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Global warming alters freeze–thaw process frequency and intensity, impacting soil carbon cycles. Four soils from a 12-year straw return experiment were used: S0 (no straw), S1 (low rate of addition), S2 (medium rate), and S3 (high rate). Ten treatments with or without temporary straw addition at different rates were conducted to explore their effects on soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) under laboratory and field freeze–thaw conditions. Compared to constant temperature, the freeze–thaw process under laboratory conditions reduced MBC (5.79%~29.9%), whereas this trend was mitigated or reversed under field conditions. The alleviating effect of straw addition on the decrease in MBC was greater in S0 than in S1, S2, and S3 by an average of 31.7%. Medium rate straw application (S2 8 t/ha) provided appropriate labile C levels, enhancing microbial activity while keeping DOC low and reducing C loss risk. The results revealed discrepancies in freeze–thaw effects on soil labile OC between laboratory and field conditions, the mitigation of freeze–thaw impacts on MBC by straw addition, and the appropriate straw return rate in Liaohe Plain. Therefore, proper nutrient management can maintain and regulate microbial activity and soil labile C in areas with freeze–thaw cycles. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) is a major component in the global carbon cycle. Yet how input of plant litter may influence the loss of SOC through a phenomenon called priming effect remains highly uncertain. Most published results about the priming effect came from short-term investigations for a few weeks or at the most for a few months in duration. The priming effect has not been studied at the annual time scale. In this study for 815 days, we investigated the priming effect of added maize leaves on SOC decomposition of two soil types and two treatments (bare fallow for 23 years, and adjacent old-field, represent stable and relatively labile SOC, respectively) of SOC stabilities within each soil type, using a natural 13C-isotope method. Results showed that the variation of the priming effect through time had three distinctive phases for all soils: (1) a strong negative priming phase during the first period (≈0-90 days); (2) a pulse of positive priming phase in the middle (≈70-160 and 140-350 days for soils from Hailun and Shenyang stations, respectively); and (3) a relatively stabilized phase of priming during the last stage of the incubation (>160 days and >350 days for soils from Hailun and Shenyang stations, respectively). Because of major differences in soil properties, the two soil types produced different cumulative priming effects at the end of the experiment, a positive priming effect of 3-7% for the Mollisol and a negative priming effect of 4-8% for the Alfisol. Although soil types and measurement times modulated most of the variability of the priming effect, relative SOC stabilities also influenced the priming effect for a particular soil type and at a particular dynamic phase. The stable SOC from the bare fallow treatment tended to produce a narrower variability during the first phase of negative priming and also during the second phase of positive priming. Averaged over the entire experiment, the stable SOC (i.e., the bare fallow) was at least as responsive to priming as the relatively labile SOC (i.e., the old-field) if not more responsive. The annual time scale of our experiment allowed us to demonstrate the three distinctive phases of the priming effect. Our results highlight the importance of studying the priming effect by investigating the temporal dynamics over longer time scales.