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Forest soil microbes play a crucial role in regulating atmospheric-soil carbon fluxes. Environmental heterogeneity across forest types and regions may lead to differences in soil CO 2 and CH 4 emissions. However, the microbial mechanisms underlying these emission variations are currently unclear. In this study, we measured CO 2 and CH 4 emissions of Robinia pseudoacacia forests along a north-south transect in the Loess Plateau. Using metagenomic sequencing, we investigated the structural and functional profiles of soil carbon cycling microbial communities. Results indicated that the forest CO 2 emissions of Robinia pseudoacacia was significantly higher in the north region than in the south region, while the CH 4 emission was oppositely. This is mainly attributed to changes in gene abundance driven by soil pH and moisture in participating carbon degradation and methane oxidation processes across different forest regions. The gene differences in carbon fixation processes between regions primarily stem from the Calvin cycle, where the abundance of rbcL, rbcS, and prkB genes dominates microbial carbon fixation in forest soils. Random forest models revealed key genes involved in predicting forest soil CO 2 emissions, including SGA1 and amyA for starch decomposition, TYR for lignin decomposition, chitinase for chitin decomposition, and pectinesterase for pectin decomposition. Microbial functional characterization revealed that interregional differences in CH 4 emissions during methane metabolism may originate from methane oxidation processes, and the associated gene abundances (glyA, ppc, and pmoB) were key genes for predicting CH 4 emissions from forest soils. Our results provide new insights into the microbial mechanisms of CO 2 and CH 4 emissions from forest soils, which will be crucial for accurate prediction of the forest soil carbon cycle in the future., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Exploring nutrient limitation in forest soil holds significant implications for forest tending and management. However, current research on nutrient limitation status of microorganisms in Robinia pseudoacacia plantations within the Loess Plateau remains insufficient. To investigate soil microbial nutrient limitation of R. pseu-doacacia plantations on the Loess Plateau, we selected R. pseudoacacia plantations with different afforestation time series (15, 25, 35, and 45 years) and a pile of barren slope cropland (control) in Yongshou County, Shaanxi Province as the research objects. We analyzed the contents of soil organic matter, total nitrogen, and total phosphorus, and the activities of β-1,4-glucosidase (BG), cellobiose hydrolase (CBH), leucine aminopeptidase (LAP), β-1,4-N-acetylglucoside (NAG) and phosphatase (AP). We analyzed the soil nutrient limitation by stoichiometry and enzyme metrology. The results showed a shift in soil pH from alkaline to acidic during vegetation restoration process, and that total phosphorus exhibited a gradual decrease over the course of 0 to 25 years. Soil orga-nic matter, total nitrogen and enzyme activities exhibited an increasing trend during the same time frame. However, between 25 and 45 years of age, soil total phosphorus, soil organic matter, total nitrogen, AP and LAP gradually declined while NAG, BG, and CBH initially increased and then decreased. Notably, the values of (BG+CBH)/(LAP+NAG), (BG+CBH)/AP and (LAP+NAG)/AP in R. pseudoacacia plantations were higher than the global average throughout the process of vegetation restoration. In the study area, the vector length was less than 1 and gradually increased, indicating that a progressive increase in microbial carbon limitation during the process of vegetation restoration. The vector angle exceeded 45° and exhibited an overall decreasing trend, suggesting that soil microorganisms were constrained by phosphorus (P) with a gradual deceleration of P limitation, without any nitrogen (N) limitation. The restoration of R. pseudoacacia plantation resulted in significant change in soil physical and chemical properties, while the time series of afforestation also influenced nutrient limitation of soil microorganisms.

Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species ( Quercus liaotungensis , Platycladus orientalis , and Robinia pseudoacacia ) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density ( J s ) of Q. liaotungensis was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in R. pseudoacacia was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in J s for P. orientalis and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m J s of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between J s and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m 3 ·m -3 for Q. liaotungensis , 0.116 m 3 ·m -3 for P. orientalis was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for 3 ·m -3 for R. pseudoacacia . Below the thresholds, J was the most capable of overcoming soil moisture constraints.s was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for Q. liaotungensis and P. orientalis reached saturation, while that of R. pseudoacacia did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, P. orientalis was the most capable of overcoming soil moisture constraints.

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration., Competing Interests: Declaration of competing interest The authors declare that they have no competing interests., (Copyright © 2023 Elsevier B.V. All rights reserved.)

During periods of water deficit, growing roots may shrink, retaining only partial contact with the soil. In this study, known mathematical models were used to calculate the root-soil air gap and water flow resistance at the soil-root interface, respectively, of Robinia pseudoacacia L. under different water conditions. Using a digital camera, the root-soil air gap of R. pseudoacacia was investigated in a root growth chamber; this root-soil air gap and the model-inferred water flow resistance at the soil-root interface were compared with predictions based on a separate outdoor experiment. The results indicated progressively greater root shrinkage and loss of root-soil contact with decreasing soil water potential. The average widths of the root-soil air gap for R. pseudoacacia in open fields and in the root growth chamber were 0.24 and 0.39 mm, respectively. The resistance to water flow at the soil-root interface in both environments increased with decreasing soil water potential. Stepwise regression analysis demonstrated that soil water potential and soil temperature were the best predictors of variation in the root-soil air gap. A combination of soil water potential, soil temperature, root-air water potential difference and soil-root water potential difference best predicted the resistance to water flow at the soil-root interface., (© The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Juglans regia, Robinia pseudoacacia, Eucalyptus sp. and Populus sp. plantations, suffering from Mn and Cu toxicity, were limed in order to reduce Cu and Mn solubility in soil. The purposes of the present work were: i) to study the changes in soil chemical properties after the addition of CaCO3, ii) to investigate the influence of liming on the reduction of Mn and Cu toxicity. After the addition of CaCO3 (three applications, during three successive years), pH and CaCO3 content were significantly increased, while organic C and N were significantly reduced. Exchangeable Ca concentrations have been slightly, or significantly, increased, while those of Mg have been decreased; in addition, ratios Ca/Mg and C/N have been significantly increased after liming. Impressive reductions of DTPA extractable Cu and Mn concentrations (more than 10 times in most cases) were recorded. It was also found that trees without Mn and Cu toxicity symptoms (healthy tress) before liming did not have, in many cases, significantly greater leaf Mn, Cu and Fe concentrations, than trees after soil liming (all the trees were healthy). This probably happened because excess Mn and Cu quantities had been accumulated into their root system. Finally, leaf Mn, Cu and Zn concentrations of trees suffering from toxicity were significantly decreased after soil liming, while leaf Fe concentrations, in all the plant species studied, were increased., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

In order to understand the carbon sequestration of ecological forests in Loess Plateau, a comparative study was made on the organic carbon density (OCD) of soil, litter, and plant organs in an 8-year-old Robinia pseudoacacia plantation and nearby barren land. Comparing with the barren land, the young R. pseudoacacia plantation had a decrease (0.26 kg x m(-2)) of soil OCD, but the OCD in its litter, root system, and aboveground organs increased by 121.1%, 202.0%, and 656. 7%, respectively, with a total carbon sequestration increased by 3.3% annually, which illustrated that R. pseudoacacia afforestation on Loess Plateau had an obvious positive effect on carbon sequestration.

Aimed to explore the effects of different vegetations and of the years of vegetation restoration on soil microbial biomass carbon and nitrogen, a comparative study was conducted, with the 5 year old Robinia pseudoacacia, Hippophae reamnoide and Prunus armeniaca plantations and the 5, 15 and 25 years old R. pseudoacacia plantation in the Yangjuangou catchment of Yanan City of Shaanxi Province, a typical hilly area of the Loess Plateau, as test objects. The results showed that among the three 5-year old plantations, H. reamnoides plantation had the highest soil organic carbon (SOC) and total nitrogen (TN) contents, while R. pseudoacacia plantation had the highest soil microbial biomass carbon (MBC) (99.56 mg x kg(-1)) and nitrogen (MBN) (28.81 mg x kg(-1)). The MBC was in the order of R. pseudoacacia > H. reamnoides > P. armeniaca, and that of MBN was of R. pseudoacacia > P. armeniaca > H. reamnoides. The MBC/SOC was in the order of R. pseudoacacia > H. reamnoides > P. armeniaca, and that of MBN/TN was of R. pseudoacacia > P. armeniaca > H. reamnoides, with the differences being significant (P < 0.05). With the increasing years of vegetation restoration, the soil pH in R. pseudoacacia plantation decreased, while the SOC, TN, electricity conductance (EC), MBC, and MBN all had an increasing trend, which illustrated that in the hilly area of Loess Plateau, planting R. pseudoacacia was more beneficial to the increase of soil MBC and MBN, and, with the increasing years of this planting, soil MBC, MBN, SOC and TN tended to be increasing.

The study on the root distribution characters of Robinia pseudoacacia and soil moisture status in the Loess Plateau showed that on shady slope, there was no obvious difference in the horizontal distribution of R. pseudoacacia fine roots within 2.0 m from the stem, and the maximal distribution depth of the roots could reach 2.0 m. In sunny slope, the vertical distribution of the roots within 0.5 m from the stem could also reach that depth. The root amount in shady slope was larger than that in sunny slope, and the affected depth of R. pseudoacacia roots on soil moisture status reached 3.3 m in shady slope, while 2.7 m in sunny slope.

With locust (Robinia pseudoacacia), the main tree species in afforestation, as test material, this paper studied the characteristics of its water physiological ecology and productivity under four habitats, i. e. , sunny, shady, semi-sunny and semi-shady hillsides, in the North Shaanxi gully areas of Loess Plateau. The mean water content in 0 - 500 cm soil layer was 8.87% (shady), 8.06% (semi-shady), 7.62% (semi-sunny), and 6.96% (sunny), respectively. There was a significant discrepancy between shady, semi-shady, semi-sunny and sunnyhilisides (a 0.01), and the difference between shady, semi-shady and semi-sunny, as well as between semi-sunny and sunny hillsides was also significant (a 0.05) . Remarkable relationships were found between leaf RWC, locus WSD and soil water content (SWC) . The leaf RWC and locust water potential under sunny hillside were lower, but those under shady hillside were higher. The daily mean transpiration was in order of shady (4.07 microg x cm(-2) x s(-1)) > semi-shady (3.89 microg x cm(-2) x s(-1)) > semi-sunny (3.05 microg x cm(-2) x s(-1)) > sunny (2.70 microg x cm(-2)s(-1)) hillside. The remarkable difference of transpiration appeared at 11:00 and 13:00, and there existed a remarkable relationship between transpiration and light intensity, RH and soil water content. All of these resulted in a diversity of locust biomass under different habitats, the highest under shady hillside, and the lowest under sunny hillside, and the differences between different habitats were all significant. It could be concluded that soil water content was the main factor affecting locust growth.