Impact of long-term fertilization practice on soil structure evolution

• Organic carbon increased from 0.015 to 0.024 kg kg− 1  with fertilization.
• Porosity followed organic carbon, increasing from 0.36 to 0.43 cm3 cm− 3.
• Soil water-holding capacity was considerably increased with fertilization.
• Level of soil aeration also increased with amount of fertilization.
• Higher earthworm activity was observed for well fertilized soils by X-ray CT.

The study characterized soil structure development and evolution in six plots that were amended with varying amounts of animal manure (AM) and NPK fertilizer over a period of 106 years in a long-term fertilization experiment in Bad Lauchstädt, Germany. Two intact soil cores (10-cm diameter and 8-cm tall) and bulk soil samples were extracted from a depth between 5 and 15-cm from each plot. Soil properties including texture, organic carbon, soil–water characteristic, air permeability and diffusivity were measured and analyzed along with X-ray computed tomography (CT) data. Long-term applications of AM and NPK had a major impact on soil organic carbon content which increased from 0.015 kg kg− 1 (unfertilized plot) to 0.024 kg kg− 1 (well fertilized plot, 30 T ha− 1 2y− 1 AM with NPK). Total porosity linearly followed the organic carbon gradient, increasing from 0.36 to 0.43 m3 m− 3. The water holding capacity of the soil was considerably increased with the increase of AM and NPK applications. Gas diffusivity and air permeability measurements clearly indicated that the level of soil aeration improved with increasing AM and NPK fertilizer amount. The three-dimensional X-ray CT visualizations revealed higher macroporosity and biological (earthworm) activity in the well fertilized areas when compared to plots without or only a small amount of fertilizer applied. A combined evaluation of the soil water characteristic, gas transport and X-ray CT results suggested that pore size distributions widened, and pore connectivity was significantly improved with increasing fertilizer amount. Furthermore, the soils fertilized with both AM and NPK showed a more aggregated structure than soils amended with AM only.