Water hyacinth (Eichhornia crassipes) mulch and urea interactions shape soil biota and nitrogen cycling

Organic amendments influence soil biological activity and nutrient availability depending on their composition and decomposition rate, and invasive plants such as water hyacinth ( Eichhornia crassipes ), one of the world’s most widespread aquatic weeds, represent a potential source of nutrient-rich organic matter that can be repurposed as a soil amendment. This study tested the effects of water hyacinth mulch, a nutrient-rich and rapidly decomposable biomass, compared with woody mulch and an unmulched control across macadamia, lychee, and grazing systems in subtropical Queensland, Australia. Plots received mulch with or without ¹⁵N-labelled urea, and soil bacteria, fungi, and nematodes were characterised using metabarcoding to assess treatment effects on microbial communities and nitrogen dynamics over seven weeks. Water hyacinth mulch reduced bacterial diversity but increased bacterial-feeding nematodes and shifted fungal composition toward saprotrophic decomposers. Bulk soil δ ¹⁵N was lower under water hyacinth mulch in fertilised macadamia soils, suggesting faster nitrogen turnover at that site, though no isotopic effect was detected at the lychee or pasture sites. These microbial shifts are consistent with accelerated nutrient mineralisation driven by labile organic matter, though effects on nitrogen cycling were context-dependent and require validation through direct process measurements. Combining water hyacinth mulch with more recalcitrant organic inputs may help moderate mineralization rates and improve nutrient use efficiency. Extended field trials are needed to evaluate whether these short-term responses translate to sustained improvements in soil function and long-term fertility. • Water hyacinth mulch reduced bacterial diversity and altered soil bacterial composition relative to woody or unmulched soils. • Urea addition suppressed diversity across systems, indicating nutrient-driven community simplification. • Stable isotope evidence and microbial trait shifts suggest water hyacinth mulch may cause context-dependent accelerated short-term nitrogen turnover. • Water hyacinth mulch shifted fungi toward saprotrophic decomposers and enriched bacterial-feeding nematodes correlated with soil potassium. • Blending water hyacinth mulch with recalcitrant inputs may moderate rapid nutrient release, though field validation needed.