Input and turnover of plant-derived lipids in arable soils

Soil organic matter plays a central role for the global and especially for the terrestrial carbon cycle. As a result of climate problems, due to the significant increase of greenhouse gases (e.g. CO2), suitable possibilities are needed to regulate the global carbon cycle. Especially the terrestrial part of the global carbon cycle seems to be suitable for regulative interventions. Until now, there is rare knowledge, which mechanisms are responsible for the fixation and mobilisation of carbon in soils. In this study, the unanswered questions shall be discussed to make recommendations, which measures can be forced in order to increase soil carbon pools sustainably. Analyses in this study were performed on soil and plant samples of several long-term agricultural trials of either urban (Halle/Saale, Germany) or rural areas (e.g. Rotthalmunster, Germany). Within the enforcement of this study a lot of new findings were made concerning the dynamics of carbon in agro ecosystems of temperate climates. The most important results are as follows: i. A reproducible, inexpensive, automated and fast extraction and separation procedure for soil lipids was successfully adopted and optimised. ii. Long-chain n-carboxylic acids were found to be best suitable for the molecular differentiation between cereal crops following distinct photosynthesis pathways (C3- and C4-plants) and the corresponding monoculture cropped soils. Alkanes and short-chain carboxylic acids are only of limited use for the differentiation between these plant types. This is due to similar distribution patterns in both plant types. Additionally, other sources of short-chain carboxylic acids in soils, like bacteria and fungi as well as fossil sources of alkanes like brown coal with similar distribution patterns, make it difficult to use these compounds as differentiation parameters. iii. As determined with the use of archived arable soil samples, significant soil carbon modifications as a result of the atmospheric Suess effect, like isotopic changes and amount of biomass incorporation, are expectable in the medium-term in arable soils. Suess effect induced modifications of soils mainly depend on amounts of biomass incorporation in soils and tillage practises applied. Within a few years of practising or low biomass incorporation these effects can be determined poorly, because marginal modifications in soil management, e.g. ploughing depth variations, may overlay modifications caused by the Suess effect in short periods. iv. For the first time turnover times of several individual lipids and lipid fractions were determined in this study. Within a few decades (20-60 years) lipids are turned over in agricultural soils, while bulk carbon is turned over significantly slower. Determined turnover times based on stable and radioactive carbon isotopes could be compared hardly, because low contributions of fossil 14C-free carbon cause high turnover times, when applying the radioactive method. For soils of the low contaminated Rotthalmunster site several lipid fractions like carboxylic acids showed similar turnover times based on stable (27 years) and radioactive (42 years) carbon isotopes. Hence, especially carboxylic acids can be classified as an uncontaminated lipid fraction, yielding realistic turnover times for predominantly plant-derived and microbial organic matter. Especially the alkane fraction, which is widely used for the calculation of new carbon proportions and source apportionment of C3- and C4-plant-derived organic matter, show high 14C ages resulting by pollution with fossil carbon (e.g. fossil fuel burning or direct input of oil and brown coal dust) for rural sites, too. Thus, alkanes can be used for turnover time calculations only with care. Additionally, actual contributions of different plant compartments to soil organic matter and their isotopic compositions directly before harvest are required for realistic turnover time determinations on a molecular level. Otherwise major errors could not be avoided. v. In contrast to previous determinations for lipids in peaty and acidic soils, lipids were found to be less stable in agricultural soils. Generally, lipids are part of the intermediate stable carbon pool in soils. As demonstrated for bulk agricultural soils, incorporation and turnover of carbon mainly depends on soil management like tillage and biomass contribution. In tilled agricultural soils a fast turnover is expectable due to annual mixing of the ploughed horizon with surficial plant residues. Contrastingly, in soils with high biomass contributions and surficial tillage, a carbon and lipid enrichment can be expected, similar to grassland or forest soils. Summarizing, sustainable management of agricultural soils in combination with large biomass contributions may lead to an increasing fixation of carbon and lipids in soils.