Element nutrient has long been identified as one of the abiotic factors constraining plant growth, ever since Liebig's ‘law of the minimum’, which describes crop production as being limited by the nutrient in the shortest supply. Among the mineral nutrient elements, nitrogen (N) is recognized as the most widely limiting nutrient for plant growth both on land and in the sea (Vitousek & Howarth, 1991). Therefore, N fertilization is widely used to improve soil N availability and increase plant growth and productivity (Frink et al., 1999). With regard to the unprecedented global climate change revealed by the four assessment reports of Intergovermental Panel of Climate Change (IPCC) since 1990, it is of great concern whether the terrestrial biosphere acts as a net C sink or source, that is, whether it poses negative or positive feedback to climate change. The third assessment report of the IPCC predicted 260–450 Pg C accumulation in terrestrial ecosystems under atmospheric CO2 and climate change, 16–34% of the expected anthropogenic CO2 emissions in an intermediate emissions scenario (Cramer et al., 2001; IPCC, 2001). Based on the theory of ecological stoichiometry (Sterner & Elser, 2002), the CO2-climate projections on C accumulation require 2.3–16.9 Pg N, which may not be met under natural conditions (Hungate et al., 2003). Elevated atmospheric CO2 concentration can increase N immobilization by long-lived plant biomass and soil organic matter by stimulating plant growth, reducing N release and return to soil from litter decomposition, and decreasing N fixation over time (Hungate et al., 2004). Consequently, soil N availability will gradually decline and progressively constrain plant growth and net primary production (NPP; Luo et al., 2004; Reich et al., 2006), leading to lower C sequestration in terrestrial ecosystems than previously predicted. Thus, the central question of carbon–climate interactions is how N availability impacts the capacity of the terrestrial ecosystem to sequester C from the atmosphere (Gruber & Galloway, 2008; Heimann & Reichstein, 2008). Therefore, better understanding of how and to what extent N restrains terrestrial plant growth and NPP (Elser et al., 2007; LeBauer & Treseder, 2008) is critical for a convincing projection of terrestrial C sequestration.
Along with global climate change, the global N cycle has also been profoundly altered by anthropogenic activities (Vitousek et al. 1997; Gruber & Galloway, 2008). Global N enrichment has been widely found to cause changes in community structure, reductions in species richness, and losses of biodiversity in various terrestrial biomes (Wedin & Tilman, 1996, Robbink et al., 1998; Gough et al., 2000; Zavaleta et al., 2003; Stevens et al., 2004; Suding et al., 2005). Given the positive relationship of biodiversity and ecosystem stability (Tilman et al., 2006), declines in plant diversity under N deposition/addition may result in greater variability (lower stability in reverse) in ecosystem function and services under environmental perturbations. Moreover, changes in community structure and biodiversity imply differential responses of growth of terrestrial plant species and their competitive ability under N addition. Therefore, understanding how terrestrial plant species respond to N addition can help to explain the changes in biodiversity, structure and functioning in terrestrial ecosystems.
There are more than 300 000 plant species across the world (Millennium Ecosystem Assessment, 2005), which can be categorized into different biological realms (e.g. seed plant, spore plant) and various functional types (e.g. growth forms, life history, and photosynthetic pathways). These categories differ in their N use strategy and grow in highly variable habitats depending on latitude and climate. Both habitats and plant functional types can impact plant responses to N addition and/or global change (Shaver & Chapin, 1980; Reich et al., 2001, 2003, 2004; Van Wijk et al., 2003). Regional and global patterns of tissue N concentrations of terrestrial plants have been widely reported along geographic and climatic gradients (Güsewell, 2004; McGroddy et al., 2004; Reich & Oleksyn, 2004; Wright et al., 2004; He et al., 2006; Lovelock et al., 2007). Moreover, N fertilization has long been conducted for a wide variety of plant species and terrestrial ecosystems (Frink et al., 1999; Reich et al., 2003). However, we still lack general response patterns of terrestrial plants at the species level to N addition (but, for NPP response, see Gough et al., 2000; Elser et al., 2007; LeBauer & Treseder, 2008), thus impeding the simulation and projection of climate-terrestrial C cycling feedback loops as well as changes in terrestrial ecosystem structure and functioning.
To reveal general response patterns of plant biomass and N concentration, we conducted a mixed-model meta-analysis using data from 456 terrestrial plant species (not including agricultural or horticultural, see Supplementary material, Tables S1–S3) in 304 published papers (see Text S1 and S2). In analyzing the response patterns of plant biomass and tissue N concentration to N addition, we incorporated factors such as geographic regions (latitude), climate (precipitation and temperature) gradients, biological realms (seed plant vs spore plant), growth forms (woody vs herbaceous species, grass, forb, shrub, and tree), life histories (annual herb vs perennial herb), photosynthetic pathways (C3 herb vs C4 herb) and other functional types (broadleaved tree vs coniferous tree, deciduous tree vs evergreen tree, legume vs nonlegume).