Abstract
We evaluated changes of different soil nitrogen forms (total N, available ammonium and nitrate, total N in microbial biomass, and soil N mineralization) after conversion of semideciduous dry tropical forest in 5- and 18-year-old pastures (YP and OP, resp.) in the western Llanos of Venezuela. This evaluation was made at early rainy season, at end rainy season, and during dry season. With few exceptions, no significant differences were detected in the total N in the three study sites. Compared to forest soils, YP showed ammonium losses from 4.2 to 62.9% and nitrate losses from 20.0 to 77.8%, depending on the season of the year. In OP, the ammonium content increased from 50.0 to 69.0% at the end of the rainy season and decreased during the dry season between 25.0 and 55.5%, whereas the nitrate content increased significantly at early rainy season. The net mineralization and the potentially mineralizable N were significantly higher (P < 0.05) in OP than in forest and YP, which would indicate a better quality of the substrate in OP for mineralization. The mineralization rate constant was higher in YP than in forest and OP. This could be associated with a reduced capacity of these soils to preserve the available nitrogen.
Highlights
Soil nitrogen (N) is a key element in primary productivity and soil fertility of ecosystems [1, 2]
The ammonium content was significantly higher in forest than in Young pasture (YP) and old pasture (OP)
It is clear that transformation of semideciduous dry tropical forest into pasture caused an increase in the total nitrogen (TN), and this is not consistent with some reported data [5, 31, 35]
Summary
Soil nitrogen (N) is a key element in primary productivity and soil fertility of ecosystems [1, 2]. Great effort has been dedicated to understand the dynamics of this element after clear-cutting and burning of these forests [1, 6]. The N mineralization involves a series of actions mainly mediated by soil microorganisms [7]. This is a sensitive process to disturbance in most forest ecosystems [1, 6]. In a tropical forest, N mineralization depends on the amount and type of organic matter and microbial activity, as well as on soil physicochemical properties and soil moisture content [8]
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