Carbon, Nitrogen, and Phosphorus Dynamics Across a Long Chronosequence in Semiarid Woodland Ecosystems, Northern ArizonaEPA Grant Number: U916251
Title: Carbon, Nitrogen, and Phosphorus Dynamics Across a Long Chronosequence in Semiarid Woodland Ecosystems, Northern Arizona
Investigators: Selmants, Paul C.
Institution: Northern Arizona University
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $99,022
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Fellowship - Terrestrial Ecology and Ecosystems , Academic Fellowships , Ecological Indicators/Assessment/Restoration
The objective of this research project is to test one of the most fundamental theories in ecosystem ecology, the Walker and Syers model of ecosystem development.
In 1976, Walker and Syers proposed a coherent explanation of how soil nutrient availability changes during long-term soil development based on a series of chronosequence (i.e., soil-age gradient) studies in New Zealand. The model distinguishes between atmospherically derived nutrients such as carbon (C) and nitrogen (N) and rock-derived nutrients such as calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P). The essence of the Walker and Syers model is that, as ecosystems develop, the stock of rock-derived nutrients is depleted over time, while atmospherically derived nutrients accumulate. Consequently, plant production and other ecosystem processes often are limited by N in young developing ecosystems, but through time, the rock-derived nutrients, especially P, become more limiting as their stock in the soil is depleted gradually. Walker and Syers viewed P as the most important of these rock-derived nutrients because P availability is frequently a primary factor governing both primary production and N accumulation within ecosystems (Walker and Syers, 1976; Vitousek and Howarth, 1991).
The Walker and Syers theory of ecosystem development was tested later in the Hawaiian Islands across a chronosequence of soil development spanning more than 4 million years (Crews, et al., 1995; Riley and Vitousek, 1995; Vitousek and Farrington, 1997). The results of these studies provided remarkable support for the Walker and Syers model; N limitation was substantial early in ecosystem development (first 20,000 years), while P availability limited plant production at the oldest site (4.1 million years). Plant production was highest at intermediate aged sites because N and P had equilibrated elative to plant requirements.
The majority of studies investigating ecosystem development as a function of soil age has been based on ecosystems with relatively mesic climates (e.g., Walker and Syers, 1976; Crews, et al., 1995; Schlesinger, et al., 1998). However, Lajtha and Schlesinger (Lajtha, 1998; Lajtha and Schlesinger, 1988) assessed changes in N and P availability along a 75,000-year soil chronosequence developed from quartz monzonite alluvium in the Jornada Desert, New Mexico. In contrast to studies in mesic ecosystems, these authors found no correlation between N and P availability and soil age, and they concluded that ecosystem development in this desert system did not fit the Walker and Syers model. Unfortunately, the Jornada Desert chronosequence was relatively short both chronologically and pedogenically; soil development should proceed much more slowly in arid climates than in more mesic climates because of lower rates of weathering. Hence, it remains unclear if the Jornada Desert chronosequence failed to fit the Walker and Syers model because the soil-age gradient was too short or because the controls on ecosystem development are truly different in more arid environments.