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Abstract Band 63

Rehbein, K. (2015): Fate of soil organic carbon in different extensive, perennial energy
cropping systems, Germany. Bonner Bodenkundliche Abhandlungen 63, 144 S.



The substitution of fossil fuels by alternative energy resources may help to counteract climate change, particularly when energy plants additionally help restoring soil and sequestering carbon. The latter may be achieved by extensive cultivation of perennial energy crops like short rotation coppice (SRC) or Miscanthus x giganteus. I hypothesized that with absence of tillage and significant organic matter return these cropping systems lead to (i) a restauration of soil structure, (ii) a recycling of soil nutrients (iii) an accumulation and (iv) a stabilization of organic material, which is accompanied by a slower turnover of soil organic carbon (SOC). Due to deep root systems, effects should not be restricted to the surface soil but extend into deeper soil layers.

To verify these hypotheses, I sampled sites, which were cultivated with SRC or Miscanthus in different regions in Bavaria, Lower Saxony and Saarland, accompanied them from the date on which they were established and collected samples every year with sites under prolonged arable cropping as reference. Additionally, I sampled a false chronosequence of former C3-derived arable fields that had been cropped with Miscanthus for 0-21 yr. To assess the implications on soil, I analyzed effects on the nutrient cycling (potassium), the soil structure (aggregate stability) and on carbon dynamics, which were achieved by the separation of particle-size fractions differing in their stability against decay and turnover times (particulate organic material, mineral associated organic material). In the Miscanthus chronosequence, the turnover of SOC was traced on the basis of natural 13C/12C isotope abundance measurements. The compound-specific isotope tracing of lignin biomarkers in bulk soil and soil fractions was used for calculating turnover times of Miscanthus-derived lignin in soils.

The results showed that relative to the reference soils, the content of plant available potassium in the surface soil of the perennial cultivation systems of SRC and Miscanthus increased initially, suggesting that these plants act as a “base pump” from the subsoil. I also detected a fast re-formation of soil aggregates, especially the amount of stable macroaggregates increased in the perennial cultivation systems. The carbon contents within the aggregates showed only little reaction to the land use change. While SOC in bulk soil of shorter chronosequences (three years) did not show significant trends, Miscanthus-derived SOC in the false Miscanthus chronosequence accumulated at a rate of 1800 kg ha-1 yr-1 down to a soil depth of 100 cm. Only about 50% of this C accrual occurred in the surface soil (0-10 cm). The C accumulation differed among particle-size fractions. Miscanthus-derived C in the coarse-particulate organic matter (POM) fraction, which represented the labile SOC fraction with a short turnover, increased fast during the first years of Miscanthus cultivation until a steady-state was reached after approximately 7 yr.

The accumulation of Miscanthus-derived C associated with the clay fraction was delayed relative to the sand fraction. As SOC associated with the clay fraction is usually regarded to have a slow turnover, it contributes to the stable carbon pool. On an absolute basis, it was thus mainly Miscanthus-derived C associated with the clay fraction that led to increasing SOC stocks.

The total contents of lignin, estimated here via the sum of vanillyl, syringyl, and cinnamyl structural units of lignin-derived phenols (VSC stocks), increased from 0.25 to 0.92 Mg ha-1 for 0-5 cm soil depth. The largest VSC stocks could again be found in the coarsest POM and in the clay fraction. The VSC in POM showed a fast saturation with longer duration of Miscanthus cultivation, whereas the stabilization of VSC in the clay fraction followed a sigmoidal shape. The mean residence time (MRT) of VSC was approximately 16 years (0-5 cm, bulk soil), as predicted by a two-pool model. Compared to published values in the literature, the MRT in perennial Miscanthus was greater than in annual cultivation systems. The deeper soil layers showed a less pronounced relation to the duration of Miscanthus cultivation. Nevertheless, SOC already accumulated below the former Ap, i.e., below a depth that would respond sensitively to a future land use change.

In conclusion, the perennial cultivation of SRC and Miscanthus has the potential to improve soil quality by increasing the availability of soil nutrients, by improving the soil structure and by increasing and stabilizing SOC stocks. While the re-formation of the soil structure happened on a short time scale, the recovery of SOC needed longer time, which confirms the suitability of chronosequences for those analyses. Overall, the cultivation of perennial energy crops such as SRC and Miscanthus lead to environmental benefits by substituting fossil fuels, by restoring degraded soils, and by accumulating atmospheric carbon dioxide sustainably in soils.