ENHANCING NUTRIENT RETENTION IN TROPICAL TREE PLANTATIONS: NO SHORT CUTS: Fid-Bau Portal

ENHANCING NUTRIENT RETENTION IN TROPICAL TREE PLANTATIONS: NO SHORT CUTS

Bigelow, Seth W. / Ewel, John J. / Haggar, Jeremy P.

In the humid tropics large quantities of nutrients can be rapidly leached when the soil is unprotected by actively growing vegetation. We established experimental plantations of three indigenous tree species on a fertile Andisol in Costa Rica and managed them under 1‐ or 4‐year cutting cycles with uncut stands as controls. Our goals were to test whether nutrient leaching was greatest under a regime of frequent disturbances that returned modest amounts of biomass to the soil surface (cutting and replanting on a 1‐yr cycle) or less frequent disturbances that returned significantly greater amounts of plant tissues to the soil (cutting and replanting on a 4‐yr cycle), and to compare those cutting cycles with nutrient leaching from uncut stands. Leaching of NO₃⁻ (over 9 yr), Ca²⁺, Mg²⁺, and K⁺ (over 4 yr) from upper soil horizons was monitored. Water balance was determined by linking Penman‐Monteith evaporation with changes in soil water storage modeled from soil physical parameters. Drainage water for solute measurement was sampled from porous ceramic cups at 1.1 m depth in the soil.

Disturbance frequency proved to be an important determinant of NO₃⁻ leaching. Average long‐term NO₃⁻ leaching losses from stands on a 1‐yr cutting cycle were extraordinarily large: 442 mmol_c·m⁻²·yr⁻¹ (62 kg·ha⁻¹·yr⁻¹ of N), compared to 187 mmol_c·m⁻²·yr⁻¹ under a 4‐yr cutting cycle and 71 mmol_c·m⁻²·yr⁻¹ from uncut stands. Elevated NO₃⁻ leaching was primarily due to increased concentration in the soil solution (rather than increased water drainage), because cutting usually resulted in a reduction of <10% in evapotranspiration. Resilience of stands decreased with continued disturbance; under a 4‐yr cutting cycle, stands tended to take longer to return to the low levels of NO₃⁻ leaching characteristic of undisturbed stands with each episode of cutting and replanting, while NO₃⁻ losses from annually cut stands became increasingly variable over time. Due to high concentrations of soil Ca at the site, the stands proved resistant to treatment‐induced losses of base cations: no increases in Ca²⁺ or Mg²⁺ leaching accompanied elevated NO₃⁻ leaching, although K⁺ leaching did increase under the 1‐yr cutting cycle. Because of the potential for massive, sustained NO₃⁻ losses, development of land use systems for these soils should focus on minimizing frequency of disturbance.