Effect of intensity and duration of freezing on soil microbial biomass, extractable C and N pools, and N sub(2)O and CO sub(2) emissions from forest soils in cold temperate region

Freezing can increase the emissions of carbon dioxide (CO sub(2)) and nitrous oxide (N sub(2)O) and the release of labile carbon (C) and nitrogen (N) pools into the soil. However, there is limited knowledge about how both emissions respond differently to soil freezing and their relationships to soil properties. We evaluated the effect of intensity and duration of freezing on the emissions of CO sub(2) and N sub(2)O, net N mineralization, microbial biomass, and extractable C and N pools in soils from a mature broadleaf and Korean pine mixed forest and an adjacent secondary white birch forest in northeastern China. These soils had different contents of microbial biomass and bulk density. Intact soil cores of 0-5 cm and 5-10 cm depth sampled from the two temperate forest floors were subjected to -8, -18, and -80 degree C freezing treatments for a short (10 d) and long (145 d) duration, and then respectively incubated at 10 degree C for 21 d. Soil cores, incubated at 10 degree C for 21 d without a pretreatment of freezing, served as control. Emissions of N sub(2)O and CO sub(2) after thaw varied with forest type, soil depth, and freezing treatment. The difference could be induced by the soil water-filled pore space (WFPS) during incubation and availability of substrates for N sub(2)O and CO sub(2) production, which are released by freezing. A maximum N sub(2)O emission following thawing of frozen soils was observed at approximately 80% WFPS, whereas CO sub(2) emission from soils after thaw significantly increased with increasing WFPS. The soil dissolved organic C just after freezing treatment and CO sub(2) emission increased with increase of freezing duration, which paralleled with a decrease in soil microbial biomass C. The cumulative net N mineralization and net ammonification after freezing treatment as well as N sub(2)O emission were significantly affected by freezing temperature. The N sub(2)O emission was negatively correlated to soil pH and bulk density, but positively correlated to soil K sub(2)SO sub(4)-extractable NO sub(3) super(-)-N content and net ammonification. The CO sub(2) emission was positively correlated to the cumulative net N mineralization and net ammonification. From the above results, it can be reasonably concluded that for a wide range of freezing temperature and freezing duration, N sub(2)O and CO sub(2) emissions after thaw were associated mainly with the changes in soil net N mineralization and the availability of substrate liberat