Plant coexistence depends on ecosystem nutrient cycles: Extension of the resource-ratio theory

Abstract

We present a model of plant-nutrient interactions that extends classical resource competition theory to environments in which essential nutrients (resources) are recycled between plant and soil pools and dissolved nutrients are lost through plant-available (i.e., inorganic forms) or plant-unavailable (i.e., complex organic forms) pathways. Losses by dissolved organic pathways can alter ratios of nutrients that are recycled and supplied within the plant-soil system, thereby influencing competition and coexistence among plant species. In special cases, our extended model does not differ from classical models, but in more realistic cases our model introduces new dynamical behavior that influences competitive outcomes. At equilibrium, coexistence still depends on nutrient supply and consumption, but nutrient supply includes recycling and is highly sensitive to whether a species promotes more organic losses of the nutrient that limits its own growth than of nutrients that limit its competitors. Because recycling operates with a time delay compared with consumption, recycling-mediated effects on competition can, under certain conditions, lead to sustained population oscillations. Our findings have implications for how we understand nutrient competition, nutrient cycles, and plant evolutionary strategies.plant competition ͉ recycling ͉ biogeochemistry ͉ nutrient losses ͉ evolution E ssential resources, such as nitrogen or phosphorus, can limit primary production of individual plants as well as entire ecosystems (1-2) and can play an essential role in plant community assembly (3-6). Therefore, understanding plantnutrient interactions constitutes a major challenge for plant community ecology and ecosystem biology.Classical models (3, 6) of exploitative competition in plant communities have long assumed that individual plants affect the abundance of nutrients only through consumption (7-9). The system of interest involves two classes of nutrients that define two functionally different compartments ( Fig. 1a): nutrients that are bound within plant biomass and inorganic nutrients (e.g., NO 3 Ϫ or PO 4 3Ϫ ) that are available for immediate plant uptake in the environmental matrix. For simplicity, the system is usually assumed to follow simple chemostat resource supply dynamics that do not incorporate species-specific effects on recycling, nutrient loss, and other such factors (9).Plant-nutrient interactions have been extensively studied from this perspective (7-10). The impact of plants on nutrients, mediated through consumption, can be illustrated (Fig. 1b) by using the traditional graphical method of the resource-ratio theory (3). If several plant species compete for two nutrients, this graphical method provides a convenient way to predict competitive outcomes as a function of nutrient supplies (3, 4, 11).On the contrary, ecosystem biology considers consumption as only one part of a larger plant-nutrient system in which nutrients cycle between plants and their environment (11-13) (Fig. 1c). This approach typically focuses ...

Nitrogen enrichment modifies plant community structure via changes to plant–soil feedback

“…Recent focus on feedback processes has highlighted their potential importance in explaining a wide range of ecological phenomena including exotic invasions (Klironomos 2002;van Grunsven et al 2007), coexistence and dominance (Bever et al 1997;Daufrense and Hedin 2005), range expansion (van Grunsven et al 2007) and successional change (van der Putten et al 1993;Kardol et al 2006Kardol et al , 2007.…”

Increased plant growth from nitrogen addition should conserve phosphorus in terrestrial ecosystems

“…1 and supporting information (SI) Appendix a simple ecosystem model in which N and P are explicitly coupled in plant biomass. Both nutrients exist in three functionally distinct pools (20,21): nutrients dissolved in solution that are available for plant uptake, including ammonium, nitrate, and directly available amino acids for N (22,23), and phosphate for P; nutrients stored in live plant biomass; and nutrients that are bound in dissolved and solid form to soil organic matter. Fluxes between the pools define the biotic nutrient cycle: The available nutrients are assimilated by plants; plant tissue loss transfers nutrients from the plant biomass to the unavailable soil organic pool; and organic matter decomposition, through biological or biochemical mineralization pathways (24), allows the transfer of plant unavailable organic nutrients into the plant available pool.…”

“…Nutrients are lost from the plant rooting zone via two functionally distinct paths (7,21): loss of plant available nutrients through leaching, together with denitrification in the case of N, and loss of organically bound unavailable nutrients through leaching of dissolved organic matter and particulate organic matter. We ignore particulate loss of P from the reactive pool, later discussing how qualitative conclusions remain unchanged where this loss pathway is included.…”

A genetic time‐delay circuitry in mammalian cells

“…Time-delayed responses to endogenous or exogenous stimuli are a fundamental motif in regulation circuitries from the single cell (Fussenegger et al, 2000a;Mangan and Alon, 2003;Covert et al, 2005) to population-wide interand intraspecies interactions (Basu et al, 2005;Daufresne and Hedin, 2005). Time-delays in biologic networks are essential patterns: (i) in oscillating systems like the cell cycle or the circadian clock (Forger and Peskin, 2003), (ii) in the spacio-temporal coordintion of cell differentiation and development (Chang et al, 2006), and (iii) in genetic circuits designed to eliminate intrinsic noise from the cellular regulation orchestra (Becskei and Serrano, 2000;Bratsun et al, 2005;Ghosh et al, 2005).…”