Soil Microbial Community Response to Nitrogen Rate in a Long-Term, No-till Continuous Corn and Corn/Soybean Rotation.

The soil fungal community is dynamic, responding to environmental factors, crop growth stage, crop rotation and nutrient management.  To assess the impact of increasing N rate on the soil fungal community under continuous corn and corn/soybean rotation, a study was conducted on a silt-loam soil in eastern Nebraska.  Five rates of N (0, 50, 100, 150 and 300 kg ha‾¹ urea) were applied to corn and soybeans grown under minimum-till management.  Soils were sampled in May, August, November, and again the following May.  Soil samples were analyzed for fatty acid methyl esters (FAMEs) as an indicator of soil microbial community composition.  FAME C18:2c9, a biomarker for saprophytic fungi was not influenced by N rate but responded to crop rotation over the sampling period.  The level of this marker in May 2009 was the same for all rotations.  In soil planted to corn, C18:2c9,12 increased between May and August, then declined to initial levels at harvest.  In soil planted to soybeans, the saprophytic marker increased throughout the growing season reaching its highest level in November.  The level of the fungal marker at harvest remained until the following spring.  The arbuscular mycorrhizal fungi (AMF) marker C16:1c11 was influenced by N rate, crop rotation and sampling date.  This marker increased sharply in the soil under corn at the August sampling date.  The values were highest for the 0 and 50 kg ha‾¹ N rates applied to continuous corn.  The addition of soybean in the rotation eliminated these sharp increases in the AMF marker for the lowest N rates.  These changes in the soil fungal community have implications for N, C, and P cycling in this system.