Shixiao Xu

Characterizing evapotranspiration over a meadow ecosystem on the Qinghai‐Tibetan Plateau

To characterize evapotranspiration (ET) over grasslands on the Qinghai-Tibetan Plateau, we examined ET and its relevant environmental variables in a Kobresia meadow from 2002 to 2004 using the eddy covariance method. The annual precipitation changed greatly, with 554, 706, and 666 mm a(-1) for the three consecutive calendar years. The annual ET varied correspondingly to the annual precipitation with 341, 407, and 426 mm a(-1). The annual ET was, however, constant at about 60% of the annual precipitation. About 85% annual ET occurred during the growing season from May to September, and the averaged ET for this period was 1.90, 2.23, and 2.22 mm/d, respectively for the three consecutive years. The averaged ET was, however, very low (< 0.40 mm/d) during the nongrowing season from October to April. The annual canopy conductance (gc) and the Priestley-Taylor coefficient (a) showed the lowest values in the year with the lowest precipitation. This study first demonstrates that the alpine meadow ecosystem is characterized by a low ratio of annual ET to precipitation and that the interannual variation of ET is determined by annual precipitation.

Radiation partitioning and its relation to environmental factors above a meadow ecosystem on the Qinghai‐Tibetan Plateau

Understanding the energy balance on the Qinghai-Tibetan Plateau is essential for better prediction of global climate change. To characterize the energy balance on the plateau, we examined the radiation partitioning over a Kobresia meadow, the most widely distributed vegetation on the plateau, for the period from 2002 to 2005. The incident solar radiation (R(s)) and net radiation (R(n)) averaged 6298 and 2779 MJ m(-2) yr(-1), respectively. The albedo averaged 0.220 annually, with a slightly low value of 0.202 in the growing season from May to September. An increase in soil water or leaf area index was correlated with a decrease of albedo over the meadow. The annual solar radiation lost 34% as longwave radiation, which was higher than values reported for lowland grasslands. The annual radiation efficiency (R(n)/R(s)) over the meadow, at an average of 0.44, was, however, much lower than that for lowland grasslands. The net longwave radiation (L(n)) and the normalized effective radiation (L(n)/R(s)) over the meadow were much higher than that for the global surface or for lowland grasslands, indicating that the longwave exchange between alpine meadow and atmosphere is the most important component of energy losses. A path analysis suggests that the water vapor pressure, air temperature, and cloud cover are the major factors governing the variations of both the net radiation and the net longwave radiation in the alpine meadow ecosystem.

More than a century of Grain for Green Program is expected to restore soil carbon stock on alpine grassland revealed by field (13)C pulse labeling.

Anthropogenic changes in land use/cover have altered the vegetation, soil, and carbon (C) cycling on the Qinghai-Tibetan Plateau (QTP) over the last ~50years. As a result, the Grain for Green Program (GfGP) has been widely implemented over the last 10years to mitigate the impacts of cultivation. To quantify the effects of the GfGP on C partitioning and turnover rates at the ecosystem scale, an in situ (13)C pulse labeling experiment was conducted on natural and GfGP grasslands in an agro-pastoral ecotone in the Lake Qinghai region on the QTP. We found that there were significant differences in the C stocks of all the considered pools in both the natural and GfGP grasslands, with higher CO2 uptake rates in the GfGP grassland than that in the natural grassland. Partitioning of photoassimilate (% of recovered (13)C) in C pools of both grasslands was similar 25days after labeling, except in the roots of the 0-15 and 5-15cm soil layer. Soil organic C (SOC) sequestration rate in the GfGP grassland was 11.59±1.89gCm(-2)yr(-1) significantly greater than that in the natural grassland. The results confirmed that the GfGP is an efficient approach for grassland restoration and C sequestration. However, it will take more than a century (119.19±20.26yr) to restore the SOC stock from the current cropland baseline level to the approximate level of natural grassland. We suggest that additional measures are needed in the selection of suitable plant species for vegetation restoration, and in reasonable grazing management.

Diurnal and monthly variations of carbon dioxide flux in an alpine shrub on the Qinghai-Tibet Plateau

Continuous CO2 flux observation with eddy covariance method conducted in the alpine shrub on the Qinghai-Tibet Plateau indicates that there are distinct diurnal and monthly variations for CO2 fluxes in the alpine shrub on the plateau. As for diurnal variation, with net CO2 influx from 08:00 to 19:00 and net CO2 efflux from 20:00 to 07:00, peak CO2 flux during warm season (July) appears around 12:00 (−1.19 g CO2 · m−2 · h−1); there is no obvious horary fluctuation for CO2 flux during cold season (January), and horary CO2 flux during most hours in a day is close to zero except for a small amount of net efflux (about 0.11 g CO2 · m−2 · h−1) from 11:00–17:00. As for monthly variation, with net CO2 influx from June to September and net CO2 efflux from January to May and October to December, the peak monthly CO2 influx and CO2 efflux appear in August and April, respectively. The total net CO2 influx from June to September and total net CO2 efflux from February to May and October to December in the alpine shrub on the Qinghai-Tibet Plateau are estimated to be 673 and 446 g CO2 · m−2. Results show that the alpine shrub on the Qinghai-Tibet Plateau is remarkable carbon dioxide sink under no grazing conditions and the total yearly CO2 influx is estimated to be 227 g CO2 · m−2.

Effect of Dietary Types on Feed Intakes, Growth Performance and Economic Benefit in Tibetan sheep and Yaks on the Qinghai-Tibet Plateau during Cold Season

Pastoralists on the Tibetan alpine rangeland suffered great economic loss in cold season, due to serious live-weight loss of domestic livestock under traditional grazing management. This study aimed to evaluate the effect of dietary types (crude protein levels) on feed intakes, growth performance and economic returns of local Tibetan sheep and yaks during cold season. Twenty-four yearling Tibetan sheep (25.29±3.95 kg LW) and twenty two-year-old yaks (100.62±4.55 kg LW) with familiar body conditions were randomly assigned to four groups, fed oats hay (OH), oats silage (OS), total mixed ration (TMR) and traditionally grazed on the local cool-season pasture (TG), respectively, over a 135-day experiment. Daily dry matter intake was determined; all animals were weighed at the beginning and every 15 days of the 135-day experiment. Then, the total live-weight gain, average daily live-weight gain, gain rate, feed efficiency and net economic benefit were calculated. Results indicated that feed and nutrient intakes (DMI, DMI/kg LW, DMI/kg LW0.75 and CPI) of TMR, OH and OS were higher than TG (P < 0.05). Grazing animals suffered serious live-weight loss, while TMR, OS and OH significantly (P < 0.05) improved total live-weight gain and gain rate in both Tibetan sheep and yaks during the entire experiment. TMR worked better in animal performance and feed efficiency, obtained the highest breeding profit in both Tibetan sheep and yaks among four treatments (P < 0.05). When expressed on net economic benefit, TMR shared the highest net economic benefit in Tibetan sheep, OH shared the highest net economic benefit in yaks, but, no significant difference of net economic benefit in yaks fed TMR and OH diets was determined (P > 0.05). Results indicated that TMR was a reasonable diet in promoting feed intakes, animal performance, feed efficiency and economic returns in domestic livestock, which should be considered by local herdsmen to increase their breeding profit during cold season.

Effects of land use and nitrogen fertilizer on ecosystem respiration in alpine meadow on the Tibetan Plateau

PurposeThere is still great uncertainty about the response of global carbon dynamics to land use change and nitrogen (N) fertilizer, especially how to balance between forage production and CO2 emission under land use change and N fertilizer times in alpine meadows.Materials and methodsThis objective of this study was to determine the effects of three land uses (i.e., abandoned cropland/pastureland (APL), perennial Elymus nutans (PEN), and annual Avena sativa L. pasture (AO)) and nitrogen fertilizer applied three times a year with the same N dose on aboveground net primary productivity (ANPP) and ecosystem respiration (Re) from 2011 to 2013.Results and discussionWe found that AO had the highest average ANPP and lowest average Re compared with APL and PEN and that there were no significant differences for them between APL and PEN during the experimental period. Nitrogen fertilization did not significantly affect ANPP, root biomass, and seasonal mean Re. Soil temperature could explain about 21.1 and 13–15% of the variation in daily Re for APL and PEN, respectively, but 20.7 and 15.0% of the variation in daily Re for APL and PEN under both with and without N fertilization, respectively. ANPP explained about 49.4% of the variation in seasonal average Re for AO, and the ratio of root biomass and ANPP explained about 45% of the variation in seasonal average Re for AO-N treatment during 2011–2013.ConclusionsLand use significantly affected ANPP, root biomass, and seasonal mean Re. However, nitrogen addition with different times did not significantly affect seasonal average Re or ANPP and root biomass for all treatments during the experimental period. Therefore, our results suggested that N could not be a limiting factor for plant production in this alpine region.

Ecological Restoration in the Typical Areas

Human activities are strongly regional and targeted, and usually tightly bound to the regional natural resources, the needs of social economic development, and ecological fragility, which cause typical regional ecosystem degradation problems. For serious degradation areas, it is an effective way to carry out regional ecological restoration and construction for restoration ecology study, which is the key to promote sustainable development and ecological security, and also the urgent requirement for the current reality. According to the demand of regional sustainable development, this chapter focused on ecological deterioration, and restoration and construction in typical areas. The chapter specially made thematic discussions on new progression in degradation problems of developing and utilizing the resource, ecological restoration practices, theory, techniques, mode, and management. This chapter was trying to provide experience and lessons, macro guidance and decision-making reference for ecological restoration in typical areas. From the regional cases of this chapter, we can see that regional ecological restoration and construction is intricate. Finally, it is necessary to point out that there are many regional ecological restoration cases, here just introduce several cases in western fragile region. The ecological restoration cases revealed in this chapter are just the first attempt to conclude the past experiences in China, which are initial identification for correlated theories and methods.

Effect of long-term organic fertilization on the soil pore characteristics of greenhouse vegetable fields converted from rice-wheat rotation fields

The shift from rice-wheat rotation (RWR) to greenhouse vegetable soils has been widely practiced in China. Several studies have discussed the changes in soil properties with land-use changes, but few studies have sought to address the differences in soil pore properties, especially for fields based on long-term organic fertilization under greenhouse vegetable system from RWR fields. This study uses the X-ray computed tomography (CT) scanning and statistical analysis to compare the long-term effects of the conversion of organic greenhouse vegetable fields (over one year, nine years, and fourteen years) from RWR fields on the soil macropore structure as well as the influencing factors from samples obtained in Nanjing, Jiangsu, China, using the surface soil layer and triplicate samples. The results demonstrated that the macropore structure became more complex and stable, with a higher connectivity, fractal dimension (FD) and a lower degree of anisotropy (DA), as the greenhouse vegetable planting time increased. The total topsoil macroporosity increased considerably, but the rate of increase gradually decelerated with time. The transmission pores (round pores ranging from 50 to 500μm) increased with time, but the biopores (>2000μm) clearly decreased after nine years of use as greenhouse vegetable fields. Soil organic matter (OM) has a significant relationship with the soil pore structure characteristics, especially for the transmission pores. In addition, organic fertilization on the topsoil had a short-term effect on the pores, but the effect stabilized and had a weak influence on the pores over longer periods. These results suggested that organic fertilization was conducive for controlling soil degradation regarding it physical quality for water and oxygen availability in the short term.

Seeding ratios and phosphate fertilizer on ecosystem carbon exchange of common vetch and oat

Few studies have investigated the effect of phosphate (P) fertilizer on net ecosystem CO2 exchange (NEE) and its components under mixed legume and grass pastures sown, which will limit our understanding about their effects on carbon (C) sequestration in soils. Our aim was to test the hypothesis that a combination of legumes with grass and P fertilizer increases NEE due to greater increases in gross primary productivity (GPP) compared to increases in ecosystem respiration (Re).We measured NEE, GPP and Re under 5 different seeding ratios of oat (Avena sativa L.) and common vetch (Vicia sativa L.) according to their thousand kernel weight and germination rate (i.e., seeding rate were: M1: 0:150, M2: 150:105, M3: 300:75, M4: 450:45 and M5: 600:0) with and P unfertilized on the Tibetan Plateau from 2011 to 2013. NEE was measured at 1-week intervals from June to September each year using a transparent chamber technique (Licor-6400). Our results showed that both P fertilizer and mixed ratio had significant effect on NEE, Re and GPP, but their interactive effects were not significant during the growing season. P fertilizer primarily increased NEE due to increased GPP for the monoculture and mixtures with a higher proportion of common vetch. NEE, Re and GPP were higher for the mixed pastures only compared with legume monoculture. From the perspective of forage yield and C sequestration, planting mixed legume-grass pastures with P fertilizer is a preferable way to balance the twin objectives of forage production and C sequestration in alpine regions.