The Origin of Soil Organic C, Dissolved Organic C and Respiration in a Long-Term Maize Experiment in Halle, Germany, Determined by13C Natural Abundance

Abstract

For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the 13 C isotope to determine the incorporation of maize residues 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 soil respiration. The maize-derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one continuous maize' and the other continuous rye' (reference site) from the long-term field experiment Ewiger Roggen' in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ 13 C of DOC and CO 2 were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize-derived C below the 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 estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m -2 and one third of the DOC derived from maize C. The specific DOC production rate from the maize-derived SOC was 2.5 times higher than that from the older humus formed by C3 plants. The total CO 2 -C emission for 16 weeks was 18 g m -2 . Fifty-eight percent of the soil respiration originated from maize C. The specific CO 2 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 CO 2 -C production was three times smaller for the young, maize-derived SOC than for the older humus formed by C 3 plants.