A paradox of global warming is that it may increase the likelihood of freezing of forest soils. The purpose of this study was to examine the effects of soil freezing on N dynamics in soil along a gradient of rates of N processing at the Fernow Experimental Forest (FEF), Tucker County, West Virginia, with particular emphasis on net nitrification. These sites were designated as low N processing (LN), moderately low N (ML), moderately high N (MH), and high N processing (HN). In addition to untreated controls, we subjected soil samples to three 7-d freezing temperature treatments: 0 C, -20 C and -80 C. We further distinguished between initial effects of freezing (based on extracts taken immediately upon thawing) and effects on N mineralization/nitrification potential (based on extracts taken following incubation at 25 C for an additional 7 d).
Results/Conclusions
Responses varied substantially between temperature treatments and along the gradient in soil N dynamics. Initial effects differed significantly among freezing temperatures for net N mineralization, but not nitrification, in soils across the gradient, which was generally maintained for all freezing treatments (i.e., rates of N processing at LN<ML<MH<HN). Net N mineralization potential was significantly higher following freezing at -20 and -80 C than the control, which was also significantly higher than the 0 C treatment. A similar pattern was observed for net nitrification potential, i.e., rates following -20 C ≈ -80 C > Control > 0 C. An exception to these patterns was evident for the LN site, wherein net nitrification remained very low across initial freezing effects and effects on nitrification potential. Indeed, net nitrification at the LN site (0.01 to 0.05 µg N/g soil/d) was consistently one to two orders of magnitude lower than at the HN site (3.0 to 5.0 µg N/g soil/d). Our data revealed that response of N mineralization to freezing differs greatly from that of nitrification, suggesting that soil freezing may serve to de-couple two processes of the soil N cycle that are otherwise tightly linked at our site. Results also suggest that soil freezing at temperatures commonly experienced at this site can further increase net nitrification in soils already exhibiting high nitrification from N saturation.