Transformed biosolids promote ryegrass growth and microbial carbon cycling at the 'cost' of soil carbon

Soil carbon supports desirable ecosystem functions for global agricultural productivity and climate resilience objectives. Wastewater biosolids can be transformed into soil amendments that return carbon and nutrients to agricultural systems in stoichiometric ratios that support carbon stabilisation. However, practicable delivery that enhances stable soil carbon and plant yield remains challenging. Soil carbon stability and nutrient availability are mediated partly by microbial community composition and function, which are poorly understood in soils amended with transformed biosolids. We conducted a 56-day study in a temperature-controlled glasshouse, growing perennial ryegrass (Lolium perenne) in pasture soil amended with straw, straw supplemented with nutrients, or transformed biosolids (composted biosolids, dried biosolids or biosolids biochar), all with equal added carbon (3500 kg ha(-1)). Control soils, with and without supplementary nutrients, were also included. Plant dry mass, soil chemical characteristics, and soil carbon fractions were measured at harvest. 16S rRNA sequencing was used to infer the composition and putative function of rhizosphere bacterial communities. Shoot dry mass increased for composted biosolids (236%) and dried biosolids (559%), but total carbon in rhizosphere soil decreased for composted biosolids (16.3%), dried biosolids (13.3%) and biosolids biochar (12.7%) when compared to unamended soils. Fine-fraction carbon in rhizosphere soil decreased for straw with supplementary nutrients (6.8%), dried biosolids (6.3%) and biosolids biochar (4.6%). Rhizosphere bacterial communities clustered by treatment, with populations correlated with fine-fraction carbon distinct from those populations correlated with shoot and root dry mass. Path analysis linked fine-fraction carbon loss with increased putative carbon cycling genes, driven by available nutrients and plant growth. Transformed biosolids can trigger a microbial response that reallocates nutrients from organic matter to plants, disrupting soil carbon-nutrient stoichiometry and facilitating carbon loss. Understanding the carbon cost of this ecosystem service is fundamental when translating benefits of transformed biosolids to end users.