In the first phase, we found that plant diversity was a significant control of the nitrogen cycle in soil. We hypothesize that the effect of plant diversity on the N cycle becomes increasingly stronger with proceeding establishment of the grassland mixtures. The reason for complementary N use in diverse plant mixtures may partly be attributable to the species-specific preferences for different inorganic and organic N forms. Furthermore, we observed that the concentrations of organic N forms in the soil solution decreased with increasing biodiversity. It is unclear, whether this is attributable to increasing uptake of organic N forms or N mineralization transforming organic to inorganic N. In this follow-up project, we have four objectives: (1) to continue to monitor all nutrient fluxes and set up a complete nutrient budget of one selected block of the Jena Experiment, (2) to explore the effect of the 60 plant species grown in monoculture on the concentrations of inorganic N, bulk organic N, and free and hydrolizable amino acids in soil solution, (3) to assess the effect of biodiversity on mineralization of organic matter and (4) to monitor the soil N concentrations in the (externally funded) mowing/N fertilizing experiment, in which it will be tested, if the N amendment influences the competition for N and thus the impact of biodiversity on soil N concentrations.

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