Variability of asymbiotic N-fixation organism activity with distance and time in North Dakota transitional no-till soils

Asymbiotic nitrogen fixation (N-fixation) is a microbial process that may result in the introduction of plant-available N into the soil. Although the process of N mineralization in soils is mediated by many microorganisms and is related mostly to soil moisture, the soil factors which regulate asymbiotic N-fixation are relatively unknown and the number of microorganisms with the ability to fix N is small relative to the whole soil microorganism pool. Soils under long-term no-till management have greater asymbiotic N-fixing organism activity compared to no-till. This study was conducted to determine the variance of N-fixing activity over distance, and the temporal variability of asymbiotic N-fixing organism activity at six sites with transitional no-till soils in eastern North Dakota over three growing seasons. Sites were sampled at the same location each month of the growing season from 2019 to 2021. At one of the sampling dates, each year additional samples were obtained at distances from the central sampling location for use in statistical analysis. The sampling over distance indicated that to characterize a large field area, multiple samples should be taken and analyzed separately or mixed together for single analysis. Monthly sampling in all 3 years indicated that peak N-fixation activity was favored by a moist, warm environment. Dry periods and excessively wet periods resulted in low activity. Heavy rains within 48 h of sampling resulted in extremely low N-fixation activity at the subsequent sampling date. Models were constructed relating the accumulative rainfall from the 30 days prior to sampling, and the mean air temperature from the 30 days prior to sampling. These data indicate that the activity of asymbiotic N-fixing organisms increased with temperature and cumulative rainfall prior to measurement.