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Soil organic carbon (SOC) is a mixture of various carbon (C) compounds with different stability, which can be distinctly affected by the priming effect (PE). However, little is known about how the PE changes with SOC stability. We address this issue by combining results from two experiments and a metaanalysis. We found that the PE increased with the prolongation of soil preincubation, suggesting that higher PE occurred for more stable SOC than for labile SOC. This was further supported by the metaanalysis of 42 observations. There were significant negative relationships between the difference in PE (ΔPE) between labile and more stable SOC and their differences in SOC, microbial biomass C and soil C : N ratio, indicating that soil C availability exerts a vital control on ΔPE. We conclude that, compared with labile SOC, stable SOC can be more vulnerable to priming once microbes are provided with exogenous C substrates. This high vulnerability of stable SOC to priming warrants more attention in future studies on SOC cycling and global change.

Fine roots play a key role in carbon, nutrient, and water biogeochemical cycles in forest ecosystems. However, inter-annual dynamics of fine root production, mortality, and turnover on the basis of long-term measurement have been less studied. Here, field scanning rhizotrons were employed for tracking fine root by branch order over a 6years period in a larch plantation. For total fine roots, from the first- to the fifth-order roots, annual root length production, length mortality, standing crops, and turnover rate varied up to 3.4, 2.3, 1.5, and 2.3-folds during the study period, respectively. The inter-annual variability of those roots indices in the first-order and the second-order roots were greater than that of the higher order (third- to fifth-order) roots. The turnover rate was markedly larger for the first-order roots than for the higher order roots, showing the greatest variability up to 20times. Seasonal dynamics of root length production followed a general concentrated pattern with peak typically occurring in June or July, whereas root length mortality followed a general bimodal mortality pattern with the dominant peak in May and the secondary peak in August or October. Furthermore, the seasonal patterns of root length production and mortality were similar across years, especially for the first-order and the second-order roots. These results from long-term observation were beneficial for reducing uncertainty of characterizing fine root demography in consideration of large variation among years. Our findings highlight it is important for better understanding of fine root dynamics and determining root demography through distinguishing observation years and root branch orders.

Aims Rhizosphere is a hotspot for soil C and N biogeochemical cycling in terrestrial ecosystems. Our understanding of interaction between C and N mineralization mainly comes from the bulk soils, yet it is unknown the interaction in the rhizosphere soils. The study aimed to identify interactions between soil C and N mineralization in paired rhizosphere soils and bulk soils. Methods We used the “soil adhering to fine roots after shaking” method to collect paired rhizosphere soils and bulk soils along an altitudinal forest gradient. Soil C mineralization rates (Cmin) and net N mineralization rates (Nmin) were determined with laboratory incubation. Results We found a strong positive relationship between Cmin and Nmin in the rhizosphere soils across sites, whereas Cmin was not correlated with Nmin in the bulk soils. Furthermore, soil properties, microbial biomass C (MBC) and extracellular enzyme activities showed substantial paths affecting Cmin and Nmin using structural equation model. The coupling of Cmin and Nmin in rhizosphere soils could be triggered by root-soil interactions, resulting in the higher level of MBC, soil total C, soil total N, extracellular enzyme activities and specific microbial community structure. By contrast, the decoupling of Cmin and Nmin in the bulk soils might be attributed to the lower level of MBC and extracellular enzyme activities. Conclusions Our results demonstrated that soil C and N mineralization coupled in the rhizosphere rather than the bulk soils. These results highlight the need to distinguish rhizosphere from bulk soils for simulating soil C and N cycling in terrestrial ecosystems.

Plant residue input plays a vital role in cropland carbon (C) balance, which can be greatly affected by climate change. Our knowledge of the effect of plant residue on soil respiration is crucial for evaluating C exchange between the atmosphere and terrestrial ecosystems, but large uncertainties remain in the effect of plant input on the temperature sensitivity (Q10) of soil organic carbon (SOC) decomposition in cropland ecosystems. Here, we tested the effect of long-term exclusion of plant input on the Q10 of SOC decomposition by incubating soils collected from two bare fallow (including a Mollisol and an Alfisol with different climate and land-use history) and adjacent old field plots at 10°C and 20°C for 815 days. The ‘equal-time’, ‘equal-C’, ‘one-pool model’ and ‘two-pool model’ methods were used to evaluate the Q10 of SOC decomposition. Results indicated that in the early stage of incubation, exclusion of plant input increased the Q10 (Alfisol) or had minor effect on the Q10 (Mollisol), which may be related to the interactions between substrate quality decrease (increase of chemical stability) and clay mineral suppression on microbial decomposition. However, in the later stage of incubation, plant residue removal decreased the Q10 values in both soils possibly due to the limitation of substrate availability on microbial decomposition. Overal, the role of plant input in the temperature sensitivity of SOC decomposition should be considered when predicting SOC stocks in a future warmer world.

The rhizosphere priming effect (RPE) is the change in decomposition of soil organic matter caused by root activity, and it can be affected by plant species, nutrient availability, and by many other factors. Although plant and soil nitrogen (N) are likely to greatly influence RPE and vice-versa, not much is known about these relationships. There is a need for more research, particularly in N-fertilized cropping systems. We examined the RPE on soil N mineralization in a 42-day greenhouse experiment. The experiment included three vegetable crops (broccoli, lettuce, and spinach) and two grain crops (sunflower and maize) and an unplanted control. These plantings were crossed with two fertilization treatments: fertilized (100 kg ha−1 of N) and unfertilized. We evaluated NO3−, NH4+, microbial N, and mobilized N, plant biomass, plant tissue N, and %RPE by planting treatment. We found that vegetables almost completely depleted NO3− in both fertilized and unfertilized soils, whereas grain crops did not. Further, vegetables took up more than twice as much N in their aboveground tissue compared to grain crops under N fertilization. Both vegetables and crops produced RPEs on N mineralization. Nitrogen fertilization significantly enhanced the RPE for vegetable crops but not for grain crops. This result corroborates the assertion that plants with higher N concentration in their tissues tend to produce a higher RPE than plants with lower N concentration. The RPE of vegetable crops were significantly enhanced with N fertilization. Our data strongly rejects the “N-mining” hypothesis which would predict a much reduced RPE with N fertilization, and suggest that this hypothesis is not a universal explanation for the observed priming phenomena.

Priming plays an important role in modifying the decomposition of soil organic matter (SOM), but there are large uncertainties in the temperature effect on priming mainly due to the variation in SOM stability. Long-term bare fallow offers a unique opportunity to isolate the relatively stabilized SOM pool and study its properties. We tested the temperature effect on priming of the relatively stabilized SOM pool by incubating soil samples collected from a bare fallow (representing the relatively stabilized SOM) and its adjacent old field (containing both stabilized SOM and labile SOM) at 10 and 20°C for 815 days. We amended the soil samples with C4 maize leaves to distinguish the CO2 source released from the soils (formed under C3 vegetation) and the substrate added (i.e. maize leaves) based on the natural abundance of δ13C. In all cases, there was a positive priming effect on native SOM decomposition when fresh organic matter (maize leaves) was added. The temperature sensitivity of priming effect (calculated as the difference in SOM decomposition due to the addition of maize leaves) in the bare fallow soil and the old field soil was quite different: increasing temperature significantly enhanced the magnitude of priming effect in the bare fallow soil, whereas had no effect on the magnitude of priming effect in the old field soil. The increase of the amount of microbial biomass C by maize leaves application was higher in the bare fallow soil than in the old field soil. Furthermore, for maize leaves-treated soil, temperature increase significantly increased the rate of microbial N mining throughout the incubation in the bare fallow soil, but had minor effect on microbial N mining in the old field soil at the end of incubation. We conclude that the priming effect of the relatively stabilized SOM was sensitive to temperature increase, which may be mainly driven by greater microbial growth and microbial demand for N. This work highlights the vulnerability of stabilized SOM to priming effect under global warming and reveals the potential role of microbes in regulating soil C dynamics under future climate change.

Microbial turnover processes are typically restricted by low substrate availability in the subsoil. We hypothesized that SOM decomposition increases with plant density and decreases with N fertilization: We expected a greater rate of C allocation to the rhizosphere in the topsoil than in the subsoil treatments. In order to simulate different degrees of rhizodeposition, wheat was planted in pots at four different densities. The plants were continuously labeled with 13C-depleted CO2. Soil CO2 efflux was partitioned for root- and SOM-derived CO2. Moreover, we determined the enzyme kinetics by measuring catalytic efficiency and enzyme stoichiometry in both topsoil and subsoil. Shoot biomass and the shoot to root ratio were significantly higher for plants grown in the topsoil compared with the subsoil, which demonstrated higher relative C allocation to root biomass in the subsoil treatment. Despite the similar size of the rhizosphere, root-derived CO2 was always higher in the topsoil compared with the subsoil treatment, indicating enhanced root exudation. Effect sizes of all enzyme activities showed stronger magnitudes for the subsoil treatments. This was in line with a two-times increase of the effect size of SOM decomposition in the subsoil relative to topsoil. Overall, the plants in the subsoil treatments allocated more C to root biomass, less C to shoot biomass, and substantially less C to root exudates. However, the effect sizes of both SOM decomposition and enzyme activities were higher in the subsoil than in the topsoil, reflecting a stronger sensitivity to C inputs.

Rhizosphere respiration (from roots and rhizosphere microbes utilizing root-derived carbon (C)) is a significant component of soil respiration, and rhizosphere priming effect (RPE, change in soil C decomposition by the presence of living roots in the rhizosphere) is crucial for regulating soil C decomposition. However, the relationships between rhizosphere respiration and RPE and root traits (such as biomass and nitrogen (N) concentration) across plant species and growing conditions are not fully resolved. In this study, we investigated rhizosphere respiration rate and RPE of three grassland species (a grass, a forb and a legume) using a continuous isotope-labeling technique. We found a significantly positive relationship between root-mass-specific rhizosphere respiration rate and root N concentration across the three species and two types of mesocosms (small pots and large buckets), and the scaling exponents were approximately one (indicating isometric scaling). Further, soil organic matter (SOM) decomposition rate was not statistically different between the planted treatments and the unplanted control, suggesting insignificant RPE at the early flowering stage (90 days after seeding) for these three species. Likely, respiration from rhizodeposition (root inputs to soil during the 90-day labeling period) was not included in SOM decomposition by the isotope labeling method, which underestimates SOM decomposition and may partly contribute to the lack of RPE of the three species. Overall, our results show that rhizosphere respiration rate is scalable with root N concentration across different plant species and growing conditions, while RPE of these grassland species at the early flowering stage was insignificant.

The rhizosphere priming effect (RPE) is increasingly recognized as an important factor in mediating soil organic matter (SOM) decomposition, which influences the CO2 release from terrestrial systems to the atmosphere. However, little is known about the long-term RPE of non-woody perennial species and the physical mechanisms underlying the RPE. Here the RPEs of three non-woody perennials (Stipa grandis, Leymus chinensis, Medicago sativa) differing in root traits and exudation were quantified using a natural 13C tracer method (C3 plant - C4 soil system) during a 476-day experiment. The results indicated that all plant species showed positive RPEs, with M. sativa showing the largest RPE and S. grandis the smallest RPE. Differences in the RPEs between species are likely associated with differences in root biomass, length, and exudates. Furthermore, the positive RPEs of the three species increased continuously with sampling time, indicating the long-term nature of the RPE. Results from structural equation modeling indicated that plant roots accelerated SOM decomposition via three mechanisms. The first mechanism (biological) was through the positive effect on dissolved organic C and microbial biomass C by the presence of the rhizosphere, consistent with the microbial activation hypothesis. The second mechanism (biological) was where plant roots significantly reduced soil mineral N and in turn promoted SOM decomposition, providing support for the microbial N-mining hypothesis. The third mechanism (physical) was where plant roots caused a net positive effect on SOM decomposition through net destruction of macroaggregates possibly as a result of high root length density and the associated drying-rewetting cycles. Overall, this study provides the first experimental evidence for the aggregate destruction hypothesis as a physical mechanism causing the RPE and highlights the importance of multiple mechanisms of the RPE on long-term SOM decomposition.

Living roots can influence microbial decomposition of soil organic matter, which has been referred to as the rhizosphere priming effect (RPE). Both microbial carbon efficiency (CUE) and microbial growth and turnover rates are associated with microbial decomposition and respiration of soil-derived C, but their linkage to the RPE remains poorly understood. Here we used a natural 13C tracer method to determine the RPE in soils planted with two wheat genotypes (249 or IAW2013) grown at high (30/24 °C during day/night) and low temperature (25/17 °C during day/night). We also determined microbial CUE, growth and biomass turnover rate using a substrate-independent H218O labeling method. The RPE varied from −2 to +455%, with significant effects of genotype, sampling date and their interaction with temperature. Compared to the unplanted control, microbial biomass C and growth/turnover rate were both enhanced in planted pots, with an average increase of 17% and 70%, respectively. Microbial CUE was lowest in pots planted with IAW2013 at low temperature, but there were no significant main effects of planting and temperature. Microbial biomass growth/turnover rate together with CUE accounted for 83% of the variation in soil-derived CO2, with a relatively larger contribution of microbial biomass growth/turnover rate (52%) than CUE (31%). Furthermore, using linear regression, we demonstrated that the RPE was significantly positively related to microbial biomass growth/turnover rate. No net soil organic C (SOC) loss or gain was detected, indicating that any increase in SOC due to increased microbial growth/turnover was counteracted by C loss caused by a higher RPE during the relatively short time of planting. These findings suggest that microbial biomass turnover associated with growth could control the loss of SOC with planting. We highlight the importance of plant-induced changes in microbial CUE and biomass growth/turnover for long-term soil C dynamics.

Rhizosphere respiration (Rroot) and rhizosphere priming effect (RPE) are crucial for regulating soil carbon dynamics. However, their responses to light intensity are not fully resolved. We investigated Rroot and RPE of soybean and sunflower using a continuous isotope-labeling technique. The two species were subjected to five levels of light intensity in a growth chamber. Plants were maintained at full light from seeding for 45 or 59 days, and switched to shading treatments for four days. Soil respiration was partitioned into root- and soil-derived CO2 during the last five days (day0, full light; day1–4, shading). Both soybean and sunflower Rroot showed significant positive relationships with light intensity, particularly after one day of shading. Moreover, both plants showed positive RPE (44–98%) at full light, but with increasing duration and intensity of shading, the RPE declined and even became negative (up to –20%). Indeed, a positive relationship between light intensity and RPE was observed in both species. Overall, our results showed that shading, by influencing light intensity and presumably photosynthesis rate and belowground carbon allocation, exerts a significant control of rhizosphere respiration rate and priming effect of the two species.

Aim: To conduct the clinical, genetic, and molecular characterization of 494 Han Chinese subjects with tic disorders (TD). Methods: In the present study, we performed the mutational analysis of 22 mitochondrial tRNA genes in a large cohort of 494 Han Chinese subjects with TD via Sanger sequencing. These variants were then assessed for their pathogenic potential via phylogenetic, functional, and structural analyses. Results: A total of 73 tRNA gene variants (49 known and 24 novel) on 22 tRNA genes were identified. Among these, 18 tRNA variants that were absent or present in Limitations: The phenotypic variability and incomplete penetrance of tic disorders in pedigrees carrying these tRNA mutations suggested the involvement of modifier factors, such as nuclear encoded genes associated mitochondrion, mitochondrial haplotypes, epigenetic, and environmental factors. Conclusion: Our data provide the evidence that mitochondrial tRNA mutations are the important causes of tic disorders among Chinese population. These findings also advance current understanding regarding the clinical relevance of tRNA mutations, and will guide future studies aimed at elucidating the pathophysiology of maternal tic disorders.

Nitrogen (N) mineralization in soils generally controls biological N availability in terrestrial ecosystems. As the pivotal first step in the overall N mineralization process, gross N mineralization (GNM, defined as the production of ammonium from microbial mineralization of organic N) is inherently coupled with microbial mineralization of soil organic carbon (C) which is commonly referred to as microbial respiration (MR), and that has often been used as a proxy of C availability. However, the pattern of GNM and its underlying mechanisms at a regional scale, and its linkage with MR remain unclear. By analyzing 100 soil samples collected across different forest types along a 3800 km long north-south transect in eastern China, we simultaneously measured the potential GNM using a 15N pool dilution method and MR using a dynamic CO2 trapping technique. We conducted a structural equation model (SEM) to examine the interactive effects of climate, soil pH, microbial substrate availability, and microbial biomass on potential GNM along the forest transect. Furthermore, we conducted a non-linear regression analysis between potential GNM and MR. We found that both potential GNM and MR varied largely, from 0.55 to 16.14 mg N kg−1 soil d−1 and from 3.64 to 24.30 mg C kg−1 soil d−1, respectively, but were not significantly affected by forest type. The SEM analysis showed that 51% of the variation in potential GNM was explained, with microbial substrate availability being the most important influencing factor. There was a positive non-linear relationship between potential GNM and MR (R2 = 0.52, P

From recent developments on how roots affect soil organic carbon (SOC) an apparent paradox has emerged where roots drive SOC stabilization causing SOC accrual, but also SOC destabilization causing SOC loss. We synthesize current results and propose the new Rhizo-Engine framework consisting of two linked components: microbial turnover and the soil physicochemical matrix. The Rhizo-Engine is driven by rhizodeposition, root turnover, and plant uptake of nutrients and water, thereby accelerating SOC turnover through both stabilization and destabilization mechanisms. This Rhizo-Engine framework emphasizes the need for a more holistic approach to study root-driven SOC dynamics. This framework would provide better understanding of plant root effects on soil carbon sequestration and the sensitivity of SOC stocks to climate and land-use changes.

The change in native soil organic carbon (SOC) decomposition caused by plant roots or the rhizosphere priming effect (RPE) is a common phenomenon. Although most of the SOC is stored in aggregates with different size classes, the RPE in aggregates and the underlying mechanisms remain unclear. In a 35‐d pot experiment, we grew Agropyron cristatum (C3 plant) in pots containing large macroaggregates (LMA), small macroaggregates (SMA), and microaggregates (MA) separated from a C4 soil. We quantified the RPE and measured microbial biomass C (MBC), oxidase activity, soil net nitrogen (N) mineralization, and aggregate dynamics at the end of the experiment. The positive RPEs ranged from 47% to 106% and were significantly lower in the SMA treatment than in the LMA and MA treatments. Planting significantly increased microbial N immobilization in all treatments, particularly in the SMA treatment. Furthermore, the positive relationship between RPE and plant‐induced changes in net N mineralization suggests that increasing microbial N immobilization could reduce RPE. Planting significantly increased MBC and oxidase activity, and the positive relationships between SOC decomposition and MBC and oxidase activity suggest that microbial activation may play an important role in the positive RPEs. Planting significantly reduced aggregate destruction in the SMA treatment but increased aggregate destruction in the MA treatment, supporting the aggregate destruction hypothesis. Overall, our results showed for the first time that the RPEs varied among aggregate size classes, with potentially important consequences for SOC dynamics in soils that have a high capacity for aggregation.

Coastal wetlands are increasingly recognized as important ecosystems for long-term carbon (C) storage. However, how soil aggregation mediates C accumulation and sequestration in these ecosystems remains unclear. Using the 13C isotope tracer from the invasion of a C4 plant, Spartina alterniflora, into the native ecosystem originally covered by C3 plants across Eastern Chinese coastal wetlands, we investigated a potential C stabilization process via soil structural protection. We quantified changes in soil aggregates, soil organic carbon (SOC), soil total nitrogen (STN), and natural 13C isotope abundance within aggregate fractions across a chronosequence of 0-, 4-, 8-, and 12-year S. alterniflora invasion. Our results showed that soil aggregate stability increased significantly along the chronosequence. Meanwhile, SOC and STN concentrations increased with invasion time in the whole soil and aggregate fractions, which were linked to increasing soil aggregate stability. The contribution of S. alterniflora-derived SOC increased from 18.96 to 40.24% in the 0–20 cm layer and from 4.66 to 32.04% in the 20–40 cm layer across the chronosequence from 4 to 12 years with the highest proportion observed in macro-aggregates. Our results indicate that invasion of S. alterniflora to coastal wetlands can sequester more C largely due to formation and stabilization of soil aggregates by soil structural protection.

The rhizosphere priming effect (RPE) is the stimulation or suppression of soil organic matter decomposition by living roots and associated rhizosphere organisms. The RPE is pivotal in regulating biogeochemical cycles in terrestrial ecosystems. However, biological mechanisms, especially soil micro-food web interactions, behind the RPE remain largely unknown. We quantified the RPE of soybean and cottonwood at three growth stages using a natural 13C tracer method, measured soil microbial and nematode community composition, and investigated their relations with the RPE. The magnitude of the RPE varied widely at different growth stages. Soybean produced a greater cumulative RPE than cottonwood. The plant species effect was also observed in the bacterial PLFA with higher values found in the soybean treatment. Mantel test analysis suggested that the variations in microbial community were closely related with the RPE, soil and plant characteristics. The nematode community affected the RPE indirectly through altering the structure of the microbial community. We demonstrated that the RPE was connected with interactions of soil micro-food webs. This connection indicates that soil micro-food web interactions in the rhizosphere may either regulate microbial turnover and/or microbial community composition, subsequently modulating the RPE.

The rhizosphere priming effect (RPE) plays an important role in the decomposition of soil organic matter (SOM) and the global carbon cycle. Although plant, soil, and environmental factors affecting the RPE have been examined, it is still unclear how soil water and nitrogen (N) status together alter the RPE. In order to examine plant, water, and N interacting effects on the RPE, we conducted a semi-natural experiment with two plant species (Leymus chinnensis and Medicago sativa) grown at two soil moisture levels (45 and 85% of water holding capacity) with or without N fertilization (0 and 10 g N m−2 y−1). We used a natural 13C abundance tracer method for measuring the RPE, and an N budgeting method for measuring net N mineralization or immobilization. Both positive (up to 253%) and negative (down to −21%) RPEs were observed in different treatment combinations. L. chinensis exhibited greater RPE than M. sativa under low soil moisture, possibly due to specific root architecture and root activity of L. chinensis. High soil moisture significantly reduced the RPE, while N fertilization significantly increased the RPE. To reconcile our current results and previous observations with existing individual hypotheses related to the RPE, we put forward a new hypothesis: when N mineralization dominates, the rhizosphere input enhances the RPE; when N immobilization dominates, the rhizosphere input reduces the RPE. Overall, this study demonstrates that the RPE on soil organic carbon decomposition is intimately linked with N processes and soil water status.

Rhizosphere priming effects (RPEs) play a central role in modifying soil organic matter mineralization. However, effects of tree species and intraspecific competition on RPEs are poorly understood. We investigated RPEs of three tree species (larch, ash and Chinese fir) and the impact of intraspecific competition of these species on the RPE by growing them at two planting densities for 140 d. We determined the RPE on soil organic carbon (C) decomposition, gross and net nitrogen (N) mineralization and net plant N acquisition. Differences in the RPE among species were associated with differences in plant biomass. Gross N mineralization and net plant N acquisition increased, but net N mineralization decreased, as the RPE on soil organic C decomposition increased. Intraspecific competition reduced the RPE on soil organic C decomposition, gross and net N mineralization, and net plant N acquisition, especially for ash and Chinese fir. Microbial N mining may explain the overall positive RPEs across species, whereas intensified plant-microbe competition for N may have reduced the RPE with intraspecific competition. Overall, the species-specific effects of tree species play an important role in modulating the magnitude and mechanisms of RPEs and the intraspecific competition on soil C and N dynamics.

Plant residue inputs play a crucial role in regulating soil carbon (C) stock and nitrogen (N) availability in cropland. However, little is known regarding how plant inputs mediate the relationships between soil C and net N mineralization, causing additional uncertainty in predicting ecosystem C and N dynamics. This study investigated the influences of long-term deprivation of plant inputs, short-term addition of maize straw and experimental warming on soil C and net N mineralization and their relationships. We conducted an 815-day laboratory incubation experiment under 10 and 20 °C using soils from a long-term bare fallow plot (without plant inputs for 23 years) and its adjacent old field plot (with continuous plant inputs). Our results showed that long-term deprivation of plant inputs decreased soil net N mineralization (per unit total N or TN) by 56% on average, but had minor effect on soil C mineralization (per unit soil organic C). Soil C and net N mineralization rates were positively correlated in the old field soil under 20 °C. However, soil C and net N mineralization rates were not correlated in the bare fallow soil, mainly due to the low level of net N mineralization. Moreover, soil C and net N mineralization rates were significantly increased by the addition of maize straw in both land-use types. When net N mineralization was 162 (or 159) μg N g−1 TN d−1, soil C and net N mineralization rates were positively correlated owing to a greater microbial mining of N from soil organic matter (SOM). Further, elevated temperature increased soil C and net N mineralization rates, and changed the relationships between soil C and net N mineralization. Taken together, this study provides evidence that plant inputs mediate the relationships between soil C and net N mineralization, and is thus critical in controlling ecosystem C and N cycling.

Plant litter inputs can influence soil organic carbon (SOC) decomposition via the priming effect. However, our understanding of the priming effect and underlying mechanisms is primarily from studies with leaf litter addition, while little is known about root litter effects, particularly of woody plants. Here, using a13C natural tracer approach, we conducted a 12-week incubation experiment to investigate litter decomposition and priming effect of mature-tree root orders (1st to 5th) of Cunninghamia lanceolata (Lamb.) Hook. We explored how litter decomposition and the priming effect were related to microbial biomass of main groups, enzyme activities, and root tissue chemistry. Root litter decomposition rates increased with increasing root order, especially during the first 4 weeks, which was likely due to higher non-structural C and lower tannin concentrations for higher order roots. A negative priming effect occurred at this initial intensive stage when microbes may have preferred utilizing litter-derived labile C. Subsequently, the priming effect switched to positive, and showed larger priming effects for the higher order roots than the lower order ones. Higher order roots also showed higher fungi to bacteria ratios and enzyme activities than the lower order roots. These patterns of fungi to bacteria ratios and enzyme activities and thus the priming effect could be attributed to the difference in carbon:nitrogen ratio among root orders. Overall, we for the first time provide strong evidence for the effect of root order on the priming effect, and thus highlight that separating root litter based on root order is necessary for accurately evaluating its influence on SOC decomposition.

Tree roots and their associated microbes can significantly influence soil organic matter (SOM) decomposition, i.e., the rhizosphere priming effect. This effect is expected to be greater in trees associated with arbuscular mycorrhizal (AM) fungi, which produce higher extracellular enzymes especially surrounding hyphae, than in trees associated with ectomycorrhizal (ECM) fungi. Here, we selected five tree species associated with AM (Juglans mandshurica Maxim. and Cunninghamia lanceolata (Lamb.) Hook.) or ECM (Picea koraiensis Nakai, Quercus mongolica Fischer ex Turcz. and Larix kaempferi (Lamb.) Carriere). We grew tree seedlings inside of cores lined with different mesh sizes to investigate how roots, hyphae and exudates influence soil carbon (C) and nitrogen (N) mineralization via the rhizosphere priming effect, using a13C natural abundance approach and a15N pool dilution method, concurrently. We found that tree seedlings significantly accelerated soil C decomposition by on average 78%, i.e., positive priming, compared to unplanted control pots. AM-associated trees induced 2.1 times greater soil C decomposition than ECM-associated trees across all mesh sizes. In contrast, gross N mineralization did not differ between tree-mycorrhizal associations. Compared to ECM counterparts, AM-associated trees had higher C- and lower N-degrading enzyme activities. Consequently, AM-associated trees induced a significantly higher C:N ratio of SOM decomposition than their ECM counterparts, which could be associated with the differences in soil enzyme activities for C and N degradation. Further, for both AM- and ECM-associated trees, we found no significant influences of mesh size on soil C decomposition, suggesting that the rhizosphere priming effect of mycorrhizal symbiosis was predominantly driven by root exudates. We conclude that SOM decomposition caused by AM-associated trees may have a higher C:N ratio than that by ECM-associated trees mainly due to differences in microbial enzyme investment. Our findings imply that tree-mycorrhizal associations are capable of modulating soil biogeochemical cycling via the rhizosphere priming effect.

Evidence from growth chamber and greenhouse experiments indicates that the rhizosphere priming effect (RPE, stimulation or suppression of soil organic matter (SOM) decomposition by live roots and their rhizospheric biota) may be sensitive to environmental factors such as temperature and level of sunlight. However, little is known about potential variations of the RPE under different latitudinal regions that vary in temperature and sunlight levels. We explored the RPE of soybean (Glycine max) and cottonwood (Populus × euramericana cv. ‘74/76’) at two different latitudinal locations under variable conditions of light and temperature for two growing seasons using a natural 13C tracer method. Different plant species at different time of season produced significantly different RPEs. The RPE of both soybean and cottonwood was small at the initial stage when the plants were small. The RPE of soybean ranged from 0.4% to135%, reached the highest level at the flowering stage and declined afterward. The RPE of cottonwood ranged from −17% to 121% and tended to increase in both growing seasons. The cumulative RPE of soybean was significantly higher than that of cottonwood at both locations and during the two growing seasons. Overall the cumulative RPE values were larger at low latitude than at high latitude. The cumulative RPE of soybean at the low latitudinal location was 1.7 times higher than at the high latitudinal location. The RPE of both plant species was significantly related to specific rhizosphere respiration, but negatively related to soil dissolved total nitrogen content. The RPE of both plant species varied at both locations during the two growing seasons. Particularly for soybean, the cumulative RPE was higher at the low latitudinal location than at the high latitudinal location, possibly because of higher light levels. Our results further indicate that plant species, time of season, specific rhizosphere respiration, and soil dissolved total nitrogen are important variables in controlling the RPE. Overall, these results provide the needed support for using RPE data in a broader context.

Rhizosphere priming is crucial for regulating soil carbon and nitrogen biogeochemical cycles. An appreciable number of studies have been conducted to quantify the rhizosphere priming effect (RPE), and have shown that the RPE is sensitive to changes of plant and soil conditions. These diverse results across individual studies offer us an opportunity to explore for potential general patterns and variability. In this study, we conducted a meta-analysis of RPE values taken from 31 publications. Our results showed that, on average, the RPE enhanced soil organic carbon mineralization rate by 59% across all studies. The magnitudes of the RPE significantly varied among plant types and soil texture. Within plant types, woody species showed the highest RPE followed by grasses while crops had the lowest level of the RPE, indicating that plant traits and physiology may exert important controls on the RPE. Soils with finer texture tended to produce stronger RPEs than soils with coarser texture, suggesting that interactions between the rhizosphere and the soil matrix may modulate the RPE. Furthermore, the level of the RPE is positively correlated with aboveground plant biomass, but surprisingly not with root biomass which is the commonly believed key variable for influencing the RPE. In addition, the RPE increased with the length of experimental duration, which implies that the RPE may persist much longer than previously believed because it impacts stabilized soil carbon more than labile carbon as the length of experimental duration increases. Overall, the results from this meta-analysis further illustrate several complex features of the RPE and call for future attentions to decipher this complexity.

Soil is an important organic carbon (C) pool in terrestrial ecosystems, but knowledge on soil organic carbon (SOC) decomposition rate and influencing factors remains limited, particularly in arid and semi-arid grasslands. Models show that global semi-arid regions are experiencing more extreme climate events, which may trigger more C loss from soils. We used the stable carbon isotope (δ13C) of plant and soil depth profile to examine decomposition rates of SOC along a 2200 km semi-arid grassland transect in northern China. Beta (β) was calculated from the relationship between δ13C and C concentrations of plant and soil profile (0–100 cm). Partial correlation analysis and structure equation models were used to analyze correlations between beta and climatic and edaphic variables. We found that the δ13C values increased from plant tissues to surface soil and then deep soil at all sites, while the SOC concentration decreased. The β values ranged from −0.33 to −2.27, with an average value at −1.37. There was a positive correlation between β value and SOC decomposition rate constant (k), supporting the hypothesis that enrichment of δ13C along soil depth profile was mainly due to isotopic fractionation during microbial SOC decomposition. The correlation between β value and aridity index was mainly due to the variations of edaphic properties such as soil C/N ratio with aridity index, pointing to the more important role of edaphic properties in β value than climatic factors. Despite uncertainties associated with the interpretation of soil δ13C along its depth profile, our results demonstrated that soil δ13C could provide an independent benchmark for examining model-based predictions of SOC decomposition in semi-arid grasslands. Incorporation of both climatic and edaphic variables into models may enhance the predictions for SOC dynamics under global climate changes.

Woody plant roots can be classified into absorptive roots and transport roots based on root functions, order and traits. While there is an emerging view that living roots actively affect soil organic carbon (SOC) decomposition via the rhizosphere priming effect (RPE), the linkages of the RPE with C allocation to absorptive roots (relative to total roots) and their functional traits across soils are virtually unknown. Here, we investigated the RPE by growing a tree species (Chinese fir, Cunninghamia lanceolata) in three isotopically-distinct C4 soil types with different soil properties such as C/nitrogen (N) ratio and texture, and by growing three tree species (Chinese fir, larch (Larix kaempferi) and ash (Fraxinus mandshurica)) with wide variations in root functional traits in one of the C4 soils. We classified living roots into absorptive roots (first and second orders) and transport roots (third and higher orders) and then quantified their C allocation (relative to total roots) and morphological and chemical traits associated with economic construction, rhizodeposition and resource acquisition. We found that the RPE of Chinese fir across the three soils decreased with an increase in soil C/N ratio. This result conflicted with the N mining hypothesis and suggests that soil C stabilization mechanisms associated with clay minerals may play an important role. Further, significant differences in the RPE among tree species were largely accounted for by the C allocation to absorptive roots. Moreover, there was a significantly negative relationship between specific surface area of absorptive roots and the specific RPE (per unit biomass of absorptive roots) among tree species, suggesting that absorptive root traits shaping the extent of the rhizosphere may regulate the RPE. Taken together, our results provide evidence that absorptive roots play a predominant role in causing the RPE. These findings present an important step toward improving our capability to predict plant effects on SOC decomposition through linking the RPE to absorptive root functional traits.

Live roots can stimulate microbial soil organic matter (SOM) decomposition and nutrient cycling, which is termed as the rhizosphere priming effect (RPE). Compared to nitrogen (N) availability, fewer studies have focused on the effect of phosphorus (P) availability on the RPE. Here we investigated the RPEs of ryegrass (Lolium perenne) and clover (Trifolium repens) with and without P fertilization (4 g P m−2) at three sampling times (Day 30, Day 44, and Day 58 after planting). A continuous 13C–CO2 labeling method was used to separate soil-derived CO2 from root-derived CO2. A nutrient budget method was applied to evaluate the rhizosphere effect on net soil N and P release for plant uptake. We found that ryegrass and clover induced positive RPEs in most plant-soil combinations, ranging from −1% to 134%. Ryegrass exhibited a larger RPE than clover by Day 30, but clover exhibited a larger RPE than ryegrass by Day 44 and Day 58, possibly due to larger shoot biomass regrowth rates, root activity, and rhizodeposition during the later stages. P fertilization significantly decreased the RPE of ryegrass by Day 44 and Day 58, but did not change the RPE of ryegrass by Day 30 and clover at all three sampling times. The reduced RPE of ryegrass with P fertilization was associated with increased microbial biomass N, more root-derived microbial C, and less shoot biomass and root-derived CO2. These findings suggest that P fertilization coupled with C supply from root exudates induced more microbial N immobilization, which reduced the RPE of ryegrass during later stages when soil N limitation negatively impacted plant growth. However, P-induced microbial N immobilization did not affect clover as much because its biological N fixation, on average 37% of total plant N, may have alleviated soil N limitation. We further observed significant positive relationships between excess net soil N and P release and the RPE by Day 58 across all planted treatments, indicating that soil N and P release by plants can be directly linked to rhizosphere C mineralization. Overall, our results demonstrate the importance of C–N–P interactions for understanding the RPE, which have significant implications for P cycling in plant-soil systems.

The influence of plant roots and the associated rhizosphere activities on decomposition of soil organic matter (SOM), the rhizosphere priming effect, has emerged as a crucial mechanism regulating global carbon (C) and nitrogen (N) cycles. However, the role of root morphology in controlling the rhizosphere priming effect remains largely unknown. To investigate the link between root hairs, a critical part of the entire root morphology, and the rhizosphere priming effect, we grew a barley wild type and a barley mutant without root hairs in a greenhouse and continuously labeled them with 13 C-depleted CO 2 . Soil CO 2 efflux was measured during tillering and head-emergence stages of plant growth. Based on its δ 13 C signature, total CO 2 was partitioned for root-derived and SOM-derived CO 2, and the SOM decomposition primed in the rhizosphere was calculated. Soil microbial biomass C and N, and the activities of six extracellular enzymes (β-cellobiohydrolase, β-glucosidase, acid phosphatase, β-xylosidase, leucin-aminopeptidase, and N-acetyl-β-glucosaminidase) were measured to test the effects of root hairs. During the early stage of development (tillering), when plants were sufficiently supplied with nutrients, the barley mutant without root hairs produced more shoot biomass. In contrast, high C costs for root-hair formation likely reduced the growth of the barley wild type. At this stage, the wild type with regular root hairs produced a positive rhizosphere priming effect (69% increase), but the mutant without root hairs produced a negative priming effect on SOM decomposition (28% decline). At the head-emergence stage, when nutrients were scarce, plant biomass production of the mutant was reduced, probably due to inefficient nutrient uptake in the absence of root hairs. At this stage, both barley types produced positive rhizosphere priming effects (72% and 209% increase for the wild type and the mutant, respectively) and the microbial biomass was higher for both planted soils compared to the tillering stage. Extracellular enzymes responsible for the decomposition of stable SOM had higher activities in cases of positive priming effects. Overall, root hairs played an important role in regulating rhizosphere priming.

Loss of plant diversity with increased anthropogenic nitrogen (N) deposition in grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of light or space is invoked as a key mechanism. Here, we provide evidence from a 9-yr N-addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply shifted plant community composition from grass-forb codominance (primarily Stipa krylovii and Artemisia frigida, respectively) to exclusive dominance by grass, with associated declines in overall species richness. Reduced abundance of forbs was linked to soil acidification that increased mobilization of soil Mn, with a 10-fold greater accumulation of Mn in forbs than in grasses. The enhanced accumulation of Mn in forbs was correlated with reduced photosynthetic rates and growth, and is consistent with the loss of forb species. Differential accumulation of Mn between forbs and grasses can be linked to fundamental differences between dicots and monocots in the biochemical pathways regulating metal transport. These findings provide a mechanistic explanation for N-induced species loss in temperate grasslands by linking metal mobilization in soil to differential metal acquisition and impacts on key functional groups in these ecosystems.

The response of soil organic carbon (SOC) pools to globally rising surface temperature crucially determines the feedback between climate change and the global carbon cycle. However, there is a lack of studies investigating the temperature sensitivity of decomposition for decadally cycling SOC which is the main component of total soil carbon stock and the most relevant to global change. We tackled this issue using two decadally (13) C-labeled soils and a much improved measuring system in a long-term incubation experiment. Results indicated that the temperature sensitivity of decomposition for decadally cycling SOC (>23 years in one soil and >55 years in the other soil) was significantly greater than that for faster-cycling SOC (

The root-driven soil aggregate turnover dynamics and rhizosphere priming effect (RPE, changes in soil organic carbon (SOC) decomposition caused by living roots) are central to the understanding of SOC cycling. However, the association between aggregate turnover and the RPE has not been illuminated in plant-soil systems because of methodological difficulties. Using rare earth oxides to trace the transformations among different aggregates and 13C natural abundance labeling, we for the first time simultaneously investigated aggregate turnover and the RPE at two phenological stages of two grass species (Agropyron cristatum and Koeleria cristata): tillering (40 days after planting, DAP40) and jointing-heading (DAP63). We found that aggregate turnover rates varied widely, with a range between 0.006 day−1 and 0.024 day−1, i.e., turnover times (the reciprocal of turnover rates) ranged from 41 to 168 days, and were significantly influenced by plant species, sampling date and their interaction. Particularly, greater aggregate turnover rates (2% ~ 68%) and transformations in breakdown and formation pathways were found for K. cristata than for A. cristatum at DAP63. The RPEs increased with plant growth and ranged from −29% to +163%. Especially, the RPE and microbial biomass C were significantly greater for K. cristata than for A. cristatum at DAP63. Root-driven aggregate turnover was tightly associated with the RPE, possibly because of the release of aggregate-protected C for microbial decomposition. There was no net C loss mainly because increased aggregate formation could have sequestrated root-derived C in macroagrgegates and thus counteracted the C loss by the positive RPE. We therefore propose a new framework of root-driven aggregate turnover for considering how plant roots influence SOC dynamics via aggregate turnover. Root-accelerated aggregate turnover acts as a “key”: enhancing SOC decomposition (i.e. RPE), while simultaneously accelerating the occlusion of root-derived C and thus facilitating new C sequestration. This framework highlights that living root-driven aggregate turnover alters the physical protection of SOC and regulates the RPE, which aligns well with the emerging perspective of SOC stabilization.

Rainfed crop production is the primary means of food security and income generation for rural households in semi-arid West Africa, which contains a high level of agro-ecological and socio-economic heterogeneity. Official management recommendations currently focus on the use of purchased certified seed and inorganic fertilizers, but are based primarily on highly controlled on-station trials that do not capture this heterogeneity and complexity of production systems. This study established hundreds of on-farm research trials across Senegal and The Gambia to test the agronomic benefit of multiple integrated practices related to seeds, inorganic fertilizers, and locally available organic amendments. All management practice were found to reliably increase yield across both the heterogeneity of this region and the diversity of the partnering farmers, and multiple integrated practices had greater effect than the currently recommended “best” practice. These alternative practices therefore offer an immediate pragmatic replacement to the existing recommendations and demonstrate the research and extension value of supervised on-farm trials. These findings also suggest that agronomic recommendations should focus on multiple “better” options rather than singular “best” practices, and farmers encouraged to select among these options based on their individual knowledge, circumstances, and preferences.

Roots and the associated rhizospheric activities regulate the mineralization of native soil organic matter (SOM), which is referred to as the rhizosphere priming effect (RPE). Although the importance of RPE for carbon cycle has increasingly been recognized, experimental evidence for how soil structural changes modulate the RPE is still unavailable. We addressed this issue by growing soybean plants (C3) in a C4-derived soil in a continuous 13C- labeling greenhouse. We hypothesized that root-induced soil structural change regulated the RPE by destabilizing soil matrix-protected organic carbon. Our results showed that the RPE was tightly coupled with plant photosynthetic activity, the disruption of coarse macro-aggregates, and the increased release of dissolved organic carbon (DOC) from the soil matrix. These findings indicate that living roots together with rhizodeposits not only can directly stimulate rhizospheric microbial activities, but also can make soil matrix-protected organic carbon available to microbial attacks and further enhance the RPE. This study suggests that the RPE on SOM mineralization is intimately linked with the dynamics of soil structures and DOC, which should be considered in future studies on mechanistic understanding and modeling of the RPE.

The decomposition of soil organic carbon (SOC) is intrinsically sensitive to temperature. However, the degree to which the temperature sensitivity of SOC decomposition (as often measured in Q10 value) varies with soil depth and labile substrate availability remain unclear. This study explores (1) how the Q10 of SOC decomposition changes with increasing soil depth, and (2) how increasing labile substrate availability affects the Q10 at different soil depths. We measured soil CO2 production at four temperatures (6, 14, 22 and 30 °C) using an infrared CO2 analyzer. Treatments included four soil depths (0–20, 20–40, 40–60 and 60–80 cm), four sites (farm, redwood forest, ungrazed and grazed grassland), and two levels of labile substrate availability (ambient and saturated by adding glucose solution). We found that Q10 values at ambient substrate availability decreased with increasing soil depth, from 2.0–2.4 in 0–20 cm to 1.5–1.8 in 60–80 cm. Moreover, saturated labile substrate availability led to higher Q10 in most soil layers, and the increase in Q10 due to labile substrate addition was larger in subsurface soils (20–80 cm) than in surface soils (0–20 cm). Further analysis showed that microbial biomass carbon (MBC) and SOC best explained the variation in Q10 at ambient substrate availability across ecosystems and depths (R2 = 0.37, P