In many grasslands, the supply rate of nitrogen (N) to plants can influence plant productivity and community structure.  Plant species can alter net N supply rates, and studies have hypothesized that species differences in carbon inputs to the soil, particularly input quantity and quality, can explain species differences in net N supply.  However, in order to evaluate species impacts on net N supply, we need to examine how plant species identity and carbon inputs impact soil microbial functioning, as the net N supply is the result of soil microbial transformations.  These underlying microbial transformations can be affected by two factors: 1) changes in microbial activity and 2) soil N content.  Microbial activity can be strongly influenced by plant carbon inputs while soil N content is a measure of the size of the substrate which is utilized by the microbial community to produce plant-usable forms of N in the soil.  Therefore we hypothesized that changes in plant identity and plant carbon inputs to the soil can indirectly change gross N mineralization rates through changes in soil microbial activity and that soil N content would be positively related to gross N mineralization.  Along a natural gradient of soil N content in successional grasslands in central MN, we conducted a survey of six grass species, which differed in their inputs to the soil microbial community.  We measured species inputs as coarse and fine root quantity (root carbon) and quality (root C:N ratio), microbial respiration as proxy for microbial activity (substrate induced respiration), gross N mineralization (isotopic-pool dilution), and total soil N content.  

Results/Conclusions

Using path analysis we found a positive relationship between plant carbon input quantity and soil microbial respiration (P=0.009).  We did not find any relationship between input quality and microbial respiration (P>0.05).  In addition we found a positive relationship between microbial respiration and gross N mineralization (P<0.001).  We also calculated indirect effects of species inputs quantity on gross N mineralization and found a significant positive indirect effect.  Finally, we found positive relationships between soil N content and microbial respiration, and gross N mineralization.  In total our results illustrate that both microbial activity and soil N content gross N mineralization.  While changes in soil N content effect the size of the substrate pool from which N is mineralized, changes in plant carbon inputs associated with different species influence microbial activity and therefore indirectly change gross N mineralization.