CARBON-ISOTOPE DYNAMICS DURING GRASS DECOMPOSITION AND SOIL ORGANIC-MATTER FORMATION

We analyzed changes in the stable C isotope composition (delta(13)C) 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 (C-4), Agropyron repens (C-3), Poa pratensis (C-3), and Agrostis scabra (C-3). Although lignin concentrations increased for all litter types during decomposition and lignin fractions were consistently depleted in C-13 compared to bulk tissues (3.6 parts per thousand more negative on average), we found neither convergence of bulk tissue delta(13)C values towards lignin delta(13)C values, nor greater stability of delta(13)C values for lignin fractions. Furthermore, delta(13)C values of C-3 and C-4 species shifted in opposite directions during decomposition. Thus, our data do not support the hypothesis that delta(13)C values decrease during decomposition because of the selective preservation of lignin and we instead suggest that 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 C-4 grass (Andropogon gerardi) growing on initially homogeneous soils with a predominantly C-3 isotopic signature, soil delta(13)C values increased 1.6-2.2 parts per thousand for the C-4 species and remained relatively unchanged for the C-3 species after 4 yr. Averaging across the C-4 species and the experimental soil organic matter gradient, 14% of the total soil C in these plots must be new C-4 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.