Asymmetrical warming of growing/non-growing season increases soil respiration during growing season in an alpine meadow.

Soil respiration (Rs) is an important carbon flux in the global carbon cycle, and understanding the influence of global warming on Rs is critical for precise prediction future climate change. Actually, global warming is expected to be seasonally asymmetric, however, it is still unclear on the response of Rs to asymmetrical warming of growing/non-growing season in alpine regions. In this study, an experiment with asymmetrical warming of growing/non-growing season (including three treatments, CK: control; GLNG: warming magnitude of growing season lower than non-growing season; GHNG: warming magnitude of growing season higher than non-growing season) was performed in an alpine meadow of the Northern Tibet since June 2015. The 'GLNG' and 'GHNG' treatments increased mean Rs by 71.22% (1.89 µmol CO2 m-2 s-1) and 34.32% (0.91 µmol CO2 m-2 s-1) during growing season in 2019, respectively. However, the 'GLNG' and 'GHNG' treatments did not significantly affect mean Rs during growing season in 2015, 2016, 2017 and 2018, respectively. The variation coefficient of growing season mean Rs was 32.95% under the CK treatment in 2015-2019. Therefore, warming may have a lagging effect on Rs. The warming scene with a greater warming during non-growing season may have a stronger effect on Rs than the warming scene with a greater warming during growing season. Inter-annual variation of Rs may be greater than the warming effect on Rs in alpine meadows.

Effects of experimental warming and increased precipitation on soil respiration in an alpine meadow in the Northern Tibetan Plateau.

Uncertainty on the response of soil respiration (Rs) to warming and increased precipitation on the Tibetan Plateau can limit our ability to predict how alpine ecosystems will respond to future climate change. Based on a warming (control, low- and high-level) and increased precipitation (control, low- and high-level) experiment, the response of Rs to experimental warming and increased precipitation was examined in an alpine meadow in the Northern Tibetan Plateau from 2014 to 2017. The low-level warming increased soil temperature (Ts) by 1.19°C and decreased soil moisture (SM) by 0.02m3m-3, whereas the high-level warming increased Ts by 2.88°C and decreased SM by 0.04m3m-3 over the four growing seasons in 2014-2017. The low- and high-level increased precipitation did not affect Ts, but increased SM by 0.02m3m-3 and 0.04m3m-3, respectively, over the four growing seasons in 2014-2017. No significant main and interactive effects of experimental warming and increased precipitation on Rs were observed over the four growing seasons in 2014-2017. In contrast, there was a significant inter-annual variation of Rs in 2014-2017. There was a marginally significant quadratic relationship between the effect of experimental warming on Rs and warming magnitude. There was a negligible difference of Rs between the low- and high-level increased precipitation over the four growing seasons in 2014-2017 and Rs also showed a quadratic relationship with precipitation. Therefore, experimental warming and increased precipitation did not change Rs and Rs responded nonlinearly to experimental warming and increased precipitation in the alpine meadow in the Northern Tibetan Plateau. Growing season precipitation may play a more important role than experimental warming and increased precipitation in affecting Rs in the alpine meadow in the Northern Tibetan Plateau.

Clipping has stronger effects on plant production than does warming in three alpine meadow sites on the Northern Tibetan Plateau.

The relative effects of warming and clipping on vegetation growth are not fully understood. Therefore, we compared the relative effects of experimental warming and clipping on the normalised difference vegetation index (NDVI), green NDVI (GNDVI), soil-adjusted vegetation index (SAVI), aboveground biomass (AGB) and gross primary production (GPP) in three alpine meadow sites (A, B and C) on the Northern Tibetan Plateau from 2013 to 2015. There were no obvious effects of experimental warming on the NDVI, GNDVI, SAVI, AGB and GPP at the three sites, which were most likely attributed to experimental warming-induced warming and drying conditions. In contrast, clipping significantly decreased the NDVI, SAVI and AGB by 27.8%, 31.3% and 18.2% at site A, by 27.1%, 31.8% and 27.7% at site B, and by 12.3%, 15.1% and 17.6% at site C, respectively. Clipping also significantly reduced the GNDVI and GPP by 11.1% and 28.2% at site A and by 18.9% and 33.7% at site B, respectively. Clipping marginally decreased the GNDVI by 8.7% (p = 0.060) and GPP (p = 0.082) by 14.4% at site C. Therefore, clipping had stronger effects on vegetation growth than did warming in the three alpine meadow sites on the Tibetan Plateau.

Environmental Humidity Regulates Effects of Experimental Warming on Vegetation Index and Biomass Production in an Alpine Meadow of the Northern Tibet.

Uncertainty about responses of vegetation index, aboveground biomass (AGB) and gross primary production (GPP) limits our ability to predict how climatic warming will influence plant growth in alpine regions. A field warming experiment was conducted in an alpine meadow at a low (4313 m), mid- (4513 m) and high elevation (4693 m) in the Northern Tibet since May 2010. Growing season vapor pressure deficit (VPD), soil temperature (Ts) and air temperature (Ta) decreased with increasing elevation, while growing season precipitation, soil moisture (SM), normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), AGB and GPP increased with increasing elevation. The growing season Ta, Ts and VPD in 2015 was greater than that in 2014, while the growing season precipitation, SM, NDVI, SAVI, AGB and GPP in 2015 was lower than that in 2014, respectively. Compared to the mean air temperature and precipitation during the growing season in 1963-2015, it was a warmer and wetter year in 2014 and a warmer and drier year in 2015. Experimental warming increased growing season Ts, Ta,VPD, but decreased growing season SM in 2014-2015 at all the three elevations. Experimental warming only reduced growing season NDVI, SAVI, AGB and GPP at the low elevation in 2015. Growing season NDVI, SAVI, AGB and GPP increased with increasing SM and precipitation, but decreased with increasing VPD, indicating vegetation index and biomass production increased with environmental humidity. The VPD explained more variation of growing season NDVI, SAVI, AGB and GPP compared to Ts, Ta and SM at all the three elevations. Therefore, environmental humidity regulated the effect of experimental warming on vegetation index and biomass production in alpine meadows on the Tibetan Plateau.

Response of soil respiration to experimental warming in a highland barley of the Tibet.

Highland barley is an important dominant crop in the Tibet and the croplands of the Tibet are experiencing obvious climatic warming. However, information about how soil respiration will respond to climatic warming in the highland barley system is still lacking. A field warming experiment using infrared heaters with two warming magnitudes was conducted in a highland barley system of the Tibet in May 2014. Five daily cycles of soil respiration was measured using a CO2 flux system (Li-8100, Li-COR Biosciences, Lincoln, NE, USA) during the period from early June to early September in 2014. The high and low experimental warming significantly increased soil temperature by 1.98 and 1.52 °C over the whole study period, respectively. The high experimental warming significantly decreased soil moisture. Soil respiration and its temperature sensitivity did not significantly change under both the high and low experimental warming. The response of soil respiration to experimental warming did not linearly correlate with warming magnitudes because a greater experimental warming resulted in a higher soil drying. Our findings suggested that clarifying the response of soil CO2 production and its temperature sensitivity to climatic warming need consider water availability in the highland barley system of the Tibet.

Response of soil C and N, dissolved organic C and N, and inorganic N to short-term experimental warming in an Alpine meadow on the Tibetan Plateau.

Although alpine meadows of Tibet are expected to be strongly affected by climatic warming, it remains unclear how soil organic C (SOC), total N (TN), ammonium N (NH4 (+)-N) , nitrate N (NO3 (+)-N), and dissolved organic C (DOC) and N (DON) respond to warming. This study aims to investigate the responses of these C and N pools to short-term experimental warming in an alpine meadow of Tibet. A warming experiment using open top chambers was conducted in an alpine meadow at three elevations (i.e., a low (4313 m), mid-(4513 m), and high (4693 m) elevation) in May 2010. Topsoil (0-20 cm depth) samples were collected in July-September 2011. Experimental warming increased soil temperature by ~1-1.4°C but decreased soil moisture by ~0.04 m(3) m(-3). Experimental warming had little effects on SOC, TN, DOC, and DON, which may be related to lower warming magnitude, the short period of warming treatment, and experimental warming-induced soil drying by decreasing soil microbial activity. Experimental warming decreased significantly inorganic N at the two lower elevations,but had negligible effect at the high elevation. Our findings suggested that the effects of short-term experimental warming on SOC, TN and dissolved organic matter were insignificant, only affecting inorganic forms.

[Soil N/P ratio distribution characteristics of alpine grassland ecosystem in Qinghai-Tibet Plateau].

The distribution characteristics of soil N/P ratio in alpine grassland ecosystem of Qinghai-Tibet Plateau were surveyed by field investigation and laboratory analysis. Horizontally, soil N/ P ratio was generally higher in west and lower in east in a manner of staggered patch distribution, with higher N/P ratios mainly centralized in the hinterland of northern part of Tibet Plateau and in the lake basin area of the northern foot of Himalayas. Significant differences in soil N/P ratio were observed among grassland types and natural transects. Vertically, the distribution of N/P ratio along the soil profile from aboveground to underground among different grass types could be categorized into five patterns, including low-high-low-high, low-high-low, low-high, high-low-high-low, and high-low-high. The N/P ratio showed a significant positive correlation with soil bulk density at 0-20 cm depth, soil water content at 20-30 cm depth, contents of soil available K and total nitrogen, respectively. However, it showed significant negative correlation with soil bulk density at 20-30 cm depth, contents of soil available P and total P, respectively.