Responses of water-stable aggregates, their associated organic carbon and crop yield to the application of biogas slurry in a fluvo-aquic soil of the North China plain

Crop productivity under intensive agriculture depends on the maintenance of fertility and soil structure. Chemical fertilizers are effective in supplying nutrients for crops but eventually lead to loss of the soil organic carbon that is important in supporting soil structure. Biogas slurry is an alternative to chemical fertilizers that potentially can both provide nutrients and promote organic carbon content. A field experiment was conducted to compare the effects of applying biogas slurry (BS), chemical fertilizer (CF) or a combination of both with 50% of nitrogen derived from each (BSCF) and no fertilizer application control (CK) to a fluvo-aquic soil of the North China Plain. All fertilizer application treatments had equal nutrient supply and were continuously applied over 4 years. Annual yields of wheat and maize were determined. Water-stable aggregates in size classes >5 mm, 2-5 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm in the topsoil of 20 cm were separated by wet sieving; mean weight diameter (MWD), geometric mean diameter (GMD), percentage of aggregates destruction (PAD) and fractal dimension (D) were derived. The concentrations of organic carbon associated with the aggregates were measured, and the distribution of soil organic carbon (SOC) was quantified. Fertilizer application treatments enhanced total crop yield by 49.1%-75.0%. Fertilizer application also increased the mass proportions of water-stable macro-aggregates by 81.6%-164.4%, MWD by 100.0%-264.3%, GMD by 54.5%-227.3%, while decreasing PAD by 15.4%-47.8% and D by 1.0%-3.8% (all relative to CK). The highest proportions of macro-aggregates, the greatest aggregate stability and the greatest crop yield all resulted from BSCF. All fertilizer application treatments substantially increased the SOC content of water-stable aggregates, with greater relative contribution in macro-aggregates. Increased SOC in macro-aggregates was mainly attributable to mass proportion changes rather than changes in their SOC concentrations. However, SOC concentration tended to account for a larger proportion of the total changes in micro-aggregates. BSCF also produced the highest total soil organic carbon stocks in aggregates (40.3% and 57.3% greater than CF and CK, respectively). Our work demonstrates that half substitution of chemical fertilizer with biogas slurry is a practical management to enhance soil aggregation and its stability that is associated with a higher crop yield than either treatment individually. It also promotes organic carbon accumulation in soil aggregates, which will support sustainable agricultural development in fluvo-aquic soil.