The origin of soil organic C, dissolved organic C and respiration in a long-term maize experiment in Halle, germany, determined by C-13 natural abundance

For a quantitative analysis of SOC dynamics it is necessary to trace the or igins of the soil organic compounds and the pathways of their transformatio ns. We used the C-13 isotope to determine the incorporation of maize residu es into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the s oil respiration. The maize-derived SOC was quantified in soil samples colle cted to a depth of 65 cm from two plots, one 'continuous maize' and the oth er 'continuous rye' (reference site) from the long-term field experiment 'E wiger Roggen' in Halle. This field trial was established in 1878 and was pa rtly changed to a continuous maize cropping system in 1961. Production rate s and delta(13)C of DOC and CO2 were determined for the Ap horizon in incub ation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the to psoil originated from maize C. The fraction of the maize-derived C below th e ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha(-1) which was equal to about 31% of the estimate d total C input via maize residues (roots and stubble). Total leaching of D OC during the incubation period of 16 weeks was 1.1 g m(-2) and one third o f the DOC derived from maize C. The specific DOC production rate from the m aize-derived SOC was 2.5 times higher than that from the older humus formed by Cg plants. The total CO2-C emission for 16 weeks was 18 g m(-)2. Fifty- eight percent of the soil respiration originated from maize C. The specific CO2 formation from maize-derived SOC was 8 times higher than that from the older SOC formed by C-3 plants. The ratio of DOC production to CO2-C produ ction was three times smaller for the young, maize-derived SC than for the older humus formed by C-3 plants.