Going with the flow: Landscape position drives differences in microbial biomass and activity in conventional, low input, and organic agricultural systems in the Midwestern U.S

Kyle Wickings, A. Stuart Grandy, Alexandra N. Kravchenko

Abstract

While there has been long-term interest in identifying agricultural practices which promote soil ecological services, there remains little consensus on how practices such as low-input and organic impact soil microbial communities and their functions. In order to improve understanding of the response of soil microbial communities to shifts in agricultural management programs, a two year field study was conducted on large-scale conventional, low-input and organic fields associate with the Kellogg Biological Station, Long Term Ecological Research site near Kalamazoo, MI. Because large commercial agricultural fields typically exhibit diverse topography, plots were established in all fields at unique landscape positions to explore interactions between management and topographical position. Over the course of two growing seasons, differences in microbial biomass, extracellular enzyme production and carbon and nitrogen mineralization were quantified at different topographical positions (summits, slopes, and depressions) in each of the three agricultural management systems. Alone, management practices had little to no impact on soil microbial traits, however, significant management by landscape position and management by time interactions were observed for extracellular enzymes. Acid phosphatase activity on hill slopes was 35–52% greater in low input systems than in conventional and organic management systems. Similarly, spring amino peptidase activity was roughly 100% higher in low input than in organic systems. Landscape position also had substantial effects on both microbial biomass and enzyme activities. Topographical depressions exhibited significantly higher activities of β-N-acetylglucosaminadase, β-glucosidase, phenol oxidase and greater microbial biomass than summits and slopes (26%, 30%, 36%, and 55%, respectively). Similarly, peroxidase activity was approximately 38% greater in depressions and on summits than on hill slopes. Differences in microbial traits among positions were also negatively correlated with soil bulk density and sand content and positively correlated with clay, silt, carbon, nitrogen and moisture content. Our study stresses the importance of accounting for topographical heterogeneity when assessing the impact of management practices on belowground ecological function. Further, while management practices alone had little effect on soil microbial biomass and function, our study demonstrates that low input management programs may be beneficial for retaining microbial resources and thus promoting microbial activity on erosion-prone hill slopes.

Keywords: Enzymes; Microbial biomass; Organic; Landscape position; Carbon; Soil organic matter.