Decomposition Of Native Soil Organic Carbon Research Articles

Low Carbon Loss from Long-Term Manure-Applied Soil during Abrupt Warming Is Realized through Soil and Microbiome Interplay.

Manure application is a global approach for enhancing soil organic carbon (SOC) sequestration. However, the response of SOC decomposition in manure-applied soil to abrupt warming, often occurring during diurnal temperature fluctuations, remains poorly understood. We examined the effects of long-term (23 years) continuous application of manure on SOC chemical composition, soil respiration, and microbial communities under temperature shifts (15 vs 25 °C) in the presence of plant residues. Compared to soil without fertilizer, manure application reduced SOC recalcitrance indexes (i.e., aliphaticity and aromaticity) by 17.45 and 21.77%, and also reduced temperature sensitivity (Q10) of native SOC decomposition, plant residue decomposition, and priming effect by 12.98, 15.98, and 52.83%, respectively. The relative abundances of warm-stimulated chemoheterotrophic bacteria and fungi were lower in the manure-applied soil, whereas those of chemoautotrophic Thaumarchaeota were higher. In addition, the microbial network of the manure-applied soil was more interconnected, with more negative connections with the warm-stimulated taxa than soils without fertilizer or with chemical fertilizer applied. In conclusion, our study demonstrated that the reduced loss of SOC to abrupt warming by manure application arises from C chemistry modification, less warm-stimulated microorganisms, a more complex microbial community, and the higher CO2 intercepting capability by Thaumarchaeota.

Functional type mediates the responses of root litter-driven priming effect and new carbon formation to warming

Input of root litter can alter soil organic carbon (SOC) dynamics via causing priming effect (PE) on native SOC decomposition and forming new SOC. However, it is unknown how functional type mediates the root litter-driven PE and new C formation as well as their response to warming, which are of pivotal for soil C budget. We mixed litter segments of absorptive roots and transport roots from a Chinese fir (Cunninghamia lanceolata) plantation into isotopically distinct soil and incubated at 19°C (local mean annual temperature) and 23°C (warming by 4°C) for 210 days. Cumulative PE was calculated via integrating the instantaneous PE rates during the incubation. And the newly formed root litter-derived SOC (SOCrl) was calculated by measuring the δ13C value of soil at the end of incubation using a two-source mixed model. We found that absorptive roots with faster decomposition rates, caused significantly higher cumulative PE and SOCrl than transport roots. The microbial biomass and enzyme activities involved in C, N and P acquisition were significantly higher in the absorptive- than the transport roots addition treatment, indicating a higher level of microbial activation caused by absorptive roots. Although warming significantly increased the litter decomposition for both of functional types, while just significantly increased the PE of transport roots, indicating a root functional type dependent sensitivity of PE to warming. However, warming had no significant effect on SOCrl either for absorptive roots or for transport roots. As a consequence, warming relatively decreased the net SOC balance (difference between PE and SOCrl) in the transport roots addition treatment. Overall, our study highlights, for the first time, that functional type primarily mediates the response of root litter-driven PE to climate warming but not the new C formation, which may advance our understanding of SOC dynamics in Chinese fir plantation under climate change.

Soil organic carbon increase via microbial assimilation or soil protection against the priming effect is mediated by the availability of soil N relative to input C

Labile C inputs into soils will be partially transformed into soil organic carbon (SOC) through microbial assimilation or physicochemical protection as such mineral-organic interactions and soil aggregation. The C inputs may stimulate the decomposition of native SOC, inducing a phenomenon known as the priming effect. Increasing C inputs may increase SOC content, yet the relative role of these mechanisms in controlling the magnitudes of SOC increase among soils remains unclear. Four soils differing in microbiology and N content were incubated with 13C-labeled glucose at the amounts of 0.5 (G0.5), 1.0 (G1.0), and 2.0 (G2.0) g C kg−1 soil for 48 days. The objectives of this study were 1) to quantify the fates of added glucose-C, the priming effect, and the changes in SOC and mineral N contents after the incubation, and 2) to identify the main mechanisms for SOC increase with the increased amount of C input and the effects of initial soil N availability and microbial composition. 13C isotope probing showed that 19 to 26 % and 37 to 96 % of added glucose were utilized for microbial respiration and growth, respectively, after the incubation in all soils, with the glucose-C use efficiency ranging from 30 to 60 %. The remaining accounted for 4 to 37 % in all cases except for UltisolHN at G1.0 and G2.0 (–23 % and −2%), which was confirmed as the fraction protected in soil or re-utilized by microbes via acid extraction. The soil-protected glucose-C was higher, while glucose-C use efficiency was lower with the increased amount of added glucose only in the soils with low N. Glucose addition induced a positive priming effect, a SOC increase, and a mineral N loss irrespective of soil type and addition amount. Glucose addition increased the cumulative fungal biomass in all cases and the cumulative bacterial biomass only at G2.0 in all soils and at G1.0 in the high-N soils. It increased the final fungal necromass only at G0.5 in MollisolLN and UltisolHN and at G2.0 in MollisolHN and the final bacterial necromass in all cases except for the low-N soils and UltisolHN at G2.0. With the increased amount of added glucose, the SOC content increased linearly and the priming effect and fungal biomass increased non-linearly, with larger magnitudes in the soils with lower N. These results indicated that glucose was quickly protected after addition and remained in the soil until soil N became a limiting factor for microbial utilization. The availability of soil N relative to input C mediated microbial growth and its controls over SOC change via the soil protection and priming effects. The carbon sequestration potential with increased input of dissolved substrates as such glucose e. g. through root exudation was likely larger in soils with lower fertility due to reduced substrate-C use efficiency and increased effect of soil protection despite an increased priming effect. Further studies are needed on more substrate types as they affect substrate-C use efficiency and soil protection mechanisms.

Mixing plant residues of different quality reduces priming effect and contributes to soil carbon retention

Cellulose-rich rice straw (RS) decomposes readily in soils, with much of C lost as CO2 and only a small fraction stabilized as soil organic carbon (SOC). Additionally, the addition of high-energy containing residues may stimulate native SOC decomposition (priming effect, PE) and the high C/N ratio of RS may lead to soil N-mining. We hypothesized that mixing high-C/N ratio RS with low-C/N ratio groundnut stover (GN) would improve the chemical composition of the input, thereby stimulating microbial growth and hence increasing the retention of residue-derived C in the soil as well as decreasing PE. To prove this, we designed a 112-day incubation experiment with unamended 13C-enriched C4 soil, soil amended with single 13C-depleted residues of C3 plants, RS and GN, or their mixture. The partitioning of applied C and native soil C between CO2 and microbial biomass was performed using their differences in 13C abundance, while the uncertainties in our estimates of soil C fluxes were quantified by a sensitivity analysis of the two-end member isotopic model. From 42 days after incorporation, the mixture reduced CO2–C loss by 9–18% compared to either residue applied alone. Compared to RS, the mixture respired less CO2, inducing a 29% reduction in PE, and increased C retention as microbial biomass by 22% during the first 56 days due to the preferential utilization of residue C. This resulted in reduced native soil C and N mining during the later decomposition stages. According to the isotopic sensitivity analysis, our conclusions were valid despite variations in isotopic composition of the chemical components of the residues. However, this might not be the case if the δ13C values of preferentially used readily available components (e.g., cellulose) and/or of whole residues are very different.

Manure application decreases soil organic carbon priming by increasing mineral protection and nitrogen availability

Rational chemical fertilizer application, especially combined with manure or straw amendment, serves as a key strategy for facilitating soil organic carbon (SOC) sequestration and soil fertility improvement. However, the effects of fertilization managements on the SOC priming induced by straw return remains unclear. Here, a microcosm was conducted to clarify the mechanisms of 13C-labeled maize straw addition effects on priming effect (PE) in two 29-year fertilized soils (Urumqi, UQ; Gongzhuling, GZL) subjected to four fertilization regimes (no fertilizer, CK; chemical fertilizers, NPK; combined application of NPK with straw, NPKS; combined application of NPK with manure, NPKM). Results showed that straw addition strongly increased native SOC decomposition by 15–44%, inducing positive PEs across sites and fertilization treatments. Compared to CK treatment, the NPK and NPKS treatments significantly increased PE by 26% and 98%, respectively, in the UQ soil, but had little effect on the PE in the GZL soil. The NPKM treatment strongly decreased PE by 51–55%, mainly due to the increased proportion of mineral-associated organic carbon (MAOC) and N availability, and decreased fungal abundances, as compared to the CK treatment in UQ and GZL soils. The PE intensities were significantly lower in GZL soil (5.1–11.7 mg CO2-C/g SOC) relative to UQ soil (13.7–54.8 mg CO2-C/g SOC), primarily due to the higher MAOC/SOC ratio and N availability in the former across fertilization regimes. Overall, manure application has the potential to mitigate soil C loss via priming, mainly resulting from enhanced SOC stabilization via mineral protection and increased N availability, thus facilitating SOC sequestration in agroecosystems.

Litter quality controls tradeoffs in soil carbon decomposition and replenishment in a subtropical forest

Abstract Species‐rich forests can produce litter of varying carbon (C) and nitrogen (N) composition (i.e. quality), which can affect decomposition and play a central role in long‐term soil organic carbon (SOC) accumulation. However, how differences in litter quality affect SOC decomposition and formation remains unclear over the full litter decomposition trajectory. We followed the in situ complete decomposition of added 13C‐labelled high‐ (low C:N) and low‐quality (high C:N) leaf‐litter and its effect on particulate organic matter (POM) and mineral‐associated organic matter (MAOM) fractions over 2 years in a natural subtropical forest. We found that during early stages of decomposition, low‐quality litter inputs decreased SOC via a positive priming effect (i.e. new C inputs favoured decomposition of native SOC), but these SOC losses were offset by SOC gains observed via a negative priming effect during decomposition of high‐quality litter. In contrast, this pattern reversed during late stages of decomposition—SOC losses via a positive priming effect induced by high‐quality litter were offset by SOC gains via a negative priming effect induced by low‐quality litter. Over the full decomposition of litter, both high‐ and low‐quality litter stimulated microbial breakdown of SOC tied to POM, but also replenished more persistent SOC that associated with soil minerals (MAOM). Altogether, we observed that low‐quality litter formed twice as much new SOC as high‐quality litter (24% vs. 12% of added litter‐C). We extend the notion of the priming effect from primarily a negative role promoting losses of native SOC, to a functional role that can replenish persistent SOC. Synthesis. Our measurements raise the possibility that, in species‐rich forests, high‐ and low‐quality litter decomposition play opposite but dynamically complementary roles in renewing POM—both by inducing its decomposition and formation—while exclusively favouring MAOM formation, which can help explain how differences in litter quality favour SOC accumulation and persistence. Global change factors that shift plant community composition may ultimately affect the fate of soil C, as changes in litter quality may force soil transitions from sinks to sources or sources to sinks of atmospheric CO2.

Effects of in situ warming and glucose addition on soil respiration in a temperate forest

As studies on priming effect (PE) under field conditions are scarce, the carbon sequestration potential of PE on soil organic carbon (SOC) turnover remains uncertain. Therefore, the present study assessed the direct effects of glucose addition and in situ warming on the extent of PE under field conditions in a temperate forest. A simple and novel technique based on Keeling plots was applied. Glucose addition significantly enhanced native SOC decomposition and induced strong PE. However, the effect of in-situ warming on the extent of PE was not significant. The present study confirms the importance of PE in regulating SOC turnover under field conditions.

Effects of combined application of nitrogen, phosphorus, and potassium fertilizers on tea (Camellia sinensis) growth and fungal community

Fertilization is an effective strategy for enhancing tea yield and quality. In practice, fertilizer application in tea plantations is often characterized by N, P, or K deficiencies, which suppress tea growth. Fertilization also exerts indirect effects on tea yield and quality by affecting the microbial community in the soil which is critical for regulating nutrient cycling and soil multiplication. Due to their tolerance to adverse environments in tea plantations including low pH and high aluminum and fluorine content, fungi are strongly associated with tea growth and are involved in the decomposition of pruning residues. To elucidate the poorly understood effects of different fertilizer combinations on soil fungi and tea, soil and plant samples were collected from a five-year (2016–2020) field experiment involving combined application of N, P, and K fertilizers (NF: no fertilizer; PK, NK, and NP: fertilization under conditions of N, P, and K deficiency, respectively; and NPK: fertilization with N, P, and K). Yield and quality differences between NPK and PK were more significant than those between NPK and NK, and NPK and NP. Of these three nutrients, N was the primary limiting factor for tea growth and N deficiency had more significant effects on fungal diversity than P or K deficiencies. N addition accelerated soil acidification and promoted native soil organic carbon decomposition, thereby decreasing the fungal diversity and modifying fungal community composition and structure. Moreover, in the PK (N deficiency) treatment, the fungal network had the lowest modularity, indicating that N deficiency reduced the resistance of fungal communities to external disturbances. The relationship between tea and fungi at different soil depths revealed that yield and amino acid content of tea were more strongly correlated with fungi in the topsoil than in the subsoil. This indicates that soil samples collected from the top layer are more informative for investigating the relationship between tea and soil. Overall, these findings deepen our understanding of how combined fertilization affects tea growth and soil quality, thereby guiding optimal fertilization strategies toward maximization of tea yield and quality and minimization of environmental risks.

Priming of soil organic carbon mineralization and its temperature sensitivity in response to vegetation restoration in a karst area of Southwest China

Plant residue input alters native soil organic carbon (SOC) mineralization through the priming effect, which strongly controls C sequestration during vegetation restoration. However, the effects of different vegetation types on SOC priming and the underlying microbial mechanisms due to global warming are poorly understood. To elucidate these unknowns, the current study quantified soil priming effects using 13C-labeled maize residue amendments and analyzed the community structure and abundances in the soils of a vegetation succession gradient (maize field (MF), grassland (GL), and secondary forest (SF)) from a karst region under two incubation temperatures (15 °C and 25 °C). Results revealed that after 120 d of incubation, vegetation restoration increased the soil priming effects. Compared to MF, the priming effects of SF at 15 °C and 25 °C increased by 142.36 % and 161.09 %, respectively. This may be attributed to a high C/N ratio and low-N availability (NO3−), which supports the “microbial nitrogen mining” theory. Variations in soil priming were linked to changes in microbial properties. Moreover, with vegetation restoration, the relative abundance of Actinobacteria (copiotrophs) increased, while Ascomycota (oligotrophs) decreased, which accelerated native SOC decomposition. Co-occurrence network analysis indicated that the cooperative interactions of co-existing keystone taxa may facilitate SOC priming. Furthermore, structural equation modeling (SEM) indicated that changes in the priming effects were directly related to the fungal Shannon index and microbial biomass C (MBC), which were affected by soil C/N and NO3−. Warming significantly decreased soil priming, which may be attributed to the increase in microbial respiration (qCO2) and decreased MBC. The temperature sensitivity (Q10) of SOC mineralization was higher after residue amendment, but significant differences were not detected among the vegetation types. Collectively, our results indicated that the intensity of priming effects was dependent on vegetation type and temperature. Microbial community alterations and physicochemical interactions played important roles in SOC decomposition and sequestration.

Global pattern of soil priming effect intensity and its environmental drivers.

The microbial priming effect-the decomposition of soil organic carbon (SOC) induced by plant inputs-has long been considered an important driver of SOC dynamics, yet we have limited understanding about the direction, intensity, and drivers of priming across ecosystem types and biomes. This gap hinders our ability to predict how shifts in litter inputs under global change can affect climate feedbacks. Here, we synthesized 18,919 observations of CO2 effluxes in 802 soils across the globe to test the relative effects (i.e., log response ratio [RR]) of litter additions on native SOC decomposition and identified the dominant environmental drivers in natural ecosystems and agricultural lands. Globally, litter additions enhanced native SOC decomposition (RR = 0.35, 95% CI: 0.32-0.38), with greater priming effects occurring with decreasing latitude and more in agricultural soils (RR = 0.43) than in uncultivated soils (RR = 0.28). In natural ecosystems, soil pH and microbial community composition (e.g., bacteria: fungi ratio) were the best predictors of priming, with greater effects occurring in acidic, bacteria-dominated sandy soils. In contrast, the substrate properties of plant litter and soils were the most important drivers of priming in agricultural systems since soils with high C:N ratios and those receiving large inputs of low-quality litter had the highest priming effects. Collectively, our results suggest that, though different factors may control priming effects, the ubiquitous nature of priming means that alterations of litter quality and quantity owing to global changes will likely have consequences for global C cycling and climate forcing.

Decomposition of carbon adsorbed on iron (III)-treated clays and their effect on the stability of soil organic carbon and external carbon inputs

The interaction of organic carbon (OC) with clay and metals stabilizes soil carbon (C), but the influence of specific clay-metal-OC assemblages (flocs) needs further evaluation. This study aimed to investigate the stability of flocs in soil as affected by external C inputs. Flocs representing OC-mineral soil fractions were synthesized using dissolved organic C (DOC) combined with kaolinite (1:1 layer structure) or montmorillonite (2:1 layer structure) clays in the absence or presence of two levels of Fe (III) (named low or high Fe). Flocs were mixed with soil (classified as Luvisol) and incubated with or without 13C labelled plant residue (i.e., ryegrass) for 30 days. The CO2 emissions and DOC concentrations as well as their 13C signatures from all treatments were examined. Total C mineralization from flocs was approximately 70% lower than non-flocced DOC. The flocs made with montmorillonite had 16–43% lower C mineralization rate than those made with kaolinite with no Fe or low Fe. However, when flocs were made with high Fe, clay mineralogy did not significantly affect total C mineralization. A positive priming effect (PE) of flocs on native soil OC was observed in all treatments, with a stronger PE found in lower Fe treatments. The high-Fe clay flocs inhibited ryegrass decomposition, while the flocs made without clay had no impact on it. Interestingly, flocs significantly decreased the PE of ryegrass on native soil OC decomposition. These results indicate that the adsorption of DOC onto clay minerals in the presence of Fe (III) stabilizes it against decomposition processes and its stability increases as Fe in flocs increases. Flocs also protect soil OC from the PE of external degradable plant C input. This study showed that Fe level and clay mineralogy play an important role in controlling soil C stability.

Decreased Soil Organic Carbon under Litter Input in Three Subalpine Forests

Soil organic carbon (SOC) is the largest terrestrial organic carbon pool. Plant litter is an important source of SOC, but the knowledge gap between SOC fractions and plant litter input remains inconsistent. Here, a litter input control experiment was conducted at three subalpine forest types (coniferous forest, mixed forest, and broadleaved forest). We assessed the variations of total organic C, active organic C (easily oxidizable C, labile organic C), recalcitrant organic C, and microbial biomass C under litter input or removal. The results showed that soil total organic C decreased greatly under litter input. It was mainly caused by the change of easily oxidizable C and labile C, while the influence of recalcitrant C was small. At the same time, this effect varied among different forest types. Among them, the effect of litter input on SOC was weak and slow in the coniferous forest with low-quality litter input, while a quick effect was observed in the mixed and broadleaved forests with high-quality litter input. Microbial biomass C declined under litter input in most cases, and its variation was strongly controlled by soil temperature and freeze-thaw events. Overall, our results provide new evidence that forest type would strongly control SOC dynamics, in concert with litter quality shifts, with potential consequences for long-term C sequestration. We highlighted that litter input could reduce microbial biomass carbon which might limit the native SOC decomposition, but the loss of active C ultimately changed the SOC in the subalpine forests. It suggested that the interaction of multiple mechanisms should be considered in the study of SOC in this region.

Effect of organic carbon addition on paddy soil organic carbon decomposition under different irrigation regimes

Abstract. Anaerobic decomposition of organic carbon (OC) in submerged rice paddies is coupled to the reduction of alternative soil electron acceptors, primarily Fe3+. During reductive dissolution of Fe3+ from pedogenic oxides, previously adsorbed native soil organic carbon (SOC) could be co-released into solution. Incorporation of crop residues could hence indirectly, i.e. through the stimulation of microbially mediated Fe3+ reduction, promote the loss of native SOC via enhanced dissolution and subsequent mineralisation to CO2 and CH4. Our aim was to estimate the relevance of such a positive feedback during the degradation of added OC, and to investigate the impact of irrigation management on this mechanism and on priming effects on native SOC decomposition in general. In a six-week pot experiment with rice plants, two Bangladeshi soils with contrasting SOC to oxalate-extractable Fe (SOC : Feox) ratios were kept under a regime of alternate wetting and drying (AWD) or continuous flooding (CF), and were either amended with maize shoots or not. The δ13C signatures of dissolved organic C and emitted CH4 and CO2 were used to infer the decomposition of added maize shoots (δ13C = −13.0 ‰) versus native SOC (δ13C = −25.4 ‰ and −22.7 ‰). Addition of maize residues stimulated the reduction of Fe as well as the dissolution of native SOC, and the latter to a larger extent under CF, especially for the soil with the highest SOC : Feox ratio. Estimated Fe-bound SOC contents denote that stimulated SOC co-release during Fe reduction could explain this positive priming effect on SOC dissolution after the addition of maize. However, priming effects on SOC mineralisation to CO2 and CH4 were lower than for SOC dissolution, and were even negative under AWD for one soil. Enhanced reductive dissolution of Fe-bound SOC upon exogenous OC addition therefore does not necessarily lead to stimulated SOC mineralisation. In addition, AWD irrigation was found to decrease the above-mentioned priming effects.

Modeling the dynamics of protected and primed organic carbon in soil and aggregates under constant soil moisture following litter incorporation

Incorporating plant litter into soil can replenish soil organic carbon (SOC) through physical protection mechanisms, such as formation of mineral-organic complexes and aggregates. Meanwhile, plant litter input may stimulate the decomposition of native SOC, known as the priming effect. It remains unclear how these physical protection mechanisms affect the priming effect, which further affects the SOC dynamics. Here, we present a novel model that can predict the dynamics of protected and primed organic carbon in soil and aggregates (PROCAAS). The PROCAAS model assumes that microbial activity will control the dynamics of both aggregate turnover and the priming effect following litter incorporation, and that litter-derived SOC will prime the decomposition of the native SOC pool as they are in the same location and in the same pool, either occluded in soil pores or associated with minerals. The PROCAAS model was well calibrated and validated using independent datasets of aggregate mass proportion and aggregated SOC contents in literature as well as soil respiration and the priming effect measured here during 76-day incubation with and without 13C labeled litter. The model results showed that litter-derived SOC was protected predominantly through occlusion in macroaggregates, while the priming effect was originated mainly from mineral-associated SOC in macroaggregates. The dynamics of priming effect could be explained by the soil structural change from soil disaggregation during litter incorporation, via the formations of macroaggregates and microaggregates within macroaggregates, to the aggregate stability during litter decomposition. Our proof-of-concept model can form the base for incorporating these biophysical mechanisms underlying soil aggregation and the priming effect into future ecosystem models.

Priming of soil organic carbon decomposition induced by exogenous organic carbon input: a meta-analysis

Priming effect (PE) of soil organic carbon (SOC) decomposition induced by exogenous organic C is an important ecological process in regulating the soil C cycle. The objective of this study was to evaluate how the PE varied among different ecosystems at the global scale and explore factors that drive the direction and magnitude of the PE. Using 2048 experimental comparisons compiled from 94 incubation studies with stable (13C) or radioactive (14C) carbon isotopic techniques, we performed a meta-analysis on the effect of exogenous organic C input on native SOC decomposition (i.e., PE) across multiple terrestrial ecosystems. In particular, the linear mixed-effect model was used to examine the relationship between the PE and potential influencing factors. The addition of exogenous organic C significantly enhanced native SOC decomposition by 47.5% (i.e., positive PE), with the highest value in cropland soils (60.9%) and the lowest value in forest soils (26.2%). The intensity of the PE decreased with increasing SOC content, soil total nitrogen content, soil C/N, incubation duration, and incubation temperature, but increased with increasing exogenous organic C addition rate and soil pH. Soil PE was not affected by the complexity of exogenous organic C. Our results indicate that positive PE is a widespread phenomenon in terrestrial ecosystems, and that the magnitude is closely related to soil properties and experimental conditions. These findings may be useful for understanding soil C priming and the effect on soil C balance under climate change scenarios.

Effect of crop rotation, fertilisation and tillage on main soil properties and its water extractable organic matter

The excessive use of fertilisers and frequent and deep tillage are not considered good agricultural practices because they increase production costs and reduce soil fertility. Water extractable organic matter (WEOM) is the fraction of soil organic matter responsible for nutrient transport and bioavailability. The aim of this work was to investigate the effect of a 2-year rotation of faba bean–wheat, cultivated for a decade, and the agricultural practices (conventional vs no tillage, and fertilisation vs no fertilisation) on selected soil parameters and WEOM quality. Results showed that the soil organic carbon (SOC) and the total nitrogen (TN) content increased after the wheat and faba bean, respectively. Plots managed with conventional tillage showed WEOM with higher OC content with respect to no-tillage plots. A higher WEOM OC content was observed also in the fertilised plots. The WEOM of fertilised and faba bean plots was derived from decomposition of native SOC, but the microbial community decomposition was the main origin of WEOM after wheat.

Susceptibility of soil organic carbon to priming after long-term CO2 fumigation is mediated by soil texture

Elevated CO2 (eCO2) may enhance soil organic carbon (SOC) sequestration via greater input of photosynthetic carbon (C). However, greater rhizodeposits under eCO2 may stimulate microbial decomposition of native SOC. This study aimed to examine the status and stability of SOC in three Australian cropping soils after long-term CO2 enrichment. Samples (0–5 cm) of Chromosol, Vertosol and Calcarosol soils were collected from an 8-year Free-air CO2 Enrichment (SoilFACE) experiment and were used to examine SOC dynamics by physical fractionation and incubation with 13C-glucose. Compared to the ambient CO2 (aCO2) (390–400 μmol mol−1), 8 years of elevated CO2 (eCO2) (550 μmol mol−1) did not increase SOC concentration of all soils, but changed SOC distribution with 12% more C in coarse soil fractions and 5% less C in fine fractions. Elevated CO2 also enhanced the susceptibility of SOC to 13C-glucose-induced priming, but this effect was only significant in the coarse-textured Calcarosol topsoil. The eCO2 history increased labile C (coarse C fraction, +13%) and soil pH (+0.25 units), and decreased available N (−30%) in the Calcarosol, which stimulated microbial biomass C by 28%, leading to an enhanced priming effect. Despite with greater total primed C, the Chromosol that had the highest amount of native C, had lower primed C per unit of SOC when compared to the low-C Calcarosol. In conclusion, the effect of long-term eCO2 enrichment on soil C and N availability in cropping soils depended on soil type with the coarse-textured Calcarosol soil being more susceptible to substrate-induced decomposition of its SOC.

Mineralization of plant residues and native soil carbon as affected by soil fertility and residue type

Crop residue return is an effective and low-cost agricultural approach for soil organic carbon (SOC) sequestration. Yet, it is largely unknown to what extent the soil fertility and residue type affect the mineralization of maize (Zea mays L.) residue carbon (C) and the decomposition of native SOC. Therefore, a better understanding of the mineralization of C derived from residues and its priming on native SOC is crucial to accurate assessment of the benefits of crop residue returning in agricultural systems. A 360-day laboratory incubation experiment was carried out with a Cambisol of low and high fertilities amended with three types of 13C-labeled maize residues (root, stem, leaf). The abundance of 13C (δ13C) in the soil samples was measured during different incubation stages. The results showed that the total mineralization of residue C was significantly higher in the low fertility soil than in the high fertility soil, but there were no significant differences among residue types. For the high fertility soil, all the residue types induced a negative priming on native SOC mineralization during the early incubation stage, but a significant total positive priming by the end of incubation, whereas for the low fertility soil, there was no significant effect of residue return on SOC mineralization. The accumulated priming by the end of incubation did not vary across residue types. Moreover, the sum of mineralization of residue C and native SOC in the high fertility soil was 1.4 times as large as that in the low fertility soil. We conclude that mineralization of crop residue C and native SOC is affected by soil fertility rather than residue type.

Negative priming effect of three kinds of biochar on the mineralization of native soil organic carbon

AbstractThis study was conducted to improve our understanding of the effect of biochar on the degradation of original soil organic carbon (SOC) and the resulting net C sequestration. A 120‐day incubation experiment was conducted to detect the CO2 evolution from soil amended with three kinds of biochar, namely, rice husk biochar (RH), pecan shell biochar (PS), and bamboo biochar (BB), at biochar to soil ratios of 0% (as control), 1%, 2%, and 5%, as well as 100% pure biochar, and to evaluate their priming effect on SOC mineralization. Results showed that the cumulative mineralized amounts of SOC decreased by 7.95–10.7% with 5% RH, PS, and BB treatments, whereas there were no significant differences in 1% or 2% biochar treatments after 120 days of incubation compared with the control. Biochar had a suppressing influence on native SOC decomposition, and the effect increased with the amount of applied biochar increasing. The negative priming effect of biochar on the mineralization of SOC was mainly observed after 15 days of incubation and increased with incubation time. The negative effects of BB and RH were significantly stronger than that of PS at the 5% application ratio. This was mainly due to the less pores and smaller specific surface area of PS, which resulted in the weaker encapsulation and adsorption protection effect of PS than RH and BB, and inhibited the degradation of the adsorbed native SOC. Therefore, BB and RH have great potential for C sequestration and mitigation, especially in soils with low organic C.

Crop residue-derived dissolved organic matter accelerates the decomposition of native soil organic carbon in a temperate agricultural ecosystem

Crop residue-derived dissolved organic matter (DOM) plays an important role in soil carbon (C) cycling. To investigate the effects of maize residue-derived DOM and urea additions on the native soil organic carbon (SOC) decomposition and soil net C balance a pot experiment was carried out during the winter wheat growing season in the North China Plain (NCP). The results showed that adding maize residue-derived DOM alone (RDOM) or together with urea (RDOM + N) accelerated the decomposition of native SOC and resulted in a net SOC loss. The net loss of SOC was 3.90 ± 0.61 and 3.53 ± 0.48 g C m−2 in RDOM and RDOM + N treatments, respectively. The stimulatory effect of per unit DOM-C addition on the native SOC decomposition was 0.25 ± 0.05 and 0.45 ± 0.07 for the RDOM and RDOM + N treatments, respectively. Increases in the microbial biomass and the activity of β-glucosidase, invertase and cellobiohydrolase as well as soil mineral N content were responsible for a more intense priming effect in DOM-amended soils. The positive relationship between primed soil C and soil available N (R = 0.76, P < 0.05) suggested that the stimulation of decomposition of native SOC by DOM addition would be enhanced by nitrogen fertilizer application.