Caiyun Luo

Effects of warming and grazing on soil N availability, species composition, and ANPP in an alpine meadow

Uncertainty about the effects of warming and grazing on soil nitrogen (N) availability, species composition, and aboveground net primary production (ANPP) limits our ability to predict how global carbon sequestration will vary under future warming with grazing in alpine regions. Through a controlled asymmetrical warming (1.2/1.7 degrees C during daytime/nighttime) with a grazing experiment from 2006 to 2010 in an alpine meadow, we found that warming alone and moderate grazing did not significantly affect soil net N mineralization. Although plant species richness significantly decreased by 10% due to warming after 2008, we caution that this may be due to the transient occurrence or disappearance of some rare plant species in all treatments. Warming significantly increased graminoid cover, except in 2009, and legume cover after 2008, but reduced non-legume forb cover in the community. Grazing significantly decreased cover of graminoids and legumes before 2009 but increased forb cover in 2010. Warming significantly increased ANPP regardless of grazing, whereas grazing reduced the response of ANPP to warming. N addition did not affect ANPP in both warming and grazing treatments. Our findings suggest that soil N availability does not determine ANPP under simulated warming and that heavy grazing rather than warming causes degradation of the alpine meadows.

Methanotrophic community structure and activity under warming and grazing of alpine meadow on the Tibetan Plateau

Knowledge about methanotrophs and their activities is important to understand the microbial mediation of the greenhouse gas CH4 under climate change and human activities in terrestrial ecosystems. The effects of simulated warming and sheep grazing on methanotrophic abundance, community composition, and activity were studied in an alpine meadow soil on the Tibetan Plateau. There was high abundance of methanotrophs (1.2–3.4 × 108pmoA gene copies per gram of dry weight soil) assessed by real-time PCR, and warming significantly increased the abundance regardless of grazing. A total of 64 methanotrophic operational taxonomic units (OTUs) were obtained from 1,439 clone sequences, of these OTUs; 63 OTUs (98.4%) belonged to type I methanotrophs, and only one OTU was Methylocystis of type II methanotrophs. The methanotroph community composition and diversity were not apparently affected by the treatments. Warming and grazing significantly enhanced the potential CH4 oxidation activity. There were significantly negative correlations between methanotrophic abundance and soil moisture and between methanotrophic abundance and NH4–N content. The study suggests that type I methanotrophs, as the dominance, may play a key role in CH4 oxidation, and the alpine meadow has great potential to consume more CH4 under future warmer and grazing conditions on the Tibetan Plateau.

Asymmetric sensitivity of first flowering date to warming and cooling in alpine plants

Understanding how flowering phenology responds to warming and cooling (i.e., symmetric or asymmetric response) is needed to predict the response of flowering phenology to future climate change that will happen with the occurrence of warm and cold years superimposed upon a long-term trend. A three-year reciprocal translocation experiment was performed along an elevation gradient from 3200 m to 3800 m in the Tibetan Plateau for six alpine plants. Transplanting to lower elevation (warming) advanced the first flowering date (FFD) and transplanting to higher elevation (cooling) had the opposite effect. The FFD of early spring flowering plants (ESF) was four times less sensitive to warming than to cooling (by −2.1 d/°C and 8.4 d/°C, respectively), while midsummer flowering plants (MSF) were about twice as sensitive to warming than to cooling (−8.0 d/°C and 4.9 d/°C, respectively). Compared with pooled warming and cooling data, warming alone significantly underpredicted 3.1 d/°C for ESF and overestimated 1.7 d/°...

Methane emission by plant communities in an alpine meadow on the Qinghai-Tibetan Plateau: a new experimental study of alpine meadows and oat pasture

Recently, plant-derived methane (CH4) emission has been questioned because limited evidence of the chemical mechanism has been identified to account for the process. We conducted an experiment with four treatments (i.e. winter-grazed, natural alpine meadow; naturally restored alpine meadow eight years after cultivation; oat pasture and bare soil without roots) during the growing seasons of 2007 and 2008 to examine the question of CH4 emission by plant communities in the alpine meadow. Each treatment consumed CH4 in closed, opaque chambers in the field, but two types of alpine meadow vegetation reduced CH4 consumption compared with bare soil, whereas oat pasture increased consumption. This result could imply that meadow vegetation produces CH4. However, measurements of soil temperature and water content showed significant differences between vegetated and bare soil and appeared to explain differences in CH4 production between treatments. Our study strongly suggests that the apparent CH4 production by vegetation, when compared with bare soil in some previous studies, might represent differences in soil temperature and water-filled pore space and not the true vegetation sources of CH4.

Plant Identity Exerts Stronger Effect than Fertilization on Soil Arbuscular Mycorrhizal Fungi in a Sown Pasture.

Arbuscular mycorrhizal (AM) fungi play key roles in plant nutrition and plant productivity. AM fungal responses to either plant identity or fertilization have been investigated. However, the interactive effects of different plant species and fertilizer types on these symbiotic fungi remain poorly understood. We evaluated the effects of the factorial combinations of plant identity (grasses Avena sativa and Elymus nutans and legume Vicia sativa) and fertilization (urea and sheep manure) on AM fungi following 2-year monocultures in a sown pasture field study. AM fungal extraradical hyphal density was significantly higher in E. nutans than that in A. sativa and V. sativa in the unfertilized control and was significantly increased by urea and manure in A. sativa and by manure only in E. nutans, but not by either fertilizers in V. sativa. AM fungal spore density was not significantly affected by plant identity or fertilization. Forty-eight operational taxonomic units (OTUs) of AM fungi were obtained through 454 pyrosequencing of 18S rDNA. The OTU richness and Shannon diversity index of AM fungi were significantly higher in E. nutans than those in V. sativa and/or A. sativa, but not significantly affected by any fertilizer in all of the three plant species. AM fungal community composition was significantly structured directly by plant identity only and indirectly by both urea addition and plant identity through soil total nitrogen content. Our findings highlight that plant identity has stronger influence than fertilization on belowground AM fungal community in this converted pastureland from an alpine meadow.

Gene or environment? Species-specific control of stomatal density and length

Stomatal characteristics are used as proxies of paleo-environment. Only a few model species have been used to study the mechanisms of genetic and environmental effects on stomatal initiation. Variation among species has not been quantified. In this paper, results from an in situ reciprocal transplant experiment along an elevation gradient in the northeast Tibetan Plateau are reported, in which the relative effects of genetics (original altitude) and environment (transplant altitude) on stomatal density (SD) and length (SL) were quantified. In Thalictrum alpinum, only the environment significantly influenced SD, with the variance component () of the environment found to be much greater than that of genetics () (). In Kobresia humillis, only genetics significantly influenced SD and SL, with the genetics variance component found to be greater than that of the environment (, for SD). These results suggest that the extent to which genetics and the environment determine stomatal initiation and development is species-specific. This needs to be considered when studying genetic or environmental controls of stomatal initiation, as well as when SD and SL are used as proxies for ancient climate factors (e.g., CO2 concentration).

Alpine Grassland Soil Organic Carbon Stock and Its Uncertainty in the Three Rivers Source Region of the Tibetan Plateau

Alpine grassland of the Tibetan Plateau is an important component of global soil organic carbon (SOC) stocks, but insufficient field observations and large spatial heterogeneity leads to great uncertainty in their estimation. In the Three Rivers Source Region (TRSR), alpine grasslands account for more than 75% of the total area. However, the regional carbon (C) stock estimate and their uncertainty have seldom been tested. Here we quantified the regional SOC stock and its uncertainty using 298 soil profiles surveyed from 35 sites across the TRSR during 2006–2008. We showed that the upper soil (0–30 cm depth) in alpine grasslands of the TRSR stores 2.03 Pg C, with a 95% confidence interval ranging from 1.25 to 2.81 Pg C. Alpine meadow soils comprised 73% (i.e. 1.48 Pg C) of the regional SOC estimate, but had the greatest uncertainty at 51%. The statistical power to detect a deviation of 10% uncertainty in grassland C stock was less than 0.50. The required sample size to detect this deviation at a power of 90% was about 6–7 times more than the number of sample sites surveyed. Comparison of our observed SOC density with the corresponding values from the dataset of Yang et al. indicates that these two datasets are comparable. The combined dataset did not reduce the uncertainty in the estimate of the regional grassland soil C stock. This result could be mainly explained by the underrepresentation of sampling sites in large areas with poor accessibility. Further research to improve the regional SOC stock estimate should optimize sampling strategy by considering the number of samples and their spatial distribution.

Temperature and Moisture Effects on Soil Respiration in Alpine Grasslands

Abstract The Tibetan Plateau, the low-latitude and high-altitude cold region, has a variety of soils rich in organic carbon (C). Climate change will have large impacts on soil carbon dioxide (CO2) efflux in the region. These impacts will subsequently affect global-scale climate and C cycle links. However, the magnitude of this feedback is still uncertain. Here we use a laboratory incubation experiment to investigate how soil temperature and moisture affected the rate and temperature sensitivity of heterotrophic respiration of three alpine ecosystems (alpine meadow [M], alpine shrubland [SB], alpine swamp [SP]) on the Tibetan Plateau. Soil samples were incubated under three temperature (0°C, 15°C, and 30°C) and two moisture (50% and 100% water-holding capacity) conditions. The response of soil respiration to temperature and moisture varied with ecosystems. Soil respiration in SP was the most temperature sensitive, and higher moisture increased its temperature sensitivity (Q10). The respiration and Q10 depended on total nitrogen in soils. Moreover, high moisture increased the dependence of Q10 on total nitrogen. Our results suggest that rising temperature in Tibetan Plateau may cause a positive feedback to the soil C cycle, particularly coupled with increasing precipitation and N addition.

Experimental Warming Increases Seasonal Methane Uptake in an Alpine Meadow on the Tibetan Plateau

Increased understanding of the response of soil methane (CH4) uptake in alpine meadow ecosystems to warming and grazing could reduce uncertainty in estimates of the soil-atmospheric CH4 budget. To determine the effects of warming and grazing on soil CH4 uptake at different timescales (that is, daily, monthly, seasonal, and annual), we conducted a controlled warming and grazing experiment [that is, no warming with no grazing (NWNG), no warming with grazing (NWG), warming with no grazing (WNG), and warming with grazing (WG)] in an alpine meadow on the Tibetan plateau from 2006 to 2009. Soil CH4 uptake was mainly affected by warming and sample date and their interaction. Warming treatment regardless of grazing significantly increased seasonal average CH4 uptake by 31-39% during the growing season (from May to September) and by 162% during the non-growing season (from October to April next year) in 2007–2008, whereas only WNG increased seasonal average CH4 uptake by 87–138% compared with NWNG during the non-growing seasons in 2006–2007 and 2008–2009. Warming in WNG and WG increased annual CH4 uptake by 50–87% compared with NWNG or NWG. Moreover, warming regardless of grazing and warming with grazing (compared with NWNG) significantly increased the contribution to annual uptake of CH4 uptake during the non-growing season in 2007–2008 and 2008–2009. However, moderate grazing did not significantly influence soil CH4 uptake, although grazing with warming decreased CH4 uptake by 43% during the growing season in 2006. Soil moisture explained 16–25% of the CH4 variation during the growing season, but there was no significant relationship between soil CH4 uptake and soil moisture during the non-growing season. Our results suggest that more attention should be paid to the stimulating effect of warming on soil CH4 uptake during the non-growing season due to its greater response to warming and different stimulating mechanisms compared to responses during the growing season in the alpine meadow.

Effects of Soil Temperature and Moisture on Soil Respiration on the Tibetan Plateau.

Understanding of effects of soil temperature and soil moisture on soil respiration (Rs) under future warming is critical to reduce uncertainty in predictions of feedbacks to atmospheric CO2 concentrations from grassland soil carbon. Intact cores with roots taken from a full factorial, 5-year alpine meadow warming and grazing experiment in the field were incubated at three different temperatures (i.e. 5, 15 and 25°C) with two soil moistures (i.e. 30 and 60% water holding capacity (WHC)) in our study. Another experiment of glucose-induced respiration (GIR) with 4 h of incubation was conducted to determine substrate limitation. Our results showed that high temperature increased Rs and low soil moisture limited the response of Rs to temperature only at high incubation temperature (i.e. 25°C). Temperature sensitivity (Q10) did not significantly decrease over the incubation period, suggesting that substrate depletion did not limit Rs. Meanwhile, the carbon availability index (CAI) was higher at 5°C compared with 15 and 25°C incubation, but GIR increased with increasing temperature. Therefore, our findings suggest that warming-induced decrease in Rs in the field over time may result from a decrease in soil moisture rather than from soil substrate depletion, because warming increased root biomass in the alpine meadow.