composting and curing; iv) adapt the CO₂
detection tube technique for compost; and v) evaluate rapid compost
stability tests (Solvita test, self-heating test and CO₂ detection tubes) for
potential use by commercial composters. Land Recovery, Inc. of Puyallup,
WA, composted yard debris under careful process control. Two compost
piles were studied for 113 d; one was subjected to continuous forced
aeration and periodic turning. The other was managed as a turned windrow.
We found that forced aeration resulted in mature compost about 20 days before windrowing. Compost pH, C content and respiration rate were all
useful indicators of compost maturity. Carbon fell from 400 k kg⁻¹ to 250 g
kg⁻¹ and pH rose from 5 to 7. The CO₂ evolution rate fell from 16 to 2 mg
CO₂-C g C⁻¹ d⁻¹. Maturity correlated somewhat with compost odor. Neither
percent germination nor an odor/color scale were reliable indicators of
maturity for theses composts. All the rapid tests were correlated with
alkaline trapping of microbially respired CO₂. The Solvita test took 4 h to
administer; values (1 to 8 Solvita scale) were 2 to 4 during active
composting, 3 to 6 during early curing and 6 to 7 during late curing. Self-heating
test values decreased from 20°C above ambient at the start of
composting to ambient (no heat production) at the end of composting. This
test took two to six days to administer. We developed a method to measure
CO₂ evolution rates of samples in sealed containers using CO₂ detection
tubes. This method took 4 h to administer, correlated well with alkaline
trapping, was quantitative and had excellent sensitivity at extreme compost
maturity. The methods shows promise as a raid test for compost respiration
and should be evaluated on a wider range of composts.
