ABSTRACT
Nutrient elements such as nitrogen (N) and phosphorus (P) enter terrestrial ecosystems such as forests through a variety of abiotic and biotic input sources, are transformed and cycled within ecosystems, and ultimately are eliminated from ecosystems through a variety of abiotic and biotic mechanisms (Figure 9.1) (Attiwill and Adams 1993). As a consequence of human activities, the inputs of N are increasing both globally (Gruber and Galloway 2008) and regionally within the tropics (Hietz et al. 2011). Altered nutrient inputs have the potential to change both species composition (Bobbink et al. 2010) and ecosystem processes (Hall and Matson 1999). Moreover, the global climate is changing as a result of anthropogenic greenhouse gas emissions (IPCC 2007a), and understanding coupled carbon (C) and nutrient cycles and their drivers is critically important for predicting climate-biosphere feedbacks (Hungate et al. 2003; Gruber and Galloway 2008; Townsend et al. 2011). Thus, investigating nutrient cycling in tropical forests has moved beyond a
CONTENTS
9.1 Introduction ................................................................................................ 141 9.2 Inputs, Outputs, and Transformations ................................................... 143
9.2.1 Inputs ............................................................................................... 143 9.2.2 Transformations ............................................................................. 144 9.2.3 Outputs ............................................................................................ 145
9.3 Patterns along Precipitation Gradients and Consequences of Seasonal Drought .................................................................................. 147 9.3.1 Insights from Precipitation Gradients ........................................ 147 9.3.2 Nutrient Pulsing and Intra-Annual Variation ........................... 149
9.4 Evidence for Nutrient Limitation ............................................................ 150 9.5 Knowledge Gaps and Future Directions ................................................ 154
question of academic curiosity to becoming essential for predicting earth system dynamics.
