Ning Zong

Responses of ecosystem CO 2 fluxes to short-term experimental warming and nitrogen enrichment in an Alpine meadow, northern Tibet Plateau.

Over the past decades, the Tibetan Plateau has experienced pronounced warming, yet the extent to which warming will affect alpine ecosystems depends on how warming interacts with other influential global change factors, such as nitrogen (N) deposition. A long-term warming and N manipulation experiment was established to investigate the interactive effects of warming and N deposition on alpine meadow. Open-top chambers were used to simulate warming. N addition, warming, N addition × warming, and a control were set up. In OTCs, daytime air and soil temperature were warmed by 2.0°C and 1.6°C above ambient conditions, but soil moisture was decreased by 4.95 m3 m−3. N addition enhanced ecosystem respiration (Reco); nevertheless, warming significantly decreased Reco. The decline of Reco resulting from warming was cancelled out by N addition in late growing season. Our results suggested that N addition enhanced Reco by increasing soil N availability and plant production, whereas warming decreased Reco through lowering soil moisture, soil N supply potential, and suppression of plant activity. Furthermore, season-specific responses of Reco indicated that warming and N deposition caused by future global change may have complicated influence on carbon cycles in alpine ecosystems.

Foliar nutrient resorption patterns of four functional plants along a precipitation gradient on the Tibetan Changtang Plateau

Abstract Nutrient resorption from senesced leaves as a nutrient conservation strategy is important for plants to adapt to nutrient deficiency, particularly in alpine and arid environment. However, the leaf nutrient resorption patterns of different functional plants across environmental gradient remain unclear. In this study, we conducted a transect survey of 12 communities to address foliar nitrogen (N) and phosphorus (P) resorption strategies of four functional groups along an eastward increasing precipitation gradient in northern Tibetan Changtang Plateau. Soil nutrient availability, leaf nutrient concentration, and N:P ratio in green leaves ([N:P]g) were linearly correlated with precipitation. Nitrogen resorption efficiency decreased, whereas phosphorus resorption efficiency except for sedge increased with increasing precipitation, indicating a greater nutrient conservation in nutrient‐poor environment. The surveyed alpine plants except for legume had obviously higher N and P resorption efficiencies than the world mean levels. Legumes had higher N concentrations in green and senesced leaves, but lowest resorption efficiency than nonlegumes. Sedge species had much lower P concentration in senesced leaves but highest P resorption efficiency, suggesting highly competitive P conservation. Leaf nutrient resorption efficiencies of N and P were largely controlled by soil and plant nutrient, and indirectly regulated by precipitation. Nutrient resorption efficiencies were more determined by soil nutrient availability, while resorption proficiencies were more controlled by leaf nutrient and N:P of green leaves. Overall, our results suggest strong internal nutrient cycling through foliar nutrient resorption in the alpine nutrient‐poor ecosystems on the Plateau. The patterns of soil nutrient availability and resorption also imply a transit from more N limitation in the west to a more P limitation in the east Changtang. Our findings offer insights into understanding nutrient conservation strategy in the precipitation and its derived soil nutrient availability gradient.

Tower-Based Validation and Improvement of MODIS Gross Primary Production in an Alpine Swamp Meadow on the Tibetan Plateau

Alpine swamp meadow on the Tibetan Plateau is among the most sensitive areas to climate change. Accurate quantification of the GPP in alpine swamp meadow can benefit our understanding of the global carbon cycle. The 8-day MODerate resolution Imaging Spectroradiometer (MODIS) gross primary production (GPP) products (GPP_MOD) provide a pathway to estimate GPP in this remote ecosystem. However, the accuracy of the GPP_MOD estimation in this representative alpine swamp meadow is still unknown. Here five years GPP_MOD was validated using GPP derived from the eddy covariance flux measurements (GPP_EC) from 2009 to 2013. Our results indicated that the GPP_EC was strongly underestimated by GPP_MOD with a daily mean less than 40% of EC measurements. To reduce this error, the ground meteorological and vegetation leaf area index (LAIG) measurements were used to revise the key inputs, the maximum light use efficiency (emax) and the fractional photosynthetically active radiation (FPARM) in the MOD17 algorithm. Using two approaches to determine the site-specific emax value, we suggested that the suitable emax was about 1.61 g C MJ−1 for this alpine swamp meadow which was considerably larger than the default 0.68 g C MJ−1 for grassland. The FPARM underestimated 22.2% of the actual FPAR (FPARG) simulated from the LAIG during the whole study period. Model comparisons showed that the large inaccuracies of GPP_MOD were mainly caused by the underestimation of the emax and followed by that of the undervalued FPAR. However, the DAO meteorology data in the MOD17 algorithm did not exert a significant affection in the MODIS GPP underestimations. Therefore, site-specific optimized parameters inputs, especially the emax and FPARG, are necessary to improve the performance of the MOD17 algorithm in GPP estimation, in which the calibrated MOD17A2 algorithm (GPP_MODR3) could explain 91.6% of GPP_EC variance for the alpine swamp meadow.

Effects of Fertilization and Grazing Exclosure on Vegetation Recovery in a Degraded Alpine Meadow on the Tibetan Plateau

Both fertilization and grazing exclosure are effective management measures for recovery of degraded grasslands. To better understand the effects of both measures and their interaction on restoration of degraded alpine grasslands, a longterm experiment of exclosure and addition of nitrogen(50 kg N hm-2 a-1, LN; 100 kg N hm-2 a-1, HN) combined with phosphorus((50 kg N + 50 kg P) hm-2 a-1, LN + P;(100 kg N + 50 kg P) hm-2 a-1, HN + P) was carried out in a degraded Kobresia pygeama alpine meadow on the Tibetan Plateau since 2008. The results showed that nitrogen(N) addition did not affect plant community coverage, while in grazing plots LN + P and HN + P treatments signifi cantly increased plant community coverage by 86% and 63%, respectively. Moreover, in grazing exclosure plots LN + P and HN + P treatments signif icantly increased 107% and 248% of plant community coverage respectively, compared with grazing plots under the same fertilization levels. N addition alone contributed to enhancing importance values(IV) of grass, and this effect was specifically improved by exclosure. However, sedge and Potentilla were much more responsive to combination of N and P addition. N addition alone under free grazing did not affect aboveground biomass for grass, sedge or herbs of Potentilla, while LN + P treatment significantly enhanced community aboveground biomass by 137%. Under exclosure conditions, N addition did not signif icantly change the aboveground biomass of sedge, but increased the aboveground biomass of grass by 8.4 times. However, N combined with Paddition in fencing plots signif icantly promoted aboveground biomass of grass, sedge and Potentilla herbs. For example, LN + P and HN + P treatments enhanced the aboveground biomass of herbs by 98% and 86%, respectively. Our results suggested that though N addition alone could not improve the condition of degraded alpine meadow, N combined with P fertilization and/or exclosure will contribute greatly to the restoration of degraded alpine meadow. Fig 3, Tab 4, Ref 54

Nutrient Enrichment Mediates the Relationships of Soil Microbial Respiration with Climatic Factors in an Alpine Meadow.

Quantifying the effects of nutrient additions on soil microbial respiration (R m) and its contribution to soil respiration (R s) are of great importance for accurate assessment ecosystem carbon (C) flux. Nitrogen (N) addition either alone (coded as LN and HN) or in combination with phosphorus (P) (coded as LN + P and HN + P) were manipulated in a semiarid alpine meadow on the Tibetan Plateau since 2008. Either LN or HN did not affect R m, while LN + P enhanced R m during peak growing periods, but HN + P did not affect R m. Nutrient addition also significantly affected R m/R s, and the correlations of R m/R s with climatic factors varied with years. Soil water content (Sw) was the main factor controlling the variations of R m/R s. During the years with large rainfall variations, R m/R s was negatively correlated with Sw, while, in years with even rainfall, R m/R s was positively correlated with Sw. Meanwhile, in N + P treatments the controlling effects of climatic factors on R m/R s were more significant than those in CK. Our results indicate that the sensitivity of soil microbes to climatic factors is regulated by nutrient enrichment. The divergent effects of Sw on R m/R s suggest that precipitation distribution patterns are key factors controlling soil microbial activities and ecosystem C fluxes in semiarid alpine meadow ecosystems.

Effects of warming and nitrogen addition on nutrient resorption efficiency in an alpine meadow on the northern Tibetan Plateau

ABSTRACT The increase in two controversial global environmental issues, climate warming and nitrogen (N) deposition, may have distinct effects on the processes and functioning of terrestrial ecosystems. Nutrient resorption is an important determinant of plant community nutrient dynamics, especially in nutrient-limited ecosystems, but information about N and phosphorus (P) resorption in alpine ecosystems is still lacking. A long-term simulated warming and exogenous N addition experiment initiated in July 2010 was conducted in an alpine meadow in Damxung County in northern Tibet. The experiment consisted of conditions of warming and no warming crossed with three N addition levels: 0 (CK), 20 (N20), and 40 (N40) kg N · ha−1 · year−1. With increasing N addition levels, the N content and the N/P ratio in plant leaves gradually increased, while the P limitation of plant growth was aggravated by N addition. The moderate N addition level (N20) increased plant N resorption efficiency (NRE), while the high N addition level (N40) had no effect on the NRE of Kobresia pygmaea or Anaphalis xylorhiza. N addition significantly increased the P resorption efficiency (PRE) in Stipa capillacea leaves. However, N addition did not change the community NRE or the community PRE. The soil N content decreased under the warming treatment. At the community level, warming significantly increased the NRE by 12% and 16%, and the PRE by 26% and 24% under the CK and N40 treatments, respectively. The NRE and PRE were higher in S. capillacea than in K. pygmaea and A. xylorhiza, especially at the high N addition level (38% and 45% higher NRE and 36% and 15% higher PRE compared to K. pygmaea and A. xylorhiza, respectively). Correlation analysis showed that the NRE and PRE in plant leaves were mainly mediated by soil inorganic N availability, and tended to decrease with increase of soil N availability, suggesting that N loss due to warming could induce changes in nutrient resorption in alpine ecosystems. The species-specific responses to N addition and the stronger competitive advantage of S. capillacea may change the community structure and subsequently affect the decomposition process in this alpine meadow under future global climate change scenarios.

The effects of warming and nitrogen addition on ecosystem respiration in a Tibetan alpine meadow: The significance of winter warming

Abstract In recent decades, global warming has become an indisputable fact on the Tibetan Plateau. Alpine ecosystems are very sensitive to global warming, and the impact may depend on the degree of atmospheric nitrogen (N) deposition. The previous studies have paid more attention to year‐round warming, but the effect of winter warming has been unstudied. In this study, a manipulative experiment was conducted, consisting of warming and N addition. It was carried out since 2010 in an alpine meadow, and three types of warming treatments were set up: no warming (NW), year‐round (YW), and winter warming (WW). Warming significantly increased air and soil temperature, but decreased soil moisture. Under no N addition, YW showed significantly decreased ecosystem respiration (Reco) in 2012, and WW decreased Reco in 2014. Under N addition, neither YW nor WW had significant effects on Reco, indicating that N addition compensated the negative effect of warming on Reco. Annually, YW and WW decreased ecosystem carbon (C) emissions, and the extent of the reduction was even larger under WW. Under no N addition, both YW and WW significantly decreased aboveground biomass. Moreover, especially under no N, YW and WW significantly decreased soil inorganic N. WW also had negative effects on soil microbial biomass C. Structure equation modeling showed that soil moisture was the most important factors controlling Reco, and soil inorganic N content and microbial biomass C could explain 46.6% and 16.8% of the variation of Reco. The findings indicate that soil property changes under warming had substantial effects on ecosystem C efflux. The inhibitory effects of winter warming on ecosystem C efflux were mainly attributed to the decline of soil N and microbial biomass. Thus, the effects of winter warming on ecosystem C emissions in this semiarid alpine meadow are not as serious as expected and largely depend on N deposition.

Satellite-Based Inversion and Field Validation of Autotrophic and Heterotrophic Respiration in an Alpine Meadow on the Tibetan Plateau

Alpine meadow ecosystem is among the highest soil carbon density and the most sensitive ecosystem to climate change. Partitioning autotrophic (Ra) and heterotrophic components (Rm) of ecosystem respiration (Re) is critical to evaluating climate change effects on ecosystem carbon cycling. Here we introduce a satellite-based method, combining MODerate resolution Imaging Spectroradiometer (MODIS) products, eddy covariance (EC) and chamber-based Re components measurements, for estimating carbon dynamics and partitioning of Re from 2009 to 2011 in a typical alpine meadow on the Tibetan Plateau. Six satellite-based gross primary production (GPP) models were employed and compared with GPP_EC, all of which appeared to well explain the temporal GPP_EC trends. However, MODIS versions 6 GPP product (GPP_MOD) and GPP estimation from vegetation photosynthesis model (GPP_VPM) provided the most reliable GPP estimation magnitudes with less than 10% of relative predictive error (RPE) compared to GPP_EC. Thus, they together with MODIS products and GPP_EC were used to estimate Re using the satellite-based method. All satellite-based Re estimations generated an alternative estimation of Re_EC with negligible root mean square errors (RMSEs, g C m−2 day−1) either in the growing season (0.12) or not (0.08). Moreover, chamber-based Re measurements showed that autotrophic contributions to Re (Ra/Re) could be effectively reflected by all these three satellite-based Re partitions. Results showed that the Ra contribution of Re were 27% (10–48%), 43% (22–59%) and 56% (33–76%) from 2009 to 2011, respectively, of which inter-annual variation is mainly attributed to soil water dynamics. This study showed annual temperature sensitivity of Ra (Q10,Ra) with an average of 5.20 was significantly higher than that of Q10,Rm (1.50), and also the inter-annual variation of Q10,Ra (4.14–7.31) was larger than Q10,Rm (1.42–1.60). Therefore, our results suggest that the response of Ra to temperature change is stronger than that of Rm in this alpine meadow.