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Daily Changes of Infections by Pythium ultimum After a Nutrient Impulse in Organic Versus Conventional Soils

First author: Department of Life and Environmental Science, Hangzhou Normal University, Hangzhou 310036, China, and Department of Environment and Resource, Zhejiang University, Hangzhou 310029, China; second author: Magazine, Chang'an University, Xi'an, Shaanxi, China, and College of Resources and Environment, China Agricultural University, Beijing, China; third author: Department of Environment and Resource, Zhejiang University, Hangzhou 310029, China; fourth author: Department of Plant Sciences, Biological Farming Systems Group, Wageningen University, P.O. Box 563, 6700 AN Wageningen, The Netherlands; fifth author: Institute of Physical-Chemical and Biological Problems in Soil Science of RAS, Institutskaya ul., 2, 142290, Pushchino, Moscow region, Russia; sixth author: Laboratory for Microorganism Cultivation Processes, Institute of Vaccines and Serums, Russian Academy of Medical Sciences, Malyi Kazennyi per., 5a, 103064 Moscow, Russia; seventh author: Department of Microbiology, Biological Faculty, Moscow State University, 119899 Vorob'evy Gory, Moscow, Russia; and eighth author: Department of Plant Pathology, University of Florida, P.O. Box 110680, Gainesville 32611-0680, and Department of Plant Sciences, Biological Farming Systems Group, Wageningen University, The Netherlands.

Bacterial populations (CFU) have been shown to oscillate in wavelike patterns after nutrient impulses in previous studies. The amplitudes and periods of oscillations could possibly be used as indicators of soil health analogous to the stability and resilience of biological populations widely accepted as indicators for ecosystem health. Limited plant and animal disease outbreaks can also be viewed as a manifestation of a healthy soil ecosystem. Two pot experiments were carried out to verify whether damping-off of beet seedlings by Pythium ultimum, measured as area under the disease progress curve (AUDPC), fluctuated over time after incorporation of organic materials into organic versus conventional soils, and to investigate whether daily dynamics of AUDPCs were linked to the dynamics of microbial populations and chemical parameters. AUDPCs oscillated significantly over time when Pythium bioassays were initiated daily after addition of ground grass and clover shoots (GC) into unplanted soils. Similar oscillations with significant harmonics of AUDPC were also observed in composted manure (CM)-amended soils but with smaller amplitudes than in GC-amended soils. The AUDPC harmonics in amended soils had periods similar to those of CFU of copiotrophic bacteria. Cross-correlation analysis demonstrated that periodic fluctuations of P. ultimum infections (AUDPCs) did not coincide with those of copiotrophic CFU but were shifted in phase. It appears that competition or antagonism from some fast-growing bacteria influenced pathogen infections, because these bacterial populations were growing and dying. Soil chemical variables, including pH, dissolved organic carbon, and NO_3⁻-N, and NH_4⁺-N contents, changed significantly in the initial 7 days after a nutrient impulse into soils. These changes were cross-correlated with copiotrophic CFU with time lags of ≈1 to 2 days but were seldom associated with daily changes in AUDPCs. Organically managed soils always had lower AUDPC ratios of amended to nonamended treatments, indicating that organic materials showed stronger suppressive abilities to P. ultimum in organic than in conventional soils. The oscillations in AUDPCs and copiotrophic CFU in amended organic soil also had smaller amplitudes than in amended conventional soil. These results suggested that organically managed soils had a greater resistance and resilience to the disturbance of the amendments and, therefore, could be considered healthier than conventionally managed soils.