Soil microbial communities and carbon utilization in diversified long-term conventional and organic management systems

Background/Question/Methods

Adoption of organic agriculture practices can alter the soil environment, with potential to develop microbial communities that differ in size, composition, and function compared to conventional systems. Microbial community response to differences in input intensity between systems (e.g., fertilizer, pesticide, and tillage) and cropping diversity and the resulting links to soil organic C turnover are not well understood. The objectives of this study were to determine the effects of conventional and organic management at two levels of cropping diversity (six-year crop rotations planted either with or without perennial alfalfa) on soil N, organic C, and the abundance and structure of the microbial community determined using PLFA analysis. This ongoing experiment was established in 1994 and was designed to investigate differences in input intensity and cropping diversity on crop production and soil quality at a field site located in Saskatchewan, Canada. To further examine differences in C utilization and activity of the microbial communities, soils collected from the long-term field study were incubated with and without ¹³C-¹⁵N labeled barley residues in a laboratory study. Over the course of the 98 d incubation, enzyme activities, nutrient levels, soil respiration, and the retention of residue-derived C in the soil microbial community using ¹³C-PLFA are being assessed.  

Results/Conclusions

There were no significant differences in either soil total N (0.23-0.25%) or organic C (2.36-2.77%) between long-term organic and conventional systems, regardless of cropping diversity. In addition, neither system nor cropping diversity had a significant effect on total PLFA or total bacterial abundance. However, fungal abundance was highest in the conventional system, regardless of cropping diversity. Abundance of Gram negative bacteria and AMF were lowest and the stress1 biomarker was highest in the annual-organic system. Ordination analysis showed that distinct microbial communities developed over 18 y of conventional and organic management; saprophytic fungi and AMF were more dominant in the conventional system, while Gram positive bacteria were more dominant in the organic system. The unique communities developed under the annual organic system may be due to differences in tillage practices—the annual organic system is tilled more frequently to control weeds, whereas the established alfalfa in the perennial system requires little tillage, and both annual and perennial conventional systems follow no-till practices. Whether these differences result in changes in the rate and pathway of microbial utilization of residue-derived C and nutrient cycling is being determined in the complementary incubation study using ¹³C-labeled barley residues and stable isotope probing.