Soil Organic Carbon and Labile Carbon Along a Precipitation Gradient and Their Responses to Some Environmental Changes

Based on data from a field survey in 2001 along the Northeast China transect (NECT), a precipitation gradient,and a short-term simulation experiment under ambient CO2 of 350 μmol mol-1 and doubled CO2 of 700 μmol mol-1with different soil moisture contents of 30%-45%, 45%-60%, and 60%-80% soil water holding capacity, the distributionof soil organic carbon and labile carbon along the NECT, their relationships with precipitation and their responses toCO2 enrichment and soil moisture changes were analyzed. The results indicated that the soil labile carbon along thegradient was significantly related to soil organic carbon (r = 0.993, P < 0.001). The soil labile carbon decreased morerapidly with depth than organic carbon. The soil organic and labile carbon along the gradient decreased with decrease inlongitude in both the topsoils and subsoils, and the coefficient of variation for the labile carbon was greater than that forthe organic carbon. Both the soil organic carbon and labile carbon had significant linear relationships with precipitation,with the correlation coefficient of soil organic carbon being lower (0.677 at P < 0.001) than that of soil labile carbon(0.712 at P < 0.001). In the simulation experiment with doubled and ambient CO2 and different moisture contents, thecoefficient of variation for soil organic carbon was only 1.3%, while for soil labile carbon it was 29.7%. With doubled CO2concentration (700 μmol mol-1), soil labile carbon decreased significantly at 45% to 60% of soil moisture content. Theseindicated that soil labile carbon was relatively more sensitive to environmental changes than soil organic carbon.

Soil bacterial characteristics between surface and subsurface soils along a precipitation gradient in the Alxa Desert, China

Bacteria in desert soil have unique phylogeny and important ecological functions, and theirresponses to changes in precipitation need further attention. However, relevant studies have mainlyfocused on the surface soil, and studies on the responses of bacteria at different soil depths to variationsin precipitation are rare. Thus, we used 16S rDNA high-throughput sequencing to investigate the changesin soil bacterial distribution along a mean annual precipitation gradient (50–150 mm) in the Alxa Desert,China, and compared the variation characteristics in the surface soil layer (0–10 cm) and subsurface soillayer (10–20 cm). Results showed that soil bacterial communities significantly changed along theprecipitation gradient in both soil layers. However, the subsurface soil layer could support bacterialcommunities with higher diversity and closer internal relationships but more internal competition than thesurface soil layer. Additionally, compared with the surface soil layer, variations in diversity andco-occurrence patterns in the subsurface soil layer were more in line with the changes in the mean annualprecipitation, while bacterial community structure was less variable in the subsurface soil layer. Comparedwith the mean annual precipitation, soil moisture had little influence on the structure and diversity of soilbacterial community but had a high correlation with intercommunity connectivity. Therefore, soilmoisture might play a complex role in mediating environmental conditions and soil bacterial communitycharacteristics. Due to the different responses of surface and subsurface soil bacteria to the changes inprecipitation, it is necessary to distinguish different soil layers when predicting the trends in desert soilbacterial conditions associated with precipitation, and prediction of subsurface soil bacteria may be moreaccurate.

Changes in morphology and soil nutrient patterns of nebkhas in arid regions along a precipitation gradient

Nebkhas, discrete mounds of sand and vegetation, are a common landscape feature critical to the stability of desert ecosystems and supported by limited precipitation. Nebkha morphology and spatial pattern vary in landscapes, but it is unclear how they change along precipitation gradients in arid and semi-arid regions. In this study we determined morphology and soil nutrient patterns of nebkha from different regions of northwestern China. The objective of this study was to understand zonal differences among nebkhas and how morphological characteristics and soil nutrient patterns of nebkha change along a precipitation gradient in northwestern China. Our results shows that mean annual precipitation(MAP) had significant effects on morphological characteristics of nebkhas such as height, area, and volume which significantly decreased with an increase in MAP. MAP had significant positive effects on shrub cover and species richness of nebkha. Soil nutrients such as soil organic matter(SOM), total carbon(TC), total nitrogen(TN), and total phosphorus(TP) in the 0-10 cm layer increased with an increase of MAP, and soil nutrient content within nebkhas was higher than in inter-nebkha areas.We concluded that nebkhas are "fertile islands" with an important role in ecosystem dynamics in study regions. Further,MAP is a key factor which determined zonal differences, morphological, and soil nutrients patterns of nebkhas. However,disturbance, such as animal grazing, and planted sand-stabilizing vegetation accelerated the degeneration of nebkha landscapes. We recommend implementation of protective measures for nebkhas in arid and semi-arid areas of China.

Plant property regulates soil bacterial community structure under altered precipitation regimes in a semi-arid desert grassland, China

Variations of precipitation have great impacts on soil carbon cycle and decomposition of soil organic matter.Soil bacteria are crucial participants in regulating these ecological processes and vulnerable to altered precipitation.Studying the impacts of altered precipitation on soil bacterial community structure can provide a novel insight into the potential impacts of altered precipitation on soil carbon cycle and carbon storage of grassland.Therefore,soil bacterial community structure under a precipitation manipulation experiment was researched in a semi-arid desert grassland in Chinese Loess Plateau.Five precipitation levels,i.e.,control,reduced and increased precipitation by 40%and 20%,respectively(referred here as CK,DP40,DP20,IP40,and IP20)were set.The results showed that soil bacterial alpha diversity and rare bacteria significantly changed with altered precipitation,but the dominant bacteria and soil bacterial beta diversity did not change,which may be ascribed to the ecological strategy of soil bacteria.The linear discriminate analysis(LDA)effect size(LEfSe)method found that major response patterns of soil bacteria to altered precipitation were resource-limited and drought-tolerant populations.In addition,increasing precipitation greatly promoted inter-species competition,while decreasing precipitation highly facilitated inter-species cooperation.These changes in species interaction can promote different distribution ratios of bacterial populations under different precipitation conditions.In structural equation model(SEM)analysis,with changes in precipitation,plant growth characteristics were found to be drivers of soil bacterial community composition,while soil properties were not.In conclusion,our results indicated that in desert grassland ecosystem,the sensitive of soil rare bacteria to altered precipitation was stronger than that of dominant taxa,which may be related to the ecological strategy of bacteria,species interaction,and precipitation-induced variations of plant growth characteristics.

THE USE OF SHEAR GRADIENT VORTICITY IN TROPICAL CYCLONE HEAVY PRECIPITATION PREDICTION:A HIGH-RESOLUTION NUMERICAL CASE STUDY

This study introduces a new dynamical quantity, shear gradient vorticity (SGV), which is defined as vertical wind shear multiplying the horizontal component of vorticity gradient, aiming to diagnose heavy precipitation induced by some strong convective weather systems. The vorticity gradient component can be used to study the collision or merging process between different vortexes or the deformation of a vortex with a sharp vorticity gradient. Vertical wind shear, another contributed component of SGV, always represents the environmental dynamical factor in meteorology. By the combined effect of the two components, overall, SGV can represent the interaction between the environmental wind shear and the evolution of vortexes with a large vorticity gradient. Other traditional vorticity-like dynamical quantities (such as helicity) have the limitation in the diagnosis of the convection, since they do not consider the vorticity gradient. From this perspective, SGV has the potential to diagnose some strong convective weather processes, such as Extratropical Transition (ET) of tropical cyclones and the evolution of multicell storms. The forecast performance of SGV for the numerical ET case of Typhoon Toraji (0108) has been evaluated. Compared with helicity, SGV has shown a greater advantage to forecast the distribution of heavy precipitation more accurately, especially in the frontal zone.

Global patterns in above-ground net primary production and precipitation-use efficiency in grasslands

The above-ground net primary production(ANPP) and the precipitation-use efficiency(PUE) regulate the carbon and water cycles in grassland ecosystems, but the relationships among the ANPP, PUE and precipitation are still controversial. We selected 717 grassland sites with ANPP and mean annual precipitation(MAP) data from 40 publications to characterize the relationships ANPP–MAP and PUE–MAP across different grassland types. The MAP and ANPP showed large variations across all grassland types, ranging from 69 to 2335 mm and 4.3 to 1706 g m^(-2), respectively. The global maximum PUE ranged from 0.19 to 1.49 g m^(-2) mm^(-1) with a unimodal pattern. Analysis using the sigmoid function explained the ANPP–MAP relationship best at the global scale. The gradient of the ANPP–MAP graph was small for arid and semi-arid sites(MAP <400 mm). This study improves our understanding of the relationship between ANPP and MAP across dry grassland ecosystems. It provides new perspectives on the prediction and modeling of variations in the ANPP for different grassland types along precipitation gradients.

Patterns in leaf traits of leguminous and non-leguminous dominant trees along a rainfall gradient in Ghana

Aims Both dominance distribution of species and the composition of the dominant species determine the distribution of traits within community.Leaf carbon(C)and nitrogen(N)isotopic composition are important leaf traits,and such traits of dominant species are associated with ecosystem C,water and N cycling.Very little is known how dominant species with distinct traits(e.g.N-fixing leguminous and non-leguminous trees)mediate resource utilization of the ecosystems in stressful environment.Methods Leaves of 81 dominant leguminous and non-leguminous trees were collected in forest(moist semi-deciduous and dry semi-deciduous ecosystems)and savanna(costal savanna,Guinean savanna and west Sudanian savanna ecosystems)areas and the transitional zone(between the forest and the savanna)along the transect from the south to the north of Ghana.We measured leaf traits,i.e.leafδ13C,leafδ15N,leaf water content,leaf mass per area(LMA)and C and N concentration.Correlation analyses were used to examine trait–trait relationships,and relationships of leaf traits with temperature and precipitation.We used analysis of covariance to test the differences in slopes of the linear regressions between legumes and non-legumes.Important Findings Leafδ13C,δ15N,leaf water content and LMA did not differ between leguminous and non-leguminous trees.Leaf N concentration and C:N ratio differed between the two groups.Moreover,leaf traits varied significantly among the six ecosystems.δ13C values were negatively correlated with annual precipitation and positively correlated with mean annual temperature.In contrast,leafδ15N of non-leguminous trees were positively correlated with annual precipitation and negatively correlated with mean annual temperature.For leguminous trees,such correlations were not significant.We also found significant coordination between leaf traits.However,the slopes of the linear relationships were significantly different between leguminous and non-leguminous trees.Our results indicate that shifts in dominant trees with distinct water-use efficiency were corresponded to the rainfall gradient.Moreover,leguminous trees,those characterized with relative high water-use efficiency in the low rainfall ecosystems,were also corresponded to the relative high N use efficiency.The high proportion of leguminous trees in the savannas is crucial to mitigate nutrient stress.