Rationale and Objectives

1. Relative influence of cover crop diversity and residue management on soil microbial community structure Sam E. Wortman, Rhae A. Drijber , and John Lindquist, University of Nebraska – Lincoln Rationale and Objectives Many studies have demonstrated microbial community response to individual cover crop species, but the effects of increasing cover crop diversity has received less attention. Moreover, there is increasing interest in conservation tillage strategies for cover crop termination and residue management. Theobjective of this study was to determine the relative influence of cover crop diversity and termination method on soil microbial community structure in an organic cropping system through the extraction of fatty acid methyl esters (FAMEs). Materials and Methods A split-plot RCBD field experiment was conducted in 2009 and 2010 near Mead, NE in a non-irrigated field. Spring-sown mixtures of 2 or 8 cover crop species were included in a sunflower – soybean – corn crop rotation. Cover crop mixtures were a 1:1 combination of legume and Brassica spp. Cover crops were planted in late-March and terminated in late-May using a field disk or sweep plow undercutter and main crops were planted within one week. Three (2009) or four (2010) soil cores (3.2 cm diameter x 20 cm depth) were sampled aseptically in all experimental units at 45 or 32 days following cover crop termination in 2009 and 2010, respectively. Lipids were extracted from 10 g of soil subsamples and microbial community structure was determined from phospholipid fatty acid methyl esters (FAMEs). Fatty acids were designated as the total number of carbon atoms followed by a colon, the number of double bonds followed by the position of the double bond from the carboxyl end of the molecule and its cis or trans configuration (e.g., C16:1c11). Differences in total FAME biomass (nmol g-1) and ratios of FAME peak area to peak area of C16:0 (nmol %) within and among microbial groups were analyzed with ANOVA, while canonical discriminant analysis was utilized to characterize changes in overall soil microbial community structure. Figure 1. Effects of cover crop treatment and termination method on total FAMEs (nmol g-1) at 45 and 32 days after cover crop termination in 2009 and 2010, respectively. Error bars represent the standard error of the mean. Figure 2. Cover crop termination via undercutter (left) and disk (right). Table 2. Mean and standard errors of ratios of FAME peak area to peak area of C16:0 (nmol %) as influenced by cover crop treatment. Figure 3. Discriminant score means for all cover crop x termination treatment combinations (a), and standardized canonical coefficients for FAMEs (b) contributing to the two significant discriminant functions DA1 and DA2. • Conclusions • FAME microbial biomass was generally greater following termination with an undercutter compared to the disk, especially following the 2 species cover crop mixture • Unmanaged spring weed communities and cover crop termination with an undercutter both led to reduced abundance of FAME biomarkers for actinomycetes • Unmanaged spring weed communities and cover crop termination with a disk both led to reduced abundance of FAME biomarkers for AMF and bacteria • Microbial community structure segregated according to the presence of cover crops (2CC and 8CC) or weeds (WD), and termination method was only important within the 2CC treatment Table 3. Mean and standard errors of ratios of FAME peak area to peak area of C16:0 (nmol %) as influenced by cover crop termination method. Table 1. Cover crop mixture and termination treatments used in 2009 and 2010.