A single high compost application accelerates organic matter restoration in marginal soil and enhances C and nutrient retention

Current German regulations on compost application rates may constrain organic C restoration and agricultural productivity in C-poor marginal soils. The objective of our study was to determine whether applying municipal organic waste compost to reclaimed soil with 0.45% organic C (dry matter), at rates higher than currently allowed, could be used to accelerate organic C enrichment without substantial C, N, and P losses over a full crop rotation. In an outdoor lysimeter experiment conducted from September 2019 to March 2023 on marginal silty loam soil under a winter wheat–winter barley–spring field pea rotation, compost was applied at either a regular rate of 4000 g m −2 (40 Mg ha −1 ) or a high rate of 8000 g m −2 (80 Mg ha −1 ) and compared with mineral-fertilized controls. We assessed crop performance, soil C and nutrient status, greenhouse gas emissions, water and nitrate leaching, and diverse microbial indicators. The regular compost application increased the soil C level by 40% and enhanced mean crop biomass by 13% compared to the control. The high compost application rate doubled the soil C content and led to the lowest inter-replicate variability in grain yield over the three-year study period. There was no obvious decline in soil organic C levels over the crop rotation. Both regular and high compost application rates reduced water leaching by about 43% and did not significantly increase nitrate leaching or nitrous oxide emissions. Compost application increased mean annual cumulative carbon dioxide emissions by about 17%, while the high compost rate showed no clear additional effect. High compost application altered microbial indicators associated with nutrient cycling and reduced the relative abundance of fungal taxa assigned as plant pathogens in the year following application. Overall, the higher compost application combined with adjusted mineral fertilization enhanced C and nutrient retention without clear increases in nutrient losses over the three-year crop rotation, supporting its potential use for organic C restoration while emphasizing the need for long-term, multi-site studies with environmental monitoring to provide a basis for revising current regulations.