Effects of thinning and understory removal on soil phosphorus fractions in subtropical pine plantations.

Forest management changes the physical environments and nutrient dynamics and then regulates the forest productivity. Soil phosphorus (P) availability is critical for productivity in tropical and subtropical forests. However, it was still poorly understood how soil P content and fraction respond to various forest management practices in these regions. Here, we measured the soil total P, available P, and Hedley's P fractions, including inorganic and organic P (Pi and Po), in subtropical pine plantations treated with understory removal (UR), non-dominant species thinning (NDST) and dominant species thinning (DST) after nine years. Compared to plantations without management (CK), treatments such as UR, NDST, and DST decreased soil total P at 0-10 cm and soil available P at 0-10 cm and 10-20 cm. Increases in resin-Pi, NaOH-Pi, and C.HCl-Pi resulted in a higher total Pi in 0-10 cm (p < 0.05) in treated plots (UR, NDST, and DST) than in CK plots. UR, NDST, and DST treatments increased NaHCO3-Po and NaOH-Po (p < 0.05) but decreased C.HCl-Po at a depth of 0-10 cm. Regardless of management treatments, soil total P, available P, and P fractions in 0-10 cm showed higher contents than those in 10-20 cm. There were positive relationships between total P and total Po (p < 0.01) and between available P and total Pi. There were also positive relationships between total P, available P, NaHCO3-Pi, and NaOH-Pi (p < 0.05). In conclusion, forest management such as UR, NDST, and DST decreased soil total P and available P, and transforming soil P fractions to available P will meet the P demand following management in the pine plantations of subtropical China.

Adaptive divergence, historical population dynamics, and simulation of suitable distributions for Picea Meyeri and P. Mongolica at the whole-genome level.

The taxonomic classification of Picea meyeri and P. mongolica has long been controversial. To investigate the genetic relatedness, evolutionary history, and population history dynamics of these species, genotyping-by-sequencing (GBS) technology was utilized to acquire whole-genome single nucleotide polymorphism (SNP) markers, which were subsequently used to assess population structure, population dynamics, and adaptive differentiation. Phylogenetic and population structural analyses at the genomic level indicated that although the ancestor of P. mongolica was a hybrid of P. meyeri and P. koraiensis, P. mongolica is an independent Picea species. Additionally, P. mongolica is more closely related to P. meyeri than to P. koraiensis, which is consistent with its geographic distribution. There were up to eight instances of interspecific and intraspecific gene flow between P. meyeri and P. mongolica. The P. meyeri and P. mongolica effective population sizes generally decreased, and Maxent modeling revealed that from the Last Glacial Maximum (LGM) to the present, their habitat areas decreased initially and then increased. However, under future climate scenarios, the habitat areas of both species were projected to decrease, especially under high-emission scenarios, which would place P. mongolica at risk of extinction and in urgent need of protection. Local adaptation has promoted differentiation between P. meyeri and P. mongolica. Genotype‒environment association analysis revealed 96,543 SNPs associated with environmental factors, mainly related to plant adaptations to moisture and temperature. Selective sweeps revealed that the selected genes among P. meyeri, P. mongolica and P. koraiensis are primarily associated in vascular plants with flowering, fruit development, and stress resistance. This research enhances our understanding of Picea species classification and provides a basis for future genetic improvement and species conservation efforts.

Physiological response and molecular mechanism of Quercus variabilis under cadmium stress.

Heavy metal pollution is a global environmental problem, and Quercus variabilis has a stronger tolerance to Cd stress than do other species. We aimed to explore the physiological response and molecular mechanisms of Q. variabilis to Cd stress. In this study, the antioxidant enzyme activities of leaves were determined, while the photosynthetic parameters of leaves were measured using Handy PEA, and ion fluxes and DEGs in the roots were investigated using noninvasive microtest technology (NMT) and RNA sequencing techniques, respectively. Cd stress at different concentrations and for different durations affected the uptake patterns of Cd2+ and H+ by Q. variabilis and affected the photosynthetic efficiency of leaves. Moreover, there was a positive relationship between antioxidant enzyme (CAT and POD) activity and Cd concentration. Transcriptome analysis revealed that many genes, including genes related to the cell wall, glutathione metabolism, ion uptake and transport, were significantly upregulated in response to cadmium stress in Q. variabilis roots. WGCNA showed that these DEGs could be divided into eight modules. The turquoise and blue modules exhibited the strongest correlations, and the most significantly enriched pathways were the phytohormone signaling pathway and the phenylpropanoid biosynthesis pathway, respectively. These findings suggest that Q. variabilis can bolster plant tolerance by modulating signal transduction and increasing the synthesis of compounds, such as lignin, under Cd stress. In summary, Q. variabilis can adapt to Cd stress by increasing the activity of antioxidant enzymes, and regulating the fluxes of Cd2+ and H+ ions and the expression of Cd stress-related genes.

[Influencing mechanism of stand age to the accumulation of microbial residue carbon in the Pinus masso-niana plantations]. / 林龄对马尾松人工林微生物残体碳积累的影响机制.

Clarifying the accumulation pattern of soil microbial residue carbon and its contribution to soil organic carbon (SOC) across stand age is helpful to understand the mechanism underlying soil carbon cycling. In this study, we analyzed the differences of amino sugar content, physicochemical properties and microbial composition in surface soil (0-10 cm) in young (6 a), middle-aged (13 a), near-mature (29 a), mature (38 a) and over-mature (57 a) Pinus massoniana plantations of subtropical China, quantified the microbial residue carbon content and its contribution to SOC, and discussed the mechanism. The results showed that SOC, total nitrogen, amorphous iron oxide and leucine aminopeptidase contents in the middle-aged plantation were significantly lower than those in the mature plantation. Soil pH and fungal/bacteria in young plantation were significantly higher than those in other age groups. Across the stand age gradient, the ranges of microbial, fungal and bacterial residue carbon were 7.52-14.63, 4.03-8.00 and 3.48-6.63 g·kg-1, respectively. The contents of all the residue carbon were significantly higher in the mature plantation than that of the middle-aged plantation, which were positively affected by soil total nitrogen content. The contribution of microbial, fungal, and bacterial residue carbon to SOC was 59.7%-72.3%, 33.4%-45.6%, and 24.3%-30.8%, respectively. The contribution of fungal residue carbon to SOC in young plantation was significantly higher than that in other age groups, and the contribution of bacterial residue carbon to SOC in middle-aged plantation was significantly higher than that in young and near-mature plantations, both of which were affected by soil inorganic nitrogen. Fungal residue carbon content was 1.2-1.7 times as that of bacterial residue carbon content, and dominated for the accumulation of microbial residue carbon. Results of the partial least squares model showed that stand age, soil environmental factors (such as leucine aminopeptidase, amorphous iron oxide, pH, and total nitrogen), bacterial residue carbon, fungal residue carbon and the contribution of bacterial residue carbon to SOC had total effects on the contribution of fungal residue carbon to SOC (-0.37, -1.16, 0.90, 1.09, and 0.83, respectively). In conclusion, stand age promoted the accumulation of microbial residue carbon but did not increase its contribution to SOC.

[Effects of nitrogen addition on acidolyzable organic nitrogen components and nitrogen mineralization in aggregates of Pinus massoniana plantations in the Three Gorges Reservoir area, China]. / æ°®æ·»åŠ å¯¹ä¸‰å³¡åº“åŒºé©¬å°¾æ¾äººå·¥æž—åœŸå£¤å›¢èšä½“æœ‰æœºæ°®ç»„åˆ†å’Œæ°®çŸ¿åŒ–çš„å½±å“.

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-2·a-1 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 associa-ted 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.

Comparison of species composition and community characteristics of Quercus forests on south and north slopes of Taibai Mountain, China.

We examined species composition, community characteristics, diversity, and community similarity of five Quercus communities composed of three Quercus species (Q. variabilis, Q. aliena var. acutiserrata, Q. wutaishanica) on the altitudinal gradient on the south and north slopes of Taibai Mountain. The results showed that there was an altitudinal transition pattern from Q. variabilis pure forest to Q. variabilis-Q. aliena var. acutiserrata mixed forest, Q. aliena var. acutiserrata pure forest, Q. aliena var. acutiserrata-Q. wutaishanica mixed forest and Q. wutaishanica pure forest on the south and north slopes of Taibai Mountain. The main companion species of Quercus community on the north slope were Pinus armandii, Castanea seguinii, and Sorbus alnifolia, and were Pinus tabuliformis, C. seguinii, Carpinus cordata, and Q. spinosa on the south slope. Species richness, woody plant density, and Quercus species dominance on the north slope of Taibai Mountain were higher than those on the south slope. α diversity of tree layer in Quercus community on the south and north slopes of Taibai Mountain increased first, then decreased and then increased with altitude. α diversity of tree layer was higher in mixed forests than pure forests. α diversity of shrub layer was higher than that of tree layer and herb layer in Quercus community on south and north slopes. ß diversity fluctuated greatly along the altitudinal gradient on the south and north slopes, indicating that species composition changed greatly with altitude. Results of redundancy analysis showed that mean warmest month temperature, altitude and tree height accounted for 79.0% of the community diversity on the north slope, and that soil water content, tree height, canopy density and mean annual temperature accounted for 79.6% of the community diversity on the south slope. Overall, Quercus dominance was higher on the north slope of Taibai Mountain, and the substitution distribution pattern of Quercus species was clearer than that on the south slope. Environmental factors related to temperature and precipitation jointly affected α diversity of Quercus communities.

Effects of gravel on the evaluation of soil organic carbon density in Pinus massoniana plantations.

Gravel (>2 mm) is one of the main parameters for estimating soil carbon pool. To assess the effects of gravel on soil bulk density (BD) and organic carbon density (SOCD) in Pinus massoniana plantations, we estimated the BD and SOCD at the 0-10, 10-20 and 20-40 cm soil depths of 131 plots under two different conditions, with and without removing gravel. The BD of each soil layer after removing gravel was 0.58-1.57, 0.60-1.67, and 0.59-1.75 g·cm-3, respectively, which was significantly lower than that before removing gravel. Gravel increased the BD by 6.5%-6.8%. The SOCD of each soil layer before removing gravel was 8.93-65.97, 7.63-59.08, and 8.79-94.53 t·hm-2, respectively, which was higher than that after removing gravel. Overall, by neglecting the effect of gravel, SOCD was overestimated by 4.9%-11.8%. As gravel content increased, the relative deviation in the estimated BD and SOCD among different methods increased. When the gravel content was higher than 20%, the estimated SOCD at soil layer of 0-40 cm showed a significant difference between neglecting gravel and removing gravel, with the former being 29.7%-47.4% higher than the latter. In conclusion, gravel markedly affected the estimations of BD and SOCD. It was recommended that SOCD should be estimated by the method that not only uses the BD after removing gravel but also considers gravel as a correction factor (especially when gravel content is above 20.0%) to avoid overestimation of soil carbon pool.

Climatic responses and variability in bark anatomical traits of 23 Picea species.

In woody plants, bark is an important protective tissue which can participate in photosynthesis, manage water loss, and transport assimilates. Studying the bark anatomical traits can provide insight into plant environmental adaptation strategies. However, a systematic understanding of the variability in bark anatomical traits and their drivers is lacking in woody plants. In this study, the bark anatomical traits of 23 Picea species were determined in a common garden experiment. We analyzed interspecific differences and interpreted the patterns in bark anatomical traits in relation to phylogenetic relationships and climatic factors of each species according to its global distribution. The results showed that there were interspecific differences in bark anatomical traits of Picea species. Phloem thickness was positively correlated with parenchyma cell size, possibly related to the roles of parenchyma cells in the radial transport of assimilates. Sieve cell size was negatively correlated with the radial diameter of resin ducts, and differences in sieve cells were possibly related to the formation and expansion of resin ducts. There were no significant phylogenetic signals for any bark anatomical trait, except the tangential diameter of resin ducts. Phloem thickness and parenchyma cell size were affected by temperature-related factors of their native range, while sieve cell size was influenced by precipitation-related factors. Bark anatomical traits were not significantly different under wet and dry climates. This study makes an important contribution to our understanding of variability in bark anatomical traits among Picea species and their ecological adaptations.

Effects of Different Donor Ages on the Growth of Cutting Seedlings Propagated from Ancient Platycladus orientalis.

The effects of tree age on the growth of cutting seedlings propagated from ancient trees have been an important issue in plant breeding and cultivation. In order to understand seedling growth and stress resistance stability, phenotypic measurements, physiological assays, and high-throughput transcriptome sequencing were performed on sown seedlings propagated from 5-year-old donors and cutting seedlings propagated from 5-, 300-, and 700-year-old Platycladus orientalis donors. In this study, the growth of cutting seedlings propagated from ancient trees was significantly slower; the soluble sugar and chlorophyll contents gradually decreased with the increase in the age of donors, and the flavonoid and total phenolic contents of sown seedlings were higher than those of cutting seedlings. Enrichment analysis of differential genes showed that plant hormone signal transduction, the plant-pathogen interaction, and the flavone and flavonol biosynthesis pathways were significantly up-regulated with the increasing age of cutting seedlings propagated from 300- and 700-year-old donors. A total of 104,764 differentially expressed genes were calculated using weighted gene co-expression network analysis, and 8 gene modules were obtained. Further, 10 hub genes in the blue module were identified, which revealed that the expression levels of JAZ, FLS, RPM1/RPS3, CML, and RPS2 increased with the increase in tree age. The results demonstrated that the age of the donors seriously affected the growth of P. orientalis cutting seedlings and that cutting propagation can preserve the resistance of ancient trees. The results of this study provide important insights into the effects of age on asexually propagated seedlings, reveal potential molecular mechanisms, and contribute to an improvement in the level of breeding and conservation of ancient germplasm resources of P. orientalis trees.

Soil microbial residue characteristics in Pinus massoniana lamb. Plantations.

A large amount of stable soil organic matter (SOM) is derived from microbial necromass, which can be assessed by quantifying amino sugar biomarkers. Pinus massoniana Lamb. Plantations are widely distributed in China and play a vital role in forest carbon sequestration. However, the patterns of soil microbial residue remain poorly understood. In this study, amino sugars were used to characterize patterns of soil microbial residues at three soil depths (0-10, 10-20, and 20-30 cm) in P. massoniana plantations of different ages (young, middle-aged, near-mature, mature, and over-mature; denoted as YG, MD, NM, MT, and OM, respectively). In the topsoil (0-10 cm), the total nitrogen (TN) content of the OM forest was the highest, whereas the soil organic carbon (SOC) content of the MT forest was the highest. Consistent with changes in SOC and TN, total microbial residue content decreased with increasing soil depth. However, the total microbial residues C to SOC contribution increased considerably with increasing depth, suggesting that more SOC was derived from microbial residues in the subsoil than that from the topsoil. The fungal residue C to SOC contribution was higher than that of bacterial residue C. Total amino sugar content in the topsoil increased with increasing age, and MT and OM had a significantly higher content than that of other forests. At all soil depths, SOC and TN content predominantly determined microbial necromass, whereas soil microbial biomass content predominantly determined microbial necromass in the topsoil; soil pH predominantly determined microbial necromass in the 10-20 cm soil layer; and soil pH and Ca2+ content were the primary factors in the soil layer below 20 cm. The study provides valuable insights into controls of microbial-derived organic C could be applied in Earth system studies for predicting SOC dynamics in forests.

Effects of Donor Ages and Propagation Methods on Seedling Growth of Platycladus orientalis (L.) Franco in Winter.

To evaluate the effects of donor ages on growth and stress resistance of 6-year-old seedlings propagated from 5-, 2000-, and 3000-year-old Platycladus orientalis donors with grafting, cutting, and seed sowing, growth indicators and physiological and transcriptomic analyses were performed in 6-year-old seedlings in winter. Results showed that basal stem diameters and plant heights of seedlings of the three propagation methods decreased with the age of the donors, and the sown seedlings were the thickest and tallest. The contents of soluble sugar, chlorophyll, and free fatty acid in apical leaves of the three propagation methods were negatively correlated with donor ages in winter, while the opposite was true for flavonoid and total phenolic. The contents of flavonoid, total phenolic, and free fatty acid in cutting seedlings were highest in the seedlings propagated in the three methods in winter. KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis of differentially expressed genes showed phenylpropanoid biosynthesis and fatty acid metabolism pathways, and their expression levels were up-regulated in apical leaves from 6-year-old seedlings propagated from 3000-year-old P. orientalis donors. In addition, hub genes analysis presented that C4H, OMT1, CCR2, PAL, PRX52, ACP1, AtPDAT2, and FAD3 were up-regulated in cutting seedlings, and the gene expression levels decreased in seedlings propagated from 2000- and 3000-year-old donors. These findings demonstrate the resistance stability of cuttings of P. orientalis and provide insights into the regulatory mechanisms of seedlings of P. orientalis propagated from donors at different ages in different propagation methods against low-temperature stress.

Nitrogen addition promotes early-stage and inhibits late-stage decomposition of fine roots in Pinus massoniana plantation.

Increasing atmospheric nitrogen (N) deposition has a profound impact on the ecosystem functions and processes. Fine root decomposition is an important pathway for the reentry of nutrients into the soil. However, the effect of N addition on root decomposition and its potential mechanism is not well understood with respect to root branch orders. In this study, we conducted a 30-month decomposition experiment of fine roots under different concentrations of N addition treatments (0, 30, 60, and 90 kg N ha-1 year-1, respectively) in a typical Pinus massoniana plantation in the Three Gorges Reservoir Area of China. In the early stage of decomposition (0-18 months), N addition at all concentrations promoted the decomposition of fine roots, and the average decomposition rates of order 1-2, order 3-4, order 5-6 fine roots were increased by 13.54%, 6.15% and 7.96% respectively. In the late stage of decomposition (18-30 months), high N addition inhibited the decomposition of fine root, and the average decomposition rates of order 1-2, order 3-4, order 5-6 fine roots were decreased by 58.35%, 35.43% and 47.56% respectively. At the same time, N addition promoted the release of lignin, carbon (C), N, and phosphorus (P) in the early-stage, whereas high N addition inhibited the release of lignin, C, N, and the activities of lignin-degrading enzyme (peroxidase and polyphenol oxidase) in the late-stage. The decomposition constant (k) was significantly correlated with the initial chemical quality of the fine roots and lignin-degrading enzyme activities. The higher-order (order 3-4 and order 5-6) fine roots decomposed faster than lower-order (order 1-2) fine roots due to higher initial cellulose, starch, sugar, C concentrations and higher C/N, C/P, lignin/N ratios and lower N, P concentrations. In addition, low N (30 kg N ha-1 year-1) treatments decreased soil organic matter content, whereas high N (90 kg N ha-1 year-1) treatment had the opposite effect. All the N treatments reduced soil pH and total P content, indicating that increased N deposition may led to soil acidification. Our findings indicated that the effect of N addition on decomposition varied with the decomposition stages. The decomposition difference between the lower-order and higher-order fine roots were controlled strongly by the initial chemical quality of the fine roots. This study provides new insights into understanding and predicting possible changes in plant root decomposition and soil properties in the future atmospheric N deposition increase scenarios.

Correlations between forest soil quality and aboveground vegetation characteristics in Hunan Province, China.

As a key component of terrestrial ecosystems, soil interacts directly with aboveground vegetation. Evaluating soil quality is therefore of great significance to comprehensively explore the interaction mechanism of this association. The purpose of this study was to fully understand the characteristics of aboveground vegetation, soil quality, and their potential coupling relationship among different forest types in Hunan Province, and to provide a theoretical basis for further exploring the mechanisms underlying soil-vegetation interactions in central China. We have set up sample plots of five kinds of forests (namely broad-leaved forest, coniferous forest, coniferous broad-leaved mixed forest, bamboo forest, and shrub forest) in Hunan Province. To explore the differences of vegetation characteristics and soil physical and chemical properties among the five stand types, variance analysis, principal component analysis, and regression analysis were used. Finally, we explored the coupling relationship between soil quality and aboveground vegetation characteristics of each forest. We found that there were significant differences in soil quality among the forest types, ranked as follows: shrub forest > bamboo forest > broad-leaved forest > mixed coniferous and broad-leaved forest > coniferous forest. In general, there was a negative correlation between vegetation richness and soil quality in the broad-leaved forest and the shrub forest, but they showed a positive correlation in the coniferous forest, the mixed coniferous and broad-leaved forest, and the bamboo forest. As a necessary habitat condition for aboveground vegetation, soil directly determines the survival and prosperity of plant species. These results indicated that for vegetation-soil dynamics in a strong competitive environment, as one aspect wanes the other waxes. However, in a weak competitive environment, the adverse relationship between vegetation and soil is less pronounced and their aspects can promote.

Living grass mulching improves soil enzyme activities through enhanced available nutrients in citrus orchards in subtropical China.

Living grass mulching (LGM) is an important orchard floor management that has been applied worldwide. Although LGM can effectively enhance soil nutrient availability and fertility, its effects on microbial-mediated soil nutrient cycling and main drivers are unclear. Meanwhile, the variation of enzyme activities and soil nutrient availability with LGM duration have been rarely studied. This study aims to explore the effects of mulching age and soil layer on enzyme activities and soil nutrients in citrus orchards. In this study, three LGM (Vicia villosa) treatments were applied, i.e., mulching for eight years, mulching for four years, and no mulching (clean tillage). Their effects on the enzyme activities and soil nutrients were analyzed in different soil layers of citrus orchards in subtropical China, i.e., 0-10, 10-20, and 20-40 cm. Compared to clean tillage, mulching for four years had fewer effects on enzyme activities and soil nutrients. In contrast, mulching for eight years significantly increased available nitrogen (N), phosphorus (P) nutrients, ß-glucosidase, and cellobiohydrolase activities in the soil layer of 0-20 cm. In the soil layer of 0-40 cm, microbial biomass carbon (C), N, P, N-acetylglucosaminidase, leucine aminopeptidase, and acid phosphatase activities also increased (P < 0.05). Mulching for eight years significantly promoted C, N, and P-cycling enzyme activities and total enzyme activities by 2.45-6.07, 9.29-54.42, 4.42-7.11, and 5.32-14.91 times, respectively. Redundancy analysis shows that mulching treatments for eight and four years had soil layer-dependent positive effects on soil enzyme activities. Microbial C and P showed the most significant positive correlation with enzyme activities, followed by moisture content, organic C, and available N (P < 0.05). Available nutrients contributed almost 70% to affect enzyme activities significantly and were the main drivers of the enzyme activity variation. In summary, LGM could improve soil enzyme activities by increasing available nutrients. The promotion effect was more significant under mulching for eight years. Therefore, extending mulching age and improving nutrient availability are effective development strategies for sustainable soil management in orchard systems. Our study can provide valuable guidelines for the design and implementation of more sustainable management practices in citrus orchards.

Variations in rhizosphere soil total phosphorus and bioavailable phosphorus with respect to the stand age in Pinus massoniana Lamb.

Phosphorus (P) is a nutrient limiting plant growth in subtropical regions. However, our understanding of how soil P responds to an increase in stand age is rather poor. In particular, little is known about how bioavailable P pools (soluble P, exchangeable P, hydrolyzable P, and ligand P) shift with a change in stand age. Moreover, the P cycle in rhizosphere soil has the most direct and significant influence on plants. The aim of the present study was to determine the concentrations of total P in various rhizosphere soil bioavailable P fractions in 5-, 9-, 19-, 29-, and 35-year-old stands of Pinus massoniana Lamb. According to the results, total P (TP) concentration and N:P ratio in rhizosphere soil first decreased, and then increased with an increase in stand age. Soluble P concentration decreased first, and then increased with an increase in stand age; exchangeable P and ligand P decreased first, and then tended to be stable with an increase in stand age, whereas hydrolyzable P increased first, and then decreased. Structural Equation Model results suggested that ligand P and soluble P were the major factor affecting the TP. In addition, soil microorganisms and acid phosphatase-driven hydrolyzable P play a crucial role in soil bioavailable P cycling. Overall, the results of our study provide a mechanistic understanding of soil bioavailable P cycling under low available P conditions, and a basis for an effective P management strategy for the sustainable development of P. massoniana plantations.

Acclimation Strategy of Masson Pine (Pinus massoniana) by Limiting Flavonoid and Terpenoid Production under Low Light and Drought.

Low light and drought often limit the growth and performance of Masson pines (Pinus massoniana) in the subtropical forest ecosystem of China. We speculated that stress-induced defensive secondary metabolites, such as flavonoids and terpenoids, might influence the growth of Masson pines, considering the existence of tradeoffs between growth and defense. However, the mechanisms of Masson pines responsive to low light and drought at the levels of these two metabolites remain unclear. In the present work, the compositions of flavonoids and terpenoids, as well as their biosynthetic pathways, were revealed through metabolome and transcriptome analyses, respectively, coupled with a study on carbon allocation using a 13CO2-pulse-labeling experiment in two-year-old seedlings under low light (LL), drought (DR), and their combined stress (DL) compared to a control (CK). A total of 35 flavonoids and derivatives (LL vs. CK: 18; DR vs. CK: 20; and DL vs. CK: 18), as well as 29 terpenoids and derivatives (LL vs. CK: 23; DR vs. CK: 13; and DL vs. CK: 7), were differentially identified in the leaves. Surprisingly, most of them were decreased under all three stress regimes. At the transcriptomic level, most or all of the detected DEGs (differentially expressed genes) involved in the biosynthetic pathways of flavonoids and terpenoids were downregulated in phloem and xylem under stress treatments. This indicated that stress treatments limited the production of flavonoids and terpenoids. The reduction in the 13C allocation to stems might suggest that it is necessary for maintaining the growth of Masson pine seedlings at the whole-plant level by attenuating energetic resources to the biosynthetic pathways of flavonoids and terpenoids when facing the occurrence of adverse environments. Our results provide new insight into understanding the acclimation strategy of Masson pines or other conifers in adverse environments.

Rotation age extension synergistically increases ecosystem carbon storage and timber production of Chinese fir plantations in southern China.

Afforestation is an effective method to increase carbon (C) sinks and address climate change. It is crucial to understand how the stand growth affects C sequestration capacity, especially when the trade-offs with timber production from plantations have not been fully examined. We used a chronosequence approach to estimate C storage in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations (including the trees, understory, litter, and soils) at seven stand ages (3, 8-11, 16, 21, 25, 29, and 32 years). Ecosystem C storage increased nonlinearly from 76.4 to 282.2 t ha-1 with stand age and was fitted with a logistic model that had a maximum C storage and age of 271.9 t ha-1 and 33 years, respectively, to reach 95% of the maximum stored C. The C increment was mainly contributed by an increase in tree biomass, which ranged from 2.8 to 177.7 t ha-1 and comprised 4-64% of the total ecosystem C. Live root C (sum of the stump, coarse, and fine root C) showed a logistic increase from 2.0 to 26.3 t ha-1 with stand age and constituted 2.5-9.3% of ecosystem C. Understory plants and litter represented a small pool (<2% of ecosystem C). The C storage in shrubs and litter slightly increased, while that in herbs decreased as the stands aged. Soil C storage was an important and relatively stable pool, ranging from 69.6 to 130.1 t ha-1. Stand volume was also best fitted with a logistic model with a maximum value of 552.6 m3 ha-1. Additionally, the time needed to reach 95% of the maximum volume was 25 years. Hence, extending the rotation age to over 30 years for Chinese fir plantations could potentially maximize the synergistic benefits of C storage to mitigate climate change and obtain timber products for economic profit.

Drought changes litter quantity and quality, and soil microbial activities to affect soil nutrients in moso bamboo forest.

Drought would significantly influence the forest soils through changing the litterfall production and decomposition process. However, comprehensive in situ studies on drought effects in subtropical forests, especially in bamboo forests, have rarely been conducted. Here, we conducted a throughfall exclusion experiment with a rainfall reduction of ~80% in moso bamboo (Phyllostachys edulis) forests to investigate effects of drought on litter quantity, quality, soil microbial and enzyme activities, and soil nutrients across two years in subtropical China. We observed that throughfall exclusion (TE) treatment significantly decreased soil moisture by 63% compared to ambient control treatment (CK). Drought significantly decreased the annual litterfall in the second treatment year, and the leaf litter decomposition rate (-30% relative to CK) over 2 years of decomposition. TE treatment significantly decreased net release rate of litter carbon (C) and the amount of litter nitrogen (N) immobilization during a 360-day decomposition period, leading an increased litter C: N ratio in TE compared to CK. There was a distinct difference in soil microbial community composition between TE and CK treatments, showing higher bacteria biomass in TE but no difference in fungal biomass between TE and CK. Structural equation modelling revealed that drought decreased the contribution of litter quantity to soil nutrients but increased that of litter quality and soil microbial community to soil nutrients. Our results suggest that increasing drought events in subtropical China will directly reduce litterfall quantity and quality on the one hand, and alter the soil enzyme activities and microbial composition on the other hand, all of which will consequently decrease litter decomposition rate, soil nutrient availability, growth rate and productivity, leading to changes in the functioning and services of subtropical bamboo forests.

Effects of Nitrogen and Phosphorus Addition on Soil Extracellular Enzyme Activity and Stoichiometry in Chinese Fir (Cunninghamia lanceolata) Forests.

Soil extracellular enzymes play an important role in microbial functions and soil nutrient cycling in the context of increasing N deposition globally. This is particularly important for Chinese fir (Cunninghamia lanceolata) forests because of the decline in soil fertility induced by successive rotation. In this study, we aimed to determine the effects of simulated N deposition (N30: 30 kg ha-2 year-1; N60: 60 kg ha-2 year-1) and phosphorus addition (P20: 20 mg kg-1; P40: 40 mg kg-1) on the activity and stoichiometry of soil extracellular enzymes related to soil C, N, and P cycling in Chinese fir. The results showed that N addition alone increased the activity of soil ß-1,4 glucosidase (BG) but decreased the activity of N-acetyl-ß-d-glucosidase (NAG) and leucine aminopeptidase (LAP). N addition increased the ratios of soil enzymes, C:N and C:P, alleviated microbial N-limitation, and aggravated microbial C-limitation. P addition alone increased enzyme activity, and P40 addition increased the ratio of BG to soil microbial biomass carbon (MBC), and (NAG + LAP):MBC activity ratio, thereby aggravating C restriction. N and P co-addition significantly affected soil extracellular enzyme activity and stoichiometry. For instance, BG activity and BG:MBC activity ratio increased significantly under the N30 + P40 treatment, which intensified C-limitation. Soil pH was the main factor influencing enzyme activity, and these variables were positively correlated. The stoichiometric relationships of enzyme reactions were coupled with soil pH, total nitrogen (TN), and available phosphorus (AP). Our results indicate that changes in soil characteristics induced by N and P inputs influence the activities of soil microorganisms and result in changes in microbial resource acquisition strategies. This study provides useful insights into the development of management strategies to improve the productivity of Chinese fir forests under scenarios of increasing N deposition.

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We examined the effects of nitrogen addition (0, 30, 60, and 90 kg N·hm-2·a-1) to soil microbial biomass, enzyme activities, and nutrient contents of the Pinus massoniana-Quercus variabilis mixed plantations in the Three Gorges Reservoir Area, with the aim to provide a theoretical basis for predicting soil carbon dynamics under the background of continuously increasing atmospheric nitrogen deposition in this area. The results showed that nitrogen addition at all levels led to a significant increase of the contents of organic carbon, total nitrogen, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass phosphorus (MBP) in the forest soil, while a decrease of soil pH-value, and no significant effect on the total phosphorus content. Nitrogen addition increased the activities of ß-1-4 glucosidase (BG), cellobiose hydrolase (CB), acid phosphatase (AP), N-acetylglucosaminosidase (NAG) and peroxidase (POD), while inhibited that of polyphenol oxidase (PPO). There was a significant seasonal variation in soil oxidase activities, in which the peroxidase activity was higher in May and August, and the polyphenol oxidase activity was the highest in August. Soil enzyme activities were significantly correlated with soil moisture and the contents of soil nutrients, MBC, MBN, and MBP. The variation of soil enzyme activities was caused by the comprehensive effects of multiple factors. The redundancy analysis (RDA) showed that the contents of total soil nitrogen and MBC were the main environmental factors driving soil enzyme activities. The continuous increase of atmosphere nitrogen deposition would lead to soil acidification and promote the turnover of soil organic carbon and nutrient cycling in the Pinus massoniana-Quercus variabilis mixed plantations of the study area.