Modeling of the long-term effects of reduced inputs of organic and inorganic fertilizers on SOC and N-balance of agricultural soils

The world's population growth requires an increase in agricultural productivity. But this must be achieved by reducing the environmental impact of agricultural practices. The European Commission's European Green Deal includes targets for reducing nitrogen (N) application. This could be achieved by reducing fertilizer amounts in regions that are heavily fertilized. A second option to reduce N emissions would be the transport of organic amendments from region with high livestock/ biogas plant density into areas where fertilization of croplands are dominated by mineral fertilization. However, only few studies exist addressing the implications of fertilizer reduction on SOC stocks, N cycling and productivity in the long-term. Biogeochemical models can help to investigate the long-term effects of reduced fertilizer application on these system properties. For model calibration, data from two 2- and 3-year experiments on sandy and clayey soils, consisting of a no-fertilization control, 3 mineral fertilizer treatments with different N levels and 3 biogas digestate treatments with corresponding rates of total N (with 60%, 80% and 100% of maximum N applied) in two cereal/maize rotations were used. The digestate was applied by trailing hoses, and directly incorporated when maize was the subsequent crop. A long-term monitoring site in Lower Saxony was used to improve and validate the SOC sub-module of the model. The dataset consists of 45 field plots with documented soil data, management data and time series of SOC content. SOC content was measured on average every 4-5 years for 20 years in the upper 0-20 cm soil horizon. The management of the sites represents general agricultural practice. The results of the experiments were used to calibrate and improve the DNDCv.Can biogeochemical model. The calibrated model was used to simulate the development of SOC stocks, N budgets and productivity for the period 2020-2060. The model was run with three future climate scenarios. It was hypothesized that (i) the N use efficiency of digestate would be inferior to that of mineral nitrogen, and therefore more N from manure would be needed to achieve the same yield, but causing higher N₂O and NH₃ emissions, (ii) those discrepancies between mineral fertilization and organic fertilization level off in the long-term, (iii) reducing N fertilizer application rates does decrease N₂O and NH₃ emissions, (iv) reduced N application decreases carbon inputs, which may lead to a long-term reduction of soil SOC. Based on the calibrated model on experimental results we compare yield, SOC, N₂O in long term (40 years) scenarios for Eastern Lower Saxony, Germany with factors a) fertilization type, b) fertilization amount, c) climate, d) soil type.