Texture and clay mineralogy as main drivers of the priming effect in temperate forest soils

This work aimed to determine how the soil parameters affect the magnitude and direction of priming effect (accelerated or decreased decomposition of native SOM under addition of new organic substrates, PE) in temperate acidic forest soils.

Thirteen topsoil samples were incubated for 163 days with the addition of maize residues. Soil respiration was measured and natural isotope labelling was used in order to separate the respiration sources (SOM, maize and PE). The effect of soil parameters on PE was studied using linear regression and structural equation modelling (SEM).

Soils with high C/N ratio showed the lowest magnitude of cumulative PE (R² = 0.321, p < 0.05) and the longest negative PE period. A positive relationship was found between PE and the pH (R² = 0.511, p < 0.05). SEM analysis showed that pH and C/N ratio has direct (β = 0.50) and indirect (β = 0.20, via modifying soil texture and mineralogy) effect on PE. Soil texture and mineralogy had a significant effect on PE: texture affects the proportions of soil respiration sources and PE was reduced by the dithionite–citrate–bicarbonate–extractable Al (Al_DCB, R² = 0.454, p < 0.05), silt + clay (R² = 0.421, p < 0.05), non-swelling clay mineral (R² = 0.575, p < 0.05) and illite (R² = 0.522, p < 0.05) contents. SEM analysis also highlighted that the Al_DCB, illite and silt + clay contents has a great effect (β=−0.59) on the PE.

The silt + clay content and mineral composition of the soil, including the Al oxide and illite contents may thus significantly inhibit the magnitude of PE, and consequently the decomposition of SOM under acidic conditions.