CROSS-POLARIZED MAGIC-ANGLE-SPINNING C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC CHARACTERIZATION OF SOIL ORGANIC-MATTER RELATIVE TO CULTURABLE BACTERIAL SPECIES COMPOSITION AND SUSTAINED BIOLOGICAL-CONTROL OF PYTHIUM ROOT-ROT

We report the use of a model system that examines the dynamics of biol ogical energy availability in organic matter in a sphagnum peat pottin g mix critical to sustenance of microorganism-mediated biological cont rol of pythium root rot, a soilborne plant disease caused by Pythium u ltimum. The concentration of readily degradable carbohydrate in the pe at, mostly present as cellulose, was characterized by cross-polarized magic-angle spinning C-13 nuclear magnetic resonance spectroscopy. A d ecrease in the carbohydrate concentration in the mix was observed duri ng the initial 10 weeks after potting as the rate of hydrolysis of flu orescein diacetate declined below a critical threshold level required for biological control of pythium root rot. Throughout this period, to tal microbial biomass and activity, based on rates of [C-14]acetate in corporation into phospholipids, did not change but shifts in culturabl e bacterial species composition occurred. Species capable of inducing biocontrol were succeeded by pleomorphic gram-positive genera and puta tive oligotrophs not or less effective in control. We conclude that su stained efficacy of naturally occurring biocontrol agents was limited by energy availability to this microflora within the organic matter co ntained in the potting mix. We propose that this critical role of orga nic matter may be a key factor explaining the variability in efficacy typically encountered in the control of pythium root rot with biocontr ol agents.