Assessing nutrient limitation in complex forested ecosystems: alternatives to large‐scale fertilization experiments

Abstract

Quantifying nutrient limitation of primary productivity is a fundamental task of terrestrial ecosystem ecology, but in a high carbon dioxide environment it is even more critical that we understand potential nutrient constraints on plant growth. Ecologists often manipulate nutrients with fertilizer to assess nutrient limitation, yet for a variety of reasons, nutrient fertilization experiments are either impractical or incapable of resolving ecosystem responses to some global changes. The challenges of conducting large, in situ fertilization experiments are magnified in forests, especially the high-diversity forests common throughout the lowland tropics. A number of methods, including fertilization experiments, could be seen as tools in a toolbox that ecologists may use to attempt to assess nutrient limitation, but there has been no compilation or synthetic discussion of those methods in the literature. Here, we group these methods into one of three categories (indicators of soil nutrient supply, organismal indicators of nutrient limitation, and lab-based experiments and nutrient depletions), and discuss some of the strengths and limitations of each. Next, using a case study, we compare nutrient limitation assessed using these methods to results obtained using large-scale fertilizations across the Hawaiian Archipelago. We then explore the application of these methods in high-diversity tropical forests. In the end, we suggest that, although no single method is likely to predict nutrient limitation in all ecosystems and at all scales, by simultaneously utilizing a number of the methods we describe, investigators may begin to understand nutrient limitation in complex and diverse ecosystems such as tropical forests. In combination, these methods represent our best hope for understanding nutrient constraints on the global carbon cycle, especially in tropical forest ecosystems.

Detecting Terrestrial Nutrient Limitation: A Global Meta-Analysis of Foliar Nutrient Concentrations after Fertilization

“…In addition, as pointed out by Sullivan et al (2014), fertilization experiments FIGURE 7 | Symbols illustrate N:P ratio, mean ± 95 CI, differences between control and fertilized (+N, +P, +NP) treatments (A-C respectively) across families. Family abbreviations used were: ACE, (Aceraceae); ALL, (Alliaceae); AST, (Asteraceae); AVI, (Avicenniaceae); BET, (Betulaceae); CEC, (Cecropiaceae); CHE, (Chenopodiaceae); CYP, (Cyperaceae); DEN, (Dennstaedtiaceae); DIC, (Dicksoniaceae); ERI, (Ericaceae); FAB, (Fabaceae); FAG, (Fagaceae); GIN, (Ginkgoaceae); LAM, (Lamiaceae); MYR, (Myrtaceae); PHY, (Phytolaccaceae); PIN, (Pinaceae); POA, (Poaceae); POL, (Polygonaceae); POS, (Posidoniaceae); PRO, (Proteaceae); RHA, (Rhamnaceae); ROS, (Rosaceae); RUB, (Rubiaceae); SAL, (Salicaceae); and WOO, (Woodsiaceae).…”

“…The primary experimental approach used to determine soil nutrient limitation has been fertilization experiments (Tanner et al, 1998;Sullivan et al, 2014). At a species level, and from an evolutionary perspective, fertilization experiments seek to examine the relative importance of intra-and interspecific competition for abiotic resources.…”

“…Most fertilization studies focus on plant biomass as the response variable, with increased growth after fertilization defining limitation (Eviner et al, 2000;Sullivan et al, 2014). However, foliar nutrient concentrations are another valuable way to assess nutrient limitation because numerous studies demonstrate that they reflect soil nutrient concentrations at spatial scales of single sites (Shaver and Melillo, 1984;Valentine and Allen, 1990) and across chronosequences, environmental gradients, and ecosystem types (Vitousek, 1998;Han et al, 2005;Parfitt et al, 2005;Townsend et al, 2007;Ordoñez et al, 2009;Cleveland et al, 2011).…”

“…When foliar nutrients are assessed as one-time measurements it is difficult to use them as evidence of nutrient limitation, because of the tremendous variation among study sites and the lack of universal cutoff values that indicate limitation. However, when foliar nutrients are assessed in relation to a fertilization event, they represent an alternative window on nutrient limitation (Güsewell and Koerselman, 2002;Wardle et al, 2004;Ågren, 2008;Sullivan et al, 2014). It is often easier and faster to collect foliar nutrient data before and after fertilization that it is to measure changes in biomass, plant height, or other size characteristics (Koerselman and Meuleman, 1996).…”

Nutrient addition effects on tropical dry forests: a mini-review from microbial to ecosystem scales

“…A major uncertainty in TDF ecology is understanding the extent to which N, P, or other elements most limit productivity, the consequences of nutrient limitation under altered nutrient deposition regimes, and how belowground communities and processes respond ( Table 1). Although many methods have been proposed to accomplish this (Sullivan et al, 2014b), the "gold standard" of ecosystem ecology remains large-scale fertilization experiments. Such experiments provide the opportunity to resolve nutrient addition effects and mechanisms across a hierarchy of scales from microbial to trees.…”