Topsoil Nutrients Drive Leaf Carbon and Nitrogen Concentrations of a Desert Phreatophyte in Habitats with Different Shallow Groundwater Depths

Phreatophytes are deep-rooted plants that reach groundwater and are widely distributed in arid and semiarid areas around the world. Multiple environmental factors affect the growth of phreatophytes in desert ecosystems. However, the key factor determining the leaf nutrients of phreatophytes in arid regions remains elusive. This study aimed to reveal the key factors affecting the ecological stoichiometry of desert phreatophytes in the shallow groundwater of three oases at the southern rim of the Taklimakan Desert in Central Asia. Groundwater depth; groundwater pH and the degree of mineralization of groundwater; topsoil pH and salt concentration; topsoil and leaf carbon, nitrogen, and phosphorus concentrations of phreatophytic Alhagi sparsifolia grown at groundwater depths of 1.3–2.2 m in the saturated aquifer zone in a desert–oasis ecotone in northwestern China were investigated. Groundwater depth was closely related to the mineralization degree of groundwater, topsoil C and P concentrations, and topsoil salt content and pH. The ecological stoichiometry of A. sparsifolia was influenced by depth, pH and the degree of mineralization of groundwater, soil nutrients and salt concentration. However, the effects of soil C and P concentrations on the leaf C and N concentrations of A. sparsifolia were higher than those of groundwater depth and pH and soil salt concentration. Moreover, A. sparsifolia absorbed more N in the soil than in the groundwater and atmosphere. This quantitative study provides new insights into the nutrient utilization of a desert phreatophyte grown at shallow groundwater depths in extremely arid desert ecosystems.

Ecological stoichiometry as a foundation for omics-enabled biogeochemical models of soil organic matter decomposition

Coupled biogeochemical cycles drive ecosystem ecology by influencing individual-to-community scale behaviors; yet the development of process-based models that accurately capture these dynamics remains elusive. Soil organic matter (SOM) decomposition in particular is influenced by resource stoichiometry that dictates microbial nutrient acquisition (‘ecological stoichiometry’). Despite its basis in biogeochemical modeling, ecological stoichiometry is only implicitly considered in high-resolution microbial investigations and the metabolic models they inform. State-of-science SOM decomposition models in both fields have advanced largely separately, but they agree on a need to move beyond seminal pool-based models. This presents an opportunity and a challenge to maximize the strengths of various models across different scales and environmental contexts. To address this challenge, we contend that ecological stoichiometry provides a framework for merging biogeochemical and microbiological models, as both explicitly consider substrate chemistries that are the basis of ecological stoichiometry as applied to SOM decomposition. We highlight two gaps that limit our understanding of SOM decomposition: (1) understanding how individual microorganisms alter metabolic strategies in response to substrate stoichiometry and (2) translating this knowledge to the scale of biogeochemical models. We suggest iterative information exchange to refine the objectives of high-resolution investigations and to specify limited dynamics for representation in large-scale models, resulting in a new class of omics-enabled biogeochemical models. Assimilating theoretical and modelling frameworks from different scientific domains is the next frontier in SOM decomposition modelling; advancing technologies in the context of stoichiometric theory provides a consistent framework for interpreting molecular data, and further distilling this information into tractable SOM decomposition models.

Ecological Stoichiometry of Bumblebee Castes, Sexes, and Age Groups

Ecological stoichiometry is important for revealing how the composition of chemical elements of organisms is influenced by their physiological functions and ecology. In this study, we investigated the elemental body composition of queens, workers, and males of the bumblebee Bombus terrestris, an important pollinator throughout Eurasia, North America, and northern Africa. Our results showed that body elemental content differs among B. terrestris castes. Young queens and workers had higher body nitrogen concentration than ovipositing queens and males, while castes did not differ significantly in their body carbon concentration. Furthermore, the carbon-to-nitrogen ratio was higher in ovipositing queens and males. We suggest that high body nitrogen concentration and low carbon-to-nitrogen ratio in young queens and workers may be related to their greater amount of flight muscles and flight activities than to their lower stress levels. To disentangle possible effects of stress in the agricultural landscape, further studies are needed to compare the elemental content of bumblebee bodies between natural habitats and areas of high-intensity agriculture.

Stoichiometry of C:N:P in the Roots of Alhagi sparsifolia Is More Sensitive to Soil Nutrients Than Aboveground Organs

The stoichiometry of carbon, nitrogen, and phosphorus (C:N:P) among leaves, stems, and roots reflects trade-offs in plants for acquiring resources and their growth strategy. The widely distributed plant Alhagi sparsifolia is an ideal species to study the ecological stoichiometry in different organs in response to the availability of nutrients and water in the desert ecosystem. However, which response of organs is most sensitive to environmental conditions is still unclear. To answer this question, we collected samples of plants and soils including not only aboveground leaves and stems, but also underground roots and soils from a wide range of arid areas during the growing season. The C, N, P, C:N, C:P, and N:P ratios in leaves, thorns, stems, and roots were derived to explore their relationship as well as their response mechanisms to nutrients and water spanning 1 m deep in the soil. The results showed that the order of N concentration was leaves > thorns > stems > roots, that the concentration of P in the leaves, thorns, and stems was similar, and that their values were higher than those in the roots. First, the C:N ratios in the leaves and stems were significantly positively correlated with the ratio in roots. The C:N ratios in each organ showed a significant relationship with the soil alkali hydrolyzable nitrogen (SAN) above a depth of 60 cm. In addition to SAN, soil available phosphorus (SAP) and soil organic carbon (SOC) affect the C:N ratio in the roots. Second, the C:P and N:P ratios in aboveground organs showed no correlations with the ratios in roots. The C:P and N:P ratios in the leaves and thorns have no relationship with soil nutrients, while the C:P ratio in roots was influenced by SAN and SOC in all soil layers. Finally, the N:P ratios in roots were also affected by nutrients in different soil depths at 0–20 and 60–80 cm. These results illustrate that the roots were more sensitive to soil nutrients than the aboveground parts. Our study of ecological stoichiometry also suggests a novel systematic approach for analyzing the sensitivity of responses of an organ to environmental conditions.

Drought stress introduces growth, physiological traits and ecological stoichiometry changes in two contrasting Cunninghamia lanceolata cultivars planted in continuous-plantation soils

Background The decrease in Cunninghamia lanceolata (Lamb.) production on continuously planted soil is an essential problem. In this study, two-year-old seedlings of two cultivars (a normal cultivar, NC, and a super cultivar, SC) were grown in two types of soil (not planted (NP) soil; continuously planted (CP) soil) with three watering regimes, and the interactive effects on plant growth and physiological traits were investigated in a greenhouse experiment. The water contents of the soil in the control (CK) (normal water content), medium water content (MWC) and low water content (LWC) treatments reached 75−80 %, 45−50 % and 20−25 % of the field water capacity, respectively. Results The results indicated that the CP soil had a negative effect on growth and physiological traits and that the LWC treatment caused even more severe and comprehensive negative effects. In both cultivars, the CP soil significantly decreased the height increment (HI), basal diameter increment (DI), dry matter accumulation (DMA), net photosynthetic rate (Pn), total chlorophyll content (TChl), carotenoid content (Caro) and photosynthetic nitrogen use efficiency (PNUE). Compared to the NP soil, the CP soil also decreased the proline and soluble protein contents, nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE) and increased the nitrogen:phosphorus ratio in roots, stems and leaves. The LWC treatment decreased growth and photosynthesis, changed ecological stoichiometry, induced oxidative stress, promoted water use efficiency and damaged chloroplast ultrastructure. Significant increases in ascorbate peroxidase (APX), peroxidase (POD), soluble protein and proline contents were found in the LWC treatment. Compared with the NC, the SC was more tolerant to the CP soil and water stress, as indicated by the higher levels of DMA, Pn, and WUE. After exposure to the CP soil and watering regimes, the decreases in biomass accumulation and gas exchange were more pronounced. Conclusions The combination of drought and CP soil may have detrimental effects on C. lanceolata growth, and low water content enhances the impacts of CP soil stress on C. lanceolata seedlings. The superiority of the SC over the NC is significant in Chinese fir plantation soil. Therefore, continuously planted soil can be utilized to cultivate improved varieties of C. lanceolata and maintain water capacity. This can improve their growth and physiological performance to a certain extent.

Linkages Between Nutrient Resorption and Ecological Stoichiometry and Homeostasis Along a Chronosequence of Mongolian Pine Plantations

Nutrient resorption is an important strategy for nutrient conservation, particularly under conditions of nutrient limitation. However, changes in nutrient resorption efficiency with stand development and the associated correlations with ecological stoichiometry and homeostasis are poorly understood. In the study, the authors measured carbon (C), nitrogen (N), and phosphorus (P) concentrations in soil and in green and senesced needles along a chronosequence of Mongolian pine (Pinus sylvestris var. mongolica) plantations (12-, 22-, 31-, 42-, 52-, and 59-year-old) in Horqin Sandy Land of China, calculated N and P resorption efficiency (NRE and PRE, respectively), and homeostasis coefficient. The authors found that soil organic C and total N concentrations increased, but soil total P and available P concentrations decreased with stand age. Green needle N concentrations and N:P ratios as well as senesced needle C:N ratios, NRE, and PRE exhibited patterns of initial increase and subsequent decline with stand age, whereas green needle C:N ratios and senesced needle N concentrations, and N:P ratios exhibited the opposite pattern. NRE was positively correlated with N concentration and N:P ratio, but negatively correlated with C:N ratio in green needles, whereas the opposite pattern was observed in senesced needles. PRE was negatively correlated with senesced needle P concentration, soil-available N concentration, and available N:P ratio. The homeostatic coefficient of N:P was greater when including all stand ages than when including only those younger than 42 years. These findings indicate that tree growth may change from tending to be N limited to tending to be P limited along the Mongolian pine plantation chronosequence. Nutrient resorption was coupled strongly to tree growth and development, whereas it played a lesser role in maintaining stoichiometric homeostasis across the plantation chronosequence. Therefore, adaptive fertilization management strategies should be applied for the sustainable development of Mongolian pine plantations.

Shoot-soil ecological stoichiometry of alfalfa under nitrogen and phosphorus fertilization in the Loess Plateau

Plants and soil interactions greatly affect ecosystems processes and properties. Ecological stoichiometry is an effective means to explore the C, N, P correlation between plants and soil and the relationship between plant growth and nutrient supply. Serious soil erosion on China’s Loess Plateau has further barrenness the soil. Fertilization solves the problem of ecosystem degradation by improving soil fertility and regulating the ecological stoichiometric between soil and plants. No fertilization (CK), nitrogen fertilization (N), phosphorus fertilization (P) and N and P combined fertilization (NP) treatments were set in an alfalfa grassland. Organic carbon (C), nitrogen (N) and phosphorus (P) nutrients and their stoichiometry were measured in shoot and soil. P and NP fertilization increased shoot C concentration (3.12%, 0.91%), and all fertilization decreased shoot N concentration (6.96%). The variation of shoot C and N concentrations resulted in a greater increase in shoot C:N under the fertilization treatment than that under CK (8.24%). Most fertilization treatments increased shoot P concentration (4.63%) at each cut, which induced a decrease of shoot C:P. Shoot N:P of most treatments were greater than 23, but it was lower under N and NP fertilization than that under CK. Fertilization only increased soil C in 2014, but had no effect on soil N. Soil P content was significantly higher under P fertilization in 2014 (34.53%), and all fertilization in the second cut of 2015 (124.32%). Shoot and soil C:P and N:P having the opposite changes to shoot and soil P, respectively. Our results suggest that the change of P after fertilization largely drove the changes of stoichiometric. The growth of alfalfa in the Loess Plateau was severely restricted by P. It is an effective method to increase the biomass of alfalfa by increasing the addition of N or NP fertilizer to alleviate P limitation.