The effect of different high carbon soil amendments on N retention capacity under winter conditions in silty clay soil with low organic carbon content: An incubation study

Average soil temperatures in winter in Germany are frequently between 0 and +10℃, usually at high soil moisture levels. Therefore, the decomposition of soil organic matter and soil nitrogen (N) cycling are still active and could be substantial under these conditions, potentially leading to high N losses both in the form of nitrate to the groundwater and nitrous oxide to the atmosphere. High carbon soil amendments (HCA) have the potential to immobilize excess mineral N in the soil due to stimulation of microbial biomass growth. However, to date, it is not sufficiently known how well this N immobilization works at lower temperatures, and how long the effect will last over winter. In order to elucidate how the application of different HCA affects N immobilization in soil under winter conditions, we conducted a 7-month laboratory incubation experiment with silty clay soil low in soil organic carbon from a recultivation area after open-cast lignite mining near Jülich, Germany. Each soil column contained about 500 g of recultivation soil sieved at 2 mm. A scenario of a typical mineral N content after harvest was created by adding 50 kg NH₄⁺-N ha^-1 to the soil before application of the different HCA, which were then added at a rate of 4 t C ha^-1. Eight different treatments were implemented: application of NH₄⁺ only (B), and then NH₄⁺ applied with wheat straw (WS), biochar (BIO), spruce sawdust (SS), lignite (LIG), cellulose (CEL), a combination of wheat straw (2 t C ha^-1) and spruce sawdust (2 t C ha^-1) (CWS), and a combination of wheat straw (2 t C ha^-1) and biochar (2 t C ha^-1) (CWB), respectively. In the three straw treatments, carbon dioxide (CO₂) emissions peaked 14 days after the start of the experiment. In all treatments, CO₂ emissions decreased with time. In the end, the CEL treatment had the highest cumulative CO₂ emission during the entire incubation period. In contrast, the CEL treatment had a significantly lower soil nitrate content than all other treatments over the whole duration of 7 months, indicating that cellulose was most effective and long-lasting in stimulating microbial N immobilization under temperate winter conditions in silty clay soil.