Nitrogen fertilization affects plant decomposition both directly and through driving evolutionary shifts in rhizobium quality

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Lauren Martin, Mia Howard and Jennifer A. Lau, Biology, Indiana University, Bloomington, IN

Background/Question/Methods
Nitrogen is an essential resource that limits productivity in most ecosystems, affecting organisms from primary producers to decomposers. Thus, nitrogen availability typically limits both plant growth and rates of decomposition. To cope with nitrogen limitation, some plants, such as clover, form mutualisms with nitrogen-fixing rhizobia. These bacteria vary in their quality as mutualists, and previous work has demonstrated that continuous nitrogen fertilization results in the evolution of less cooperative rhizobia that provide fewer growth benefits to their host plants. As these evolutionary changes in mutualist quality affect host growth and tissue quality, we investigated whether these effects would scale up to alter the decomposition rates of their host plants. We hypothesized that decomposition rates are primarily driven by organic matter quality, and as such predicted that plants partnered with the more cooperative mutualists would decompose the fastest under both high and low soil nitrogen conditions. To test this, we grew clover in a greenhouse and inoculated them with strains of rhizobia that had evolved in fields with ambient (low) levels of nitrogen or that had been repeatedly fertilized with nitrogen for over two decades. We then let these plants decompose under high and low soil nitrogen conditions in a long-term nitrogen addition field experiment at the Kellogg Biological Station in Michigan and measured total biomass lost after six weeks.
Results/Conclusions
Plants tended to decompose faster in nitrogen-fertilized fields. Overall, the amount of decomposition was strongly correlated with plant size. When correcting for differences in initial plant sizes (e.g., greenhouse-fertilized plants and plants inoculated with high quality mutualist strains tended to be larger), we found that plants that had been inoculated with rhizobium strains evolved in low N environments tended to decompose faster than their counterparts inoculated with strains from fertilized environments, though the effects were small. These results indicate that nitrogen fertilization can not only speed up contemporary decomposition rates, but can also select for rhizobia that provide less growth benefits to their plant hosts and make them decompose more slowly.