EFFECT OF FREEZE-THAW EVENTS ON MINERALIZATION OF SOIL-NITROGEN

In humid regions of the United States there is considerable interest in the use of late spring (April-June) soil NO3- concentrations to estimate fertilizer N requirements. However, little information is available on the environmental factors that influence soil NO3- concentrations in late winter/early spring. The influence of freeze-thaw treatments on N mineralization was studied on several central Iowa soils. The soils were subjected to temperatures of -20-degrees-C or 5-degrees-C for 1 week followed by 0 - 20 days of incubation at various temperatures. The release of soluble ninhydrin-reactive N, the N mineralization rate, and net N mineralization (mineral N flush) were observed. The freeze-thaw treatment resulted in a significant increase in the N mineralization rate and mineral N flush. The N mineralization rate in the freeze-thaw treated soils remained higher than in non-frozen soils for 3 - 6 days when thawed soils were incubated at 25-degrees-C and for up to 20 days in thawed soils incubated at 5-degrees-C. The freeze-thaw treatments resulted in a significant release of ninhydrin-reactive N. These values were closely correlated with the mineral N flush (r2 = 0.84). The release of ninhydrin-reactive N was more closely correlated with biomass N (r2 = 0.80) than total N (r2 = 0.65). Our results suggest that freeze-thaw events in soil disrupt microbial tissues in a similar way to drying and re-wetting or chloroform fumigation. Thus the level of mineral N released was directly related to the soil microbial biomass. We conclude that net N mineralization following a spring thaw may provide a significant portion of the total NO3- present in the soil profile.