Asymbiotic Nitrogen Fixation is Greater in Soils under Long‐Term No‐Till Versus Conventional Tillage

Abstract

A series of N‐rate experiments previously conducted in spring wheat, corn, and sunflower in North Dakota indicated that less N was required when fields were in six years or more continuous no‐till compared to conventional till. The objective of this study was to determine whether part of the reason for the decreased requirement for N was the greater activity of asymbiotic N‐fixing organisms. Twelve paired‐samplings were conducted in 2018. A surface 0‐ to 5‐cm deep sample was obtained in a long‐term no‐till field directly across the fence or road from a similar soil in conventional till. Samples were incubated in an acetylene‐reduction procedure to estimate N fixation rate. Ten of twelve paired samplings had greater asymbiotic N fixation compared to the conventional till counterpart. This indicates that long‐term no‐till soils support greater N production from soil microorganisms than conventional till soils, which would result in lower input costs to no‐till farmers. Core Ideas Microbial communities are different between conventional till and no‐till managed soils. Asymbiotic N‐fixation was greater in farmer‐managed no‐till than neighboring conventional till. N recommendations in long‐term continuous no‐till should be separate from conventional till N recommendations.

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

“…One of the reasons why transitional no‐till sites were chosen for this study was that a trend to greater N‐fixing activity was expected over time. However, each season was unique due to soil moisture and temperature regime, and although previous work indicated that long‐term no‐till supports greater activity compared to conventional tillage (Franzen et al., 2019), the trend toward greater N‐fixation is likely to be observed at a longer time‐span than this 3‐year sampling study was capable of showing. One might speculate that it would take 10 years to produce enough data to discern a trend because of year‐to‐year variation in rainfall patterns and spring warm‐up conditions.…”

“…One of the reasons why transitional no-till sites were chosen for this study was that a trend to greater N-fixing activity was expected over time. However, each season was unique due to soil moisture and temperature regime, and although previous work indicated that long-term no-till supports greater activity compared to conventional tillage (Franzen et al, 2019), the trend toward greater N-fixation is likely to be observed at a F I G U R E 1 0 Exponential relationship between N-fixing activity and mean air temperature, 30 days preceding sampling. longer time-span than this 3-year sampling study was capable of showing.…”

“…Asymbiotic N-fixation has been shown to have a higher rate in soils under long-term no-till management than in conventional tillage (Franzen et al, 2019;Lamb et al, 1987). In a wheat-fallow system in western Nebraska (Lamb et al, 1987), soil sampling for asymbiotic organism activity was conducted on three dates in 1983, andin April, May, June, July, andSeptember, 1984. In 1983, the soil was very dry all season (0.21-0.26 m 3 m −3 ), which was probably why there were no differences in activity between dates.…”

“…The sample jars were transported during the sampling expeditions in a Styrofoam cooler with ice packs and placed in a refrigerator for up to 3 days at 3˚C before shipment. The samples were shipped overnight with ice packs to the University of Florida Wetland Biogeochemical Laboratory, Gainesville, FL, where an incubation acetylene reduction procedure was performed, as detailed in Franzen et al (2019). At the second sampling, samples were obtained from the initial Global Positioning System (GPS) sampling location, and 3, 6, 15, and 30 m away from center in two directions from the GPS sampling location in 2019 and 2020, and from a 30 and 61 m distance in two directions in 2021 (Figure 2).…”

Tillage intensity and plant rhizosphere selection shape bacterial-archaeal assemblage diversity and nitrogen cycling genes

“…Additionally, Obscuribacteria, a family within the phylum Cyanobacteria; Nitrosotaleaceae a family of archaea that can oxidize ammonia (Prosser and Nicol, 2015); and Phycisphaeraceaea, a family potentially involved in anammox (Rios-Del Toro, et al, 2018), were all higher in NT treatments compared to MP. Increased activity of free-living N2-fixing microorganisms may also occur in NT soils (Franzen et al, 2019). Changes to community composition in bulk soils managed under NT favor bacteria and archaea that cycle N and bypass the denitrification pathway.…”