Enzyme stoichiometry reveals microbial nitrogen limitation in stony soils.

Resource limitation for soil microorganisms is the crucial factor in nutrient cycling and vegetation development, which are especially important in arid climate. Given that rock fragments strongly impact hydrologic and geochemical processes in arid areas, we hypothesized that microbial resource (C and N) limitation will increase along the rock fragment content (RFC) gradient. We conducted a field experiment in Minjiang river arid valleys with four RFC content (0 %, 25 %, 50 %, and 75 %, V V-1) and four vegetation types (Artemisia vestita, Bauhinia brachycarpa, Sophora davidii, and the soil without plants). Activities of C (ß-1,4-glucosidase, BG), N (ß-1,4-N-acetyl-glucosaminidase, NAG; L-leucine aminopeptidase, LAP), and P (acid phosphatase, ACP) acquiring enzymes were investigated to assess the limitations by C, N or P. In unplanted soil, the C acquiring enzyme activity decreased by 43 %, but N acquiring enzyme activity increased by 72 % in 75 % RFC than those in rock-free soils (0 % RFC). Increasing RFC reduced C:N and C:P enzymatic ratios, as well as vector length and vector angle (< 45°). Plants increased the activities of C and N acquiring enzymes in soils, as well as C:P and N:P enzyme activities, as well as vector length (by 5.6 %-25 %), but decreased vector angle (by 13 %-21 %). Enzyme stoichiometry was dependent on biotic and abiotic factors, such as soil water content, soil C:N, and total content of phospholipid fatty acids, reflecting microbial biomass content. Increased RFC shifted enzymatic stoichiometry toward lower C but stronger N limitation for microorganisms. Vegetation increased microbial C and N limitation, and impacted the enzymatic activities and stoichiometry depending on shrub functional groups. Consequently, the direct effects of vegetation, nutrient availability and microbial biomass content, as well as indirect effects of soil properties collectively increased microbial resource limitations along the RFC gradient.

Shifting patterns in fine root distribution of four xerophytic species across soil structural gradients and years of growth.

Fine root (diameter < 2 mm) distribution influences the potential for resource acquisition in soil profiles, which defines how plants interact with local soil environments; however, a deep understanding of how fine root vertical distribution varies with soil structural variations and across growth years is lacking. We subjected four xerophytic species native to an arid valley of China, Artemisia vestita, Bauhinia brachycarpa, Sophora davidii, and Cotinus szechuanensis, to increasing rock fragment content (RFC) treatments (0%, 25%, 50%, and 75%, v v-1) in an arid environment and measured fine root vertical profiles over 4 years of growth. Fine root depth and biomass of woody species increased with increasing RFC, but the extent of increase declined with growth years. Increasing RFC also increased the degree of interannual decreases in fine root diameter. The limited supply of soil resources in coarse soils explained the increases in rooting depth and variations in the pattern of fine root profiles across RFC. Fine root depth and biomass of the non-woody species (A. vestita) in soil profiles decreased with the increase in RFC and growth years, showing an opposite pattern from the other three woody species. Within woody species, the annual increase in fine root biomass varied with RFC, which led to large interannual differences in the patterns of fine root profiles. Younger or non-woody plants were more susceptible to soil environmental changes than the older or woody plants. These results reveal the limitations of dry and rocky environments on the growth of different plants, with woody and non-woody plants adjusting their root vertical distribution through opposite pathways to cope with resource constraints, which has management implications for degraded agroforest ecosystems.

A trade-off between leaf hydraulic efficiency and safety across three xerophytic species in response to increased rock fragment content.

Limited information is available on the variation of plant leaf hydraulic traits in relation to soil rock fragment content (RFC), particularly for xerophytes native to rocky mountain areas. In this study, we conducted a field experiment with four gradients of RFC (0, 25, 50 and 75% ν ν-1) on three different xerophytic species (Sophora davidii, Cotinus szechuanensis and Bauhinia brachycarpa). We measured predawn and midday leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), Ψleaf induced 50% loss of Kleaf (P50), pressure-volume curve traits and leaf structure. A consistent response of hydraulic traits to increased RFC was observed in three species. Kleaf showed a decrease, whereas P50 and turgor loss point (Ψtlp) became increasingly negative with increasing RFC. Thus, a clear trade-off between hydraulic efficiency and safety was observed in the xerophytic species. In all three species, the reduction in Kleaf was associated with an increase in leaf mass per area. In S. davidii, alterations in Kleaf and P50 were driven by leaf vein density (VLA) and Ψtlp. In C. szechuanensis, Ψtlp and VLA drove the changes in Kleaf and P50, respectively. In B. brachycarpa, changes in P50 were driven by VLA, whereas changes in both Kleaf and P50 were simultaneously influenced by Ψtlp. Our findings suggest that adaptation to increased rockiness necessarily implies a trade-off between leaf hydraulic efficiency and safety in xerophytic species. Additionally, the trade-off between leaf hydraulic efficiency and safety among xerophytic species is likely to result from processes occurring in the xylem and the outside-xylem hydraulic pathways. These findings contribute to a better understanding of the survival strategies and mechanisms of xerophytes in rocky soils, and provide a theoretical basis for the persistence of xerophytic species in areas with stony substrates.

Age Determination and Growth Characteristics of the Potentilla griffithii: A Comparison of Two Different Habitats in Western Sichuan Plateau, China.

This study proposes a rapid and non-destructive technique for determining the age of Potentilla griffithii individuals in the field by observing the sequence of leaf scars. Based on two- to three-year-old P. griffithii seedlings, planted in a common garden in the western Sichuan Plateau, China, the study found that the rates of basal leaf production were consistent, with leaves growing from March to April and falling off from October to December, leaving behind basal leaf scars. Thus, the age of individuals in situ could be determined by counting the leaf scars. Through this method, we determined the age structure and growth strategy of P. griffithii populations in two typical habitats in the western Sichuan Plateau. In open land habitats, the age structure of P. griffithii populations was relatively younger compared to understory habitats. In open land, P. griffithii tends to allocate more photosynthate terminal organs (leaves and fine roots) to absorbing more resources, as well as to its reproductive organs (flower stems and aggregate fruits), to expand the population. The P. griffithii population in the understory habitat is in its middle-age stage and concentrates more photosynthate in the coarse root part (e.g., the high coarse root mass fraction (FRMF)) to support the plant. Additionally, we found a significant correlation between P. griffithii plant age and various traits in open land habitats. Therefore, we conclude that plant age can be used as a good predictor of plant growth condition in open land. These results allow for predicting ecological processes, based on the ages and traits of P. griffithii plants, providing a theoretical basis to support the large-scale breeding of P. griffithii.

Within leaf nitrogen allocation regulates the photosynthetic behavior of xerophytes in response to increased soil rock fragment content.

There is limited information on how plant functional traits vary with soil rock fragment content (RFC), especially for xerophytes growing in stony soils. We examined leaf functional traits of three xerophytes (Sophora davidii; Cotinus szechuanensi; and Artemisia vestita) grown under an RFC gradient in a heavy loamy soil. Our results show that photosynthetic capacity increased linearly with RFC in S. davidii, whereas unimodal patterns were observed for the other two species. The RFC that maximized photosynthetic capacity (Asat) and photosynthetic N use efficiency (PNUE) were achieved by allocating more N to photosynthetic apparatus at the expense of cell walls. For C. szechuanensis, the increased fraction of photosynthetic N allocated to carboxylation (PC) bioenergetics (PB), and thylakoid light-harvesting components (PL) together contributed to the higher Asat and PNUE values. As for S. davidii, both PC and PB mainly contributed to higher Asat and PNUE, whereas for A. vestita only PB was the main contributor. Our results suggest that increased non-capillary porosity of high RFC soil conditions through promoting the root growth of S. davidii and C. szechuanensis ensures sufficient water and N supply for photosynthetic capacity. In shallow-rooted species A. vestita, low RFC soil maintained higher nitrate N in the topsoil, enhancing leaf photosynthetic capacity. We conclude that rock fragments promoted leaf photosynthetic capacity in the studied loamy soil system, but the promoting effect was species-specific. The results highlight the relevance of consideration of soil rock fraction in evaluation of photosynthetic behavior of xerophytes in heterogeneous rocky soils.

Large-scale patterns of understory biomass and its allocation across China's forests.

Plant biomass storage and its allocation reflect the ecosystem productivity and adaptation to different environments. Understory vegetation is a significant component of any forest ecosystem and plays a vital role in biodiversity maintenance and the ecosystem's carbon cycle. Although many studies have addressed the relationships of climate, stand structure and resource availability with understory biomass and its allocation at local scales, the large-scale variation of understory biomass and allocation and their underlying mechanisms remain unclear. We compiled a large database of understory biomass at the community level across China's forests to explore the large-scale patterns of understory biomass and R/S ratio, and to quantify the relative importance of drivers. Understory biomass and R/S ratio varied largely with forest types, and decreased with increasing longitude, but increased with elevation. Understory biomass increased with increasing latitude, mean annual temperature (MAT), and mean annual precipitation (MAP), while the R/S ratio decreased with latitude, MAT, and MAP. Stand structure had a strong effect on the variations in understory biomass. MAP was the most important driver in determining R/S ratio. This synthesis provides a first assessment of the large-scale patterns of understory biomass and allocation and sheds new light on the mechanisms underlying the variations in understory biomass and its allocation over a broad geographic scale. These findings will improve predictions of understory community dynamics in response to climate change and aid in further optimizing ecosystem process models.

Soil carbon, nitrogen and phosphorus ecological stoichiometry shifts with tree species in subalpine plantations.

Understanding ecological stoichiometric characteristics of soil nutrient elements, such as carbon (C), nitrogen (N) and phosphorus (P) is crucial to guide ecological restoration of plantations in ecologically vulnerable areas, such as alpine and subalpine regions. However, there has been only a few related studies, and thus whether and how different tree species would affect soil C:N:P ecological stoichiometry remains unclear. We compared soil C:N:P ecological stoichiometry of Pinus tabulaeformis, Larix kaempferi and Cercidiphyllum japonicum to primary shrubland in a subalpine region. We observed strong tree-specific and depth-dependent effects on soil C:N:P stoichiometry in subalpine plantations. In general, the C:N, C:P and N:P of topsoil (0-10 cm) are higher than subsoil (>10 cm) layer at 0-30 cm depth profiles. The differences in C:N, N:P and C:P at the topsoil across target tree species were significantly linked to standing litter stock, tree biomass/total aboveground biomass and Margalef's index of plant community, respectively, whereas the observed variations of C:N, N:P and C:P ratio among soil profiles are closely related to differences in soil bulk density, soil moisture, the quantity and quality of aboveground litter inputs as well as underground fine root across plantations examined. Our results highlight that soil nutrients in plantation depend on litter quantity and quality of selected tree species as well as soil physical attributes. Therefore, matching site with trees is crucial to enhance ecological functioning in degraded regions resulting from human activity.

High nitrogen resorption efficiency of forest mosses.

BACKGROUND AND AIMS: Nutrient resorption from senescing tissue is a key mechanism for plants to conserve nutrients, and can affect the nutrient dynamics of ecosystems. Yet, our limited knowledge of nitrogen (N) resorption and release from mosses hampers our understanding of the role of mosses as N sources and, thereby, N cycling in moss-dominated ecosystems. The aims of this study were to estimate N resorption efficiency (NRE) of two moss species, identify the pathways of N release from the mosses and to provide a better understanding of N cycling and budgeting strategies of mosses. METHODS: The dynamics of N allocation along annual moss segments of two dominant moss species (Actinothuidium hookeri and Hylocomium splendens) were assessed in old-growth fir forests using an in situ15N tracer experiment. KEY RESULTS: The NRE of A. hookeri and H. splendens was 61 and 52 %, respectively. While the mosses lost 23 and 33 % N from live tissues via leaching, 15 and 14 % of N remained in senesced tissues (>3 years old) in A. hookeri and H. splendens, respectively. CONCLUSIONS: Both mosses resorbed the majority of their tissue N, but a considerable amount of N was lost from live segments. Our results highlight the crucial role mosses play as N sinks in ecosystems, since N retention (resorbed and sequestered in senescent tissue) outweighed N loss via leaching. However, the sink strength depends on temperature and precipitation, which will change in a future climate. The values for NRE, leaching, etc. estimated here can help improve biogeochemical models aiming to complete N budgets for moss-abundant ecosystems.

Drivers of tree carbon storage in subtropical forests.

Tropical and subtropical forest ecosystems play an important role in the global carbon regulation. Despite increasing evidence for effects of biodiversity (species diversity, functional diversity and functional dominance), stand structural attributes, stand age and environmental conditions (climate and topography) on tree carbon storage, the relative importance of these drivers at large scale is poorly understood. It is also still unclear whether biodiversity effects on tree carbon storage work through niche complementarity (i.e. increased tree carbon storage due to interspecific resource partitioning) or through the mass-ratio effect (tree carbon storage regulated by dominant traits within communities). Here we analyze tree carbon storage and its drivers using data of 480 plots sampled across subtropical forests in China. We use multiple regression models to test the relative effects of biodiversity, stand structural attributes, stand age and environmental conditions on tree carbon storage, and use a partial least squares path model to test how these variables directly and/or indirectly affect tree carbon storage. Our results show that tree carbon storage is most strongly affected by stand age, followed by climate, biodiversity and stand structural attributes. Stand age and climate had both direct and indirect (through species diversity, functional dominance and stand structural attributes) effects. We find that tree carbon storage correlates with both species diversity and functional dominance after stand age and environmental drivers are accounted for. Our results suggest that niche complementarity and the mass-ratio effect, not necessarily mutually exclusive, both play a role in maintaining ecosystem functioning. Our results further indicate that biodiversity conservation might be an effective way for enhancing tree carbon storage in natural, species-rich forest ecosystems.

Linking leaf hydraulic properties, photosynthetic rates, and leaf lifespan in xerophytic species: a test of global hypotheses.

PREMISE OF THE STUDY: Leaf venation and its hierarchal traits are crucial to the hydraulic and mechanical properties of leaves, reflecting plant life-history strategies. However, there is an extremely limited understanding of how variation in leaf hydraulics affects the leaf economic spectrum (LES) or whether venation correlates more strongly with hydraulic conductance or biomechanical support among hierarchal orders. METHODS: We examined correlations of leaf hydraulics, indicated by vein density, conduit diameter, and stomatal density with light-saturated photosynthetic rates, leaf lifespan (LLS), and leaf morpho-anatomical traits of 39 xerophytic species grown in a common garden. KEY RESULTS: We found positive relationships between light-saturated, area-based photosynthetic rates, and vein densities, regardless of vein orders. Densities of leaf veins had positive correlations with stomatal density. We also found positive relationships between LLS and vein densities. Leaf area was negatively correlated with the density of major veins but not with minor veins. Most anatomical traits were not related to vein densities. CONCLUSIONS: We developed a network diagram of the correlations among leaf hydraulics and leaf economics, which suggests functional trade-offs between hydraulic costs and lifetime carbon gain. Leaf hydraulics efficiency and carbon assimilation were coupled across species. Vein construction costs directly coordinated with the LLS. Our findings indicate that hierarchal orders of leaf veins did not differ in the strength of their correlations between hydraulic conductance and biomechanical support. These findings clarify how leaf hydraulics contributes to the LES and provide new insight into life-history strategies of these xerophytic species.

Thinning-induced canopy opening exerted a specific effect on soil nematode community.

Changes in microclimate, soil physicochemical properties, understory vegetation cover, diversity, and composition as well as soil microbial community resulting from silvicultural practices are expected to alter soil food webs. Here, we investigated whether and how contrasting-sized canopy openings affect soil nematode community within a 30 year-aged spruce plantation. The results indicated that the responses of soil nematodes to canopy opening size were dependant on their feeding habit. The abundance of total nematodes and that of free-living nematodes was negatively correlated with soil bulk density, whereas the abundance of omnivore-predators was negatively correlated with soil bulk density and shrubs cover, respectively. The ratio of the sum abundance of predators and omnivores to the plant parasites' abundance, Simpson's dominance index, Pielou's evenness index, and sigma maturity index, maturity index (MI), MI 2-5, basal index, enrichment index, and structure index was sensitive to alteration in canopy opening size. Multivariate analysis indicated that thinning-induced gap size resulted in contrasting nematode assemblages. In conclusion, soil nematodes should be integrated as an indicator to monitor soil multifunctionality change due to thinning.

Comparisons of photosynthesis-related traits of 27 abundant or subordinate bryophyte species in a subalpine old-growth fir forest.

Bryophyte communities can exhibit similar structural and taxonomic diversity as vascular plant communities, just at a smaller scale. Whether the physiological diversity can be similarly diverse, and whether it can explain local abundance patterns is unknown, due to a lack of community-wide studies of physiological traits. This study re-analyzed data on photosynthesis-related traits (including the nitrogen, phosphorus and chlorophyll concentrations, photosynthetic capacities, and photosynthetic nutrient use efficiencies) of 27 bryophyte species in a subalpine old-growth fir forest on the eastern Tibetan Plateau. We explored differences between taxonomic groups and hypothesized that the most abundant bryophyte species had physiological advantages relative to other subdominant species. Principal component analysis (PCA) was used to summarize the differences among species and trait values of the most abundant and other co-occurring subdominant species. Species from the Polytrichaceae were separated out on both PCA axes, indicating their high chlorophyll concentrations and photosynthetic capacities (axis 1) and relatively high-light requirements (axis 2). Mniaceae species also had relatively high photosynthetic capacities, but their light saturation points were low. In contrast, Racomitrium joseph-hookeri and Lepidozia reptans, two species with a high shoot mass per area, had high-light requirements and low nutrient and chlorophyll concentrations and photosynthetic capacities. The nutrient concentrations, photosynthetic capacities, and photosynthetic nutrient use efficiencies of the most abundant bryophyte species did not differ from co-occurring subdominant species. Our research confirms the links between the photosynthesis-related traits and adaptation strategies of bryophytes. However, species relative abundance was not related to these traits.

Does thinning-induced gap size result in altered soil microbial community in pine plantation in eastern Tibetan Plateau?

Although the effects of gap formation resulting from thinning on microclimate, plant generation and understory plant community have been well documented, the impact of thinning on soil microbial community and related ecological functions of forests particularly in subalpine coniferous region is largely unknown. Here, the effects of thinning on soil microbial abundance and community structure using phospholipid fatty acid (PLFA) in pine plantations were investigated 6 years after thinning. The experimental treatments consisted of two distinct-sized gaps (30 m2 or 80 m2 in size) resulting from thinning, with closed canopy (free of thinning) as control. Soil temperature as well as the biomass of actinomycete and unspecific bacteria was sensitive to gap formation, but all these variables were only responsive to medium gap. Nonmetric multidimensional scaling confirmed that soil microbial community was responsive to gap size. In addition, gap size exerted contrasting effect on bacteria-feeding nematode and fungi-feeding nematodes. In conclusion, thinning-induced gap size would affect soil microbial community through changing soil temperature or the abundance of fungi-feeding nematodes.

The 'plant economic spectrum' in bryophytes, a comparative study in subalpine forest.

PREMISE OF THE STUDY: Tradeoffs among functional traits of vascular plants are starting to be better understood, but it is unclear whether bryophytes possess similar tradeoffs or how trait relationships, or the 'economic spectrum', differ between the two groups. METHODS: We determined functional-trait values [including shoot mass per area (SMA), light-saturated assimilation rate (Amass), dark respiration rate (Rdmass), N and P concentrations (Nmass and Pmass), and photosynthetic N and P use efficiency (PNUE and PPUE)] and their bivariate relationships for 28 bryophytes growing in a subalpine old-growth fir forest on the eastern Tibetan Plateau. Trait values and scaling relationships of these bryophytes were compared with data for vascular plant leaves from the Global Plant Trait Network (GLOPNET) dataset. KEY RESULTS: We found that the Amass, Nmass, N:P, PNUE and PPUE of bryophyte shoots were lower than those of vascular plant leaves. In contrast, bryophytes possessed higher Pmass and the two groups had similar values of SMA and Rdmass. The Nmass and Pmass were closely associated with Amass and Rdmass, and these traits were all significantly negatively related to SMA. Metabolic rates increased faster with nutrient concentrations in bryophytes than in vascular plants. CONCLUSIONS: Our research indicates that bryophytes have similar trait relationships as vascular plant leaves, although the slopes of the relationships differ for most trait combinations. This study confirms a functional-trait tradeoff in bryophytes, and reveals that bryophytes allocate greater proportions of N and P into the metabolic pools.

Higher photosynthetic capacity and different functional trait scaling relationships in erect bryophytes compared with prostrate species.

Ecophysiological studies of bryophytes have generally been conducted at the shoot or canopy scale. However, their growth forms are diverse, and knowledge of whether bryophytes with different shoot structures have different functional trait levels and scaling relationships is limited. We collected 27 bryophyte species and categorised them into two groups based on their growth forms: erect and prostrate species. Twenty-one morphological, nutrient and photosynthetic traits were quantified. Trait levels and bivariate trait scaling relationships across species were compared between the two groups. The two groups had similar mean values for shoot mass per area (SMA), light saturation point and mass-based nitrogen (N(mass)) and phosphorus concentrations. Erect bryophytes possessed higher values for mass-based chlorophyll concentration (Chl(mass)), light-saturated assimilation rate (A(mass)) and photosynthetic nitrogen/phosphorus use efficiency. N(mass), Chl(mass) and A(mass) were positively related, and these traits were negatively associated with SMA. Furthermore, the slope of the regression of N(mass) versus Chl(mass) was steeper for erect bryophytes than that for prostrate bryophytes, whereas this pattern was reversed for the relationship between Chl(mass) and A(mass). In conclusion, erect bryophytes possess higher photosynthetic capacities than prostrate species. Furthermore, erect bryophytes invest more nitrogen in chloroplast pigments to improve their light-harvesting ability, while the structure of prostrate species permits more efficient light capture. This study confirms the effect of growth form on the functional trait levels and scaling relationships of bryophytes. It also suggests that bryophytes could be good models for investigating the carbon economy and nutrient allocation of plants at the shoot rather than the leaf scale.

The capsid protein p38 of turnip crinkle virus is associated with the suppression of cucumber mosaic virus in Arabidopsis thaliana co-infected with cucumber mosaic virus and turnip crinkle virus.

Infection of plants by multiple viruses is common in nature. Cucumber mosaic virus (CMV) and Turnip crinkle virus (TCV) belong to different families, but Arabidopsis thaliana and Nicotiana benthamiana are commonly shared hosts for both viruses. In this study, we found that TCV provides effective resistance to infection by CMV in Arabidopsis plants co-infected by both viruses, and this antagonistic effect is much weaker when the two viruses are inoculated into different leaves of the same plant. However, similar antagonism is not observed in N. benthamiana plants. We further demonstrate that disrupting the RNA silencing-mediated defense of the Arabidopsis host does not affect this antagonism, but capsid protein (CP or p38)-defective mutant TCV loses the ability to repress CMV, suggesting that TCV CP plays an important role in the antagonistic effect of TCV toward CMV in Arabidopsis plants co-infected with both viruses.

Three new phenolic compounds from the leaves of Rosa sericea.

Two new flavonoids, quercetin-3-O-ß-d-xylopyranosyl-(1→2)-α-d-ribopyranoside (1) and kaempferol-3-O-ß-d-xylopyranosyl-(1→2)-α-d-ribopyranoside (2), and one new phenolic derivative, gallicin-p-O-(6'-O-caffeoyl)-ß-d-glucoside (3), together with twelve known compounds were isolated from the leaves of Rosa sericea (Rosaceae family). The structures of the new compounds were established by means of spectroscopic analysis including one- and two-dimensional NMR spectroscopy. Some of the isolated compounds were tested for the cytotoxicity of a breast cancer cell (MCF-7) line. The results showed that rubanthrone A (4) has moderate cytotoxicity against the MCF-7 cell line.

[Short-term effects of low intensity thinning simulated by gap on ground microclimate and soil nutrients of pure spruce plantation].

Taking a dense spruce pure plantation as test object and simulating the formation of natural forest gap, this paper studied the effects of low intensity thinning by gap creation on the ground temperature, ground humidity, and nutrient contents in different soil layers of the plantation. In the first year of gap creation, the mean diurnal temperature in the gap across the growth season (May - September) increased, while the mean diurnal humidity decreased. The soil organic matter (SOM) and NH4(+) -N contents in O-horizon (humus layer) increased by 19.62% and 283.85%, and the dissolved organic carbon (DOC) and NO3(-) -N contents decreased by 77.86% and 23.60%, respectively. The SOM, total nitrogen (TN), and NO3(-) -N contents in 0-10 cm soil layer increased by 45.77%, 37.14%, and 75.11%, and the NH4(+) -N, DOC, and total phosphorus (TP) contents decreased by 48.56%, 33.33%, and 13.11%, respectively. All the results suggested that low intensity thinning by gap creation could rapidly improve the ground microclimate of the plantation, and consequently, promote the soil microbial activity and mineralization processes in O-horizon, the release of soil nutrients, and the restoration of soil fertility.

[Effects of environmental stress on seedlings root growth and nodulation of leguminous shrubs in the dry valley of Minjiang River].

A field investigation was made to understand the seedlings root nodulation, biomass accumulation, root length, and fine root percentage of Sophora davidii, Indigofera lenticellata and Campylotropis polyantha along an altitudinal gradient on two contrasting sloped hills (north Zongqu and south Jingzhoushan) in the dry valley of Minjiang River. In the meantime, a greenhouse experiment was conducted to study the adaptation responses of 2 month-old S. davidii and C. polyantha seedlings root nodulation to different soil moisture regimes (80%, 60%, 40%, and 20% WHC). For the three test shrub species, fewer nodules were observed at lower altitude (1600-1950 m) areas, the nodule number per plant of S. davidii, I. lenticellata, and C. polyantha being 0.1 +/- 0.1, 0.9 +/- 0.5, and 5.7 +/- 1.9, and the non-nodulation plant accounting for 65.1%, 12.3% and 17.6%, respectively. The nodule number of the three species increased with increasing altitude, and correlated positively with root length and fine root percentage. However, there were no significant differences in the plant growth and biomass at different altitudes. When the soil moisture content was lower than 60% WHC, the nodule number and the fresh and dry mass of both S. davidii and C. polyantha decreased markedly, and at 20% WHC, no nodule and only 9.8 +/- 3.6 nodules were observed for S. davidii and C. polyantha, respectively, indicating that in this dry valley, the root nodulation capability of endemic leguminous shrubs was very low. Comparing with S. davidii, C. polyantha had higher root nodulation capability and drought-resistance. Prior to introducing these shrub species in forestation practices, to keep the soil moisture content higher than 40% WHC was recommended for relatively efficient biological nitrogen fixation.