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.

Soil bacteria, fungi and nematodes are three of the most abundant groups in the belowground micro-food web, yet how they change together during forest recovery after disturbance remains poorly understood. We examined a temperate forest secondary successional sequence in northeastern China, measuring soil nutrients and the diversity, taxonomic composition and functional composition of the three groups, and used structural equation modelling to test how their linking network changes among successional stages. Nutrients did not accumulate uniformly. Available potassium increased stage by stage, carbon and nitrogen fluctuated without a clear trend, and phosphorus peaked at the poplar stage. The diversity of each group tracked a characteristic nutrient most closely, potassium for fungi, nitrogen for nematodes, and carbon broadly for both microbial groups. Taxonomic and functional responses were decoupled. Bacterial and fungal communities diverged strongly among stages, while nematode taxonomic composition stayed largely unchanged, yet nematode feeding composition shifted markedly and fungivorous nematodes reached their highest share at the intermediate broadleaf stage. The pathway network linking nutrients, microorganisms and nematodes kept the same structure in all four stages, so succession changed the strength of these associations rather than the pathways themselves. For soil nematodes, recovery therefore reshapes what the community consumes rather than which taxa it contains, and feeding traits rather than taxonomic counts better capture how the nematode community tracks forest recovery.