Huajun Yin

Warming effects on growth and physiology in the seedlings of the two conifers Picea asperata and Abies faxoniana under two contrasting light conditions

The short-term effects of two levels of air temperature (ambient and warmed) and light (full light and ca. 10% of full light regimes) on the early growth and physiology of Picea asperata and Abies faxoniana seedlings was determined using open-top chambers (OTC). The OTC manipulation increased mean air temperature and soil surface temperature by 0.51°C and 0.34°C under the 60-year plantation, and 0.69°C and 0.41°C under the forest opening, respectively. Warming, with either full-light or low-light conditions, generally caused a significant increase in plant growth, biomass accumulation, and stimulated photosynthetic performance of P. asperata seedlings. However, the warming of A. faxoniana seedlings only significantly increased their growth under low-light conditions, possibly as a result of photoinhibition caused by full light, which may shield and/or impair the effects of warming manipulation, per se, on the growth and physiological performance of A. faxoniana seedlings. In response to warming, P. asperata seedlings allocated relatively more biomass to roots and A. faxoniana more to foliage under similar environments. This might provide A. faxoniana with an adaptive advantage when soil moisture was not limiting and an advantage to P. asperata if substantial moisture stress occurred. Warming markedly increased the efficiency of PSII in terms of the increase in Fv/Fm and photosynthetic pigment concentrations for the two conifer seedlings, but the effects of warming were generally more pronounced under low-light conditions than under full-light conditions. On balance, this study suggested that warming had a beneficial impact on the early growth and development of conifer seedlings, at least in the short term. Consequently, warming may lead to changes in forest regeneration dynamics and species composition for subalpine coniferous ecosystems under future climate change.

Effects of nutrient heterogeneity and competition on root architecture of spruce seedlings: implications for an essential feature of root foraging.

Background We have limited understanding of root foraging responses when plants were simultaneously exposed to nutrient heterogeneity and competition, and our goal was to determine whether and how plants integrate information about nutrients and neighbors in root foraging processes. Methodology/Principal Findings The experiment was conducted in split-containers, wherein half of the roots of spruce (Picea asperata) seedlings were subjected to intraspecific root competition (the vegetated half), while the other half experienced no competition (the non-vegetated half). Experimental treatments included fertilization in the vegetated half (FV), the non-vegetated half (FNV), and both compartments (F), as well as no fertilization (NF). The root architecture indicators consisted of the number of root tips over the root surface (RTRS), the length percentage of diameter-based fine root subclasses to total fine root (SRLP), and the length percentage of each root order to total fine root (ROLP). The target plants used novel root foraging behaviors under different combinations of neighboring plant and localized fertilization. In addition, the significant increase in the RTRS of 0–0.2 mm fine roots after fertilization of the vegetated half alone and its significant decrease in fertilizer was applied throughout the plant clearly showed that plant root foraging behavior was regulated by local responses coupled with systemic control mechanisms. Conclusions/Significance We measured the root foraging ability for woody plants by means of root architecture indicators constructed by the roots possessing essential nutrient uptake ability (i.e., the first three root orders), and provided new evidence that plants integrate multiple forms of environmental information, such as nutrient status and neighboring competitors, in a non-additive manner during the root foraging process. The interplay between the responses of individual root modules (repetitive root units) to localized environmental signals and the systemic control of these responses may well account for the non-additive features of the root foraging process.

Effects of temperature on soil net nitrogen mineralisation in two contrasting forests on the eastern Tibetan Plateau, China

Intact soil cores from two adjacent forest ecosystems (natural coniferous forest and dragon spruce plantation) were incubated in the laboratory to examine effects of temperature, reforestation and their interactions on rates of nitrogen (N) mineralisation, nitrification and ammonification in the subalpine forest of the eastern Tibetan Plateau. Two contrasting soils were incubated at five temperatures (–5, 0, 5, 15 and 25°C) for 4 weeks. Rates of N mineralisation and nitrification were insensitive to temperature at lower temperatures (0°C and 5°C) but increased over higher temperatures (15°C and 25°C). Large amounts of ammonium were released for each incubation time in both soils when the incubation temperature was –5°C. Therefore, the rates of mineralisation and ammonification at –5°C were significantly higher than at the other temperatures. Both the accumulations of inorganic N and rates of N transformation were significantly higher in the natural forest than in the plantation. Moreover, temperature sensitivity of net nitrification and N mineralisation were greater in the natural forest than the spruce plantation. Effects of temperature on accumulations of inorganic N and rates of N transformation were dependent on incubation time and forest ecosystem. Our results suggested that –5°C might be a key low temperature for N mineralisation in subalpine forest on the eastern Tibetan Plateau; the effect of projected warming on soil N transformation rate may be less significant in plantation forests than natural forests in this specific region.

Analysis of the phenolic compounds in root exudates produced by a subalpine coniferous species as responses to experimental warming and nitrogen fertilisation

In terrestrial ecosystems, plant root exudates clearly play a crucial role in the belowground ecosystem. However, there have been few reports on root exudates from field-grown plants or mature trees in situ, especially when exposed to experimental warming. In this study, we adopted and modified a culture-based cuvette system developed especially for root exudation collection in the field to collect soluble root exudates of a subalpine coniferous species, Abies faxoniana, under experimental warming and nitrogen fertilisation treatments. We then analysed the chemical composition and relative abundance of root exudates using gas chromatography-mass spectrometry (GC-MS). The major chemical constituents of root exudates were phenols and their derivatives of all the different treatments, such as 2,6-di-tert-butyl-4-methylphenol. Experimental warming had significant effects on the relative contents of major compounds and an increase effect on the total phenolic acid compounds. By contrast, there were small significant effects of N fertilisation on root exudation and no significant effects of the warming×N fertilisation interaction. Meanwhile, warming also markedly increased soil polyphenol oxidase activity and it may be soil ecological adjustment response to changes of root exudation under global climate warming.

Effects of two root-secreted phenolic compounds from a subalpine coniferous species on soil enzyme activity and microbial biomass

In order to evaluate the ecological consequences and potential mechanisms of specific C compounds on soil microbial processes under climate warming, we injected solutions of two modelled root exudates, 2,6-di-tert-butyl-4-methylphenol (BHT) and 1,2-benzenedicarboxylic acid, dibutyl ester (DBP), respectively, into soil at two concentrations (20 and 1000 µg g−1 soil). For all treatments, soils amended with the two phenolic compounds were incubated at two temperatures (20°C and 30°C) for 30 days. The responses of soil enzyme activity and microbial property to modelled root exudates to some extent depended on temperature regime, exudation component, and addition concentration. For example, the addition of BHT tended to decrease the soil enzyme activities. However, DBP addition generally increased the two metabolic enzyme activities at 30°C, and tended to decrease the two enzyme activities at 20°C, but a significant reduction was observed only at a high concentration at 20°C. The microbial biomass and enzyme activity were generally lower at 30°C compared to those at 20°C, when averaged across all treatment combinations. Taken together, our results indicated that the amounts and quality of liable root-derived C can differentially affect microbial processes, and various environmental changes will greatly complicate root–microbe–soil interactions in forests.

Exudate components exert different influences on microbially mediated C losses in simulated rhizosphere soils of a spruce plantation

Background and aimsRoot exudates play a vital role in driving ecosystem carbon (C) cycling; however, few studies have examined the degree to which a specific exudate component affects soil C loss. The objective was to examine the impacts of different exudate components on microbially-mediated C decomposition and their underlying mechanisms.MethodsIn a well-controlled simulated rhizosphere system, we added exudate chemicals (glucose, glycine and oxalic acid) to spruce (Picea asperata) plantation soils over a 35-day period. The total C contents, net N mineralization rates, microbial communities and extracellular enzymes were measured.ResultsThe three exudate components induced different C losses by different mechanisms. Oxalic acid promoted net C loss by accelerating microbial mineralization of soil organic matter (SOM). In contrast, glucose resulted in a net C accumulation, which challenged the assumption that glucose serves as a co-metabolite in driving SOM decomposition to lose C. Glycine increased the total C content via negative priming effects.ConclusionsExudate-induced rhizosphere priming effects are not entirely dependent on the energy properties of root exudates. Different exudates may affect SOM decomposition differently, thus the component-specialized rhizosphere processes induced by individual exudate components on soil C dynamics should be integrated into forest C cycle-climate feedbacks under environmental changes.

Experimental warming effects on root nitrogen absorption and mycorrhizal infection in a subalpine coniferous forest

ABSTRACT Climate warming may change soil nutrient supply and affect the biogeochemical processes, especially the nitrogen (N) cycle in forest ecosystems. However, little is still known about how root N uptake responds to climate warming, and whether forests will experience more intense N limitation under warming. We studied the N absorption and mycorrhizal infection rate in the roots of seedlings of a subalpine coniferous species, Picea asperata Mast., under experimental warming, four years after the seedlings were transplanted in the experimental plots. We found that warming had a significant positive effect on root N absorption, with increases of 151.1%, 99.6%, and 71.9% in May, July, and October of 2011, respectively, when compared to the control treatment. In addition, warming increased the N element content and reduced the C:N ratio of the roots over the warming period. Warming also increased mycorrhizal infections by 30.5%, 12.3%, and 108.1% in May, July, and October of 2011, respectively, when compared to the control treatment. Increases in the N absorption and mycorrhizal infection rates in roots may be an important adjustment to meet plant N demand in the subalpine coniferous forest under warming.

Soil respiration and carbon pools across a range of spruce stand ages, Eastern Tibetan Plateau

Abstract To understand the effects of reforestation on soil carbon (C) dynamics, we measured soil carbon dioxide (CO2) effluxes and soil C pools in dragon spruce (Picea asperata Mast.) stands of various ages (22-, 47- and 68-year-old dragon spruce plantations, and a 150-year-old primeval coniferous forest) in the eastern Tibet Plateau. The soil respiration rate of all the stands increased from March, followed by a peak in late July to August with temporal fluctuations, and then dramatically declined in September. The annual total soil CO2 effluxes (Rtot), soil heterotrophic CO2 effluxes (Rh), and soil autotrophic CO2 effluxes (Ra) were higher in the 22- (1004, 656 and 348 g C m−2 year−1, respectively) and 150-year-old stands (1070, 712 and 358 g C m−2 year−1, respectively), and lower in the 47- (792, 543 and 249 g C m−2 year−1, respectively) and 68-year-old stands (952, 607 and 345 g C m−2 year−1, respectively). The soil total organic carbon (TOC), easily oxidizable organic C (EOC), microbial biomass C (MBC), and particulate organic C (POC) pools at depths ranging from 0 cm to 30 cm decreased within 68 years after tree planting. The TOC and its different components in the 0 cm to 10 cm soil layer were significantly and positively related with Rh and Rtot (p < 0.01). The TOC contents (0 cm to 10 cm soil layer) in the primary forest (77.6 g kg−1) were about 1.87, 2.51 and 3.14 times higher than those in the 22-, 47- and 68-year-old plantations, respectively. Our results indicated soil respiration and soil C contents firstly decreased but then increased with stand age, and 68 years of reforestation has caused substantial depletion of soil CO2 effluxes and carbon pools.

Short-term Effects of Night Warming and Nitrogen Addition on Soil Respiration of Subalpine Coniferous,Western Sichuan,China

Responses of soil respiration to experimental warming and nitrogen addition can provide insights into C source/sink functions of forest ecosystems in climates change.An infrared heater was used to determine the short-term effects of two levels of air temperature(ambient and warmed) combined with nitrogen addition(N,0 and 25 g m-2 a-1) on soil respiration in Picea asperata plots and Abies faxoniana plots.The results showed that the warming manipulation increased mean air temperature and soil temperature at 5 cm depth by 2.03℃ and 4.10 ℃,respectively.Soil respiration displayed strong seasonal and diurnal patterns.In general,simulated warming increased soil respiration,while nitrogen addition decreased the soil respiration of the two conifer plots during the whole experiment.However,the responses of soil respiration to experimental warming and nitrogen addition were strongly dependent on both tree species and seasons.Warming increased the carbon fluxes of soil total respiration by 22.30% and 8.82% in the unfertilized and fertilized P.asperata plots,and 4.85% and 4.45% in the unfertilized and fertilized A.faxoniana plots,respectively.Significant exponential relationships were found between the soil respiration rates and soil temperature at 5 cm depth.Warming and nitrogen addition both increased the Q 10 values in the two conifer plots,but Q 10 value was generally higher in P.asperata plots.Therefore,the soil respiration in P.asperata plots was more sensitive to temperature changes than A.faxoniana plots.As a result,the response differences to warming and nitrogen addition between tree species add more uncertainty and complexity in C sink/source dynamics in changing climate,and the underlying mechanism should be further studied.

C: N: P stoichiometry of Ericaceae species in shrubland biomes across Southern China: influences of climate, soil and species identity

Aims Carbon (C), nitrogen (N) and phosphorus (P) stoichiometry strongly affect functions and nutrient cycling within ecosystems. However, the related researches in shrubs were very limited. In this study, we aimed to investigate leaf stoichiometry and its driving factors in shrubs, and whether stoichiometry significantly differs among closely related species. Methods We analyzed leaf C, N and P concentrations and their ratios in 32 species of Ericaceae from 161 sites across southern China. We examined the relationships of leaf stoichiometry with environmental variables using linear regressions, and quantified the interactive and independent effects of climate, soil and species on foliar stoichiometry using general linear models (GLM). Important Findings The foliar C, N and P contents of Ericaceae were 484.66, 14.44 and 1.06 mg g(-1), respectively. Leaf C, N and P concentrations and their ratios in Ericaceae were significantly related with latitude and altitude, except the N:P insignificantly correlated with latitude. Climate (mean annual temperature and precipitation) and soil properties (soil C, N and P and bulk density) were significantly influenced element stoichiometry. The GLM analysis showed that soil exerted a greater direct effect on leaf stoichiometry than climate did, and climate affected leaf traits mainly via indirect ways. Further, soil properties had stronger influences on leaf P than on leaf C and N. Among all independent factors examined, we found species accounted for the largest proportion of the variation in foliar stoichiometry. These results suggest that species can largely influence foliar stoichiometry, even at a lower taxonomic level.