Publisher Summary This chapter discusses a number of factors influencing the throughfall and stemflow quality and their variation. The effect of the canopy in altering the precipitation quality is also discussed. The annual nutrient return to the forest soil for the elements K, Na, and S is predominantly via throughfall and stemflow and little because of litterfall. Stemflow transfers only 5–20% of the total in precipitation-borne solutes, yet it is the major nutrient input to restricted areas of the forest floor. Because throughfall and stemflow are associated with precipitation events, the transport of nutrients contained in throughfall and stemflow depends on the magnitude, timing, and form of the precipitation. Thus, a reliable estimate of throughfall nutrient flux demands a good forest hydrological budget. The quantity and distribution of throughfall and stemflow depends on microscale features of canopy structure, such as crown density, closeness of the foliar elements, distance from the nearest bole, or open spaces in the canopy.

Throughfall and Stemflow in the Forest Nutrient Cycle

Analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the Amazon River system draining past Obidos (80% of the basin) at the time of peak discharge in June. Amazon precipitation chemistry can be devided into two principal components: marine and terrestrial. The marine component (determined from analyses of marine rain) consists of Na, K, Mg, Ca, and Cl in approximately sea-salt proportions, with S doubly enriched. The excess sulfur is probably derived from gas phase inputs. The terrestrial component makes an important contribution of K, Ca, S, and N, much of which can be related to biological emissions. The emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural ‘acid rain’ in the Amazon region with a pH from 4.7 to 5.7. This is about one tenth the acidity of polluted urban rain. The chloride content of lowland rivers, which drain regions lacking significant geologic sources of chloride, shows a systematic decrease in chloride with increasing distance from the ocean. This trend is used to define the cyclic salt background for Amazonian surface waters. Cyclic salts, in general, make only a minor contribution, relative to terrestrial inputs, to the chemistry of Amazon Basin rivers, even those draining intensely weathered terrains. An estimated 17.6%-Cl, 6.9%-Na, 1.3%-Mg, 3.6%-S, 0.4%-K, and 0.1%-Ca of the dissolved load at Obidos during peak discharge is cyclic.

Geochemistry of the Amazon: 1. Precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge

Nutrient poor forests are more common in the tropics than in the temperate regions, but most of the work on nutrient cycling in the tropics has been carried out on nutrient rich sites such as those in Puerto Rico and Costa Rica rather than nutrient poor sites such as those in central and eastern Amazonia. Consequently, there has been confusion as to whether nutrients are critical in tropical forests. Productivity and nutrient cycling in nutrient rich and nutrient poor forest ecosystems do not differ greatly as long as the ecosystems are undisturbed. However, when the forests are cleared for agricultural purposes, the nutrient poor systems quickly lose their productive potential, whereas the nutrient rich systems do not. The nutrient poor ecosystem is able to maintain its productivity under undisturbed conditions through a variety of nutrient conserving mechanisms, the most important of which are associated with the humus and root layer on top of the mineral soil. These mechanisms are destroyed when the nu...

Tropical Rain Forests: Are Nutrients Really Critical?

Atmospheric gains and hydrologic losses of calcium, magnesium, potassium, phosphorus and nitrogen for 25 tropical forest sites on a variety of geological substrates are reviewed. The data set comprised 19 lowland and six montane sites. Twenty studies were subjected to further analysis after initial quality control. These were subdivided into forests on (1) very infertile soils (N = 5), (2) (moderately) infertile soils (N = 5), (3) moderately fertile soils (N = 4) and (4) fertile soils (N = 4). Two studies pertaining to large river basins were treated separately. Although variation in nutrient fluxes was large, reflecting both natural and methodological factors, scatter plots of annual calcium, magnesium and potassium losses v. annual runoff for small catchment areas revealed four groups with characteristic nutrient export patterns that corresponded closely with soil fertility levels. Element losses from the two large basins were much higher than those recorded for small basins in the same areas and were interpreted in terms of depths of weathering front, river incision and root network. Phosphorus accumulated in virtually all cases, reflecting the low mobility of the element. Nitrogen budgets were generally very incomplete. There is a need for more and careful studies of tropical forest nutrient budgets, especially for nitrogen. Standardization of methodology is essential if comparability of results is to be improved.

Nutrient input–output budgets of tropical forest ecosystems: a review

Rainfall interception from a lowland tropical rainforest in Brunei

/pdf/water-and-cation-movement-in-a-tropical-rainforest-2ojljj84sr.pdf

S. Nortcllff

Papers

Water and cation movement in a tropical rainforest environment. I. Objectives, experimental design and preliminary results.