Effects of freeze-thaw cycles and initial soil moisture content on soil aggregate stability in natural grassland and Chinese pine forest on the Loess Plateau of China

Abstract

Ongoing global warming is decreasing the thickness of snow covers and increasing initial soil moisture content (SMC), which increase the number of freeze-thaw cycles (FTCs) at mid-high latitudes and high elevations, especially at temperate regions. FTCs substantially affect the stability of soil aggregates, which may increase soil erosion. A comprehensive understanding of aggregate stability under different types of vegetation restoration in response to FTCs, however, has not yet been attained. We evaluated the effects of number of FTCs (0, 1, 3, and 9) and initial SMC (40, 60, and 80% field capacity) on aggregate distribution and stability in cropland, natural grassland, and Chinese pine forestland, the three typical types of vegetation on the Loess Plateau in China. The experiment was conducted under simulated conditions in the laboratory using disturbed soil samples. Most aggregate-size fractions and mean weight diameters (MWDs) were significantly (P < 0.05) affected by FTCs, initial SMC, and vegetation types. FTCs significantly decreased MWD by 3.6–18.1% through disrupting larger macro-aggregates, which increased with SMC. Increased SMC increased MWD by 2.0–53.0% through binding soil particles, the effect of which was much larger than the disruptive effects under the freeze-thaw conditions, especially at high SMC. MWD was in the order of cropland < Chinese pine forestland < natural grassland under each freeze-thaw condition. The results indicated that natural vegetation succession was better than Chinese pine forest plantation for resisting seasonal FTCs and that aggregate stability may increase due to increased initial SMC under scenarios of future global warming.