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Soil samples were collected in June and October from areas with three land-use types, i.e., Robinia pseudoacacia L. (RP), Caragana korshinskii Kom. (CK), and abandoned land (AL), of which the former two were afforested areas, whereas the latter was not. These areas were converted from similar farmlands 40 years prior. Illumina sequencing of 16S rRNA gene and fungal ITS gene was used to analyze soil bacterial and fungal diversity. Additionally, plant communities, soil properties, fine root biomass, and C, N, and P levels in fine root and microbial biomass were estimated. Compared to AL, the C:N:P stoichiometry in fine root and microbial biomass in the afforested lands was synchronously changed, especially the N:P ratio. Soil microbial diversities were affected by afforestation and were more related to N:P ratio than C:P and C:N ratios. Moreover, Alpha-proteobacteria, Gamma-proteobacteria, and Bacteroidetes were significantly more abundant in afforested soils than in the AL soil, and the abundances of Actinobacteria, Chloroflexi, Cyanobacteria, and Nitrospirae ranked as AL > RP or CK. For fungal taxa, Ascomycota abundance responded positively to afforestation, whereas Basidiomycota abundance responded negatively. Changes of soil microbial taxa were significantly correlated with the N:P ratio in fine root and microbial biomass, which explained 54.1 and 55% of the total variation in bacterial and fungal taxa, respectively. Thus, our results provide evidence that compositions of soil microbial communities are linked to the N:P ratio in the plant-soil system. [ABSTRACT FROM AUTHOR]

Ecological stoichiometry (C:N:P ratios) in soil plays an important role in ecosystem dynamics and functioning; however, its relationship with below-ground microbial diversity following afforestation remains poorly understood. To illustrate the linkage of C, N, and P in soil and microbial biomass, and the effect on the soil bacterial community, soil samples were collected from farmland and from three afforested land, namely Robinia pseudoacacia L., Caragana korshinskii Kom, and abandoned land, which have been arable for the past 40 years. Quantitative PCR and Illumina sequencing of the 16S rRNA genes were used to analyze soil bacterial abundance, diversity, and composition. Additionally, soil properties and C, N, and P levels in soil and microbial biomass were estimated. The results revealed that C, N, and P levels in soil and microbial biomass increased following afforestation, with a significant correlation observed, especially for the N:P ratio. Additionally, a rise in 〈alpha〉- and 〈beta〉-diversity of soil bacteria was observed in response to afforestation, and was linked to soil C:P and N:P ratios. Soil bacterial phyla with high relative abundance (relative abundance > 1%) across all samples showed inconsistent directional trends in the composition response after afforestation. An increased abundance of Proteobacteria, Acidobacteria, and Nitrospirae were observed, while the abundance of Chloroflexi was opposite (P < 0.05) following afforestation. Those were largely influenced by changes in the N:P ratio in soil and microbial biomass. Taken together, significantly increased N:P ratio in soil reflected the deficient P that would be limited for increased microbial biomass and diversity. Thus, P status should be an important factor for sustainable restoration of the forest during the process of afforestation. [ABSTRACT FROM AUTHOR]

In the Loess Hilly Region of China, the widespread conversion of cropland to forestland and grassland has resulted in great increased in organic carbon (C), nitrogen (N) and phosphorus (P) stocks in the shallow soil layers. However, knowledge regarding changes in C, N, and P in deep soil is still limited. To elucidate the responses of deep soil C, N, and P stocks and stoichiometry in response to changes in land use, the soil from a 0–200 cm soil profile was collected from the following three typical land use patterns in the heartland of the region: forestland, grassland, and cropland. Compared with cropland, forestland and grassland had improved soil organic carbon (SOC) and total nitrogen (TN) contents and stocks at most soil depths but decreased total phosphorus (TP) contents and stocks. At soil depths of 0–200 cm in the forestland and grassland, the cumulative SOC stocks were improved by 34.97% and 7.61%, respectively, and the TN stocks were improved by 54.54% and 12.47%, respectively. The forestland had higher SOC, TN and TP contents and stocks compared to the grassland in almost all soil layers. The soil depths of 100–200 cm contained the highest percentages of SOC, TN and TP stocks (47.80%–49.93%, 46.08%–50.05% and 49.09%–52.98%, respectively). Additionally, the forestland and grassland showed enhanced soil C:P, N:P and C:N:P ratios, and the forestland had higher C:P, N:P and C:N:P ratios compared to the grassland. Furthermore, the SOC and TN stocks had significant impacts on the soil C:N, C:P and N:P ratios. It was concluded that afforestation was the best choice for soil nutrient restoration of degraded land, and deep soil provided an extremely important resource for evaluating soil C, N and P pools and cycling. [ABSTRACT FROM AUTHOR]

Abstract The effects of vegetation restoration on plant characteristics and soil physicochemical properties have been widely documented; however, knowledge of the variation in soil ecoenzymatic activity and stoichiometry remains limited, particularly with respect to their relationship with plant and soil variables in restored ecosystems. Here, the vegetation and soil from one farmland and two restored land-use types (Robinia pseudoacacia plantations: RP and abandoned lands: AL) of four age classes (10, 18, 28, and 43-years) were investigated and sampled on the Loess Plateau, China. The activity of C, N, and P-acquiring enzymes (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; leucine aminopeptidase, LAP; and alkaline phosphatase, AP) and other major influencing factors (soil physical properties, soil nutrient contents, and microbial biomass) were determined. It was found that both restoration time and vegetation types significantly affect plant characteristics, soil physical properties, soil nutrient content, and microbial biomass. Soil BG, NAG + LAP, and AP enzyme activities were higher in RP and AL than in farmland, and increased with restoration time in AL. For RP, the activity of these enzymes increased from year 10 to year 28, then decreased from year 28 to year 43. Soil ecoenzymatic C:P and N:P activity ratios were ordered: farmland > AL > RP, and decreased with restoration time in AL and RP; thus, P limitation was stronger in RP than in AL and increased with restoration time in both AL and RP. Soil ecoenzymatic C:N:P acquisition ratios of farmland, AL, and RP deviated from the 1:1:1 ratio and depended on the availability of environmental nutrients and demand for microbial nutrients. The vegetation characteristics and soil physical properties were closely related to the nutrient acquisitions of microbes and, ultimately, contributed towards shaping soil ecoenzymatic activity and stoichiometry (particularly vegetation coverage, belowground biomass, soil water content, soil bulk density, and pH). Furthermore, variation in soil ecoenzymatic activity and stoichiometry was better accounted by dissolved nutrients in the soil (particularly C and N) and microbial biomass (particularly N and P) than by plant characteristics and soil properties. Overall, this study demonstrates that the C:N:P stoichiometry of soil microbes and ecoenzymatic activity is nutrient dependent, rather homeostatic, with the potential to influence nutrient cycling on the Loess Plateau. Graphical abstract Image 1 Highlights • Soil ecoenzymatic C:N:P acquisition ratios deviated from the 1:1:1. • Soil ecoenzymatic activity ratio was not in homeostasis but resource dependent. • P limitation of microbial metabolisms increased with restoration time. • Soil ecoenzymatic stoichiometry were influenced by both plant and soil properties. • Soil dissolved nutrients and microbial biomass have stronger effect on soil ecoenzymatic stoichiometry than plant and soil properties. [ABSTRACT FROM AUTHOR]

Ecological stoichiometry (C:N:P ratios) has been widely used to indicated the complicated microbial ecology system and microbial community, however, limited researches have been done in the anaerobic digestion process. In this study, different responses of microbial community to C:N:P stoichiometry were investigated. Results showed that bacterial diversity were higher than archaea; Beta diversity was decreased then remained stable and strongly correlated to C:N:P ratios, while it was not significant for alpha diversity. Furthermore, considering the archaeal dominant order, Methanomicrobiales showed no significant correlation with C:N:P stoichiometric ratios, but closely related to C, Methanosarcinales was positively influenced by C:N ratio, and Methanobacteriales was significantly and negatively impacted by N:P ratio. For bacteria, Clostridiales and Sphaerochaetales closely and positively related to N, Bacteroidales significantly and negatively related to P, but positively to C:N ratio, and Spirochaetales positively related to C. Moreover, changes of microbial taxa were significantly correlated with all considered process variables, which explained 94.1% and 96.6% of total variation in archaea and bacteria taxa, respectively. It is worthy note that these results could be used to predict the process performance and indicate the complicated microbial community by using the simple C:N, C:P and N:P ratios. • As digestion prolonged, C:N ratio decreased, while C:P and N:P increased. • Microbial diversity strongly related to process properties and C:N:P ratios. • Both archaeal and bacterial dominant order significantly responded to C:N:P ratios. • Process variables explained 94.1% (archaea) and 96.6% (bacteria) of total variation. [ABSTRACT FROM AUTHOR]

Abstract C:N:P stoichiometry in the plants and soils is an important indicator of biogeochemical cycles and functioning in ecosystems; however, the response of plant growth to ecological stoichiometry following afforestation remains unclear. To illustrate the C:N:P stoichiometry connections between the plants and soils and their effects on the growth of plants, soil and plant samples were collected from one farmland site and four Robinia pseudoacacia (RP) forests that were planted for 10, 15, 25, and 35 years. Plant community characteristics, growth of overstory trees and understory plants, physical and chemical properties of the soil, and C:N:P stoichiometry in plants and soil were measured. The results revealed that soil organic C, total N, total P, dissolved organic N, C:P and N:P ratios, and water content significantly increased, whereas soil bulk density, available P, and pH decreased with afforestation age. Leaf C:P and N:P ratios in RP were significantly higher than that in understory biomass because P concentrations gradually decreased in RP leaves and increased in understory biomass. The C, N, and P contents and stoichiometry in the soils and plants were significantly correlated, particularly for N:P ratio that can be used to reveal close coupling between plant and soil nutrients. The N:P ratio in RP leaf and understory biomass increased with afforestation age and varied from 14.44 to 20.81 and 7.59 to 10.96, respectively, suggesting that P limitation gradually increased in RP, and N limitation gradually declined in understory biomass. Furthermore, the responses of overstory trees and understory plant growth to N:P ratios were contradictory, because of their different nutrient acquisition patterns and nutrient-use efficiencies. Therefore, this finding provides evidence that the strong correlation between the plants and soils were tightly coupled to N and P concentrations and N:P ratios, and therefore, have the potential to influence the growth of forests on the Loess Plateau. Graphical abstract Unlabelled Image Highlights • Robinia pseudoacacia afforestation improved soil and plant community characteristics. • Leaf C:P and N:P ratios were higher for Robinia pseudoacacia than for understory plants. • N:P ratios indicate a tight coupling between soil and plant. • P limitation increased in Robinia pseudoacacia and N limitation declined in understory biomass. • N:P ratios negatively correlated with Robinia pseudoacacia growth, but positively correlated with understory biomass. [ABSTRACT FROM AUTHOR]

The afforestation of abandoned land could offer opportunities to sequester soil organic carbon (SOC), promote nutriment elements cycling, improve plant diversity in the plantation understory and provide ecosystem services. The objectives of this study were to identify plant diversity in the plantation understory, quantify the changes in SOC and total nitrogen (TN) storage in deep soil, assess the SOC, TN, and total phosphorus (TP) stoichiometries, and investigate their relationships in the Loess Plateau Region (LPR) undergoing long-term afforestation. Soil samples were collected at a soil depth of 0–200 cm under 30-yr old Robinia pseudoacacia L. and adjacent abandoned sites, and SOC, TN and TP were determined in different soil depth. Additionally, plant composition and diversity in the plantation understory were evaluated. The results showed that land subjected to long-term afforestation had greater plant coverage, plant density, richness index ( R ) and Shannon–Wiener diversity ( H ) compared to abandoned land communities ( P < 0.05). SOC, TN and TP contents in afforested sites were significantly increased in surface soil (0–30 cm) as well as in the underlying soil (100 cm) compared to the corresponding abandoned land sites ( P < 0.05) in most cases. Meanwhile, SOC, TN, and TP stoichiometry in afforested areas were higher than those of abandoned lands and significantly related to understory vegetation diversity ( P < 0.05). In addition, lands subjected to long-term afforestation effectively increased SOC and TN storages compared to abandoned land at soil depths of 0–30 cm and 100–200 cm and were also significantly related to understory vegetation diversity ( P < 0.05). These findings demonstrating that afforestation not only affects SOC and TN stocks in surface soil, but also strongly influences that in deep soil. And it is also indicating that long-term afforestation could greatly affect soil R CN , R CP , and R NP ratios. [ABSTRACT FROM AUTHOR]

• N limitation for microbe increased with increasing elevation. • The rates of SOC decomposition and microbial respiration decreased with elevation. • Global warming relieve microbial N limitation and lead to increased soil carbon release. • DOC:TDN explained more variation for microbial N limitation and microbial processes. Soil microbes have a great influence on the feedbacks of carbon (C)-climate, and their metabolic activities are limited by resource availability. Altitudinal gradients strongly affect soil microbial communities, but the effects on microbial resource limitation and their regulation for C dynamics remain unclear. In this study, we designed an altitudinal gradient experiment that included six altitudinal sites from 1308 m to 2600 m in the Qinling Mountains, China. The enzymatic stoichiometry was determined and modeled to investigate microbial resource limitations and major microbial metabolism processes (e.g., organic C decomposition rate and microbial respiration rate) along the elevation gradient. Other environmental variables including mean annual temperature (MAT) and mean annual precipitation (MAP), the C: nitrogen (N): phosphorus (P) ratio in soil total nutrients, available nutrients, and microbial biomass were also measured. The results showed that soil microbes suffered from N limitation in our study and microbial N limitation significantly increased with increasing elevation. But the rates of both organic C decomposition and microbial respiration greatly decreased with increased elevation. These trends suggest that warming induced by elevation change might relieve N limitation for microbes and lead to increased soil C release. Redundancy analysis (RDA) showed that MAT and soil nutrient stoichiometry, particularly for the DOC: TDN ratio, explained more variations for changes in microbial N limitation and major microbial processes. Collectively, our study demonstrated that the higher microbial N limitation at high elevation may be beneficial to soil carbon accumulation by changing the C: N ratio, which provided insights into microbially mediated soil carbon release under global warming. [ABSTRACT FROM AUTHOR]