Agriculture is facing the challenge of feeding a growing world population. At the same time, concerns about the environmental effects of current and historical fertiliser use, as well as global climate change, requires re-thinking of how food is produced.
This PhD study focused on how historical nutrient inputs from synthetic fertilisers and animal manure, and variable soil fertility conditions, affect growth, productivity, and resilience of cereal crops, and examined the effects on soil phosphorus pools and soil microbial communities. The study included 20 year year of experimental field data, new field experimentation and lab and green-house trials with soils from the Long-Term Nutrient Depletion Trial at the KU experimental farm in Taastrup, Denmark. The field was purposely depleted of nutrients for 30 years before the introduction of different permanent nutrient application treatments with synthetic fertilisers and animal manure, which have by now been on-going for 20 years and can therefore provide relevant information for future management strategies.

In most of the years, reasonably good yields could be attained, but it was clear that sufficient nutrient availability is essential at critical growth stages of the crop, especially the early establishment phase. In combination with a detailed study of soil P availability and forms, these results give a preliminary indication that on this soil, shifting from the traditional cropping methods to new strategies with lower P inputs may be feasible.
Long-term animal slurry applications resulted in a higher soil pH, greater C accumulation and microbial P concentrations, and had a strong enhancing effect on soil bacterial diversity and microbial substrate use. All in all, it is clear that these inputs have additional benefits, over and above the nutrient contents.

Finally, plants that are better at taking up P from the soil under limiting conditions could play an important role in improving sustainability, but experiments with modern cereal varieties suggest that modern breeding has resulted in varieties with rather similar ability for P uptake under low availability conditions. Breeding programmes should focus on improving current crop varieties to perform better under low-P conditions.