Distribution of organic carbon and nitrogen in soil aggregates of aspen (Populus simonii Carr.) woodlands in the semi-arid Loess Plateau of China

The distribution and turnover of organic carbon (OC) and nitrogen (N) associated with aggregates in soils is critical for understanding the behaviour of C and N in soils. We collected soil samples from aspen (Populus simonii Carr.) woodland in the semi-arid Loess Plateau of China to investigate the distribution of aggregate-associated OC and N. The distribution of aggregates and aggregate-associated OC and N were measured, and OC and N stocks in each aggregate fraction were calculated. Across the sites and soil depths, microaggregates and the silt + clay fraction dominated the distribution of soil aggregates, which varied with site. Organic C and N accumulated mainly in the macro- and micro-aggregate fractions in loamy soils but in the silt + clay fractions in sandy soils. The OC and N stocks in the bulk soil of aspen woodland were determined by the OC and N stocks associated with silt + clay fraction. The results of this study indicate that soil texture may play an important role in assessing the distributions of soil OC and N in both bulk soils and aggregate size fractions in aspen woodland, especially in semi-arid regions. Furthermore, the establishment of aspen woodland would result in greater accumulation of OC and N in loam soils than in sandy soils.