Leaf nitrogen resorption is more important than litter nitrogen mineralization in mediating the diversity–productivity relationship along a nitrogen-limited temperate forest succession chronosequence

The resorption of nutrients by plants before litter fall and the mineralization of nutrients from plant litter by soil processes are both important pathways supporting primary productivity. While the positive relationship between plant biodiversity and primary productivity is widely accepted for natural ecosystems, the roles of nutrient resorption and mineralization in mediating that relationship remains largely unknown. Here, we quantified the relative importance of nitrogen(N) resorption and N mineralization in driving plant community N investment and the correlation between species diversity and community productivity along an N-limited successional chronosequence of the mixed broadleaved–Korean pine(Pinus koraiensis) forest in northeastern China. Leaf N resorption efficiency(NRE) at the community level increased significantly along the successional chronosequence,whereas litter N mineralization rate decreased significantly. Leaf NRE was more important than litter N mineralization rate in driving the diversity–productivity relationship. However, higher leaf NRE led to less N mineralization as succession progressed along the chronosequence. Our results highlight the importance of the N resorption pathway rather than the N mineralization pathway for forest N acquisition with community succession,and they provide mechanistic insights into the positive effects of biodiversity on ecosystem functioning. In future forest management practices, we recommend appropriate application of N fertilizer to mitigate the adverse effects of N-poor soil on seedling regeneration during late succession and thus maintain the sustainable development of temperate forest ecosystems.

Nitrogen Cycling and Losses Under Rice-Wheat Rotations with Coated Urea and Urea in the Taihu Lake Region

A lysimeter experiment with undisturbed soil profiles was carried out to study nitrogen cycling and losses in a paddy soll with applications of coated urea and urea under a rice-wheat rotation system in the Taihu Lake region from 2001 to 2003. Treatments for rice and wheat included urea at conventional, 300 （rice） and 250 （wheat） kg N ha^-1, and reduced levels, 150 （rice） and 125 （wheat） kg N ha^-1, coated urea at two levels, 100 （rice） and 75 （wheat） kg N ha^-1, and 150 （rice） and 125 （wheat） kg N ha^-1, and a control with no nitrogen arranged in a completely randomized design. The results under two rice-wheat rotations showed that N losses through both NH3 volatilization and runoff in the coated urea treatments were much lower than those in the urea treatments. In the urea treatments N runoff losses were significantly （P 〈 0.001） positively correlated （r = 0.851） with applied N. N concentration in surface water increased rapidly to maximum two days after urea application and then decreased quickly. However, if there was no heavy rain within five days of fertilizer application, the likelihood of N loss by runoff was not high. As the treatments showed little difference in N loss via percolation, nitrate N in the groundwater of the paddy fields was not directly related to N leaching. The total yield of the two rice-wheat rotations in the treatment of coated urea at 50% conventional level was higher than that in the treatment of urea at the conventional level. Thus, coated urea was more favorable to rice production and environmental protection than urea.

Contrasting resilience of soil microbial biomass,microbial diversity and ammonification enzymes under three applied soil fumigants

Fumigation is a widely applied approach to mitigate the soil-borne diseases.However,the potential effects of currently applied fumigants on ammonification remain unclear.An 84-day incubation experiment was conducted based on non-fumigated soil(CK)and fumigated soil using three common fumigants,i.e.,chloropicrin(CP),1,3-dichloropropene(1,3-D),and metam sodium(MS).The results showed that,the three fumigants all decreased the microbial C,and the largest reduction(84.7%)occurred with the application of CP.After fumigation,the microbial diversity in the CP treatment rapidly recovered,but that in the 1,3-D treatment decreased and did not recover by the end of the experiment.The application of MS showed no impact on the microbial diversity during the assay,indicating that significantly different microbial diversity can be achieved by choosing different fumigants.Futhermore,the three fumigants showed divergent effects on the enzymes involved in ammonification.The analysis showed that the enzyme variation with CP application was mainly associated with the changed microbial C and N(P<0.05),and not with the microbial community,which was different from the observed effects of 1,3-D or MS application.In addition,the soil quality index showed that CP was still significantly harmful at the end of incubation compared with the good resilience of MS,indicating that CP may not be a suitable fumigant.

A Process-based Model of N_2O Emission from a Rice-Winter Wheat Rotation Agro-Ecosystem:Structure,Validation and Sensitivity

In order to numerically simulate daily nitrous oxide （N2O） emission from a rice-winter wheat rotation cropping system, a process-based site model was developed （referred to as IAP-N-GAS） to track the movement and transformation of several forms of nitrogen in the agro-eeosystem, which is affected by climate, soil, crop growth and management practices. The simulation of daily N2O fluxes, along with key daily environmental variables, was validated with three-year observations conducted in East China. The validation demonstrated that the model simulated well daily solar radiation, soil temperature and moisture, and also captured the dynamics and magnitude of accumulated rice aboveground biomass and mineral nitrogen in the soil. The simulated daily N2O emissions over all three years investigated were generally in good agreement with field observations. Particularly well simulated were the peak N2O emissions induced by fertilizations, rainfall events or mid-season drainages. The model simulation also represented closely the inter-annuM variation in N2O emission. These validations imply that the model has the capability to capture the general characteristics of N2O emission from a typical rice-wheat rotation agro-ecosystem. Sensitivity analyses revealed that the simulated N2O emission is most sensitive to the fertilizer application rate and the soil organic matter content, but it is much less sensitive to variations in soil pH and texture, temperature, precipitation and crop residue incorporation rate under local conditions.

Evaluation of Nitrogen Cycling Associated with Agricultural Production and Environmental Load in a Mountain Region,in Hokkaido,Japan

This study examined the nitrogen cycling associated with agricultural production and environmental load in central Hokkaido. The nitrogen (N) budget analysis model offers a new set of tools for evaluating N cycling in agro-ecosystems. The cycling index (CI) is a useful tool for estimating optimal N flows in farmlands. The fertilization index (FI) is a useful indicator for characterizing the N flows related to farms. Using these parameters, we analyzed all farm systems to estimate the optimal N cycling for minimizing N pollution in groundwater and maximizing agricultural production in mountain regions of Japan. The results showed that the critical N application rate (chemical fertilizer + manure) was 143.3 kg N ha-1 y-1. The critical inter-system input (chemical fertilizer N, imported food and feed N, and natural supplied N) was 169.2 kg N ha-1 y-1.

Mycorrhizal fungi mitigate nitrogen losses of an experimental grassland by facilitating plant uptake and soil microbial immobilization

Nitrogen(N)is one of the most limited nutrients of terrestrial ecosystems,whose losses are prevented in tightly coupled cycles in finely tuned systems.Global change-induced N enrichment through atmospheric deposition and application of vast amounts of fertilizer are now challenging the terrestrial N cycle.Arbuscular mycorrhizal fungi(AMF)are known drivers of plant-soil nutrient fluxes,but a comprehensive assessment of AMF involvement in N cycling under global change is still lacking.Here,we simulated N enrichment by fertilization(low/high)in experimental grassland microcosms under greenhouse conditions in the presence or absence of AMF and continuously monitored different N pathways over nine months.We found that high N enrichment by fertilization decreased the relative abundance of legumes and the plant species dominating the plant community changed from grasses to forbs in the presence of AMF,based on aboveground biomass.The presence of AMF always maintained plant N:phosphorus(P)ratios between 14 and 16,no matter how the soil N availability changed.Shifts in plant N:P ratios due to the increased plant N and P uptake might thus be a primary pathway of AMF altering plant community composition.Furthermore,we constructed a comprehensive picture of AMF’s role in N cycling,highlighting that AMF reduced N losses primarily by mitigating N leaching,while N_(2)O emissions played a marginal role.Arbuscular mycorrhizal fungi reduced N_(2)O emissions directly through the promotion of N_(2)O-consuming denitrifiers.The underlying mechanism for reducing N leaching is mainly the AMF-mediated improved nutrient uptake and AMF-associated microbial immobilization.Our results indicate that synergies between AMF and other soil microorganisms cannot be ignored in N cycling and that the integral role of AMF in N cycling terrestrial ecosystems can buffer the upcoming global changes.

Regional coupled C-N-H2O cycle processes and associated driving mechanisms

From a molecular level to an ecosystem scale,different coupling mechanisms take place during coupled carbonnitrogen-water(C-N-H2O)cycle,of which essential are water flux and related biogeochemical processes through physicochemical reactions associated with terrestrial and aquatic ecosystems.Meanwhile,regional coupled C-N-H2O cycle will subsequently impact regional gross primary productivity(GPP)and C and N exchanges during air-water interactions that occur downstream of watersheds.This study aimed to first synthetically analyze the regional dynamics of C,N and H2O cycles in ecosystems and determine their interactional relationships;second,to specify regional C-N-H2O coupled relationships of ecosystems and their theoretical ecological principles;third,to classify coupled regional response and adaptation of the C-N-H2O cycle to climatic and environmental changes under anthropogenic activities,providing a theoretical basis to fully understand and make adjustments to interactional C,N and H2O cycling relationships at different ecosystem scales and under associated coupling processes.

Impacts of wetting-drying cycles on short-term carbon and nitrogen dynamics in Amynthas earthworm casts

Effects of earthworm casts on soil nutrient dynamics and their responses to changing moisture availability in subtropical ecosystems remain poorly understood.This study aimed to examine short-term carbon(C)and nitrogen(N)dynamics and their interactions with wetting-drying cycles in three different structural forms(i.e.,granular,globular,and heap-like)of Amynthas earthworm casts.The rates of C and N mineralization in the earthworm casts were examined under two different wetting-drying cycles(i.e.,2-d and 4-d wetting intervals)using a rainfall simulation experiment.After three simulated rainfall events,subsamples of the earthworm casts were further incubated for 4 d for the determination of CO2 and N2O fluxes.The results of this study indicated that the impacts of wetting-drying cycles on the short-term C and N dynamics were highly variable among the three cast forms,but wetting-drying cycles significantly reduced the cumulative CO2 and N2O fluxes by 62%-83%and 57%-85%,respectively,when compared to the control without being subjected to any rainfall events.The C mineralization rates in different cast forms were affected by the amount of organic substrates and N content in casts,which were associated with the food preference and selection of earthworms.Meanwhile,the cumulative N2O fluxes did not differ among the three cast forms.Repeated wetting and drying of casts not only enhanced aggregate stability by promoting bonds between the cast particles,but also inhibited microbial survival and growth during the prolonged drying period,which together hindered decomposition and denitrification.Our findings demonstrated that the interactions between the structural forms,aggregate dynamics,and C and N cycling in the earthworm casts were highly complex.

THEOREM OF EXISTENCE OF EXACT n LIMIT CYCLES FOR LINARD'S EQUATION

Liénard’s equation is a kind of important ordinary differential equations frequently appearing in engineering and technology, and hence receives great attention of many mathematicians. In 1949, H. J. Eckweiler conjectured that the equation +μsin+x=0 has infinite number of limit cycles. Then H. S. Hochstadt and B. Stephan, R. N. D’Heedene and others proved that this equation has at least n limit cycles in the interval |x|<(n+1)π for specified parameter μ. In 1980, Professor Zhang Zhifen proved that this equation has exact n limit cycles in the interval |x|<(n+1)π for any nonzero parameter μ, and thus pushed the related work forward greatly. In this paper, we shall prove that the Liénard’s equation has exact n limit cycles in a finite interval under a class of very general condition.

Ammonia Volatilization in a Semi-arid Rangeland in Inner Mongolia

为了了解草原生态系统氨气挥发情况、历史放牧的影响及其影响机制,2005和2006年生长季期间,在长达17年放牧处理的样地上测定了5个放牧强度下的氨气挥发量。结果表明:氨气挥发随季节变化波动很大,春季和初秋高,晚秋和夏季低,变化范围为0.88-3.52 gN ha-1d-1。土壤pH值、NH4+-N浓度、湿度以及容重对氨气挥发控制作用较强。但是容重的影响与其他3个因子的影响作用相反,而容重的大小直接受不同放牧强度影响。虽然不同的放牧强度对氨气释放的影响在统计学上来说并不显著,但是,相对不放牧的情况而言,氨气挥发量在放牧情况下更大。而土壤氮浓度、植物生物量却在放牧的情况下更低。这些都意味着放牧草地生态系统比不放牧草地生态系统恢复起来会更慢。这种关于草原生态系统氨气挥发的研究意义重大,因为它有助于加强了解氮素丢失对草原各进程的抑制作用,同时对制订适宜的放牧及施肥措施从而达到合理利用资源的目的也具有重要的参考价值。

Soil Enzyme Activities on Eroded Slopes in the Sichuan Basin, China

Determining how soil erosion affects enzyme activity may enhance our understanding of soil degradation on eroded agricultural landscapes. This study assessed the changes in enzyme activity with slope position and erosion type by selecting water and tillage erosion-dominated slopes and performing analyses using the 1376s technique. The 137Cs data revealed that soil loss occurred in the upper section of the two eroded slope types, while soil accumulation occurred in the lower section. The invertase activity increased downslope and exhibited a pattern similar to the 137Cs data. The spatial patterns of urease and alkaline phosphatase activities were similar to the 137Cs inventories on the water and tillage erosion-dominated slopes, respectively. On both the eroded slope types, the invertase activity and soil organic carbon content were correlated, but no correlation was observed between the alkaline phosphatase activity and total phosphorus content. Nevertheless, the urease activity was correlated with the total nitrogen content only on the water erosion-dominated slopes. The enzyme activity-to-microbial biomass carbon ratios indicated high activities of invertase and urease but low activity of phosphatase on the water erosion-dominated slopes compared with the tillage erosion-dominated slopes. Both the invertase activity and the invertase activity-to-microbial biomass carbon ratio varied with the slope position. Changes in the urease activity-to-microbial biomass carbon ratio were significantly affected by the erosion type. These suggested that the dynamics of the invertase activity were linked to soil redistribution on the two eroded slope types, whereas the dynamics of the urease and alkaline phosphatase activities were associated with soil redistribution only on the water or tillage erosion-dominated slopes, respectively. The erosion type had an obvious effect on the activities of invertase, urease and alkaline phosphatase. Soil redistribution might influence the involvement of urease in the N cycle and alkaline phosphatase in the P cycle. Thus, enzyme activity-to-microbial biomass ratios may be used to better evaluate microbiological activity in eroded soils.

Diurnal and spatial variations of soil NO_x fluxes in the northern steppe of China

NOx emissions from biogenic sources in soils play a significant role in the gaseous loss of soil nitrogen and consequent changes in tropospheric chemistry. In order to investigate the characteristics of NOx fluxes and factors influencing these fluxes in degraded sandy grasslands in northern China, diurnal and spatial variations of NOx fluxes were measured in situ. A dynamic flux chamber method was used at eight sites with various vegetation coverages and soil types in the northern steppe of China in the summer season of 2010.Fluxes of NOx from soils with plant covers were generally higher than those in the corresponding bare vegetation-free soils, indicating that the canopy plays an important role in the exchange of NOx between soil and air. The fluxes of NOx increased in the daytime,and decreased during the nighttime, with peak emissions occurring between 12：00 and14：00. The results of multiple linear regression analysis indicated that the diurnal variation of NOx fluxes was positively correlated with soil temperature（P 〈 0.05） and negatively with soil moisture content（P 〈 0.05）. Based on measurement over a season, the overall variation in NOx flux was lower than that of soil nitrogen contents, suggesting that the gaseous loss of N from the grasslands of northern China was not a significant contributor to the high C/N in the northern steppe of China. The concentration of NOx emitted from soils in the region did not exceed the 1-hr National Ambient Air Quality Standard（0.25 mg/m^3）.

Soil Carbon and Nitrogen in Response to Perennial Bioenergy Grass, Cover Crop and Nitrogen Fertilization

Cover crop and nitrogen （N） fertilization may maintain soil organic matter under bioenergy perennial grass where removal of aboveground biomass for feedstock to produce cellulosic ethanol can reduce soil quality. We evaluated the effects of cover crops and N fertilization rates on soil organic carbon （C） （SOC）, total N （STN）, ammonium N （NH4-N）, and nitrate N （NO3-N） contents at the 0-5, 5-15, and 15-30 cm depths under perennial bioenergy grass from 2010 to 2014 in the southeastern USA. Treatments included unbalanced combinations of perennial bioenergy grass, energy cane （Saccharum spontaneum L.） or elephant grass （Pennisetum pur- pureum Schumazh.）, cover crop, crimson clover （Trifolium incarnatum L.）, and N fertilization rates （0, 100, and 200 kg N ha-l）. Cover crop biomass and C and N contents were greater in the treatment of energy cane with cover crop and 100 kg N ha-1 than in the treatment of energy cane and elephant grass. The SOC and STN contents at 0-5 and 5-15 cm were 9%-20% greater in the treatments of elephant grass with cover crop and with or without 100 kg N ha-1 than in most of the other treatments. The soil NO3-N content at 0--5 cm was 31%-45% greater in the treatment of energy cane with cover crop and 100 kg N ha-1 than in most of the other treatments. The SOC sequestration increased from 0.1 to 1.0 Mg C ha-1 year-1 and the STN sequestration from 0.03 to 0.11 Mg N ha-1 year-1 from 2010 to 2014 for various treatments and depths. In contrast, the soil NH4-N and NO3-N contents varied among treatments, depths, and years. Soil C and N storages can be enriched and residual NO3-N content can be reduced by using elephant grass with cover crop and with or without N fertilization at a moderate rate.

Vertex-Distinguishing E-Total Coloring of the Graphs mC_3 and mC_4

Let G be a simple graph. A total coloring f of G is called E-total-coloring if no two adjacent vertices of G receive the same color and no edge of G receives the same color as one of its endpoints. For E-total-coloring f of a graph G and any vertex u of G, let Cf （u） or C（u） denote the set of colors of vertex u and the edges incident to u. We call C（u） the color set of u. If C（u） ≠ C（v） for any two different vertices u and v of V（G）, then we say that f is a vertex-distinguishing E-total-coloring of G, or a VDET coloring of G for short. The minimum number of colors required for a VDET colorings of G is denoted by X^evt（G）, and it is called the VDET chromatic number of G. In this article, we will discuss vertex-distinguishing E-total colorings of the graphs mC3 and mC4.

Mammalian herbivory in post-fire chaparral impacts herbaceous composition but not N and C cycling

Aims Classical theory predicts that herbivores impact herb assemblages and soil nitrogen(N)cycling through selective plant consumption and the deposition of N-rich waste,with effects dependent upon ecosystem N availability.Herbivores are predicted to accelerate N cycling when N availability is high and decelerate cycling when availability is low.However,experimental tests of these theories in natural systems are limited and have yielded contradictory results.California’s widespread chaparral shrublands provide a tractable system in which to test these theories.They are prone to periodic crown fire,which temporarily removes living shrub cover,deposits mineral N on soils and allows diverse herbaceous assemblages to dominate the landscape for 3–5 years.Chaparral is also increasingly vulnerable to herbaceous invasion;mammalian herbivory may limit the establishment of non-native herbs in the shrub understory.Methods We implemented a 2-year herbivore-exclosure experiment(Hopland,CA)to assess the impact of mammalian herbivory during early post-fire chaparral succession,both on herbaceous plant assemblages and soil N and C cycling.We predicted that,in high-N post-fire conditions,mammalian herbivory would not demonstrate a strong preference for N-fixing herbs,would accelerate N cycling and would reduce the abundance of non-native herbs.Important Findings Excluding mammalian herbivores increased herb standing biomass by 54%,but changed neither the relative abundance of N-fixing vs.non-N-fixing herbs nor any measure of N or C cycling.Herbivore impacts on nutrient cycling may not be significant over the 2-year time scale of the experiment and physical effects of herbivore activity could have counteracted the influence of plant litter and animal dung/urine inputs.Mammalian herbivores concentrated their feeding on typical non-native herbs,slightly decreasing their relative abundance;however,mammalian herbivory was not sufficient to stem the invasion of chaparral by invasive herbs or alter C and N cycling over the first 2 years after fire.