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Cover crops provide multiple agronomic and environmental benefits, such as enhanced soil carbon sequestration, aggregation, water infiltration, and reduced erosion and nutrient leaching compared with no cover crop. However, little is known regarding the effect of cover crop species, biomass quality and quantity, and method of residue placement on greenhouse gas (GHG) emissions. Using meta-analysis, this study examined the effect of cover crop species, quality and quantity of biomass, and residue management on response ratios (RRs) of cover crops to no cover crop on CO2 and N2O emissions following cash crops. All cover crop species increased CO2 emissions, but reduced N2O emissions compared with no cover crop, except legumes which increased N2O emissions. Cover crop biomass explained 63% of variability in increased CO2 emissions and 55% of variability in decreased N2O emissions. Both CO2 and N2O emissions decreased polynomially with increased cover crop biomass C/N ratio, with greater rate of decline for legumes than nonlegumes. Cover crop residue incorporated into the soil increased CO2 and N2O emissions compared with the residue placed at surface or removed from the soil. Cover crops emitted lower CO2 and N2O emissions than no cover crops in silty loam and sandy clay loam soils, respectively, compared to clay loam and silty clay loam soils. Both soil organic carbon and total nitrogen increased with cover crop compared to no cover crop. Although CO2 and N2O emissions varied with cover crop species, using legume and nonlegume cover crop mixture to enhance residue C/N ratio compared to legumes and placing the residue at the surface instead of incorporating into the soil can reduce GHG emissions. Because of enhanced soil C and N storage and other known benefits, improvement in soil health and environmental quality due to cover crop may outweigh CO2 emissions compared to no cover crop.

Soil microbial communities are closely associated with aboveground plant communities and soil properties. This study explored the dynamics of soil bacterial communities during the plant growing season and their interaction with soil carbon and nitrogen, physicochemical properties, plant biomass and diversity following afforestation. The study investigated afforested lands containing Robinia pseudoacacia L (RP) and Caragana korshinskii Kom (CK), as well as abandoned land (AL) and farmland (FL) in the Loess Hilly Region of northern China. After establishment on former FL for 40 years, the Richness and Shannon's index, above-ground herbaceous biomass (AGB) and below-ground biomass (BGB) in RP and CK increased by 0.5, 0.1, 1.2 and 1.7 times, respectively, compared with AL. Additionally, the soil organic carbon (SOC), total nitrogen (TN), dissolved organic carbon/nitrogen (DOC/N) and microbial biomass carbon/nitrogen (MBC/N) increased by 1.7, 1.9, 0.8 and 2.5 times, respectively, while the soil bacterial abundance and alpha diversity (Shannon index) increased by 4.7% and 2.0% compared with FL. During the growing season, the soil carbon and nitrogen content was higher in August or October than in April or June. Particularly, the AGB, DOC and MBC showed a temporal pattern of normal distribution that was 36.7%, 48.2% and 74.9% higher in August than the rest of the study period. The BGB also increased by 123.5% in October compared with June in all study areas. The soil bacterial abundance in afforested lands was also higher in October, and beta bacterial diversity (Principal coordinates analysis plots: PCoA) was greater in June and August than in April and October. The dominant bacterial taxa, Proteobacteria and Bacteroidetes, were significantly higher in afforested lands than in FL, and Proteobacteria increased from April to October, while Actinobacteria and Acidobacteria peaked in June or August. These seasonal changes in the soil bacterial community were significantly correlated with carbon and nitrogen levels of soil and plant biomass, especially with DOC and BGB. Additionally, soil variables explained 41.2% of the total variation in bacterial-dominant phyla, while plant biomass only explained 20.2% of it, which indicates that soil properties are more important controlling factors of the bacterial community than plant biomass. In conclusion, soil variables change seasonally, which drives the composition and diversity of the soil bacterial community to exhibit a temporal pattern over the growing season after return of AL to forest.

Mulching enhances soil C and N fractions compared to no mulching, but its impact on soil microbial communities and their relations to C and N fractions with mulching are not clear. We studied the 9-yr effect of no mulching (CK), straw mulching (SM), and plastic film mulching (PM) on soil bacterial and fungal community structure and their relationships to soil C and N fractions under dryland winter wheat in the Loess Plateau of China. The SM did not affect bacterial diversity and richness, but enhanced fungal diversity and richness compared to CK in subsoil layers. The PM also increased fungal diversity and richness, but reduced bacterial diversity and richness compared to SM and CK. Compared to CK, the relative abundance of Actinobacteria was lower with SM at 0–10 cm, but the abundances of Nitrospirae, Firmicutes, WS3, and Zygomycota were greater with PM at most soil depths. Bacterial diversity correlated with soil C and N fractions, and fungal richness with potential N mineralization and microbial biomass C and N. Although plastic film mulching enhanced fungal diversity and richness compared to no mulching, straw mulching was more effective in maintaining soil microbial diversity and richness and enhancing soil C and N fractions under dryland winter wheat.

Secondary succession has great impact on plant and soil characteristics, however, the trends of microbial patterns and the influencing factors during grassland succession without human disturbance remains unclear. Therefore, we investigated the changes of bacterial community in sloped farmlands abandoned for 0, 20, 30, and 40 years (GL-0 yr, GL-20 yr, GL-30 yr, and GL-40 yr). Additionally, plant traits (coverage, diversity, richness, evenness, biomass, and biomass carbon) and soil nutrients were also determined. The results showed that soil bacterial alpha diversity was positively and significantly correlated with the succession time, and the secondary succession greatly affected soil bacterial beta diversity, in contrast, the effects on soil bacterial beta diversity at the late succession time (GL40 and GL30) were larger than that at the early succession time (GL20). For the bacterial taxa, the dominant phyla including Actinobacteria (34.8%), Proteobacteria (26.0%), Acidobacteria (15.0%), Chloroflexi (7.5%), Gemmatimonadetes (8.7%), Nitrospirae (1.6%), Bacteroidetes (2.1%), Verrucomicrobia (1.1%), and Planctomycetes (1.0%) were found. Particularly, the relative abundance of Proteobacteria was higher at the late time (RP40), while the Actinobacteria was higher at the early time (RP20). Such different responses of bacterial diversity and taxa were largely explained by plant traits and soil nutrients, especially for TOC and TN. Collectively, our results indicate that plant secondary succession shifts the bacterial community structure, largely driven by changes in soil nutrients and plant diversity and composition, and also supported the growing view that soil bacterial community are the key determinants of aboveground and belowground linkages that functionally control terrestrial ecosystems.

Straw mulching has been widely adopted in dryland cropping but its effect on soil respiration and microbial communities under warming are not well understood. Soil samples were collected from a corn field with straw mulching (SM) for nine years and without straw mulching (CK), and incubated at 15°C, 25°C, and 35°C for 60 days. Soil respiration, C fractions and bacterial and fungal community structure were measured SM had greater soil organic carbon and potential C mineralization and a similar microbial biomass carbon throughout the incubation when compared with CK. Soil respiration increased with increasing temperature and its temperature sensitivity (Q10) was lower with SM than CK. Similar microbial community composition was found in the soils with SM and CK before incubation. However, SM had a greater bacterial richness and the relative abundances of Proteobacteria, Acidobacteria, Nitrospirae, Planctomycetes, Bacteroidetes, and Basidiomycota, but lower relative abundances of Actinobacteria, Chloroflexi, and Ascomycota than CK after incubation. Bacterial richness and diversity were greater at 15°C and 25°C than 35°C, but there was no difference in fungal richness and diversity among the incubation temperatures. As temperature increased, the relative abundances of Chloroflexi, Acidobacteria, and Bacteroidetes decreased, but Gemmatimonadetes and Ascomycota increased, and were significantly correlated with soil C fractions and respiration. These findings indicated that the effect of straw mulching on soil C cycling and microbial community structure can be highly modified by increasing temperature.

Soil aggregation is one of the most important factors affecting soil organic carbon (SOC) stabilization, and the stability of aggregates depends in part on soil microbial diversity and composition. Interactions between the soil bacterial community and SOC content in soil aggregates after afforestation are poorly understood. In this study, we investigated difference in the diversity of soil bacterial with high-throughput 16S rRNA sequencing, as well as the SOC content in soil aggregates representing a chronosequence of 42, 27, and 17 years of Robinia pseudoacacia L. succession (RP42, RP27, and RP17), and in farmland (FL) soil for comparison (millet (Setaria italica) and soybean (Glycine max) rotation).The SOC content in RP17, RP27, and RP42 plots were significantly higher than that of FL by an average of 85.57%, 142.37%, and 76.69% in large macro-aggregates (>1 mm), small macro-aggregates (0.25–1 mm), and micro-aggregates (

Afforestation has been shown to strongly affect substrate stoichiometry and profoundly influence the microbial community. However, the degree to which microbial and activity are linked to soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry in afforested ecosystems remains unclear. In this study, soil samples were collected from Robinia pseudoacacia L. (RP42, RP27, and RP17) and farmland (FL) sites with a chronosequence of 42, 27, and 17 years. We determined the microbial biomass C (Cmic), N (Nmic), Cmic to organic C (Csoil) ratio (Cmic:Csoil), Nmic to total nitrogen (Nsoil) ratio (Nmic:Nsoil), and metabolic quotient (Rmic:Cmic) to investigate microbial. We also measured soil b-glucosidase (BG), N-acetylglucosaminidase (NAG), acid phosphatase (AP), as well as Csoil, Nsoil, and total phosphorus (Psoil). The results showed that, compared with FL, Csoil:Psoil and Nsoil:Psoil were increased during aggradation, whereas Csoil:Nsoil increased in RP27 and RP17 but decreased in RP42. Cmic:Csoil and Nmic:Nsoil were 94% and 182% higher in RP42 than FL, respectively. However, Rmic:Cmic was lower in RP42, RP27, and RP17 than FL by 64%, 36% and 25%, respectively. Moreover, we found that BG, NAG, AP, BG:NAG, and BG:AP in RP42, RP27, and RP17 were higher than in FL. Compared with global soil (0.62 and 0.13), the lower BG:AP (0.42) and BG:NAG (0.03) ratios in present study indicated that Psoil may be limited. Additionally, redundancy analysis (RDA) revealed that the Csoil: Nsoil was positively correlated with Rmic: Cmic but negatively with Cmic: Csoil and Nmic: Nsoil, while Csoil:Psoil and Nsoil:Psoil were also significantly and negatively correlated with BG:AP, BG:NAG, and NAG. Therefore, our results indicated that afforested ecosystem are highly susceptible to changes in soil microbial and enzyme stoichiometry during aggradation and may become P-limited. Such changes were modulated by soil nutrient stoichiometry.

Straw mulching has been used to conserve soil water and sustain dryland crop yields, but the impact of the quantity and time of mulching on soil C fractions are not well documented. We studied the effects of various amounts and times of wheat (Triticum aestivum L.) straw mulching on soil C fractions at 0–10- and 10–20-cm depths from 2009 to 2017 in the Loess Plateau of China. Treatments were no mulching (CK), straw mulching at 9.0 (HSM) and 4.5 Mg ha−1 (LSM) in the winter wheat growing season, and straw mulching at 9.0 Mg ha−1 in the summer fallow period (FSM). Soil C fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). All C fractions at 0–10 and 10–20 cm were 8–27% greater with HSM and LSM than FSM and CK. Both SOC and POC at 0–10 cm increased at 0.32 and 0.27 Mg ha−1 year−1 with HSM and at 0.40 and 0.30 Mg C ha−1 year−1 with LSM, respectively, from 2009 to 2017. Winter wheat grain yield was lower with HSM and LSM, but total aboveground biomass was greater with HSM than other treatments. All C fractions at most depths were correlated with the estimated wheat root residue returned to the soil and PCM at 0–10 and 0–20 cm was correlated with wheat grain yield. Wheat straw mulching during the growing season increased soil C sequestration and microbial biomass and activity compared with mulching during the fallow period or no mulching, regardless of mulching rate, due to increased C input, although it reduced wheat grain yield. Continuous application of straw mulching over time can increase soil C sequestration by increasing nonlabile C fractions while decreasing labile fractions. Straw mulching at higher rate and mulching during the summer fallow period had no additional benefits in soil C sequestration.

Stimulation of root growth from mulching may enhance soil C fractions under maize (Zea mays L.). We studied the 5-yr straw (SM) and plastic film (PM) mulching effect on soil C fractions and maize yield compared with no mulching (CK) in the Loess Plateau of China. Soil samples collected from 0- to 10- and 10- to 20-cm depths after maize harvest in the fall, 2011 to 2015, were analyzed for soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At both depths, all C fractions were 7 to 35% greater with SM than PM and CK. At 0 to 20 cm, SOC increased at 0.87 Mg C ha–¹ yr–¹ with PM and POC increased at 0.18 and 0.54 Mg C ha–¹ yr–¹ with SM and PM, respectively, from 2011 to 2015. Maize grain yield and aboveground biomass were 5 to 33% greater with PM and SM than CK. The PCM and MBC at all depths were negatively correlated with maize grain yield, but SOC and POC at 10 to 20 cm were positively correlated with estimated maize root residue returned to the soil. Plastic film mulching increased maize yield, but reduced soil C fractions compared with straw mulching. Because of favorable effect on soil C fractions and maize yield, straw mulching can enhance soil C sequestration by increasing labile and intermediate C fractions and sustain maize yield compared with no mulching under dryland cropping systems.

Land-use change can modify terrestrial ecosystem processes with potentially important effects on below-ground carbon dynamics. Soil microbes are considered the rate-limiting factor in carbon decomposition. However, the effect of land-use change on soil microbial community and the mechanism of soil carbon dynamics remain unclear. In this study, soil samples were collected during four periods (April, June, August, and October) at sites in the Loess Plateau in China with different land- use types: Robinia pseudoacacia L. (RP) and abandoned land (AL); these areas were converted 40 years ago from similar farmlands, while the millet (Setaria italica) farmlands (FL) were selected as a control in our study. Quantitative PCR and Illumina sequencing of the 16S rRNA and ITS genes were performed to analyze the abundance, diversity, and compositions of the soil microbes (bacteria and fungi). Additionally, soil organic carbon fractions (soil organic carbon: SOC, dissolved organic carbon: DOC, microbial biomass carbon: MBC) and soil respiration components (soil respiration: SR, heterotrophic respiration: HR, autotrophic respiration: AR) were evaluated. The results showed that SOC fractions and soil respiration increased after land-use change, with significant correlation being observed. In particular, DOC was more related to SR and HR than to the other fractions. Moreover, the abundance and diversity of the microbes (bacteria and fungi) were greatly affected by the land- use change; both of them were significantly and positively correlated with soil organic carbon fractions and soil respiration components. For dominant bacterial phyla, both Proteobacteria and Bacteroidetes were significantly more abundant in the afforested soil than in the FL, while the abundances of Actinobacteria and Chloroflexi ranked as FL > AL > RP. For dominant fungal phyla, Ascomycota responded positively to land- use changes, whereas Basidiomycota responded negatively. Such changes in the abundances of microbial phyla were significantly correlated with the linkage of soil organic carbon fractions and respiration components. Altogether, these results suggest that the changes in components of soil respiration may be highly susceptible to soil organic carbon fractions, especially to DOC, and this linkage is largely modulated by microbial community across land-use changes.

Soil bacterial community structure, enzyme activities and their relationships to soil carbon and nitrogen in response to long-term fertilization remain poorly understood. Therefore, the objective of this study was to evaluate soil carbon and nitrogen fractions, enzyme activity, and bacterial community structure at 0–15, 15–30, and 30–60 cm depths after 34-yr of continuous application of manure and inorganic fertilizers. The study had a randomized complete block design with six treatments and three replications. Treatments were inorganic nitrogen fertilizer only (N), nitrogen plus phosphrous fertilizers (NP), manure (M), nitrogen plus manure (NM), nitrogen plus phosphorus plus manure (NPM), and unfertilized control (CK) in a winter wheat (Triticum aestivum L.) monoculture system. Most soil carbon and nitrogen fractions at 0–15 and 15–30 cm were greater with M, NM, and NPM, and winter wheat yield was greater with NPM than other treatments. The NPM increased β-glucosidase, β-xylosidase, and β-N-acetylglucosidase compared to other treatments at all depths. Soil bacterial Shannon index was similar among treatments at 0–15 and 15–30 cm and lower in N and NP than other treatments at 30–60 cm. Compared to CK, inorganic and manure fertilization increased relative abundances of Gemmatimonadetes and Bacteroidetes but decreased those of Nitrospirae, Planctomycetes, and Latescibacteria. Increases in soil enzyme activities and bacterial communities after long-term application of inorganic N and P fertilizers and manure was related to increased substrate availability. Overall, a combination of chemical fertilizers and manure can enhance soil health and quality through increased soil organic matter component, enzyme activity, and bacterial abundance.

Altered precipitation regimes have a great impact on global climate change, with potentially important effects on below-ground carbon dynamics. Soil microbes and carbon (C)-degrading extracellular enzymes activities (EEAs) are considered as the rate-limiting step in C decomposition. However, the effect of altered precipitation on the microbial biomass, microbial EEAs, and mechanism for soil C dynamics has not been established. In current study, we synthesized the responses of microbial biomass, C-degrading EEAs, soil organic carbon (SOC), dissolved organic carbon (DOC), and soil respiration (SR) to altered precipitation from 70 published studies. The results showed that increased precipitation significantly enhanced soil microbial biomass and oxidative C-degrading EEAs (Ox-EEAs) by 16.18% and 6.58%, respectively, but had no effects on hydrolytic C-degrading EEAs (Hy-EEAs). Decreased precipitation led to a significant decline of soil microbial biomass and Ox-EEAs by 11.61% and 10.99%, respectively; however, Hy-EEAs increased by 25.79%. Furthermore, increased precipitation stimulated soil microbial biomass in shrub and grassland but had no effects in forest, while decreased precipitation repressed soil microbial biomass but increased Hy-EEAs in forest. The response ratios (RRs) of microbial biomass to altered precipitation were negatively correlated with the mean annual precipitation (MAP) and mean annual temperature (MAT); however, the RRs of Hy-EEAs were positively correlated with MAP. Particularly, in low MAP (≤600 mm), increased precipitation significantly increased soil microbial biomass by 21.40% but decreased precipitation did not affect soil microbial biomass; In contrast, in high MAP (>600 mm), decreased precipitation significantly declined soil microbial biomass by 15.37%, and significantly increased Hy-EEAs by 29.31% but increased precipitation did not affect both of them. Moreover, the RRs of microbial biomass and Ox-EEAs were significantly correlated with SOC, DOC, and SR; however, we only observed a negative relationship between RRs of Hy-EEAs and RRs of SOC, suggesting the concurrent responses of microbial biomass and C-degrading EEAs for below-ground C dynamics under simulated precipitation changes, and that a large amount of recalcitrant C in the soils would be highly susceptible to changing precipitation.

Soil microorganisms regulate carbon (C) transfer from terrestrial sources to the atmosphere, therefore playing a pivotal role in soil C dynamics. Worldwide, grazing is one of the most prevalent grassland management strategies, yet the effects of grazing on soil microbial community size and soil respiration (SR) are still active areas of debate. We conducted a meta-analysis of 71 publications to synthesize the responses of soil microbial community size and SR to grazing. Our results showed that grazing significantly decreased soil total microbial, bacterial and fungal community size by 11.74, 8.85 and 11.45%, respectively. However, this effect were differed when the studies were grouped by the grazing intensity. Briefly, light and moderate grazing intensity had no effect on soil microbial, bacterial and fungal community size, but heavy grazing intensity significantly reduced soil’s total microbial, bacterial and fungal community size by 14.79, 16.48 and 28.12%, respectively. The responses of microbial community size to grazing were positively correlated with those of SR both under moderate and heavy grazing intensity. Our findings indicate that soil microbial community size could be an important underlying mechanism involved in determining soil C dynamics under grazing. Hence better understanding of the responses of soil microbial community size would greatly contribute to our understanding of soil C dynamics. Lastly, our results underscore the importance of factoring grazing intensity into consideration to further improve the model’s projection of soil C dynamics.

Afforestation is recognized as an important driving force for soil organic C (SOC) dynamics and soil element cycling. To evaluate the relationships between soil C:N:P stoichiometry and SOC fractions, soil C:N:P stoichiometry distributions at 0–200 cm soil depths were analyzed and the contents of SOC fractions were evaluated in 9 typical land-use systems on the Loess Plateau of China. The contents of light fraction organic C, particulate organic C (> 53, 53–2 000, and > 2 000 µm), labile organic C, microbial biomass C, and dissolved organic C decreased with increasing soil depth and were higher in afforested soil than in slope cropland soil. Compared with the slope cropland, different vegetation types influenced soil C:N, C:P, and N:P ratios, especially when C:P and N:P ratios were significantly higher (P < 0.05). Moreover, SOC fractions at the 0–10 and 10–40 cm depths were particularly affected by soil C:P ratio, whereas those at the 40–100 and 100–200 cm soil depths were significantly affected (P < 0.05) by soil N:P ratio. These results indicate that changes in SOC fractions are largely driven by soil C:P and N:P ratios at different soil depths after afforestation.

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.

Microbial biomass, extracellular enzyme activity, and their stoichiometry in soil play an important role in ecosystem dynamics and functioning. To better understand the improvement of sand soil quality and the limitation of soil nutrients after adding feldspathic sandstone, we investigated changes in soil microbial activity after 10 months of mixing feldspathic sandstone and sand, and compared the dynamics with soil properties. We used fumigation extraction to determine soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and microplate fluorometric techniques to measure soil &beta, 1,4-glucosidase (BG), &beta, 1,4-xylosidase (BX), &beta, D-cellobiohydrolase (CBH), N-acetyl-&beta, glucosaminidase (NAG), and Alkaline phosphatase (AKP). We also measured soil organic carbon (SOC), pH, electrical conductivity (EC), soil inorganic carbon (SIC), and soil water content (SWC). Our results showed that the soil microbial biomass C, N, P, and individual extracellular enzyme activities significantly increased in mixed soil. Similarly, the soil microbial biomass C:N, C:P, N:P, MBC:SOC, and BG:NAG significantly increased by 54.3%, 106.3%, 33.1%, 23.0%, and 65.4%, respectively. However, BG:AKP and NAG:AKP decreased by 19.0% and 50.3%, respectively. Additionally, redundancy analysis (RDA) and Pearson&rsquo, s correlation analysis showed that SWC, SOC, porosity and field capacity were significantly associated with soil microbial biomass indices (i.e., C, N, P, C:N, C:P, N:P in microbial biomass, and MBC:SOC) and extracellular enzyme activity metrics (i.e., individual enzyme activity, ecoenzymatic stoichiometry, and vector characteristics of enzyme activity), while pH, EC, and SIC had no correlation with these indices and metrics. These results indicated that mixing feldspathic sandstone and sand is highly susceptible to changes in soil microbial activity, and the soil N limitation decreased while P became more limited. In summary, our research showed that adding feldspathic sandstone into sand can significantly improve soil quality and provide a theoretical basis for the development of desertified land resources.

Variability in soil enzyme activity may have important implications for the knowledge of underground ecosystem functions driven by soil extracellular enzymes. To illustrate the temporal variation in soil enzyme activity after afforestation, we collected soil samples during different vegetative growth periods in three Caragana korshinskii Kom. stands of different ages (20, 30, and 40 years) and in a slope cropland in the Loess Plateau. These samples were used to analyze the catalase, sucrase, urease and alkaline phosphatase activities, the soil water content and the available soil nutrients (i.e., dissolved organic carbon, dissolved organic nitrogen, and available phosphorus). The results illustrated that the soil enzyme activities significantly increased following afforestation and varied with temporal variation. Overall, soil enzyme activities were higher in June and August, particularly, and both alkaline phosphatase and sucrase were more sensitive to temporal variation than the other two enzymes. In addition, redundancy analysis showed that soil enzyme activities were greatly correlated with soil nutrients, especially for dissolved organic carbon and dissolved organic nitrogen. Therefore, the results highlighted the importance of soil enzyme activities to soil nutrients under temporal variation following afforestation in the Loess Plateau, which may have practical significance for forest managers&rsquo, fertilization management of plantation in different seasons and different stand ages.

Plant nitrogen (N) and phosphorus (P) resorption is an important strategy to conserve N and P in the face of nutrient limitation. However, little is known about the variation of N and P resorption efficiency (NRE and PRE) and their correlation with leaves and soil C:N:P stoichiometry in black locust forests (Robinia pseudoacacia L.) of different ages. In this study, we measured C, N, and P concentrations in soil, green leaves, and senesced leaves from black locust forests of different ages (i.e, 10-, 20-, 30-, 36-, and 45-year-old), and calculated the NRE, PRE, and C:N:P stoichiometry ratios. The NRE and PRE tended to increase and then decrease with stand age, ranging from 46.8% to 57.4% and from 37.4% to 58.5%, with averages of 52.61 and 51.89, respectively. The PRE:NRE decreased with increased stand ages. The C:P and N:P of soil and green leaves increased with stand ages, indicating the increase of P limitation. In the senesced leaves, C:P and N:P were lower than in green leaves and first increased and then decreased with stand age. The PRE was significantly negatively correlated with the C:P and N:P of soil and green leaves. The NRE was significantly correlated with the C concentration of green leaves, P of the senesced leaves, and C:N. Results suggested that the NRE and PRE responded differently to soil and plant nutrients in black locust forests of different ages. In addition, the black locust plantations would alter the conservation and use strategy of nutrients in the ecosystem through a plant-mediated pathway. Future studies should elucidate the central nutrient utilization strategy of black locust in response to a nutrient-poor environment and determine how it is involved in regulating nutrient resorption.

The distribution patterns of the microbial community and enzyme activity in soil systems along an elevation gradient have attracted considerable attention; however, the differences in microbial diversity and enzyme activity between the rhizosphere and bulk soil and their drivers are still unclear. Here, we used an elevation gradient that covered six elevation levels and ranged from 1308 to 2600 m above sea level. Illumina MiSeq sequencing of the 16S rRNA gene and ITS-1 gene was used to analyze the community of bacteria, total fungi, ectomycorrhizal (EcM) fungi, and saprotrophic fungi in both rhizosphere and bulk soil; in addition, the soil enzyme activity (β-glucosidase, N-acetyl-glucosaminidase, leucine aminopeptidase, and acid phosphatase) was investigated. The results revealed that the elevation significantly affected the diversity of the bacterial, total fungal, EcM, and saprotrophic fungal community, as well as the enzyme activity dynamics. The difference in the microbial diversity and enzyme activity between rhizosphere and bulk soil diminished as the elevation increased, except for the saprotrophic fungal diversity. Similarly, the dominant phyla from the compositions of bacteria, fungi, EcM fungi, and saprotrophic fungi, such as Proteobacteria, Acidobacteria, Actinobacteria, Basidiomycota, and Ascomycota, also changed with elevation and rhizosphere. In addition, the elevation-dependent differences in the microbial community and enzyme activity between the rhizosphere and bulk soil were affected mainly by climatic factors (mean annual temperature and precipitation) and soil properties, such as the bulk density, ammonium nitrogen, and total phosphorus. The effects of the climatic factors were greater than those of the soil properties along the elevation gradient. These results suggest that changes in climatic factors, such as temperature, with elevation may affect the microbial interaction between roots and the soil. The result highlight the importance of the ecological roles of the microbial community in climate change.

Microbial C/N/P stoichiometry ratios are of interest because of important ecosystem fluxes of C, N, and P, such as mineralization and immobilization, and they can help to characterize the cycling of soil elements, especially regarding soil C sequestration. However, it is not clear how soil microbial C/N/P stoichiometry influences C cycling. In this study, the concentrations of microbial biomass C, microbial biomass N, microbial biomass P, and soil C fractions were measured in 45-, 40-, and 25-yr-old black locust (Robinia pseudoacacia L.) forest soils and a sloped cropland. The results showed that soil organic C (SOC) concentrations or stocks and the percentage increment of the SOC stocks were highest under 45-yr-old black locust (RP45a). The concentrations of each C fraction in the black locust forest soils (45a, 40a, and 25a) were higher than that of sloped cropland in the 0- to 30-cm soil profile. The concentrations of microbial C, N, and P in RP45a were also higher by an average of 29.6 to 232.9, 9.2 to 17.9, and 0.6 to 1.1 mg kg⁻¹, respectively, compared with the same concentrations in sloped cropland at the 0- to 30-cm soil depth. Redundancy analysis indicated that soil C fractions, especially for particulate organic C (53–2000 and >2000 μm), were significantly correlated with the microbial C/P and N/P ratios, and the “best” model selection indicated that the microbial N/P and C/P ratios significantly changed with soil C fractions with regard to stand age and soil depth in the black locust forests. Therefore, the present study demonstrated that C fractions respond to microbial C, N, and P stoichiometry after farmland-to-forest conversion and hence have the potential to affect C storage in the loess hilly region.

Temporal variation in soil enzyme activities has important significance for soil quality after afforestation. This study investigated the temporal variation in soil enzyme activities and their response to changes in soil properties following afforestation. Soil samples were collected during different periods of vegetation growth at sites in the Loess Plateau with different land use types: 40 year-old Robinia pseudoacacia L. (RP40a), Caragana Korshinskii Kom (CK40a), and abandoned land (AL40a), as well as millet ( Setaria italica ) farmland (FL). Activities of four enzymes (catalase, saccharase, urease and alkaline phosphatase) that are involved in C, N, P cycling and soil water content (SWC), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and available phosphorus (AP) were measured. The results revealed that temporal variation and land-use have significant effects on the patterns of SWC, DOC, and DON, although not on that of AP. In addition, the activities of soil enzymes were highest in June, indicating that there are differences in the temporal variation of soil enzyme activities within a given land-use type, especially for alkaline phosphatase. The consistent ranking of soil enzyme activities was also significantly increased by afforestation. Furthermore, nonmetric multidimensional scaling analysis showed that the influence degree of afforestation on activities of the four enzymes was higher than that of temporal variation. Significant correlations between soil enzyme activities and soil properties indicated that soil enzyme activities are closely related to soil nutrients dynamics, particularly with respect to DON. The present study accordingly demonstrates that soil enzyme activities respond to farmland-to-forest conversion and hence have the potential to affect soil qualities in the Loess Plateau.

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

In the Loess Hill region of China, a large amount of arable land has been replaced by forest plantation, resulting in increased storage of soil organic C (SOC). To elucidate the mechanisms of SOC storage among the afforested lands, we separated SOC into four specific size/density fractions: coarse free particulate organic C (cfPOC), fine free POC (ffPOC), intra-microaggregate POC (iPOC), and mineral–associated organic C (MOC). Soils were collected from arable land and from areas with four tree species—robinia (Robinia pseudoacacia L.), poplar (Populus tomentosa Carriere), caragana (Caragana korshinskii Kom.), and buckthorn (Hippophae rhamnoides L.)—in 15-yr-old stands. The total soil C stock to a depth of 100 cm was in the order robinia > poplar > caragana = buckthorn, and C increased by 7.9 to 18.2 Mg C ha⁻¹ compared with the arable land. Stocks of the cfPOC, iPOC, and MOC in the robinia and poplar land were also higher than that of the caragana and buckthorn land. Compared with the arable land, the MOC and cfPOC increased by 4.8 to 11.9 and 1.9 to 5.1 Mg C ha⁻¹, respectively, in all of the afforested lands, and the iPOC only increased by 0.8 Mg C ha⁻¹ in the robinia and poplar areas. However, the ffPOC increased by 0.9 Mg C ha⁻¹ in all four afforested lands. The differences of the SOC fractions among the four afforested lands occurred primarily because the planting of robinia and poplar increased the cfPOC and MOC in the 0- to 100-cm soil profile, whereas caragana and buckthorn only increased cfPOC and MOC at depths of 0 to 20 cm and 0 to 60 cm, respectively. The total SOC fraction was composed of MOC (75.8%) > cfPOC (15.8%) > iPOC (4.0%) = ffPOC (4.3%). We conclude that the afforestation of former arable land with trees (robinia and poplar) can provide a greater increase in the SOC in the physical structure of the soil than shrubland (caragana and buckthorn), especially for the MOC.

Soil respiration (SR) is an important process in the carbon cycle. However, the means by which changes in understory plant community traits affect this ecosystem process is still poorly understood. In this study, plant species surveys were conducted and soil samples were collected from forests dominated by black locust (Robinia pseudoacacia L.), with a chronosequence of 15, 25, and 40 years (RP15, RP25, and RP40, respectively), and farmland (FL). Understory plant coverage, evenness, diversity, and richness were determined. We investigated soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and stoichiometry (MBC:MBN, MBC:MBP, and MBN:MBP). Soil enzyme assays (catalase, saccharase, urease, and alkaline phosphatase), heterotrophic respiration (HR), and autotrophic respiration (AR) were measured. The results showed that plant coverage, plant richness index (R), evenness, and Shannon-Wiener diversity were higher in RP25 and RP40 than in RP15. SR, HR, and AR were significantly higher in the forested sites than in farmland, especially for SR, which was on average 360.7%, 249.6%, and 248.2% higher in RP40, RP25, and RP15, respectively. Meanwhile, catalase, saccharase, urease, and alkaline phosphatase activities and soil microbial C, N, P, and its stoichiometry were also higher after afforestation. Moreover, significant Pearson linear correlations between understory plants (coverage, evenness, diversity, and richness) and SR, HR, and AR were observed, with the strongest correlation observed between plant coverage and SR. This correlation largely depended on soil enzymes (i.e., catalase, saccharase, urease, and alkaline phosphatase), and soil microbial biomass C, N, and P contents and its stoichiometry, particularly urease activity and the MBC:MBP ratio. Therefore, we conclude that plant communities are drivers of soil respiration, and that changes in soil respiration are associated with shifts in soil enzyme activities and nutrient stoichiometry.

The depth of sampling is an important factor for evaluating soil stability. The objective of this study was to test soil aggregate particle-size fractions and soil organic carbon (SOC) in water-stable aggregate by vegetation restoration through 0-60 cm soil profile. We collected soil samples in 30 years old Robinia psendoacacia (Rr); Platycladus orientalis (Po); Pinus tabulaeformis (Pt); abandoned land (Ab), and slope cropland (Sc), which were separated into > 2, 2-1, 1-0.25, 0.25-0.053, and 0.25 mm water-stable aggregates (WSA) and mean weight diameter (MWD) were calculated in 0-60 cm soil depth. Results showed that soil aggregate fractions (> 0.25 mm) of four vegetation types were significantly ( P 0.25 mm) under Rr, Po, Pt, and Ab was higher more than 37.7, 92.4, 92.5, 79.1%, respectively in 40-60 cm compared with Sc additionally, > 0.25 mm WSA and MWD was significantly higher in Pt soil in 20-40 cm, 40-60 cm soil depth ( P < 0.05 ). The results demonstrated that soil stability was enhanced and SOC content was increased after converting slope cropland to forest, especially under Pt forest that greatly influenced the subsoil.

Forest development gradually changes the above-ground plant community, but the potential effects on the below-ground microbial community remain unclear, particularly at the various succession stages. Here, we investigated six vegetation types that covered two succession stages (early and late stages) during forest development. The early stage showed changes from two pioneer communities (Betula platyphylla and Populus davidiana) to two mixed communities (Populus davidiana with Pinus tabuliformis), while the late stage showed changes from the two mixed communities to two climax communities (Quercus wutaishanica and Pinus tabuliformis). Illumina sequencing of the 16S rRNA gene and ITS gene were carried out to analyze soil microbial (bacterial and fungal) diversity and composition. Soil properties (i.e., moisture, bulk density, texture, ammonium nitrogen, nitrate nitrogen, organic carbon, total nitrogen, and total phosphorus) were also determined. The results showed that forest development had positive effects on microbial diversity. Particularly, bacterial diversity successions proceed faster than that of fungi at the early stage. Proteobacteria and Bacteroidetes significantly increased, but Actinobacteria and Acidobacteria significantly decreased during forest development. In contrast, the effect sizes of Proteobacteria, Actinobacteria, and Acidobacteria at the early stage were larger than those at the late stage. The dominant fungi phyla (i.e., Ascomycota, Basidiomycota, and Zygomycota) across all soil samples responded insignificantly to forest development. Such differential responses of the microbial diversity and dominant phyla were significantly correlated with soil texture, soil organic carbon, and total nitrogen. In contrast, the effect size of soil properties on bacterial phyla was larger than that on fungal phyla. Collectively, these results emphasize that the responses of the microbial community to forest development depend on the succession stage, and also suggest that bacteria and fungi may not follow the same successional trajectories due to the differed responses of bacteria and fungi to changing soil texture and carbon/nitrogen contents during forest development.

In the present paper, we investigated the effects of afforestation on nitrogen fractions and microbial communities. A total of 24 soil samples were collected from farmland (FL) and three afforested lands, namely Robinia pseudoacacia L (RP), Caragana korshinskii Kom (CK), and abandoned land (AL), which have been arable for the past 40 years. Quantitative PCR and Illumina sequencing of 16S rRNA genes were used to analyze soil bacterial abundance, diversity, and composition. Additionally, soil nitrogen (N) stocks and fractions were estimated. The results showed that soil N stock, N fractions, and bacterial abundance and diversity increased following afforestation. Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla of soil bacterial compositions. Overall, soil bacterial compositions generally changed from Actinobacteria (Acidobacteria)-dominant to Proteobacteria-dominant following afforestation. Soil N fractions, especially for dissolved organic nitrogen (DON), were significantly correlated with most bacterial groups and bacterial diversity, while potential competitive interactions between Proteobacteria (order Rhizobiales) and Cyanobacteria were suggested. In contrast, nitrate nitrogen (NO3−-N) influenced soil bacterial compositions less than other N fractions. Therefore, the present study demonstrated that bacterial diversity and specific species respond to farmland-to-forest conversion and hence have the potential to affect N dynamic processes in the Loess Plateau.

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.

Artificial vegetation restoration can induce variations in accumulation and distribution of soil carbon (C), nitrogen (N) and phosphorus (P). However, little is known about variations in soil C, N and P nutrient fraction stratification following artificial vegetation in Loess Plateau China. Based on the hypothesis that re-vegetated can improve soil quality and stratification ratios (SR) can be used as an indicator to evaluate soil quality. This study measured contents and storages of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) and their SRs in topsoil (0–20 cm) and subsoil (20–60 cm) in three 30-year re-vegetated lands that had been converted from arable land (Robinia pseudoacacia L., Caragana Korshinskii Kom. and abandoned cropland with low interferences and few management measures) and one slope cropland (SC) as a control for three soil profiles(0–20 cm, 20–40 cm and 40–60 cm) from June 2009 to June 2013. The results showed that the contents and storages of SOC, TN and TP in re-vegetated land were significantly higher than those in the SC in both topsoil and subsoil. The storages of SOC, TN and TP in the topsoil (0–20 cm) of the re-vegetated lands increased by 16.2%-26.4%, 12.7%-28.4% and 16.5%-20.9%, respectively, and increased by smaller but significant amounts in subsoil from 2009 to 2013. The SRs for SOC, TN and TP in the re-vegetated lands were mostly >2 (either for 0–20:20–40 cm or 0–20:40–60 cm) and greater than that in the SC. The SRs showed an increasing trend with increasing restoration age. The results also showed that the land use type and soil depth were the most influential factors for the SRs and storages, and the SRs of SOC and TN had significantly positive correlations with their storages. The SRs were concluded to be a good indicator for evaluating the soil quality, which can be significantly enhanced through vegetation restoration. Moreover, vegetation restoration can significantly enhance SOC, TN and TP accumulation in both topsoil and subsoil.