Properties of soils in the Swedish long-term fertility experiments: VII. Changes in topsoil and upper subsoil at Örja and Fors after 50 years of nitrogen fertilization and manure application

Abstract

We tested the hypothesis whether fertilization has an impact on subsoil development using data from two long-term soil fertility experiments. We chose treatments without nitrogen (N) and high N fertilizer addition in two crop rotations with and without ley and farmyard manure application. The soil properties measured included soil organic matter, microbial composition, aggregate stability, readily dispersible clay and micronutrient content in topsoil (0–30 cm) and upper subsoil (30–40 cm). Studies of two adjacent soil profiles revealed that the upper subsoil of an N-fertilized treatment had more soil organic matter, darker colour, more aggregation, more fine roots and was 10 cm thicker than that of the no-N treatment. This pronounced subsoil development in treatments with higher yields (high N input) was corroborated by carbon mass determinations based on systematic sampling including bulk density. Soil organic matter contents were higher in N-fertilized than unfertilized treatments, either with or without manure. Nitrogen fertilization caused distinct differences in microbial composition at one study site. Phospholipid fatty acid analysis revealed: (1) that mineral N fertilizer promoted nitrifiers; and (2) that bacteria were favoured over fungi in treatments with N supply probably due to nitrogen-rich crop residues. Soil aggregate stability (water dispersion) was correlated to soil organic matter and clay content. We also tested the hypothesis whether trace element contents in the top- and subsoil were affected by N fertilization through removal by large yields. Despite long-term depletion, crop removal of micronutrients was too small to cause significant differences in soil between treatments.