Interactive effects of water and nitrogen addition on soil microbial communities in a semiarid steppe

Aims Better understanding of microbial compositional and physiological acclimation mechanisms is critical for predicting terrestrial ecosystem responses to global change. The aim is to assess variations in soil microbial communities under future scenarios of changing precipitation and N deposition in a semiarid grassland of northern China.Methods In order to explicitly estimate microbial responses, a field experiment with water and N addition was established in April 2005 and continuously conducted for 4 years. Specifically, soil microbial community composition and microbial C utilization potential were determined by phospholipid fatty acid (PLFA) and community-level physiological profiles, respectively.Important findings Water addition had no effects on the PLFA concentrations of gram-positive (GP) and negative bacteria (GN), total bacteria and fungi. However, N addition caused significant reductions in the PLFA concentrations of GP, GN, total bacteria and fungi and thus decreased total PLFA of microbial communities. Moreover, there were interactive effects of water and N addition on GN/GP and the ratio of fungal to bacterial PLFA (F/B). In addition, synergistic effects were found between water and nitrogen in affecting microbial C utilization potentials, which implies that microbial C utilization potentials tend to be enhanced when both N and water availability are sufficient. Overall, the microbial responses to water and N addition support our hypothesis that water and N addition may be combined together to affect microbial communities in the semiarid grassland.     

Increasing water and nitrogen availability enhanced net ecosystem CO2 assimilation of a temperate semiarid steppe

Boron (B) is an essential micronutrient, whose deficiency is common in boreal forests. Our aim was to investigate the effects of the B supply on the retranslocation of micro- and macro nutrients in seedlings of Betula pendula Roth. One-year-old seedlings were grown under three different levels of B: 0%, 30% and 100% of the standard level for complete nutrient solution. Half of the seedlings were harvested after summer period and another half when leaves abscised. Boron was not resorbed in significant amounts from senescing birch leaves prior to abscission. The only micronutrients resorbed were Zn and Ni. Three macronutrients, N, P, and S, were resorbed efficiently from senescing leaves and accumulated into the stems. The resorption of nutrients was the mostly pronounced in B0 seedlings and minimal in B30 seedlings, which, however, showed the highest accumulation of nutrients during autumn period at least partly independently from the resorption from senescing leaves. Boron was shown to be an immobile element in silver birch seedlings that was not withdrawn from senescing leaves prior to abscission. This may increase the B availability for other tree species but also increase the potential for its leaching.     

Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co-occurring environmental changes. We examined the effects of urbanization and fragmentation on oak-dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.     

Testing nitrogen and water co-limitation of primary productivity in a temperate steppe

Background and aims

Studies have found significant differences in methane (CH₄) emissions among rice cultivars; however, it is unclear whether this difference is related to radial oxygen loss (ROL) from the roots.

Methods

Based on a 2-year in situ field study and solution culture experiments on 16 rice cultivars, we investigated CH₄ emission levels and their dependence on ROL.

Results

We detected significant differences in CH₄ emission and ROL among rice cultivars. The lowest and highest CH₄ emission levels were 4.10 and 7.35 g m^?2 for early rice, and 14.36 and 23.33 g m^?2 for late rice, respectively. The maximum and minimum ROL values were 3.77 and 1.73 mmol plant^?1 h^?1 for early rice, and 4.18 and 2.08 mmol plant^?1 h^?1 for late rice, respectively. Seasonal total CH₄ emission was negatively correlated with ROL in the early rice season (p?<?0.01), and (p?<?0.01) in the late rice season. ROL was positively correlated with the number of roots per plant (RN), root tips per plant (RT), and root volume per plant (RV).

Conclusions

We suggest that ROL can be used as a predictive index for CH₄ emissions. RN, RT, and RV were the most important factors influencing ROL in rice cultivars.

     

Differential responses of auto‐ and heterotrophic soil respiration to water and nitrogen addition in a semiarid temperate steppe

Evaluating how autotrophic (SR_A), heterotrophic (SR_H) and total soil respiration (SR_TOT) respond differently to changes of environmental factors is critical to get an understanding of ecosystem carbon (C) cycling and its feedback processes to climate change. A field experiment was conducted to examine the responses of SR_A and SR_H to water and nitrogen (N) addition in a temperate steppe in northern China during two hydrologically contrasting growing seasons. Water addition stimulated SR_A and SR_H in both years, and their increases were significantly greater in a dry year (2007) than in a wet year (2006). N addition increased SR_A in 2006 but not in 2007, while it decreased SR_H in both years, leading to a positive response of SR_TOT in 2006 but a negative one in 2007. The different responses of SR_A and SR_H indicate that it will be uncertain to predict soil C storage if SR_TOT is used instead of SR_H to estimate variations in soil C storage. Overall, N addition is likely to enhance soil C storage, while the impacts of water addition are determined by its relative effects on carbon input (plant growth) and SR_H. Antecedent water conditions played an important role in controlling responses of SR_A, SR_H and the consequent SR_TOT to water and N addition. Our findings highlight the predominance of hydrological conditions in regulating the responses of C cycling to global change in the semiarid temperate steppe of northern China.     

冻融对土壤氮素损失及有效性的影响

土壤冻融交替是寒冷生态系统土壤氮素循环的重要驱动力。已有研究表明冻融交替作用能够促进氮素周转,从而缓解因土壤有效氮素缺乏而引起的植物生长限制。即便如此,冻融环境下土壤有效氮素供应量远高于其利用量,过剩的氮素会通过气态(N2O-N)排放、淋溶和径流等途径损失。论述了季节冻融环境和模拟冻融条件下土壤氮素损失状况;同时分析了影响冻融土壤N2O生产的相关因素、产生途径及冻融期N2O大量排放的机制;针对冻融交替过程中土壤氮素有效性问题,探讨了氮矿化、可溶性有机氮(DON)和微生物量氮(MBN)与氮素损失的关系。评述了土壤冻融研究中存在的不足,认为模型研究、土壤微生物功能、氮素转化中间产物、土壤-植物界面过程是未来值得关注和深入探讨的研究方向。     

Differential effects of grazing,water, and nitrogen addition on soil respiration and its components in a meadow steppe

Background and aims

Understanding the influences of environmental variation and anthropogenic disturbance on soil respiration (R_S) is critical for accurate prediction of ecosystem C uptake and release. However, surprisingly, little is known about how soil respiration and its components respond to grazing in the context of global climate change (i.e., precipitation or nitrogen deposition increase).

Methods

We conducted a field manipulative grazing experiment with water and nitrogen addition treatments in a meadow grassland on the Songnen Plain, China, and assessed the combined influences of grazing and global change factors on R_S, autotrophic respiration (R_A), and heterotrophic respiration (R_H).

Results

Compared with the control plots, R_S, R_A and R_H all exhibited positive responses to water or nitrogen addition in the wet year, while a similar effect occurred only for R_H in the dry year. The responses of R_S to precipitation regimes were dominated by both frequency and amount. However, grazing significantly inhibited both soil respiration and its components in all subplots. Further analysis demonstrated that the plant root/shoot ratio, belowground biomass and microbial biomass played dominant roles in shaping these C exchange processes.

Conclusion

These findings suggest that changes in precipitation regimes, nitrogen deposition, and land utilization may significantly alter soil respiration and its component processes by affecting local carbon users (roots and soil microorganism) and carbon substrate supply in meadow steppe grasslands. The future soil carbon sequestration in the studied meadow steppe will be benefited more by the moderate grazing disturbance.

     

The effects of a 9-year nitrogen and water addition on soil aggregate phosphorus and sulfur availability in a semi-arid grassland

Previous studies have demonstrated that higher nitrogen (N) and water availability affect both above- and below-ground communities, soil carbon and N pools, and microbial activity in semi-arid grasslands of Inner Mongolia. However, how soil phosphorus (P) and sulfur (S) pools, and related soil enzyme activities (as indicators of P and S cycles) respond to long-term N and water addition has still remained unclear. Since 2005, a field experiment with urea and water amendments has been conducted to examine their effects on total and available P and S concentrations and alkaline phosphomonoesterase (PME) and aryl-sulfatase (ArS) activities in three soil aggregate fractions: large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), and microaggregates (<0.25 mm) in an Inner Mongolia semi-arid grassland. Normalized to aggregate mass, microaggregates retained the highest total P and S concentrations. Both N and water additions increased the available P (by up to 84.5%) and the available S (by up to 150%) in the soil aggregate fractions. Soil acidification, as a result of the N addition, decreased both alkaline PME and ArS activities by up to 62.9% and 39.6%, respectively, while the water addition increased their activities. Our observations revealed that soil acidification (under the N addition) and elevated enzyme activity (under the water addition) played important roles in the levels of soil available P and S. The depression of P- and S-acquiring enzymes with soil acidification may decrease P and S availability, potentially impacting ecosystem processes and limiting the restoration of these grassland systems. The water addition was shown to be a more effective practice than the urea amendment for improving soil structure, supplying available P and S, and maintaining the sustainability of this semi-arid grassland.     

连续氮添加14年对温带典型草原土壤碳氮组分及物理结构的影响

全球氮沉降对生态系统造成了深远的影响,研究长时间氮沉降对草地生态系统土壤理化特征的影响有助于加强生态系统对氮沉降响应的长效机制的理解。通过连续14年长期施加N0（0 g N m^-2 a^-1）、N2（2 g N m^-2 a^-1）、N4（4 g N m^-2 a^-1）、N8（8 g N m^-2 a^-1）、N16（16 g N m^-2 a^-1）、N32（32 g N m^-2 a^-1）六种浓度尿素模拟氮沉降,并将土壤分成0-10、10-20和20-40 cm三个深度土层,研究温带草原生态系统土壤碳氮组分及物理结构对氮添加的响应及其相互关系,结果表明：（1）氮添加显著降低0-10 cm土壤酸碱度及土壤微生物量碳含量,N32相比N0分别下降了27.63%和58.40%（P<0.05）;各土层总有机碳和全氮含量对氮添加处理无显著响应,0-10 cm土层显著高于20-40 cm土层。（2）同一土层深度不同梯度氮添加处理显著增加土壤无机氮离子含量（P<0.05）,0-10 cm土层铵态氮含量N32相比N0增加了88.72%,20-40 cm土层硝态氮含量N32相比N0增加了19.55倍,土壤深度与氮添加对无机氮离子含量影响具有显著的交互效应。（3）同一土壤深度不同梯度氮添加处理土壤粒度分形维数及土壤团聚体差异不显著,相关分析表明土壤碳氮元素含量与土壤结构显著相关。土壤碳氮组分在适宜浓度氮添加的增加趋势说明氮添加在一定程度上可能促进土壤理化性质的改良,氮添加对土壤物理结构的影响还需要进一步的深入研究。     

水氮添加对内蒙古温带典型草原植被氮含量季节和年际动态的影响

作为对全球变化响应最敏感的生态系统类型之一,草原生态系统植被氮含量的季节、年际变化及其对气候变化(氮沉降、降水格局改变)的响应研究相对匮乏。基于内蒙古温带典型草原5年的氮添加(10 g N m^-2 a^-1)和水添加(添加量80 mm,分2 mm×40次、5 mm×16次、10 mm×8次、20 mm×4次、40 mm×2次5种处理)控制试验分析了水氮添加后植被氮含量在生态系统和物种两个水平的季节和年际变化。结果表明,高强度水添加处理有降低(10 mm/次和40 mm/次)生态系统氮含量的趋势,但不显著,小强度水添加处理(2 mm/次、5 mm/次)在不同年份之间无一致的升高或降低趋势,但所有水添加处理有降低两种优势物种整个生长季氮含量的趋势。氮添加促进生态系统和两种优势物种整个生长季的氮含量,但该促进作用可被水添加抵消,且这种抵消作用随水氮添加年限的延长而加剧。水氮添加均增加了生态系统氮含量的年际变异,但对特定物种季节内变异的影响在湿润和干旱年份存在一定差异。本研究将为预测草原生态系统对未来氮沉降增加和降水格局改变的响应及模型改进提供科学依据...     

The effects of warming and nitrogen addition on soil nitrogen cycling in a temperate grassland, northeastern China

Background

Both climate warming and atmospheric nitrogen (N) deposition are predicted to affect soil N cycling in terrestrial biomes over the next century. However, the interactive effects of warming and N deposition on soil N mineralization in temperate grasslands are poorly understood.

Methodology/Principal Findings

A field manipulation experiment was conducted to examine the effects of warming and N addition on soil N cycling in a temperate grassland of northeastern China from 2007 to 2009. Soil samples were incubated at a constant temperature and moisture, from samples collected in the field. The results showed that both warming and N addition significantly stimulated soil net N mineralization rate and net nitrification rate. Combined warming and N addition caused an interactive effect on N mineralization, which could be explained by the relative shift of soil microbial community structure because of fungal biomass increase and strong plant uptake of added N due to warming. Irrespective of strong intra- and inter-annual variations in soil N mineralization, the responses of N mineralization to warming and N addition did not change during the three growing seasons, suggesting independence of warming and N responses of N mineralization from precipitation variations in the temperate grassland.

Conclusions/Significance

Interactions between climate warming and N deposition on soil N cycling were significant. These findings will improve our understanding on the response of soil N cycling to the simultaneous climate change drivers in temperate grassland ecosystem.     

Nitrogen effects on net ecosystem carbon exchange in a temperate steppe

It has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO₂ exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m⁻² yr⁻¹ with NH₄NO₃ alone or in combination with phosphorous (P, 5 g P₂O₅ m⁻² yr⁻¹) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing-season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N-induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species ( Stipa krylovii ) and standing litter accumulation induced light limitation on the lower canopy species ( Artemisia frigida ). Thus, N-induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.     

The effects of nitrogen, light and water availability on tropic leaf movements in soybean (Glycine max)

Abstract. Rapid, tropic leaf movements and photo-synthetic responses of the heliotropic plant, soybean, Glycine max cv. Cumberland, grown under two different nitrogen, three different light and two different water treatments were examined. Measurements of leaf orientation during midday periods outdoors, and tropic reorientation of leaflets in response to vertical illumination indoors, revealed a positive, linear relationship between leaf water potential and the cosine of the angle of incidence between the leaf and the direct beam of the excitation light. This relationship was altered by nitrogen availability, such that a lower cosine of incidence (lower leaf irradiance) for a given leaf water potential was measured for plants grown under low nitrogen compared to those grown under high nitrogen. Additionally, plants grown under low nitrogen and low water availability showed more rapid rates of leaf movement compared to plants receiving high levels of these resources. Light regime during growth had no effect on the relationship between the cosine of incidence and leaf water potential. Reduced water and nitrogen availabilities during growth resulted in lower photosaturated rates of photosynthesis and stomatal conductance, as well as alterations in the relationship between these parameters. Thus, higher values for the ratio of intercellular CO₂/ambient CO₂ were measured for low-N grown plants (higher nitrogen use efficiencies) and lower values of this ratio for water stressed plants (higher water use efficiencies). The results show that environmental growth conditions other than water availability have the potential to modify leaf orientation responses to vectorial light in heliotropic legumes such as soybean. This has implications for the potential of heliotropic movements to minimize environmental stress-induced damage to the photosynthetic apparatus, and to modulate leaf-level resource use efficiencies.     

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

Background and Aims

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown.

Methods

Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China.

Key Results

Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production.

Conclusions

These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.     

Interactive effects of disturbance and nitrogen availability on phosphorus dynamics of southern Appalachian forests

Understanding the main and interactive effects of chronically altered resource availability and disturbance on phosphorus (P) availability is increasingly important in light of the rapid pace at which human activities are altering these processes and potentially introducing P limitation. We measured P pools and fluxes in eighteen mixed forest stands at three elevations (low, mid, high) subjected to increasing atmospheric N deposition, where hemlock (Tsuga canadensis) was absent or declining due to infestation by the exotic hemlock woolly adelgid (Adelges tsugae). While total soil P was similar across the study area, phosphorus fractionation revealed distinct differences in the distribution of soil P fractions as elevation and N availability increased. Soils from high elevation plots where N availability was greatest had 139 % larger organic P pools and 55 % smaller residual and refractory P pools than soils from low elevation plots with less N availability, suggesting that increased N availability has driven the depletion of recalcitrant P pools by stimulating biotic demand and sequestration. These differences in P distribution among fractions influenced how tree mortality affected P dynamics. At high elevations, plots containing declining hemlocks had significantly greater foliar P concentrations and fluxes of P from the forest floor than reference plots at similar elevations, whereas at low and mid-elevations there were no consistent differences between plots. Across all elevation classes, hardwood foliar N:P ratios were lower in plots with declining hemlocks. Collectively, these results suggest that increased N availability enhances bioavailable P, which is sequestered in vegetation until disturbances liberate it.     

Soil microbial properties and plant growth responses to carbon and water addition in a temperate steppe: the importance of nutrient availability

Background

Global climatic change is generally expected to stimulate net primary production, and consequently increase soil carbon (C) input. The enhanced C input together with potentially increased precipitation may affect soil microbial processes and plant growth.

Methodology/Principal Findings

To examine the effects of C and water additions on soil microbial properties and plant growth, we conducted an experiment lasting two years in a temperate steppe of northeastern China. We found that soil C and water additions significantly affected microbial properties and stimulated plant growth. Carbon addition significantly increased soil microbial biomass and activity but had a limited effect on microbial community structure. Water addition significantly increased soil microbial activity in the first year but the response to water decreased in the second year. The water-induced changes of microbial activity could be ascribed to decreased soil nitrogen (N) availability and to the shift in soil microbial community structure. However, no water effect on soil microbial activity was visible under C addition during the two years, likely because C addition alleviated nutrient limitation of soil microbes. In addition, C and water additions interacted to affect plant functional group composition. Water addition significantly increased the ratio of grass to forb biomass in C addition plots but showed only minor effects under ambient C levels. Our results suggest that soil microbial activity and plant growth are limited by nutrient (C and N) and water availability, and highlight the importance of nutrient availability in modulating the responses of soil microbes and plants to potentially increased precipitation in the temperate steppe.

Conclusions/Significance

Increased soil C input and precipitation would show significant effects on soil microbial properties and plant growth in the temperate steppe. These findings will improve our understanding of the responses of soil microbes and plants to the indirect and direct climate change effects.     

Hierarchical responses of plant stoichiometry to nitrogen deposition and mowing in a temperate steppe

Background and aims

Stoichiometric relations drive powerful constraints on many ecosystem processes. However, our understanding of the hierarchical responses of plant C:N:P stoichiometry at different levels of biological organization to global change factors remains limited.

Methods

we examined the plant C:N:P stoichiometric responses to N deposition and mowing (hay making) at both species- and community-level by carrying out a 4-year field experiment in the temperate steppe of northern China.

Results

Our results showed that N addition and mowing resulted in higher plant N concentrations, lower C:N, and higher N:P at both species- and community-level. Mowing had a limited negative influence on the effects of N addition. We observed divergent responses of both plant P concentrations and C:P to N addition at species-level and community-level: N addition led to higher plant P and lower C:P at species-level, but this effect was not observed at the community-level.

Conclusions

Our results indicate that stoichiometric responses at community-level to N addition and mowing diverge from more traditionally examined species-specific responses. Our results suggest that the hierarchical responses of plant stoichiometry to anthropogenic disturbance deserves more attention when we model the interactions of terrestrial ecosystem C, N, and P cycling under scenarios of increasing N availability concomitantly occurring with active land management.     

Oxygen stress in Salvinia natans: Interactive effects of oxygen availability and nitrogen source

The ability of Salvinia natans (L.) All. to tolerate growth in oxic, hypoxic and anoxic nutrient solutions when supplied with either NH₄⁺ or NO₃^? were studied in the laboratory to test the hypothesis that inorganic N-source affects the response of the plants to O₂ deprivation. The relative growth rate (RGR) was significantly reduced in the anoxic treatment, but in the hypoxic treatment RGR was only slightly affected. The NH₄⁺ fed plants generally had a higher shoot to root ratio than the NO₃^? fed plants, and highest in the anoxic treatment. Plants had more roots and larger leaves when supplied with NH₄⁺ as compared with NO₃^?, particularly in the oxic treatment, and root length was most affected by O₂ deprivation for NO₃^? fed plants. Cell walls in the endodermis, the bundle sheath and the cortex adjacent to endodermis developed thickened sclerenchymatous walls when deprived of O₂, and more so in plants supplied with NO₃^?. Plants lost chlorophylls, had lower rates of photosynthetic electron transport (ETR_max) and lower quantum yields (Fv/Fm ratios) when grown in anoxic solutions, and the negative effects were mildest for NO₃^? fed plants suggesting that NO₃^? may be used as an alternative e^?-acceptor in non-cyclic electron transport in the chloroplasts. Overall S. natans grew best on NH₄⁺, but it also grew well on NO₃^?, and the O₂ stress symptoms differed somewhat between NH₄⁺ fed and NO₃^? fed plants. However, because N-form itself significantly influenced morphology and cell metabolism, it was impossible to conclusively identify the role of N-form for the O₂ stress reactions. S. natans is not well-adapted to grow in O₂ deficient waters and will not tolerate completely anoxic conditions as will prevail in waters receiving high loadings of organic pollutants such as livestock wastewater.