Fatty acids (FAs) are abundant lipids in plants, microorganisms and soil. Depending on chain length they provide potential for evaluating different sources of C in soil: shoots, roots and microorganisms. This, together with their fast turnover and transformation in living and decaying plant tissues, suggests the use of FA molecular ratios as source indicators in soil. To evaluate the applicability of FAs as source indicators, their dynamics in plant tissue and soil were traced during a laboratory experiment using the highly productive perennial C4 energy grass Miscanthus x giganteus (Greef et Deu.). For the comprehensive use of FAs as source indicators various ratios were calculated: fatty acid ratio (originally defined as carboxylic acid ratio: CAR), carbon preference index (CPI), average chain length (ACL) and unsaturated vs. saturated C18 acids. The FA composition was specific for individual plant tissues as indicated by the CAR, with high values in roots and lower ones in the above ground plant tissue. Based on ACL values of rhizosphere, soil and roots, an enrichment in root derived FAs vs. root-free soil could be estimated. The rhizosphere contained 35–70% more plant derived FAs than root-free soil. The ACL showed potential for estimating root derived carbon in the rhizosphere. The study documents for the first time very fast spatial processes in soil related to plant growth, thereby strongly influencing the FA composition of soil.Highlights► Fatty acid composition of Miscanthus tissues changes in the short term with growth. ► Differentiation of Miscanthus root and shoot biomass using ACL and CPI values. ► Molecular proxies reveal changes in plant and microbial FAs in soil and rhizosphere. ► Root-derived FAs were more strongly enriched (30–75%) in rhizosphere than bulk soil.

Plant-derived lipids are regarded to be mainly incorporated into soil via above ground biomass. The contribution of root-derived lipids to soil organic matter (SOM) is not easily accessible, so this incorporation pathway is mostly underestimated, whereas it is common knowledge that the contribution of rhizodeposits to SOM is of major importance for the turnover of organic carbon. Not only the contribution, but also the incorporation rates and turnover time of exclusively root-derived lipids remain unknown. We determined for the first time the incorporation rates of rhizodeposit-derived lipids into soil planted with Lolium perenne,   using a multiple 14CO214CO2 pulse labelling experiment carried out under controlled laboratory conditions. Additionally, we accessed differences in lipid composition between the rhizosphere and root-free soil to evaluate the direct contribution of root-derived lipids to SOM. The lipid composition in the rhizosphere clearly showed a greater abundance of microbial lipids like C16:1 and C18:1, as well as root-derived C18:2+3 fatty acids (FAs) than the initial and root-free soil. The incorporation rates of total lipids (kLip) and FAs (kFA) based on 14C data revealed a very fast incorporation into SOM (rhizosphere: kLip 0.82 year−1; kFA 0.31 year−1; root-free soil: kLip 0.70 year−1; kFA 0.48 year−1) after the first 14C pulse for young plants. Thereafter, incorporation rates decreased until the end of the experiment (rhizosphere: kLip 0.17 year−1; kFA 0.03 year−1; root-free loess: kLip 0.11 year−1; kFA 0.06 year−1). The incorporation rates resulting from the 14C pulse labelling experiment are comparable to turnover rates of total, i.e. above ground and root-derived, and lipids from field experiments using 13C labelling approaches. The fast allocation of root-derived lipids to substrate beneath, and distant from, roots gives new insights into the carbon incorporation of OM on a molecular level. This implies that incorporation of lipid compounds, especially into soil deeper than the uppermost few centimetres or the ploughing layer, is mainly due to root-derived OM, in contrast to common knowledge.

Two powerful approaches are frequently used to trace incorporation and degradation of plant derived C in soil: 14C labelling/chasing and analysis of lipid composition. In this study, we coupled these approaches in order to trace short term incorporation of plant derived lipids into rhizosphere and non-rhizosphere soil. Methodological optimization was required and implied 14C liquid scintillation counting improvement for plant lipid extracts taking into account organic solvents, solvent-to-scintillation cocktail ratio, and amount of lipids. Following method optimization, 14C data of fatty acids indicated a notable contribution of root derived lipids to rhizosphere and non-rhizosphere soil. Coupling of 14C labelling/chasing with lipid analysis is a powerful and cheap approach for tracing of root derived C in soil allowing for estimation of C budget, for determination of C formation and translocation within plants and from plant to soil, as well as for identification of short term dynamics of specific compound classes within soil.

Future climatic conditions may coincide with an increased potential for wildfires in grassland and forest ecosystems, whereby charred biomass would be incorporated into soils. Molecular changes in biomass upon charring have been frequently analysed with a focus on black carbon. Aliphatic and aromatic hydrocarbons, known to be liberated during incomplete combustion of biomass have been preferentially analysed in soot particles, whereas determinations of these compounds in charred biomass residues are scarce. We discuss the influence of increasing charring temperature on the aliphatic and aromatic hydrocarbon composition of crop grass combustion residues. Straw from rye, representing C3 grasses and maize, representing C4 grasses, was charred in the presence of limited oxygen at 300, 400 and 500 °C. Typical n-alkane distribution patterns with a strong predominance of long chain odd-numbered n-alkanes maximising at C31 were observed in raw straw. Upon combustion at 300 °C aliphatic hydrocarbons in char were dominated by sterenes, whereas at 400 °C sterenes disappeared and medium chain length n-alkanes, maximising around n-C20, with a balanced odd/even distribution were present. At a charring temperature of 500 °C n-alkane chain length shifted to short chain homologues, maximising at C18 with a pronounced predominance of even homologues. Even numbered, short chain n-alkanes in soils may thus serve as a marker for residues of charred biomass. Aromatic hydrocarbons indicate an onset of aromatization of biomass already at 300 °C, followed by severe aromatization upon incomplete combustion at 400–500 °C. The diagnostic composition of aliphatic and aromatic hydrocarbons from charred biomass affords potential for identifying residues from burned vegetation in recent and fossil soils and sediments.

Future enrichment of atmospheric CO2 and its effect on ecosystems were studied using grassland free air CO2 enrichment (FACE) experiments. Plant waxes have been shown to be directly modified under elevated CO2 concentration. Lipids, as major components of plant waxes, are important constituents of plant surfaces and their position at the plant/atmosphere interface makes them susceptible to environmental change. The main focus of this study was to improve knowledge about modifications to stable carbon isotopic (δ13C) values of individual lipids within plant biomass and soils as a result of the increased atmospheric CO2 concentration, implying an addition of 13C labelled CO2. The isotopically labelled biomass facilitates turnover time determination of lipids in soils due to the direct comparison of identical plants grown under ambient and 13C-depleted atmospheric conditions. We demonstrate which lipids were influenced by modified CO2 concentration and how the lipid isotopic values of plant biomass and soil were influenced under elevated vs. ambient CO2 conditions. Most plant carboxylic acids and alkanes were uniformly depleted in 13C by ca. 6‰ when compared to plant biomass bulk isotope values. In soil, short chain carboxylic acids (< C20), derived mainly from microbial sources, revealed a lower depletion in isotope value than plant-derived long chain acids (⩾C20). The isotopic differences between individual compounds in soil under ambient vs. elevated CO2 conditions varied significantly between 2 and 6‰ for individual acids and 0–6‰ for individual alkanes. This argues against plant/soil turnover determinations for individual compounds. Preferably, weighted mean average isotopic values of the most abundant lipids provide reliable calculation of replaced carbon proportions and turnover times. Carboxylic acids were turned over fastest in grassland soil, followed by bulk carbon, whereas alkanes exhibited the slowest turnover times. This is in contrast to previous studies of arable soil, but confirms observations made on peaty soil indicating that alkanes may be part of the relatively stable carbon fraction in soils. The turnover of total organic carbon, carboxylic acids and long chain alkanes was observed to be significantly greater in soil under Lolium perenne (ryegrass) than in soil under the leguminose plant Trifolium repens (white clover).

Grassland soils are regarded as one potential sink for atmospheric CO2 via photosynthetic fixation in plant biomass and subsequent transformation into soil organic matter upon degradation. In the future, an enrichment in atmospheric CO2 concentration is expected, leading to modified photosynthetic activity in plant biomass. Free air CO2 enrichment (FACE) experiments provide an opportunity for investigating under field conditions plant behaviour expected under future atmospheric composition. Lipid components are important constituents of plant surfaces, whereby their position at the plant/atmosphere interface leads to a high susceptibility towards environmental change. The main focus of this study was an investigation of the modification in lipid distribution patterns within plant biomass and the translocation of these lipids towards, and fixation within, soil organic matter as a result of enhanced CO2 concentration. We demonstrate which lipids are mainly influenced under modified CO2 concentrations and show how this affects the lipid composition of plant biomass and soil. Carboxylic acid, alcohol and aliphatic hydrocarbon distribution patterns of plant biomass and soils are discussed. While short chain acids reveal a uniform depletion in unsaturated C18 acids in plants and soils under enhanced CO2 concentration, the alcohol fraction shows diverse trends for Lolium perenne and Trifolium repens plants and soil. Long chain alcohols increase in abundance for L. perenne and decrease for T. repens samples. The n-alkanes in soil, as degradation products of plant-derived acids and alcohols, exhibit minor compositional variation. Decreasing amounts of plant-derived acids vs. increasing concentrations of alcohols are noted for T. repens samples. The study demonstrates the response on the molecular level of selected plants under enhanced atmospheric CO2 concentration. Lipid compositional variation is modified by photosynthetic activity and adapted biosynthesis under future atmospheric conditions may be expected.