Soil water stable aggregates and carbon and nitrogen storage enhanced by conversion of farmland to shrub and grass in China Loess Plateau: the influence of conversion cultivation time Effects of straw mulching and Vetiver grass hedgerows on the size distribution of the soil water stable aggregates and aggregate-associated organic carbon in red soil The role of increasing soil water-stable aggregates with diameter > 0.25mm by vegetation restoration in enhancement of soil organic carbon in the Loess Plateau