Biao Zhu

Changes in vegetation net primary productivity from 1982 to 1999 in China

[1]xa0Terrestrial net primary production (NPP) has been a central focus of ecosystem science in the past several decades because of its importance to the terrestrial carbon cycle and ecosystem processes. Modeling studies suggest that terrestrial NPP has increased in the northern middle and high latitudes in the past 2 decades, and that such increase has exhibited seasonal and spatial variability, but there are few detailed studies on the temporal and spatial patterns of NPP trend over time in China. Here we present the trends in Chinas terrestrial NPP from 1982 to 1999 and their driving forces using satellite-derived NDVI (Normalized Difference Vegetation Index), climate data, and a satellite-based carbon model, CASA (Carnegie -Ames-Stanford Approach). The majority of China (86% of the study area) has experienced an increase in NPP during the period 1982–1999, with an annual mean increase rate of 1.03%. This increase was resulted primarily from plant growth in the middle of the growing season (June to August) (about 43.2%), followed by spring (33.7%). At the national and biome levels, the relative increase is largest in spring (March–May), indicating an earlier onset of the growing season. The changes in the phase of Chinas seasonal NPP curve may primarily be the result of advanced growing season (earlier spring) and enhanced plant growth in summer. During the past 2 decades the amplitude of the seasonal curve of NPP has increased and the annual peak NPP has advanced. Historical NPP trends also indicated a high degree of spatial heterogeneity, coupled with regional climate variations, agricultural practices, urbanization, and fire disturbance.

Altitudinal changes in carbon storage of temperate forests on Mt Changbai, Northeast China

A number of studies have investigated regional and continental scale patterns of carbon (C) stocks in forest ecosystems; however, the altitudinal changes in C storage in different components (vegetation, detritus, and soil) of forest ecosystems remain poorly understood. In this study, we measured C stocks of vegetation, detritus, and soil of 22 forest plots along an altitudinal gradient of 700–2,000xa0m to quantify altitudinal changes in carbon storage of major forest ecosystems (Pinus koraiensis and broadleaf mixed forest, 700–1,100xa0m; Picea and Abies forest, 1,100–1,800xa0m; and Betula ermanii forest, 1,800–2,000xa0m) on Mt Changbai, Northeast China. Total ecosystem C density (carbon stock per hectare) averaged 237xa0txa0Cxa0ha−1 (ranging from 112 to 338xa0txa0Cxa0ha−1) across all the forest stands, of which 153xa0txa0Cxa0ha−1 (52–245xa0txa0Cxa0ha−1) was stored in vegetation biomass, 14xa0txa0Cxa0ha−1 (2.2–48xa0txa0Cxa0ha−1) in forest detritus (including standing dead trees, fallen trees, and floor material), and 70xa0txa0Cxa0ha−1 (35–113xa0t Cxa0ha−1) in soil organic matter (1-m depth). Among all the forest types, the lowest vegetation and total C density but the highest soil organic carbon (SOC) density occurred in Betula ermanii forest, whereas the highest detritus C density was observed in Picea and Abies forest. The C density of the three ecosystem components showed distinct altitudinal patterns: with increasing altitude, vegetation C density decreased significantly, detritus C density first increased and then decreased, and SOC density exhibited increasing but insignificant trends. The allocation of total ecosystem C to each component exhibited similar but more significant trends along the altitudinal gradient. Our results suggest that carbon storage and partitioning among different components in temperate forests on Mt Changbai vary greatly with forest type and altitude.

Forest biomass carbon stocks in China over the past 2 decades: Estimation based on integrated inventory and satellite data

[1]xa0Forests are major contributor of terrestrial ecosystem carbon (C) pools, and are thus crucial components for assessing the global C budget. On the basis of forest inventory data for three inventory periods of 1984–1988, 1989–1993, and 1994–1998, and synchronous NDVI (Normalized Difference Vegetation Index) data, we developed a satellite-based approach for estimating Chinas forest total biomass C stocks. Using this approach, we analyzed the changes in forest C stocks over the last 2 decades to identify the size and distribution of C sinks/sources in the forests. The total forest biomass of China averaged 5.79 Pg C (1 Pg = 1015 g) during the study period, with an average biomass density of 45.31 Mg C/ha (1 Mg = 106 g). The forest biomass C density showed a large spatial heterogeneity: high in southwestern and northeastern areas, and low in the eastern coastal regions. Over the past 2 decades, the total forest biomass C stock increased from 5.62 Pg C in the early 1980s (average for 1981–1983) to 5.99 Pg C by the end of the 1990s (average for 1997–1999), giving a total increase of 0.37 Pg C and an annual sequestration rate of 0.019 Pg C/yr. The C sink appeared mainly in regions with lower C density. Both environmental changes and human activities are likely major drivers of such spatiotemporal patterns.

NDVI-indicated decline in desertification in China in the past two decades

[1]xa0In this study, we explore the trend in desertification in China from 1982 to 1999 by investigating the changes in area and normalized difference vegetation index (NDVI) of arid and semiarid regions, using NDVI time series data sets and climatic variables. We use Thornthwaite moisture index (Im) to define the arid and semiarid region as Im ≤ −40 and −40 < Im ≤ −20, respectively. Rainy season NDVI (May to October NDVI) increased in most areas of arid and semiarid regions over the past two decades, accounting for 72.3% and 88.2% of total area of arid and semiarid regions, respectively. Compared to that in the early 1980s, the area of arid and semiarid regions decreased by 23 × 104 km2 (6.9%) and 7 × 104 km2 (7.9%) by the end of the 1990s, suggesting a reversal of desertification processes in these two climate regions. Transformation from warm-arid to warm-wet climate and weakened disturbance from human activities may be the major causes of this declined trend.

Complementarity in nutrient foraging strategies of absorptive fine roots and arbuscular mycorrhizal fungi across 14 coexisting subtropical tree species

In most cases, both roots and mycorrhizal fungi are needed for plant nutrient foraging. Frequently, the colonization of roots by arbuscular mycorrhizal (AM) fungi seems to be greater in species with thick and sparsely branched roots than in species with thin and densely branched roots. Yet, whether a complementarity exists between roots and mycorrhizal fungi across these two types of root system remains unclear. We measured traits related to nutrient foraging (root morphology, architecture and proliferation, AM colonization and extramatrical hyphal length) across 14 coexisting AM subtropical tree species following root pruning and nutrient addition treatments. After root pruning, species with thinner roots showed more root growth, but lower mycorrhizal colonization, than species with thicker roots. Under multi-nutrient (NPK) addition, root growth increased, but mycorrhizal colonization decreased significantly, whereas no significant changes were found under nitrogen or phosphate additions. Moreover, root length proliferation was mainly achieved by altering root architecture, but not root morphology. Thin-root species seem to forage nutrients mainly via roots, whereas thick-root species rely more on mycorrhizal fungi. In addition, the reliance on mycorrhizal fungi was reduced by nutrient additions across all species. These findings highlight complementary strategies for nutrient foraging across coexisting species with contrasting root traits.

Footprint of temperature changes in the temperate and boreal forest carbon balance

In this study, we use net ecosystem productivity (NEP) measurement data across several forest sites and a simple conceptual model to investigate the linkage between temperature and NEP by considering either temperature change in the recent past or current mean annual temperature (MAT) as a forcing. After removing the effect of stand age, forest NEP is only weakly correlated with MAT. However, temperature changes during the period of 1980-2002 do explain a very significant fraction of the current spatial patterns of NEP, although the response of the terrestrial carbon balance to temperature changes varies with season. Changes in spring temperature having the highest correlation with annual NEP. We also show that temperature changes before the 1970s had a limited influence on the current NEP, and that the impact of recent temperature changes within the last decade on NEP are not strong enough to be observable. Overall, our analysis indicates not only that temperature changes in the recent past is one of the important drivers of todays forest carbon balance in the Northern Hemisphere, but also that the ongoing global warming will contribute significantly to the near-future evolution of the Northern Hemisphere carbon sink. A non-equilibrium framework must be taken into account when studying the impacts of temperature change on current or future forest net carbon balance.

13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

Stable carbon isotopes are used extensively to partition total soil CO2 efflux into root-derived rhizosphere respiration or autotrophic respiration and soil-derived heterotrophic respiration. However, it remains unclear whether CO2 from rhizosphere respiration has the same δ13C value as root biomass. Here we investigated the magnitude of 13C isotope fractionation during rhizosphere respiration relative to root biomass in six plant species. Plants were grown in a carbon-free sand-perlite medium inoculated with microorganisms from a farm soil for 62xa0days inside a greenhouse. We measured the δ13C value of rhizosphere respiration using a closed-circulation 48-hour CO2 trapping method during 40~42 and 60~62xa0days after sowing. We found a consistent depletion in 13C (0.9~1.7‰) of CO2 from rhizosphere respiration relative to root biomass in three C3 species (Glycine max L. Merr., Helianthus annuus L. and Triticum aestivum L.), but a relatively large depletion in 13C (3.7~7.0‰) in three C4 species (Amaranthus tricolor L., Sorghum bicolor (L.) Moench and Zea mays L. ssp. mays). Overall, our results indicate that CO2 from rhizosphere respiration is more 13C-depleted than root biomass. Therefore, accounting for this 13C fractionation is required for accurately partitioning total soil CO2 efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.

Labile substrate availability controls temperature sensitivity of organic carbon decomposition at different soil depths

The decomposition of soil organic carbon (SOC) is intrinsically sensitive to temperature. However, the degree to which the temperature sensitivity of SOC decomposition (as often measured in Q10 value) varies with soil depth and labile substrate availability remain unclear. This study explores (1) how the Q10 of SOC decomposition changes with increasing soil depth, and (2) how increasing labile substrate availability affects the Q10 at different soil depths. We measured soil CO2 production at four temperatures (6, 14, 22 and 30xa0°C) using an infrared CO2 analyzer. Treatments included four soil depths (0–20, 20–40, 40–60 and 60–80xa0cm), four sites (farm, redwood forest, ungrazed and grazed grassland), and two levels of labile substrate availability (ambient and saturated by adding glucose solution). We found that Q10 values at ambient substrate availability decreased with increasing soil depth, from 2.0–2.4 in 0–20xa0cm to 1.5–1.8 in 60–80xa0cm. Moreover, saturated labile substrate availability led to higher Q10 in most soil layers, and the increase in Q10 due to labile substrate addition was larger in subsurface soils (20–80xa0cm) than in surface soils (0–20xa0cm). Further analysis showed that microbial biomass carbon (MBC) and SOC best explained the variation in Q10 at ambient substrate availability across ecosystems and depths (R2xa0=xa00.37, Pxa0<xa00.001), and MBC best explained the variation in the change of Q10 between control and glucose addition treatment (R2xa0=xa00.14, Pxa0=xa00.003). Overall, these results indicate that labile substrate limitation of the temperature sensitivity of SOC decomposition, as previously shown in surface soils, is even stronger for subsoils. Understanding processes controlling the labile substrate availability (e.g., with rising atmospheric CO2 concentration and land use change) should advance our prediction of the fate of subsoil SOC in a warmer world.

Vegetation and Soil 15N Natural Abundance in Alpine Grasslands on the Tibetan Plateau: Patterns and Implications

The natural abundance of nitrogen (N) stable isotopes (δ15N) has the potential to enhance our understanding of the ecosystem N cycle at large spatial scales. However, vegetation and soil δ15N patterns along climatic and edaphic gradients have not yet been fully understood, particularly for high-altitude ecosystems. Here we determined vegetation and soil δ15N in alpine grasslands on the Tibetan Plateau by conducting four consecutive regional surveys during 2001–2004, and then examined their relationships with both climatic and edaphic variables. Our results showed that both vegetation and soil N in Tibetan alpine grasslands were more 15N-enriched than global averages. Vegetation δ15N did not exhibit any significant trend along the temperature gradient, but decreased significantly with an increase in precipitation amount. In contrast, soil δ15N did not vary with either mean annual temperature or precipitation. Our results also indicated that soil δ15N exhibited a slight increase with clay content, but decreased with soil carbon:nitrogen ratio. A general linear model analysis revealed that variations in vegetation δ15N were dominantly determined by climatic variables, whereas soil δ15N was related to edaphic variables. These results provide clues for potential climatic and edaphic regulations on ecosystem N cycle in these high-altitude regions.

Effects of nitrogen deposition on soil microbial communities in temperate and subtropical forests in China

Increasing nitrogen (N) deposition has aroused large concerns because of its potential negative effects on forest ecosystems. Although microorganisms play a vital role in ecosystem carbon (C) and nutrient cycling, the effect of N deposition on soil microbiota still remains unclear. In this study, we investigated the responses of microbial biomass C (MBC) and N (MBN) and microbial community composition to 4-5years of experimentally simulated N deposition in temperate needle-leaf forests and subtropical evergreen broadleaf forests in eastern China, using chloroform fumigation extraction and phospholipid fatty acid (PLFA) methods. We found idiosyncratic effects of N addition on microbial biomass in these two types of forest ecosystems. In the subtropical forests, N addition showed a significant negative effect on microbial biomass and community composition, while the effect of N addition was not significant in the temperate forests. The N addition decreased MBC, MBN, arbuscular mycorrhizal fungi, and the F/B ratio (ratio of fungi to bacteria biomass) in the subtropical forests, likely due to a decreased soil pH and changes in the plant community composition. These results showed that microbial biomass and community composition in subtropical forests, compared with the temperate forests, were sensitive to N deposition. Our findings suggest that N deposition may have negative influence on soil microorganisms and potentially alter carbon and nutrient cycling in subtropical forests, rather than in temperate forests.