The effect of long-term, extremely high organic fertilization on the composition of soil organic matter

Soil organic matter (SOM) composition is influenced by the input of organic materials. The chemical composition of SOM resulting from the input and turnover of large amounts of manure and waste is insufficiently known to weigh up the risks against the possible benefits. Therefore, we aimed to determine the molecular-chemical composition of SOM in four different treatments of NPK (control), compost, farmyard manure, and sewage sludge from a Danish experimental field by pyrolysis-field ionization mass spectrometry (Py-FIMS) and size/density fractionation. The amounts were applied over 17 years and correspond to inputs that in total exceed what can be legally applied over a period of > 100 years. The order of soil C-content was household waste compost (CHA) > cattle manure (CMA) > sewage sludge (SA) > NPK. All three organic amendments resulted in larger proportions of lignin dimers, lipids and free fatty acids in the soil compared to the NPK treatment. The treatments CHA and CMA added disproportionally large amounts to the proportions of particulate organic matter (POM; >63 µm, <2 g cm⁻³) that is considered labile and potentially mineralizable. However, enrichments occurred in relatively recalcitrant compound classes like lignin dimers and lipids, and in the thermally stable proportions of other compound classes (carbohydrates and various N-containing compounds), that are interpreted as being mineral-associated. This provides evidence for the presence of stable organic matter originating from the organic amendments. The increase in SOC stocks, relative to the total carbon input over the course of the experiment, indicates that the SOC building potential of the materials differed in the following order: CHA > SA > CMA. Large-scale application of compost from household waste and sewage sludge, in addition to the traditional farm-scale soil amendment with cattle manure, could therefore contribute to mitigate climate change caused by elevated CO₂ concentrations in the atmosphere.