Nutrient Limitation of Ecosystem Production

Nutrient limitation is a fundamental concept in ecological research and, at its core, is a question of ecological stoi-chiometry. Primary production is usually stimulated by nutrient addition. Increased production following fertilization has been identified by some as one of the most repeatable and predictable features of nature and one humans rely upon for their food supply. Modern research into nutrient limitation can be traced to Justus Liebig, who developed the ''law of the minimum'' to explain why crop yield was most often stimulated by a single element, though that element may change from one system to the next. Simply put, the law of the minimum states that the nutrient present in the lowest amount relative to organism demand will limit growth and production. Liebig's law rests on an implicit assumption that an organism's elemental composition is relatively constrained and that its production will reduce elemental pools until one element becomes rare enough to limit production.

As a modern research topic, nutrient limitation spans levels of organization from individuals to ecosystems. At the ecosystem scale, investigations of nutrient limitation tend to focus on primary rather than secondary production because it represents the interface between the physical and chemical environment and the resident biological community. Rates of material flow across this boundary and the factors that influence those rates are fundamental topics in ecosystem science. From a stoichiometric perspective, limitation status is a product of relative nutrient supply and demand for those elements by biota capable of 'using' inorganic nutrient forms (e.g., plants, bacteria, and fungi). At the scale of an organism, relative nutrient requirements are controlled by growth rate, specific physiological processes, and plasticity therein. At the ecosystem scale, nutrient demand is also influenced by trophic structure (e.g., relative abundance of primary producers), nonassimilative biological processes (e.g., deni-trification) and various physical and chemical processes (e.g., precipitation, weathering, leaching). Nevertheless, we frequently speak of biological production in forests, grasslands, lakes, and streams as nitrogen or phosphorus limited because nutrient imbalances between inorganic pools and organism stoichiometry are pronounced or experimental manipulations have indicated that the system responded in some way to nutrient addition.

In terrestrial ecosystems, N limitation is most common though P-limitation has been observed. Global scale patterns in soil nutrient availability suggest that N limitation is most pronounced in recently glaciated temperate areas with P-poor soils dominating tropical regions. More current research indicates that foliar chemistry tracks this pattern with lower C:N and higher N:P ratios typical of leaf tissue from tropical versus temperate ecosystems. A frequent explanation for N limitation is the rarity of N in mineral substances relative to P and the difficulty of transforming dinitrogen gas to forms available to plants and microorganisms (biologically mediated N-fixation). A secondary explanation of N limitation in terrestrial ecosystems relates to the relative mobility of dissolved forms of N and P. Phosphorus mobility in soils is poor due to sorption kinetics of dissolved P-forms. Therefore, as P cycles from organic compartments to inorganic forms (via mineralization), there is a tendency for it to remain in the local environment. Nitrogen is mineralized from organic material as ammonium (NH4 +), which like dissolved P has poor mobility in soils. However, microbial activity rapidly converts ammonium to a much more mobile form, nitrate (NO3 -), via nitrification, which can be rapidly leached from soils. In addition, loss of N in dissolved organic forms (e.g., humic and fulvic acids) has been shown to be an important avenue of N loss. The development of P-poor soils and the potential for P-limitation become pronounced as landscapes and associated ecosystems age. Research on geological chron-osequences in Hawaii and New Zealand indicates that terrestrial ecosystems move toward P-limitation with age due to the establishment of N-fixing organisms, accumulation of organic N in soils, and long-term losses of P via weathering and subsequent leaching. Tropical ecosystems have also been shown to tend toward P-limitation for similar reasons (i.e., highly weathered soils and abundant N-fixation). In the modern era, a human-induced change in the abundance of biologically available N is also likely to push terrestrial systems away from N limitation (potentially toward P-limitation) even in relatively undisturbed ecosystems (via atmospheric deposition).

In aquatic habitats, the prevailing contention has historically been that phosphorus tends to limit gross primary production in freshwater ecosystems, whereas nitrogen tends to limit production in marine environments. However, recent reviews indicate that this conclusion is an oversimplification and the nature of nitrogen and phosphorus limitation of gross primary production in aquatic ecosystems appears more complicated than previously thought. Unlike the terrestrial case presented above, anthropogenic nutrient enrichment of lakes indicates greater relative additions of P versus N. In contrast, human impacts on stream and river ecosystems tend to increase nitrogen availability relative to phosphorus. Recent reviews also suggest gross primary production in marine offshore ecosystems is often limited by phosphorus or micronutrients, whereas gross primary production in coastal marine ecosystems tends to be N limited. An unfortunate stoichiometric outcome of human activity has arisen from elevated N in rivers alleviating primary producer limitation in coastal waters. Excess production in many estuaries has led to extensive 'dead zones' created by anaerobic conditions that develop as excess production is mineralized.

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