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Natural vegetation succession is a process of interactions between plants and various environmental factors. The dynamics of soil microbes during successional stages have been intensively explored in the past two decades. The effect of rhizosphere interactions of interspecies on microbial properties, however, has received less attention. During the early stage of succession on a Loess Plateau soil from China, we investigated microbial properties in the rhizospheres of three dominant grass species: Artemisia capillaris , Artemisia sacrorum and Stipa bungeana . Experiments with monocultured and polycultured potted plants were designed to determine the effects of rhizosphere interactions on plant growth and soil microbial properties. The results showed that root biomass, shoot biomass, plant height, microbial biomass carbon (C), microbial biomass nitrogen (N), invertase and urease activities, Gram-negative and Gram-positive bacteria were significantly higher for A. capillaris than for A. sacrorum and S. bungeana in both monocultures and polycultures. Lower root biomass, shoot biomass, plant height, microbial biomass C, microbial biomass N, invertase and alkaline phosphatase activities, and levels of microbial phospholipid-derived fatty acids in polycultures relative to the monocultures indicated that rhizosphere interactions led to lower plant growth and soil microbial activities. Our study suggests that rhizosphere interactions had a significant effect on the microbial properties of plant rhizospheres. The advantage of A. capillaris over A. sacrorum and S. bungeana in growth characteristics and rhizosphere microbial conditions is likely responsible for the dominance of A. capillaris in the early stage of succession in abandoned cropland on the Loess Plateau. [ABSTRACT FROM AUTHOR]

The Grain for Green Project (GGP) has increased the area of forest plantations on the Loess plateau region of China, which has created a need to understand the potential for long-term carbon (C) sequestration. The objectives of this work were to investigate the response of biomass C and soil organic carbon (SOC) densities to the age of these forest plantations and to determine their contribution to the C sources/sink within these regional forest ecosystems. In this project three different species of plants, Caragana korshinskii Kom., Hippophae rhamnoides and Prunus davidiana (Carr.) Franch, were studied by selecting 3 plantation areas where each plant had grown for 9-years and 3 separate plantation areas where each plant had grown for 26-years. All three of these species were widely planted in the case study area. A total of 54 quadrants (2 m × 2 m) were randomly selected in 18 plots (3 plots for each species × 3 species × 2 stand ages) from these plantations for measuring the above- and below-ground biomass as well as the biomass C concentration and density of the three species. A bucket auger was used to collect soil samples from 0 to 100 cm soil depth to determine the SOC concentration and density in each plot. Biomass density, biomass C and SOC increased with growth between 9 and 26 years for all three plant species. The biomass densities for P. davidiana , C. korshinskii and H. rhamnoides increased from 27.3 ± 7.5 to 49.9 ± 11.9 Mg ha − 1 , 16.6 ± 3.8 to 39.0 ± 18.2 MgC ha − 1 , and 14.4 ± 4.4 to 23.7 ± 5.7 Mg ha − 1 , respectively. The biomass C for P. davidiana , C. korshinskii and H. rhamnoides also increased during this time from 9.5 ± 2.6 to 18.7 ± 4.1 Mg ha − 1 , 6.2 ± 1.1 to 14.6 ± 6.6 Mg ha − 1 , and 6.2 ± 1.9 to 8.5 ± 2.0 Mg ha − 1 , respectively. Similarly, the SOC for P. davidiana , C. korshinskii and H. rhamnoides during this time increased from 114.1 ± 13.9 to 185.4 ± 21.2 Mg ha − 1 , 80.0 ± 9.0 to 93.3 ± 10.7 Mg ha − 1 , and 62.4 ± 3.5 to 81.6 ± 15.4 Mg ha − 1 , respectively. Significant differences ( P < 0.05) were found for the biomass C and SOC densities between the 9- and 26-year old plantations, demonstrating that biomass production continued to significantly impact the SOC storage with increasing age of the forest plantations investigated. This work suggested that both the stored biomass C and SOC in these plantations did not decrease during the growth period from 9- to 26-years. The results of this study highlighted the importance of forest age in understanding the C storage of these forest plantations. [ABSTRACT FROM AUTHOR]

The effects of reforestation on carbon (C) sequestration in China's Loess Plateau ecosystem have attracted much research attention in recent years. Black locust trees ( Robinia pseudoacacia L.) are valued for their important use in reforestation and water and soil conservation efforts. This forest type is widespread across the Loess Plateau, and must be an essential component of any planning for C sequestration efforts in this fragile ecological region. The long-term effects of stand age on C accumulation and allocation after reforestation remains uncertain. We examined an age-sequence of black locust forest (5, 9, 20, 30, 38, and 56 yr since planting) on the Loess Plateau to evaluate C accumulation and allocation in plants (trees, shrubs, herbages, and leaf litter) and soil (0-100 cm). Allometric equations were developed for estimating the biomass of tree components (leaf, branch, stem without bark, bark and root) with a destructive sampling method. Our results demonstrated that black locust forest ecosystem accumulated C constantly, from 31.42 Mg C/ ha (1 Mg = 10 g) at 5 yr to 79.44 Mg C/ha at 38 yr. At the 'old forest' stage (38 to 56 yr), the amount of C in plant biomass significantly decreased (from 45.32 to 34.52 Mg C/ha) due to the high mortality of trees. However, old forest was able to accumulate C continuously in soil (from 33.66 to 41.00 Mg C/ha). The C in shrub biomass increased with stand age, while the C stock in the herbage layer and leaf litter was age-independent. Reforestation resulted in C re-allocation in the forest soil. The topsoil (0-20 cm) C stock increased constantly with stand age. However, C storage in sub-top soil, in the 20-30, 30-50, 50-100, and 20-100 cm layers, was age-independent. These results suggest that succession, as a temporal factor, plays a key role in C accumulation and re-allocation in black locust forests and also in regional C dynamics in vegetation. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic carbon (SOC) is an important component of agricultural soil. Understanding the storage of carbon (C) and nitrogen (N) helps us understand how ecosystems would respond to natural and anthropogenic disturbances under different management strategies. Although the quantity of SOC stocks in the Loess Plateau has been previously investigated, the distribution and storage of C in its deep soil layers has been poorly studied to date. The present study was conducted in the Zhifanggou Watershed on the Loess Plateau to evaluate the effect of the types of land use on the distribution and storage of SOC and total nitrogen (TN) at different soil depths. Ten types of land use were investigated: woodland (Robinia pseudoacacia), shrubland (Caragana korshinskii; Hippophae rhamnoides), natural grassland, artificial grassland (Medicago sativa), orchard, check-dam cropland, terraced cropland (Setaria italica; Zea mays), and sloped cropland. The results show that land use significantly affects the SOC and TN content. The reconversion of sloped croplands into forestlands and grasslands improve their SOC and TN content. Natural grassland, terraced cropland with Z. mays, and the artificial grassland had the highest SOC content within 0–60 cm, 60–100 cm, and 150–500 cm layers, respectively. The SOC and TN content in the top layer were higher than those in the deep layer. Natural grassland had the highest SOC and TN storage within 0–40 cm layer, followed by shrubland I (C. korshinskii), check-dam cropland, woodland, and shrubland II (H. rhamnoides), orchard and artificial grassland, whereas the sloped cropland, and the terraced cropland with S. italica had the lowest values. The SOC and TN storage between the top (0–20 cm) and deep layers (0–100 cm, 0–300 cm, and 0–500 cm) can be modeled using linear function (y = ax + b). The results suggest that natural grasslands may be the optimal choice for SOC and TN sequestration in the Zhifanggou Watershed. Shrublands, woodland, artificial grasslands, orchards, terrace cropland with Z. mays and check-dam croplands are also recommended for their great contribution to SOC and TN storage in the area. [Copyright &y& Elsevier]

Background and aims: Afforestation is considered one important means of mitigating climate change. However, it is still controversial whether mixed-species plantations (MP) were more conducive to the soil organic carbon (SOC) stocks than monoculture plantations (PP). Methods: We conducted a meta-analysis based on 21 publications to assess the effects of different afforestation modes as well as controlling factors of SOC stocks on the Loess Plateau of China. Results: Compared with monoculture plantations, mixed-species plantations could significantly increase the response size of SOC stocks by 28%. For different species combinations, tree-shrub mixtures were more conducive to increasing the SOC stocks. In climate-limited regions (MAT <8 °C, MAP <500 mm), the response size of SOC stocks of mixed-species plantations was 34% higher than that of monoculture plantations. Furthermore, significant differences were found in the response size of SOC stocks at young (< 10 yr) and mature stages (> 20 yr). Compared with abandoned land, afforestation on cropland, especially mixed-species plantations, could increase the SOC stocks by 10%. Additionally, compared with monoculture plantations, a stronger coupling relationship was observed between the soil total nitrogen (STN) and SOC stocks in mixed-species plantations. Conclusion: Our results suggest that for the Loess Plateau of China, planting mixed-species plantations containing nitrogen-fixing plants is a more effective approach to enhancing the SOC stocks than monoculture plantations. [ABSTRACT FROM AUTHOR]

Aims: Understanding the accumulation and mineralization of soil organic carbon (SOC) in aggregates under long-term fertilization conditions is of great significance in comprehending the dynamic changes in farmland C pools on the Loess Plateau. Methods: The long-term field experiment has been established for 26 years and includes six fertilization treatments: no fertilization (CK), organic fertilizer (M), organic and nitrogen fertilizers (MN), organic and phosphorus fertilizers (MP), NP and MNP. We determined nutrient content, mineralization characteristics, temperature sensitivity (Q10), and extracellular enzyme activities of soil aggregates (< 0.25 mm, 0.25–1 mm, 1–2 mm, > 2 mm). Results: (1) M, MN, MP, and MNP significantly increased the SOC content of aggregates by 92.58%, 82.84%, 73.55% and 83.23% compared to NP. The SOC in < 0.25 mm aggregates was significantly highest than other particles. (2) Organic fertilizer treatments (M, MN, MP, and MNP) significantly increased the cumulative mineralization of different-sized aggregates by 97.10–178.75% compared to NP. The average mineralization rate and the cumulative mineralization of < 0.25 mm aggregates was the lowest. (3) The mineralization of aggregates is directly and significantly affected by the available soil nutrients and enzyme activities. (4) Fertilization and aggregate size had significant effects on Q10 and the Q10 of MNP was lower than that of M, MP, and MN. Conclusion: < 0.25 mm aggregates had the strongest C sequestration effect. MNP reduced the amount of mineralization by regulating enzyme activities and promoted the accumulation of SOC in farmland on the Loess Plateau by increasing the percentage of < 0.25 mm aggregates. [ABSTRACT FROM AUTHOR]

• A trade-off exists between tree biomass and soil water storage in artificially planted trees. • The soil water storage and tree biomass of mixed trees are significantly higher than those of pure trees. • The trade-off in mixed trees is significantly lower than that in pure trees. • Changes in trade-offs are influenced by climate, plantation age and soil properties. Afforestation is an effective measure to combat land degradation and mitigate climate change. However, afforestation tends to consume a tremendous amount of soil moisture while increasing vegetation biomass and coverage. Although convincing evidence indicates that mixed trees have higher resistance and resilience to climate change and natural disturbance than pure trees, it is unclear how tree mixtures affect the relationship between soil water storage and tree biomass. By conducting a meta-analysis of 86 observations from 27 studies, we showed that there was a trade-off between soil water storage and tree biomass in artificially planted trees on the Loess Plateau, with an average root mean square deviation (RMSD) value of 0.22. The RMSD value between soil water storage and tree biomass for mixed trees was significantly lower than that for pure trees, and the relative benefits of the trade-off were biased toward soil water storage. In dry areas (aridity index (AI) < 0.3), the RMSD value between soil water storage and tree biomass of artificially planted trees was the highest, and this value of mixed trees was significantly lower than that of pure trees. In terms of different plantation ages, the RMSD value between soil water storage and tree biomass for mixed trees was significantly lower than that for pure trees in plantations aged >20 years. Additionally, RMSD values between soil water storage and tree biomass under different afforestation patterns were negatively correlated with the initial water storage and soil organic carbon content. Our results show that tree mixtures optimize the trade-off between soil water storage and tree biomass in arid and semiarid areas. [ABSTRACT FROM AUTHOR]

Grassland is a major carbon sink in the terrestrial ecosystem. The dynamics of grassland carbon stock profoundly influence the global carbon cycle. In the published literatures so far, however, there are limited studies on the long-term dynamics and influential factors of grassland carbon stock, including soil organic carbon. In this study, spatial-temporal substitution method was applied to explore the characteristics of Medicago sativa L. (alfalfa) grassland biomass carbon and soil organic carbon density (SOCD) in a loess hilly region with different growing years and management patterns. The results demonstrated that alfalfa was the mono-dominant community during the cutting period (viz. 0-10 year). Community succession began after the abandonment of alfalfa grassland and then the important value of alfalfa in the community declined. The artificial alfalfa community abandoned for 30 years was replaced by the S. bungeana community. Accordingly, the biomass carbon density of the clipped alfalfa showed a significant increase over the time during 0-10 year. During 0-30 year, the SOCD from 0-100 cm of the soil layer of all 5 management patterns increased over time with a range between 5.300 ± 0.981 kg/m and 12.578 ± 0.863 kg/m. The sloping croplands had the lowest SOCD at 5.300 ± 0.981 kg/m which was quite different from the abandoned grasslands growing for 30 years which exhibited the highest SOCD with 12.578 ± 0.863 kg/m. The ecosystem carbon density of the grassland clipped for 2 years increased 0.1 kg/m compared with the sloping cropland, while that of the grassland clipped for 10 years substantially increased to 10.30 ± 1.26 kg/m. Moreover, the ecosystem carbon density for abandoned grassland became 12.62 ± 0.50 kg/m at 30 years. The carbon density of the grassland undisturbed for 10 years was similar to that of the sloping cropland and the grassland clipped for 2 years. Different management patterns imposed great different effects on the accumulation of biomass carbon on artificial grasslands, whereas the ecosystem carbon density of the grassland showed a slight increase from the clipping to abandonment of grassland in general. [ABSTRACT FROM AUTHOR]

Oxidizable organic carbon is a valuable indicator of soil quality, early changes in soil organic carbon (SOC) content, and changes in stabilities of SOC induced by soil fertilization practices. To improve SOC accumulation under soybean-corn cropping system in flat farmland on the Loess Plateau, we examined the effects of nine fertilization treatments [bare land (BL), control (CK), nitrogen (N), phosphorus (P), N + P (NP), manure (M), M + N (MN), M + P (MP), M + N + P (MNP)] on dynamics of SOC and four fractions of oxidizable organic carbon (OC) [very labile (C VL ), labile (C L ), less labile (C LL ) and recalcitrant (C NL )], and stability of SOC over the period of 1997 to 2013. Although SOC content in treatments without fertilization or with inorganic fertilization remained relatively stable between 1997 and 2013, long-term no fertilization or inorganic fertilization caused a depletion of the very labile C (C VL ) fraction and increased the stability of SOC. Long-term application of manure promoted an accumulation of SOC in the topsoil between 1997 and 2013, mainly by sequestering labile C (C VL and C L ) in the 0–20-cm soil layer, and the dynamics of SOC and C VL contents under these treatments fitted a logistic regression model, which decreased the stability of SOC between 2008 and 2013. SOC content in the subsoil (20–40 cm) under cultivation with organic fertilization increased between 1997 and 2006, but then decreased between 2007 and 2013. Organic fertilization increased C VL and C L contents in the 20–40-cm layer between 1997 and 2010, but then decreased them between 2010 and 2013, and increased the stability of SOC between 2011 and 2013. Our findings indicated that manure fertilization was more effective in promoting SOC enrichment of the soil surface layers by increasing C VL , relative to inorganic fertilization and no fertilization, whereas long-term manure fertilization was associated with a risk of decreasing labile carbon in the 20–40-cm layer and the stability of SOC in the 0–20-cm layer. Manure fertilizer should thus be used cautiously in farmland on the Loess Plateau to guarantee long-term soil carbon stock. [ABSTRACT FROM AUTHOR]

• Warming decreased the stability of soil aggregate in shrubland. • Warming decreased the soil aggregate associated carbon storage in meadow but increased it in shrubland. • Warming enhanced the stability of soil aggregate associated carbon. • The associated carbon storage of macroaggregates is more sensitive to warming. Global warming is increasingly impacting the structure and functioning of the ecosystems in the Tibetan plateau. However, information about warming effects on the stability of carbon pools in soil aggregates is limited. Here, we implemented a 21-year long-term open-top-chamber (OTCs) warming experiment to reveal the warming effects on the stability of topsoil aggregate structure and carbon pools in a Tibetan alpine meadow and shrubland. The results demonstrated that long-term warming significantly reduced the soil aggregate stability of the shrubland, but had no significant effect on the alpine meadow. Although warming enhanced the stability of carbon pools in topsoil aggregate fractions for both the alpine meadow and shrubland, a negative response of the carbon management index (CMI) and carbon pool storage in the alpine meadow was found, while shrubland showed a positive response to warming. The response of carbon fractions to warming depended on soil aggregate size, with a reduction of labile C fraction 1 (C1) and stable carbon fraction (C4) in large (LMGA: >2 mm) and small macroaggregates (SMGA: 2–0.25 mm) of the alpine meadow, and unchanged carbon fractions in microaggregates (MIGA: <0.25 mm) for both the alpine meadow and shrubland. Long-term warming significantly decreased C-acquiring enzyme activity in shrubland, but had no significant effect on the meadow. Enzyme activity and glomalin-related soil protein (GRSP) concentration decreased as the size of soil aggregates increased in the alpine meadow and shrubland, which could be attributed to the structural characteristics of microaggregates, which have larger specific surface area and stronger microbial protection ability. Our findings provide a new perspective in understanding the response of alpine meadow and shrubland soil structure and carbon pool stability to long-term warming at the soil aggregate level. [ABSTRACT FROM AUTHOR]

Slope aspect, as an important topographic factor on the Loess Plateau in China, may influence the productivity and relative characteristics of Artemisia sacrorum in this area. Three slope aspects (sunny, half-sunny, and shady slopes) were chosen to study the effects of slope aspect on the biomass, carbon:nitrogen:phosphorus (C:N:P) stoichiometry, and non-structural carbohydrate (NSC) concentrations in A. sacrorum in 2014. The maximum A. sacrorum biomass was detected in the half-sunny slope, and the ratio of belowground to aboveground biomass significantly changed in the different slope aspects. Slope aspect significantly affected the N:P ratio in the aboveground and the C:P and N:P ratios in the belowground. The maximum C:P and N:P ratios in the rhizosphere and non-rhizosphere soils were reached in the sunny slope, whereas no significant change was observed in C:N ratio. The maximum NSC concentrations occurred in different A. sacrorum tissues, that is, aboveground in the sunny slope and belowground in the half-sunny slope. The ratio of soluble sugar to starch concentrations in the aboveground significantly differed among the three slope aspects, but that in the belowground showed no significant difference. Slope aspect was the main environmental factor affecting the plant nutrient and stoichiometry on the Loess Plateau in China. [ABSTRACT FROM AUTHOR]

The ecological linkage between above- and belowground parts of the terrestrial ecosystem is of interest to ecological and management fields. However, the knowledge on how the loss of plant functional groups impacts multitrophic interactions across soil biota and associated ecological functioning knowledge is lacking. This study aimed to evaluate responses of soil nematode communities to PFG (Gramineae, Cyperaceae, Leguminosae, and other Forbs) identity and richness loss after 9 years by comparing 15 treatments. Specifically, the loss of PFG richness significantly decreased carbon biomass and the abundance of bacterivores and negatively affected footprints of bacterivores and omnivore-predators (OP), leading to a decline in the enrichment and structure footprints. The response of nematode community structure and functional composition at different trophic levels to PFG richness loss varied with PFG identity. Gramineae removal reduced the community structural footprint positively associated with the biomass carbon and footprint of OP, suggesting carbon and energy enrichment are likely to be lower in soil micro-food webs. Although the removal of Cyperaceae reduced carbon biomass and the footprint of bacterivores, it did not cause significant shifts in carbon and energy enrichment and structural footprints in the soil food web, similar to the other two PFGs. Notably, Gramineae strongly controlled carbon and energy enrichment regulations in soil micro-food webs than other PFGs. The present study highlights the key role and influence of PFG richness and identification on food web ecosystem services. It provides a basis for the development of sustainable strategies for grazed alpine meadow ecosystem's management. [Display omitted] • The responses of nematodes community among trophic levels to the loss of PFG differ. • The negative effects of PFG loss on nematode community and functional conditions were conservative. • The positive effect of Gramineae on the soil carbon and energy enrichment over other PFG identities [ABSTRACT FROM AUTHOR]

• Recovery years increased soil enzyme activity in the steppe and forest-steppe zone. • Recovery years decreased SOC stability in the steppe and forest-steppe zone. • The forest zone might have the most stable soil enzyme activity and SOC stability. • Key factors regulating these changes are different among the three vegetation zones. The effects of cropland abandonment on soil enzyme activity and soil organic carbon (SOC) stability, along with the driving factors, are poorly understood. Here, we aimed to systematically and comprehensively evaluate soil enzyme activity, SOC stability, and the associated driving factors in different vegetation zones after cropland abandonment on the Loess Plateau, China. We selected grasslands with different recovery times along a rainfall gradient encompassing the steppe zone (SZ), forest-steppe zone (FSZ), and forest zone (FZ). We measured and compared the changes in soil enzyme activity (saccharase, polyphenol oxidase, urease, phosphatase, and catalase) and SOC stability as a function of recovery time; we also evaluated the relationships between these two parameters. In SZ and FSZ, soil enzyme activity, fractions of oxidizable carbon (including very labile [C1], labile [C2], and less labile [C3]), and the carbon management index (CMI) increased with recovery time, whereas the SOC stability index (SI) decreased. Conversely, in FZ, polyphenol oxidase activity increased linearly, urease and catalase activities decreased linearly, and the change in saccharase activity was represented by a cubic equation regression. SI showed no obvious changes with recovery time, whereas C1, C2, and C3 initially decreased and then increased. Redundancy analysis showed that, in FSZ and FZ, soil enzyme activity, C1, C2, C3, and SI were influenced by vegetation diversity, coverage, and soil nutrient levels. In comparison, in SZ, these parameters were mainly influenced by soil nutrient levels. Soil enzyme activity was strongly correlated with C1 and C3 in SZ and FSZ, but not in FZ. Overall, in SZ and FSZ, soil enzyme activity increased with recovery time, whereas SOC stability decreased. In contrast, both parameters were relatively stable in FZ, which had higher mean annual precipitation and mean annual temperature. [ABSTRACT FROM AUTHOR]

• SOC stability in different vegetation zones was evaluated in the Loess Plateau, China. • Overall SOC stability decreased after vegetation restoration. • SOC stability was strongly affected by climate (precipitation and temperature). • A basis for the long-term monitoring of SOC stability in the Loess Plateau is provided. Vegetation restoration is considered to be an important pathway for ecosystem improvement; however, information remains limited about how the soil organic carbon (SOC) stability changes after vegetation restoration. This could be evaluated by measuring the response of SOC stability to soil physicochemical properties, soil enzyme activity, and environmental factors [recovery years, vegetation types, mean annual precipitation (MAP) and mean annual temperature (MAT)]. Here, the soils of shrub and forest land in different vegetation zones (steppe zone [SZ], forest-steppe zone [FSZ], and forest zone [FZ]) in the Loess Plateau of China were evaluated over different time periods. As the number of years increased, the active carbon fractions [very labile (C1) and labile (C2) fractions of oxidizable carbon] and carbon management index (CMI) increased, whereas the SOC stability index (SI) decreased for both vegetation types in the three vegetation zones. However, the factors driving the changes in SOC stability varied with respect to the vegetation type and zone. Specifically, the rate of change in active carbon fractions was greater in FZ, whereas the rate of change of SI was higher in FSZ, and was correlated with higher MAP and MAT. Overall, the SOC stability decreased after vegetation restoration, with MAP, MAT, vegetation type, and years of recovery representing the main factors driving the changes in SOC stability. These results are expected to facilitate future studies on SOC aimed at enhancing the management of different vegetation zones during recovery in the Loess Plateau of China, in response to global climate change. [ABSTRACT FROM AUTHOR]