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

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 ¹⁵N-labelled (NH₄)₂SO₄ 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 ¹⁵N 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 ¹⁵N in N_ret was approximately twice that of the Control. In sandy acidic soils, HCA increased the recovery of ¹⁵N 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.