In extremely acidic mining sediments of the Lusatian mining district, the alkalinisation process relies on organic C, which can serve as electron donor for microbially induced sulfate reduction. Plant material of the pioneer plantJuncus bulbosus is an important organic matter source in lake sediments. Therefore, decomposition of the plant tissue was assessed during the exposure of litterbags for 30 months in the 0 – 5 cm layer of waterlogged mining sediments, which have a pH between 2.5 and 3. The ash free dry weight (AFDW) and elemental content of the plant tissue were recorded several times during the exposure. Changes in chemical structure were analyzed by solid-state13 C cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy and the lignin component characterized by wet-chemical CuO oxidation. The AFDW accounted for about 34% of initial biomass after field exposure for 30 months. Mass loss of biomass occurred in two phases with decomposition rates varying between 30 and 430 mg AFDW d21. The mass loss increased considerably after 5 – 7 months when litterbags were invaded by fresh J. bulbosus plants. With respect to higher mass loss,13 C CPMAS NMR spectroscopy, showed slight changes of the bulk chemical composition after 11 months, indicating that microorganisms present in the sediments or in the rhizosphere degrade plant material as a whole, rather than selectively. During the second phase from about 11 months until the end of the exposure period, contribution of O-alkyl C most probably assignable to easily degradable polysaccharides decreased. In contrast, the contribution of alkyl, aromatic and carboxyl C increased. CuO oxidation showed that the lignin component of J. bulbosusis degraded oxidatively during field exposure. Our results indicate that the exposed plant material is decomposed in the sediment due to changes in sediment conditions that followed plant invasion of the litterbags. It is suggested that the rhizosphere of J. bulbosus by its influence on the redox potential, pH and the microbial component plays a crucial role in organic matter degradation in acidic mining sediments.