Nitrogen and phosphorus addition promote invasion success of invasive species via increased growth and nutrient accumulation under elevated CO2.

In the context of the resource allocation hypothesis regarding the trade-off between growth and defence, compared with native species, invasive species generally allocate more energy to growth and less energy to defence. However, it remains unclear how global change and nutrient enrichment will influence the competition between invasive species and co-occurring native species. Here, we tested whether nitrogen (N) and phosphorus (P) addition under elevated CO2 causes invasive species (Mikania micrantha and Chromolaena odorata) to produce greater biomass, higher growth-related compounds and lower defence-related compounds than native plants (Paederia scandens and Eupatorium chinense). We grew these native and invasive species with similar morphology with the addition of N and P under elevated CO2 in open-top chambers. The addition of N alone increased the relative growth rate (RGR) by 5.4% in invasive species, and its combination with P addition or elevated CO2 significantly increased the RGR of invasive species by 7.5 or 8.1%, respectively, and to a level higher than that of native species (by 14.4%, P < 0.01). Combined N + P addition under elevated CO2 decreased the amount of defence-related compounds in the leaf, including lipids (by 17.7%) and total structural carbohydrates (by 29.0%), whereas it increased the growth-related compounds in the leaf, including proteins (by 75.7%), minerals (by 9.6%) and total non-structural carbohydrates (by 8.5%). The increased concentrations of growth-related compounds were possibly associated with the increase in ribulose 1,5-bisphosphate carboxylase oxygenase content and mineral nutrition (magnesium, iron and calcium), all of which were higher in the invasive species than in the native species. These results suggest that rising atmospheric CO2 concentration and N deposition combined with nutrient enrichment will increase the growth of invasive species more than that of native species. Our result also suggests that invasive species respond more readily to produce growth-related compounds under an increased soil nutrient availability and elevated CO2.

Effects of elevated CO2 concentration and nitrogen addition on foliar phosphorus fractions of Mikania micranatha and Chromolaena odorata under low phosphorus availability.

Invasive plants rapidly spread in habitats with low soil phosphorus (P) availability and have triggered a sharp decline in the diversity of native species. However, no studies have explored how widespread invasive species acclimate to low soil P availability via changing foliar P fractions, especially under elevated atmospheric CO2 concentrations ([CO2 ]) and nitrogen (N) deposition. Here, an open-top chamber experiment was conducted to explore the effect of nutrient addition and elevated [CO2 ] on leaf traits and foliar functional P fractions (i.e., Pi, metabolite P, lipid P, nucleic acid P, and residual P) of two aggressive invasive species (Mikania micranatha and Chromolaena odorata). We found that foliar N/P ratios were more than 20, and P addition significantly increased plant biomass. Both results indicated P-limited plant growth at our studied site. Elevated [CO2 ], N and N + P addition greatly increased plant biomass, photosynthetic rates, and photosynthetic P-use efficiency (PPUE) in invasive species, but PPUE decreased with increasing P addition. Nitrogen addition slightly decreased the concentration of leaf total P, decreased foliar residual P, but increased metabolite P concentrations in invasive species. Similar changes in foliar P fractions were found under N + P addition. Phosphorus addition increased foliar P concentrations, which was strongly correlated with an increase in metabolite P concentrations in invasive species. Elevated [CO2 ] alleviated these effects and increased PPUE. The present results suggest that future elevated [CO2 ] and N deposition allow the invasive species to acclimate to low soil P availability and support their successful invasion by greatly reducing P allocation to non-metabolite foliar P fractions (i.e., nucleic acid P and residual P) to meet their demand of metabolite P for photosynthesis and exhibit a high PPUE.

Latitudinal patterns of terrestrial phosphorus limitation over the globe.

Phosphorus limitation on terrestrial plant growth is being incorporated into Earth system models. The global pattern of terrestrial phosphorus limitation, however, remains unstudied. Here, we examined the global-scale latitudinal pattern of terrestrial phosphorus limitation by analysing a total of 1068 observations of aboveground plant production response to phosphorus additions at 351 forest, grassland or tundra sites that are distributed globally. The observed phosphorus-addition effect varied greatly (either positive or negative), depending significantly upon fertilisation regime and production measure, but did not change significantly with latitude. In contrast, phosphorus-addition effect standardised by fertilisation regime and production measure was consistently positive and decreased significantly with latitude. Latitudinal gradient in the standardised phosphorus-addition effect was explained by several mechanisms involving substrate age, climate, vegetation type, edaphic properties and biochemical machinery. This study suggests that latitudinal pattern of terrestrial phosphorus limitation is jointly shaped by macro-scale driving forces and the fundamental structure of life.

Global meta-analysis shows pervasive phosphorus limitation of aboveground plant production in natural terrestrial ecosystems.

Phosphorus (P) limitation of aboveground plant production is usually assumed to occur in tropical regions but rarely elsewhere. Here we report that such P limitation is more widespread and much stronger than previously estimated. In our global meta-analysis, almost half (46.2%) of 652 P-addition field experiments reveal a significant P limitation on aboveground plant production. Globally, P additions increase aboveground plant production by 34.9% in natural terrestrial ecosystems, which is 7.0-15.9% higher than previously suggested. In croplands, by contrast, P additions increase aboveground plant production by only 13.9%, probably because of historical fertilizations. The magnitude of P limitation also differs among climate zones and regions, and is driven by climate, ecosystem properties, and fertilization regimes. In addition to confirming that P limitation is widespread in tropical regions, our study demonstrates that P limitation often occurs in other regions. This suggests that previous studies have underestimated the importance of altered P supply on aboveground plant production in natural terrestrial ecosystems.

Photosynthesis of subtropical forest species from different successional status in relation to foliar nutrients and phosphorus fractions.

The ecophysiological linkages of leaf nutrients to photosynthesis in subtropical forests along succession remain elusive. We measured photosynthetic parameters (Amax, Vcmax, Jmax, PPUE), leaf phosphorus (P) and nitrogen (N), foliar P fractions and LMA from 24 species (pioneer, generalist, and climax). Amax was significantly related to N and P for the pooled data, while significant relationship between Amax and P was only found in climax species. The mixed-effect model including variables (N, P, and SLA or LMA) for predicting Vcmax and Jmax best fitted but varied remarkably across succession. Climax species had higher N: P ratios, indicating an increasing P limitation at later succession stage; photosynthesis, however, did not show stronger P than N limitations across all species. Nevertheless, climax species appeared to increase nucleic acid P allocation and residual P utilization for growth, thereby reducing the overall demand for P. Our results indicate that the scaling of photosynthesis with other functional traits could not be uniform across succession, growth variables (e.g. photosynthesis) and species trait identity (e.g. successional strategy) should be considered in combination with N: P ratio when we investigate P limitation in subtropical forests, and variations in P allocation state further influencing photosynthetic rates and P-use efficiency.

Effects of climate on soil phosphorus cycle and availability in natural terrestrial ecosystems.

Climate is predicted to change over the 21st century. However, little is known about how climate change can affect soil phosphorus (P) cycle and availability in global terrestrial ecosystems, where P is a key limiting nutrient. With a global database of Hedley P fractions and key-associated physiochemical properties of 760 (seminatural) natural soils compiled from 96 published studies, this study evaluated how climate pattern affected soil P cycle and availability in global terrestrial ecosystems. Overall, soil available P, indexed by Hedley labile inorganic P fraction, significantly decreased with increasing mean annual temperature (MAT) and precipitation (MAP). Hypothesis-oriented path model analysis suggests that MAT negatively affected soil available P mainly by decreasing soil organic P and primary mineral P and increasing soil sand content. MAP negatively affected soil available P both directly and indirectly through decreasing soil primary mineral P; however, these negative effects were offset by the positive effects of MAP on soil organic P and fine soil particles, resulting in a relatively minor total MAP effect on soil available P. As aridity degree was mainly determined by MAP, aridity also had a relatively minor total effect on soil available P. These global patterns generally hold true irrespective of soil depth (≤10 cm or >10 cm) or site aridity index (≤1.0 or >1.0), and were also true for the low-sand (≤50%) soils. In contrast, available P of the high-sand (>50%) soils was positively affected by MAT and aridity and negatively affected by MAP. Our results suggest that temperature and precipitation have contrasting effects on soil P availability and can interact with soil particle size to control soil P availability.

Altitudinal patterns and controls of plant and soil nutrient concentrations and stoichiometry in subtropical China.

Altitude is a determining factor of ecosystem properties and processes in mountains. This study investigated the changes in the concentrations of carbon (C), nitrogen (N), and phosphorus (P) and their ratios in four key ecosystem components (forest floor litter, fine roots, soil, and soil microorganisms) along an altitudinal gradient (from 50 m to 950 m a.s.l.) in subtropical China. The results showed that soil organic C and microbial biomass C concentrations increased linearly with increasing altitude. Similar trends were observed for concentrations of total soil N and microbial biomass N. In contrast, the N concentration of litter and fine roots decreased linearly with altitude. With increasing altitude, litter, fine roots, and soil C:N ratios increased linearly, while the C:N ratio of soil microbial biomass did not change significantly. Phosphorus concentration and C:P and N:P ratios of all ecosystem components generally had nonlinear relationships with altitude. Our results indicate that the altitudinal pattern of plant and soil nutrient status differs among ecosystem components and that the relative importance of P vs. N limitation for ecosystem functions and processes shifts along altitudinal gradients.

Phosphatase activity in relation to key litter and soil properties in mature subtropical forests in China.

Phosphatase-mediated phosphorus (P) mineralization is one of the critical processes in biogeochemical cycling of P and determines soil P availability in forest ecosystems; however, the regulation of soil phosphatase activity remains elusive. This study investigated the potential extracellular activities of acid phosphomonoesterase (AcPME) and phosphodiesterase (PDE) and how they were related to key edaphic properties in the L horizon (undecomposed litter) and F/H horizon (fermented and humified litter) and the underlying mineral soil at the 0-15cm depth in eight mature subtropical forests in China. AcPME activity decreased significantly in the order of F/H horizon>L horizon>mineral soil horizon, while the order for PDE activity was L horizon=F/H horizon>mineral soil horizon. AcPME (X axis) and PDE (Y axis) activities were positively correlated in all horizons with significantly higher slope in the L and F/H horizons than in the mineral soil horizon. Both AcPME and PDE activities were positively related to microbial biomass C, moisture content and water-holding capacity in the L horizon, and were positively related to soil C:P, N:P and C:N ratios and fine root (diameter≤2mm) biomass in the mineral soil horizon. Both enzyme activities were also interactively affected by forest and horizon, partly due to the interactive effect of forest and horizon on microbial biomass. Our results suggest that modulator(s) of the potential extracellular activity of phosphatases vary with horizon, depending on the relative C, P and water availability of the horizon.

Nutrient limitation on ecosystem productivity and processes of mature and old-growth subtropical forests in China.

Nitrogen (N) is considered the dominant limiting nutrient in temperate regions, while phosphorus (P) limitation frequently occurs in tropical regions, but in subtropical regions nutrient limitation is poorly understood. In this study, we investigated N and P contents and N:P ratios of foliage, forest floors, fine roots and mineral soils, and their relationships with community biomass, litterfall C, N and P productions, forest floor turnover rate, and microbial processes in eight mature and old-growth subtropical forests (stand age >80 yr) at Dinghushan Biosphere Reserve, China. Average N:P ratios (mass based) in foliage, litter (L) layer and mixture of fermentation and humus (F/H) layer, and fine roots were 28.3, 42.3, 32.0 and 32.7, respectively. These values are higher than the critical N:P ratios for P limitation proposed (16-20 for foliage, ca. 25 for forest floors). The markedly high N:P ratios were mainly attributed to the high N concentrations of these plant materials. Community biomass, litterfall C, N and P productions, forest floor turnover rate and microbial properties were more strongly related to measures of P than N and frequently negatively related to the N:P ratios, suggesting a significant role of P availability in determining ecosystem production and productivity and nutrient cycling at all the study sites except for one prescribed disturbed site where N availability may also be important. We propose that N enrichment is probably a significant driver of the potential P limitation in the study area. Low P parent material may also contribute to the potential P limitation. In general, our results provided strong evidence supporting a significant role for P availability, rather than N availability, in determining ecosystem primary productivity and ecosystem processes in subtropical forests of China.

Structural and physiological responses of two invasive weeds, Mikania micrantha and Chromolaena odorata, to contrasting light and soil water conditions.

To better understand the requirement of light and soil water conditions in the invasion sites of two invasive weeds, Mikania micrantha and Chromolaena odorata, we investigated their structural and physiological traits in response to nine combined treatments of light [full, medium and low irradiance (LI)] and soil water (full, medium and low field water content) conditions in three glasshouses. Under the same light conditions, most variables for both species did not vary significantly among different water treatments. Irrespective of water treatment, both species showed significant decreases in maximum light saturated photosynthetic rate (P (max)), photosynthetic nitrogen-use efficiency, and relative growth rate under LI relative to full irradiance; specific leaf area, however, increased significantly from full to LI though leaf area decreased significantly, indicating that limited light availability under extreme shade was the critical factor restricting the growth of both species. Our results also indicated that M. micrantha performed best under a high light and full soil water combination, while C. odorata was more efficient in growth under a high light and medium soil water combination.

[A preliminary study on the chemical properties of precipitation, throughfall, stemflow and surface run-off in major forest types at Dinghushan under acid deposition].

Studies on the chemical properties of precipitation, throughfall, stemflow and surface run-off in major forest types at Dinghushan under acid deposition showed that the pH value of precipitation was about 4.90, and the frequency of acid rain was over 62%. In broad-leaved forest, the pH value of precipitation was lower than that of throughfall, but higher than that of stemflow and especially the surface run-off, indicating that the soil was naturally acidified. In mixed forest, both throughfall and surface run-off had a higher pH value, but stemflow had a lower pH value than precipitation. The throughfall and stemflow were more acidified than precipitation in coniferous pine forest, but the surface run-off had a higher pH value than precipitation. These results suggested that among the three major forest types at Dinghushan, the canopy of broad-leaved forest had the highest buffering ability, whereas for the soil, the coniferous forest had the highest soil buffering capacity. The concentrations of nutrient elements, such as P, K, Ca, Na and Mg in the throughfall, stemflow and surface run-off were higher than those in bulk precipitation in all forests at Dinghushan, some even 10 times higher, indicating that a large amount of nutrients were leached from the canopy. The concentrations of nutrient elements in stemflow were higher than those in throughfall in all forests, and the concentration of nutrient elements in surface water was higher than those in atmospheric rainfall. Coniferous forest had a higher concentration of nutrients in the throughfall and stemflow and a lower nutrient concentration in the surface run-off than other forest types, which implied that nutrient loss was more serious in broad-leaved and mixed forests than in coniferous forests.