Effects of freeze–thaw on aggregate stability and the organic carbon and nitrogen enrichment ratios in aggregate fractions

Soil samples from the Hexi Corridor located in the arid regions of Northwestern China were collected from a site that had received fertilizer applications for 23 years. Effects of freeze–thaw on aggregate stability and the organic carbon (OC) and nitrogen (N) enrichment ratios in water stable aggregate (WSA) fractions were investigated. In treatments combining the application of N fertilizer with green manure (GN) or straw (SN), the percentage of >0.25 mm WSA fraction was not significantly different from the control soil that received no fertilizer or organic amendment. After a freeze–thaw cycle, the percentages of the >0.25 mm WSA fraction in the GN and SN treatments showed no change, but the size of this fraction in the other treatments decreased. In addition, the organic carbon (OC) and N enrichment ratios in the >0.25 mm WSA fraction in GN and SN treatments increased after a freeze–thaw cycle. However, in this size fraction, the OC and N enrichment ratios decreased in other treatments. Both the changes of the percentages of the >0.25 mm WSA fraction and the OC and N enrichment ratios in this fraction under freeze–thaw in the GN and SN treatments exhibited the most significant increases compared with other treatments (P