Impacts of Soil Management on Microbial Assemblages Involved in Nitrogen Transformations in Agroecosystems in Tennessee, USA

Nutrient reduction, particularly with respect to nitrogen (N) losses, is an important goal for sustainably managed agroecosystems. Soil N-cycling microbial populations that modulate these processes are affected by agricultural management regimes. This research focused on the controls and dynamics of the major N-cycling microbial populations in high-input cotton field under agricultural management regimes and low-input native C₄ forage grass systems under pasture management practices to determine the effects of management regimes on in situ seasonal dynamics of the functional microbes responsible for N fixation, nitrification, and denitrification processes. Molecular microbial ecology methods were combined with soil physicochemical properties and biogeochemical process rates to address research questions.

The results showed that for soils in both cropland and grassland, the variability of the activity of N-cycling microbial populations was consistently and significantly higher than for functional potential. Agricultural season was a key factor affecting the variability of both functional potential and activity of N-cycling microbial populations regardless of agroecosystem types. In cotton fields, leguminous cover crops like hairy vetch promoted functional activity of N-cycling populations through the growing season, equaling or exceeding the effects of inorganic N fertilizer, suggesting the potential of leguminous cover crops for replacing inorganic N fertilization from the perspective of microbial ecology. In native C₄ grasslands, high N fertilization rate changed community composition of both diazotrophs and ammonia-oxidizing bacteria (AOB), inhibited diazotrophs functional activity and diversity of diazotrophs but promoted AOB abundance, activity, and diversity as well as N₂O emission and nitrification potential, indicating the negative influence of excessive N fertilization on soil functional microbial community, N fixation and reservation, and soil and downstream environmental health. Therefore, the significance of reduced N fertilization may be substantial both economically and environmentally. Our findings highlighted the fact that both genetic and transcriptional assessments can provide important information about the microbial populations and functions. Together, our work revealed insights into important relationships between microbial functional capacity and activity and associated nitrogen pools and processes in the field, adding to our understanding of the effects of soil health management practices on soil N-cycling microbial ecology.