Abstract

JOHN, B., LUDWIG, B. and FLESSA, H.

Soil Biol. Biochem., 35:1193-1202 (2003)

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 ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-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 °C and irrigated with 2 mm 10^-2 M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance.The percentage of maize-derived carbon was highest in CO₂ (42 to 79%), followed by C_mic (23 to 46%), DOC (5 to 30%) and SOC (5 to 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 to 0.08% for DOC, 1.6 to 2.6% for CO₂-C and 1.9 to 2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03 to 0.05% for DOC, 0.1 to 0.2% for CO₂-C and 0.3 to 0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r³0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r ≤ 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.