Carboxylic acid distribution patterns of temperate C3 and C4 crops

Agricultural grasses cover a major part of the land surface in temperate agro-ecosystems and contribute significantly to the formation of soil organic matter. Crop-derived lipids are assumed to be responsible for fast carbon turnover in soils. Differences in lipid distribution patterns between crops following C₃ and C₄ photosynthesis pathways have rarely been described, but could be useful for source apportionment of crop-derived input into soils or sediments. The distribution of long chain n-carboxylic acids (C₂₂, C₂₄, C₂₆) reveals significant differences between crop plants following either the C₃ or the C₄ photosynthetic carbon fixation pathway. The plant compartments leaves, stems and roots of C₄ plants contain relatively large proportions (> 40%) of n-C₂₄ carboxylic acid when compared to C₃ plants. These reveal larger relative proportions of n-C₂₂ and n-C₂₆ acids, whose relative abundance is subject to change between different plant compartments and during the growing season. The carboxylic acid ratio [CAR = n-C₂₄/(n-C₂₂ + n-C₂₆) carboxylic acids] provides distinct ratios for C₄ (> 0.67) and C₃ crops (< 0.67) and can thus be used as a molecular marker for the differentiation of crop plant biomass. In combination with the bulk stable carbon isotopic composition (δ¹³C) the CAR can be used as a tool for the estimation of the C₄ derived carbon proportion in soils or sediments.