Abstract: Biochar, an environmentally friendly soil conditioner, is produced using several thermochemical processes. It has unique characteristics like high surface area, porosity, and surface charges. This paper reviews the fertilizer value of biochar, and its effects on soil properties, and nutrient use efficiency of crops. Biochar serves as an important source of plant nutrients, especially nitrogen in biochar produced from manures and wastes at low temperature (≤ 400 °C). The phosphorus, potassium, and other nutrient contents are higher in manure/waste biochars than those in crop residues and woody biochars. The nutrient contents and pH of biochar are positively correlated with pyrolysis temperature, except for nitrogen content. Biochar improves the nutrient retention capacity of soil, which depends on porosity and surface charge of biochar. Biochar increases nitrogen retention in soil by reducing leaching and gaseous loss, and also increases phosphorus availability by decreasing the leaching process in soil. However, for potassium and other nutrients, biochar shows inconsistent (positive and negative) impacts on soil. After addition of biochar, porosity, aggregate stability, and amount of water held in soil increase and bulk density decreases. Mostly, biochar increases soil pH and, thus, influences nutrient availability for plants. Biochar also alters soil biological properties by increasing microbial populations, enzyme activity, soil respiration, and microbial biomass. Finally, nutrient use efficiency and nutrient uptake improve with the application of biochar to soil. Thus, biochar can be a potential nutrient reservoir for plants and a good amendment to improve soil properties.

Abstract: Numerous studies have demonstrated that fertilization with nutrients such as nitrogen, phosphorus, and potassium increases plant productivity in both natural and managed ecosystems, demonstrating that primary productivity is nutrient limited in most terrestrial ecosystems. In contrast, it has been demonstrated that heterotrophic microbial communities in soil are primarily limited by organic carbon or energy. While this concept of contrasting limitations, that is, microbial carbon and plant nutrient limitation, is based on strong evidence that we review in this paper, it is often ignored in discussions of ecosystem response to global environment changes. The plant-centric perspective has equated plant nutrient limitations with those of whole ecosystems, thereby ignoring the important role of the heterotrophs responsible for soil decomposition in driving ecosystem carbon storage. To truly integrate carbon and nutrient cycles in ecosystem science, we must account for the fact that while plant productivity may be nutrient limited, the secondary productivity by heterotrophic communities is inherently carbon limited. Ecosystem carbon cycling integrates the independent physiological responses of its individual components, as well as tightly coupled exchanges between autotrophs and heterotrophs. To the extent that the interacting autotrophic and heterotrophic processes are controlled by organisms that are limited by nutrient versus carbon accessibility, respectively, we propose that ecosystems by definition cannot be 'limited' by nutrients or carbon alone. Here, we outline how models aimed at predicting non-steady state ecosystem responses over time can benefit from dissecting ecosystems into the organismal components and their inherent limitations to better represent plant-microbe interactions in coupled carbon and nutrient models.

Abstract: Biofertilizers consists of the microorganisms bringing about the improvement of the nutrients of the soil enhancing their accessibility to the crops. Plant nutrients form the most vital components of the sustainable agriculture. Producing healthy crops for the fulfillment of the demands of the world's growing population is completely dependent upon kind of the fertilizers being used to provide the plants with all the major nutrients but more dependability on the chemical fertilizers is destroying the environmental ecology and negatively influencing the health of humans. Thus, using microbes as bioinoculants is believed to be the best substitute of chemical fertilizers as eco-friendly manner for plant growth and soil fertility. These microbes are known to be the potent tool to provide substantial benefits to crops for sustainable agriculture. The beneficial microbes colonize the plant (epiphytic, endophytic and rhizospheric) systems of crops and plays significant role in nutrient uptake from surrounding ecosystems of plants. The plant associates microbes have ability to promote growth of plant under the natural as well as extreme conditions. These plant growth promoting microbes enhance the plant growth by various direct and indirect plant growth promoting mechanisms such as biological nitrogen fixation, the production of various plant growth hormones, siderophores, HCN, various hydrolytic enzymes and solubilization of potassium, zinc, and phosphorus. Extensive work on the biofertilizers has been done and even available which clearly reveals that these microbes possess the potential of providing the vital nutrients to the crops in adequate quantities for the enrichment of yield of the crops without disturbing the environment.

Abstract: As primary producers, plants rely on a large aboveground surface area to collect carbon dioxide and sunlight and a large underground surface area to collect the water and mineral nutrients needed to support their growth and development. Accessibility of the essential nutrients nitrogen (N) and phosphorus (P) in the soil is affected by many factors that create a variable spatiotemporal landscape of their availability both at the local and global scale. Plants optimize uptake of the N and P available through modifications to their growth and development and engagement with microorganisms that facilitate their capture. The sensing of these nutrients, as well as the perception of overall nutrient status, shapes the plant's response to its nutrient environment, coordinating its development with microbial engagement to optimize N and P capture and regulate overall plant growth.

Abstract: Plant growth often occurs under a range of stressful conditions, including soil acidity and alkalinity Hydrogen ion concentration, which determines pH of the soil, regulates the entire chemistry of plant nutrient colloidal solutions Beyond certain levels of pH multiple stresses such as hydrogen ion toxicity, and nutrient imbalance, toxicities and deficiencies are induced in plants Breeding for stress coupled with suitable agronomic practices has been a way to deal with this situation in agriculture However, plant growth promoting microbes (PGPM) have shown potential as sustainable plant growth enhancers and have potential to help with a range of environmental stresses Considering the long-term evolutionary relationships between plants and microbes, it is probable that much remains unknown about potential benefits of microbes that could be harnessed from PGPM This article reviews the current understanding of acidity and alkalinity stress effects on plants and various approaches have or could address these stresses This review further provides a detailed account of the current understanding regarding the role of PGPM in acidity and alkalinity stress management, including when agronomic practices and plant breeding are combined Approaches already evaluated have shown limitations because acidity and alkalinity in soils are gradual and progressive conditions Greater exploitation of PGPM in this regard, would be interesting to explore as they have the potential to address multiple stresses in a more sustainable fashion Future crop production will require further breeding for pH stress tolerance, but also implementation of microbial technologies that provide enhanced tolerance to pH stress

Abstract: Revegetation of semiarid lands depends upon soil microbial communities to supply nutrients for successive plant species, but microbial activity can be constrained by insufficient water. The objective of this study was to quantify the metabolic limitation of microbes by extracellular enzymatic stoichiometry, and to determine how this affected microbial carbon use efficiency (CUE) with biogeochemical equilibrium model. The study occurred in long-term revegetation experiment with seven successional stages (0, 11, 35, 60, 100, 130 and 150 years) in the Loess Plateau, China. Microbes maintained stoichiometric homeostasis in all successional stages, but plants did not. Microbial metabolism was limited by low soil phosphorus (P) concentration throughout the succession, whereas plants were limited by low soil P during the late successional stages (from 60 to 150 years) only. An increase in soil moisture during succession was associated with greater P limitation in microbes and plants. There was less microbial P limitation at the 35-year successional stage, and the greatest microbial P limitation occurred at the 130-year successional stage. The microbial C limitation followed a unimodal pattern through the vegetation succession and reached a maximum at 100 years of succession (the early forest stage). This coincided with the lowest microbial CUE at 100 years of succession (CUE was from 0.24 to 0.41), suggesting a change in the physiological responses from microbes (such as enzyme synthesis and the priming effect), that tended to reduce soil C sequestration. Our results indicate that soil moisture regulated microbial C and P metabolism during the vegetation succession in this semiarid region, which has implications for understanding how microbial metabolism affects soil C dynamics under vegetation restoration.

Abstract: As part of a successful agronomic strategy, adequate nutrient management of the potato crop is essential throughout the whole growth period. In this review, the importance of balanced fertilisation for potato yield formation and yield security is addressed by taking advantage of the results of field trials and existing literature. The most important roles of the macronutrients nitrogen, phosphorous, potassium, magnesium, calcium and sulphur in the plant and their importance for yield formation in potato are reviewed. Fertilisation practices in potato production are discussed. Due to their various functions in plant metabolism, the impact of plant nutrients on potato yield is complex. Therefore, interactions with abiotic and biotic factors, for instance interactions between two different plant nutrients in the soil and the plant, are taken into account.

Abstract: Silicon is one of the most significant elements in plants under abiotic stress, so we investigated the role of silicon in alleviation of the detrimental effects of salinity at two concentrations (1500 and 3000 ppm sodium chloride) in sweet pepper plants in two seasons (2018 and 2019). Our results indicated that relative water content, concentrations of chlorophyll a and b, nitrogen, phosphorus and potassium contents, number of fruits plant−1, fruit fresh weight plant−1 (g) and fruit yield (ton hectare−1) significantly decreased in salt-stressed sweet pepper plants as compared to control plants. In addition, electrolyte leakage, proline, lipid peroxidation, superoxide (O2−) and hydrogen peroxide (H2O2) levels, soluble sugars, sucrose, and starch content as well as sodium content significantly increased under salinity conditions. Conversely, foliar application of silicon led to improvements in concentrations of chlorophyll a and b and mineral nutrients, water status, and fruit yield of sweet pepper plants. Furthermore, lipid peroxidation, electrolyte leakage, levels of superoxide, and hydrogen peroxide were decreased with silicon treatments.

Abstract: In this work, we have proposed a new formulation of a hybrid nanofertilizer (HNF) for slow and sustainable release of nutrients into soil and water. Urea-modified hydroxyapatite was synthesized, wh...

Abstract: Microorganisms mediate nutrient cycling in soils, and thus it is assumed that they largely control responses of terrestrial ecosystems to anthropogenic nutrient inputs. Therefore, it is important to understand how increased nitrogen (N) and phosphorus (P) availabilities, first, affect soil prokaryotic and fungal community composition and second, if and how changes in the community composition affect soil element cycling. We measured soil microbial communities and soil element cycling processes along a nine-year old experimental N-addition gradient partially crossed with a P-addition treatment in a temperate grassland. Nitrogen addition affected microbial community composition, and prokaryotic communities were less sensitive to N addition than fungal communities. P addition only marginally affected microbial community composition, indicating that P is less selective than N for microbial taxa in this grassland. Soil pH and total organic carbon (C) concentration were the main factors associated with prokaryotic community composition, while the dissolved organic C-to-dissolved N ratio was the predominant driver of fungal community composition. Against our expectation, plant biomass and plant community structure only explained a small proportion of the microbial community composition. Although microbial community composition changed with nutrient addition, microbial biomass concentrations and respiration rates did not change, indicating functional redundancy of the microbial community. Microbial respiration, net N mineralization, and non-symbiotic N2 fixation were more strongly controlled by abiotic factors than by plant biomass, plant community structure or microbial community, showing that community shifts under increasing nutrient inputs may not necessarily be reflected in element cycling rates. This study suggests that atmospheric N deposition may impact the composition of fungi more than of prokaryotes and that nutrient inputs act directly on element-cycling rates as opposed to being mediated through shifts in plant or microbial community composition.

Abstract: Drought is a very common abiotic stress worldwide in arid and semiarid areas. It decreases the growth and yield of the crops. Due to drought stress, there is insufficient intake of the nutrients, the low rate of photosynthesis and limited supply of water in plants. Inoculating crops with plant growth-promoting rhizobacteria (PGPR) mitigates the deleterious effects of stress by promoting beneficial effects such as helping them in the acquisition of less available nutrients, increasing the levels of plant growth regulators, improving the physiological health of the plants. In the present study, drought-tolerant phosphorus-solubilizing rhizobacteria with multifunctional plant growth-promoting attributes were isolated from different cereal crops grown in the Divine Valley of Baru Sahib, Himachal Pradesh, using a different nutrient combination. A total of 86 bacteria were isolated from different growth media. All 86 could tolerate 5% PEG, and 19, 6 and 6 isolates could tolerate 6%, 7% and 8% PEG-8000, respectively. Among 86, 48 drought-adapted and P-solubilizing strains were selected and screened for diverse PGP attributes such as solubilization of potassium and zinc, production of siderophores, hydrogen cyanide, ammonia and 1-aminocyclopropane-1-carboxylate deaminase. The efficient drought-adaptive P-solubilizing strain was used for seed germination and plant growth-promoting ability in different pot assays under laboratory and greenhouse conditions at different water regimes. The strain EU-LWNA-33 positively influencing the growth parameters and physiological parameters was identified using 16 S rRNA gene sequencing as Pseudomonas libanensis. To our knowledge, this is the first report for P. libanensis EU-LWNA-33 to solubilize a considerable amount of P under the water-deficient conditions. The use of stress-adaptive and P-solubilizing PGPR provides significant promise to overcome the challenges of sustainable agriculture in stressed environmental conditions.

Abstract: One of the main obstacles to plant growth is the lack of the availability of nutrient elements in many agricultural environments in the world, especially the tropics where soils can be extremely low in nutrients. Using different mechanisms of action, plant growth-promoting rhizobacteria (PGPR) participate in geochemical nutrition cycles and determine their access to plants and the microbial community of the soil. Use of these bacteria as bio-inoculants will increase the availability of nutrient elements in soil, help to minimize the chemical fertilizer application, reduce environmental pollution, and promote sustainable agriculture. Considering comprehensive reviews previously published on plant growth enhancement mechanisms, this review focuses on what is known about the action mechanisms underlying the increase of the availability of nutrient elements as an effect of microbial colonization especially PGPR. In this chapter, some of the most important mechanisms and processes regarding the effects of PGPR on the availability and hence uptake of nutrient elements by plant are reviewed. The awareness of such mechanisms can be important for the selection and hence production of microbial inoculums, which are appropriate for biological fertilization as substituting or decreasing the need of using chemical fertilizers in crops. In this review, special consideration is given to the role of PGPR in the availability of nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) as macronutrients and iron (Fe) and manganese (Mn) as micronutrients.

Abstract: Among the extreme habitats, drought is most harsh abiotic stress affecting growth, development and productivity of crops. Plants also face limitations of certain nutrients such as phosphorus required for different physiological and metabolic activities. Stress-adaptive phosphorus-solubilizing microbes in rhizospheric soil can help plants to combat water scarcity and overcome the problem of phosphorus unavailability to plant systems. The present investigation deals with the isolation of drought stress adaptive and P-solubilizing microbes from rhizospheric soil of different cereals and pseudocereals and their role in mitigation of drought stress in great millet. A total of 193 rhizospheric microbes were isolated and screened for their capability to solubilize phosphorus under drought stress. Twenty isolates exhibited P-solubilizing attribute under drought stress, which were further screened for plant growth promoting (PGP) traits such as solubilization of zinc and potassium; production of Fe-chelating compounds, indole acetic acid, hydrogen cyanide and ammonia. On basis of multifunctional PGP traits, two efficient and potential microbes were evaluated for PGP in great millet in vitro under green house with different water regimes. The isolates were found to be efficient in terms of enhancing accumulation of different osmolytes such as glycine betaine, proline, sugars, increased chlorophyll content, and decreasing lipid peroxidation. The isolates were identified by 16S/18S rRNA gene sequencing as Streptomyces laurentii EU-LWT3-69 and Penicillium sp. strain EU-DSF-10. To best of our knowledge Streptomyces laurentii has been reported first time as PGP and drought adaptive bacterium. PGP drought-adaptive phosphorus solubilizers could be used as bioinoculants for crops under water scarcity ecosystems.

Abstract: Nitrogen (N) and phosphorus (P) availability strongly affects carbon (C) cycling and storage in terrestrial ecosystems. Nutrient addition can increase C inputs into soil via increased above- and belowground plant productivity, but at the same time can accelerate organic matter decomposition in the soil. The mechanisms underlying these effects on soil organic C (SOC) dynamics remain unclear, especially in nutrient-limited alpine ecosystems that have been subjected to increasing N and P availability in recent decades. The aim of this study was to clarify the mechanisms underlying SOC decomposition and stabilization in an alpine grassland soil after four years of N and P additions. The soil aggregate size distribution, microbial community structure (lipid biomarkers), microbial C use efficiency (CUE) and microbial necromass composition (amino sugar biomarkers) were analyzed. Nutrient addition increased dominance of fast-growing bacteria (copiotrophs), while P addition alone intensified the competitive interactions between arbuscular mycorrhizal and saprotrophic fungi. These changes led to decreases in the microbial CUE of glucose by 1.6–3.5% and of vanillin by 8.5%, and therefore, reduced SOC content in the topsoil. The total microbial necromass remained unaffected by nutrient addition, but the contribution of fungal necromass to SOC increased. The increased abundance of arbuscular mycorrhizal fungi and fungal necromass under elevated N availability raised the mass proportion of soil macroaggregates (>250 μm) by 16.5–20.3%. Therefore, fungi were highly involved in macroaggregation following N addition, and so, moderated the SOC losses through enhanced physical protection. Overall, the complex interactions between microbial physiology (CUE), necromass composition (amino sugars) and physical protection (macroaggregation) in mediating SOC dynamics in response to nutrient enrichment were disentangled to better predict the capability of alpine grassland soils to act as a C sink or source under global change.

Abstract: Variations in soil microbial metabolism currently represent one of the greatest areas of uncertainty with regard to soil nutrient cycles and the control of terrestrial carbon (C) and nitrogen (N) loss and are poorly understood in agricultural ecosystems with intensive farming practices. In this study, extracellular enzymatic stoichiometry models and quantitative PCR techniques were used to examine microbial metabolic limitation and its relationship with N-cycling gene expression in semi-arid agricultural ecosystems considering four N fertilization levels (N 0, N 100, N 250, and N 400 kg N ha−1) and two agronomic strategies (film mulching and no mulching). Film mulching increased microbial C limitation (reflecting microbial C metabolism size; 0.189 of the total effects), while very small effects on microbial phosphorus (P) limitation were observed (-0.007 of the total effects). N fertilization increased the microbial demand for P (microbial P limitation; 0.504 of the total effects). Increased microbial C metabolism was mainly attributed to increased soil moisture content after film mulching, which enhanced microbial decomposition of organic C (high C-acquiring enzyme activities). Changes in nutrient stoichiometry and the increase in N availability due to N fertilization were largely responsible for increased microbial P limitation. Furthermore, microbial P limitation negatively affected the abundance of AOA amoA, AOB amoA (involved in nitrification), nirK, nirS, nosZ (involved in denitrification) genes, strongly inhibiting nitrification and denitrification potential (-0.743 and -0.761 of the total effects, respectively). The present results suggest that agricultural ecosystems with film mulching are conducive to organic residue decomposition, while appropriate P limitation under N fertilization could reduce the loss of N due to nitrification and denitrification in soil. This study highlights the importance of elemental stoichiometry-driven microbial metabolic variation in understanding soil nutrient cycles and optimizing agricultural practices.

Abstract: A synthesis of available agronomic datasets and peer-reviewed scientific literature was conducted to: (1) assess the status of micronutrients in sub-Saharan Africa (SSA) arable soils, (2) improve the understanding of the relations between soil quality/management and crop nutritional quality and (3) evaluate the potential profitability of application of secondary and micronutrients to key food crops in SSA, namely maize (Zea mays L.), beans (Phaseolus spp. and Vicia faba L.), wheat (Triticum aestivum L.) and rice (Oryza sativa L.). We found that there is evidence of widespread but varying micronutrient deficiencies in SSA arable soils and that simultaneous deficiencies of multiple elements (co-occurrence) are prevalent. Zinc (Zn) predominates the list of micronutrients that are deficient in SSA arable soils. Boron (B), iron (Fe), molybdenum (Mo) and copper (Cu) deficiencies are also common. Micronutrient fertilization/agronomic biofortification increases micronutrient concentrations in edible plant organs, and it was profitable to apply fertilizers containing micronutrient elements in 60-80% of the cases. However, both the plant nutritional quality and profit had large variations. Possible causes of this variation may be differences in crop species and cultivars, fertilizer type and application methods, climate and initial soil conditions, and soil chemistry effects on nutrient availability for crop uptake. Therefore, micronutrient use efficiency can be improved by adapting the rates and types of fertilizers to site-specific soil and management conditions. To make region-wide nutritional changes using agronomic biofortification, major policy interventions are needed.

Abstract: Soil enzymes produced by microorganisms transform substrates in the soil carbon (C) and nutrient cycles. Limitations in C and other nutrients could affect microbial biosynthesis processes, so we expect that soil enzyme activity will reflect microbial deficiencies in C, nitrogen (N) and phosphorus (P) at a large spatial scale. We collected soil from nutrient addition trials in eight forest ecosystems, ranging from temperate forests to tropical forests in eastern China, and conducted vector analysis of the soil enzymatic stoichiometry to examine the spatial extent of soil microbial C and nutrient limitations. We also determined whether nutrient addition could alleviate nutrient limitation or otherwise impact soil microbial resource use. Soil microbial C vs. nutrient limitation (thereafter C limitation) was greater in the temperate forests than in the tropical forests, but did not vary with soil depth. Soil microbial P vs. N limitation (thereafter nutrient limitation) decreased with latitude, and increased with soil depth. We found a negative relationship between soil microbial C limitation and nutrient limitation, which was more pronounced in the topsoil than in deeper soil depths. Furthermore, we found that climate (mean annual precipitation and temperature), soil pH and soil nutrients were significantly correlated with soil microbial C (explaining about 23% of the variation) and nutrient limitation (responsible for about 87% of the variation). Nutrient addition represented ~1% of the variation in soil microbial C and nutrient limitations and thus did not alleviate nutrient deficiencies. We conclude that soil microbial C and nutrient limitations are more likely driven by climate and soil physicochemical properties than by nutrient addition in eight forest ecosystems. Since soil microbial C and nutrient limitations result from long-term adaptation of soil microbial communities to site-specific soil and environmental conditions, the soil enzyme activity is not modified by short-term changes in nutrient availability resulting from fertilizer application.

Abstract: Although the effects of fertilization and microbiota on plant growth have been widely studied, our understanding of the chemical fertilizers to alter soil chemical and microbiological properties in woody plants is still limited. The aim of the present study is to investigate the impact of long-term application of chemical fertilizers on chemical and microbiological properties of root-associated soils of walnut trees. The results show that soil organic matter (OM), pHkcl, total nitrogen (TN), nitrate-nitrogen (NO3-), and total phosphorus (TP) contents were significantly higher in non-fertilized soil than after chemical fertilization. The long-term fertilization led to excessive ammonium-nitrogen (NH4+) and available phosphorus (AP) residues in the cultivated soil, among which NH4+ resulted in soil acidification and changes in bacterial community structure, while AP reduced fungal diversity. The naturally grown walnut trees led to an enrichment in beneficial bacteria such as Burkholderia, Nitrospira, Pseudomonas, and Candidatus_Solibacter, as well as fungi, including Trichoderma, Lophiostoma, Phomopsis, Ilyonectria, Purpureocillium, Cylindrocladiella, Hyalorbilia, Chaetomium, and Trichoglossum. The presence of these bacterial and fungal genera that have been associated with nutrient mobilization and plant growth was likely related to the higher soil OM, TN, NO3-, and TP contents in the non-fertilized plots. These findings highlight that reduced chemical fertilizers and organic cultivation with beneficial microbiota could be used to improve economic efficiency and benefit the environment in sustainable agriculture.

Abstract: In order to fully exploit the nutrient density concept, thorough understanding of the biological activity of single nutrients in their interaction with other nutrients and food components from whole foods is important This review provides a narrative overview of recent insights into nutrient bioavailability from complex foods in humans, highlighting synergistic and antagonistic processes among food components for two different food groups, ie, dairy, and vegetables and fruits For dairy, bioavailability of vitamins A, B2, B12 and K, calcium, phosphorous, magnesium, zinc and iodine are discussed, whereas bioavailability of pro-vitamin A, folate, vitamin C and K, potassium, calcium, magnesium and iron are discussed for vegetables and fruits Although the bioavailability of some nutrients is fairly well-understood, for other nutrients the scientific understanding of uptake, absorption, and bioavailability in humans is still at a nascent stage Understanding the absorption and bioavailability of nutrients from whole foods in interaction with food components that influence these processes will help to come to individual diet scores that better reflect absorbable nutrient intake in epidemiologic studies that relate dietary intake to health outcomes Moreover, such knowledge may help in the design of foods, meals, and diets that aid in the supply of bioavailable nutrients to specific target groups

Abstract: Microbial detritus contributes substantially to the soil organic matter (SOM). Analysis of global literature indicated that microbial detritus carbon (C) contributed 59 and 64% of total soil C in arable agricultural and grassland systems respectively, with a 2.5% greater contribution of bacterial-derived detritus in grasslands and with no difference in the proportional contribution of fungal detritus. Total soil C and nitrogen (N) content was higher in grasslands with an average of 2.8 and 1.6 g N kg−1 soil and 28.8 and 16.8 g C kg−1 soil in grassland and arable systems, respectively. Soil N content explained 11 to 28% of the variance in microbial detritus contribution to soil C. Further, total soil N and C content explained more variance than other factors which are commonly considered to mediate SOM content including precipitation, acidity and clay. Microbial biomass C assimilation and re-metabolism of SOM are affected by nutrient supply and the dissimilarity of the C to N, phosphorus (P) and sulfur (S) ratios between fresh organic matter (FOM), SOM and microorganisms (C:N:P:S 10,000:261:32:48, 10,000:833:200:143, and 10,000:1,494:458:154, respectively). In agricultural systems, stoichiometrically balanced nutrient addition to FOM can increase C transfer to SOM by 6 to 52% and importantly reduce the mineralization of pre-existing SOM by 24 to 50%. Future research to quantify economic and environmental implications is warranted with need for a paradigm shift in thinking to focus on the nutrient requirements of the whole soil–plant system rather than the agronomic requirements of crops alone.

Abstract: Hydropower development is the key strategy in many developing countries for energy supply, climate-change mitigation and economic development. However, it is commonly assumed that river dams retain nutrients and therefore reduce downstream primary productivity and fishery catches, compromising food security and causing trans-boundary disputes. Contrary to expectation, here we found that a cascade of reservoirs along the upper Mekong River increased downstream bioavailability of nitrogen and phosphorus. The dams caused phytoplankton density to increase with hydraulic residence time and stratification of the stagnant reservoirs caused hypoxia at depth. This allowed the release of bioavailable phosphorus from the sediment and an increase in dissolved inorganic nitrogen as well as a shift in nitrogen species from nitrate to ammonium, which were transported downstream by the discharge of water from the base of the dam. Our findings provide a new perspective on the environmental impacts of river dams on nutrient cycling and ecosystem functioning, with potential implications for sustainable development of hydropower worldwide.

Abstract: Soil microbial metabolism is vital for nutrient cycling and ecosystem stability. To quantify microbial metabolism and nutrient limitation during plant secondary succession, we measured soil physicochemical properties, microbial biomass, and four enzyme activities (β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), L-leucine aminopeptidase (LAP), and alkaline phosphatase (AP)) and identified the changes of soil ecoenzymatic stoichiometry and microbial nutrient limitation along a secondary succession series in typical arid and semi-arid ecosystems on the Loess Plateau, China. Soil enzyme activities increased in the first 17a succession and then decreased with plant secondary succession. The ln(BG):ln(NAG + LAP) ratio and ln(BG):ln(AP) ratio showed a decreasing trend in the first 22a succession and then increased. Most soil nutrient contents and nutrient stoichiometry were significantly correlated with enzyme activities and enzymatic stoichiometry. Moreover, vector analysis of soil enzymes indicated that microbial community were co-limited by C and P during secondary succession. Linear regression of C:N and C:P between the soil nutrient and microbial community showed that soil microbial community maintained stoichiometry homeostasis during plant secondary succession. The threshold elemental ratio revealed that the microbial nutrient metabolisms were co-limited by N and P, particularly P during plant secondary succession. Therefore, microbial communities were co-limited by C, N, and P, particularly C and P during plant secondary succession, and the limitation was mainly associated with soil nutrient status.