Characterization of compost composition and its relationship to the suppression of Pythium root rot of cucumber

Sawdust-bedded dairy manure solids were composted in windrows for 286 days. Compositional changes were investigated by FTIR and {dollar}sp{lcub}13{rcub}{dollar}C CPMAS NMR spectroscopy and related to conventional compost maturity indices. Forty-two and seventy-five percent of initial lignin and carbohydrate contents, respectively, were degraded by day 143. The compost reached maturity by day 196 as determined by a ryegrass maturity bioassay. Most changes in DR-FTIR subtraction spectra occurred by day 186. The ryegrass bioassay, total C:N, soluble organic C, soluble organic N, and DR-FTIR subtraction spectra could be utilized as maturity indices. DR-FTIR cellulose spectral fitting estimated carbohydrate degradation during the composting process.; Compost was amended to sand and incubated in pots. As the compost decomposed further, the relationships between the mass and composition (by DR-FTIR and {dollar}sp{lcub}13{rcub}{dollar}C CPMAS NMR spectroscopy) of the light fraction (LF), and the collapse of suppression to Pythium root rot caused by Pythium ultimum were investigated for 506 days. Suppressiveness was sustained through day 375. During this period, microbial biomass declined but there was no significant change in FDA activity. A significant decline in total LF accompanied by very little change in LF chemistry occurred, indicating a non-selective degradation of organic matter. Gram negative PLFAs increased and gram positives and actinomycete PLFAs declined in proportion. Between days 375 and 426, suppression was lost, FDA activity declined, and significant reductions in lignin and aliphatics concentrations (DR-FTIR), or O-alkyl and alkyl-C contents ({dollar}sp{lcub}13{rcub}{dollar}C CPMAS NMR) were observed, and there was an increase in proportion of fungal PLFAs and a decline in proportion of gram positive and actinomycete PLFAs.; Suppressive compost-derived LF was compositionally similar to forest organic horizons and unprotected LF (ULF) in field soils, as determined by {dollar}sp{lcub}13{rcub}{dollar}C CPMAS NMR spectroscopy. As suppression was lost, compost-LF O-alkyl-C content declined to a level below that of ULF and closer to protected LF. This work indicates that ULF may play a role in suppression of Pythium root rots.