Plant effects on soil N mineralization are mediated by the composition of multiple soil organic fractions
Despite the topic of soil nitrogen (N) mineralization being well-studied, very few studies have addressed the relative contribution of different plant and soil variables in influencing soil N mineralization rates, and thus the supply of inorganic N to plants. Here, we used data from a well-studied N-limited grassland to address the relative effects of six plant and soil variables on net and on gross rates of soil N mineralization. We also addressed whether plant effects on soil N mineralization were mediated by changes in C and N concentrations of multiple soil organic matter (SOM) fractions. Regression analyses show that key plant traits (i.e., plant C:N ratios and total root mass) were more important than total C and N concentrations of bulk soil in influencing N mineralization. This was mainly because plant traits influenced the C and N concentration (and C:N ratios) of different SOM fractions, which in turn were significantly associated with changes in net and gross N mineralization. In particular, C:N ratios of a labile soil fraction were negatively related to net soil N mineralization rates, whereas total soil C and N concentrations of more recalcitrant fractions were positively related to gross N mineralization. Our study suggests that changes in belowground N-cycling can be better predicted by simultaneously addressing how plant C:N ratios and root mass affect the composition and distribution of different SOM pools in N-limited grassland systems.
KeywordsEcosystem process Nitrogen cycling Soil density fractionation Soil organic matter
We thank Stephen Hart and Dan Binkley, who helped greatly in the interpretation of our results. Saran P. Sohi gave us helpful suggestions on how to perform soil density fractionation analyses. This research was supported by a grant from the University of Minnesota’s Initiative on Renewable Energy and the Environment, by the LTER program of the US National Science Foundation (NSF/DEB-0620652) and a Marie Curie Outgoing Fellowship issued to D.A.F. within the Work Programme 2004, “Structuring the European Research Area” (2002–2006).
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