ObjectiveThe spring freezing-thawing period is a critical period connecting winter and growing season, during which intense temperature changes may profoundly affect soil ecological processes. We studied the effects of soil freezing-thawing on soil nitrogen mineralization in spring, and revealed the law of the effects of freezing-thawing on soil nitrogen mineralization in the cold temperate zone of northeastern China, so as to provide theoretical basis for the study of forest ecosystem nitrogen and forest productivity evaluation in the cold temperate zone.

MethodIn this study, the dynamic changes of soil inorganic nitrogen (NH₄⁺-N, NO₃⁻-N), net ammoniation rate, net nitrification rate and nitrogen mineralization rate during spring thawing were measured and analyzed by resin core method in three typical forests (Larix gmelinii forest, Pinus sylvestris var. mongolica forest, and Betula platyphylla forest) in the cold temperate zone.

ResultThe contents of inorganic nitrogen in the soil of the three types of forests in the spring freezing-thawing period of the cold temperate zone all showed release characteristics, and increased significantly at the end of freezing-thawing period. However, the variation rules of the three types of forests were different. The content of ammonium nitrogen in the soil of the three types of forests accounted for 83.91%−97.22% of the content of inorganic nitrogen, which was the main existing form of soil inorganic nitrogen in the spring freezing-thawing period. During the freezing-thawing cycle, the net nitrogen mineralization rate of Larix gmelinii forest, Pinus sylvestris var. mongolica forest, and Betula platyphylla forest in 0−10 cm soil layer increased by 1.86 times, 6.18 times and 0.25 times, respectively. The net nitrogen mineralization rate of Pinus sylvestris var. mongolica forest, and Betula platyphylla forest in 10−20 cm soil layer increased by 4.09 times and 2.25 times, respectively, excepting for the decrease of Larix gmelinii forest. Moreover, the net ammonia mineralization rate of soil accounted for 73.47%−96.76% of the net nitrogen mineralization rate of soil, and the main nitrogen mineralization in soil was ammonia. Soil moisture content was the main influencing factor for soil organic nitrogen mineralization.

ConclusionFreezing-thawing in the cold temperate zone is beneficial to the mineralization of soil organic nitrogen, and the response of soil nitrogen mineralization in broadleaved forest to freezing-thawing cycle is stronger than that in coniferous forest.