Carbon Isotope Dynamics During Grass Decomposition and Soil Organic Matter Formation

Abstract

We analyzed changes in the stable C isotope composition (° 1 3 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 13 C compared to bulk tissues (3.6% more negative on average), we found neither convergence of bulk tissue ° 13 C values towards lignin ° 13 C values, nor greater stability of ° 13 C values for lignin fractions. Furthermore, ° 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 ° 13 C 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 C 4 grass (Andropogon gerardi) growing on initially homogeneous soils with a predominantly C 3 isotopic signature, soil ° 13 C values increased 1.6—2.2 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.