Algae growth under different fertilizer regimes in Swiss rice paddies

As rice cultivation expands into temperate regions such as Switzerland, understanding ecological dynamics within rice paddies becomes crucial for sustainable agricultural practices. This thesis explores the interactions between macroscopic green algae and rice cultivation, focusing on algal composition of Swiss rice paddies, influence of fertilizer on algae growth, and crop performance under algae presence. Through an integrative approach, the study was divided into three sub-projects: field monitoring of Swiss rice paddies (1), fertilizer experiment in the paddy of La Sauge (2), and a greenhouse experiment (3). In sub-project 1, field observations across multiple sites in the canton of Aargau identified Chara spp. and Hydrodictyon spp. as the dominant algal genera. Algal presence varied between sites, influenced by factors such as paddy age, water depth, nutrient inputs, and shading from rice or floating plants. While some paddies showed high algal coverage early in the season, none exhibited viable algal communities when rice was maturing, suggesting a natural die-off due to shading, nutrient limitations, or initiated drying of the paddy. Sub-project 2 focused on the impact of varying fertilizer regimes on algae. Despite high initial algal coverage across all treatments, no significant effect of fertilizer amount or timing on algal biomass was observed. Algal decline over time was attributed to rice canopy shading. Water chemistry only varied slightly across treatments, with pH and temperature decreasing, and conductivity increasing over the growing season. Sub-project 3 provided insights into causal relationships between algae, fertilization, and rice performance. Here, algae flourished in unfertilized and organically fertilized conditions but declined under mineral fertilization, likely due to nutrient imbalances or competitive exclusion by fast-growing rice. Algae altered water parameters, particularly in unfertilized systems, by raising pH and temperature, while reducing conductivity. Interestingly, algae increased rice biomass in nutrient-poor conditions but had negligible or slightly negative effects under high fertilization (110 kg N ha-1). Algal presence also affected nitrogen dynamics by reducing Nitrogen Uptake Efficiency of rice plants in fertilized treatments. Further, contributing to ammonia volatilization, thus reducing nitrogen availability. Collectively, this study demonstrates that macroscopic green algae have a measurable influence on nutrient dynamics and rice quality in Swiss paddies. These findings highlight the importance of considering algal dynamics in the management of temperate rice paddies, to optimize nutrient efficiency, and promote sustainable cultivation practices.