Maize residue decomposition measurement using soil surface carbon dioxide fluxes and natural abundance of carbon-13

The decomposition rate of crop residues in soils directly impacts organic m atter content and nutrient cycling. We hypothesized that natural abundance C-13 analyses could be used with soil CO2 flux measurements to quantify the short-term decomposition rates of maize (Zea mays L.) residues under undis turbed field conditions. For this purpose, maize was grown in a sandy loam (Umbric Dystrochrept) that developed under C3 vegetation. Residues were ret urned to the field at the end of the growing season. During the following s nowfree period (May to November), the maize residue decomposition rate was calculated for plots that were either under no-till or moldboard plowed, us ing the C isotope ratio (C-13/C-12) of the soil CO2, the C isotope ratio of the plant and soil substrates, and the soil respiration rate. The incorpor ation of residue-derived C into the soil microbial biomass was also evaluat ed. Maize residue decomposition increased the C isotope ratio of the soil C O2 by 2 to 7 parts per thousand relative to unamended control plots. Decomp osition rates peaked in June (2-3 g C m(-2) d(-1)) and were low at both the beginning and end of the growing season (<0.5 g C m(-2) d(-1)). For a give n soil temperature, the decomposition was more active early than late in th e season because of decreased substrate availability as decomposition proce eded. The decomposition rate of maize-derived C correlated with the fractio n of the microbial biomass derived from maize residues. This active pool re presented 9% of microbial biomass and showed a high level of specific activ ity. The total maize residue-C losses during the study corresponded with 35 % of the added residue C under no-till plots and 40% with moldboard plowing . Natural abundance C-13 analyses mag be successfully used with respiration measurements to quantify crop residue decomposition rates under undisturbe d field conditions.