Biogas slurry application offers an alternative to chemical fertilizer in realizing ecologically recycling agriculture. However, the responses of soil fertility and microbial communities to long term use of biogas slurry need to be explored in different soil types and regions. We investigated the effects of repeated applications over six years on the soil properties and microbial characteristics of a fluvo-aquic soil in the North China Plain. The experiment, with equivalent nitrogen inputs, comprised: biogas slurry (BS), chemical fertilizer (CF) or substitution of half the chemical fertilizer with biogas slurry (BSCF); a control treatment had no fertilizer addition. Soil samples, at a depth of 0–20 cm, were collected for their physicochemical properties. Microbial community diversity and composition was investigated using high-throughput sequencing. Biogas slurry application treatments tended to lower the soil bulk density while increasing the water-holding capacity and the water-stable aggregate mean weight diameter. Organic carbon and available nutrient concentrations (nitrogen, potassium and phosphorus) were enhanced in all fertilization treatments relative to the control, especially in the BSCF treatment. Significant differences in microbial community composition were detected between the control and all of the fertilization treatments. BSCF resulted in the greatest diversity and most evenly balanced assemblages of both bacteria and fungi at the phylum level. There were clear associations between microbial composition and changes in soil environmental variables caused by the fertilization treatments. Bacterial community composition and alpha diversity were associated particularly with differences in soil total nitrogen, pH, and available potassium, whereas fungal communities were more related to available potassium. Half substitution of the chemical fertilizer by biogas slurry gave the greatest improvement in soil structure and nutrient availability and this was associated with greater microbial diversity and better balanced microbial communities. Our results suggest that partial substitution with biogas slurry is an alternative to complete chemical fertilizer and that it offers clear benefits for the topsoil structure and fertility in fluvo-aquic soils. It also represents a promising approach to a biogas-linked agroecosystem that restores sustainable coordination between cropping and animal husbandry under an intensive production regime.

Excessive use of nitrogen fertilizers is a common problem for intensively cultivated cropping systems. Introducing cover crops during fallow periods is a good practice for encouraging the sustainable use of N resources in such cropping systems. Although individual previous studies assessing the interception of soil residual N by cover crops and its mineralization for subsequent N supply abound, a comprehensive synthesis of cover crop effects on N retention and supply from such studies is still lacking, especially in fertilizer-intensive cropping systems. This paper aims to assess the influences of cover crop practices on soil N retention and supply on the basis of published literatures. Cover crop biomass accumulation, N absorption capacity, root-induced rhizosphere deposition, N-stratification and biological nitrification inhibition all play a part in temporal N storage and leaching reduction. Excessive pre-absorption, residue-induced immobilization or the negative priming effect can result in a negative after-effect, increasing the risk of yield reduction and reducing the likelihood of cover crop introduction. Comparisons between different cover crops, especially the mostly reported leguminous and non-leguminous species, are carried out in this review and the influences of cover crop management practices including species selection, planting and termination time are also discussed. There is a need to develop adaptive cover crop management practice for specific cropping systems based on its effects on N retention and supply.