Plant Nutrients Appears Decoupled from Soil: Increasing Influences from Changing Climate in the Grassland’s of Inner Mongolia, China

Extremes in weather episodes seem to be the new normal. We need to better understand how changing climatic conditions alter plant growth in grasslands, especially macro nutrient uptake and stoichiometry. However, few studies have examined how warmer/colder or wetter/drier climates influence the nutrient decoupling between plants and soils at the ecosystem level. Here, we investigated the changes in carbon (C), nitrogen (N), and phosphorus (P) concentrations and their stoichiometric ratios in plants and soils from 65 grassland sites along a geographic gradient of temperature and aridity in northern China. Often, we saw inverse responses between plant and soil nutrients with respect to temperature and aridity. Soil C and N were negatively correlated with temperature and aridity. Soil P was negatively correlated with aridity. Plant N was positively correlated with aridity and plant P was negatively correlated with temperature, while plant C had no relationship with either. Temperature and aridity were positively correlated with C:N and negatively correlated with C:P and N:P ratios in soils. However, aridity was negatively correlated with plant C:N ratios. Plant N:P ratios were positively correlated with temperature and aridity, whereas plant C:P ratios had no relationship with either. Our findings suggest at a broad geographic scale, plant nutrients do not always reflect soil nutrient availability. It is conceivable that rapid climate shifts and the resulting changes in element availability, turnover rates, absorption, and use efficiency might cause decoupling of C, N, and P cycles between plants and soils.

Nutrient Storage and Stoichiometry of the Forest Floor Organic Matter in Japanese Forests

Nutrient storage in the forest floor is regulated through litter decomposition and nutrient cycling. Stoichiometry of nutrients can provide characterization of the forest floor. To quantify nutrient storage in the forest floor and to determine stoichiometry among different forest types, available data on nutrients were meta-analyzed. The data on nutrients—nitrogen, phosphorus, potassium, calcium, and magnesium—were collected from published reports and original data on Japanese forests. The relationship between nutrient storage and forest floor mass was also examined. Japanese cypress and cedar plantations had small N and P storage in the forest floor with high C:N and C:P ratios, whereas subalpine conifers had large N and P storage in the forest floor with low C:N and C:P ratios; cedar plantations showed large Ca-specific storage in the forest floor. The stoichiometry of the forest floor varied between different forest types, namely C:N:P ratios were 942:19:1 for cedar and cypress plantations, 625:19:1 for broad-leaved forests, and 412:13:1 for subalpine conifers and fir plantations. N storage was closely correlated; however, P and other mineral storages were weakly correlated with the forest floor mass. Nutrient storage and stoichiometry can provide a better perspective for the management of forest ecosystem.

Macro-Nutrient Stoichiometry of Glacier Algae From the Southwestern Margin of the Greenland Ice Sheet

Glacier algae residing within the surface ice of glaciers and ice sheets play globally significant roles in biogeochemical cycling, albedo feedbacks, and melt of the world’s cryosphere. Here, we present an assessment of the macro-nutrient stoichiometry of glacier algal assemblages from the southwestern Greenland Ice Sheet (GrIS) margin, where widespread glacier algal blooms proliferate during summer melt seasons. Samples taken during the mid-2019 ablation season revealed overall lower cellular carbon (C), nitrogen (N), and phosphorus (P) content than predicted by standard microalgal cellular content:biovolume relationships, and elevated C:N and C:P ratios in all cases, with an overall estimated C:N:P of 1,997:73:1. We interpret lower cellular macro-nutrient content and elevated C:N and C:P ratios to reflect adaptation of glacier algal assemblages to their characteristic oligotrophic surface ice environment. Such lower macro-nutrient requirements would aid the proliferation of blooms across the nutrient poor cryosphere in a warming world. Up-scaling of our observations indicated the potential for glacier algal assemblages to accumulate ∼ 29 kg C km2 and ∼ 1.2 kg N km2 within our marginal surface ice location by the mid-ablation period (early August), confirming previous modeling estimates. While the long-term fate of glacier algal autochthonous production within surface ice remains unconstrained, data presented here provide insight into the possible quality of dissolved organic matter that may be released by assemblages into the surface ice environment.

The Variation in the Stoichiometric Characteristics of the Leaves and Roots of Karst Shrubs

Currently, vegetation restoration is being implemented in the ecologically fragile karst areas in southwest China; however, the stoichiometry of the dominant shrubs and their relationship with the environmental factors in the degraded habitats is still unclear. In this study, we investigated the stoichiometry of C, N, and P, their internal correlations, and influencing factors in 23 shrub species in the Huanjiang County in northwest Guangxi Province, China. We found that the mean contents of C, N and P in leaves were higher than those in roots. In addition, the N:P ratio in the leaves was significantly higher than that in the roots, but the opposite was observed for the C:N and C:P ratios. Except for Leaf C and Root C, significant positive or negative correlations were observed across the stoichiometry of the shrub leaves and roots. A factor analysis of variance demonstrated that the differences across species had higher explanatory power than the topography and soil nutrients in terms of the shrub leaf and root stoichiometry. Hence, our results can improve the understanding of the distribution patterns of these vital elements, as well as of the interactions and influencing factors in the different organs of the karst shrubs.

A critical assessment of the stoichiometric knife-edge: no evidence for artifacts caused by the experimental P-supplementation of algae

The stoichiometric knife-edge refers to the reduced performance of consumers encountering food with excess phosphorus (P) relative to carbon (C) or nitrogen (N). Studies that provide evidence for such knife-edge in aquatic systems often apply phosphate supplementation to create P-rich food treatments. However, this method may suffer from artifacts, because after uptake algae may store P in a form different from the P-rich biomolecules typically consumed by zooplankton. Our aim was to test if P supplementation results in potential biases. We experimentally exposed populations of the herbivore rotifer species, Brachionus calyciflorus (Pallas), to four different food quality treatments: algae grown under P-saturating (HPchem, molar C:P ratio = 59.7 ± 2.7) and P-sufficient (MPchem, molar C:P = 116.3 ± 5.2) conditions in chemostats, and algae grown under P-limiting conditions, but with molar C:P ratios equal to HPchem and MPchem treatments, respectively (HPLP+P, molar C:P = 59.8 ± 0.14; MPLP+P, molar C:P = 121.0 ± 4.3). The latter two treatments were achieved through P-supplementation of P-limited algae. Results show that for rotifers fed algae with either excess or intermediate P content, population growth rates were consistently higher on algae grown in chemostats than algae treated with the P supplementation method. Importantly, growth rates were also consistently lower in HP than in MP treatments and the magnitude of this negative impact was independent on algal growth history. The latter result confirms the existence of a stoichiometric knife-edge and indicates that P supplementation is a reliable method to study the relative effect of excess P on zooplankton performance in a standardized way.

Catchment landscape components alter relationships between discharge and stream water nutrient ratios in the Xitiao River Basin China

The terrestrial environment of a watershed is a source of potential carbon (C), nitrogen (N), and phosphorus (P) exports, and the hydrological regime provides the mechanism to turn the potential exports into reality when water is available. However, the extent to which the terrestrial environment alters the strength and nature of streamflow in transporting stream water nutrient ratios remains largely unknown. This study combined monthly stream discharge data with synchronously sampled stream water C:N:P ratios in 14 catchment streams in the Xitiao River Basin (XRB) in Zhejiang Province, China. The transport effect of streamflow on C:N:P ratios varied depending on the nutrient element, flow condition, and terrestrial environment. In the lower reaches of the XRB, there were negative relationships between C:N ratios, C:P ratios and watershed discharge, and positive relationships between N:P ratios and watershed discharge in both high and low flow conditions. In the middle and upper reaches of the XRB, the C:N-discharge relationship changed from negative to positive when the streamflow conditions altered from low to high flow. The C:P- and N:P-discharge relationships were negative regardless of high or low flows, but the regression coefficient significantly decreased with increasing streamflow. The C:N-discharge correlation over the course of the year shifted from negative to positive, as urban areas expanded within the catchment. The C:P-discharge relationship altered from negative to positive with more cropland and wetland but from positive to negative with a greater forest percentage and mean percentage slope. Our results indicate that changes in the terrestrial environment (e.g., the proportion of a particular land cover within a watershed) generally produced a threshold flow above which the coupling relationships between element fluxes from the terrestrial to riverine ecosystem changed sharply.

Silicon modifies C:N:P stoichiometry, and increases nutrient use efficiency and productivity of quinoa

Recognizably, silicon has a beneficial effect on plant growth and productivity. In this respect, it is also known that the C, N and, P stoichiometric ratios and nutrient conversion efficiency allow identifying the interactions between elements while helping to understand the role Si plays in plant growth. This study aims to investigate whether increasing Si concentrations (0, 1, 2, and 3 mmol L−1) supplied in the nutrient solution is uptaken by quinoa, modifies the C:N:P stoichiometry while increasing nutritional efficiency and crop productivity as well. Our results revealed that the Si supply by promoting a decline in the C levels, associated with greater uptake of N and P, especially decreased the C:N and C:P ratios, favoring the C metabolism efficiency, and modulated the N and P use efficiency for biomass accumulation. This improved nutritional performance and greater use efficiency of C directly favored quinoa productivity. The future perspective is to encourage new field studies with this species to adjust silicon fertilization management to different soils aiming at enhancing quinoa productivity on a sustainable basis.

What controls microbial growth in tropical soils? The role of carbon and phosphorus.

<p>Tropical forest ecosystems are important components of global carbon (C) and nutrient cycles. Many tropical rainforests grow on old and highly weathered soils depleted in phosphorus (P) and other rock-derived nutrients. While plants in such forests are usually P limited, it remains unclear if heterotrophic microbial communities are also limited by P or rather by C or energy. Elemental limitations of microorganisms in soil are often approached by measurements of changes in respiration rates or microbial biomass in response to additions of nutrients or carbon. However, it has been argued lately, that microbial growth rather than respiration or biomass should be used to assess microbial limitations.</p><p> </p><p>In this study we asked the question whether the growth of heterotrophic microbial communities in tropical soil is limited by available P or by C. We sampled soils along a topographic gradient (plateau, slope, bottom) differing in soil texture and total and available P concentrations from a highly weathered site in French Guiana. We incubated these soils in the laboratory with cellulose as a C source, phosphate (pH adjusted) and with a combination of both. We determined microbial growth by measuring the incorporation of <sup>18</sup>O from labelled water into microbial DNA.</p><p> </p><p>In general, plateau soils were higher in microbial C, while bottom soils were higher in microbial P, leading to increased microbial C:P ratios in plateau soils compared to bottom soils. Microbial C, N and P did not respond to the addition of cellulose. Microbial P on the other hand was significantly increased by P additions, with no interactive effect between cellulose and P. Although microbial C was significantly higher in plateau soils, respiration rates were similar to those of bottom soils. This led to similar mass specific respiration rates in plateau and slope soils, with bottom soils being significantly higher. Moreover, we found that C and P addition increased mass specific respiration rates and both nutrient additions showed a positive interactive effect. Gross microbial growth rates were stimulated by P additions but were unresponsive to C additions alone. However, the addition of carbon further stimulated the effect of P on growth.</p><p> </p><p>The observed interactive effect of C and P additions on gross microbial growth rates suggests a co-limitation of microorganisms by C and P in highly weathered soils. We argue that co-limitation bears significant ecological advantages for microorganisms as it minimizes the investments in acquiring nutrients for growth.We further conclude that microorganisms in tropical soils are highly efficient in taking up and storing P from the environment. In our experiment, microbial P almost doubled in the six days after P addition, while microbial C was not enhanced. This also means that the microbes were not homeostatic with regard to their C:P ratios. Finally, our study demonstrates the importance of investigating gross microbial growth rates, rather than respiration or biomass, for inferring nutrient limitations.</p>

Divergent Adaptation Strategies of Vascular Facultative Epiphytes to Bark and Soil Habitats: Insights from Stoichiometry

Understanding the stoichiometric traits of plants is critical for studying their ecological adaptation strategies. Facultative epiphytes (which can also live on the ground) are an important component of epiphytic flora of montane forest ecosystems. However, a key gap persists in our understanding how facultative epiphytes can adapt different nutritional conditions of ground and canopy habitats? To study adaptive strategies of facultative epiphytes and the characteristics of the content and stoichiometric homeostasis of C, N, and P elements, we conducted a field experiment and a greenhouse N and P additions cultivation experiment. We found that epiphytic individuals of facultative epiphytes showed lower C:N and C:P ratios, higher variation in elemental composition, and more pronounced N limitation than terrestrial individuals. Moreover, facultative epiphytes showed strong control over the elemental composition of leaves, and their stoichiometric homeostasis of leaves and stems were stronger than roots. Furthermore, the homeostasis of facultative epiphytes decreased in the order N > P. Our results indicated that epiphytic and terrestrial individuals of facultative epiphytes have difference in nutrient limitation, and they use plastic strategies in different habitats. Epiphytic individuals survive in the intermittent habitat through luxury consumption of nutrient while terrestrial individuals were relatively conservative nutrient users. Furthermore, our results implied that facultative epiphytes maintain stable metabolic leaf activity via variable element concentrations of roots to adapt to highly heterogeneous forest habitats.