RAPID WATER FLOW AND TRANSPORT OF INORGANIC AND ORGANIC NITROGEN IN A HIGHLY AGGREGATED TROPICAL SOIL

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Abstract
Microaggregated tropical soils have shown high water conductivity even under unsaturated conditions in laboratory experiments. It is not clear, however, what depth the infiltrating soil water reaches during storm events under humid tropical conditions and how this relates to losses of N by leaching. Dynamics and fluxes of water and applied 15N were determined with high temporal resolution to a depth of 5 m in a Xanthic Hapludox of central Amazonia, Brazil. The soil water percolated to a depth of 0.9 m within 2 h of a rainfall event of 48 mm. Water fluxes were significantly slower below 0.9 m (17% of infiltration at 0–0.9 m) due to higher bulk densities. Percolation not only started rapidly after a rainfall event when soil water suction reached a certain threshold (ca. 20–30 hPa) but was also reduced to background levels less than 1 h after the rain had ended. Traces of labeled N reached 5 m within a few days, and 15N maintained high levels to a depth of 1.2 m throughout the rainy season. Organic N was a large proportion (36–44%) of the total N leaching and the proportion increased with depth. However, organic N percolated more slowly than nitrate. The demonstrated extreme short-term dynamics of water fluxes have implications for measurement design of water availability and solute leaching in microaggregated tropical soil that require correct time integrals of solution concentrations and soil water dynamics. Measurement intervals of 30 min or less were necessary in our study. Rapid water flows explain the observed high N losses from the topsoil of microaggregated tropical soil and the large nitrate accumulation in the deep soil to a depth of at least 5 m.