Acidification, N-mineralisation and carbon sequestration in a fruit orchard soil at Klein-Altendorf after 55 years


In the strive for sustainable and resilient horticulture, the objective of this study was to investigate long-term (55 years) effects of different fertilisation viz orchard management strategies at Klein-Altendorf, University of Bonn (50°N), Germany (ca. 600 mm annual precipitation; 9.6°C; fertile luvisol on loess). Both the grass mulch from the alleyways and the prunings remain in the orchard. The four treatments of 40 trees each were 1) control trees without fertilisation; 2) lime (1 t CaO ha-1 10 years-1 to stabilize the pH near the optimum for stone fruit; 3) lime as above plus organic fertiliser; and 4) lime with inorganic NPK – on the herbicide strip. Results after 55 years are: 1) Control apple and cherry trees without fertilisation grew well without nutrient deficiency symptoms or any yield depression; 2) lime (1 t CaO ha-1 10 years-1) increased soil pH from pH 6.3 to pH 6.7, whereas the pH in the un-fertilised plot dropped to pH 5.7-5.9, thereby confirming the proposed slow soil acidification in the herbicide strip; 3) lime as above plus organic fertiliser (3 t DM pig manure ha-1) in the first two decades and then BGK certified green compost (10 t DM ha-1 a-1) in the last 12 years increased humus and nitrogen mobilisation in spring; 4) lime as above plus inorganic NPK fertiliser (40 kg N ha-1 year-1) increased both pH and soil nutrient content; 5) lime as above plus organic fertiliser accumulated the largest total nitrogen (Nt) with the largest nitrogen mineralisation (28-55 kg Nmin ha-1) in spring, double that of the other plots (18-28 kg Nmin ha-1) primarily as NO3 and, to a lesser extent, as NH3; and 6) lime plus organic fertiliser increased the humus content by ca. 2.5 fold from 1.8% (in 1968) to 4.3% (in 2021) indicating an annual rate of 0.028% SOC year-1 and a potential for carbon sequestration, at a rate of 0.7 t CO2 ha-1 year, if organic matter was regularly supplied. Conclusion: tillage-free fertile soils, particularly stonefruit orchards, offer the possibility of carbon sequestration, if organic matter with limited nutrients (plus lime) is regularly applied at a moderate rate ( extless10 t DM ha-1 a-1).