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Nitrogen (N) resorption is an important pathway of N conservation, contributing to an important proportion of plant N requirement. However, whether the ratio of N resorption to N requirement may be affected by environmental factors, mycorrhizal types or atmospheric CO2 concentration remains unclear. Here, we conducted a meta‐analysis on the impacts of environmental factors and mycorrhizal types on this ratio. We found this ratio in ectomycorrhizal (EM) trees decreased with mean annual precipitation, mean annual temperature, soil total N content and atmospheric CO2 concentration and was significantly lower than that in arbuscular mycorrhizal (AM) trees. An in situ15N tracing experiment further confirmed that AM trees have a stronger reliance on N resorption than EM trees. Our study suggests that AM and EM trees potentially have different strategies for alleviation of progressive N limitation, highlighting the necessity of incorporating plant mycorrhizal types into Earth System Models. [ABSTRACT FROM AUTHOR]

Evidence is emerging that microbial products and residues (necromass) contribute greatly to stable soil organic matter (SOM), which calls for the necessity of separating the microbial necromass from other SOM pools in models. However, the understanding on how microbial necromass stabilizes in soil, especially the mineral protection mechanisms, is still lacking. Here, we incubated 13C‐ and 15N‐labelled microbial necromass in a series of artificial soils varying in clay minerals and metal oxides. We found the mineralization, adsorption and desorption rate constants of necromass nitrogen were higher than those of necromass carbon. The accumulation rates of necromass carbon and nitrogen in mineral‐associated SOM were positively correlated with the specific surface area of clay minerals. Our results provide direct evidence for the protection role of mineral in microbial necromass stabilization and provide a platform for simulating microbial necromass separately in SOM models. [ABSTRACT FROM AUTHOR]

The organic carbon (C) in soil is mainly from plants via litter decomposition. Here, we developed a new litter decomposition submodel incorporating the microbial biomass effect on the decomposition rate based on the Michaelis‐Menten kinetics. This new submodel was coupled with the existing plant and soil submodels to simulate C cycling in natural ecosystems in the continental United States. The C transfer efficiency (EFF), defined as the percentage of C transferred to the next layer in the plant‐litter‐soil continuum, was quantified in different types of natural ecosystems. We estimated that on average 48.1% of gross primary productivity (GPP) was transferred from plant to litter and 15.1% of litterfall was transferred from litter to soil, meaning that the C that finally enters soil was on average approximately 7.3% of GPP. Ecosystems with a drier climate and lower GPP had higher EFF from plant to soil. The EFF concept we proposed provides an empirical proxy for diagnosing ecosystem C cycling and a framework for projecting the change of C fluxes and C pool sizes in response to climate change. If C transfer can represent energy transfer analogous to Lindeman Efficiency, our results suggest a pattern of resource and energy transfer in nature: higher resource or energy availability usually means lower resource or energy transfer efficiency. Plain Language Summary: Carbon sequestration by ecosystems is important for mitigating global warming. During the transfer of carbon from atmospheric CO2 to plants, then to litter, and then to soil, carbon is continuously lost to the atmosphere via CO2. Here, we adopted the idea of Lindeman Efficiency to express this efficiency of carbon transfer between different layers in the plant‐litter‐soil systems in the continental US. We found that ecosystems with higher productivity have lower carbon transfer efficiency, indicating the tradeoff between the two. Therefore, it is hard to find an ecosystem with both high productivity and high carbon transfer efficiency to soil for carbon sequestration. Our attempt to understand carbon transfer efficiency provides an intuitive proxy of C cycling and a framework for future research on ecosystem carbon sequestration and global warming. Key Points: The transfer of carbon from plant to litter than to soil is more efficient in ecosystems with higher productivityOur litter submodel can directly simulate carbon input fluxes into soil, has intermediate complexity, and shows good performanceThe C transfer efficiency concept we proposed provides an empirical proxy for diagnosing ecosystem C cycling [ABSTRACT FROM AUTHOR]

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer‐term (decadal to century) feedbacks between climate, CO2 and plant N uptake could emerge to reduce ecosystem‐level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N2 fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ15N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem‐level N cycling responses to rising atmospheric CO2 concentrations. We find that pre‐industrial increases in global atmospheric CO2 concentrations corresponded with a decrease in the δ15N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ15N of Sphagnum decreases with increasing N2‐fixation rates. These findings suggest that plant‐microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO2 concentrations, highlighting an ecosystem‐level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome. [ABSTRACT FROM AUTHOR]

Soil microbial biomass and microbial stoichiometric ratios are important for understanding carbon and nutrient cycling in terrestrial ecosystems. Here, we compiled data from 12245 observations of soil microbial biomass from 1626 published studies to map global patterns of microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), and their stoichiometry using a random forest model. Concentrations of MBC, MBN, and MBP were most closely linked to soil organic carbon, while climatic factors were most important for stoichiometry in microbial biomass ratios. Modeled seasonal MBC concentrations peaked in summer in tundra and in boreal forests, but in autumn in subtropical and in tropical biomes. The global mean MBC/MBN, MBC/MBP, and MBN/MBP ratios were estimated to be 10, 48, and 6.7, respectively, at 0–30 cm soil depth. The highest concentrations, stocks, and microbial C/N/P ratios were found at high latitudes in tundra and boreal forests, probably due to the higher soil organic matter content, greater fungal abundance, and lower nutrient availability in colder than in warmer biomes. At 30–100 cm soil depth, concentrations of MBC, MBN, and MBP were highest in temperate forests. The MBC/MBP ratio showed greater flexibility at the global scale than did the MBC/MBN ratio, possibly reflecting physiological adaptations and microbial community shifts with latitude. The results of this study are important for understanding C, N, and P cycling at the global scale, as well as for developing soil C‐cycling models including soil microbial C, N, and P as important parameters. [ABSTRACT FROM AUTHOR]

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Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO 2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer-term (decadal to century) feedbacks between climate, CO 2 and plant N uptake could emerge to reduce ecosystem-level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N 2 fixation-the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ 15 N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO 2 concentrations. We find that pre-industrial increases in global atmospheric CO 2 concentrations corresponded with a decrease in the δ 15 N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ 15 N of Sphagnum decreases with increasing N 2 -fixation rates. These findings suggest that plant-microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO 2 concentrations, highlighting an ecosystem-level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome., (© 2022 John Wiley & Sons Ltd.)