Effects of freeze-thaw cycles resulting from winter climate change on soil nitrogen cycling in ten temperate forest ecosystems throughout the Japanese archipelago

In temperate forest ecosystems, accelerated freeze-thaw cycles caused by winter climate change are expected to affect nitrogen (N) cycling in soils. Net N mineralization and nitrification rates were investigated via incubations of sieved soils transplanted from ten temperate forest ecosystems to two northern Japan sites with natural snowfall gradients. This was done to address: 1) how freeze-thaw cycles affect N mineralization and nitrification in temperate forest soils; 2) whether freeze-thaw cycles change the soil N transformation rates in the following growing season; and 3) which soil characteristics affect the response of the N transformation rates to freeze-thaw cycles. The effect of freeze-thaw cycles on inorganic N and dissolved organic carbon productions differed among soils, that is, some soils produced more inorganic N and dissolved organic carbon in the conditions imposed by freeze thaw cycles than in the non-frozen treatment but the others did not. The response to the freeze-thaw cycles was explained by soil microbial activity (gross N mineralization and nitrification rate) and soil fertility (inorganic N pools in the early spring and water soluble ions). Freeze-thaw cycles significantly increased N transformation rates in the following growing season, suggesting that winter climate change might also affect nutrient availability for vegetation and soil microbes in the growing season. The magnitude and frequency of freeze-thaw cycles were considered to be important indicators of N transformation rates during the growing season, suggesting that the higher intensity of freeze-thaw cycles in the original locations of soils changed the microbial communities and functions with high tolerance to freeze-thaw cycles; this resulted in greater N transformation rates in the following growing season. Microbial activity, soil fertility and climate patterns in the original locations of soils are believed to have an effect on the response to winter climate change and to cause large variability of soil response of N transformation rates to freeze-thaw cycles in both the dormant and growing seasons.