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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]

• Climate change and restoration policies are the primary and secondary factors to the greening of the Loess Plateau. • The difference-in-differences method suggest lowered contribution of the restoration policy to vegetation greening than previously thought. • Using spatially specific starting time of the policy implementation instead of a fixed year can help better estimate the effects of restoration programs. The vegetation greening on the Loess Plateau, China, over recent decades, has been primarily driven by a series of vegetation restoration programs (VRPs) and other natural environmental changes (including climate change, rising atmospheric CO 2 concentration, and nitrogen deposition, etc.). However, accurate determination of the contributions of natural environmental change and the VRPs to this greening has been challenging due to the lack of the exact time of policy implementation in different areas. Herein, we extracted the starting time of greening (SOG) from different sources of remote sensing data and used it as the starting time of the VRPs. Then, we employed a difference-in-differences (DID) method to ascertain the relative contributions of the VRPs on the observed vegetation greening. Results showed that the contribution of the VRPs to vegetation greening on the Loess Plateau was 23.8–35.8 %, with a greater contribution potentially originating from natural environmental factors. Our findings contribute to a better understanding of the complex relationships among vegetation restoration policies and climate change, and offer a platform to quantify the climate effects on vegetation greening. [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]

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]

Nitrogen (N) resorption is a strategy for plant N conservation through which plants withdraw N from senescing leaves prior to litterfall and its underlying mechanisms are important for better understanding of N cycling. However, most current studies focused on the impacts of soil and leaf nutrients on leaf N resorption efficiency (NRE), and plant physiological regulation that is species dependent is still unclear.Here, we conducted a field experiment to investigate the variations of leaf NRE along an altitudinal gradient in a temperate forest of Northeastern China.Results showed that leaf NRE of Quercus mongolica and Fraxinus mandshurica increased with altitude, while leaf NRE of Tilia amurensis, Acer mono and Acer pseudosieboldianum exhibited an opposite trend, although the relationships were not significant for F. mandshurica and A. mono. The inconsistent responses of leaf NRE of different species to increasing altitude were primarily due to the effect of leaf Glutamate dehydrogenase (GDH), an enzyme responsible for N translocation. Leaf GDH activity in senescing leaves explained the variation of NRE more than soil and climate factors did, suggesting that different plant species had different physiological regulation strategies for their N conservation under similar environment.Synthesis. Our study highlights the role of leaf enzyme as a pivotal regulator of leaf NRE and helps us better understand and predict N cycling under climate change in forest ecosystems. [ABSTRACT FROM AUTHOR]