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

We analyzed changes in the stable C isotope composition (delta(13)C) o f 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), Agro pyron repens (C-3), Poa pratensis (C-3), and Agrostis scabra (C-3). Al though lignin concentrations increased for all litter types during dec omposition and lignin fractions were consistently depleted in C-13 com pared to bulk tissues (3.6 parts per thousand more negative on average ), we found neither convergence of bulk tissue delta(13)C values towar ds lignin delta(13)C values, nor greater stability of delta(13)C value s 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 du ring decomposition because of the selective preservation of lignin and we instead suggest that isotopic shifts are caused by the incorporati on of new C from soil organic matter into litter by microbial decompos ers. 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 monoculture s of these four species plus another C-4 grass (Andropogon gerardi) gr owing on initially homogeneous soils with a predominantly C-3 isotopic signature, soil delta(13)C values increased 1.6-2.2 parts per thousan d for the C-4 species and remained relatively unchanged for the C-3 sp ecies after 4 yr. Averaging across the C-4 species and the experimenta l 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 tha t this amount of new soil C equals 30% of NPP summed over 4 yr in thes e plots.