Rapid Nitrogen Supply Increases N2O Losses in Organic Potato Systems

Nitrous oxide (N₂O) emissions are a major contributor to the greenhouse gas footprint of agricultural systems and are strongly influenced by nitrogen management. With the ongoing expansion and specialization of organic farming in Germany, an increasing number of farms operate without livestock, raising new challenges for nutrient supply. In stockless organic systems, clover grass is used in alternative ways instead of animal feed, resulting in different organic fertilizer forms with contrasting nitrogen availability, which may strongly affect N₂O emissions. However, field-based empirical data on these effects are still scarce.

This study assessed N₂O emissions from potato cultivation within a long-term organic field experiment established in 2017 at the experimental farm of the University of Kassel, Germany. The experiment compares different organic farm types and fertilization strategies, with a focus on stockless systems. During the 2024 growing season (May–September), N₂O fluxes were measured in three farm types using dynamic, non-transparent PVC chambers installed on permanently embedded soil frames. Chambers were equipped with internal fans, temperature sensors, vent, and pressure opening to ensure stable measurement conditions. In addition to ridge measurements, small PVC sampling tubes installed between ridges allowed spatially differentiated flux measurements across ridge and inter-ridge positions. Chamber air was continuously analyzed in real time using laser-based direct absorption spectroscopy (MIRA Ultra N₂O/CO₂, AERIS Technologies, USA).

The investigated systems included (i) a bio-vegan Cut & Carry system fertilized with tofu whey and fresh clover grass mulch, (ii) a soil fertility–oriented system fertilized with clover grass compost, and (iii) a mixed-farm system fertilized with cattle manure compost, with nitrogen application rates ranging from 55 to 67 kg N ha^-1. Across all systems, N₂O emissions exhibited pronounced temporal dynamics, with the highest fluxes occurring after spring fertilization, incorporation of organic fertilizers, and mechanical disturbance such as ridge harrowing. Additional emission peaks were observed after ridging operations and after harvest.

Cumulative N₂O emissions over the growing season were consistently higher on ridges than between ridges (42.7 %). The bio-vegan Cut & Carry treatment showed the highest cumulative N₂O emissions (average across on- and between ridge positions: 85 mg m^-2), attributed to rapidly available nitrogen from tofu whey combined with fresh clover grass mulch. In contrast, compost-based fertilization strategies resulted in lower emissions (average across on- and between ridge positions:  53–60 mg m^-2), likely due to higher C/N ratios and slower nitrogen release. Despite these differences, potato yields did not differ significantly among systems.

The results demonstrate that rapid nitrogen availability in stockless organic systems can substantially increase N₂O losses without providing yield benefits. Compost-based fertilization strategies appear more effective in mitigating N₂O emissions while maintaining productivity, highlighting the importance of carefully designed clover grass utilization and nutrient transfer strategies for climate mitigation in stockless organic farming systems.