Abstract
Examining foliar nutrient concentrations after fertilization provides an alternative method for detecting nutrient limitation of ecosystems, which is logistically simpler to measure than biomass change. We present a meta-analysis of response ratios of foliar nitrogen and phosphorus (RRN, RRP) after addition of fertilizer of nitrogen (N), phosphorus (P), or the two elements in combination, in relation to climate, ecosystem type, life form, family, and methodological factors. Results support other meta-analyses using biomass, and demonstrate there is strong evidence for nutrient limitation in natural communities. However, because N fertilization experiments greatly outnumber P fertilization trials, it is difficult to discern the absolute importance of N vs. P vs. co-limitation across ecosystems. Despite these caveats, it is striking that results did not follow “conventional wisdom” that temperate ecosystems are N-limited and tropical ones are P-limited. In addition, the use of ratios of N-to-P rather than response ratios also are a useful index of nutrient limitation, but due to large overlap in values, there are unlikely to be universal cutoff values for delimiting N vs. P limitation. Differences in RRN and RRP were most significant across ecosystem types, plant families, life forms, and between competitive environments, but not across climatic variables.
Highlights
Soil nutrient availability is a factor that defines the structure, function, and dynamics of many terrestrial ecosystems, in those where nutrients are limited
We found N:P ratios to increase significantly after N fertilization [t(79) = 7.94, P < 0.0001] and decrease significantly after P fertilization [t(49) = −3.81, P < 0.0004], while no difference in N:P ratios resulted from NP fertilization [t(127) = −1.38, P = 0.1705; Figure 1B]
After NP fertilization we found that the majority of changes across ecosystems, families, and life forms were driven by P concentrations, due to the significant decreases rather than increases in N:P ratios (Figures 6, 7)
Summary
Soil nutrient availability is a factor that defines the structure, function, and dynamics of many terrestrial ecosystems, in those where nutrients are limited. The primary experimental approach used to determine soil nutrient limitation has been fertilization experiments (Tanner et al, 1998; Sullivan et al, 2014). At ecosystem and global levels, fertilization experiments confirm biogeographic patterns of nutrient limitation, relating patterns to climate, substrate age, and other physical factors (Crews et al, 1995; Laliberté et al, 2012). 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). 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
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