Browsing by Author "Heil, B."

The study of natural isotopic abundance signatures is useful to gain further insights in the processes resulting in depthwise changes in the composition of soil organic matter (SOM). Objectives were to describe the delta(13)C and delta(15)N abundances of SOM with depth in soils from a 153-year old beech (B1), a 119-year old spruce (F1) and a 61-year old spruce (F2) stand at Selling, north-west Germany, and to study, how podzolisation affects the isotopic abundances of C-13 and N-15 in the SOM. The degree of podzolisation decreased in the order F1 > B1 > F2. At the surface of the humus layer of all three sites, delta(13)C values are approximately 1 to 4 parts per thousand higher than in the leaves and needles, probably mainly due to the discrimination of C-13 by microbial decomposition. C-13 abundances in the organic layers of Fl and F2 increased only slightly from -27.6 parts per thousand PDB (B1, L) to -27.2 parts per thousand PDB (B1, Oh) and from -26.3 parts per thousand PDB (F2, L) to -25.9 parts per thousand PDB (F2, Oh), suggesting that biotic activity resulted in mixing of organic matter. At Fl, however, C-13 abundance increased from -27.5 parts per thousand PDB (L) to -26.0 parts per thousand PDB (Oh) which reflects the lack of mixing by animals. In the upper 2-4 cm of the mineral soil, i.e., in the eluvial horizons Aeh, C-13 values showed a minimum at the spruce sites which was presumably related to a translocation of C-13 enriched fulvic acids. Depthwise changes in delta(15)N values were not related to podzolisation processes. At all three sites, a N-15 enrichment with depth occurred in the mineral soil which is the result of the discrimination of N-15 by microbial decomposition.

Dissolved organic carbon (DOC) in seepage water can combine with organic pollutants, with Al and heavy metal ions and transport them through the soil profile with a potential to contaminate groundwater. We studied the production of DOC in aerobic decomposition experiments at 8 degrees C and moisture close to field capacity in soils from two sites with different microbial activities (spodic dystric Cambisols with moder (SLB) and mor-moder (SLS) layers) using C-13-depleted plants of differing decomposability (Epilobium angustifolium and Calamagrostis epigeios). Additionally, we investigated the DOC transformation during soil passage in decomposition experiments and in the field for the sites SLB and SLS. For SLS, decomposition of Epilobium resulted in a cumulative CO2 production of 14% of the added C within 128 days. Priming effects were negligible. CO2: production for the experiments using Calamagrostis was less with 11% for SLB and 10% for SLS. Cumulative DOC production was markedly high in the Epilobium decomposition experiment, being 25 g m(-2), out of which 11 g m(-2) were Epilobium-derived (2% of the added C). For the Calamagrostis experiments, cumulative productions of DOC and Calamagrostis-derived DOC (0.1% of the added C for SLS and SLB) were much less. During the soil passage, much of the DOC was removed by sorption or decomposition processes. Field studies at SLS and SLB using C-13 natural abundance showed that C-13 distribution of soil organic matter increased with depth, probably mainly due to a discrimination of C isotopes by decomposing microorganisms. DOG, however, showed a depletion of C-13 from -28 parts per thousand PDB to -29 parts per thousand (SLB at 40 cm) or --28 to -30 parts per thousand (SLS at 20 cm) with depth, owing to preferential decomposition of C-13-enriched substances or preferential adsorption. This study indicates that DOC production is strongly affected by litter composition and that significant changes in DOC composition may occur during its passage through a soil depth of 40 cm.

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 C-13 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 delta(13)C of DOC and CO2 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 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 production was three times smaller for the young, maize-derived SC than for the older humus formed by C-3 plants.