Nutrient retention by tropical ecosystems: soil adsorption and plant absorption as synergistic processes

Laboratory and growth-chamber experiments were used to evaluate whether there was evidence for nutrient retention by tropical terrestrial ecosystems being a two-stage process involving first soil adsorption and then plant absorption. Quartz sand with and without Fe and Al oxide coatings were treated with nutrient solution, then subjected to a leaching regime that simulated early wet-season conditions at a tropical location. Nutrient cations applied were rapidly lost in the initial leaches from quartz sand without oxide coatings, but showed a more gradual loss from oxide-coated sand, indicating temporary adsorption by the latter. In a second experiment, oxide-coated sand with and without seedlings of Grevillea robusta (a non-mycorrhizal tree species) were subjected to a similar treatment and leaching losses were compared. The presence of seedlings significantly reduced the losses of all nutrient cations, with the effect being minimal for Na and greatest for K, confirming that plants can gain access to temporarily adsorbed nutrients. More typical tropical soil-vegetation systems are likely to possess properties that magnify both the adsorptive and absorptive processes that have been documented in these experiments, justifying extrapolation of the experimental results to these natural systems. The existence of a two-stage process of nutrient retention provides a plausible explanation for the resistance of most tropical ecosystems to rapid loss of nutrients following events such as fires, which provide acute nutrient loading to the system.