Soil organic matter and glomalin-related soil protein contents do not explain soil aggregate stability after freeze-thaw cycles at contrasting soil moisture contents

Soil organic carbon (SOC) and glomalin-related soil protein (GRSP) contributes to the formation and stability of soil aggregates, but it remains unknown whether the frequency of freeze-thaw cycles (FTCs) and soil moisture content (SMC) at freezing changes aggregate stability and whether SOC and GRSP regulate these changes. Effects of FTCs (0, 1, 3 and 9 times) and initial SMC (40% and 80% field capacity) on aggregate distribution, SOC, and easily extractable, difficultly extractable and total GRSP content in soil aggregates (> 2000 μm, 250–2000 μm, and <250 μm) were evaluated in Chinese pine forest soil. FTCs significantly reduced MWD by disrupting large macro-aggregates. On the other hand, higher SMC increased MWD by binding soil particles, the effect of which was much greater than the disruptive effects of FTCs. SOC in bulk soil and aggregate fractions slightly decreased with the frequency of FTCs. Bulk and aggregate-associated GRSP fractions first decreased, and then increased with the frequency of FTCs. SOC and GRSP fractions correlated insignificantly with MWD. Our results indicated that increasing SMC could augment soil aggregate stability, and that SOC and GRSP might not be the primary control on aggregate stabilization during freeze-thaw conditions in Chinese pine forest soil.