Carbon Isotope Dynamics During Grass Decomposition and Soil Organic Matter Formation

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Abstract
We analyzed changes in the stable C isotope composition (°13C) of bulk tissues and lignin fractions during a 2—yr decomposition study in east—central Minnesota (USA) of aboveground and belowground litter from four perennial grass species: Schizachyrium scoparium (C4), Agropyron repens (C3), Poa Pratensis (C3), and Agrostis scabra (C3). Although lignin concentrations increased for all litter types during decomposition and lignin fractions were consistently depleted in 13C compared to bulk tissues (3.6% more negative on average), we found neither convergence of bulk tissue °13C values towards lignin °13C values, nor greater stability of °13C values for lignin fractions. Furthermore, °13C values of C3 and C4 species shifted in opposite directions during decomposition. Thus, our data do not support the hypothesis that °13C values decrease during decomposition because of the selective preservation of lignin and we instead suggest the isotopic shifts are caused by the incorporation of new C from soil organic matter into litter by microbial decomposers. We estimate that this new C comprised 12—19% of the total litter C, depending on species, at the point of 70% mass loss. In monocultures of these four species plus another C4 grass (Andropogon gerardi) growing on initially homogeneous soils with a predominantly C3 isotopic signature, soil °13C values increased 1.6—2.2 for the C4 species and remained relatively unchanged for the C3 species after 4 yr. Averaging across the C4 species and the experimental soil organic matter gradient, 14% of the total soil C in these plots must be new C4 C to account for this isotopic shift. We estimate that this amount of new soil C equals 30% of NPP summed over 4 yr in these plots.

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