Using Real-Time Quantitative PCR to Examine the Dynamic Role of Soil Fungi in C and N Cycling of Rangeland Soils

Recent work has revealed that the soils of southwestern rangelands represent strong seasonal sources and sinks of atmospheric trace gases, and that the activities of drought-tolerant soil fungi contribute strongly to C and N cycling during periods of extreme dryness. Many of these findings have been established using soil incubations with added C and N substrates and/or selective microbial inhibitors, methods that effectively measure community activity, but do not allow quantification of specific microbial groups and/or species-specific identification. This project used real-time quantitative PCR to examine the influences of soil moisture status, livestock grazing, and mesquite management on ratios of fungal to bacterial biomass in the Santa Rita Experimental Rangeland (SRER) of southeastern Arizona. Soil samples were collected at two different depths (0-5 and 5-10 cm) from plots established in long-term (100-yr) grazing exclosures and in adjacent grazed areas under established mesquite and in sites with no historic mesquite occupation. Sampling was performed in the pre-monsoon, immediately following the first monsoon rainfall, and six weeks following monsoon onset. Results showed that fungal biomass dominated in surface (0-5 cm) soils under mesquite, but no impacts of grazing on fungal to bacterial biomass ratios were detected. Bacterial biomass was strongly reduced under extremely dry soil conditions, while fungal biomass did not show a similar trend, confirming that soil fungi are less susceptible to extreme drying in rangeland soils. This work confirms that fungi contribute strongly to nutrient cycling in southwestern rangeland soils.