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Afforestation can modify terrestrial carbon (C) pools, some of which are primarily fixed in the plant dead biomass and then incorporated into the microbial dead biomass. Soil microorganisms exert a critical role in C flow and potentially influence C balance through the degradation of plant and microbial dead biomass. Here, we compared sites along a 45-year Robinia pseudoacacia (RP) afforestation chronosequence on the Loess Plateau of China. Subsequently, the trends of microbial carbohydrate-active enzymes (CAZyme) and their responses to the decomposition of dead biomass of different origins were studied using metagenomics. The results show that soil microbial CAZyme families, which degrade the plant- and microbial-derived components, significantly increased after afforestation, with a significant peak at the 20-year site. The dominant bacterial phyla (i.e., Actinobacteria, Proteobacteria, and Acidobacteria) mineralized C sources from plant and microbial biomass components through their corresponding CAZyme families. Moreover, the increased abundance of CAZymes involved in the decomposition of plant-derived components (e.g., cellulose, hemicellulose, and lignin) contributed to the formation of C pools. In the case of microbial-derived components, the abundance of CAZymes encoding the bacterial-derived components (peptidoglycan) was larger than that encoding fungal-derived components (chitin and glucans) and was more associated with microbial metabolic activity (qCO2 and Cmic: Corg ratio), indicating a higher investment of bacterial-derived components for microbial carbon turnover following afforestation. Overall, our study compares plant- and microbial-derived biomass to illustrate the differential contributions of dead biomass to C accumulation and confirms the importance of the bacterial community and derived biomass for C turnover following afforestation.

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.

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.

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.

The priming effect is a pivotal mechanism for microbial regulation of soil C cycling; however, the microbial mechanisms underlying priming effects remain elusive. Here, we combined an isotopic approach with metagenomic sequencing to investigate priming effects at five forest sites along an elevational gradient. Positive priming effects were found across the Quercus aliena var. acutiserrata (low elevation), Q. wutaishanica (low-mid elevation), Betula albosinensis (mid elevation), Abies fargesii Franch (mid-high elevation), and Larix chinensis Beissn (high elevation) forest sites. A significant positive correlation was found between the abundance of microbial C decomposition genes and priming effects (p

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.

Cover cropping is practiced to enhance soil health and sustain succeeding crop yield; however, the effect of cover crop on soil water storage, succeeding crop yield, and water-use efficiency (WUE) may not be consistent in all regions. A meta-analysis was carried out to evaluate the effect of cover crop on precipitation storage efficiency (PSE, the percent of precipitation that is stored in the soil during the fallow period), soil water storage at succeeding crop planting (SWSP), succeeding crop yield, and WUE from data collected from 117 studies across the world. Cover crop decreased PSE by 33.4% and soil water storage for the whole profile (SWSPT) at soil depth by 13.2%, but increased water storage to a depth of 30 cm (SWSP30) by 6.0% (P

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.

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.

Cover crops have been increasingly grown for improving soil health and crop production and minimizing environmental impact compared to no cover crop. Systematic documentation of cover cropping effects on soil microbial abundance and community structure, however, is scarce. A meta-analysis including data from 81 available studies was conducted to elucidate the effect of “cover crop” versus “no cover crop” on soil microbial community abundance and structure. Microbial biomass C and N (MBC and MBN) and total phospholipid-derived fatty acids (PLFA) were taken as proxies for soil microbial abundance, and total fungi, total bacteria, gram-positive and -negative bacteria, actinomycete, and arbuscular mycorrhizal fungi (AMF) for microbial community structure. Compared to no cover crop, cover crop overall enhanced PLFA, MBC, and MBN by 24, 40, and 51%, respectively. Soil total bacteria and total fungi, and the groups in them increased by 7–31% with cove crop compared to no cover crop. Fungi were affected more by cover crop than bacteria as indicated by the greater fungi/bacteria ratio. In depth categorical meta-analyses revealed that the legume and nonlegume cover crop mixture reduced MBC, PLFA, and actinomycete compared to legume or nonlegume cover crop alone. Legume cover crop enhanced actinomycete in comparison to nonlegume or the cover crop mixture. Incorporation of cover crop residue into the soil increased PLFA, total bacteria, AMF root colonization, and spore density, but decreased gram-positive and -negative bacteria and AMF compared to residue placed at the surface or removed from the soil. Microbial parameters due to cover crop compared to no cover crop were related to soil properties and annual precipitation. Medium-textured soils showed greater response of cover crop on PLFA, total bacteria and fungi, and actinomycete than fine- or coarse-textured soils. We conclude that cover crops enhance soil microbial community biomass and affected community structure compared to no cover crop and the responses of microbial parameters to cover crop varied with soil and climatic conditions. Cover crops can enhance biological soil health by enhancing microbial community abundance compared to no cover crop.

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.

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.

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.

Land use change is widely considered as a major factor that affects soil organic carbon (SOC) sequestration. Conversion of sloped croplands to perennial vegetation could increase SOC accumulation. The Grain-to-Green Program (GTGP) implemented in 1999, which converts low-yield sloped croplands into forest, shrub, and grassland, is not only a nationwide ecological recovery project but also the largest payment-for-ecosystem-service program in China. Based on data concerning carbon change in GTGP-related zones collected from published literature, this report (i) provides an estimate of the SOC sequestration potential and the rate of SOC sequestration caused by the GTGP in China; (ii) illustrates the differences in SOC sequestration in zones with different land use types (i.e., forest, shrub, and grassland), different precipitation rates, and restoration ages. Soil organic carbon sequestration caused by the GTGP was estimated to be 14.46 TG C year (a rate of 0.54 Mg C ha year). The SOC sequestration significantly increased in forest lands (P < 0.05), which was greater than 18.7% and 42.9% in shrublands and grasslands, respectively. However, the average rate of SOC accretion was greater in grassland, followed by forest lands. Annual average precipitation and restoration age greatly affected the SOC sequestration. The average SOC sequestration increased with restoration age, whereas the average rates of SOC sequestration decreased. The GTGP resulted in increased SOC storage, making significant contribution to carbon sequestration in China.

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.

Conversion of slope cropland to perennial vegetation has a significant impact on soil organic carbon (SOC) stock in A horizon. However, the impact on SOC and its fraction stratification is still poorly understood in deep soil in Loess Hilly Region (LHR) of China. Samples were collected from three typical conversion lands, Robinia psendoacacia (RP), Caragana Korshinskii Kom (CK), and abandoned land (AB), which have been converted from slope croplands (SC) for 30 years in LHR. Contents of SOC, total nitrogen (TN), particulate organic carbon (POC), and labile organic carbon (LOC), and their stratification ratios (SR) and carbon management indexes (CMI) were determined on soil profiles from 0 to 200 cm. Results showed that the SOC, TN, POC and LOC stocks of RP were significantly higher than that of SC in soil layers of 0-10, 10-40, 40-100 and 100-200 cm (P2.0 in most cases of RP, CK and AB. Moreover, CMI values of RP, CK, and AB increased by 11.61-61.53% in soil layer of 100-200 cm compared with SC. Significant positive correlations between SOC stocks and CMI or SR values of both surface soil and deep soil layers indicated that they were suitable indicators for soil quality and carbon changes evaluation. The Grain-to-Green Program (GTGP) had strong influence on improving quantity and activity of SOC pool through all soil layers of converted lands, and deep soil organic carbon should be considered in C cycle induced by GTGP. It was concluded that converting slope croplands to RP forestlands was the most efficient way for sequestering C in LHR soils.