Nutrient leaching and deep drainage under Eucalyptus plantations managed in short rotations after afforestation of an African savanna: Two 7-year time series

Abstract African savannas developed an efficient strategy of nutrientuptake and control of nitrification making it possible to avoid large losses of nutrientby drainage. The conversion of native savanna into commercial forest plantations is an important global change driver that potentially impacts element cycles. Afforestationof native Congolese savannas by Eucalyptusstarted 30 years ago. Large amounts of nutrientsare removed through biomass harvesting every 6–7 years. Losses of nutrientsby deep drainage might be a serious threat for the sustainability of these plantations, established on sandy soils with high hydraulic conductivities. We compared the soil N-mineralisation, the nutrientfluxes and deep drainage beneath savanna and Eucalyptusplantation in Congo. Then, we discussed the strategy of nutrientrecycling. The water fluxes at a depth of 400 cm were approximately 20% higher in the savanna than in Eucalyptusplantation. Although the nitrification rate and solution chemistry exhibited strong modifications during the first year following both savanna afforestationand the harvesting of the Eucalyptusstand, the losses of nutrientsby deep drainage remained unexpectedly low. The largest fluxes of drainage at a depth of 6 m were found for NH 4 + -N , which reached a maximum of 0.4 g m −2 yr −1 in the second year following savanna afforestationand 1 g m −2 yr −1 in the first year after clear cutting. The deep drainage of NO 3 - -N , Mg 2+ , Ca 2+ and K + did not exceed 0.2 g m −2 yr −1 in the savanna and at any stage of plantation development. These results are discussed regarding (i) the roots distribution in the soil of each ecosystem and (ii) the nutrientaccumulation in biomass. The limited changes between the nutrientfluxes in both ecosystems were the result of fast root growth in the deep soil layers after planting, combined with an intense uptake of the tree roots to satisfy the large nutrientand water requirements for the development of tree crowns. Intense uptake and cycling of nutrientsmitigated the risk due to clear cut and N fertilisation in this forest plantation. Nevertheless, forest managers must carefully fit fertilisation regimes to the nutrientrequirements of new clone selected by breeding programmes and reduce as much as possible the delay between harvesting and re-planting to avoid high losses by deep drainage.