Effects of freeze-thaw cycles and soil moisture content on soil available micronutrients on aggregate scale in natural grassland and Chinese pine forestland on the Loess Plateau, China

PURPOSE: Micronutrient presence and concentration are strongly influenced by soil aggregate size. Seasonal freeze-thaw patterns are known to modify soil aggregate distributions, thereby contributing to the transformation and redistribution of available soil micronutrients. Few studies, however, have evaluated the response of aggregate-associated available micronutrients to freeze-thaw cycles (FTCs) under different types of vegetation restoration. MATERIALS AND METHODS: We designed a laboratory experiment to simulate the three typical types of ecosystems on the Loess Plateau in China (cropland, Chinese pine forestland, and natural grassland). In this experiment, we measured the effects of number of FTCs (0, 1, 3, and 9) and soil moisture content (SMC) (40 and 80% field capacity) on available micronutrients (Cu, Fe, Mn, and Zn) in bulk soil and three soil aggregate size classes. RESULTS AND DISCUSSION: FTCs significantly increased available micronutrient content in bulk soil by increasing > 2000 μm aggregate-associated available micronutrient contents and decreasing < 250 μm aggregate-associated available micronutrient contents. In most cases, the influences of FTCs on bulk soil and aggregate-associated available micronutrient content were enhanced by increased SMC. Compared with cropland soil, the bulk soil available Cu, Fe, Mn, and Zn in natural grassland soil and available Cu in Chinese pine forestland soil were significantly increased. The increasing effects of FTCs on the available micronutrient content in cropland and natural grassland soils were larger than that in Chinese pine forestland soil. CONCLUSIONS: The results indicated that increasing FTCs and SMC was beneficial for alleviating available micronutrient deficiencies in soil, and natural grassland was better than Chinese pine forestland for increasing available micronutrients in this loess-derived soil.