Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem

Experimentation with dynamics of soil carbon pools as affected by elevated CO₂ can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ¹³C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO₂ enrichment (FACE) experiment, in which elevated CO₂ (FACE: 560 μmol mol⁻¹) and ambient CO₂ (Control: 360 μmol mol⁻¹) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO₂ transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO₂ was higher than that under ambient CO₂, indicating that elevated CO₂ reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO₂ enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO₂ and mitigating harmful CO₂ under future atmospheric conditions.     

Priming effects in different soil types induced by fructose, alanine, oxalic acid and catechol additions

It is well established that certain substrate additions to soils may accelerate or retard the mineralisation of soil organic matter. But up to now, research on these so called ‘priming effects’ was almost exclusively conducted with arable soils and with plant residues or glucose as additives. In this study, the effects of the uniformly ¹⁴C-labelled substrates fructose, alanine, oxalic acid and catechol on the mineralisation of soil organic carbon (SOC) from different horizons of two forest soils (Haplic Podzol and Dystric Cambisol) and one arable soil (Haplic Phaeozem) under maize and rye cultivation were investigated in incubation experiments for 26 days. Apart from the controls, all samples received substrate additions of 13.3 μg substrate-C mg⁻¹ C_org. During the incubation, CO₂-evolution was measured hourly and the amount of ¹⁴CO₂ was determined at various time intervals. In almost all soils, priming effects were induced by one or several of the added substrates. The strongest positive priming effects were induced by fructose and alanine and occurred in the Bs horizon of the Haplic Podzol, where SOC mineralisation was nearly doubled. In the other soil samples, these substrates enhanced SOC mineralisation by +10 to +63%. Catechol additions generally reduced SOC mineralisation by −12 to −43% except in the EA horizon of the Haplic Podzol where SOC-borne CO₂-evolution increased by +46%. Oxalic acid also induced negative as well as positive priming effects ranging from −24 to +82%. The data indicate that priming effects are ubiquitously occurring in surface and subsoil horizons of forest soils as well as in arable soils. Although a broad variety of soils was used within this study, relationships between soil properties and priming effects could not be ascertained. Therefore, a prediction on occurrence and magnitude of priming effects based on relatively easily measurable chemical and physical soil properties was not possible. Nevertheless, the data suggest that positive priming effects are most pronounced in forest soils that contain SOC of low biodegradability, where the added substrates may act as an important energy source for microbial metabolism.     

Controls on soil carbon accumulation during woody plant encroachment: Evidence from physical fractionation, soil respiration, and δC of respired CO2

Woody plant encroachment into grasslands and savannas is a globally extensive land-cover change that alters biogeochemical processes and frequently results in soil organic carbon (SOC) accrual. We used soil physical fractionation, soil respiration kinetics, and the isotopic composition of soil respiration to investigate microbial degradation of accrued SOC in sandy loam soils along a chronosequence of C₃woody plant encroachment into a C₄-dominated grassland in southern Texas. Our previous work in this system demonstrated significant changes in the chemistry and abundance of lignin and aliphatic biopolymers within particulate soil fractions during the first 40 yrs of woody plant encroachment, indicating selective accrual of purportedly more recalcitrant plant chemicals. However, during the long-term soil laboratory incubation presented herein, a greater proportion of SOC was mineralized in soils from older woody stands (34-86 yrs) than in soils from younger woody stands (14-23 yrs) and grasslands, providing no evidence for greater biochemical recalcitrance as a controlling mechanism for SOC accrual. In addition, δ¹³C values of respired CO₂ indicate that the mineralized SOC was predominately of C₃ origin from all woody stands along the chronosequence, and that respired CO₂ was primarily derived from the free light fraction (density <1.0 g/cm³) and macroaggregate-sized soil fraction. Our data suggested that the location of SOC among soil fractions was more important than plant polymer chemistry in determining SOC turnover rates during incubation. Surprisingly, estimates of the size and turnover rate of the active SOC pool based on respiratory kinetics did not increase with woody encroachment, and the turnover rate of the slower SOC pool decreased, again supporting the notion that increases in biochemically recalcitrant biopolymers did not hinder decomposition in the lab. These data indicate environmental conditions that may allow for C accrual in the field were alleviated during the controlled incubation. Therefore, C accrual in these sandy loam soils following woody encroachment should not be assumed stable, and this factor should be taken into account when considering responses of SOC to climate change or when making management decisions regarding land cover impacts on SOC.     

Land use change exerts a strong impact on deep soil C stabilization in subtropical forests

Purpose

It has been widely recognized that land use changes can cause significant alterations of soil organic matter (SOM) of various ecosystems. Forest conversion, a common land use change, and its effects on SOM have been a hot research topic during the past two decades. However, the mechanisms of the effects of forest conversion on SOM dynamics, particularly in deep soils, largely remain uncertain. This study aimed to examine the impacts of forest conversion on SOM stabilization through the analysis of soil aggregate and density fractionation, microbial composition, and functions in deep soils.

Materials and methods

Soil C and microbes were sampled in soil layers of 0–20 and 60–80 cm under broadleaved secondary forest and two coniferous plantations (Cunninghamia lanceolata and Pinus massoniana). Aggregate and density fractionation techniques were used to analyze C accumulation in non-protected, physically, chemically, and biochemically protected C fractions. A 90-day laboratory mineralization incubation experiment with and without 400-mg C kg^?1 soil glucose and phenol was conducted to determine the potential mineralizable C, utilization of substrate capacity, and metabolic quotient (qCO₂).

Results and discussion

Conversion of secondary forests into coniferous plantations significantly decreased bulk soil C, especially in the deep soils. Forest conversion significantly decreased non-protected, physically, and chemically protected C fractions in both topsoil and deep soil and biochemically protected C fraction in deep soils. The soil organic carbon (SOC) of topsoils was dominated by non-protected fraction while in deep soil which was dominated by protected fraction. Compared with the topsoils, soil microbes in the deep soils tend to preferentially use labile soil organic matter with lower substrate use efficiency (higher values of qCO₂), which indicates that a r-strategy dominates of microbes. The increased respiration rate in the deep soils caused by forest conversion, when normalized to soil C, indicates that deep SOM may be more prone to decomposition and destabilization than top SOM.

Conclusions

Forest conversion can cause a significant alteration of SOC stabilization through the changes of physically, chemically, and biochemically protected SOC fractions. The mechanisms for the changes in non-protected or/and protected SOC fractions may be associated with the redistribution of r-strategy- and K-strategy-dominated microbes due to changes in litter inputs and priming effects.

     

Plant-biochar interactions drive the negative priming of soil organic carbon in an annual ryegrass field system

There is a knowledge gap on biochar carbon (C) longevity and its priming effects on soil organic carbon (SOC) and recent root-derived C under field conditions. This knowledge would allow the potential of biochar in long-term soil C sequestration to be established. However, most studies on biochar C longevity and its priming effect have been undertaken in plant-free laboratory incubations.A 388 d field study was carried out in the presence of an annual ryegrass (C₃) growing on a rhodic ferralsol with established C₃/C₄ plant-derived SOC (δ¹³C: −20.2‰) in a subtropical climate. A ¹³C-depleted hardwood biochar (δ¹³C: −35.7‰, produced at 450 °C) was applied at 0 and 30 dry t ha⁻¹ and mixed into the top 100-mm soil profile (equivalent to 3% w/w). We report on the differentiation and quantification of root respiration and mineralisation of soil-C and biochar-C in the field. Periodic ¹³CO₂ pulse labelling was applied to enrich δ¹³C of root respiration during two separate winter campaigns (δ¹³C: 151.5–184.6‰) and one summer campaign (δ¹³C: 19.8–31.5‰). Combined soil plus root respiration was separated from leaf respiration using a novel in-field respiration collar. A two-pool isotope mixing model was applied to partition three C sources (i.e. root, biochar and soil). Three scenarios were used to assess the sensitivity associated with the C source partitioning in the planted systems: 1) extreme positive priming of recent SOC derived from the current ryegrass (C₃) pasture; 2) equivalent magnitude of priming of SOC and labile root C; and 3) extreme positive priming of the native C4-dominant SOC.We showed that biochar induced a significant negative priming of SOC in the presence of growing plants but no net priming was observed in the unplanted soil. We also demonstrated the importance of experimental timeframe in capturing the transient nature of biochar-induced priming, from positive (day 0–62) to negative (day 62–388). The presence/absence of plants had no impact on biochar-C mineralisation in this ferralsol during the measurement period. Based on a two-pool exponential model, the mean residence time (MRT) of biochar varied from 351 to 449 years in the intensive pasture system to 415–484 years in the unplanted soils.     

Factors influencing the stability of labelled microbial materials in soils

The effect of freshly added substrate on carbon turnover of a microbial population and the priming action on stabilized soil organic constituents were investigated in the laboratory. ¹³C-labelled glucose. NH₄NO₃, or both were added to samples of a Brown Chernozemic soil which had been initially amended with ¹⁴C-glucose and incubated 2 months under field conditions. At the end of 14 days laboratory incubation. 39 per cent and 33 per cent of the ¹³C had been respired as CO₂ from the glucose and glucose plus NH₄NO₃ treatments, respectively. These two treatments resulted in a marked priming of native ¹²C during the second and third days of incubation and a second priming peak during the fifth day. In contrast, there was only a small priming action of the ¹⁴C-labelled materials. Addition of NH₄NO₃ by itself had no effect on the amount of ¹²C or ¹²C respired.Appreciable amounts of ¹⁴C were mineralized following treatments known to partially sterilize soil. Freezing and thawing was more effective than wetting and drying, but less effective than CHCl₃ vapour in releasing stabilized ¹⁴C materials. The amount of labelled-¹⁴C mineralized during incubation after treatment with chloroform vapour was greater than could he accounted for by the decrease in soil biomass.     

Natural 13C abundance and soil carbon dynamics under long‐term residue retention in a no‐till maize system

Residue retention and reduced tillage are both conservation agricultural practices that may enhance soil organic carbon (SOC) stabilization in soil. We evaluated the long‐term effects of no‐till (NT) and stover retention from maize on SOC dynamics in a Rayne silt loam Typic Hapludults in Ohio. The six treatments consisted of retaining 0, 25, 50, 75, 100 and 200% of maize residues on each 3 × 3 m plot from the crop of previous year. Soil samples were obtained after 9 yrs of establishing the experiment. The whole soil (0–10 and 10–20 cm of soil depths) samples under different treatments were analysed for total C, total N, recalcitrant C (NaOCl treated sample) and ¹³C isotopic abundance (0–10 cm soil depth). Complete removal of stover for a period of 9 yrs significantly (P < 0.01) decreased soil C content (15.5 g/kg), whereas 200% of stover retention had the maximum soil C concentration (23.1 g/kg). Relative distribution of C for all the treatments in different fractions comprised of 55–58% as labile and 42–45% as recalcitrant. Retention of residue did not significantly affect total C and N concentration in 10–20 cm depth. ¹³C isotopic signature data indicated that C₄‐C (maize‐derived C) was the dominant fraction of C in the top 0–10 cm of soil layer under NT with maize‐derived C accounting for as high as 80% of the total SOC concentration. Contribution of C₄‐C or maize‐derived C was 71–84% in recalcitrant fraction in different residue retained plots. Residue management is imperative to increase SOC concentrations and long‐term agro‐ecosystem necessitates residue retention for stabilizing C in light‐textured soils.     

Pyrogenic carbon additions to soil counteract positive priming of soil carbon mineralization by plants

Important due to both its role in fire-affected ecosystems, and also its proposed intentional production and application for carbon (C) management, pyrogenic organic matter (PyOM) is thought to contain very stable forms of C. However, the mechanisms behind its interactions with non-PyOM soil organic C (SOC) remain speculative, with studies often showing short-term positive and then long-term negative “priming effects” on SOC decomposition after PyOM applications. Furthermore, studies of these interactions to date have been limited to systems that do not include plants. This study describes results from a 12-week greenhouse experiment where PyOM-SOC priming effects with and without plants were investigated using stable isotope partitioning. In addition, we investigated the optimal δ¹³C proxies for sources of SOC, PyOM, and plant-derived CO₂ emissions. The two-factorial experiment included the presence or absence of corn plants and of ¹³C-labelled PyOM. In order to control for pH and nutrient addition effects from PyOM, its pH was adjusted to that of the soil and optimal nutrient and water conditions were provided to the plants. The δ¹³C of PyOM sub-components were significantly different. Significant losses of 0.4% of the applied PyOM-C occurred in the first week. We find evidence for a “negative priming” effect of PyOM on SOC in the system (SOC losses are 48% lower with PyOM present), which occurred primarily during the first week, indicating it may be due to transient effects driven by easily mineralizable PyOM. Additionally, while the presence of corn plants resulted in significantly increased SOC losses (“positive priming”), PyOM additions counteract this effect, almost completely eliminating net C losses either by decreasing SOC decomposition or increasing corn C additions to soil. This highlights the importance of including plants in studies of PyOM-SOC interactions.     

Priming effects in soil size fractions of a podzol Bs horizon after addition of fructose and alanine

It is well known that the addition of easily available substrates to soils can affect microbial activity and thus the mineralization of soil organic carbon (SOC). Up to now, little is known about the processes leading to these priming effects and which fractions of organic matter (OM) are affected. The objectives of this study were to determine if SOC associated with isolated soil size fractions showed different susceptibility to priming effects, whether these pools are easily depleted, or whether the amount of substrate addition affects the extent of priming effects. In an incubation experiment, the effect of the uniformly ¹⁴C‐labeled substrates fructose and alanine on the mineralization of the SOC of a Bs horizon of a Haplic Podzol was investigated. The soil sample was fractionated into the three soil size fractions sand, silt, and clay by a mild sonication followed by sieving and sedimentation. Additionally, nonfractionated soil of the horizon was included in the experiment. Every soil sample received four substrate additions repeated at weekly intervals with 3.325 μg substrate‐C (mg SOC)^–1 and a final addition of 13.3 μg substrate‐C (mg SOC)^–1 after 4 weeks. The respiration was determined hourly and ¹⁴CO₂ was analyzed every 2, 4, and 7 d after the respective substrate addition. After 56 d, between 42% and 58% of the added substrates had been mineralized. Both substrates strongly increased the mineralization of the OM in all fractions (positive priming effects). The priming effects were always higher after the addition of the high substrate dose than during the first 4 weeks when four small doses were added. In general, the priming effects increased with decreasing particle size. Alanine generally caused higher priming effects than fructose in the soil size fractions (up to 280% vs. 231%, respectively). This indicates that alanine serves not only as an energy substrate but also as a N source and, thus, also promotes microbial growth. The strong priming effects in the silt and clay fraction (133% and 125% with fructose, 172% and 168% with alanine) showed, that not only the labile pool of OM is affected, but also a more stable pool characterized by higher ¹⁴C ages. We assume that the stability of the OM in these fractions is not only due to recalcitrance or to interactions with the minerals, but that it may also be caused by a substrate limitation of the degrading microorganisms.     

中国西南喀斯特地区周期性温度波动对土壤有机碳矿化的影响

The diurnal fluctuation in soil temperature may influence soil organic carbon (SOC) mineralization, but there is no consensus on SOC mineralization response to the cyclical fluctuation in soil temperature. A 56-d incubation experiment was conducted to investigate the effects of constant and variable temperatures on SOC mineralization. Three soils were collected from the karst region in western Guizhou Province, southwestern China, including a limestone soil under forest, a limestone soil under crops and a yellow soil under crops. According to the World Reference Base (WRB) classification, the two limestone soils were classified as Haplic Luvisols and the yellow soil as a Dystric Luvisol. These soils were incubated at three constant temperatures (15, 20 and 25 ^oC) and cyclically fluctuating temperatures (diurnal cycle between 15 and 25 ^oC). The results showed that the 56-d cumulative SOC mineralized (C₅₆) at the fluctuating temperatures was between those at constant 15 and 25 ^oC, suggesting that the cumulative SOC mineralization was restricted by temperature range. The SOC mineralization responses to the fluctuating temperatures were different among the three soils, especially in contrast to those at constant 20 ^oC. Compared with constant 20 ^oC, significant (P < 0.05) decreases and increases in C₅₆ value were found in the limestone soil under forest and yellow soil under crops at the fluctuating temperatures, respectively. At the fluctuating temperatures, the forest soil with lower temperature coefficient Q₁₀ (the relative change in SOC mineralization rate as a result of increasing the temperature by 10 ^oC) had a significantly (P < 0.05) lower SOC mineralization intensity than the two cropland soils. These indicated that differences in temperature pattern (constant or fluctuating) could significantly influence SOC mineralization, and SOC mineralization responses to the fluctuating temperatures might be affected by soil characteristics. Moreover, the warmer temperatures might improve the ability of soil microbes to decompose the recalcitrant SOC fraction, and cyclical fluctuations in temperature could influence SOC mineralization through changing the labile SOC pool size and the mineralization rate of the recalcitrant SOC in soils.     

Short-term temperature dependence of heterotrophic soil respiration after one-month of pre-incubation at different temperatures

Quantification of microbial activities involved in soil organic carbon (SOC) decomposition is critical for the prediction of the long-term impact of climate change on soil respiration (SR) and SOC stock. Although the temperature sensitivity of SR is especially critical in semi-arid regions, such as North West Tunisia, where the SOC stock is low, little research has been carried out in these environments. More needs to be known about factors, such as SOC availability that influence temperature sensitivity. In this study, soil samples were incubated with and without glucose addition for 28 days after a 28-day pre-incubation period. Pre-incubation and incubation was carried out at 20 °C, 30 °C, 40 °C and 50 °C. Respiration measurements were taken with temperature, glucose addition and incubation time as independent variables. The highest pre-incubation temperature reduced the temperature sensitivity of SR during the subsequent incubation period, both with and without glucose addition. Soil samples pre-incubated at 50 °C had the lowest SR at all subsequent incubation temperatures and the lowest temperature sensitivity of SR, even after glucose addition. However, after glucose addition, the effect of a high pre-incubation temperature on soil respiration lasted only two days. Measuring the water-soluble carbon (WSC) in soil samples suggested that the high pre-incubation temperature may have killed part of the microbial biomass, modified microbial communities or solubilized SOC. For quantifying the possible effect of global warming, in particular heat waves, on soil respiration in the soil studied, the results indicate a moderate response of soil respiration to temperature at high temperatures, as shown by Q₁₀ close to 1.7, even in the range 40-50 °C.     

Carbon quality and the temperature sensitivity of soil organic carbon decomposition in a tallgrass prairie

The temperature sensitivity of soil organic carbon (SOC) decomposition will influence the accuracy of the quantitative prediction of carbon (C) balance between ecosystem C fixation and decomposition in a warmer world. However, a consensus has not yet been reached on the temperature sensitivity of SOC decomposition with respect to SOC quality. The fundamental principles of enzyme kinetics suggest that temperature sensitivity of decomposition is inversely related to the C quality of the SOC. This “C quality-temperature” hypothesis was tested in a 170-day laboratory experiment by incubating soil samples with changing temperature (low-high-low) at a ±5 °C step every 24 h. Soil samples were collected from a long-term warming experiment in a tallgrass prairie. There were four treatments of soil samples before lab incubation: control (C), warmed (W), field incubation (FI, litter exclusion), and warmed plus field incubation (WFI). Results showed that SOC decomposition rates were influenced by labile organic C (LOC) content, which were low in the soils under field incubation and decreased with increasing lab incubation time. Field warming and field incubation increased the temperature sensitivity of SOC decomposition in the 1st two lab incubation cycles but the treatment effects diminished as decomposition proceeded, probably due to increased contribution of recalcitrant C. In line with the hypothesis, we found that the lower the SOC quality, the higher the Q₁₀ values. This relationship held across treatments and lab incubation cycles, regardless of whether the differences in SOC quality resulted from inherent differences in SOC chemistry or from differences in the extent of SOC decomposition. Treatment effects of field warming and field incubation on SOC quality and Q₁₀ values also negatively correlated with each other. Our results suggest that dynamics of low-quality SOC have the highest potential to impact long-term C stocks in soils. Potential decreases in SOC quality in response to warming and consequent shifting species composition may result in a positive feedback of SOC to climate change in the future.     

Effect of temperature on biochar priming effects and its stability in soils

Recent studies have shown both increased (positive priming) and decreased (negative priming) mineralisation of native soil organic carbon (SOC) with biochar addition. However, there is only limited understanding of biochar priming effects and its C mineralisation in contrasting soils at different temperatures, particularly over a longer period. To address this knowledge gap, two wood biochars (450 and 550 °C; δ¹³C −36.4‰) were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol; δ¹³C −17.3 to −28.2‰) at 20, 40 and 60 °C in the laboratory. The proportions of biochar- and soil-derived CO₂–C were quantified using a two-pool C-isotopic model.Both biochars caused mainly positive priming of native SOC (up to +47 mg CO₂–C g⁻¹ SOC) in the Inceptisol and negative priming (up to −22 mg CO₂–C g⁻¹ SOC) in the other soils, which increased with increasing temperature from 20 to 40 °C. In general, positive or no priming occurred during the first few months, which remained positive in the Inceptisol, but shifted to negative priming with time in the other soils. The 550 °C biochar (cf. 450 °C) caused smaller positive priming in the Inceptisol or greater negative priming in the Entisol, Oxisol and Vertisol at 20 and 40 °C. At 60 °C, biochar caused positive priming of native SOC only in the first 6 months in the Inceptisol. Whereas, in the other soils, the native SOC mineralisation was increased (Entisol and Oxisol) and decreased (Vertisol) only after 6 months, relative to the control. At 20 °C, the mean residence time (MRT) of 450 °C and 550 °C biochars in the four soils ranged from 341 to 454 and 732−1061 years, respectively. At 40 and 60 °C, the MRT of both 450 °C biochar (25−134 years) and 550 °C biochar (93−451 years) decreased substantially across the four soils. Our results show that biochar causes positive priming in the clay-poor soil (Inceptisol) and negative priming in the clay-rich soils, particularly with biochar ageing at a higher incubation temperature (e.g. 40 °C) and for a high-temperature (550 °C) biochar. Furthermore, the 550 °C wood biochar has been shown to persist in soil over a century or more even at elevated temperatures (40 or 60 °C).     

Changes in CO<Subscript>2</Subscript>, <Superscript>13</Superscript>C abundance,inorganic nitrogen, β-glucosidase,and oxidative enzyme activities of soil during the decomposition of switchgrass root carbon as affected by inorganic nitrogen additions

This study was conducted to investigate the effect of inorganic nitrogen (N) and root carbon (C) addition on decomposition of organic matter (OM). Soil was incubated for 200 days with nine treatments (three levels of N (no addition (N0) = 0, low N (NL) = 0.021, high N (NH) = 0.083 mg N g⁻¹ soil) × three levels of C (no addition (C0) = 0, low C (CL) = 5, high C (CH) = 10 mg root g⁻¹ soil)). The carbon dioxide (CO₂) efflux rates, inorganic N concentration, pH, and potential activities of β-glucosidase and oxidative enzyme were measured during incubation. At the beginning and the end of incubation, the native soil organic carbon (SOC) and root-derived SOC were quantified by using a natural labeling technique based on the differences in δ ¹³C between C3 and C4 plants. Overall, the interaction between C and N was not significant. The decomposition of OM in the NH treatment decreased. This could be attributed to the formation of recalcitrant OM by N because the potentially mineralizable C pool was significantly lower in the NH treatment (3.1 mg C g⁻¹) than in the N0 treatment (3.6 mg C  g⁻¹). In root C addition treatments, the CO₂ efflux rate was generally in order of CH > CL > C0 over the incubation period. Despite no differences in the total SOC concentration among C treatments, the native SOC in the CH treatment (18.29 mg C g⁻¹) was significantly lower than that in the C0 treatment (19.16 mg C g⁻¹).     

Five-year warming does not change soil organic carbon stock but alters its chemical composition in an alpine peatland

Climate warming may promote soil organic carbon(SOC) decomposition and alter SOC stocks in terrestrial ecosystems, which would in turn affect climate warming. We manipulated a warming experiment using open-top chambers to investigate the effect of warming on SOC stock and chemical composition in an alpine peatland in Zoigê on the eastern Tibetan Plateau, China. Results showed that 5 years of warming soil temperatures enhanced ecosystem respiration during the growing season, promoted above-and be...     

Effect of extracellular-enzyme activities on solubilization rate of soil organic nitrogen

In a sandy soil containing ¹⁵N-labeled active (soluble and easily degradable) and non-labelled passive (recalcitrant) fractions of soil organic matter, the rate of net N mineralization (solubilization) was determined during a 55-day incubation at 25°C, 63% water-holding capacity and different levels of soil extracellular-enzyme activities. The active fraction of soil N was labelled by preincubation (at 5°C and 74% water-holding capacity for 6 months) of soil amended with ¹⁵N-labeled plant material. Increases in the activity of extracellular-enzymes in soil were induced by the addition of glucose and KH₂PO₄ at the beginning of the incubation. The results show that the contents of total soluble N (NO ₃ ^– –N+NH ₄ ⁺ –N + soluble organic N) were significantly higher in glucose-amended soil compared to the unamended soil. The increases in soluble N in soil amended with 1 and 2 mg glucose g^-1 dry soil corresponded to a mean rate of net solubilization of 7.9±1.4 and 18.8±0.7 nmol N g^-1 dry soil day^-1, respectively. The mean rate of net N solubilization (3.6±1.0 nmol N g^-1 dry soil day^-1) in unamended soil was significantly lower than those of glucose amended soils. The content of ¹⁵N in total soluble N in soil amended with 2 mg glucose, for example, was diluted from 3.11±0.08 atom% before the incubation to 2.77±0.03 atom% after 55 days. This indicates that 89% of soluble-N accumulated in soil by the end of the incubation originated from the active fraction of soil N and the rest, estimated at 11%, originated from the passive fraction. The activities of soluble and total proteases as well as the rate of N solubilization in the soil increased with the application of glucose. The activity of these extracellular enzymes was highly correlated with the rates of net N solubilization. Thus, increases in extracellular-enzyme activities in glucose-amended soils had a priming effect on the solubilization of ¹⁵N-labeled active and non-labeled passive fractions of soil organic N. It seems that the activity of extracellular-enzymes expressed in terms of total and soluble protease activities could be a rate-limiting factor in the processes of soil organic N solubilization.     

Initial soil organic carbon concentration influences the short-term retention of crop-residue carbon in the fine fraction of a heavy clay soil

Among factors controlling decomposition and retention of residue C in soil, effect of initial soil organic C (SOC) concentration remains unclear. We evaluated, under controlled conditions, short-term retention of corn residue C and total soil CO₂ production in C-rich topsoil and C-poor subsoil samples of heavy clay. Topsoil (0–20 cm deep, 31.3 g SOC kg^?1 soil) and subsoil (30–70 cm deep, 4.5 g SOC kg^?1 soil) were mixed separately with ¹³C–¹⁵N-labeled corn (Zea mays L.) residue at rates of 0 to 40 g residue C kg^?1 soil and incubated for 51 days. We measured soil CO₂–C production and the retention of residue C in the whole soil and the fine particle-size fraction (<50 μm). Cumulative C mineralization was always greater in topsoil than subsoil. Whole-soil residue C retention was similar in topsoil and subsoil at rates up to 20 g residue C kg^?1. There was more residue C retained in the fine fraction of topsoil than subsoil at low residue input levels (2.5 and 5 g residue C kg^?1), but the trend was reversed with high residue inputs (20 and 40 g residue C kg^?1). Initial SOC concentration affected residue C retention in the fine fraction but not in the whole soil. At low residue input levels, greater microbial activity in topsoil resulted in greater residue fragmentation and more residue C retained in the fine fraction, compared to the subsoil. At high residue input levels, less residue C accumulated in the fine fraction of topsoil than subsoil likely due to greater C saturation in the topsoil. We conclude that SOC-poor soils receiving high C inputs have greater potential to accumulate C in stable forms than SOC-rich soils.     

添加玉米秸秆对黄棕壤有机质的激发效应

玉米秸秆还田是培肥地力的一项重要措施,但是玉米秸秆添加后会改变土壤原有有机质的矿化过程,即引起激发效应,从而影响土壤碳平衡和周转。因此,适量秸秆还田将是高效且环境友好的提升土壤生产潜力的关键。本试验以黄棕壤为研究对象,设不添加玉米秸秆对照(CK)和添加占干土重的1%、5%和9%的粉碎玉米秸秆处理进行室内培养,分析土壤CO_2释放动态及激发效应。试验结果表明,添加不同量玉米秸秆后,土壤CO_2释放速率和累积量呈现出抛物线型变化趋势。在培养前期,各处理土壤CO_2释放速率表现为9%5%1%CK,到培养的第8天左右,添加5%玉米秸秆的土壤CO_2释放速率超过了添加9%玉米秸秆的土壤,在培养后期,所有处理的土壤CO_2释放速率慢慢地趋于一致。从累积CO_2释放量来看,添加5%玉米秸秆的处理比9%玉米秸秆的处理土壤总CO_2释放量高,表明添加5%秸秆的处理对微生物群落和微生物活性的作用最大。在整个培养阶段,玉米秸秆添加对土壤有机质的激发效应均为负值,而加入的玉米秸秆并没有完全矿化,从而使土壤有机碳含量因添加玉米秸秆而升高。这些结果表明,添加玉米秸秆有利于提高黄棕壤土壤有机碳含量,在本试验的短期培养过程中以土壤干重9%的添加量增加最多。     

Effects of NH4+ and NO3− on litter and soil organic carbon decomposition in a Chinese fir plantation forest in South China

Soil organic carbon (SOC) dynamics and nutrient availability determine the soil quality and fertility in a Chinese fir plantation forest in subtropical China. Uniformly ¹³C-labeled Chinese fir (Cunninghamia lanceolata) and alder (Alnus cremastogyne) leaf litter with or without 100 mg NH₄⁺ or NO₃⁻ were added to the soil. The purpose was to investigate the influence of N availability on the decomposition of the litter and native SOC. The production of CO₂, the natural abundance of ¹³C–CO₂, and the inorganic N dynamics were monitored. The results showed that Chinese fir (with a high C:N ratio) and alder (with a low C:N ratio) leaf litter caused significant positive priming effects (PEs) of 24% and 42%, respectively, at the end of the experiment (235 d). The PE dynamics showed that positive PE can last for at least 87 d. However, the possible occurrence of a significant negative PE with a sufficient incubation period is difficult to confirm. The application of both NH₄⁺ and NO₃⁻ was found to have a stimulating effect on the decomposition of Chinese fir and alder leaf litter in the early stage (0–15 d) of incubation, but an adverse effect in the late stage. Compared with NO₃⁻, NH₄⁺ caused a greater decrease in the PE induced by both Chinese fir and alder leaf litter. The effects of NH₄⁺ and NO₃⁻ on the PE dynamics had different patterns for different incubation stages. This result may indicate that the stability or recalcitrance of SOC, especially in such plantation forest soils, strongly depends on available leaf litter and application of N to the soil.     

Priming effect of bamboo (Phyllostanchys edulis Carrière) biochar application in a soil amended with legume

Abstract

Biochar application to soils can mitigate carbon dioxide (CO₂) by increasing soil carbon (C) sink, but also causes increased CO₂ released from soils through priming effects of soil organic carbon (SOC). However, priming effects of biochar application on SOC are complex, showing inconsistent results, and further complicated when applied with other substrates such as organic amendment (OA). Incubation experiments were conducted using Typic Durudand with bamboo (Phyllostanchys edulis Carrière) biochar (400°C) and OA (crotalaria) applied individually, simultaneously or with biochar applied 5 weeks prior to OA application. After 56 d of incubation, cumulative CO₂ released from soils with no amendments (control), biochar only (BC), OA only (OA), simultaneous (BC+OA), and differently timed (BCP+OA) applications reached 313, 326, 1270, 1535 and 1311 mg CO₂ kg^?1, respectively. The OA application distinctly increased CO₂ released from the soils due to its decomposition. The OA decomposition rates were comparable with OA and BC+OA, while those with BCP+OA were lower than those with other treatments during early incubation. Net CO₂ (CO_{2-(treatment)} ? CO_2-control) from soils with BC, OA, BC+OA and BCP+OA yielded 13, 957, 1222 and 998 mg CO₂ kg^?1, respectively. Primed CO_2-BC of 13 mg CO₂ kg^?1 was equivalent to 4.2% of priming effect relative to CO_2-control. Primed CO_2-BC+OA [net CO_2-BC+OA ? (net CO_2-BC + net CO_2-OA)] and primed CO_2-BCP+OA were 252 and 28 mg CO₂ kg^?1, equivalent to 26% and 2.9% of priming effects relative to sum of net CO_2-BC + net CO_2-OA, respectively. The priming effect with BC was negligible likely because of limited amounts of biochar labile C to induce co-metabolism, while BC+OA showed a modest priming effect most likely as a result of co-metabolism induced by additional mineralization of presumably SOC and/or biochar, because the OA decomposition rates were not affected by biochar application. The priming effect with BCP+OA was comparable to that with BC likely due to changes in soil properties caused by biochar application prior to OA, likely from slowed decomposition rates of OA.