Norwegian soil: exploring soil health at the plot-, field- and regional scale

Soil health reflects the physical, chemical, and biological condition of the soil and its ability to support multiple ecosystem services, including food production, water regulation, climate regulation, and biodiversity. However, various degradation processes caused by human activities and climate change threaten soil health and its functions. Erosion, carbon loss, and soil compaction are examples of processes that negatively affect soil health and hinder optimal soil functioning. Norwegian soils, shaped by climate, topography, and geological history, vary significantly across regions. For instance, Western Norway’s soils, rich in organic matter due to high rainfall, differ from the clay-rich soils of Trøndelag and the Oslo region, formed from post-glacial marine deposits. These variations influence soil properties, such as water retention and nutrient availability, and underscore the need for regionally adapted soil health assessments. This dissertation explores soil health across multiple scales—regional, field, and plot. Using an existing database, chemical soil indicators were analysed to develop region specific-reference curves for Southeast- and Mid-Norway (Chapter 2). These curves provide a novel framework for assessing soil health, allowing a farm's soil to be benchmarked against the typical range for its specific soil type and climate. The curves need further development to incorporate more soil functions beyond productivity. On-farm experiments (Chapter 3) revealed that conservation practices, such as reduced tillage, cover cropping, and crop diversification, enhanced soil structure and biological activity—evidenced by higher aggregate stability and a fourfold increase in earthworms compared to a conventionally managed field. While these findings are significant, the study highlights that further research comparing more farms is needed to strengthen and validate these results. A plot experiment (Chapters 4 & 5) further examined key soil functions, showing that cover crops, were successfully established even at 63.9°N latitude, without reducing barley yields. The research demonstrated that cover crops contributed to nutrient cycling; for instance, they captured significant amounts of phosphorus in their biomass, while the high carbon to phosphorus ratio (C:P ratio) of certain mixtures was shown to cause a temporary immobilization of plant-available phosphorus. Furthermore, the research revealed that root biomass was the key driver for the formation of stable, mineral-associated organic matter (MAOM). These findings emphasize that management practices must be tailored to specific objectives and local conditions. A key outcome of this research is a set of recommendations for improving soil stewardship in Norway. The thesis identifies an urgent need for a national soil health monitoring framework and concludes that future assessments must prioritize plant-soil interactions and utilize adaptive, pedo-climatic benchmarks to be effective (Chapter 6).