Carbon dynamics determined by natural C-13 abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural C-13 abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C-3-plant) since 1878 and maize (C-4-plant) since 1961 with and without NPK fertilization from the long-term experiment 'Ewiger Roggen' in Halle, Germany, were incubated for 230 days at 8 degreesC and irrigated with 2 mm 10(-2) M CaCl2 per day. Younger, C-4-derived and older, C-3-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C-mic) and CO2 from heterothropic respiration were determined by natural C-13 abundance. The percentage of maize-derived carbon was highest in CO2 (42-79%), followed by C-mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO2-C and C-mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO2-C and 1.9-2.7% for Cmic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO2-C and 0.3-0.5% for C-mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C-4-derived C-mic and C-4-derived SOC, DOC and CO2-C (r greater than or equal to 0.90), whereas the relationship between C-3-derived C-mic and C-3-derived SOC, DOC and CO2-C was not as pronounced (r less than or equal to 0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil. (C) 2003 Elsevier Ltd. All rights reserved.