Mineralisation of Miscanthus-Derived C in Three Differently Textured Soils and its Incorporation into Microbial Biomass.
Katja Schneckenberger and Yakov Kuzyakov. Institute of Soil Science and Land Evaluation, Univ of Hohenheim, Emil-Wolff-Str. 27, 70599 Stuttgart, Germany
Natural abundance of 13C is often used to investigate C turnover in soil after a change of the C3 vegetation to C4 crops. Most studies have been carried out with maize as a very common agriculturally used annual C4 plant. Focussing research only on maize may result in biased information for other cultivated C4 plants. To study C dynamics in soil, we applied 13C natural abundance approach by cultivation of the perennial C4 energy grass Miscanthus x giganteus. Such investigations are especially important because growing of energy grasses is assumed as a measure to sequester C in soil. Studying dynamics of individual C pools provide more specific information about their turnover rates and C sequestration potential compared to bulk soil analyses. Microbial biomass is one of such C pools, which has fast turnover. Using 13C natural abundance approach after C3 to C4 vegetation change allows to estimate turnover rates of microbial biomass. Coupling microbial biomass turnover with CO2 efflux and its C isotopic composition allows evaluation of contribution of new (C4) and old (C3) soil organic matter to test the decomposability of SOM pools. Many studies with maize showed, that soil texture strongly affects C turnover. Aim of this study was to test the effect of soil texture on mineralisation of old C (>12 years) and new C (<12 years). We used different textured soils with Miscanthus cultivation for 12 years and 1) determined the contribution of Miscanthus-derived C to microbial biomass and 2) compared the mineralisation of old C3-C and new C4-C (Miscanthus-derived C) by incubation under controlled conditions. Samples were taken from 0-10 cm and 10-20 cm depth from the Miscanthus and from neighboured grassland plots from three soils textured as 1) fine loamy sand, 2) coarse loamy sand, or 3) silty loam. δ13C values of total SOM and microbial biomass estimated by chloroform-fumigation-extraction were measured to calculate the portion of Miscanthus-derived C. Soil samples were incubated for half a year in air-tied glass vessels at 20 °C and 70% water holding capacity. Microbial biomass and its δ13C value were measured 3 weeks after the start and at the end of the incubation. Evolved CO2 was trapped in 1 M NaOH and changed twice a month during the incubation. Total evolved CO2 trapped in NaOH was measured by titration and its δ13C value was determined after precipitation as SrCO3 with IRMS. Soil texture affected the incorporation of Miscanthus-derived C into microbial biomass as well as the mineralisation of C3-C and Miscanthus-derived C4-C. The content of microbial biomass was lower in sandy soils, but it contained higher portions of Miscanthus-derived C compared with loamy soil. Poor aggregation and lower clay content in sandy soil led to a faster mineralisation of soil organic carbon (SOC) than in loamy soil. Mineralisation rate of total SOC decreased significantly in all soils with increasing incubation time and the differences between the soils were alleviated. In all soils mineralisation of new Miscanthus-derived C was faster than the mineralisation of old C (C3), which was better stabilized. Our hypothesis regarding the effect of soil texture was confirmed by CO2 efflux and its C isotopic composition. General statement about the possibility of C sequestration in soil by cultivation of energy grasses cannot be confirmed without further investigations of the individual locations. Incorporation of new C4-C into microbial biomass was 1.5 to 3 times higher than C4-C content in total SOM.