ENHANCING NUTRIENT RETENTION IN TROPICAL TREE PLANTATIONS: NO SHORT CUTS

Abstract

In the humid tropics large quantities of nutrients can be rapidly leached when the soil is unprotected by actively growing vegetation. We established experimental plan- tations 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_-m-2.yr-1 (62 kg-ha-1.yr-1 of N), compared to 187 mmolc.m-2.yr-1 under a 4-yr cutting cycle and 71 mmolc.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 undis- turbed 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.