The Effects of Nutrient Sources On Soil Microbial Communities and Switchgrass Production.

Sustainable crop production can be influenced by belowground microbial communities. A field study was conducted in Perkins and Lake Carl Blackwell, Oklahoma to determine how soil microbial communities varied by nutrient sources applied to the soil, and how it affected switchgrass biomass production. The experimental design was a split plot with four replications.  Soil and plant samples were taken in April and November 2011.  Soil microbial communities, organic carbon (OC), NO₃, plant available P and K under dual purpose and sole feedstock “Alamo” switchgrass were characterized following applications of cattle manure, poultry litter, inter-seeded legumes, or inorganic fertilizers. Microbial biomass was determined using the phospholipid fatty acid analysis (PLFA) and the arbuscular mycorrhizal colonization was determined using the grid intersection method. Soil NO₃-N was determined by 1 M KCl extraction, P and K by Mehlich 3 extraction, OC by dry combustion and soil pH by an electrode in 1:1 soil:water suspension.

Percent arbuscular mycorrhizal root colonization was similar for all fertilizer treatments at both sites and both samplings. This could indicate that arbuscular mycorrhizal fungi (AMF) form a similar association with switchgrass with or without amendments to the soil. Root density at the upper 5 cm of the soil also did not significantly differ among fertilizer treatments. This could be due to switchgrass tendency to accumulate roots on the upper part of the soil.  Cow manure had the largest fungal:bacterial ratio, with the April sampling of the inter-seeded legume showing the lowest ratio at Perkins. However, in November all fungal:bacterial ratios generally indicated a decrease, as compared to the April sampling. The fungal:bacterial ratio of the inter-seeded legume treatment was generally not affected by sampling date, possibly indicating that legumes create a more stable microbial community as compared to the other fertilizer treatments. Total microbial biomass was lower in April, as compared to November, possibly indicating that microbial activity followed the pattern of the growth of the plant, as switchgrass senesces in winter. Soil pH generally increased with incorporation of animal manure, bringing our acidic soils to neutral. Mehlich P and K were significantly increased by manure application based on crop nitrogen needs Results of our research indicate time of sampling effects soil microbial communities, and these communities may also be affected by nutrient source.