Winter nocturnal warming affects the freeze-thaw frequency, soil aggregate distribution, and the contents and decomposability of C and N in paddy fields

Abstract

Climate warming is expected to cause greater increases in nocturnal temperatures than daytime temperatures, thereby altering freeze-thaw cycles. Although the importance of freeze-thaw cycles in regulating soil aggregate stability and nutrient availability has attracted increasing attention, little is known about how winter nocturnal warming modulates freeze-thaw frequency, soil aggregate distribution, or the contents and mineralization of soil organic carbon (SOC) and total nitrogen (TN) in paddy fields. The nocturnal soil temperature in the upper 0–2 cm layer in a paddy field was elevated by approximately 2 °C using a passive nocturnal warming method during winter. An anaerobic experiment with a first-order reaction model was conducted to measure the C decomposition (C0) and N mineralization (N0) potentials in bulk soil and four soil aggregate fractions. Winter nocturnal warming significantly decreased freeze-thaw frequency and affected soil aggregate distribution and SOC and TN contents in <0.25 mm aggregate. Both SOC and TN fractions were significantly increased in the 0.25–1 mm aggregate but decreased in the >2 mm aggregate due to winter nocturnal warming. Winter nocturnal warming did not affect C0, N0, C0/SOC, and N0/TN in bulk soil. However, it decreased C0 and C0/SOC in all aggregates except the 0.25–1 mm aggregate, and increased N0 and N0/TN in all aggregates except the >2 mm aggregate. In the nocturnal warming treatment, the highest C0 and N0 values were found in the <0.25 mm aggregate, but only the N0 in the <0.25 mm aggregate was significantly larger than that in the other three soil aggregates. Our study indicated that winter nocturnal warming would reduce the freeze-thaw frequency and change C and N distributions in soil aggregates, resulting in increased soil N availability in the subsequent rice growth season.