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 Boron was shown to be an immobile element in silver birch seedlings that was not withdrawn from senescing leaves prior to abscission.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.     

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.     

Soil water availability effects on seed germination account for species segregation in semiarid roadslopes

Previous studies report that the low colonisation success on eroded roadslopes of semiarid environments is controlled by microsite limitations. We predicted that soil water availability, through its effect on seed germination, is a determinant factor in the colonisation process of roadslopes in semiarid environments. Moreover, we predicted that the success of species establishment on the harshest roadslope conditions (i.e., south-facing roadcuts) is either due to the ability of seeds to germinate fast at low water potentials (colonising species) or to the ability of plants to sprout (resistant species). Specifically we present evidence for: (1) soil drying occurs faster on roadcuts than on roadfills after a rainfall event; (2) germination is a filtering process that influences the success of species establishment on roadslopes; (3) species able to colonise successfully south-facing roadcuts have higher germination rates and a shorter time to germination under water-stress conditions than species able to colonise successfully but exclusively the most favourable roadslopes (i.e., roadfills); (4) species that live on south-facing roadcuts and have the ability to sprout do not necessarily germinate with germinating rates and speeds as high as species that colonise successfully these slopes but are unable to sprout. To test these hypotheses we compared water dynamics in the soil among roadslope types and aspects as well as the seed ability to germinate at low water potentials among species showing different regeneration strategies and establishment success on roadslopes. Soil water availability after rainfalls occurring during the germination period played a major role in the germination of seeds. The patterns of seed germination under water-stress conditions were consistent with the success of colonising species on roadslopes and with the distribution of adult plants in the roadslopes 8 years after these latter were built. We discuss the usefulness of these results for the improvement of revegetation projects in semiarid areas by means of an appropriate selection of species adapted to the local environmental conditions. We suggest that the ability of species to germinate under water stress could be an indication of a species’ potential for success under semiarid conditions. Responsible Editor: John McPherson Cheeseman.     

Nutrient availability and limitation in traditionally mown and in abandoned limestone grasslands: a bioassay experiment

Management is necessary for the conservation of limestone grasslands. However, the traditional management of hay-making every year in July is no longer profitable for farmers. Hence many species-rich grasslands have become abandoned. The aim of this study is (a) to investigate the consequences of abandonment (as compared with annual mowing) on the availability of soil nutrients, and (b) to determine the type of nutrient limitation. The soil was taken from a long-term experimental site set up 22 years ago in northern Switzerland. The availability of soil N and P was assessed in a bioassay where soil from mown and unmown plots was supplied with different nutrient solutions or deionised water as control. Seedlings of Galium mollugo s.str. L. and Raphanus sativus ssp. oleiferus (DC) Metzg. were used as phytometer plants. Their growth in the control treatment was limited by N and P almost to the same extent, indicating that the availability of both elements was very low in this soil. P limited plant growth slightly more, but was overcome in the case of Raphanus by a high P content in the seeds. The N and P availability responded differently to management. The availability of N was slightly higher in soil from the abandoned plots, whereas the P availability did not differ significantly. Accumulation of nitrogen in the soil after abandonment did presumably not have any decisive effect on the vegetation because the amount was small and because the vegetation seemed primarily P-limited. The direct effect of mowing or abandonment on plants is therefore likely to be much more important for species composition than the minor changes in soil nutrients.     

Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration

Herbivores may influence the nitrogen (N) recycling rates and consequently increase or decrease the productivity of grasslands. Plant N concentration emerged as a critical parameter to explain herbivore effects from several conceptual models, which predict that herbivores decrease soil N availability when plant N concentration is low whereas they increase it when plant N concentration is high ( Hobbs 1996 , Ritchie et al. 1998 , Pastor et al. 2006 ). However, a broader cross-site comparison among published studies to test these predictions is hampered by the different methodologies used to measure soil N availability or a proxy thereof, and a lack of measurements of plant N concentration. Therefore it remains unclear whether these model predictions are generally valid across a range of grasslands.
We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6–8  year old exclosures (with an unfenced control) of vertebrate herbivores (>1  kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confirms the importance of plant N concentration as a predictor of herbivore effect on soil N availability across grasslands, but contradicts the models. A possible explanation may be that the results represent a transient situation as the exclosures were relatively young whereas the models may refer to an equilibrium state. Simultaneous measurements of both plant N concentration and herbivore effect on soil N availability from more grassland sites, preferably with contrasting plant N concentrations and including exclosures of different ages, should resolve the contrast between model predictions and our field measurements.     

Interactive effects of soil nitrogen and water availability on leaf mass loss in a temperate steppe

Although the link between leaf mass loss and assessment of ecosystem nutrient use efficiency and plant nutrient resorption efficiency has received considerable attention in various ecosystems, there has been relatively little effort to assess plant leaf mass loss during senescence, especially for herbaceous species. We conducted experimental studies to assess leaf mass loss during senescence in five dominant herbaceous species and examined the effects of increasing nitrogen (N) and water availability on leaf mass loss of four species in a temperate steppe in northern China. We nondestructively estimated mature leaf mass based on leaf length and width. Leaf mass loss varied substantially among species, ranging from 20–50%. On average across all species, N and water addition increased leaf mass loss by 30% and 19%, respectively. N and water addition interacted to affect leaf mass loss, as water addition had a significant positive effect on leaf mass loss under enriched N conditions but showed no effect under ambient N levels. We conclude that leaf mass loss of herbaceous plants was considerable and can potentially be more pronounced with increasing N and water availability. It is notable that the responses of plant species to N and water addition were variable. We suggest that leaf mass loss during senescence should be given full consideration in assessing nutrient use and resorption efficiency in semi-arid areas.     

Impact of hemiparasitic Rhinanthus angustifolius and R. minor on nitrogen availability in grasslands

Root hemiparasites like Rhinanthus angustifolius C.C. Gmel and R. minor L. have a potential to accelerate the restoration of semi-natural grasslands because they may decrease above-ground biomass of the vegetation. This, in turn, may be beneficial for species diversity. It is known that hemiparasites often accumulate high nutrient concentrations in their above-ground parts, resulting in high quality litter. Because of the short life cycle of many parasitic plants, litter is released early in the season and the main part is not removed from the grassland by hay-making. This has been shown to yield an increased nutrient availability locally. We performed an introduction experiment with R. angustifolius and R. minor in three semi-natural grasslands in Flanders (Belgium). In the second year after sowing, the above-ground nitrogen (N) content of the grasses and of the potential host vegetation (excluding the hemiparasite), was increased in the parasitized plots. The reduction of grass (and legume) above-ground biomass in parasitized plots resulted in a decrease in the total above-ground N uptake of grasses, host and total vegetation (ex- and including the parasite, respectively) of the parasitized plots compared to the control. Furthermore, with a tracer experiment (¹⁵N), we demonstrated that the N from the added tracer was relatively less available in parasitized plots, suggesting larger soil N pools in these treatments. This is probably the consequence of increased mineralization, resulting from the high-quality, parasitic litter. Further experiments should be conducted to investigate the impact of hemiparasitic Rhinanthus spp., e.g. on the availability of other nutrients such as phosphorus.     

Interspecific effects on plant size inequality: evidence from a temperate savanna community

The influences of intraspecific competition on plant size inequality have been well documented, but interspecific effects on this topic remain little understood. Here we examined the effects of canopy shading and fine roots of the trees (Elaeagnus angustifolia) on size inequality of the grasses (Achnatherum splendens) in a temperate savanna community in northwest China. Three study plots of 400 m² were divided into 4-m² quadrats, within each of which (1) canopy shading was quantified by modeling cumulative direct solar radiation (CDSR) and (2) the root effect was quantified using an empirical relationship between tree fine root density (TFRD) and relative distance to tree bases. Morphological traits were measured to represent grass size. Redundancy analysis (RDA) was conducted to examine the relative influences of grass density, CDSR and TFRD on the coefficient of variation of grass size. Results showed that no significant correlation occurred between grass density and grass size inequality. Both CDSR and TFRD had significant negative correlations with grass size inequality, suggesting that canopy shading and the presence of fine roots of trees can, respectively, increase and reduce grass size inequality. Canopy shading and TFRD played competitive roles in determining grass size inequality, where the root effect was a stronger factor than canopy shading. The tree effects can substantially alter the intensity of water stress. In response, size inequality of the grasses could be influenced through size-specific growth/mortality and slowed size divergence. These mechanisms could operate together in the savanna community.     

氮沉降对草地土壤及团聚体元素有效性的影响

深入理解土壤及团聚体元素有效性对氮沉降的响应机制,是研究全球变化背景下土壤养分供应及生态系统结构和功能的关键.本研究综合评述了草地生态系统土壤表土及团聚体内元素分布及其对氮沉降的响应机制.总体而言,草地表土内碳、氮、磷、硫有效性研究较多,且研究结果因氮添加形态、添加时间及生态系统类型而异.氮沉降通过改变碳、氮、磷、硫等生源要素的转化过程及其在土壤团聚体内的再分配,而影响这些元素的生物有效性.然而,氮沉降影响草地土壤交换性盐基及有效态微量元素的研究较少.氮沉降促进土壤酸化,导致各团聚体内钙、镁差异性流失,其中大粒径团聚体内盐基元素更易流失;酸化还有助于提高团聚体内铁、锰、铜、锌有效性.土壤小粒径团聚体内的养分对外界环境变化响应不敏感.当前研究的不足之处在于,较少关注氮沉降对土壤团聚作用及团聚体元素有效性的影响.今后应加强团聚体元素有效性与土壤酶活性耦合变化关系的研究,并分析氮沉降背景下土壤物理结构和化学组成的变化对植物群落的反馈作用.     

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

Global nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.     

半干旱地区不同地膜覆盖时期对土壤氮素有效性的影响

在年降水量415mm的半干旱地区黄绵土上,以春小麦为供试作物进行大田试验,研究地膜覆盖(包括不覆膜、播种后覆膜30d、覆膜60d和全程覆膜)对土壤水分、温度和氮素有效性的影响.结果表明,覆膜对2m土层贮水量基本没有影响,能显著提高0～20cm土层的含水量;覆膜对5cm处土壤温度的影响呈"U”型变化,即在作物生长前期和后期影响显著,中期影响较小;覆膜能增加土壤呼吸和有机氮的矿化,因而显著影响土壤剖面中NO-3-N的累积表现为在收获时覆膜30d和60d处理剖面中NO-3-N的累积明显下降,而全程覆膜处理剖面中NO-3-N的累积显著增加,在覆膜的基础上施用氮肥,这种作用会更加突出.显然全程覆膜易造成有机质的大量矿化和NO-3-N的淋溶损失.     

黄土高原半干旱草地封育后土壤碳氮矿化特征

土壤有机碳和全氮的分布与矿化是退化草地封育后土壤生态效应研究的重要内容和指标。结合野外调查和室内培养实验,研究了半干旱黄土区不同封育年限草地土壤有机碳和全氮的含量变化及其矿化特征。结果表明,封育对半干旱黄土区退化草地土壤有机碳和全氮的影响主要体现在0-40 cm土层封育超过17a后,封育年限的影响逐渐减弱。封育显著增加了土壤有机碳矿化速率和C_(min)/C_0封育对有机碳矿化速率的影响与封育年限和土层深度无关,而对C_(min)/C_0的影响则与封育年限和土层深度有关。封育显著提高了0-40 cm土层土壤氮素矿化速率,但是降低了40-80 cm土层土壤氮矿化速率,并且降低了080 cm土层N_(min)/N_0。碳氮矿化速率与有机碳和全氮之间显著相关,而与碳氮比之间的相关性较小。这些结果表明,退化草地封育后土壤碳氮元素的转化主要受土层深度、封育年限以及土壤碳氮含量的影响。     

Leaf water potential-leaf water deficit relationship for ten species of a semiarid grassland community

The relationship between water potential and relative water content (water content in percentage of full hydration) is a characteristic of plant tissues, that may vary with environmental conditions. It is used here to compare leaf water relations of ten species coexisting in a semiarid grassland community (Festucetum vaginatae danubiale) in Hungary. Three groups of species can be distinguished. In two of these leaf water potential changes only moderately with decreasing leaf water content. These are either short-lived, drought escaping spring plants relying on seasonally favourable water supply (group 1) or xerophytes with very deep root system having access to permanent water resources (group 2, only one species studied here). Xerophytes with moderately deep roots (group 3) display a rapid drop of leaf water potential with increasing leaf water deficit. This generates a steep water potential gradient in the soil-plant continuum that in turn enhances water uptake by roots. There is a positive correlation between the rate of water potential decline and degree of sclerophylly (proportion of dry material in the water-saturated leaf), and both variables show seasonal change in perennial species.     

A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen

We investigated the effect of climate change on Poa secunda Presl. and soils in a shrub‐steppe ecosystem in south‐eastern Washington. Intact soil cores containing P. secunda were reciprocally transplanted between two elevations. Plants and soils were examined, respectively, 4.5 and 5 years later. The lower elevation (310 m) site is warmer (28.5 °C air average monthly maximum) and drier (224 mm yr^?1) than the upper elevation (844 m) site (23.5 °C air average monthly maximum, 272 mm yr^?1). Observations were also made on undisturbed plants at both sites. There was no effect of climate change on plant density, shoot biomass, or carbon isotope discrimination in either transplanted plant population. The cooler, wetter environment significantly reduced percent cover and leaf length, while the warmer, drier environment had no effect. Warming and drying reduced percent shoot nitrogen, while the cooler, wetter environment had no effect. Culm density was zero for the lower elevation plants transplanted to the upper site and was 10.3 culms m^?2 at the lower site. There was no effect of warming and drying on the culm density of the upper elevation plants. Culm density of in situ lower elevation plants was greater than that of the in situ upper elevation plants. Warming and drying reduced total soil carbon 32% and total soil nitrogen 40%. The cooler, wetter environment had no effect on total soil C or N. Of the C and N that was lost over time, 64% of both came from the particulate organic matter fraction (POM, > 53 µ m). There was no effect of warming and drying on the upper population of P. secunda while exposing the lower population to the cooler, wetter environment reduced reproductive effort and percent cover. With the warmer and drier conditions that may develop with climate change, total C and N of semiarid soils may decrease with the active fraction of soil C also rapidly decreasing, which may alter ecosystem diversity and function.     

Irrigation and enhanced soil carbon input effects on below-ground carbon cycling in semiarid temperate grasslands

Global climate change is generally expected to increase net primary production, resulting in increased soil carbon (C) inputs. To gain an understanding of how such increased soil C inputs would affect C cycling in the vast grasslands of northern China, we conducted a field experiment in which the responses of plant and microbial biomass and respiration were studied. Our experiment included the below-ground addition of particulate organic matter (POM) at rates equivalent to 0, 60, 120 and 240 g C m(-2), under either natural precipitation or under enhanced precipitation during the summer period (as predicted for that region in recent simulations using general circulation models). We observed that addition of POM had a large effect on soil microbial biomass and activity and that a major part of the added C was rapidly lost from the system. This suggests that microbial activity in the vast temperate grassland ecosystems of northern China is energy-limited. Moreover, POM addition (and the associated nutrient release) affected plant growth much more than the additional water input. Although we performed no direct fertilization experiments, the response of plant productivity to POM addition (and associated release of nutrients) leads us to believe that plant productivity in the semiarid grassland ecosystems of northern China is primarily limited by nutrients and not by water.     

Response of a Mediterranean semiarid community to changing patterns of water supply

Climate change may cause profound effects on terrestrial ecosystems. Changes in rainfall patterns may have large effects on a wide range of biological processes such as seed germination, seedling establishment, plant growth, community composition, and population and community dynamics. Climate change models for the Mediterranean region forecast reduced annual precipitation and more extreme rain events (i.e., fewer rainy days and longer drought periods between rainfall events), along with seasonal changes. We experimentally addressed the response of a semiarid Mediterranean community to higher aridity and changes in seasonal rainfall patterns in two glasshouse experiments in which we manipulated water supply. We simulated a delay in the onset of autumn rainfalls (i.e., a longer summer drought period), decreased watering amount and frequency as predicted by climate models, and manipulated the seasonality of water supply. We found that delayed watering led to decreases in plant community productivity and to delays in flowering time, in terms of both date and number of days of water supply. Decreased watering amount and frequency, and accentuated seasonality, also diminished biomass and individuals recruited, but did not change flower phenology. Species diversity was not affected by watering delays; however, it was reduced by changes in frequency, amount and seasonality. Overall, these data underline the need to consider rainfall patterns as an important element that might alter community dynamics and ecosystem structure and functioning. Therefore, the analysis of climate change consequences must not depend on climatic means-based scenarios but must take into account expected seasonal changes in rainfall quantity and frequency.     

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.     

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.