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Soil priming is a microbial-driven process, which determines key soil–climate feedbacks in response to fresh carbon inputs. Despite its importance, the microbial traits behind this process are largely undetermined. Knowledge of the role of these traits is integral to advance our understanding of how soil microbes regulate carbon (C) emissions in forests, which support the largest soil carbon stocks globally. Using metagenomic sequencing and C-glucose, we provide unprecedented evidence that microbial traits explain a unique portion of the variation in soil priming across forest biomes from tropical to cold temperature regions. We show that microbial functional profiles associated with the degradation of labile C, especially rapid simple sugar metabolism, drive soil priming in different forests. Genes involved in the degradation of lignin and aromatic compounds were negatively associated with priming effects in temperate forests, whereas the highest level of soil priming was associated with β-glucosidase genes in tropical/subtropical forests. Moreover, we reconstructed, for the first time, 42 whole bacterial genomes associated with the soil priming effect and found that these organisms support important gene machinery involved in priming effect. Collectively, our work demonstrates the importance of microbial traits to explain soil priming across forest biomes and suggests that rapid carbon metabolism is responsible for priming effects in forests. This knowledge is important because it advances our understanding on the microbial mechanisms mediating soil–climate feedbacks at a continental scale., This work were financially supported by the National Natural Science Foundation of China (41907031), the Chinese Academy of Sciences “Light of West China” Program for Introduced Talent in the West, the National Natural Science Foundation of China (31570440, 31270484), the Key International Scientific and Technological Cooperation and Exchange Project of Shaanxi Province, China (2020KWZ-010), the 2021 First Funds for Central Government to Guide Local Science and Technology Development in Qinghai Province (2021ZY002), the i-LINK +2018 (LINKA20069) from CSIC, and a Ramón y Cajal grant from the Spanish Ministry of Science and Innovation (RYC2018-025483-I)

In this study, we determined the variations in the diversity and structure of nitrogen-fixing microorganisms during afforestation and how this variability correlates with the soil physicochemical factors. Soils were sampled from replicated plots representing three revegetation types (Robinia pseudoacacia L. [Rp], Caragana korshinskii Kom [Ck], and abandoned land) with adjacent farmland as the control at a research site in Ansai District, Yan’an City, Shaanxi Province, China. Using high-throughput sequencing of the nifH rRNA gene, we found that revegetation significantly increased the Shannon diversity index, and for the richness index Chao1 was highest in Ck and have no significant difference in other three lands. From farmland to vegetation types abandoned land, Ck, and Rp, the dominant phylum of nifH microorganisms changed from Actinobacteria to Proteobacteria. Furthormore, variation in the Chao1 richness index was significantly influenced by pH (p = 0.002), soil organic carbon (SOC; p = 0.025) and total nitrogen (TN; p = 0.015). The Shannon diversity index was significantly influenced by SOC, nitrate nitrogen (NN), C to P ratio, TN, AN, N to P ratio, TP, and sand. The changes of nifH microorganism composition at the phylum level were significantly correlated with the soil N to P ratio, which explained 53.1% of the variation among environmental factors (F = 11.8, p = 0.002). The ratio of N to P (interpretation rate 65.2%, F = 18.8, p = 0.002) was also a key factor that influenced the composition at the genus level. Based on the diversity and composition, Ck was a more advantageous recovery method to increase the capacity for N fixation. This study provides a basis for the study of N-fixation characteristics in the Loess Hilly Region.

Organic carbon (OC) sequestration through soil aggregation is an important aspect of land use change/conversion (LUCC) influencing the terrestrial ecosystem C cycle, although little is known on the changes in aggregate dynamics and their contributions to OC accumulation after LUCC in regions with serious soil erosion. Therefore, bulk soil samples under four land uses (farmland and three vegetated soils converted from farmland 42 years ago: Robinia pseudoacacia [RP42yr], Caragana korshinskii [CK42yr], and abandoned land [AL42yr]) in the Loess Plateau, China, was collected, separated into seven aggregate size fractions, and examined for OC content. Farmland conversion into AL42yr, CK42yr, and RP42yr increased macroaggregate (>2 mm) and mesoaggregate (2–0.25 mm) proportions, mean weight diameter, and geometric mean diameter but decreased microaggregates (0.25–0.053 mm) amount. Bulk soil and aggregates OC content and stock varied with soil depth and land use types but were usually highest in RP42yr. Mesoaggregates contained higher OC content and stock than other aggregates at 0‐ to 20‐cm depth under all land uses. Increases in the OC stocks of mesoaggregates accounted for 46% and 85% of the increase in bulk soil OC stocks at 0‐ to 20‐ and 20‐ to 40‐cm depth, respectively. Thus, soil OC accumulation after LUCC is mainly due to increased OC stock within mesoaggregates, which is further attributed to increased mesoaggregate proportions. Overall, vegetation restoration promotes the physical protection of OC by increasing soil aggregation, being a management option to enhance the C sequestration potential in ecological fragile regions.

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.

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.