Effect of intensity and duration of freezing on soil microbial biomass,extractable C and N pools,and N2O and CO2 emissions from forest soils in cold temperate region

Freezing can increase the emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O) 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 CO₂ and N₂O, 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°C freezing treatments for a short (10 d) and long (145 d) duration, and then respectively incubated at 10°C for 21 d. Soil cores, incubated at 10°C for 21 d without a pretreatment of freezing, served as control. Emissions of N₂O and CO₂ 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 N₂O and CO₂ production, which are released by freezing. A maximum N₂O emission following thawing of frozen soils was observed at approximately 80% WFPS, whereas CO₂ emission from soils after thaw significantly increased with increasing WFPS. The soil dissolved organic C just after freezing treatment and CO₂ 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 N₂O emission were significantly affected by freezing temperature. The N₂O emission was negatively correlated to soil pH and bulk density, but positively correlated to soil K₂SO₄-extractable NO₃ ^--N content and net ammonification. The CO₂ 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, N₂O and CO₂ emissions after thaw were associated mainly with the changes in soil net N mineralization and the availability of substrate liberated by freezing as well as other soil properties that influence porosity.