Impact of grassland degradation on the distribution and bioavailability of soil silicon: Implications for the Si cycle in grasslands.

Grassland ecosystems play an important role in the global terrestrial silicon (Si) cycle, and Si is a beneficial element and structural constituent for the growth of grasses. In previous decades, grasslands have been degraded to different degrees because of the drying climate and intense human disturbance. However, the impact of grassland degradation on the distribution and bioavailability of soil Si is largely unknown. Here, we investigated vegetation and soil conditions of 30 sites to characterize different degrees of degradation for grasslands in the agro-pastoral ecotone of northern China. We then explored the impact of grassland degradation on the distribution and bioavailability of soil Si, including total Si and four forms of noncrystalline Si in three horizons (0-10, 10-20 and 20-40 cm) of different soil profiles. The concentrations of noncrystalline Si in soil profiles significantly decreased with increasing degrees of degradation, being 7.35 ± 0.88 mg g, 5.36 ± 0.39 mg g, 3.81 ± 0.37 mg g and 3.60 ± 0.26 mg g in non-degraded, lightly degraded, moderately degraded and seriously degraded grasslands, respectively. Moreover, the storage of noncrystalline Si decreased from higher than 40 t ha to lower than 23 t ha. The corresponding bioavailability of soil Si also generally decreased with grassland degradation. These processes may not only affect the Si pools and fluxes in soils but also influence the Si uptake in plants. We suggest that grassland degradation can significantly affect the global grassland Si cycle. Grassland management methods such as fertilizing and avoiding overgrazing can potentially double the content and storage of noncrystalline Si in soils, thereby enhancing the soil Si bioavailability by >17%.