Fertilized graminoids intensify negative drought effects on grassland productivity.

Present study was aimed at examining the effects of spatial scale, plant identity and management on the relationship between diversity and productivity in an old semi-natural grassland in the Solling uplands, Germany. The study was conducted in the framework of the Grassland Management (GrassMan) experiment which is a part of the Excellence cluster „Functional Biodivesity Research“ at the University of Goettingen. The experimental field is a Lolio-cynosuretum semi-natural permanent grassland with more than a hundred-year old history of extensive agricultural use. The three experimental factors (sward composition, fertilization and cutting frequency) results in 12 different treatments and are set in Latin Rectangle, comprising 6 replications of each treatment. Experimental approach that we used, the so called „removal experiment“, is aimed at studying the effects of removal itself and recovery of the vegetation after disturbance, as well as the different aspects of ecosystem functioning
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\nIn the first chapter we investigate the effects of sampling scale on the relationship between species diversity and productivity. So far, many observational studies, conducted in semi-natural grasslands, explored the relationship between species diversity and productivity at the common size of vegetation surveys of 1 m² or larger, selected according to the species minimum areal. Experimental studies, on the other hand, referred to the small-scale effects of diversity and productivity relationship which often caused the problem of extrapolating and generalizing of their results to more natural plant communities. We studied the effects of spatial scale on the biomass production and diversity relationship by selecting four spatial scales: small (0.04 m² and 0.16 m²), medium (1 m²), large (9 m²), and very large (225 m²) and comparing the power of this relationship, including the effects of agricultural management. We found that the relationship between diversity and productivity of a semi-natural grassland differed across the scales of sampling and that harvesting of the biomass at small spatial scales did not always fully reflect the relationship between the two variables (which often turned into insignificant at larger spatial scales). The most common size of plots for vegetation surveys, being 1 m², in this study showed high variation, both in vegetation composition and harvested biomass. Management system established at the field seemed to play a role in the direction of this relationship (positive or negative). So, plots cut three times a year, becoming more homogeneous (more even) in vegetation composition showed a positive relationship between diversity and productivity. We suggest that selecting an appropriate spatial scale is therefore very important in heterogeneous natural grasslands, also those agriculturally managed. While in more homogeneous environments rather small spatial scale is adequate for describing the composition and many aspects of ecosystem functioning, in more heterogeneous habitats it is important to include this parameter in the analysis.
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\nIn the second chapter we present the results of a study on the effects of management intensification in a permanent grassland and the response of overall and dominant species diversity. A removal experiment in the Solling uplands, Germany (three sward types: control, dicot-reduced and monocot-reduced) employed four different levels of management intensity resulting from a combination of two factors: fertilization (no and 180-30-100 kg ha -1 year -1 of N-P-K, respectively) and cutting frequency (cut once and three times a year). This study was conducted over two years (2010, 2011), starting with a third year after introducing the management treatments. We defined species diversity by species number per plot as well as evenness and identified dominant species, making up about 80% share of the yield. We collected information on several plant functional traits for each of the dominant species: plant height, leaf dry matter content, stem dry matter content, leaf specific area, green leaves / total leaves ratio, stem specific density, and calculated additionally the ratio of stem specific density and plant height. Further measures of functional diversity included functional group shares, functional diversity index, defined as the total branch length of the traits-species cluster diagram, and aggregated plant functional traits for each plot. We found that management intensification did not affect the total species number, but affected species evenness and functional diversity of dominant species, including their number and identity. Correlations of above-ground biomass and several dominant species’ traits were responsible for fertilization effects on above-ground productivity in this grassland. This indicates the importance of monitoring not only species richness but also other measures of diversity, as well as including management aspects in studies of plant functional traits in grasslands. 
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\nIn the third chapter of the thesis we present the results from the whole investigation period and summarize the findings of the GrassMan experiment regarding the relationship between species richness and productivity, as well as the changes in species number over time and the main determinants of productivity. We analyzed the overall effects of species diversity expressed in species number, functional group composition and species identity effects on the above-ground biomass production. We found that the effects of species richness on the productivity were rather weak while the functional group diversity was a better predictor of productivity in some years. Intensifying the management, however, caused higher above-ground biomass production. It also affected species composition and evenness: increasing cutting frequency increased the evenness while increasing fertilization decreased it. We suggest that functional group richness might be important for better use of available resources. We conclude that existing species composition under appropriate agricultural management seems to have a potential for sustainable forage production without significant species losses, when not used and fertilized too intensively, and without the need of being converted to arable land or manipulating the species composition. The changes in species diversity should, however, be monitored, including not only species number but also other parameters, such as vegetation composition and functional group shares. 
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\nWe finally discuss that our findings do not necessarily support the evidence from experimental studies on sown grasslands which often found that species richness had a defining role in biomass production. While overall species richness was of relatively less importance than management in this grassland, species composition was changing beyond just the number of species. We thus underline the importance of bringing biodiversity experiments to the „real-world“ ecosystems and suggest that thorough consideration of spatial aspects of the diversity-productivity relationship, as well as incorporating multiple measures of diversity in the experiments, conducted in agricultural grasslands under appropriate management strategies, might give better insights in their functioning and serve as motivation for farmers to conserve existing species diversity. Apart from the number of important ecosystem functions, providing fodder for herbivores and ruminants, conserving natural vegetation composition contributes to delivering further ecosystem services, which could support cultural and biodiversity benefits of the agricultural landscapes.

Species of the tree genus Pine (Pinus L.) exist all over the world and no other group contains so many attractive forms (Curtis & Bausor, 1943) . Scots pine (Pinus sylvestris L.) is currently the most widely distributed pine and occurs throughout all of Eurasia. In the central alpine valleys, Scots pine is growing at the dry border of its distribution range, which involves overcoming periods with extreme low water availability. Although the species is known for its ability to grow on dry and nutrient poor soils, several extreme droughts during the last two decades have caused a 50% dieback of Scots pine in the dry valleys of the Central Alps in Switzerland. The ability of trees to survive drought is determined by their initial health and their resilience to drought, as well as on the characteristics of a drought event – i.e. timing, duration and intensity. The mechanisms underlying drought-induced mortality are still unclear, as well as the recovery process after soil rewetting. Furthermore, possible mitigation or aggravation of drought effects by elevated nutrient availability in the soil has not been studied before. The carbon (C) balance in trees is used as an indicator for C assimilation, growth, defense and storage processes. When trees are exposed to drought, to changes in soil nutrition or sudden defoliation, the C balance may change. In this thesis, the main objective was thus to combine effects of drought and fertilization to study the C and nitrogen (N) dynamics in Scots pine trees.
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\nIn the first chapter, I give an overview of the state-of-the-art in research on drought-affected C and N dynamics in trees. The aim of the second chapter was to assess the effects of long-term drought release on growth and non-structural carbohydrate (NSC) concentrations of adult P. sylvestris trees. A long-term (13 years) irrigation experiment was conducted in the Pfynwald, a Scots pine dominated forest located at the dry distribution margin of the species in southern Switzerland. I measured growth, NSC, N and phosphorus (P) concentrations, as well as the natural abundance of 13C isotopes on trees with different leaf area in control and irrigation plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with lower leaf area in both control and irrigated trees, but NSC did not correlate with leaf-level gas exchange indices such as foliar δ13C, which is an indicator for water use efficiency, N or P, which are both stimulants of photosynthesis. Trees with initially low leaf area had limited ability to respond to the long-term irrigation, indicating a legacy effect of previously low crown condition. The NSC constancy across treatments suggests that carbohydrate storage may stay constant when changes in climate are slow enough to allow acclimation. Moreover, total leaf area, rather than leaf gas exchange per unit leaf area, drives variation in whole-tree carbohydrate dynamics in this system.
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\nThe main focus of the third chapter was the mitigation or aggravation of drought effects by nutrient availability in the soil. Three year-old P. sylvestris saplings were exposed to drought during two subsequent years, using four different water and two soil nutrient regimes, and drought was released thereafter. In addition, partial and full needle removal was performed in order to assess effects of changes in source:sink ratio. Biomass, leaf gas exchange and tissue NSC were measured during and after the first and second growing season. Extreme drought reduced stomatal conductance, photosynthesis, biomass and NSC, whereas intermediate drought only slightly affected biomass and NSC. Defoliation stimulated photosynthesis and fertilization increased growth and root biomass fraction, but mainly in the two intermediate drought levels. Only extreme drought pushed P. sylvestris trees to mortality. The third chapter concludes that tree mortality under severe drought periods will not be mitigated, but that the effects of low intensity drought stress could be compensated by increased nutrient availability and decreased source:sink ratio. 
\nThe aim of the fourth chapter was to assess the C and N allocation underlying the biomass changes that were found in chapter 3. I hypothesized that, during drought, increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. I therefore conducted a 15N and 13C labelling experiment in July and August 2016 respectively, on the saplings described above. 15N labelling was conducted with fertilized saplings from all water regimes, while 13C labelling was only conducted with saplings (both nutrient regimes) from two out of four water regimes (well-watered and mild drought). I assessed the abundance of 15N and 13C in the roots, stem and needles after the first growing season and during the second year. C uptake was slightly lower in drought stressed trees, and extreme drought inhibited largely the N uptake and transport. Carbon allocation to belowground tissues was decreased under drought, but not in combination with fertilization. The results indicate a potential positive feedback loop, where fertilization improved the metabolism and functioning of the roots, stimulating source activity and hence C allocation to belowground tissues. We can thus conclude that soil nutrients might play an important role in mitigating drought stress of trees.
\nOverall this thesis shows that the impairment of tree functioning and mortality can be explained with thresholds: long-term drought causes a reduction in tree vigor and leaf area, and if a threshold of approximately 60 – 70% loss of leaf area is reached, trees may follow a trajectory towards mortality, even if drought is released in the soil. In the controlled experiment, soil moisture thresholds were visualized. The impairment of C allocation belowground under mild drought, the reduction of NSC in and impairment of 15N uptake by the roots under extreme drought indicate that roots might be the first tissue to lose function and eventually die off during drought stress. Additional nutrient supply can sustain root functioning under drought, indicating that soil moisture tipping points are not fixed, but can be modified. In general, trees have a strongly coordinated supply – demand regulation for C and N, enabling homeostatic C balances as long as changes in climate are slow or mild enough for trees to acclimate. 
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Plants are frequently exposed to a variety of stress conditions such as drought, salinity, heavy metal toxicity, low temperature, flooding, extremes of soil pH and heat which causes reduction of plant growth and productivity. Such abiotic stresses may cause metabolic impairment, nutrient imbalance, reduced synthesis of photosynthetic pigments which are closely related with biomass production in plant, thus, causing serious loss in crop productivity. The present experiment was undertaken to study the biochemical and physiological effects of salinity, drought and heavy metal (copper and lead) stress on seed germination in ricebean [Vigna umbellata (Thunb.) Ohwi and Ohashi] variety Bidhan 1. For studying the effect of iso-osmotic potential of salinity and drought stress, the solutions of NaCl and PEG 6000 with − 0.2, − 0.4 and − 0.8 MPa osmotic potential were used whereas the solutions of 50, 100 and 200 µM Cu and Pb supplemented in the form of CuSO4.·5H2O and Pb(NO3)2 respectively were used to study the effects of equimolar concentrations of copper and lead. Drought was found to produce more adverse effects on germination %, as well as speed of germination, in the seeds of ricebean. The seed protein content was significantly higher under all the treatments of salinity, drought stress as well as metal stress. The highest intensity of copper stress was found to produce more adverse effects than lead in respect of water uptake % in germinating seeds and root elongation rates of ricebean seeds in the present experiment. The presence of copper in the germinating medium produced more detrimental effects on activities of antioxidative enzyme ascorbate peroxidase and guaiacol peroxidase than equimolar concentrations of lead.

In grasslands, climate change has the potential to disrupt a range of ecosystem services, including agricultural production, carbon (C) storage and nutrient cycling. In particular, climate change is likely to increase the frequency and severity of extreme climate events, such as drought and the subsequent rewetting event. Yet the effect of drought events will not be consistent across grassland communities, instead likely varying with grassland properties. One such property may be the level of nutrient availability, which brings about changes in plant productivity, plant community composition, and soil microbial composition and function. In this thesis, the effect of reduced precipitation on C cycling in UK species-rich grasslands is investigated in two field experiments, with varying long-term grassland restoration treatments and short-term nutrient addition, and a glasshouse experiment with reduced soil moisture. It was hypothesised that changes in plant and soil microbial communities, brought about by differences in nutrient availability, would modulate above and belowground C cycling responses to drought. This thesis found that the level of nutrient availability was important for modulating how C is cycled in response to drought in plants, soil microbial communities and whole ecosystem CO2 fluxes. For plants, the effect of drought and nutrient availability differed between functional groups, species and due to intraspecific trait variation. For soil microbial communities, the effect of drought on carbon use efficiency was modulated by short-term nutrient addition. Increased nutrient availability and drought therefore interact to determine how C is cycled and stored in plants and soil microbial communities, revealing the importance of agricultural practices in modulating whole community responses to climate change. Overall, this thesis shows the mechanisms by which drought may alter C cycling and its potential feedbacks to climate are complex, but at least in part, depend on the level of nutrient availability.

Contrasting effects of nitrogen and phosphorus additions on nitrogen competition between coniferous and broadleaf seedlings

Plant growth, a fundamental biological process that underpins terrestrial ecosystem function, is susceptible to nutrient availability. Despite extensive research on lowland ecosystems, the responses of alpine plant growth to nutrient addition remain poorly understood, particularly given the heightened sensitivity of alpine ecosystems to global change. To investigate the effects of nitrogen (N) and phosphorus (P) additions on the growth rates of alpine plants and the underlying mechanisms of how these nutrient additions influence plant growth rates, we conducted an experiment in an alpine grassland on the Qinghai-Tibet Plateau, targeting 14 common plant species. Growth rates were measured using biomass and height, with plant height and soil physicochemical properties recorded biweekly during the growing season. We assessed the effects of nitrogen and phosphorus additions on growth rates, their seasonal dynamics, and their relationships with soil physicochemical properties. Results showed that phosphorus addition and combined nitrogen-phosphorus additions significantly increased the relative growth rate based on height (RGRH). In contrast, nutrient additions had no significant effect on the relative growth rate based on biomass (RGRB). RGRH decreased from June and early July to August, exhibiting species-specific responses to nutrient additions. Additionally, RGRH was significantly influenced by the interaction of nitrogen and phosphorus additions, species, and seasonal dynamics (p < 0.05). Soil available N, available P, and moisture were significantly positively correlated with RGRH (p < 0.05), while soil temperature (ST), total nitrogen (TN), and soil organic carbon (SOC) exhibited significant negative correlations (p < 0.05). Nutrient additions altered the hierarchy, as well as the direct and indirect factors that influence RGRH, revealing the opposing regulatory effects of total and available nitrogen. These findings highlight the critical roles of nitrogen and phosphorus, suggesting phosphorus is a potential limiting factor for plant growth in this alpine region. This study offers a comprehensive analysis of how nitrogen and phosphorus additions affect alpine plant growth rates and clarifies the underlying mechanisms in these sensitive ecosystems.

Nutrient addition alters drought resistance and resilience via forb abundance in a temperate grassland

Anthropogenic global change alters the activity and functional composition of soil communities that are responsible for crucial ecosystem functions and services. Two of the most pervasive global change drivers are drought and nutrient enrichment. However, the responses of soil organisms to interacting global change drivers remain widely unknown. We tested the interactive effects of extreme drought and fertilization on soil biota ranging from microbes to invertebrates across seasons. We expected drought to reduce the activity of soil organisms and fertilization to induce positive bottom-up effects via increased plant productivity. Furthermore, we hypothesized fertilization to reinforce drought effects through enhanced plant growth, resulting in even drier soil conditions. Our results revealed that drought had detrimental effects on soil invertebrate feeding activity and simplified nematode community structure, whereas soil microbial activity and biomass were unaffected. Microbial biomass increased in response to fertilization, whereas invertebrate feeding activity substantially declined. Notably, these effects were consistent across seasons. The dissimilar responses suggest that soil biota differ vastly in their vulnerability to global change drivers. Thus, important ecosystem processes like decomposition and nutrient cycling, which are driven by the interdependent activity of soil microorganisms and invertebrates, may be disrupted under future conditions.

In the present study, four provenances were studied and (i) soil parameters by localities, (ii) Morphology of fruits, seeds and leaves, (iii) Germination percentage under temperature and osmotic potential, (iv) Growth and biomass production under drought stress, (v) proline and total soluble sugars contents under drought stress were determined. The germination responses of seeds were determined over a wide range of constant temperatures (15 to 50 °C) and polyethylene glycol (PEG6000) solutions of different osmotic potentials (0 to -1,2 MPa). The effect of drought on several physiological parameters are studied by submitted plants to water defcit (10 and 20 days of withheld irrigation). Morphological study indicating a large-scale diversity among the provenances. Germination was inhibited by either an increase or decrease in temperature from the most suitable temperature found (40 °C). The highest germination percentages (95%) were obtained under control conditions without PEG, and increasing moisture stress progressively inhibited seed germination, which was less than 18% at -1.2 MPa. The results showed that growth parameters and biomass production in leaves, stem and roots were not impacted under medium drought stress. The applied drought stresses increased the proline and soluble sugars contents of the plant. Cultivating Ziziphus spina-christi under drought or salt stresses could be a good alternative to valorize antioxidant molecules from this species in industrial applications.

Climate change scenarios forecast increased aridity in large areas worldwide with potentially important effects on nutrient availability and plant growth. Plant nitrogen and phosphorus concentrations (plant [N] and [P]) have been used to assess nutrient limitation, but a comprehensive understanding of drought stress on plant [N] and [P] remains elusive. We conducted a meta-analysis to examine responses of plant [N] and [P] to drought manipulation treatments and duration of drought stress. Drought stress showed negative effects on plant [N] (-3.73%) and plant [P] (-9.18%), and a positive effect on plant N:P (+ 6.98%). Drought stress had stronger negative effects on plant [N] and [P] in the short term (< 90 d) than in the long term (> 90 d). Drought treatments that included drying-rewetting cycles showed no effect on plant [N] and [P], while constant, prolonged, or intermittent drought stress had a negative effect on plant [P]. Our results suggest that negative effects on plant [N] and [P] are alleviated with extended duration of drought treatments and with drying-rewetting cycles. Availability of water, rather than of N and P, may be the main driver for reduced plant growth with increased long-term drought stress.

Background The decline in soil organic carbon accumulation caused by intensified nitrogen deposition is concerning. Although phosphorus input may alleviate the negative impacts, there is still a research gap regarding the mechanisms, particularly those involving the soil biota, that drive the stability of soil organic carbon. Methods We conducted a 2-year nitrogen (0, 30 and 90 kg N ha – 1 yr – 1 ) and phosphorus (0, 30 kg P ha – 1 yr – 1 ) addition experiment with six treatments in a 25-year-old Pinus massoniana plantation in subtropical China. Results The addition of external nutrients improved soil nutrient availability but led to a decrease in pH. Low nitrogen input promoted the particulate organic carbon (POC) and total organic carbon, whereas high nitrogen input had the opposite effect. Phosphorus addition alleviated these negative impacts to some extent. Nitrogen and phosphorus addition significantly affected the dissimilarity of soil biological communities. Nitrogen treatments generally reduced the alpha diversity index of soil bacteria, while the trend for fungi was the opposite. Arthropods showed a rise followed by a decline, with phosphorus addition weakening these effects. Soil respiration decreased with increasing nitrogen addition, and phosphorus addition didn’t alter this trend. The POC was primarily influenced by the soil environment-microorganism-respiration and environment-microorganism pathways, whereas the mineral-associated organic carbon (MAOC) was mainly influenced by the soil environment-arthropod pathway. POC (Path coefficient, pc = 0.524) and MAOC (pc = 0.237) directly determine the accumulation of organic carbon. This conceptual model explained 59.4% of the variation in total organic carbon (Goodness-of-fit, GOF = 0.594), thereby delineating the integrated mechanisms underlying SOC accumulation. Conclusions Excessive nitrogen input was unfavorable for organic carbon accumulation, while phosphorus addition partially mitigated the negative effects of nitrogen excess. Under this context, active organic carbon was significantly influenced by soil microorganisms and soil respiration, whereas stable organic carbon was primarily affected by soil arthropods. Graphical Abstract

Effects of experimental nitrogen and/or phosphorus additions on soil nematode communities in a secondary tropical forest

Soil nutrients and lowered source:sink ratio mitigate effects of mild but not of extreme drought in trees

Drought and salinity are the major abiotic stress factors negatively affecting the morphophysiological, biochemical, and anatomical characteristics of numerous plant species worldwide. The detrimental effects of these environmental factors can be seen in leaf and stem anatomical structures including the decrease in thickness of cell walls, palisade and spongy tissue, phloem and xylem tissue. Also, the disintegration of grana staking, and an increase in the size of mitochondria were observed under salinity and drought conditions. Drought and salt stresses can significantly decrease plant height, number of leaves and branches, leaf area, fresh and dry weight, or plant relative water content (RWC%) and concentration of photosynthetic pigments. On the other hand, stress-induced lipid peroxidation and malondialdehyde (MDA) production, electrolyte leakage (EL%), and production of reactive oxygen species (ROS) can increase under salinity and drought conditions. Antioxidant defense systems such as catalase, peroxidase, glutathione reductase, ascorbic acid, and gamma-aminobutyric acid are essential components under drought and salt stresses to protect the plant organelles from oxidative damage caused by ROS. The application of safe and eco-friendly treatments is a very important strategy to overcome the adverse effects of drought and salinity on the growth characteristics and yield of plants. It is shown that treatments with plant growth-promoting bacteria (PGPB) can improve morphoanatomical characteristics under salinity and drought stress. It is also shown that yeast extract, mannitol, proline, melatonin, silicon, chitosan, α-Tocopherols (vitamin E), and biochar alleviate the negative effects of drought and salinity stresses through the ROS scavenging resulting in the improvement of plant attributes and yield of the stressed plants. This review discusses the role of safety and eco-friendly treatments in alleviating the harmful effects of salinity and drought associated with the improvement of the anatomical, morphophysiological, and biochemical features in plants.

Aims Nutrient resorption is a crucial component of plant nutrient use strategy, yet the controls on the responses of community-level nutrient resorption to altered nutrient availability remain unclear. Here, we addressed two questions: (1) Did leaf and stem nutrient resorption respond consistently to increased nutrient availability? (2) Was community-level plant nutrient resorption response after nutrient enrichment driven by the intraspecific plasticity in plant nutrient resorption or by altered species composition? Methods We investigated the changes in aboveground biomass, and leaf and stem nutrient resorption of individual species after 3-year nitrogen (N) and phosphorus (P) additions, and assessed community-level nutrient resorption response to 3-year nutrient additions in a graminoid-dominated temperate wetland, Northeast China. Important Findings For both leaves and stems, N and P additions did not affect nutrient resorption efficiency, but they decreased respective nutrient resorption proficiency. Similarly, community-level N and P resorption proficiency declined with respective nutrient addition. Community-level N and P resorption efficiency was reduced by N addition primarily due to altered community composition and declined leaf:stem ratio. These results suggest that leaf and stem nutrient resorption processes exhibit consistent responses to increasing nutrient availability in the temperate wetland. These findings highlight the importance of altered species composition and biomass allocation between leaf and stem in driving community-level nutrient resorption response to nutrient enrichment.