Acidification Induced by Different Nitrogen Sources in Columns of Selected Tropical Soils

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Abstract
Sustainability in agriculture is a rising concern in the tropics where fertilizers are introduced to meet increasing food demands. The adverse effects of long‐term fertilizer use were monitored through changes in the soil exchange complex in packed columns of an Oxic Paleustalf, a Typic Paleudult, and a Tropeptic Haplustox. Following each application of N at rates of 0, 50, or 100 kg ha−1 as urea, ammonium sulfate (AS), or calcium ammonium nitrate (CAN), the soils were flushed with distilled water, representing the water draining from these soils in their respective tropical environments during a growing season. The procedure was repeated 30 times during 2 yr with 6‐wk rest periods after each five cycles. The rate of acidification reflected the rate of application and acid‐producing capacity of the N source. In the Oxisol and Ultisol, further acidification resulted in an increase in exchangeable Al and Mn, which were eventually found in the effluent, suggesting mineral dissolution. In the Alfisol, acidification resulted in a loss of cation‐exchange sites and the increasing predominance of Al and Mn. This acid‐consuming process caused the development of a descending acidification front, in which soil pH dropped from 6.7 to 5.0. Leachates remained neutral until the acidification front reached the bottom of the soil column. Applied N was quantitatively recovered in the leachates of all soils. Although plant growth would moderate these processes, the results show that indiscriminate N use on such fragile soils may eventually cause soil degradation and groundwater pollution.
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