When waste becomes fertilizer: Can sludge-derived liquids reshape aquatic life in farmlands?

A new study provides critical insights into the environmental trade-offs of recycling sludge-derived liquids in agricultural systems.

Hydrothermal carbonization (HTC) is an emerging technology for converting sewage sludge and other high-moisture organic wastes into reusable products without energy-intensive drying. In addition to producing hydrochar, HTC generates a nutrient-rich liquid byproduct known as HAP, into which a substantial fraction of organic matter, nitrogen, and phosphorus is transferred.

Owing to its high nutrient content, HAP has been proposed as a soil amendment to reduce synthetic fertilizer inputs and enhance crop productivity. However, agroecosystems—particularly flooded systems like rice paddies—rely on complex microbial communities at the soil-water interface. How HAP affects periphyton biofilms and their ecological functions remains insufficiently understood.

A study (DOI:10.48130/aee-0025-0012) published in Agricultural Ecology and Environment on 29 December 2025 by Huifang Xie’s team, Nanjing University of Science and Technology, reveals how sludge-derived hydrothermal byproducts reshape microbial networks and ecosystem multifunctionality, providing a mechanistic basis for evaluating the ecological risks of nutrient recycling in agricultural systems.

Microcosm experiments

Using controlled microcosm experiments, researchers exposed periphyton communities to gradient concentrations of sludge-derived HAP and comprehensively assessed water physicochemical properties, microbial diversity and composition (Shannon, Chao1, NMDS), community assembly processes (niche breadth and normalized stochasticity ratio, NST), interdomain bacterial–eukaryotic networks, trophic functional profiles (FUNGuild, FAPROTAX), ecosystem multifunctionality, and predicted metabolic pathways (MetaCyc).

HAP rapidly altered water chemistry, initially suppressing dissolved oxygen—especially at high concentrations—while sharply increasing nitrogen loads; however, DO gradually recovered through photosynthesis, and NH4+-N, TN, and COD declined over time, with removal rates reaching up to 55% for COD and 35% for ammonium, demonstrating partial purification capacity.

Although periphyton biomass decreased with increasing HAP, α-diversity remained stable, whereas β-diversity shifted significantly, with enrichment of Bdellovibrionota and Chlorophyta and declines in Firmicutes and Mucoromycota. Niche breadth narrowed, particularly for eukaryotes, and bacterial assembly became more stochastic under higher HAP stress.

Network analyses

Network analyses revealed reduced connectivity, density, and complexity, alongside intensified competition and functional shifts toward chemoheterotrophy and nitrogen fixation. Environmental variables explained over 70% of community variation. Importantly, ecosystem multifunctionality declined significantly with increasing HAP and was strongly associated with community structure, niche breadth, and network complexity rather than species richness.

Metabolic predictions further showed suppression of key biosynthetic and nutrient metabolism pathways and enhancement of stress-related pathways, indicating adaptive but insufficient compensation. Overall, the findings demonstrate that HAP reshapes community assembly, trophic interactions, and functional expression in periphyton, with network integrity emerging as the primary driver of ecosystem functioning.

Dual nature

This study underscores the dual nature of sludge-derived HAP as both a nutrient resource and an ecological stressor. While periphyton can partially buffer HAP inputs by maintaining nutrient removal capacity, excessive application disrupts microbial network integrity and reduces ecosystem multifunctionality.

These findings highlight the need for ecological risk assessments that move beyond nutrient removal efficiency to include trophic interactions and interdomain network complexity. Careful optimization of HAP dosage and monitoring of microbial indicators will be essential to achieve sustainable nutrient recycling without compromising agroecosystem resilience.