Impacts of Soil Additives on Crop Yield and C-Sequestration in Post Mine Substrates of Lusatia, Germany

Opencast lignite mining in the Lusatia region of Germany has resulted in large scale landscape disturbances, which require suitable recultivation techniques in order to promote plant growth and establishment in the remaining nutrient-poor substrates with low water-holding capacity. Thus, the effects of two commercial soil additives (CSA), a hydrophilic polymer mixed with volcanic rock flour and bentonite (a-CSA), and digester solids from biogas plants enriched with humic acids and bentonite (b-CSA), on soil organic carbon (SOC) storage, plant yields and root biomass were assessed after cultivating perennial crops (Dactylis glomerata L.) in monoculture and Helianthus annuus L.-Brassica napus L. in crop rotation systems. The CSA were incorporated into the top 20 cm soil depth using a rotary spader. The results indicated that a-CSA led to a significant increase in plant yield during the first year, and improved root biomass in the following year. As a result, SOC stocks increased, especially in the 0–10 cm soil layer. No significant sequestration of additional SOC was observed on b-CSA-amended plots at the end of both years. Bulk density values decreased in all treatments under the monoculture system. It can be concluded that application of a-CSA enhanced soil water availability for plant uptake and consequently promoted plant growth and organic carbon sequestration. The relative enrichment of organic matter without effects on water-holding capacities of b-CSA treatments suggested that it was not suitable for rapid land reclamation.