Soil organic carbon accrual potential of anaerobically digested organic fertilizers 100%

Anaerobically digested organic fertilizers such as digested slurry, liquid and solid digestate are progressively becoming popular alternatives to slurry or manure in organic agriculture. The additional anaerobic digestion step reduces labile organic carbon contents in the fertilizers compared to undigested slurry and manure. However, little is known about the soil organic carbon accrual potential of such fertilizers compared to undigested organic fertilizers under field conditions.

The project Recycle4Bio (jointly managed by FiBL and Agroscope) aims to evaluate and further optimize the agronomical performance of anaerobically digested organic fertilizers with particular attention to their nitrogen-use-efficiency and their climate impact. In a field trial in Wallbach (CH), different organic fertilizers have been applied yearly since February 2018. The goal of this thesis is to assess the potential for soil organic carbon accrual resulting from field application of anaerobically digested organic fertilizers over six years (2018-24).

In this thesis, soil organic carbon stocks will be quantified for:

• 9 treatments (each with 4 field replicates): 2 controls (unfertilized, minerally fertilized), 4 liquid fertilizers (cattle slurry, digested slurry ± biochar, liquid digestate), 3 solid fertilizers (solid digestate, composted solid digestate ± biochar)
• 3 soil depths: 0-20 cm, 20-40 cm, 40-60 cm
• 2 soil organic carbon fractions: particulate organic matter (POM), mineral-associated organic matter (MAOM).

In addition, physical and biological soil quality parameters will be determined in the topsoil (0-20 cm).

Collected data aims to address the following research questions:

1. Does the application of anaerobically digested organic fertilizers enhance soil organic carbon accrual compared to cattle slurry application (business as usual reference)?
2. Does the application of anaerobically digested organic fertilizers enhance soil organic carbon accrual in the MAOM fraction compared to cattle slurry application (business as usual reference)?
3. Does the application of anaerobically digested organic fertilizers influence aggregate stability, soil water tension and microbial activity?

Earliest February 2024, latest July 2024

Dr. Else Bünemann-König

Lucilla Agostini

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