Effect Of Cover Crop Decomposition On Soil Microbial
Communities, Nutrient Cycling And Suppressiveness To Damping-Off
Pathogens
Niklaus J. Grünwald, Shuijin Hu, and Ariena H.C. VanBruggen
Department of Plant Pathology, University of California at
Davis CA 95616
We studied the effect of cover crop decomposition stage on soil carbon and nitrogen cycling, soil microbial community dynamics and suppressiveness to two fungal pathogens (Pythium aphanidermatum and Rhizoctonia solani). We conducted three experiments, two of which also compared organic and conventionally farmed soil (Student Farm at UC Davis; SAFS project, UC Davis) while the third experiment compared plots amended with and without Pythium aphanidermatum in a microplot experiment (Plant Pathology Field Station, UC Davis). Oat-vetch in mixture was used throughout all experiments.
Nutrient cycling processes were monitored as follows: crude fiber analysis, total N and total C of debris; NH4-N, NO3-N, total N and total C. Microbial processes were investigated with the following methods: fluorescein diacetate hydrolysis, potentially mineralizable N, and microbial biomass C; counts of total and active actinomycetes, fungi and bacteria; metabolic diversity of soil microbial communities using BIOLOG plates. Receptivity of soil to the 2 fungi was measured with the agar-ring method. A positive correlation was obtained between the in vitro agar-ring test and suppressiveness test for P. Aphanidermatum conducted in the growth chamber.
Suppressiveness was highest 3 to 5 weeks after incorporation and
was generally higher in the organically managed system. Stepwise
discriminant analysis was conducted to assess which of the variables
measured best describe reduction of growth of the pathogens: in
vitro selected active biomass of bacteria and fungi, soil nitrate
or ammonium content, C/N ratio of incorporated debris, and microbial
activity as measured with fluorescein diacetate hydrolysis. Metabolic
diversity patterns of soil microbial communities were more affected
by cover cropping than by farming system. Cover cropping had an
overwhelming effect on the system and tended to eliminate differences
between systems. Planting of a cash crop after cover crop incorporation
seems best 3 to 5 weeks after incorporation.
Expected Publications
Grünwald, N. J., F. Workneh, S. Hu and A. H. C. VanBruggen.
1996. Comparison of an in-vitro and a growth chamber assay to
test conventional and organic soils for suppressiveness to
Pythium aphanidermatum. Phytopathology, to be submitted in
March.
Hu, S., N. J. Grünwald, and A. H. C. VanBruggen. 1996. Short-term
effects of cover crop incorporation on soil C pools and nitrogen
dynamics. Soil Sci. Soc. Am. J., to be submitted in March.
Grünwald, N. J., and A. H. C. VanBruggen. 1996. Carbon source
utilization patterns during cover crop decomposition of an oat-vetch
residue in soils from two farming systems. Soil Microbiol. Biochem.,
in preparation.
Grünwald, N. J., S. Hu and A. H. C. VanBruggen. 1996. Soil
microbial and nutrient cycling variables associated with different
levels of Pythium aphanidermatum growth. Phytopathology,
in preparation.
Cover Crop Research and Education Summaries