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.
... mehr
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.