Effects of stand density on the structure of soil microbial functional groups in Robinia pseudoacacia plantations in the hilly and gully region of the Loess Plateau, China.

Elucidating the responses of soil microbial functional groups to changes in stand density is crucial for understanding the sustainability of forest development. In this study, we obtained soil samples from Robinia pseudoacacia plantations of three different stand densities (low, middle, and high densities of 750, 1125, and 1550 trees ha-1, respectively) in the hilly and gully region of the Loess Plateau, China. We sought to determine the effects of stand density on the structure of soil microbial functional groups. Stand density had no significant effects on species diversity indices of fungal trophic modes or bacterial functional groups involved in carbon (C) cycling and nitrogen (N) cycling. However, differences in stand density substantially altered the composition of fungal functional groups. In low-density plantations, saprophytic fungi were the main trophic mode, with a high relative abundance of ∼62 %, whereas the fungal communities associated with middle- and high-density plantations were dominated by other fungi with a combined trophic mode, which accounted for ∼43 % and ∼41 % of the fungal trophic modes, respectively. Furthermore, we detected increases in the relative abundance of plant pathogens, nitrifiers, and nitrous oxide-denitrifying bacteria with increasing stand density. Results of the Monte Carlo test showed that soil pH influenced the composition of soil fungal (but not bacterial) groups. These findings suggested that a high density of trees might inhibit the decomposition of recalcitrant organic material and stimulate nitrous oxide emission, consequently decreasing soil nutrient availability and stimulating soil N loss. Moreover, high-density stands might increase the potential risk for plant disease. Overall, the present study suggested that reducing stand density to coverage between 750 and 1125 trees ha-1 would increase soil nutrient availability and prevent N loss from the soil. To verify these suppositions, further research is needed to determine the links between microbial functional groups composition and soil biogeochemistry.

[Response of Soil Respiration Rates to Soil Temperature and Moisture at Different Soil Depths of Caragana korshinskii Plantation in the Loess-Hilly Region].

It is of great significance to clarify the influence of soil temperature and moisture on soil respiration rate and its characteristics in ecologically fragile regions under the background of climate change for the accurate assessment and prediction of carbon budgets in this region. The average CO2 concentration and soil temperature and moisture at different soil depths (10, 50, and 100 cm) were measured using a CO2 analyzer and temperature and moisture sensors. The soil respiration rate was calculated using Fick's first diffusion coefficient method. The dynamic characteristics of soil temperature, soil moisture, and soil respiration rate in different soil depths were explored, and the response of soil respiration rate to soil temperature and moisture were further analyzed. The results showed that the diurnal variation in soil respiration rate decreased significantly with the increase in soil depth (P<0.05), and the peak time lagged behind. Soil respiration rate in adjacent soil depths (10, 50, and 100 cm) lagged 1 h from top to bottom. The monthly variation in soil respiration rate was a multi-peak curve, in which the maximum soil respiration rates of 10, 50, and 100 cm soil depths were on July 25th, August 6th, and August 10th, reaching 13.96, 2.96, and 1.47 µmol·(m2·s)-1, respectively. The effect of soil temperature on soil respiration rate decreased with the increase in soil depth. Soil temperature at 50 cm and below had no significant effect on soil respiration rate (P>0.05). The fitting index of 10 cm soil depth was the best (R2=0.96), but the fitting indexes of 50 cm and 100 cm soil depths were poor (R2=0.00 and R2=0.01, respectively). The temperature sensitivity coefficient Q10 decreased with the increase in soil depth. Soil moisture in different soil depths had significant effects on soil respiration rate (P<0.05), and the quadratic fitting indicated that 50 cm (R2=0.35)>10 cm (R2=0.22)>100 cm (R2=0.31). The combined effects of soil temperature and moisture in different soil depths could explain 96%, 6%-50%, and 22%-24% of soil respiration rate, respectively. In summary, the effects of soil temperature and moisture at different soil depths of the Caragana korshinskii plantation in the loess-hilly region on soil respiration rate differed. The soil respiration rate of the 10 cm soil depth was affected by the comprehensive effect of soil temperature and moisture; however, the relative contribution of soil temperature was higher, and soil moisture at and below a soil depth of 50 cm was the key factor. These results could help improve predictions on the impact of future climate change on the carbon cycle of terrestrial ecosystems in the region and provide a theoretical basis for greenhouse gas regulation in the future.

Rainfall pulse response of carbon fluxes in a temperate grass ecosystem in the semiarid Loess Plateau.

Rainfall pulses can significantly influence carbon cycling in water-limited ecosystems. The magnitude of carbon flux component responses to precipitation may vary depending on precipitation amount and antecedent soil moisture, associated with nonlinear responses of plants and soil microbes. The present study was carried out in a temperate grass ecosystem during 2013-2015 in the semiarid Loess Plateau of China, to examine the response of carbon fluxes to precipitation using the "threshold-delay" model. The unique contribution of environmental variables such as precipitation amount and antecedent soil moisture before rainfall (SWC_antecedent) to carbon fluxes in response to rainfall was also investigated. The lower threshold of effective rainfall was 6.6 mm for gross ecosystem production (GEP), 8.5 mm for net ecosystem production (NEP), and 4.5 mm for ecosystem respiration (RE); and the upper threshold of effective rainfall was 21.4 mm for GEP and NEP, and 16.8 mm for RE. Rainfall amount was positively affected the relative rainfall responses of GEP, NEP, and RE. However, SWC_antecedent at 20 cm soil depth offset the response of GEP to rainfall pulses, and SWC_antecedent at 5 cm soil depth offset the response of NEP and RE to rainfall pulses, with corresponding partial slopes of linear regressions of -0.50, -0.40, and -0.52. These results indicated that NEP was more sensitive to rainfall pulses and RE was more sensitive to SWC_antecedent. These results demonstrate the importance of rainfall events of <10 mm and that the negative effect of SWC_antecedent should also be considered when estimating ecosystem carbon fluxes in this semiarid region.

Sap flow characteristics and responses to summer rainfall for Pinus tabulaeformis and Hippophae rhamnoides in the Loess hilly region of China.

As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density (Fd) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψm) and leaf stomatal conductance (Gs) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized Fd values exhibited a single-peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized Fd for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized Fd for H. rhamnoides in response to rainfall in June and August (p < .05), although no significant differences were observed in July. The normalized Fd values for P. tabulaeformis and H. rhamnoides fitted well with the derived variable of transpiration, an integrated index calculated from the vapor pressure deficit and solar radiation (Rs), using an exponential saturation function. The differences in fitting coefficients suggested that H. rhamnoides showed more sensitivity to summer rainfall (p < .01) than P. tabulaeformis. Furthermore, during the study period, P. tabulaeformis reduced Gs as soil water decreased, maintaining a relatively constant ψm; while H. rhamnoides allowed large fluctuations in ψm to maintain Gs. Therefore, P. tabulaeformis and H. rhamnoides should be considered isohydric and anisohydric species, respectively. And more consideration should be taken for H. rhamnoides in the afforestation activities and the local plantation management under the context of the frequently seasonal drought in the loess hilly region.

The coupling of leaf, litter, and soil nutrients in warm temperate forests in northwestern China.

The nutrient ecological stoichiometry of plants and soil is important for the growth and dynamics of species, but the stoichiometric relationships among leaf, litter, and soil remain poorly understood. We analyzed the carbon (C), nitrogen (N), and phosphorus (P) stoichiometry of the leaves, litter, and soil for 31 species at 140 sites in warm temperate forests in northwestern China to document the patterns of nutrient traits and their relationships with climatic factors. The average concentrations of C, N, and P in the combined forests were 462.97, 18.04, and 1.32 g kg-1 for leaves, 365.12, 12.34, and 0.87 g kg-1 for litter, and 15.72, 1.29, and 0.54 g kg-1 for soil, respectively. The concentrations differed significantly among the leaves, litter, and soil. Leaf and soil nutrients were not significantly correlated, whereas leaf and litter nutrients and litter and soil nutrients were significantly correlated, indicating that litter provided a link between leaves and soil and demonstrating the nutrient associations among leaves, litter, and soil. Soil nutrients were strongly correlated with climatic factors, and precipitation had a larger impact than temperature on the plants and soil. This study will help to predict the growth and dynamics of species under environmental changes.

[C, N, P stoichiometric characteristics of tree, shrub, herb leaves and litter in forest community of Shaanxi Province, China].

A total of 121 sampling sites were selected to study the C, N, P stoichiometric characteristics among the leaves of tree, shrub, herb and the litter of the majority forest community of Shaanxi Province, China. We also studied their relationships with geographical factors. The results showed that C, N concentrations were highest in tree leaf, P concentration was highest in herb leaf, and the C, N, P concentrations were lowest in litter. Leaf C: N: P ratios of tree, shrub, herb leaves and litter were 439.4:14.2:1, 599.2:13.5:1, 416.5:13.3:1, 504.8:15.5:1, respectively. The N:P ratios of tree, shrub and herb leaves were not significantly different, but they were all significantly higher than that of litter layer, indicating N:P ratio of different living life forms' leaves was stable. Compared with N, C and P had better relationships in each layer, and the relationships of leaf C, N, P stoichiometry between tree and herb leaves were better than those between tree and shrub leaves. The correlations of C, N, P stoichiometry between litter and the tree or herb leaves were highly significantly positive, and the correlations of P stoichiometry between litter and shrub leaves were highly significantly positive. Shrub leaf N, P concentration increased with the increasing latitude, and herb leaf N, P concentrations decreased with the increasing longitude. Compared with the longitude and latitude, the effect of altitude was smaller, and only tree leaf N:P decreased with the increasing latitude. Multivariate linear regression analysis results showed that the influence of geographical factors on herb leaf was longitude > latitude > altitude, while was latitude > longitude > altitude for tree, shrub leaves and litter.

Temporal-Spatial Pattern of Carbon Stocks in Forest Ecosystems in Shaanxi, Northwest China.

The precise and accurate quantitative evaluation of the temporal and spatial pattern of carbon (C) storage in forest ecosystems is critical for understanding the role of forests in the global terrestrial C cycle and is essential for formulating forest management policies to combat climate change. In this study, we examined the C dynamics of forest ecosystems in Shaanxi, northwest China, based on four forest inventories (1989-1993, 1994-1998, 1999-2003, and 2004-2008) and field-sampling measurements (2012). The results indicate that the total C storage of forest ecosystems in Shaanxi increased by approximately 29.3%, from 611.72 Tg in 1993 to 790.75 Tg in 2008, partially as a result of ecological restoration projects. The spatial pattern of C storage in forest ecosystems mainly exhibited a latitude-zonal distribution across the province, increasing from north (high latitude) to south (low latitude) generally, which signifies the effect of environmental conditions, chiefly water and heat related factors, on forest growth and C sequestration. In addition, different data sources and estimation methods had a significant effect on the results obtained, with the C stocks in 2008 being considerably overestimated (864.55 Tg) and slightly underestimated (778.07 Tg) when measured using the mean C density method and integrated method, respectively. Overall, our results demonstrated that the forest ecosystem in Shaanxi acted as a C sink over the last few decades. However, further studies should be carried out with a focus on adaption of plants to environmental factors along with forest management for vegetation restoration to maximize the C sequestration potential and to better cope with climate change.

[Storage and allocation of carbon and nitrogen in Pinus tabuliformis plantations on the south slope of the East Qinling Mountains, China].

The objective of this study was to study carbon and nitrogen storages and distributions in Pinus tabuliformis plantations along an age chronosequence of 8-, 25-, 35-, 42- and 61-year-old on the south slope of the East Qinling Mountains; China. Results showed that the carbon content and nitrogen contents ranged from 441.40 to 526.21 g . kg-1 and from 3.13 to 3.99 g . kg-1 in arbor layer, from 426.06 to 447.25 g . kg-1 and from 10.62 to 12.45 g . kg-1 in shrub layer, from 301.37 to 401.52 g . kg-1 and from 10.35 to 13.33 g kg-1 in herb layer, from 382.83 to 424.71 g . kg-1, and from 8.69 to 11.90 g . kg-1 in litter layer, and from 1.51 to 18.17 g . kg-1 and from 0.29 to 1.45 g . kg-1 in soil layer (0-100 cm) , respectively. The largest carbon and nitrogen storages in arbor layer were trunks and branches, which made up 48.5% to 62.7% and 39.2% to 48.4% of the total storage, respectively. Carbon and nitrogen storages of P. tabuliformis plantations were obviously age-dependent. Carbon storage at first increased with stand Age before the stand was ripe. It was the highest (146.06 t . hm-2) when the stand was 35 year-old, after which the carbon storage de- creased. The nitrogen storage reached the peak value of 10.99 t . hm-2 at 25 year-old. The average carbon and nitrogen storages were 45.33 t . hm-2 and 568.55 kg . hm-2 in the plant layer and, 73.12 and 8.57 t . hm-2 in soil layer, respectively. Moreover, carbon and nitrogen were accumulated at higher levels in the surface soil layer. In addition, the storages of carbon and nitrogen were mainly distributed in soil layer and arbor layer in this region. The average carbon storage in different components followed an order as soil layer (64.1%) > arbor layer (30.0%) > shrub-herb and litter layers (5.9%), while the nitrogen storage followed as: soil layer (93.2%) > arbor layer (5.3%) > shrub-herb and litter layers (1.5%).

[Storage and allocation of carbon and nitrogen in Robinia pseudoacacia plantation at different ages in the loess hilly region, China].

The 9-, 17-, 30- and 37-year-old Robinia pseudoacacia plantations in the loess hilly region were investigated to study the dynamics and allocation patterns of carbon and nitrogen storage. The results showed that the ranges of carbon and nitrogen contents were 435.9-493.4 g x kg(-1) and 6.8-21.0 g x kg(-1) in the arbor layer, 396.3-459.2 g x kg(-1) and 14.2-23.5 g x kg(-1) in the herb and litter layer, and 2.7-10.7 g x kg(-1) and 0.2-0.7 g x kg(-1) in the soil layer, respectively. The branch was the major carbon and nitrogen pool in the arbor layer, accounting for 46.9%-63.3% and 39.3%-57.8%, respectively. The maximum storage values were 30.1 and 1.8 Mg x hm(-2) for carbon and nitrogen, respectively, in the 0-20 cm soil layer in the 37-year-old R. pseudoacacia plantation. The total carbon and nitrogen storage in the R. pseudoacacia plantation ecosystem increased with increasing forest age, and the maximum values were 127.9 Mg x hm(-2) and 6512.8 kg x hm(-2) for carbon and nitrogen storage, respectively, in the 37-year-old R. pseudoacacia plantation. Soil layer was the major carbon and nitrogen pool of R. pseudoacacia plantation ecosystem, accounting for 63.3%-83.3% and 80.3%-91.4%, respectively.

Chromatic dispersion correction in planar waveguide using one-layer volume holograms based on three-step exposure.

We propose a novel method to correct the chromatic dispersion in a planar waveguide with volume holograms fabricated by the three-step exposure technique. The 532 nm green laser is used to illuminate the holographic plate in three groups of different angles for achieving the desired holograms. When it is used in the planar waveguide, the chromatic dispersion of the original display can be corrected and an image with the real color can be obtained. The experiments are performed, and the results are in good agreement with the theory. It is believed that this technique is a good way to correct the chromatic problems in the display systems in the future.

[Effects of artificial seabuckthorn forest on soil and water conservation in loess hilly region].

Seabuckthorn is regarded as a main eco-economical tree species, and plays an increasing important role in eco-environmental construction in Northwest, Northeast and North China. Our study on artificial seabuckthorn forest in loess hilly region showed that the average rainfall interception rate of 7-10 ages seabuckthorn canopy was 8.5%, and the litter layer of 5-10 ages seabuckthorn forest could intercept 0.89 mm rainfall. Seabuckthorn forest could improve soil infiltration and anti-strike ability through improving soil physical and chemical properties, and the numbers of its hair roots and the depth of its litter layer were the main indices of soil anti-strike ability. The effects of seabuckthorn forest on soil and water conservation increased with its increasing age. In 2-3 ages stage, the effects were weak, and the runoff and sediment were mainly affected by the characters of rainfall. In 4-5 ages stage when the forest became maturing, the annual runoff depth and annual erosion modulus were 1.8-3.2 mm and 24.64 t x km(-2), respectively. In 6-12 ages stage when the forest matured, the runoff and sediment on seabuckthorn woodland changed slowly, the annual runoff depth and annual erosion modulus being 0.3 -3.4 mm and 0-6.75 t x km(-2), respectively, and the characters of rainfall had much less effect on them. In the stage from young (2-5 ages) to mature forest, the sediment charge in runoff changed sharply, ranged from 77. 31 kg x m(-3) to 9.12 kg x m(-3), but in 6-12 ages stage, the sediment content in runoff changed very slowly, and the range was 0-5.09 kg x m(-3).

[Ecological benefits of artificial seabuckthorn stands in semi-arid hilly region of Loess plateauion soil- and water conservation and soil moisture].

There is a remarkable function on decreasing runoff and sediment in seabuckthorn and its mixed stands, but the effects on soil- and water conservation are different due to different structure and patterns of seabuckthorn and its mixed stands. The intensity of soil water use by seabuckthorn forest was different along with the month in growing season. In growing season, soil moisture in 0-500 cm layer was 5.1%, the lowest in the end of May and 8.8%, the highest in the end of October. This intensity was also different with forest age. 8 ages seabuckthorn forest consumed 231.2 mm of soil stored water in 0-500 cm layer annually, and soil moisture was 5.6%. Therefore, it should be cut for increasing soil moisture. In end of the third year after cutting, soil moisture in 0-160 cm layer could recover, and the value would be 10.3%-14.6%. Seabuckthorn forest could reduce the effect of slope location on soil moisture. Seabuckthorn and its mixed stands have similar intensity of soil water use, and there was a soil dry layer phenomenon in their stands.