Soil carbon response to long‐term biosolids application

Summary

This 20-year field study evaluated soil carbon accumulation in three California agroecosystems following long-term biosolids amendment, with cumulative applications ranging from 74 to 359 Mg ha⁻¹. Contrary to a simple dose-response relationship, the site receiving the lowest biosolids loading (Solano) achieved the highest carbon storage rate (1.0 ± 0.2 Mg C per Mg biosolids per year) and 50% greater SOC to 100 cm depth. The findings emphasise that local soil conditions controlling carbon stabilisation are more critical than amendment quantity alone for predicting climate mitigation benefits, and that measuring carbon changes beyond 30 cm depth is essential for assessing true sequestration potential.

UK applicability

These findings may be relevant to UK agricultural policy regarding waste-to-land practices and carbon accounting methodologies, particularly as biosolids application is regulated under the Quality Protocol. However, the Californian semi-arid climate, soil types, and management practices differ from UK conditions, so site-specific validation would be necessary before applying these carbon sequestration rates to UK agroecosystems.

Key measures

Soil organic carbon (SOC) and total nitrogen concentrations (0-100 cm); microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) (0-30 cm); net changes in soil carbon stocks (Mg C ha⁻¹ yr⁻¹); carbon storage efficiency per unit biosolids applied

Outcomes reported

The study measured soil organic carbon (SOC) concentrations, total nitrogen, microbial biomass carbon and nitrogen, and net changes in soil carbon stocks to 100 cm depth across three California sites that received biosolids applications over 20 years. Sites showed variable carbon storage rates per unit biosolids applied, ranging from 0.04 to 1.0 Mg C per dry Mg biosolids per year, with greatest increases at the site receiving the lowest cumulative biosolids loading.

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