Nitrogen addition promotes terrestrial plants to allocate more biomass to aboveground organs: A global meta-analysis

A significant increase in reactive nitrogen (N) added to terrestrial ecosystems through agricultural fertilization or atmospheric deposition is considered to be one of the most widespread drivers of global change. Modifying biomass allocation is one primary strategy for maximizing plant growth rate, survival, and adaptability to various biotic and abiotic stresses. However, there is much uncertainty as to whether and how plant biomass allocation strategies change in response to increased N inputs in terrestrial ecosystems. Here, we synthesized 3516 paired observations of plant biomass and their components related to N additions across terrestrial ecosystems worldwide. Our meta-analysis reveals that N addition (ranging from 1.08 to 113.81 g m-2  year-1 ) increased terrestrial plant biomass by 55.6% on average. N addition has increased plant stem mass fraction, shoot mass fraction, and leaf mass fraction by 13.8%, 12.9%, and 13.4%, respectively, but with an associated decrease in plant reproductive mass (including flower and fruit biomass) fraction by 3.4%. We further documented a reduction in plant root-shoot ratio and root mass fraction by 27% (21.8%-32.1%) and 14.7% (11.6%-17.8%), respectively, in response to N addition. Meta-regression results showed that N addition effects on plant biomass were positively correlated with mean annual temperature, soil available phosphorus, soil total potassium, specific leaf area, and leaf area per plant. Nevertheless, they were negatively correlated with soil total N, leaf carbon/N ratio, leaf carbon and N content per leaf area, as well as the amount and duration of N addition. In summary, our meta-analysis suggests that N addition may alter terrestrial plant biomass allocation strategies, leading to more biomass being allocated to aboveground organs than belowground organs and growth versus reproductive trade-offs. At the global scale, leaf functional traits may dictate how plant species change their biomass allocation pattern in response to N addition.

Elevated ozone decreases the multifunctionality of belowground ecosystems

Elevated tropospheric ozone (O₃ ) affects the allocation of biomass aboveground and belowground and influences terrestrial ecosystem functions. However, how belowground functions respond to elevated O₃ concentrations ([O₃ ]) remains unclear at the global scale. Here, we conducted a detailed synthesis of belowground functioning responses to elevated [O₃ ] by performing a meta-analysis of 2395 paired observations from 222 publications. We found that elevated [O₃ ] significantly reduced the primary productivity of roots by 19.8%, 16.3%, and 26.9% for crops, trees and grasses, respectively. Elevated [O₃ ] strongly decreased the root/shoot ratio by 11.3% for crops and by 4.9% for trees, which indicated that roots were highly sensitive to O₃ . Elevated [O₃ ] impacted carbon and nitrogen cycling in croplands, as evidenced by decreased dissolved organic carbon, microbial biomass carbon, total soil nitrogen, ammonium nitrogen, microbial biomass nitrogen, and nitrification rates in association with increased nitrate nitrogen and denitrification rates. Elevated [O₃ ] significantly decreased fungal phospholipid fatty acids in croplands, which suggested that O₃ altered the microbial community and composition. The responses of belowground functions to elevated [O₃ ] were modified by experimental methods, root environments, and additional global change factors. Therefore, these factors should be considered to avoid the underestimation or overestimation of the impacts of elevated [O₃ ] on belowground functioning. The significant negative relationships between O₃ -treated intensity and the multifunctionality index for croplands, forests, and grasslands implied that elevated [O₃ ] decreases belowground ecosystem multifunctionality.

Interactive effects of ozone exposure and nitrogen addition on tree root traits and biomass allocation pattern: An experimental case study and a literature meta-analysis

Ground-level ozone (O₃) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O₃ and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O₃ concentrations levels. Here, we explored the interactive impact of O₃ (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O₃ via a synthesis of published literature. Elevated O₃ inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O₃. No interactive effects between O₃ and N were observed on root biomass and R:S ratio. The slope of log-log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O₃. The analysis of published literature further revealed that the O₃-induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O₃ levels. N addition does not markedly alter the sensitivity of root biomass of trees to O₃. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O₃ and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O₃-polluted and N-enriched environments.

The effects of elevated ozone on the accumulation and allocation of poplar biomass depend strongly on water and nitrogen availability

Ozone (O₃) pollution can alter carbon allocation and reduce tree growth - both above and below ground, but the extent of these effects depends on the variation in soil water and nutrient availability. Here we present the accumulation and allocation of biomass in poplar clone 546 (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial') for one growing season at two O₃ concentrations (charcoal-filtered air [CF] and non-filtered air + 40 ppb of O₃ [E-O₃]), two watering regimes (well-watered [WW] and reduced watering at 40% of WW irrigation [RW]) and two soil nitrogen addition treatments (no addition [N0] and the addition of 50 kg N ha^-1 year^-1 [N50]). We found that the deleterious effects of E-O₃ depended on the supply of water and nitrogen. Specifically, when the supplies of water and/or N (WW and/or N50) were abundant, E-O₃ significantly reduced whole plant biomass by >15% but had no significant effect on biomass when these supplies were limited (RW and N0). A significant reduction of biomass by E-O₃ occurred earlier in fine roots than in other plant organs, indicating greater sensitivity of fine root to E-O₃. These results suggest that rising O₃ concentrations may not ubiquitously lead to a large reduction in plant biomass since plant growth is often jointly constrained by water and nutrients.

Structure and function of rhizosphere and non-rhizosphere soil microbial community respond differently to elevated ozone in field-planted wheat

To assess the responses of the soil microbial community to chronic ozone (O3), wheat seedlings (Triticum aestivum Linn.) were planted in the field and exposed to elevated O3 (eO3) concentration. Three treatments were employed: (1) Control treatment (CK), AOT40=0; (2) O3-1, AOT40=1.59 ppm•h; (3) O3-2, AOT40=9.17 ppm•h. Soil samples were collected for the assessment of microbial biomass C, community-level physiological profiles (CLPPs), and phospholipid fatty acids (PLFAs). EO3 concentration significantly reduced soil microbial carbon and changed microbial CLPPs in rhizosphere soil, but not in non-rhizosphere soil. The results of the PLFAs showed that eO3 concentrations had significant effects on soil community structure in both rhizosphere and non-rhizosphere soils. The relative abundances of fungal and actinomycetous indicator PLFAs decreased in both rhizosphere and non-rhizosphere soils, while those of bacterial PLFAs increased. Thus the results proved that eO3 concentration significantly changed the soil microbial community function and composition, which would influence the soil nutrient supply and carbon dynamics under O3 exposure.

Non-stomatal limitation to photosynthesis in Cinnamomum camphora seedings exposed to elevated O3

Ozone (O3) is the most phytotoxic air pollutant for global forests, with decreased photosynthesis widely regarded as one of its most common effects. However, controversy exists concerning the mechanism that underlies the depressing effects of O3 on CO2 assimilation. In the present study, seedlings of Cinnamomum camphora, a subtropical evergreen tree species that has rarely been studied, were exposed to ambient air (AA), ambient air plus 60 [ppb] O3 (AA+60), or ambient air plus 120 [ppb] O3 (AA+120) in open-top chambers (OTCs) for 2 years. Photosynthetic CO2 exchange and chlorophyll a fluorescence were investigated in the second growing season (2010). We aim to determine whether stomatal or non-stomatal limitation is responsible for the photosynthesis reduction and to explore the potential implications for forest ecosystem functions. Results indicate that elevated O3 (E-O3) reduced the net photosynthetic rates (PN) by 6.0-32.2%, with significant differences between AA+60 and AA+120 and across the four measurement campaigns (MCs). The actual photochemical efficiency of photosystem II (PSII) in saturated light (Fv'/Fm') was also significantly decreased by E-O3, as was the effective quantum yield of PSII photochemistry (ΦPSII). Moreover, E-O3 significantly and negatively impacted the maximum rates of carboxylation (Vcmax) and electron transport (Jmax). Although neither the stomatal conductance (gs) nor the intercellular CO2 concentration (Ci) was decreased by E-O3, PN/gs was significantly reduced. Therefore, the observed reduction in PN in the present study should not be attributed to the unavailability of CO2 due to stomatal limitation, but rather to the O3-induced damage to Ribulose-1,5-bisphosphate carboxylase/oxygenase and the photochemical apparatus. This suggests that the down-regulation of stomatal conductance could fail to occur, and the biochemical processes in protoplasts would become more susceptible to injuries under long-term O3 exposure, which may have important consequences for forest carbon and water budget.