Xiaobing Zhou

Nitrogen deposition and its ecological impact in China: an overview.

Nitrogen (N) deposition is an important component in the global N cycle that has induced large impacts on the health and services of terrestrial and aquatic ecosystems worldwide. Anthropogenic reactive N (N(r)) emissions to the atmosphere have increased dramatically in China due to rapid agricultural, industrial and urban development. Therefore increasing N deposition in China and its ecological impacts are of great concern since the 1980s. This paper synthesizes the data from various published papers to assess the status of the anthropogenic N(r) emissions and N deposition as well as their impacts on different ecosystems, including empirical critical loads for different ecosystems. Research challenges and policy implications on atmospheric N pollution and deposition are also discussed. China urgently needs to establish national networks for N deposition monitoring and cross-site N addition experiments in grasslands, forests and aquatic ecosystems. Critical loads and modeling tools will be further used in N(r) regulation.

Sensitivity of growth and biomass allocation patterns to increasing nitrogen: a comparison between ephemerals and annuals in the Gurbantunggut Desert, north-western China.

BACKGROUND AND AIMSnBiomass accumulation and allocation patterns are critical to quantifying ecosystem dynamics. However, these patterns differ among species, and they can change in response to nutrient availability even among genetically related individuals. In order to understand this complexity further, this study examined three ephemeral species (with very short vegetative growth periods) and three annual species (with significantly longer vegetative growth periods) in the Gurbantunggut Desert, north-western China, to determine their responses to different nitrogen (N) supplements under natural conditions.nnnMETHODSnNitrogen was added to the soil at rates of 0, 0.5, 1.0, 3.0, 6.0 and 24.0 g N m(-2) year(-1). Plants were sampled at various intervals to measure relative growth rate and shoot and root dry mass.nnnKEY RESULTSnCompared with annuals, ephemerals grew more rapidly, increased shoot and root biomass with increasing N application rates and significantly decreased root/shoot ratios. Nevertheless, changes in the biomass allocation of some species (i.e. Erodium oxyrrhynchum) in response to the N treatment were largely a consequence of changes in overall plant size, which was inconsistent with an optimal partitioning model. An isometric log shoot vs. log root scaling relationship for the final biomass harvest was observed for each species and all annuals, while pooled data of three ephemerals showed an allometric scaling relationship.nnnCONCLUSIONSnThese results indicate that ephemerals and annuals differ observably in their biomass allocation patterns in response to soil N supplements, although an isometric log shoot vs. log root scaling relationship was maintained across all species. These findings highlight that different life history strategies behave differently in response to N application even when interspecific scaling relationships remain nearly isometric.

Responses of microbial activities and soil physical-chemical properties to the successional process of biological soil crusts in the Gurbantunggut Desert, Xinjiang

Biological soil crusts (BSCs) are capable of modifying nutrient availability to favor the establishment of biogeochemical cycles. Microbial activities serve as critical roles for both carbon and nutrient transformation in BSCs. However, little is known about microbial activities and physical-chemical properties of BSCs in the Gurbantunggut Desert, Xinjiang, China. In the present research, a sampling line with 1-m wide and 20-m long was set up in each of five typical interdune areas selected randomly in the Gurbantunggut Desert. Within each sampling line, samples of bare sand sheet, algal crusts, lichen crusts and moss crusts were randomly collected at the depth of 0–2 cm. Variations of microalgal biomass, microbial biomass, enzyme activities and soil physical-chemical properties in different succession of BSCs were analyzed. The relationships between microalgal biomass, microbial biomass, enzymatic activities and soil physical-chemical properties were explored by stepwise regression. Our results indicate that microalgal biomass, microbial biomass and most of enzyme activities increased as the BSCs developed and their highest values occurred in lichen or moss crusts. Except for total K, the contents of most soil nutrients (organic C, total N, total P, available N, available P and available K) were the lowest in the bare sand sheet and significantly increased with the BSCs development, reaching their highest values in moss crusts. However, pH values significantly decreased as the BSCs developed. Significant and positive correlations were observed between chlorophyll a and microbial biomass C. Total P and N were positively associated with chlorophyll a and microbial biomass C, whereas there was a significant and negative correlation between microbial biomass and available P. The growth of cyanobacteria and microorganism contributed C and N in the soil, which offered substrates for enzyme activities thus increasing enzyme activities. Probably, improvement in enzyme activities increased soil fertility and promoted the growth of cyanobacteria, eukaryotic algae and heterotrophic microorganism, with the accelerating succession of BSCs. The present research found that microalgal-microbial biomass and enzyme activities played important roles on the contents of nutrients in the successional stages of BSCs and helped us to understand developmental mechanism in the succession of BSCs.

Divergence in physiological responses between cyanobacterial and lichen crusts to a gradient of simulated nitrogen deposition

Background and aimsNitrogen (N) deposition in arid lands is known to be increasing. However, N deposition gradients have unclear effects on physiological characteristics of biocrusts. This study tested if physiological characteristics are stimulated by low levels of N deposition and reduced by high levels of N deposition.MethodsWe simulated N deposition at various rates to plots of cyanobacterial and lichen biocrusts in the Gurbantunggut Desert and measured indicators of growth and stress.ResultsIn cyanobacterial crusts, most evidence suggests that biomass and growth are unaffected by lower levels of N but suppressed at the highest level. Biomass and growth of lichen crusts were less sensitive to N addition but, in the case of actual photochemical efficiency, were also suppressed at the highest N addition rate. Most osmotic adjustment substances of cyanobacterial and lichen crusts did not show significant responses after N addition. In the two crusts, minor increase in antioxidative enzyme activity was found in some N addition rates but no similar trends between the crust types were observed.ConclusionsPhysiological performance of cyanobacterial crusts was more sensitive to high levels of N addition than that of lichen crusts. Increasing N deposition might greatly affect cyanobacterial crusts before lichen crusts.

Differential responses of dinitrogen fixation, diazotrophic cyanobacteria and ammonia oxidation reveal a potential warming-induced imbalance of the N-cycle in biological soil crusts

N2 fixation and ammonia oxidation (AO) are the two most important processes in the nitrogen (N) cycle of biological soil crusts (BSCs). We studied the short-term response of acetylene reduction assay (ARA) rates, an indicator of potential N2 fixation, and AO rates to temperature (T, -5°C to 35°C) in BSC of different successional stages along the BSC ecological succession and geographic origin (hot Chihuahuan and cooler Great Basin deserts). ARA in all BSCs increased with T until saturation occurred between 15 and 20°C, and declined at 30–35°C. Culture studies using cyanobacteria isolated from these crusts indicated that the saturating effect was traceable to their inability to grow well diazotrophically within the high temperature range. Below saturation, temperature response was exponential, with Q10 significantly different in the two areas (~ 5 for Great Basin BSCs; 2–3 for Chihuahuan BSCs), but similar between the two successional stages. However, in contrast to ARA, AO showed a steady increase to 30–35°C in Great Basin, and Chihuhuan BSCs showed no inhibition at any tested temperature. The T response of AO also differed significantly between Great Basin (Q10 of 4.5–4.8) and Chihuahuan (Q10 of 2.4–2.6) BSCs, but not between successional stages. Response of ARA rates to T did not differ from that of AO in either desert. Thus, while both processes scaled to T in unison until 20°C, they separated to an increasing degree at higher temperature. As future warming is likely to occur in the regions where BSCs are often the dominant living cover, this predicted decoupling is expected to result in higher proportion of nitrates in soil relative to ammonium. As nitrate is more easily lost as leachate or to be reduced to gaseous forms, this could mean a depletion of soil N over large landscapes globally.

Sensitivity of the xerophytic moss Syntrichia caninervis to prolonged simulated nitrogen deposition

BACKGROUND AND AIMSnBiological soil crusts, comprising assemblages of cyanobacteria, fungi, lichens and mosses, are common in dryland areas and are important elements in these ecosystems. Increasing N deposition has led to great changes in community structure and function in desert ecosystems worldwide. However, it is unclear how moss crusts respond to increased atmospheric N deposition, especially in terms of growth and physiological parameters. The aim of this study was to understand how Syntrichia caninervis, a dominant species in moss crusts in many northern hemisphere desert ecosystems, responds to added N.nnnMETHODSnThe population and shoot growth, and physiological responses of S. caninervis to six different doses of simulated N deposition (0, 0·3, 0·5, 1·0, 1·5 and 3·0u2009g N m(-2) year(-1)) were studied over a 3 year period.nnnKEY RESULTSnLow amounts of added N increased shoot length and leaf size, whereas high doses reduced almost all growth parameters. Moss shoot density increased, but population biomass decreased with high N. Low N augmented chlorophyll b, total chlorophyll content and soluble protein concentrations, but not chlorophyll a or chlorophyll fluorescence. High N was detrimental to all these indices. Soluble sugar concentration declined with increased N, but proline concentration was not affected significantly. Antioxidant enzyme activities generally decreased with low N additions and increased with high doses of simulated N deposition.nnnCONCLUSIONSnLow amounts of added N (0-0·5u2009g N m(-2) year(-1)) may enhance moss growth and vitality, while higher amounts have detrimental effects.