Aims： Under the background of global climate change, frequent extreme precipitation events will affect the soil nitrogen transformation process in the semi-arid steppe. However, how the key processes of soil nitrogen transformation respond to precipitation change and the sensitivity of total nitrogen mineralization rate to different precipitation levels remains unclear. Methods： In this study, the gross nitrogen mineralization rate (GNM), and gross nitrification rate (GN) of soil were measured by the 15N pool dilution method based on meadow steppe, typical steppe, and desert steppe of the Global Change Network test platform of northern China Steppe. And related biological (microbial biomass carbon, microbial biomass nitrogen, background biomass (BGB)), and abiotic (soil temperature (ST), soil water content (SWC)) soil substrate (soil ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N, soluble organic carbon, soluble organic nitrogen) indexes. Important findings：The results showed that there were significant differences in GNM among different steppe types. The highest GNM was in meadow steppe (3.284 ± 0.613mg·kg-1·d-1), followed with typical steppe (1.370 ± 0.167 mg·kg-1·d-1) and desert steppe (0.724 ± 0.216 mg·kg-1·d-1). However, A 50% decrease in rainfall had no significant effect on GNM and GN of the three grasslands. The sensitivity of GNM and GN to precipitation reduction in typical steppe and desert steppe soil were significantly higher than that of precipitation increase, while the sensitivity of GNM and GN in meadow steppe soil was not significantly different between precipitation increase and decrease. Structural equation model (SEM) analysis revealed that soil moisture was the main factor affecting soil GNM. These results indicate that short-term extreme precipitation has no significant effect on GNM and GN of three important grasslands in northern China, but changes their sensitivity to precipitation change. How the increase of extreme precipitation events will affect the soil nitrogen conversion process in terrestrial ecosystems in the future needs to be systematically studied on long-term scales and large spatial patterns.

Aims  Pinus sylvestris var. mongolica, an evergreen coniferous tree species, plays a pivotal role in ecological restoration efforts in the deserts of northern China. This study aimed to elucidate the community assembly of belowground fungi and the intricate relationships between P. sylvestris var. mongolica and fungi in P. sylvestris var. mongolica plantations. The findings would provide the novel microbial perspectives for sustainable management strategies of P. sylvestris var. mongolica plantations.

Methods  Pinus sylvestris var. mongolica plantations of different stand ages (26 a, 37 a, and 46 a) in the Hulun Buir Sandy Land were selected to examine the diversity, composition and assembly pattern of root-associated fungi (RAF), rhizosphere soil fungi (RhSF) and non-rhizosphere soil fungi (NRhSF).

Important findings  (1) Stand ages and niches significantly influenced fungal diversity. The fungal community richness and diversity indices ranked as follows: 46 a > 26 a > 37 a, and the dissimilarity gradually increased with the increase of the stand age. Among the different niches, the richness, diversity indexes, and dissimilarity were the highest in NRhSF, the middle in RhSF and the lowest in RAF. (2) The belowground fungi were assigned to 14 phyla and 592 genera. The belowground fungal communities of 26 a, 37 a, and 46 a plantations had 3, 1, and 5 abundant genera respectively, and they had symbiotic capability of endophytic or ectomycorrhizal fungi. RAF, RhSF, and NRhSF had 3, 8, and 5 abundant genera, respectively, and the proportions of Mortierellomycota and saprotrophic fungi increased from root to soil. (3) The primary assembly processes of belowground fungal communities were the dispersal limitation (63.54%), drift (22.06%) and homogeneous selection (12.90%). Stand age significantly correlated with structure of belowground fungi. Soil total nitrogen content, soil total nitrogen and phosphorus content, and soil organic matter content were the main factors influencing RAF, RhSF, and NRhSF, respectively. This study highlights temporal and spatial heterogeneity of fungal community diversity and composition in P. sylvestris var. mongolica plantations. Stochastic processes mainly were dispersal limitations, shaping these communities, while the deterministic processes were influenced by host selection and environmental filtering.

Aims In the saline–alkaline grassland of the agro-pastoral ecotone with limited nutrients, the increase of nitrogen input induced by agricultural fertilization and the change of land use patterns usually cause changes in soil available nutrients, which further affect the net primary productivity of plants. However, due to the impact of inter-annual precipitation, there is still great uncertainty about whether the response of net primary productivity of plants to nitrogen addition and mowing will vary with changes in rainfall. Methods This study takes saline-alkali grassland in the agro-pastoral ecotone of northern China as the research object, relies on the experimental platform for nitrogen forms and mowing at the National Research Station of Grassland Ecosystems on the Loess Plateau in Youyu, Shanxi Province, and experiments with six treatments including control and simulated nitrogen deposition experiments with two common nitrogen compounds (ammonium nitrate and urea), with and without mowing. The response of plant net primary productivity to nitrogen addition and mowing was studied. Important findings The results showed that: (1) Regardless of mowing or un-mowing treatments, short-term addition of ammonium nitrate and urea significantly increased the content of inorganic nitrogen in the soil, thereby increasing the aboveground (ANPP), belowground (BNPP), and total net primary productivity (NPP); (2) ANPP, BNPP, NPP, inorganic nitrogen content, and soil water content showed significant interannual differences, with higher values observed in the wet year (2018) than in the dry year (2017); (3) The interaction between short-term nitrogen addition and year had a significant impact on NPP. In the wet year, NPP was significantly higher under nitrogen addition than in the dry year. In the wet year, the positive effect of nitrogen addition on NPP was significantly higher than that in the dry year, which was mainly related to the synergistic effect of soil nitrogen and water; (4) Mowing decreased NPP and had a significant interactive effect with the year on the BNPP/ANPP. In the dry year, mowing generally decreased BNPP/ANPP. However, in the wet year, this negative effect gradually weakened and even turned into a positive effect. These results emphasize the crucial role of natural precipitation in regulating the response of net primary productivity of saline-alkali grassland in the agro-pastoral ecotone to anthropogenic disturbances, and further indicate that the saline-alkali grassland ecosystem in the agro-pastoral ecotone is jointly limited by nitrogen and water.

Changes in subsurface ecological processes in the context of global change have become one of the hot issues of concern in the field of ecology. Mycorrhiza are symbiotic relationships between plant roots and mycorrhizal fungi, widely distributed in terrestrial ecosystems. As two major types of mycorrhizal fungi related to trees, arbuscular mycorrhizal fungi and ectomycorrhizal fungi exhibit significant differences in morphology and function. Root exudates, as an important medium for exchange of matter, energy, and information between plants and soil, play a crucial role in soil carbon dynamics. The root exudates of tree species associated with different mycorrhizal types can actively respond to environmental changes by continuously adjusting their quantity and chemical composition, and influence the belowground carbon dynamics and cycling processes of forest ecosystems. Currently, the composition and function of root exudates from different types of mycorrhizal fungi, as well as the variation and impact on plant and soil, remain unclear. Therefore, the article combines current frontier dynamics in the field both domestically and internationally, and summarizes the root exudate characteristics, influencing mechanisms, and rhizosphere effects of tree species associated with different mycorrhizal types. This review is expected to provide references for further research on the response and adaptation mechanism of root systems and exudates to global changes. In addition, it also proposes directions for future research on root exudates among different types of mycorrhizal fungi that require further investigation: (1) strengthening systematic research on root exudates among different types of mycorrhizal fungi; (2) studying the influence mechanism of mycorrhizal type on root exudates in combination with other environmental factors; (3) using more precise technological means to comprehensively understand the changes in root exudate characteristics among different types of mycorrhizal fungi; (4) deeply revealing the influencing mechanism of root exudates among different types of mycorrhizal fungi from the perspective of plant physiology and metabolism; (5) conducting long-term dynamic monitoring and simulation experiments on different types of mycorrhizal fungi to predict their impact on soil ecological processes.

Aims: Drought effects on the carbon balance are considered as the major factor of tree mortality and are as-sumed to be regulated by soil nutrient (e.g., nitrogen) availability. However, the effects of nitrogen addition on trees carbon and nitrogen utilization between aboveground and belowground and on coupling relationship of carbon and nitrogen in various organs in response to drought are still unclear in trees. Methods: A two-year full factorial microcosm experiment was set up with sessile oak (Quercus petraea). Nitrogen addition was performed in the first year, and drought was conducted in the second year. The isotope 15N labeling and 13C labeling were carried out before drought and during drought, respectively. Three consecutive samplings were conducted after dual labeling of 13C and 15N labelling in the second year, and the effects of ni-trogen addition on carbon and nitrogen allocation dynamics during progressive drought were tested. Important findings: Our results showed that previous nitrogen addition promoted aboveground photosyn-thetic carbon fixation and nitrogen allocation, increased root nitrogen uptake, reduced the non-structural carbo-hydrates (NSC) contents in all organs and changed the relationships of carbon and nitrogen in aboveground and belowground organs in sessile oak. However, drought had minor effects on nitrogen and carbon allocation be-tween aboveground and belowground, and had minor effects on the relationship of carbon with nitrogen in all organs (represented by the ratio of 13C to 15N in all organs). Drought only significantly reduced the content of NSC in sessile oak. During drought (from day 40 to 73), previous nitrogen addition led sessile oak to prioritize belowground carbon and nitrogen allocation. Our results indicate that sessile oak can change its carbon and ni-trogen allocation strategies to adapt to drought well, while previous nitrogen addition may increase its drought sensitivity.

Aims Plant sexual systems is an important reproductive role that affects plant mating, genetics, evolution, and species distribution. They are classified into three categories based on the floral organ's pistil and stamen arrangement: hermaphroditism, dioecy, and monoecy. Methods This article takes forest communities in Daiyun Mountain (elevation 900-1600 m) as the research object, analyzing changes in the quantitative characteristics of woody plant sexual systems and investigating their correlation with environmental factors via Mantel correlation analysis. Important findings The study area encompassed 85 woody plant species, with 49 hermaphroditic (57.6%), 26 dioecious (30.6%), and 10 monoecious (11.8%) species. Altitudinal increase led to a significant rise in the number of hermaphroditic and dioecious individuals, with no significant change in their species ratio, while monoecious individuals' number and species ratio significantly declined. The Shannon Wiener index and Pielou evenness index for sexual systems generally decreased with altitude, whereas the Simpson dominance index remained stable. Effective phosphorus and soil temperature were identified as the primary drivers of altitudinal changes in sexual systems characteristics. In conclusion, the quantitative characteristics of the sexual systems of woody plants on the altitudinal gradient of Daiyun Mountain showed significant differences, and the altitudinal distribution of the sexual systems was highly sensitive to environmental changes, indicating that plants adapt to environmental changes by regulating the composition of their reproductive systems, thereby ensuring the continuous survival and reproduction of their populations.