Authors: Nathan Stacey, Washington State University (WSU)-Puyallup; Ricky Lewis & Tarah Sullivan, WSU-Pullman and Joan Davenport, WSU-Irrigated Agriculture Research & Extension Center Prosser

Bacterial and fungal organisms, collectively known as soil microbial communities, are responsible for 90% of soil organic decomposition, and therefore play large roles in the cycling of carbon (C) and nitrogen (N).  As both C and N are essential for plant productivity, the function of the soil microbiome is of fundamental interest for agricultural and horticultural crop production.  

It is estimated that one gram of soil can harbor tens of thousands of bacterial species and hundreds of meters of fungal hyphae.  Newer molecular methods have allowed researchers to characterize this incredible diversity by identifying individual species (also called operational taxonomic units – OTUs) and then clustering these OTUs into related groups (i.e., bacteria and fungi).  What results is a molecular picture of the community, defined as its structure (who and how many?).  This microbial community structure is known to change with variation in cropping systems, land use change, and various soil properties (e.g., pH and soil moisture).

Understanding shifts in the soil microbial community structure may aid in predicting soil nutrient availability (particularly N).  To assess whether organic amendments, like a composted biosolid, are responsible for shifts in bacterial and fungal community structure, a field experiment was established on fairways at Eagles Pride Golf Course, a Joint Base Lewis-McChord (JBLM) facility.  Soil samples were collected from compost amended and unamended plots and then analyzed for bacterial and fungal community structural differences.  Picture above shows a golf course fairway following compost amendment.

Both bacterial and fungal community structure were unchanged following compost amendment.  Instead, the observed community shifts (bacterial and fungal) were related to site conditions (the difference between fairways).  It is unclear what caused the shifts in bacterial and fungal community structure.  Factors shaping the communities may be related to soil properties, turfgrass species or management, or a combination of all three.