Global change progressively increases foliar nitrogen-phosphorus ratios in China's subtropical forests.

Globally increased nitrogen (N) to phosphorus (P) ratios (N/P) affect the structure and functioning of terrestrial ecosystems, but few studies have addressed the variation of foliar N/P over time in subtropical forests. Foliar N/P indicates N versus P limitation in terrestrial ecosystems. Quantifying long-term dynamics of foliar N/P and their potential drivers is crucial for predicting nutrient status and functioning in forest ecosystems under global change. We detected temporal trends of foliar N/P, quantitatively estimated their potential drivers and their interaction between plant types (evergreen vs. deciduous and trees vs. shrubs), using 1811 herbarium specimens of 12 widely distributed species collected during 1920-2010 across China's subtropical forests. We found significant decreases in foliar P concentrations (23.1%) and increases in foliar N/P (21.2%). Foliar N/P increased more in evergreen species (22.9%) than in deciduous species (16.9%). Changes in atmospheric CO2 concentrations ( P CO 2 $$ {\mathrm{P}}_{{\mathrm{CO}}_2} $$ ), atmospheric N deposition and mean annual temperature (MAT) dominantly contributed to the increased foliar N/P of evergreen species, while P CO 2 $$ {\mathrm{P}}_{{\mathrm{CO}}_2} $$ , MAT, and vapor pressure deficit, to that of deciduous species. Under future Shared Socioeconomic Pathway (SSP) scenarios, increasing MAT and P CO 2 $$ {\mathrm{P}}_{{\mathrm{CO}}_2} $$ would continuously increase more foliar N/P in deciduous species than in evergreen species, with more 12.9%, 17.7%, and 19.4% versus 6.1%, 7.9%, and 8.9% of magnitudes under the scenarios of SSP1-2.6, SSP3-7.0, and SSP5-8.5, respectively. The results suggest that global change has intensified and will progressively aggravate N-P imbalance, further altering community composition and ecosystem functioning of subtropical forests.

Elevated CO2 and nitrogen addition enhance the symbiosis and functions of rhizosphere microorganisms under cadmium exposure.

Soil microbes are fundamental to ecosystem health and productivity. How soil microbial communities are influenced by elevated atmospheric carbon dioxide (eCO2) concentration and nitrogen (N) deposition under heavy metal pollution remains uncertain, despite global exposure of terrestrial ecosystems to eCO2, high N deposition and heavy metal stress. Here, we conducted a four year's open-top chamber experiment to assess the effects of soil cadmium (Cd) treatment (10 kg hm-2 year-1) alone and combined treatments of Cd with eCO2 concentration (700 ppm) and/or N addition (100 kg hm-2 year-1) on tree growth and rhizosphere microbial community. Relative to Cd treatment alone, eCO2 concentration in Cd contaminated soil increased the complexity of microbial networks, including the number links, average degree and positive/negative ratios. The combined effect of eCO2 and N addition in Cd contaminated soil not only increased the complexity of microbial networks, but also enhanced the abundance of microbial urealysis related UreC and nitrifying related amoA1 and amoA2, and the richness of arbuscular mycorrhiza fungi (AMF), thereby improving the symbiotic functions between microorganisms and plants. Results from correlation analysis and structural equation model (SEM) further demonstrated that eCO2 concentration and N addition acted on functions and networks differently. Elevated CO2 positively regulated microbial networks and functions through phosphorus (P) and Cd concentration in roots, while N addition affected microbial functions through soil available N and soil organic carbon (SOC) concentration and microbial network through soil Cd concentration. Overall, our findings highlight that eCO2 concentration and N addition make microbial communities towards ecosystem health that may mitigate Cd stress, and provide new insights into the microbiology supporting phytoremediation for Cd contaminated sites in current and future global change scenarios.

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.

Contrasting responses in growth, photosynthesis and hydraulics of two subtropical tree species to cadmium contamination as affected by elevated CO2 and nitrogen addition.

Plant growth, photosynthesis, and hydraulics are affected by heavy metals but also by elevated atmospheric CO2 concentration (e[CO2]) and nitrogen (N) deposition. However, few studies have investigated the response of woody species to the combined effects of these three factors. We conducted an open-top chamber experiment with two common subtropical trees (Acacia auriculiformis and Syzygium hainanense) to explore the effects of cadmium (Cd)-contamination, e[CO2], and N addition on plant eco-physiological traits. We found that the growth of A. auriculiformis was insensitive to the treatments, indicating that it is a Cd-tolerant and useful afforestation species. For S. hainanense, in contrast, e[CO2] and/or N addition offset the detrimental effects of Cd addition by greatly increasing plant biomass and reducing the leaf Cd concentration. We then found that e[CO2] and/or N addition offset the detrimental Cd effects on S. hainanense biomass by increasing its photosynthetic rate, its N concentration, and the efficiency of its stem water transport network. These offsetting effects of e[CO2] and/or N addition, however, came at the expense of reduced xylem hydraulic safety resulting from wider vessels, thinner vessel walls, and therefore weaker vessel reinforcement. Our study suggests that, given future increases in global CO2 concentration and N deposition, the growth of Cd-tolerant tree species (like A. auriculiformis) will be probably stable while the growth of Cd-sensitive tree species (like S. hainanense) might be enhanced despite reduced hydraulic safety. This also suggests that both species will be useful for afforestation of Cd-contaminated soils given future global change scenarios.

Effects of elevated CO2 concentration and nitrogen addition on the chemical compositions, construction cost and payback time of subtropical trees in Cd-contaminated mesocosm soil.

Rising atmospheric CO2 concentration ([CO2]) and nitrogen (N) deposition are changing plant growth, physiological characteristics and chemical compositions; however, few studies have explored such impacts in a heavy metal-contaminated environment. In this study, we conducted an open-top chamber experiment to explore the impacts of 2 years of elevated atmospheric [CO2] and N addition on the growth, physiological characteristics and chemical compositions of five subtropical tree species in a cadmium (Cd)-contaminated environment. Results showed that N addition significantly increased concentration of leaf N and protein in five tree species and also decreased payback time (PBT) and leaf carbon:nitrogen ratios and increased tree relative height growth rate (RGR-H) and basal diameter growth rate (RGR-B) in Liquidambar formosana Hance and Syzygium hainanense Chang et Miau. Elevated [CO2] increased leaf maximum photosynthetic rate (Amax) and concentration of total non-structural carbohydrates and shortened PBT to offset the negative effect of Cd contamination on RGR-B in Acacia auriculiformis A. Cunn. ex Benth. The combined effects of elevated [CO2] and N addition did not exceed their separate effects on RGR-H and RGR-B in Castanopsis hystrix Hook. f. & Thomson ex A. DC. and Cinnamomum camphora (L.) presl. The addition of N significantly increased the concentration of leaf Cd by 162.1% and 338.0%, and plant Cd bio-concentration factor by 464% and 861% in C. hystrix and C. camphora, respectively, compared with only Cd addition. Among the five tree species, the decrease in PBT and the increase in Amax, RGR-B and concentrations of leaf protein in response to N and Cd addition under elevated [CO2] were on average 86.7% higher in A. auriculiformis than other species, suggesting that the mitigation of the negative effects of Cd pollution by elevated [CO2] and N addition among five species was species-specific. Overall, we concluded that N addition and elevated [CO2] reduced Cd toxicity and increased the growth rate in A. auriculiformis, S. hainanense and L. formosana, while it maintained the growth rate in C. hystrix and C. camphora by differently increasing photosynthetic rate, altering the leaf chemical compositions and shortening PBT.

Physiological and proteomic analyses reveal the effects of exogenous nitrogen in diminishing Cd detoxification in Acacia auriculiformis.

Cadmium (Cd) has toxic effects on plants. Nitrogen (N), an essential element, is critical for plant growth, development and stress response. However, their combined effects on woody plants, especially in N-fixing tree species is still poorly understood. Our previous study revealed that the fast-growing Acacia auriculiformis showed strong Cd tolerance but the underlying mechanisms was not clear, which constrained its use in mine land reclamation. Herein, we investigated the physiological and proteomic changes in A. auriculiformis leaves to reveal the mechanisms of Cd tolerance and toxicity without N fertilizer (treatment Cd) and with excess N fertilizer (treatment CdN). Results showed that Cd tolerance in A. auriculiformis was closely associated with the coordinated gas exchange and antioxidant defense reactions under Cd treatment alone. Exogenous excessive N, however, inhibited plant growth, increased Cd concentrations, and weaken photosynthetic performance, thus, aggregated the toxicity under Cd stress. Furthermore, the aggregated Cd toxicity was attributed to the depression in the abundance of proteins, as well as their corresponding genes, involved in photosynthesis, energy metabolism (oxidative phosphorylation, carbon metabolism, etc.), defense and stress response (antioxidants, flavonoids, etc.), plant hormone signal transduction (MAPK, STN, etc.), and ABC transporters. Collectively, this study unveils a previously unknown physiological and proteomic network that explains N diminishes Cd detoxification in A. auriculiformis. It may be counterproductive to apply N fertilizer to fast-growing, N-fixing trees planted for phytoremediation of Cd-contaminated soils.

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.

Effects of plant growth regulator and chelating agent on the phytoextraction of heavy metals by Pfaffia glomerata and on the soil microbial community.

Pfaffia glomerata is a candidate for the remediation of heavy metal-contaminated soil, but phytoremediation efficiency requires enhancement. In this study, we evaluated how application of DA-6, EDTA, or CA affected the growth and heavy metal accumulation of P. glomerata and soil microorganisms. We found that P. glomerata removed more Cd and Zn than Pb or Cu from contaminated soil. When compared to the control, application of DA-6, CA, or CA + DA-6 increased plant biomass and increased stem Cd concentration by 1.28-, 1.20-, and 1.31-fold respectively; increased leaf Cd concentration by 1.25-, 1.28-, and 1.20-fold, respectively; and increased the total quantity of Cd extracted by 1.37-, 1.37-, and 1.38-fold, respectively. When compared to the control, application EDTA or EDTA + DA-6 significantly increased the soil available metal and Na concentrations, which harmed plant growth. Application of EDTA or EDTA + DA-6 also significantly decreased the Cd concentration in roots and stems. 16S rRNA high-throughput sequencing analysis revealed that application of EDTA or CA alone to soil significantly reduced the richness and diversity of soil bacteria, while foliar spraying of DA-6 combined with EDTA or CA slightly alleviated this reduction. EDTA or CA addition significantly changed the proportion of Actinobacteria and Proteobacteria. In addition, EDTA or CA addition caused changes in soil properties (e.g. heavy metal availability, K concentration, Na concentration, soil pH, soil CEC, and soil DOC concentration) that were associated with changes in the bacterial community. EDTA addition mainly affected the soil bacterial community by changing soil DOC concentration, the soil available Pb and Na concentration, and CA addition mainly affected the soil bacterial community by changing the soil available Ca concentration.

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.

Effects of forest conversion on carbon-degrading enzyme activities in subtropical China.

The mineralization of soil organic carbon (SOC) is primarily mediated by carbon (C) degrading enzyme. In the current study, we determined how the activities of four soil C-degrading enzymes, the hydrolases ß-glucosidase (BG) and cellobiohydrolase (CBH) and the oxidases polyphenol oxidase (PPO) and peroxidase (POD), responded to forest conversion of natural broadleaf forests (BF) to secondary forests (SF) and plantation forests (PF) in subtropical China. We also quantified SOC, dissolved organic C (DOC), permanganate oxidase organic C (PXC), recalcitrant C (RC), microbial biomass C (MBC), mineral-associated C (MOC), soil particle-sizes distribution, pH, and moisture content, and C: nitrogen (N) ratio. Results showed that, the activities of all four C-degrading enzymes (BG, CBH, PPO and POD) decreased by 23.1, 9.5, 6.9 and 1.8%, respectively by forest conversion of BF to SF and 30.5, 15.3, 28.1 and 27.8%, respectively by conversion of BF to PF and were higher in the topsoil than in the subsoil. Relative to SF and PF, BF had higher hydrolase activities, which were related to its higher concentrations of MBC, DOC, and PXC, and to its lower C:N ratio. The BF also had higher oxidase activities, which were related to its higher concentrations of MBC, RC, and MOC, and to its lower C:N ratio. PF had higher specific enzyme activities (i.e., enzyme activities per unit of SOC) than BF and SF, indicating faster C turnover rates in PF. In addition to being affected by the concentrations of SOC and SOC components, forest conversion-induced changes in soil enzyme activities were affected by clay content and soil moisture content. These results revealed the different underlying mechanisms between soil hydrolases and oxidases in their responses to forest conversion.

Effects of elevated atmospheric CO2 and nitrogen deposition on leaf litter and soil carbon degrading enzyme activities in a Cd-contaminated environment: A mesocosm study.

Rising atmospheric CO2 concentration and nitrogen (N) deposition are changing terrestrial carbon (C) cycle; however, little has been known about such impacts in a heavy-metal-contaminated environment. This study conducted an open-top chamber experiment to explore the impacts of rising atmospheric CO2 concentration and N deposition on the leaf litter and soil C cycle in cadmium (Cd)-contaminated environment. The experiment include five treatments: control, Cd (30 g ha-1 yr-1) addition, Cd addition under elevated CO2 (700 ppm CO2), Cd and N(100 kg ha-1 yr-1) additions, and Cd and N additions under elevated CO2, with three replicates per treatment. Leaf litter and soil C cycle were indexed by microbial biomass C concentration and the activities of four key C-degrading enzyme (ß-glucosidase (BG), cellobiohydrolase (CBH), polyphenol oxidase (PPO), and peroxidase (POD)) in litter and soil. Results showed that, after one year treatment, Cd addition negatively affected the activities of all four C-degrading enzyme in litter and soil; while elevated CO2 and N addition essentially alleviated these negative effects. Elevated CO2 and N addition increased C-degrading enzyme activities more of the non-legume (i.e., Cinnamomum camphora) litter than those of the legume (i.e., Acacia auriculiformis) litter. Elevated CO2, N addition, and Cd addition all affected C-degrading enzyme activities via their effects on the microbial biomass C concentration and C and N availability of the litter and soil samples. We suggest that rising atmospheric CO2 concentration and N deposition can offset the detrimental effect of Cd on the litter and soil C-degrading enzyme activities in forest ecosystems.