Long-term dynamics of mycorrhizal root tips in a loblolly pine forest grown with free-air CO2 enrichment and soil N fertilization for 6 years

Fine root production, mortality, and turnover in response to simulated nitrogen deposition in the subtropical Abies georgei (Orr) forest

Increased nitrogen deposition has important effects on below-ground ecological processes. Fine roots are the most active part of the root system in terms of physiological activity and the main organs for nutrient and water uptake by plants. However, there is still a limited understanding of how nitrogen deposition affects the fine root dynamics in subtropical Abies georgei (Orr) forests. Consequently, a three-year field experiment was conducted to quantify the effects of three forms of nitrogen sources ((NH4)2SO4, NaNO3, and NH4NO3) at four levels (0, 5, 15, and 30 kg N·ha-1·yr-1) on the fine root dynamics in Abies georgei forests using a randomized block-group experimental design and minirhizotron technique. The first year of nitrogen addition did not affect the first-class fine roots (FR1, 0 < diameter < 0.5 mm) and second-class fine roots (FR2, 0.5 < diameter < 1.0 mm). The next two years of nitrogen addition significantly increased the production, mortality, and turnover of FR1 and FR2; the three year of nitrogen addition did not affect the dynamics of the third- class fine roots (FR3, 1.0 < diameter < 1.5 mm) and fourth- class fine roots (FR4,1.5 < diameter < 2.0 mm). Nitrogen addition positively affected the dynamics of FR1, FR2, FR3 and FR4 by positively affecting the carbon, nitrogen, and phosphorus contents of fine roots and indirectly affecting the soil pH. Increased carbon allocation to FR1 and FR2 may represent a phosphorus acquisition strategy when nitrogen is not the limiting factor. The nitrogen addition forms and levels affected the fine root dynamics in the following orde: NH4NO3 > (NH4)2SO4 > NaNO3 and high nitrogen > medium nitrogen > low nitrogen. The results suggest that the different-diameter fine root dynamics respond differently to different nitrogen addition forms and levels, and linking the different-diameter fine roots to nitrogen deposition is crucial.

Increased leaf area index and efficiency drive enhanced production under elevated atmospheric [CO2 ] in a pine-dominated stand showing no progressive nitrogen limitation

Enhancement of net primary production (NPP) in forests as atmospheric [CO2 ] increases is likely limited by the availability of other growth resources. The Duke Free Air CO2 Enrichment (FACE) experiment was located on a moderate-fertility site in the southeastern US, in a loblolly pine (Pinus taeda L.) plantation with broadleaved species growing mostly in mid-canopy and understory. Duke FACE ran from 1994 to 2010 and combined elevated [CO2 ] (eCO2 ) with nitrogen (N) additions. We assessed the spatial and temporal variation of NPP response using a dataset that includes previously unpublished data from 6 years of the replicated CO2  × N experiment and extends to 2 years beyond the termination of enrichment. Averaged over time (1997-2010), NPP of pine and broadleaved species were 38% and 52% higher under eCO2 compared to ambient conditions. Furthermore, there was no evidence of a decline in enhancement over time in any plot regardless of its native site quality. The relation between spatial variation in the response and native site quality was suggested but inconclusive. Nitrogen amendments under eCO2 , in turn, resulted in an additional 11% increase in pine NPP. For pine, the eCO2 -induced increase in NPP was similar above- and belowground and was driven by both increased leaf area index (L) and production efficiency (PE = NPP/L). For broadleaved species, coarse-root biomass production was more than 200% higher under eCO2 and accounted for the entire production response, driven by increased PE. Notably, the fraction of annual NPP retained in total living biomass was higher under eCO2 , reflecting a slight shift in allocation fraction to woody mass and a lower mortality rate. Our findings also imply that tree growth may not have been only N-limited, but perhaps constrained by the availability of other nutrients. The observed sustained NPP enhancement, even without N-additions, demonstrates no progressive N limitation.

Mycorrhizal mycelium as a global carbon pool

For more than 400 million years, mycorrhizal fungi and plants have formed partnerships that are crucial to the emergence and functioning of global ecosystems. The importance of these symbiotic fungi for plant nutrition is well established. However, the role of mycorrhizal fungi in transporting carbon into soil systems on a global scale remains under-explored. This is surprising given that ∼75% of terrestrial carbon is stored belowground and mycorrhizal fungi are stationed at a key entry point of carbon into soil food webs. Here, we analyze nearly 200 datasets to provide the first global quantitative estimates of carbon allocation from plants to the mycelium of mycorrhizal fungi. We estimate that global plant communities allocate 3.93 Gt CO₂e per year to arbuscular mycorrhizal fungi, 9.07 Gt CO₂e per year to ectomycorrhizal fungi, and 0.12 Gt CO₂e per year to ericoid mycorrhizal fungi. Based on this estimate, 13.12 Gt of CO₂e fixed by terrestrial plants is, at least temporarily, allocated to the underground mycelium of mycorrhizal fungi per year, equating to ∼36% of current annual CO₂ emissions from fossil fuels. We explore the mechanisms by which mycorrhizal fungi affect soil carbon pools and identify approaches to increase our understanding of global carbon fluxes via plant-fungal pathways. Our estimates, although based on the best available evidence, are imperfect and should be interpreted with caution. Nonetheless, our estimations are conservative, and we argue that this work confirms the significant contribution made by mycorrhizal associations to global carbon dynamics. Our findings should motivate their inclusion both within global climate and carbon cycling models, and within conservation policy and practice.

Remarkable Similarity in Timing of Absorptive Fine-Root Production Across 11 Diverse Temperate Tree Species in a Common Garden

Long-term minirhizotron observations of absorptive fine roots provide insights into seasonal patterns of belowground root production and carbon dynamics. Our objective was to compare root dynamics over time across mature individuals of 11 temperate trees species: five evergreen and six deciduous. We analyzed the timing and growth on 1st-and 2nd-order roots in minirhizotron images down to a vertical depth of 35 cm, as well as monthly and total annual length production. Production patterns were related to total annual precipitation of the actual and previous year of root production over 6 years. The main or largest peak of annual fine-root production occurred between June and September for almost all species and years. In most years, when peaks occurred, the timing of peak root production was synchronized across all species. A linear mixed model revealed significant differences in monthly fine-root length production across species in certain years (species x year, P < 0.0001), which was strongly influenced by three tree species. Total annual root production was much higher in 2000-2002, when there was above-average rainfall in the previous year, compared with production in 2005-2007, which followed years of lower-than-average rainfall (2003-2006). Compared to the wetter period all species experienced a decline of at least 75% in annual production in the drier years. Total annual root length production was more strongly associated with previous year's (P < 0.001) compared with the actual year's precipitation (P = 0.003). Remarkably similar timing of monthly absorptive fine-root growth can occur across multiple species of diverse phylogeny and leaf habit in a given year, suggesting a strong influence of extrinsic factors on absorptive fine-root growth. The influence of previous year precipitation on annual absorptive fine-root growth underscores the importance of legacy effects in biological responses and suggests that a growth response of temperate trees to extreme precipitation or drought events can be exacerbated across years.

Ectomycorrhizal Fungi: Participation in Nutrient Turnover and Community Assembly Pattern in Forest Ecosystems

Ectomycorrhizal fungi (EcMF) are involved in soil nutrient cycling in forest ecosystems. These fungi can promote the uptake of nutrients (e.g., nitrogen (N) and phosphorus (P)) and water by host plants, as well as facilitate host plant growth and resistance to stresses and diseases, thereby maintaining the aboveground primary productivity of forest ecosystems. Moreover, EcMF can acquire the carbon (C) sources needed for their growth from the host plants. The nutrient regulation mechanisms of EcMF mainly include the decay of soil organic matter via enzymatic degradation, nonenzymatic mechanism (Fenton chemistry), and priming effects, which in turn promote C and N cycling. At the same time, EcMF can secrete organic acids and phosphatases to improve the availability of soil P, or increase mycelium inputs to facilitate plant acquisition of P. The spatiotemporal distribution of EcMF is influenced by a combination of historical factors and contemporary environmental factors. The community of EcMF is associated with various factors, such as climate change, soil conditions, and host distribution. Under global climate change, investigating the relationships between the nutrient cycling functions of EcMF communities and their distribution patterns under various spatiotemporal scales is conducive to more accurate assessments of the ecological effects of EcMF on the sustainable development of forest.

Effects of Elevated CO2 Concentration and Nitrogen Addition on Soil Respiration in a Cd-Contaminated Experimental Forest Microcosm

Forests near rapidly industrialized and urbanized regions are often exposed to elevated CO2, increased N deposition, and heavy metal pollution. To date, the effects of elevated CO2 and/or increased N deposition on soil respiration (Rs) under heavy metal contamination are unclear. In this study, we firstly investigated Rs in Cd-contaminated model forests with CO2 enrichment and N addition in subtropical China. Results showed that Rs in all treatments exhibited similar clear seasonal patterns, with soil temperature being a dominant control. Cadmium addition significantly decreased cumulative soil CO2 efflux by 19% compared to the control. The inhibition of Rs caused by Cd addition was increased by N addition (decreased by 34%) was partially offset by elevated CO2 (decreased by 15%), and was not significantly altered by the combined N addition and rising CO2. Soil pH, microbial biomass carbon, carbon-degrading hydrolytic enzymes, and fine root biomass were also significantly altered by the treatments. A structural equation model revealed that the responses of Rs to Cd stress, elevated CO2, and N addition were mainly mediated by soil carbon-degrading hydrolytic enzymes and fine root biomass. Overall, our findings indicate that N deposition may exacerbate the negative effect of Cd on Rs in Cd-contaminated forests and benefit soil carbon sequestration in the future at increasing atmospheric CO2 levels.

Unaltered phenology but increased production of ectomycorrhizal roots of Pinus elliottii under 4 years of nitrogen addition

Timing (phenology) and amount (production) are two integral facets of root growth, and their shifts have profound influences on below-ground resource acquisition. However, the environmental control and the effects of nitrogen (N) deposition on the production and phenology of ectomycorrhizal (ECM) roots remain unclear. Using a 4 yr minirhizotron experiment, we explored the control of the production and three phenophases (initiation, peak, and cessation of growth) of ECM roots in two soil layers and investigated their dynamic responses to N addition in a seasonally dry subtropical Pinus elliottii forest. We found a stronger control of water availability on the production and a stronger control of temperature on the phenology of ECM roots under ambient conditions. Temperature was correlated positively with initiation and negatively with cessation, especially in the shallow layer. N addition did not affect the phenology of ECM roots but increased their production by modifying N and phosphorus (P) stoichiometry in the soil and foliage. Our findings suggest a greater sensitivity of production than phenology of ECM roots to N addition. The increased production of ECM roots under N addition could be driven by N-induced P limitation or some combination of below-ground resources (P, N, water).

Plant-mycorrhizal interactions mediate plant community coexistence by altering resource demand

As the diversity of plants increases in an ecosystem, so does resource competition for soil nutrients, a process that mycorrhizal fungi can mediate. The influence of mycorrhizal fungi on plant biodiversity likely depends on the strength of the symbiosis between the plant and fungi, the differential plant growth responses to mycorrhizal inoculation, and the transfer rate of nutrients from the fungus to plant. However, our current understanding of how nutrient-plant-mycorrhizal interactions influence plant coexistence is conceptual and thus lacks a unified quantitative framework. To quantify the conditions of plant coexistence mediated by mycorrhizal fungi, we developed a mechanistic resource competition model that explicitly included plant-mycorrhizal symbioses. We found that plant-mycorrhizal interactions shape plant coexistence patterns by creating a tradeoff in resource competition. Especially, a tradeoff in resource competition was caused by differential payback in the carbon resources that plants invested in the fungal symbiosis and/or by the stoichiometric constraints on plants that required additional, less-beneficial, resources to sustain growth. Our results suggested that resource availability and the variation in plant-mycorrhizal interactions act in concert to drive plant coexistence patterns. Applying our framework, future empirical studies should investigate plant-mycorrhizal interactions under multiple levels of resource availability.

Nitrogen deposition increases root production and turnover but slows root decomposition in Pinus elliottii plantations

Fine roots of woody plants comprise multiple root orders, which can be functionally partitioned into two pools: absorptive fine roots (AFRs, orders 1, 2) and transport fine roots (TFRs, orders 3-5). However, the function-based fine-root dynamics and especially their responses to increased nitrogen (N) availability remain unclear. We explored dynamic responses of both AFRs and TFRs of Pinus elliottii to N addition in subtropical China based on a 4-yr minirhizotron experiment and a two-stage - early (0.5 yr) vs late (4 yr) - decomposition experiment. N addition increased the production, mortality, and turnover of AFRs but not TFRs. High rates of N persistently inhibited AFR decomposition but affected TFR decomposition differentially at the early (no effect) and late (negative effect) stages. The increased production of AFRs was driven by N-induced decrease in foliar and soil phosphorus (P) concentrations. The decreased decomposition of AFRs might be due to the increased acid-unhydrolyzable residues in decomposing roots. AFRs are the resource-acquiring module, the increased carbon allocation to AFRs may represent a P-acquiring strategy when N no longer limits growth of P. elliottii. Our results suggest that AFRs and TFRs respond differently to N deposition, both in terms of production, mortality, and turnover and in terms of decomposition.

Dynamics of soil CO2 efflux under varying atmospheric CO2 concentrations reveal dominance of slow processes

We evaluated the effect on soil CO₂ efflux (F_CO2 ) of sudden changes in photosynthetic rates by altering CO₂ concentration in plots subjected to +200 ppmv for 15 years. Five-day intervals of exposure to elevated CO₂ (eCO₂ ) ranging 1.0-1.8 times ambient did not affect F_CO2 . F_CO2 did not decrease until 4 months after termination of the long-term eCO₂ treatment, longer than the 10 days observed for decrease of F_CO2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of F_CO2 following the initiation of eCO₂ . The reduction of F_CO2 upon termination of enrichment (~35%) cannot be explained by the reduction of leaf area (~15%) and associated carbohydrate production and allocation, suggesting a disproportionate contraction of the belowground ecosystem components; this was consistent with the reductions in base respiration and F_CO2 -temperature sensitivity. These asymmetric responses pose a tractable challenge to process-based models attempting to isolate the effect of individual processes on F_CO2 .

Variations of Soil Microbial Community Structures Beneath Broadleaved Forest Trees in Temperate and Subtropical Climate Zones

Global warming has shifted climate zones poleward or upward. However, understanding the responses and mechanism of microbial community structure and functions relevant to natural climate zone succession is challenged by the high complexity of microbial communities. Here, we examined soil microbial community in three broadleaved forests located in the Wulu Mountain (WLM, temperate climate), Funiu Mountain (FNM, at the border of temperate and subtropical climate zones), or Shennongjia Mountain (SNJ, subtropical climate). Although plant species richness decreased with latitudes, the microbial taxonomic α-diversity increased with latitudes, concomitant with increases in soil total and available nitrogen and phosphorus contents. Phylogenetic NRI (Net Relatedness Index) values increased from -0.718 in temperate zone (WLM) to 1.042 in subtropical zone (SNJ), showing a shift from over dispersion to clustering likely caused by environmental filtering such as low pH and nutrients. Similarly, taxonomy-based association networks of subtropical forest samples were larger and tighter, suggesting clustering. In contrast, functional α-diversity was similar among three forests, but functional gene networks of the FNM forest significantly (P < 0.050) differed from the others. A significant correlation (R = 0.616, P < 0.001) between taxonomic and functional β-diversity was observed only in the FNM forest, suggesting low functional redundancy at the border of climate zones. Using a strategy of space-for-time substitution, we predict that poleward climate range shift will lead to decreased microbial taxonomic α-diversities in broadleaved forest.

Changes in root architecture under elevated concentrations of CO₂ and nitrogen reflect alternate soil exploration strategies

Predicting the response of fine roots to increased atmospheric CO₂ concentration has important implications for carbon (C) and nutrient cycling in forest ecosystems. Root architecture is known to play an important role in how trees acquire soil resources in changing environments. However, the effects of elevated CO₂ on the fine-root architecture of trees remain unclear. We investigated the architectural response of fine roots exposed to 14 yr of CO₂ enrichment and 6 yr of nitrogen (N) fertilization in a Pinus taeda (loblolly pine) forest. Root traits reflecting geometry, topology and uptake function were measured on intact fine-root branches removed from soil monoliths and the litter layer. CO₂ enrichment resulted in the development of a fine-root pool that was less dichotomous and more exploratory under N-limited conditions. The per cent mycorrhizal colonization did not differ among treatments, suggesting that root growth and acclimation to elevated CO₂ were quantitatively more important than increased mycorrhizal associations. Our findings emphasize the importance of architectural plasticity in response to environmental change and suggest that changes in root architecture may allow trees to effectively exploit larger volumes of soil, thereby pre-empting progressive nutrient limitations.

The rhizosphere and hyphosphere differ in their impacts on carbon and nitrogen cycling in forests exposed to elevated CO₂

While multiple experiments have demonstrated that trees exposed to elevated CO₂ can stimulate microbes to release nutrients from soil organic matter, the importance of root- versus mycorrhizal-induced changes in soil processes are presently unknown. We analyzed the contribution of roots and mycorrhizal activities to carbon (C) and nitrogen (N) turnover in a loblolly pine (Pinus taeda) forest exposed to elevated CO₂ by measuring extracellular enzyme activities at soil microsites accessed via root windows. Specifically, we quantified enzyme activity from soil adjacent to root tips (rhizosphere), soil adjacent to hyphal tips (hyphosphere), and bulk soil. During the peak growing season, CO₂ enrichment induced a greater increase of N-releasing enzymes in the rhizosphere (215% increase) than in the hyphosphere (36% increase), but a greater increase of recalcitrant C-degrading enzymes in the hyphosphere (118%) than in the rhizosphere (19%). Nitrogen fertilization influenced the magnitude of CO₂ effects on enzyme activities in the rhizosphere only. At the ecosystem scale, the rhizosphere accounted for c. 50% and 40% of the total activity of N- and C-releasing enzymes, respectively. Collectively, our results suggest that root exudates may contribute more to accelerated N cycling under elevated CO₂ at this site, while mycorrhizal fungi may contribute more to soil C degradation.

Root length, biomass, tissue chemistry and mycorrhizal colonization following 14 years of CO2 enrichment and 6 years of N fertilization in a warm temperate forest

Root systems serve important roles in carbon (C) storage and resource acquisition required for the increased photosynthesis expected in CO2-enriched atmospheres. For these reasons, understanding the changes in size, distribution and tissue chemistry of roots is central to predicting the ability of forests to capture anthropogenic CO2. We sampled 8000 cm(3) soil monoliths in a pine forest exposed to 14 years of free-air-CO2-enrichment and 6 years of nitrogen (N) fertilization to determine changes in root length, biomass, tissue C : N and mycorrhizal colonization. CO2 fumigation led to greater root length (98%) in unfertilized plots, but root biomass increases under elevated CO2 were only found for roots <1 mm in diameter in unfertilized plots (59%). Neither fine root [C] nor [N] was significantly affected by increased CO2. There was significantly less root biomass in N-fertilized plots (19%), but fine root [N] and [C] both increased under N fertilization (29 and 2%, respectively). Mycorrhizal root tip biomass responded positively to CO2 fumigation in unfertilized plots, but was unaffected by CO2 under N fertilization. Changes in fine root [N] and [C] call for further study of the effects of N fertilization on fine root function. Here, we show that the stimulation of pine roots by elevated CO2 persisted after 14 years of fumigation, and that trees did not rely exclusively on increased mycorrhizal associations to acquire greater amounts of required N in CO2-enriched plots. Stimulation of root systems by CO2 enrichment was seen primarily for fine root length rather than biomass. This observation indicates that studies measuring only biomass might overlook shifts in root systems that better reflect treatment effects on the potential for soil resource uptake. These results suggest an increase in fine root exploration as a primary means for acquiring additional soil resources under elevated CO2.