Sixty-year impact of manure and NPK on soil aggregate stability

Understanding how long-term land use affects soil quality and resistance to degradation is essential for identifying sustainable management practices in different soil types. This study's aim was to evaluate how different fertilization approaches influence soil aggregate stability (SAS) and some associated soil properties. The experiment was established in 1955 at three sites with different soil types (Chernozem on loess, Phaeozem on loess, and Cambisol on gneiss) and diverse climatic conditions. Three fertilization scenarios were selected for the study conducted during 2014–2021: i) farmyard manure (40 t ha⁻¹) once every 4 years; ii) NPK mineral fertilizer every year plus farmyard manure once every 4 years, and iii) no fertilizer (control).

Farmyard manure positively affected stability of the Cambisol soil aggregates in both cases of fertilization (i and ii). On the other hand, manure had negligible impact upon SAS of the other two soils. In addition, significantly lower SAS values were measured for soils fertilized also by the mineral fertilizer (ii) than for the other scenarios (i and iii). Manure treatment and combined fertilization showed a significant increase in hot water extractable carbon and total carbon content at all sites compared to the unfertilized treatments. A positive relationship between SAS and total organic carbon was confirmed, however, only for the Cambisol spring samples. In some years, composition of organic matter or content of glomalin was also investigated to reveal their effects on SAS. A positive impact of hydrophobicity on SAS was proven for the Phaeozem and Chernozem, but not for the Cambisol. An unexpected negative effect was observed for glomalin. For both spring and summer sampling events, the SAS values were strongly and negatively correlated with the field (sampling) soil water content, which partly masked effects of other soil properties on SAS. These results underscore the importance of complex long-term studies for understanding mutual interactions affecting the stability of soil aggregates in individual soil types and different climatic conditions.