Effects of mineral versus organic fertilizers on soil fertility and organic carbon stocks in agricultural topsoil and subsoil

The main goals of sustainable agricultural practices are to rebuild soil organic carbon (SOC) stocks and to sustain soil fertility. The use of organic amendments such as manure, slurry and biogas digestate, as sources of carbon and nutrients, is one of the levers to achieve these goals, as an alternative to the use of mineral fertilizers. However, the effects of organic amendments compared with traditional mineral fertilizers on topsoil and subsoil SOC stocks and soil fertility are still uncertain. Hence, we aimed at investigating the effects of mineral and organic fertilizers (i.e., manure, pig slurry and biogas digestate) on topsoil and subsoil biogeochemistry, and soil structure, after seven years of application. To this end, we sampled soil cores down to 1 m depth in a randomized field experiment in North Germany, running since 2011. We quantified the SOC and nitrogen stocks, as well as some nutrient contents (e.g., nitrate, available phosphorus). Selected samples were further analysed for aggregate size distribution, as well as organic carbon and nitrogen contents within these aggregates. A hyperspectral camera in the range of Vis-NIR was used to scan undisturbed core-samples in order to reveal hotspots of carbon storage along the soil profile. Soil carbon distribution was predicted as a function of spectral response coupled with a machine learning ensemble. Overall, the mean SOC stocks were low (53 t ha-1), reflecting the sandy loam texture of the Northeast German soils under permanent cropping. The application of organic fertilizers (whatever their nature) resulted in higher SOC contents in the first 10 cm (+26 %) and from 20-40 cm (+30%), as compared to the mineral fertilizer treatments. The application of mineral fertilizer or digestate, as compared to the control, resulted in higher relative amount of microaggregates (versus macroaggregates) (+ 19-40 %) in the soil down to 80 cm. These results will provide essential information to develop management strategies that could increase nutrient recycling as well as SOC stocks.