Industrial eucalyptus plantations of southeastern Brazil may be crucial to provisioning wood while preserving adjacent remnants of the biodiverse Cerrado and Atlantic Forest ecosystems. The sustainability of tree plantations depends on their capacity to maintain soil fertility over repeated harvests. However, nutrient input-output budgets for individual management units (stands) from five plantation companies indicate compounding nitrogen losses over successive harvests, as the fast-growing trees mine native soil organic matter. Inputs and outputs of other nutrients, such as calcium, may also be imbalanced. A prior analysis found that eucalyptus stands had less soil carbon than adjacent Atlantic Forest vegetation, raising the question of how stocks of carbon, as well as plant nutrients, continue to change over time within established plantations.
Here, we assess whether and where plant macronutrients and soil carbon decrease or accumulate with repeated fertilization and harvest cycles, and whether these patterns suggest nutrient limitation of future productivity. We compared soil samples collected from the same stands and adjacent native vegetation in 2004 and in 2016, 1-2 harvests later, to a depth of 100 cm. Total C and N were assessed by combustion and total Ca, Mg, P, and K by X-ray fluorescence spectroscopy.
Results/Conclusions
Nitrogen stocks decreased and C:N ratios increased between 2004 and 2016 in the top 20 cm of soil, consistent with our hypothesis of net soil nitrogen loss. Soil carbon concentrations increased in the top 10 cm, although stocks to 20 cm did not change significantly. Large changes in near-surface stocks of calcium and potassium in some stands indicate that chemical fertilizers strongly affect soil nutrient content within a harvest cycle, but the added nutrients may not be retained in the soil over multiple cycles.
Ongoing work will assess the redistribution of nutrients within the soil profile. Accumulation at the surface would indicate efficient biological cycling of both mined and added nutrients, while accumulation at greater depths would suggest that leaching and sorption restrict trees’ nutrient uptake. In two stands, nitrogen and potassium losses from the top 20 cm alone exceed the estimated removals in harvested biomass, suggesting greater physical transport of nutrients than we would expect given the plantations’ high nutrient uptake and transpiration. Continued nitrogen losses after repeated harvest cycles imply that the soil still contains adequate nitrogen for the trees to use, but increasing C:N ratios could slow nutrient cycling and limit nitrogen access for future cycles.