Michael Kaiser

(Isolation and characterization of soil organic matter fractions different in solubility as a possibility to evaluate and to improve C-pools in C-turnover models)

Carbon-turnover models use C-pools different in stability to predict changes in soil organic matter (OBS) content due to changes in management. It is known that soil organic matter will be stabilized by different chemical and physical processes. The aim of this study is the characterisation of chemically protected and unprotected soil organic matter fractions isolated by sequential extraction and the validation of the C-pools determined by a C-turnover model with the experimentally obtained data of the isolated soil organic matter fractions. Long-term field experiments were used for this investigations as changes in soil organic matter content need more than 30 years. Soil organic matter fractions different in stability were isolated by sequential extraction, assuming that the water soluble fraction represents the unstable and the sodium pyrophosphate soluble fraction a stable part of soil organic matter. The amount of the isolated fractions were quantified and compared with the corresponding stable C-pool (HUM) simulated with the C-turnover model ROTH-C. Fourier Transform Infrared spectra of the soil organic matter fractions show that their composition is influenced by different long-term fertilization. The observed difference in soil organic matter composition is related to their sorption properties. Therefore, the isolation of the different soil organic matter fractions allows on one hand the validation of C-pools simulated with a carbon turnover model and gives on the other hand information on changes in their sorption behavior related to their composition due to changes in management.

Effect of grazing intensity and soil characteristics on soil organic carbon and nitrogen stocks in a temperate long-term grassland

ABSTRACT The effects of different grazing pressures (GPs) on soil properties are not sufficiently understood. The objectives were to analyse the effects of three different extensive GPs on stocks of soil organic C and total N, soil microbial biomass C, basal respiration and mineral N in three different soil depths of a long-term pasture in Central Germany (FORBIOBEN field trial). No significant (p ≤ 0.05) effects of GP on weighted stocks of soil organic C, total N, soil microbial biomass C, mineral N and basal respiration rate were observed, suggesting that the C and N cycles are coupled in the three grazing treatments. Oxalate soluble Fe contents explained a marked part of the variation of soil organic C (multiple linear regression: R2 = 0.64) and total N contents (R2 = 0.64) in the soils, whereas almost all of the variability of soil microbial biomass C contents and basal respiration was explained by soil organic C contents. Overall, variabilities of soil organic C and N contents were largely explained by oxalate soluble Fe contents, whereas grazing intensity did not affect the C and N dynamics.

Effects of fine root characteristics of beech on carbon turnover in the topsoil and subsoil of a sandy Cambisol

S . V o r m s t e i n a, M . K a i s e r a, H .P . P i e p h o b, R . G . J o e r g e n s e n c & B . L u d w i g a aDepartment of Environmental Chemistry, University of Kassel, Nordbahnhofstr. 1a, 37213, Witzenhausen, Germany, bBiostatistics Unit, Institute of Crop Science, University of Hohenheim, Fruwirthstr. 23, 70599, Stuttgart, Germany, and cDepartment of Soil Biology and Plant Nutrition, University of Kassel, Nordbahnhofstr. 1a, 37213, Witzenhausen, Germany

Organic matter distribution and retention along transects from hilltop to kettle hole within an agricultural landscape

In agricultural landscapes, the spatio-temporal distribution of organic matter (OM) varies greatly across landscape structures and soil types. We investigated patterns of organic carbon (OC) content, polyvalent cations, and isotopic values for specific OM fractions along transects spanning topographic positions from erosional to depositional areas, including aquatic sediments within a single kettle hole. We hypothesized different drivers exist at different scales. At the transect scale, we hypothesized (1) landscape form and land management to explain patterns of isotopic and OC content from different OM fractions. At the aggregate scale, (2) we expected different OM-mineral associations to explain stabilized OM. We also hypothesized, (3) that shallow sediment δ13C and δ15N of the kettle hole reflected different terrestrial sources. We found that distinct differences in the OM turnover rates existed between the fractions suggesting that different processes are affecting the transformation rates that are recorded in the isotopic composition patterns. Erosion along with plant productivity drive mineral-associated fractions over the transect, while microbial decomposition and slurry influence freely available and aggregated OM fractions. The type and magnitude of OM-mineral associations changed along the transect while binding OM of different decomposition status. OM in mineral-associated fractions in kettle hole sediments were derived from clay- and silt-sized particles from the field, whereas OM in freely available and aggregated fractions potentially originated from macrophytes. We conclude that kettle holes constitute important sinks for terrestrial OM across the landscape.