The influence of landscape‐scale soil processes on microbial activity associated with carbon and nitrogen cycling in agroecosystems

Background/Question/Methods

Alternative biomass cropping systems offer the potential to enhance both the productivity and environmental quality of agroecosystems by providing a range of ecosystem services. Much work has been done to quantify the aboveground productivity of these systems but less is known about the specific effects of these systems on belowground carbon (C) and nitrogen (N) cycling and whether these effects vary at scales relevant to land management. Understanding the effect of biomass cropping systems on nutrient cycling provides a mechanistic link between agroecosystem management and ecosystem services. The main objective of this study was to assess C and N cycling in five biomass cropping systems and the extent to which nutrient cycling varies with landscape position at the Landscape Biomass Project, Iowa State University. We hypothesized that aspects of cropping system such as fertilization regime, perennial vs. annual crops, and cover crops would influence microbial activity associated with C and N mineralization and denitrification potential. Furthermore, we hypothesized that microbial responses would be dependent on the position of these systems on the landscape due to differences in soil water saturation, soil organic C content, and soil texture.

Results/Conclusions

Denitrification enzyme potential and activity of six extracellular enzymes corresponding to mineralization of C and N differed across the landscape (P<0.0001 for all) with two patterns of variation in activity: a gradient response increasing downslope or high activity on the floodplain with no change in activity at other landscape positions. Cropping system influenced the activity of cellobiohydrolase and beta xylosidase, which could result in differences in C cycling among cropping systems. In contrast to soil microbial data, soil chemical properties (extractable organic C and inorganic N) responded to both cropping system and landscape position but no interactions were detected. The implications of these data for management of ecosystem services associated with nutrient cycling will be discussed, in addition to biomass-specific activity and enzyme ratios. Results suggest that, despite contrasting cropping systems, belowground cycling of C and N by microorganisms at this site is influenced most strongly by landscape-scale soil processes at this time.