High nitrogen retention potential of cellulose and starch applied to four soils under simulated post-harvest conditions

• Labile high-carbon soil amendments (HCA) can improve post-harvest N retention in four soils. • Starch stimulates N immobilization faster but for a shorter period of time than cellulose. • Microbial-mediated retention of N min is regulated by soil pH and soil organic carbon (SOC). • HCA promote the formation of amino acids as important sinks for excess post-harvest N min . Conventional agriculture often leaves surplus mineral nitrogen (N min ) after harvest, increasing the risk of nitrogen (N) loss. Applying high soil carbon amendments (HCA) can promote microbial N retention when catch crops are not feasible. However, the effects of soil type on microbial N immobilization in response to HCA application are insufficiently understood. Here, we applied 50 kg N ha −1 as 15 N-labelled (NH 4 ) 2 SO 4 and 4 t C ha −1 as starch or cellulose to the Ap horizon of four contrasting agricultural soils in different soil organic carbon (SOC) and texture: alkaline silty Luvisol (SOC 0.93 %) and Regosol (SOC 0.35 %), and acidic sandy Luvisol (SOC 0.58 %) and Gleysol (SOC 1.72 %). Soils were incubated for 98 days at 8.6 °C and 65 % water-holding capacity, simulating post-harvest conditions in North Rhine-Westphalia, Germany. After 98 days under post-harvest condition, cellulose application reduced N min by 50–140 kg N ha −1 depending on the soil type, while starch induced faster N min retention, followed by gradual re-release of N min towards the end of the incubation period. HCA-amended soil showed that most of the 15 N was recovered in the soil N pool inaccessible to multiple extractions (N ret ), rather than in microbial biomass. In HCA-amended clay soils, the recovery of 15 N in N ret was approximately twice that of the Control. In sandy acidic soils, HCA increased the recovery of 15 N in N ret to levels comparable to those in alkaline soils, while in SOC-rich sandy loam soils, only cellulose caused a slight increase in N ret . After incubation, up to 41 % of labeled N min was stabilized in the form of amino acids, with a more pronounced effect when HCA was added. We conclude that labile organic C can promote the conversion of excess N min into more stable N forms, such as amino acids or peptides, that are able to stabilize N beyond the post-harvest period.