Dynamics of Culturable Soil Bacterial Communities During Pre‐Planting and Growing Soybean Season as Evaluated by Community Level Physiological Profiles

Abstract

The agricultural practices used today for farming produce crops generate major effects on microbial ecosystems. At the same time, soil microbes can stimulate a wide range of impacts on growing crops. Since changes in soil microbial communities may reflect changes in soil quality or differences in soil management practices, we examined the effects of agricultural farming on the dynamics of soil microbial biomass and community structure. Soil samples were collected prior to planting (May 2019) and during the growing season (September 2019) from a non‐tilled agricultural soybean farm in Greencastle, IN. We quantified microbial biomass using phospholipid phosphate and quantified community structure using community level physiological profiling (CLPP) with Biolog EcoPlates. Soil samples were collected from four subplots in the agricultural field and a control sample from an adjacent woods plot. There was not a significant difference in the C/N ratios between the pooled agricultural plots (9.84) and the woods (9.96) in the pre‐planting season. We also did not see significant differences in C/N ratios among the agricultural subplots. Moisture content (Woods: 36.06% to 42.74%; Farm: 20.18% to 15.46%, from pre‐planting to growing season) was higher in the woods with a stable temperature regime. Consequently, there were no significant differences in the microbial biomass from pre‐planting (37.74 nmol PO□ gdw) to the growing season (41.48 nmol PO□ gdw) in the farm plots, but the woods (Pre‐planting: 44.63 nmol PO□ gdw; Growing: 119.57 nmol PO□ gdw) showed much more temporary fluctuation in biomass. In addition, there was a higher microbial biomass in the control plot compared to the agricultural plots during both the sampling periods. It is possible that seasonal changes are not reflected in microbial biomass on the agricultural plots due to the consistent C/N ratio or the agricultural farming of soybeans. The principal component analysis (PCA) of the CLPP indicated a broad scale of different metabolic functions of the soil microbial communities between the agricultural plots and the control. However, we did not observe any significant difference between the microbial communities of the agricultural plots from pre‐planting to the growing season. The dominant bacterial group present in the agricultural plots was Pseudomonadaceae, a common bacteria in soil, while in the control plot, the most dominant bacterial group was Enterobacteriaceae. Our results imply that a consistent organic input (C/N ratio) might sustain a stable microbial community and thus promote soil health, however, diverse factors such as crop growth stage, residue inputs, and soil physical and chemical conditions may contribute to soil quality and microbial communities. In order to determine the effects of root detritus of soybeans after harvest on soil health and microbial communities, it is recommended that sample collection and analysis occur following the harvest of the soybean crop.Support or Funding InformationScience Research Fellows Program; DePauw University