Soil type modulates the response of C, N, P stocks and stoichiometry after native forest substitution by exotic plantations

Natural forests conversion to intensively managed forest plantation is an expanding land-use change that could profoundly impact C and nutrient terrestrial cycles. Here we assessed how different soil types respond to changes in the C, N, and P major biogeochemical pools by comparing paired natural broadleaf deciduous forest and pine plantation across five different soils with contrasting mineralogy. We quantified major biogeochemical stocks in all soil compartments to a depth up to 240 cm, including the forest floor. Our result showed that soils' C, N, and P stocks and stoichiometry do not respond equally to this land-use change. Deep soil biogeochemical pools (120-240 cm) contributed substantially to the total soil stock in most sites (up to 40% for C, 47% for P, and 52% for P). Elemental depth distribution was significantly modified because of forest substitution, and some sites displayed a significant stock reduction in deep soil compartments (>120 cm). Whole mineral soil C and N stocks were generally lower in the planted forest, while these stocks in litter layers were significantly different between soil and forest types (p < 0.001). C and N stocks in soils with low clay content or low activity clays were more significantly affected by forest substitution. On the other hand, soils with moderate to high clay content and dominant high activity clay were only mildly affected by this change. Phosphate and ammonium available pools were more affected in low activity and low clay content soil. P pools, and consequently C:P and N:P ratios in mineral soils, vary significantly between soil types. Our results corroborate that soils with contrasting properties respond differently to native forest substitution to intensively managed plantations and that the magnitude of this response seems to be controlled mostly by clay mineralogy.