Jinhua Li

Effects of Nitrogen and Phosphorus Fertilization on Soil Carbon Fractions in Alpine Meadows on the Qinghai-Tibetan Plateau

In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization at 5, 10, 15 g m−2 yr−1 (N5, N10, N15, P5, P10, P15, N5P5, N10P10, and N15P15) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0–20 cm and 20–40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m−2 yr−1 decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N5, N10, P5, and P10 in 0–20 cm, and increased in N10 and P5 while decreased in other treatments in 20–40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO2) in 0–20 cm but increased qCO2 in 20–40 cm. Fertilization increased soil microbial functional diversity (except N15) but decreased cumulative C mineralization (except in N15 in 0–20 cm and N5 in 20–40 cm). Soil organic C (SOC) decreased in P5 and P15 in 0–20 cm and for most of the fertilization treatments (except N15P15) in 20–40 cm. Overall, these results suggested that soils will not be a C sink (except N15P15). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m−2 yr−1 may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.

Effect of legume species introduction to early abandoned field on vegetation development

One of the most important areas in ecology is to elucidate the factors that drive succession in ecosystems. The purpose of our study was to assess the effects of legume species (Medicago sativa, Melilotus suaveolens and Astragalus adsurgens) introduction to abandoned arable land on vegetation development in the Loess Plateau, China. Results from our study showed that addition of legume species strongly affected the composition of recently abandoned-field vegetation. Legume species were effective at reducing the number and dominance of natural colonizers (mainly weeds from the seed bank). The introduction of legume species into newly abandoned fields maintained high total cover and above-ground biomass and could improve soil organic carbon and total nitrogen. However, the effects of the treatments were species-specific. Melilotus suaveolens turned out to be severely suppressive to natural colonizers (weed species). Also, Melilotus suaveolens-adding maintained the highest cover and above-ground biomass and was helpful to improve later succession species, e.g. Stipa breviflora and Astragalus polycladus, to invade and establish. Medicago sativa-adding was superior in enhancing the soil organic carbon and total nitrogen. The present results suggested that addition of legume species with greater cover and biomass strongly suppressed the dominance of the weedy species in early succession and the course of old-field succession may be accelerated by introduction of legume species at least temporarily. However, the experimental period was too short to assess to what extent succession may be affected in the longer term.

Responses of Caragana korshinskii Kom. to shoot removal: mechanisms underlying regrowth

Caragana korshinskii Kom. a dominant member of desert flora in north-western China, is often subjected to aboveground shoot destruction but is very successful in its rapid recovery. We investigated the physiological basis for resprouting by comparing shoot elongation, leaf-nutrient content, pre-dawn leaf-water potential (LWP), root non-structural carbohydrate (TNC), and photosynthetic rate of first-year resprouts with those of adjacent undamaged individuals. C. korshinskii resprouts had a significantly higher rate of shoot elongation. Plant responses associated with enhanced shoot elongation included (1) improved water status, (2) drawing upon more TNC from roots to support aboveground shoot regrowth, (3) enhanced photosynthetic rate owing to improved water status and increased nutrient content in leaves, and (4) allocating more photosynthate to vegetative production without reproduction costs. Maintaining an active root system after shoot removal may be the foundation which engenders these mechanisms underlying rapid regrowth of C. korshinskii in the disturbed environment.

Plant Communities, Soil Carbon, and Soil Nitrogen Properties in a Successional Gradient of Sub-Alpine Meadows on the Eastern Tibetan Plateau of China

To assess the recovery trajectory and self-maintenance of restored ecosystems, a successional gradient (1, 3, 5, 15, and 30xa0years after abandonment) was established in a sub-alpine meadow of the eastern Tibetan Plateau in China. Plant communities and soil carbon and nitrogen properties were investigated and analyzed. Regression analyses were used to assess the models (linear or quadratic) relating measures of species richness, soil carbon and nitrogen properties to fallow time. We found that species richness (S) increased over the first 20xa0years but decreased thereafter, and aboveground biomass showed a linear increase along the fallow time gradient. The richness of different functional groups (forb, grass and legume) changed little along the fallow time gradient, but their corresponding above ground biomass showed the U-shaped, humped or linear pattern. Soil microbial carbon (MBC) and nitrogen (MBN) in the upper 20xa0cm showed a U-shaped pattern along the fallow time gradient. However, soil organic carbon (Corg) and total nitrogen (TN) in the soil at depth greater than 20xa0cm showed significant patterns of linear decline along the fallow time gradient. The threshold models of species richness reflected best the recovery over the 15xa0year fallow period. These results indicated that fallow time had a greater influence on development of the plant community than soil processes in abandoned fields in sub-alpine meadow ecosystem. These results also suggested that although the succession process did not significantly increase soil C, an increase in microbial biomass at the latter stage of succession could promote the decomposability of plant litter. Therefore, abandoned fields in sub-alpine meadow ecosystem may have a high resilience and strong rehabilitating capability under natural recovery condition.

Differential Effects of Legume Species on the Recovery of Soil Microbial Communities, and Carbon and Nitrogen Contents, in Abandoned Fields of the Loess Plateau, China

Plant–soil interactions are known to influence a wide range of ecosystem-level functions. Moreover, the recovery of these functions is of importance for the successful restoration of soils that have been degraded through intensive and/or inappropriate land use. Here, we assessed the effect of planting treatments commonly used to accelerate rates of grassland restoration, namely introduction of different legume species Medicago sativa, Astragalus adsurgens, Melilotus suaveolens, on the recovery of soil microbial communities and carbon and nitrogen contents in abandoned fields of the Loess Plateau, China. The results showed effects were species-specific, and either positive, neutral or negative depending on the measure and time-scale. All legumes increased basal respiration and metabolic quotient and had a positive effect on activity and functional diversity of the soil microbial community, measured using Biolog EcoPlate. However, soil under Astragalus adsurgens had the highest activity and functional diversity relative to the other treatments. Soil carbon and nitrogen content and microbial biomass were effectively restored in 3–5xa0years by introducing Medicago sativa and Astragalus adsurgens into early abandoned fields. Soil carbon and nitrogen content were retarded in 3–5xa0years and microbial biomass was retarded in the fifth year by introducing Melilotus suaveolens. Overall, the restoration practices of planting legumes can significantly affect soil carbon and nitrogen contents, and the biomass, activity, and functional diversity of soil microbial community. Therefore, we propose certain legume species could be used to accelerate ecological restoration of degraded soils, hence assist in the protection and preservation of the environment.

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant Reaumuria soongorica during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (PN), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, Fv/Fm), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.

Functional group dominance and not productivity drives species richness

Background: There is a lack of consensus about the productivity–richness relationship, with several recent studies suggesting that it is not productivity but other factors that are the important drivers that determine species richness. Aims: Here, we examine the relationship between productivity, functional group dominance and plant species richness at the plot scale in Tibetan Plateau meadows. These alpine meadows are ideal to examine the species productivity-richness relationship because they have a very high species richness, a large gradient in productivity, and can be dominated by either graminoids (grasses and sedges) or forbs. Methods: We measured plant species richness and above-ground biomass along a natural gradient of functional group abundance in 44 plots distributed across five natural, winter-grazed but otherwise undisturbed sites in the eastern part of the Qing-Hai Tibetan Plateau, in Gansu province, China in 2008. Results: Graminoid abundance (i.e. graminoid biomass as percent of the total above-ground biomass) explained 39% of plot differences in species richness while neither productivity nor the biomass of the three most abundant plant species, either individually or combined, were a significant predictor of species richness. Conclusions: Our results show that within these alpine meadows, a shift from graminoid to forb dominance, rather than the individual dominant species or productivity itself, is strongly correlated with species richness. Thus, differences in functional group abundance can be a strong driver of observed plant species richness patterns.

Responses of Plant Community Composition and Eco-Physiological Characteristics of Dominant Species to Different Soil Hydrologic Regimes in Alpine Marsh Wetlands on Qinghai-Tibetan Plateau, China

Shifts in soil water content affect seasonal wetland plant communities worldwide, but little is known about the responses and influences of plant communities to soil water content on Qinghai–Tibetan Plateau. To determine the relationship between soil water content and plant community structure in seasonally wet alpine marshes, we investigated plant community structures at different soil water content amounts in the field station at sampling sites. We selected and determined eco-physiological characteristics of species belonging to three different functional types (sedges, grasses, forbs) exposed to the same soil water regimes as characteristics in field station and under experimental flooding conditions in controlled treatments on pots plants. The field investigation indicated that decreased soil moisture (from 57 to 43xa0%xa0m3xa0m−3) during the growing season was significantly associated with reductions in aboveground biomass, average plant height, and species richness. A shift in dominant plant species within the communities from sedges and grasses to some forbs accompanied the decrease in soil moisture. The controlled treatments demonstrated that the sedge and grass species had higher net photosynthetic rates and higher instantaneous water-use efficiencies than the forb species in the studied communities. The results indicate that shifts in wetland plant community structure and function are the result of longer dry periods and more intense rainfall events. This positive feedback suggests that changes in plant community composition could intensify soil drought conditions in seasonally wet alpine marshes in the future.

Effect of loss of plant functional group and simulated nitrogen deposition on subalpine ecosystem properties on the Tibetan Plateau

Biodiversity loss impacts on ecosystem functioning can vary greatly among ecosystems types and different ecosystem processes can respond differently. Here we conducted a plant functional group removal experiment with and without nitrogen (N) addition (5gNm-2year-1) to examine the effects of plant functional group types presence, nitrogen deposition and their interaction effects on plant diversity, aboveground biomass, soil nutrients, soil microbial biomass and soil enzyme activity. After 4years, the removal of dominant grass did increase subordinates, forb richness, and decreased total aboveground biomass significantly. However, the removal of forb resulted in a rapid decline in species richness, which did not change strongly the aboveground biomass, regardless of N addition. This pattern suggests that the dominant grass can compensate for the loss of forb removal with respect to production, but cannot compensate with respect to species loss. Forb cannot compensate for grass removal with respect to production, but can compensate with respect to species loss. Nitrogen addition only has a small effect on species richness, and also not enhances aboveground biomass. In addition, the majority of soil properties did not respond to either plant functional group removal, or N addition. Only soil CO2 efflux and soil NO3--N content significantly changed with plant functional group removal. Soil respiration rate was positively correlated with both plant species richness (R2=0.97) and aboveground biomass (R2=0.64). Our results show that the short-term losses of plant functional group have significant effects on plant diversity and productivity, and only minor impact on soil properties.

Plant–soil feedbacks in a sub-alpine meadow ecosystem with high plant diversity on the Qinghai-Tibetan Plateau

Most plant–soil feedback studies infer mechanisms based on microbes directly affecting plant performance and have been carried out in grasslands with relatively low plant diversity; however, little is known about how plant–soil feedbacks directly affect soil chemistry in grasslands with high plant diversity. In our study, five types of species-origin soils (3 from an old-field and 2 from an undisturbed meadow) were collected, and used to establish an outdoor feedback experiment. Plant performance in home versus away soil, and soil abiotic and biotic properties before and after the feedback experiment was measured to examine patterns of plant–soil feedbacks, and their mechanisms depending on that affecting soil N availability directly. Both old-field and meadow species performed better in old-field soil than meadow soil. On species level, there was no consistent “home” or “away” advantage. In pre-feedback experiment, old-field was substantially higher in soil total phosphorus and NH4+. All soil properties differed significantly among plant species beneath which the soil was collected; however, there was no consistent pattern among species. In post-feedback experiment, old-field soil had larger increases in NO3−, microbial biomass C and N (MBC and MBN) while larger decreases in NH4+ than meadow soil. Each old-field species had the largest decrease in NH4+ in its own soil, while all species had the largest increases in NO3−, MBC, and MBN in Artemisia’s soils. Plant species influenced MBC and MBN, and these microbial biomass differences in turn drove N mineralization and nitrification differences among species. In total, our results showed that plant–soil feedbacks, in a very diverse alpine meadow, were complex and species specific, but were not driven by plant-induced N mineralization and nitrification.