Changes in Microbial Respiration and Nitrogen Cycling Following Soil Freezing

Caroline B. Nielsen

Microbial respiration and nitrogen cycling activities have been shown to increase after soil freezing events. These events have also been correlated with canopy dieback in maple trees. The goal of the current study was to examine some of the factors which might influence the effect of soil freezing on microbial processing. Soil samples from three different soil horizons (O_e, O_a, and A₁), were taken from plots dominated by either sugar maple or yellow birch, at the Hubbard Brook Experimental Forest in New Hampshire. The samples were frozen in the laboratory at two different temperatures (^-13° C and ^-3° C) for 10 days. Soil respiration, nitrogen mineralization, and nitrification were measured for three weeks following freezing. Overall rates of microbial processes differed significantly among soil horizons, with higher rates in the O_e horizon and lower rates in the A₁. Freezing to ^-3° C had no significant effect on any of the measured responses. The effects of freezing to ^-13° C varied by both horizon and species. Freezing at ^-13° C increased respiration in the O_a and A₁ horizons of both species and mineralization in the maple plots. In the birch plots, freezing decreased mineralization in the O_e horizon. Nitrification exhibited little response to freezing in any of the horizons.