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Low P (LP) levels in leaves can affect their photosynthetic N-use efficiency (PNUE), internal N allocation, and mesophyll conductance to CO2 (gm). The changes in leaf internal N allocation and gm in N-fixing trees and the consequent changes in PNUE under low soil P treatments are not well understood. In this study, we exposed seedlings of Dalbergia odorifera, Erythrophleum fordii (N-fixing trees), Castanopsis hystrix, and Betula alnoides (non-N-fixing trees) to three levels of soil P. The effects were not consistent among species, and LP had no specific effect on N-fixing species. Saturated net CO2 assimilation rate (Asat) values in D. odorifera and C. hystrix were remarkably lower under LP than under high P (HP) because Cc in D. odorifera and Vcmax and Jmax in C. hystrix were reduced. Narea values in D. odorifera and C. hystrix were also reduced under LP, and the degree of reduction of Narea was larger than that of Asat, which resulted in decreased PNUE in these species. PR and gm in D. odorifera and PR, PB, and gm in C. hystrix significantly decreased under LP and were internal factors affecting the variation in PNUE in these two trees. PCW was significantly and linearly related to PR only in C. hystrix, indicating that more N was invested in the cell walls to resist the damage caused by low soil P, at the expense of Rubisco N. Our results showed that soil P deficiency affected leaf N utilization, photosynthetic efficiency, and seedling growth. [ABSTRACT FROM AUTHOR]

Quantifying the allocation of photosynthetic products among different carbon (C) pools is critical for understanding and predicting plant C turnover response to climate change. A field experiment with ectomycorrhizal fungi (EMF) and nitrogen (N) was established to investigate the effects on allocation of photosynthetic products in Pinus massoniana (Lamb.) seedlings given increased N deposition. Seedlings were subjected to N addition and symbiosis with EMF, and the short-term allocation of a 13C photosynthetic pulse into leaves, branches, stems, roots, and soil was traced. Photosynthetic rate and root respiration were measured. It was found that N addition changed the allocation pattern of photosynthetic products in various organs of P. massoniana. Furthermore, N addition, mycorrhizal symbiosis, and interaction of N and EMF, all increased the amount of C produced by photosynthesis. N application less than 60 kg N hm–1 a–1 could promote the transfer and allocation of photosynthetic products in P. massoniana organs, which peaks at 60 kg N hm–1 a–1, and the highest N treatment began to decrease at 90 kg N hm–1 a–1. EMF inoculation could expand the absorption area of plant roots to obtain more nutrients and synthesize more C and N compounds for promoting the growth of itself and the host plant, improving the net photosynthetic rate and the distribution of C produced by photosynthesis in various organs. This forms a benign C and N cycle, thereby reducing the effect of high N addition on plants. The optimal N addition concentration was 60 kg N hm–1 a–1, and the optimal EMF was Pt, which provides a theoretical basis for inoculating EMF during increasing N deposition in the future climate change scenario. This enables plants to distribute more photosynthetic products to their roots, thus affecting their own C distribution for promoting growth.

We sieved soils from a Pinus massoniana plantation in the Three Gorges Reservoir area into four aggregate sizes, including aggregates of 2000-8000 μm (large macroaggregates), 1000-2000 μm (coarse aggregates), 250-1000 μm (small macroaggregates), and <250 μm ( microaggregates). We analyzed the differences in the acidolyzable organic N components and net N mineralization of the aggregates under different N addition levels (30, 60, and 90 kg N ⋅ hm-2 ⋅ a-1, representing by N30, N60 and N90, respectively). The results showed that net nitrification rate of the aggregates ranged from 0.30-3.42 mg N ⋅ kg-1 and accounted for more than 80% of net nitrogen mineralization. Compared with the control, addition of 30, 60, and 90 kg N ⋅ hm ⋅ a increased total N by 24.1%-45.5%, 6.4% -34.3%, and 7.9%-42.4% in the large aggregates, coarse aggregate, small macroaggregates, and microaggregates, increased net N mineralization rate by 1.3-7.2, 1.4-6.6, and 1.8-12.9 times, but decreased the contents of available phosphorus by 9.3%-36.9%, 12.2%-56.7%, and 19.2%-61.9%, respectively. The contents of total acidolyzable N, soil organic matter, and rates of net ammonification, net nitrification, and net N mineralization increased as the aggregate size decreased, while available phosphorus contents showed an opposite trend. The levels of acid-hydrolyzable N components were ranked as acidolyzable amino acid N > acidolyzable ammonia N > acidolyzable unknown N> acidolyzable amino sugar N. Total N was the dominant contributor to the increases in acid-hydrolyzable N components. Results of stepwise multiple regression analyses showed that acidoly-zable amino acid N and acidolyzable amino sugar N were predictors of net ammonification rate. Acidolyzable amino sugar N, acidolyzable amino acid N, and acidolyzable ammonia N were predictors of net nitrification, net nitrogen mineralization rate, and net nitrogen mineralization accumulation. The physical structure of aggregates was associated with soil net N mineralization. Addition of N increased the contents and bioavailability of acidolyzable organic N, a large amount of which contributed to soil organic matter levels and the decrease in available phosphorus. [ABSTRACT FROM AUTHOR]

Analyzing the molecular and physiological processes that govern the uptake and transport of nitrogen (N) in plants is central to efforts to fully understand the optimization of plant N use and the changes in the N-use efficiency in relation to changes in atmospheric N deposition changes. Here, a field experiment was conducted using the ectomycorrhizal fungi (EMF), Pisolithus tinctorius (Pt) and Suillus grevillei (Sg). The effects of N deposition were investigated using concentrations of 0 kg·N·hm−2a−1 (N0), a normal N deposition of 30 kg·N·hm−2a−1 (N30), a moderate N deposition of 60 kg·N·hm−2a−1 (N60), and a severe N deposition of 90 kg·N·hm−2a−1 (N90), with the goal of examining how these factors impacted root activity, root absorbing area, NH4+ and NO3− uptake kinetics, and the expression of ammonium and nitrate transporter genes in Pinus massoniana seedlings under different levels of N deposition. These data revealed that EMF inoculation led to increased root dry weight, activity, and absorbing area. The NH4+ and NO3− uptake kinetics in seedlings conformed to the Michaelis–Menten equation, and uptake rates declined with increasing levels of N addition, with NH4+ uptake rates remaining higher than NO3− uptake rates for all tested concentrations. EMF inoculation was associated with higher Vmax values than were observed for non-mycorrhizal plants. Nitrogen addition resulted in the upregulation of genes in the AMT1 family and the downregulation of genes in the NRT family. EMF inoculation under the N60 and N90 treatment conditions resulted in the increased expression of each of both these gene families. NH4+ and NO3− uptake kinetics were also positively correlated with associated transporter gene expression in P. massoniana roots. Together, these data offer a theoretical foundation for EMF inoculation under conditions of increased N deposition associated with climate change in an effort to improve N absorption and transport rates through the regulation of key nitrogen transporter genes, thereby enhancing N utilization efficiency and promoting plant growth. Synopsis: EMF could enhance the efficiency of N utilization and promote the growth of Pinus massoniana under conditions of increased N deposition. [ABSTRACT FROM AUTHOR]

Low light intensity can lead to a decrease in photosynthetic capacity. However, could N-fixing species with higher leaf N contents mitigate the effects of low light? Here, we exposed seedlings of Dalbergia odorifera and Erythrophleum fordii (N-fixing trees), and Castanopsis hystrix and Betula alnoides (non-N-fixing trees) to three irradiance treatments (100%, 40%, and 10% sunlight) to investigate the effects of low irradiance on leaf structure, leaf N allocation strategy, and photosynthetic physiological parameters in the seedlings. Low irradiance decreased the leaf mass per unit area, leaf N content per unit area (Narea), maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), light compensation point, and light saturation point, and increased the N allocation proportion of light-harvesting components in all species. The studied tree seedlings changed their leaf structures, leaf N allocation strategy, and photosynthetic physiological parameters to adapt to low-light environments. N-fixing plants had a higher photosynthesis rate, Narea, Vcmax, and Jmax than non-N-fixing species under low irradiance and had a greater advantage in maintaining their photosynthetic rate under low-radiation conditions, such as under an understory canopy, in a forest gap, or when mixed with other species. [ABSTRACT FROM AUTHOR]

Investigations about phenotypic plasticity and adaptive traits of species facing rapidly changing stressful conditions are particularly relevant in context of rapid climate change. Morphological and physiological traits of Quercus aquifolioides plants growing at 2600 m and 3500 m above sea level (in the vicinity of the tree line) were studied. Leaf mesophyll conductance (gm) and stomatal conductance (gs) decreased, whereas carbon isotope composition (δ13C), nitrogen concentration and dark respiration increased with elevation. The specific leaf area (SLA) did not change, whereas photosynthetic capacity was dramatically inhibited at higher elevation. Differences in decline of photosynthesis (∼64%) and gm (∼80%) at elevations were reflected by similar chloroplast (Pc) to atmospheric (Pa) CO2 partial pressure ratio between the two populations. Therefore, δ13C changes were not associated to either SLA or Pc/Pa, d13C furnished an estimation of long-term Pc/Pa and, in turn, of long-term water use efficiency. Air temperature, which decreased consistently with altitude, significantly affected the long-term Pc/Pa. Plausibly, low temperature is the main determinant affecting δ13C at high altitude. In conclusion, phenotypic plasticity enabled Q. aquifolioides to maintain a positive carbon balance in response to dramatic environmental changes. [ABSTRACT FROM AUTHOR]

The study showed that at low elevation of Three Gorges Reservoir area, Pinus massoniana, Cupressus funebris and Cunninghamia lanceolata were the main coniferous forests, which were respectively divided into 14, 9 and 5 community types based on field investigation. The whole trend of their species richness index, species diversity index and species evenness index was that shrub layer and herb layer were larger than tree layer. The diversity indices had no obvious trend along the low elevation gradients, which was probably relevant to the high intensity of human activities in the area.