Priming Of Soil Organic Carbon Research Articles

Distinct biophysical and chemical mechanisms governing sucrose mineralization and soil organic carbon priming in biochar amended soils: evidence from 10 years of field studies

While many studies have examined the role of biochar in carbon (C) accrual in short-term scale, few have explored the decadal scale influences of biochar on non-biochar C, e.g., native soil organic C (SOC) and added substrate. To address this knowledge gap, soils were collected from decade-old biochar field trials located in the United Kingdom (Cambisol) and China (Fluvisol), with each site having had three application rates (25–30, 50–60 and 75–100 Mg ha−1) of biochar plus an unamended Control, applied once in 2009. We assessed physicochemical and microbial properties associated with sucrose (representing the rhizodeposits) mineralization and the priming effect (PE) on native SOC. Here, we showed both soils amended with biochar at the middle application rate (50 Mg ha−1 biochar in Cambisol and 60 Mg ha−1 biochar in Fluvisol) resulted in greater substrate mineralization. The enhanced accessibility and availability of sucrose to microorganisms, particularly fast-growing bacterial genera like Arenimonas, Spingomonas, and Paenibacillus (r-strategists belonging to the Proteobacteria and Firmicutes phyla, respectively), can be attributed to the improved physicochemical properties of the soil, including pH, porosity, and pore connectivity, as revealed by synchrotron-based micro-CT. Random forest analysis also confirmed the contribution of the microbial diversity and physical properties such as porosity on sucrose mineralization. Biochar at the middle application rate, however, resulted in the lowest PE (0.3 and 0.4 mg of CO2-C g soil−1 in Cambisol and Fluvisol, respectively) after 53 days of incubation. This result might be associated with the fact that the biochar promoted large aggregates formation, which enclosed native SOC in soil macro-aggregates (2–0.25 mm). Our study revealed a diverging pattern between substrate mineralization and SOC priming linked to the biochar application rate. This suggests distinct mechanisms, biophysical and physicochemical, driving the mineralization of non-biochar carbon in a field where biochar was applied a decade before.

Soil priming effects and involved microbial community along salt gradients

Abstract. Soil salinity mediates microorganisms and soil processes, like soil organic carbon (SOC) cycling. Yet, how soil salinity affects SOC mineralization via shaping bacterial community diversity and composition remains elusive. Therefore, soils were sampled along a salt gradient (salinity at 0.25 %, 0.58 %, 0.75 %, 1.00 %, and 2.64 %) and incubated for 90 d to investigate (i) SOC mineralization (i.e., soil priming effects induced by cottonseed meal, as substrate) and (ii) the responsible bacteria community by using high-throughput sequencing and natural abundance of 13C isotopes (to partition cottonseed-meal-derived CO2 and soil-derived CO2). We observed a negative priming effect during the first 28 d of incubation that turned to a positive priming effect after day 56. Negative priming at the early stage might be due to the preferential utilization of cottonseed meal. The followed positive priming decreased with the increase in salinity, which might be caused by the decreased α diversity of microbial communities in soil with high salinity. Specifically, soil pH and electrical conductivity (EC) along the salinity gradient were the dominant variables modulating the structure of the microbial community and consequently SOC priming (estimated by distance-based multivariate analysis and path analysis). By adopting two-way orthogonal projections to latent structures (O2PLS), priming effects were linked with specific microbial taxa; e.g., Proteobacteria (Luteimonas, Hoeflea, and Stenotrophomonas) were the core microbial genera that were attributed to the substrate-induced priming effects. Here, we highlight that the increase in salinity reduced the diversity of the microbial community and shifted dominant microorganisms (Actinobacteria and Proteobacteria: Luteimonas, Hoeflea, and Stenotrophomonas) that determined SOC priming effects, which provides a theoretical basis for understanding SOC dynamics and microbial drivers under the salinity gradient.

Deciphering Biotic and Abiotic Mechanisms Underlying Straw Decomposition and Soil Organic Carbon Priming in Agriculture Soils Receiving Long-Term Fertilizers.

Straw-related carbon (C) dynamics are central for C accrual in agro-ecosystems and should be assessed by investigating their decomposition and soil organic carbon (SOC) priming effects. Our understanding of biotic and abiotic mechanisms underpinning these two C processes, however, is still not sufficiently profound. Soils that had received organic and mineral fertilizers for 26 years were sampled for a 28 day incubation experiment to assess 13C-labeled straw decomposition and SOC priming effects. On the basis of analyzing physicochemical properties, fungal taxonomic (MiSeq sequencing) and functional (metagenomics) guilds, we quantified the contributions of biotic and abiotic attributes to straw decomposition and SOC priming. Here, we propose two distinct mechanisms underlying straw decomposition and SOC priming in agriculture soils: (i) accelerated straw mineralization in manure-treated soils was mainly driven by biotic forces, while (ii) larger SOC priming in NPK-amended soils was through abiotic regulation.

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.

Priming effects decrease with the quantity of cover crop residues – Potential implications for soil carbon sequestration

Meta-analyses suggest a global potential of cover crops to increase soil organic carbon (SOC) stocks, yet with a large variation across studies, which underlines the need to understand the effect of cover crops on carbon (C) sequestration under specific soil and climate conditions. We studied the C sequestration potential from cover crops, based on a Danish long-term field experiment (LTE) initiated in 1997, where SOC and C in the fractions of particulate organic matter (POM) and mineral associated organic matter (MAOM) was measured to 1-m depth. Next, we performed a mesocosm study where the fate of 14C-labeled cover crop residues (fodder radish, Raphanus sativus L.) and SOC priming were traced in two texturally similar soils from the LTE with different SOC concentrations (2.0 vs. 2.6% SOC). The results showed that cover cropping for up two decades had negligible effect on SOC in POM and MAOM fractions. Yet, the mesocosm study showed considerable overall SOC increases (20–25% of added C) when the cover crop C input exceeded rates of 0.2–0.3 mg C g−1 in the two soils. This was due to a combination of new SOC formation and priming effects shifting from positive to negative. The input rates of 0.2–0.3 mg C g−1 correspond to the C input from cover crops with an aboveground yield of approximately 0.7–1.1 Mg dry matter ha−1, which is a level not always achieved at the field site. The combined observations from the field and mesocosm study suggest that SOC buildup was not constrained by soil C saturation, but rather by low cover crop productivity and/or positive priming effects. Therefore, agricultural management practices (e.g., species choice and sowing time) should be adopted to achieve a sufficient cover crop C input to secure that the positive priming effect is not exceeding the rate of SOC formation.

Exogenous Carbon Addition Reduces Soil Organic Carbon: The Effects of Fungi on Soil Carbon Priming Exceed Those of Bacteria on Soil Carbon Sequestration

Soil organic carbon (SOC) forms the largest terrestrial organic carbon (C) pool, which is regulated by complex connections between exogenous C input, microbial activity, and SOC conversion. Few studies have examined the changes in natural abundance C due to microbial activity after exogenous C inputs in karst lime soils in China. In this research, the 13C isotope tracer technique was employed to investigate the priming effect of SOC on typical lime soil (0~20 cm) of 13C_litter and 13C_calcium carbonate (CaCO3) through a mineralization incubation experiment. Samples were collected at 5, 10, 20, 40, 60, and 80 days of incubation and analyzed for SOC mineralization, SOC distribution across fractions (>250 μm, 53~250 μm, and <53 μm), and soil microbial diversity. A control consisting of no exogenous C addition was included. SOC mineralization and SOC priming were considerably higher (15.48% and 61.00%, respectively) after litter addition compared to CaCO3. The addition of either litter or CaCO3 reduced the total organic C (TOC) and macroaggregate (>250 μm) and microaggregate (53~250 μm) C fractions by 2150.13, 2229.06, and 1575.06 mg C kg−1 Cbulk on average and increased the mineral particulate C fraction (<53 μm) by 1653.98 mg C kg−1 Cbulk. As the incubation time extended, a significantly positive correlation was apparent between SOC priming and soil fungal diversity, as well as between the mineral particulate C fraction and soil bacterial diversity. The effect of soil fungal diversity on SOC priming (R = 0.40, p = 0.003) significantly exceeded that of bacterial diversity on SOC sequestration (R = 0.27, p = 0.02). Our results reveal that after adding litter or CaCO3, soil fungi stimulate SOC mineralization and decomposition and soil bacteria enhance SOC sequestration, with the effects of fungi being more pronounced. These findings can provide a theoretical basis for understanding C sequestration and emission reduction in karst lime soils.

Trait-based assembly of arbuscular mycorrhizal fungal communities determines soil carbon formation and retention.

Fungi are crucial for soil organic carbon (SOC) formation, especially for the more persistent mineral-associated organic C (MAOC) pool. Yet, evidence for this often overlooks arbuscular mycorrhizal fungi (AMF) communities and how their composition and traits impact SOC accumulation. We grew sudangrass with AMF communities representing different traits conserved at the family-level: 1) competitors, from the Gigasporaceae family; 2) ruderals, from the Glomeraceae family; or 3) both families combined. We labeled sudangrass with 13 C-CO2 to assess AMF contributions to SOC, impacts on SOC priming, and fungal biomass persistence in MAOC. Single-family AMF communities decreased total SOC by 13.8%, likely due to fungal priming. Despite net SOC losses, all AMF communities contributed fungal C to soil but only the Glomeraceae community initially contributed to MAOC. After a month of decomposition, both the Glomeraceae and mixed-family communities contributed to MAOC formation. Plant phosphorus uptake, but not hyphal chemistry, was positively related to AMF soil C and MAOC accumulation. AMF contribution to MAOC is dependent on the specific traits of the AMF community and related to phosphorus uptake. These findings provide insight into how variations in AMF community composition and traits, and thus processes like environmental filtering of AMF, may impact SOC accumulation.

A synthesis of soil organic carbon mineralization in response to biochar amendment

Biochar amendment can alter native soil organic carbon (SOC) mineralization via the priming effect (PE); however, its direction, intensity, and controls over a broad geographic scale are not clear, undermining the predictions of SOC dynamics as impacted by biochar inputs. Here, we synthesized 5,720 measurements of CO2 effluxes from 329 soil samples with and without 13C/14C labeled-biochar additions to quantify the spatial patterns and temporal dynamics of the PE and assess the underlying environmental drivers. Across all data, biochar amendment has led to a slight but significant positive PE (i.e., 56 mg C kg−1 soil), with stronger PEs in natural ecosystems than in agricultural soils. Negative PE occurred in the short term (i.e., <9 days after biochar addition) and subsequently shifted to a strong positive PE (i.e., 364–966 days) and remained in an insignificant PE thereafter (i.e., >1450 days). Notably, soils from rainfed croplands had the lowest negative PE (−28.76 mg C kg−1 soil). Grass-derived biochar produced at a low pyrolysis temperature (i.e., 300–400 °C) induced the strongest positive PE (244 mg C kg−1 soil). The results of our structural equation model indicated that biochar pyrolysis temperature and soil C:N ratio had the largest negative and direct association with biochar-induced PE, whereas incubation temperature and microbial biomass C exerted the greatest positive and direct effects on the PE. Variance partitioning analyses further revealed that both biochar and soil properties together accounted for 73% of the explained variance in biochar-induced PE. Overall, these results add to our understanding of biochar-induced SOC priming as impacted by different land-use types, soils, and biochar properties. The amendment of wood-derived biochars produced at high pyrolysis temperatures (>500 °C) to rainfed croplands could serve as a promising strategy to achieve maximum soil C sequestration.

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.

Microbial community structure and functional genes drive soil priming effect following afforestation

Afforestation substantially modifies native soil organic carbon (SOC) decomposition via plant carbon inputs (the priming effect), and in turn, triggers vital biogeochemical processes that influence the regulation of soil carbon dynamics. Soil microbes are crucial in regulating the direction and magnitude of the priming effect. In the present study, we performed metagenomic sequencing and 13C-glucose labeling analyses of microbial communities and priming effects across a Robinia pseudoacacia afforestation chronosequence (14-, 20-, 30-, and 45-year-old stands) in the Loess Plateau in China, with adjacent farmland being selected as a control. Our results revealed that the cumulative priming effect across five sites along the afforestation chronosequence initially increased and approached a peak value in the 20-year-old stand, after which it declined. The priming effect was predominantly driven by the microbial community structure (i.e., the fungal-to-bacterial ratios and relative abundances of Proteobacteria and Actinobacteria), and stable C decomposition genes and C-degrading enzymes. Specifically, among the key functional genes correlated with priming effect, which were identified in orders Rhizobiales and Pseudonocardiales, considerably promoted SOC priming. Overall, our findings indicate that afforestation alters soil microbial community structure and function, particularly with respect to enhancing stable soil C decomposition genes, which may promote SOC priming. The findings of the present study could enhance our understanding of fresh C input-induced changes associated with C mineralization in the context of the revegetation of ecologically fragile areas.

Soil properties and substrate quality determine the priming of soil organic carbon during vegetation succession

Soil properties and substrate quality determine the priming of soil organic carbon during vegetation succession

Liming and priming: the long-term impact of pH amelioration on mineralisation may negate carbon sequestration gains.

Acidity negatively impacts upon soil capability and conditions across approximately 50% of the world's arable land. Plant growth and nutrient cycling are known to respond positively to the addition of lime to decrease soil acidity. However, the interactions between liming and soil carbon dynamics remain incompletely understood. The nexus of soils, food security and climate change make this topic an urgent concern for investigation. This study utilised soils (Cambisols) from three long-term lime field trials (13-39 years) on farms. Soils (0-10 cm) were incubated in the laboratory for 3 months with and without 13C-labelled wheat crop residue. This approach enabled direct quantification of the CO2-C originating from three different processes; decomposition of extant soil organic carbon (SOC), decomposition of the added crop residue, and SOC priming i.e. the additional decomposition of SOC stimulated by addition of plant material. Biological and chemical soil properties were also quantified, with a novel application of measurement of the abundance of the functional genes involved in SOC decomposition. Priming was significantly increased in limed soils (P<0.01). The abundance of both archaeal rRNA genes, and the laccase-like functional gene involved in lignin decomposition correlated significantly and positively with SOC priming (Rho 0.46, P<0.01). Decomposition of extant SOC was positively correlated with pH and N (Rho 0.65, P<0.001). Decomposition of the added crop residue was also correlated with pH and inorganic N, albeit less strongly than for SOC (Rho 0.43, P< 0.05). These results support the theory that the application of lime to ameliorate soil acidity improves conditions for soil microorganisms as well as for plants. Archaea, and organisms harbouring SOC-decomposing laccases, would appear to play a key role in releasing nutrients stored in extant SOC, when priming is prompted by the addition of new plant material. This work highlights the importance of taking account of multiple response factors when assessing potential solutions within the food security-climate change nexus.

Residue decomposition and priming of soil organic carbon following different NPK fertilizer histories

AbstractThe effect of mineral fertilizer practices on increasing crop yield has been well studied, but few studies have examined the influence of various long‐term subtractive fertilization designs on the decomposition of crop residues or the influence on soil organic carbon (SOC) levels. To assess the combined effects of the long‐term (27‐yr) subtractive fertilizer inputs on SOC mineralization rates, we studied the change in added 13C‐labeled corn (Zea mays L.) stem and leaf residue decomposition and the magnitude of priming effect (PE) on SOC in a 56‐d lab incubation. The long‐term history of nitrogen (N), phosphorus (P), and potassium (K) combinations showed that coupling N and P fertilization had a strong effect on SOC content which increased by 6% relative to the unfertilized treatment. Soil organic C mineralization rates with no residue added were highest in soils that received N, P, and K. Cumulative CO2 production in soils amended with residue was significantly affected by residue type and was 9–19% higher for leaves than stems across treatments with varied fertilization histories. After a 56‐d incubation 14–16% of leaf C and 18–19% of stem C was mineralized. The cumulative PE was significantly higher in the unfertilized soil with leaf addition but was not affected by residue type in other treatments. Our data showed that crop residue decomposition and SOC is affected by historical N, P, and K fertilization, with long‐term complete NPK application resulting in greater SOC content and slower SOC mineralization rates with corn leaf addition.

Interactive effects of crop residue quality and nitrogen fertilization on soil organic carbon priming in agricultural soils

Soil organic carbon (SOC) priming affects C sequestration in soils, the intensity of which differs depending on residue quality. N fertilization could also alter SOC priming. However, the interaction of crop residue quality and N fertilization on the SOC priming is still not clear. To address this gap in knowledge, we conducted this study. We undertook a 110-day laboratory incubation experiment to evaluate the SOC priming and sequestration induced by maize shoot and root residues with and without the application of mineral fertilizer-N in two types of agricultural soils (Andisol and Entisol). Application rates of maize residue and N were 3 g C kg−1 soil and 60 mg N kg−1 soil, respectively. 13C-labeled maize residue allowed quantifying residue decomposition and calculating SOC priming and sequestration. After 110 days of incubation, the cumulative intensity of priming effect was higher for root residue than shoot residue. Addition of N results in contrasting effects on the priming effect induced by root and shoot residue in both types of soils; with root residue, it reduced the intensity of priming effect and resulted in a higher net C sequestration because of reduced N mining, whereas it had little effect with shoot residue, where co-metabolism is the likely explanation for the positive priming effect. Crop residue quality and N fertilization can interactively affect the SOC priming. N fertilization is beneficial for soil C sequestration when soil is treated with low-quality crop residue (e.g., root residue) because of lowering of the intensity of priming effect and crop residue decomposition.

Trade-off between ‘new’ SOC stabilisation from above-ground inputs and priming of native C as determined by soil type and residue placement

Due to the geographical expanse of grasslands with depleted organic matter stocks, there has been growing interest in the management of these ecosystems for C sequestration to help mitigate climate change. It is generally accepted that management practices intending to increase forage production (e.g. decreasing grazing density) result in increased soil C stocks by increasing the return of biomass inputs to the soil organic carbon (SOC) pool. However, the contribution of C inputs to stable SOC versus GHG losses, and how this is affected by soil properties, remains largely unknown, particularly within subtropical biomes. To investigate the role of soil texture and mineralogy on SOC stabilisation, we identified three different soil types with varying physical properties in close proximity (< 2 km2) to each other. We used isotopically labelled plant material (13C), placed on the soil surface versus incorporated within the mineral soil, to trace the fate of fresh residue inputs into SOM fractions that differed in their degree of protection and mechanistic interactions with the soil matrix. Weekly GHG measurements (CO2, N2O and CH4) were taken to understand the overall GHG balance resulting from C inputs (i.e. SOC accrual versus GHG losses in CO2 equivalents). In finer textured soils with a greater smectite content, SOC accrual was greater but was significantly outweighed by GHG losses, primarily from native SOC priming. The incorporation of residue within the soil increased residue-derived SOC accrual by 4- to 5-fold, whilst also suppressing the priming of native SOC. This improved understanding of how soil texture and residue placement affect the global warming mitigation potential of subtropical grassland soils will be important in determining identifiable regions that should be targeted for SOC restoration efforts by increasing C inputs.

Priming of soil organic carbon induced by sugarcane residues and its biochar control the source of nitrogen for plant uptake: A dual 13C and 15N isotope three-source-partitioning study

Sugarcane (Saccharum spp.) farming systems globally have largely transitioned away from burning the crop prior to harvest. Harvesting the sugarcane crop ‘green’ results in large volumes of biomass residues being left on the soil. Despite this, there is little evidence for increased soil organic carbon stocks. We investigated the role of surface application or incorporation (0–200 mm soil layer) of harvest residues (15 t dry weight residues ha−1) and its biochar (5.4 t ha−1 based on the quantity of resource recovered after pyrolysis) on the priming of native soil organic carbon (SOC), the mineralisation of the organic amendments and the source of crop nitrogen (N) uptake (soil, organic amendment or urea). All treatments received urea at 180 kg N ha−1. To achieve the separation of C and N sources, dual 13C and 15N-enriched sugarcane residues and corresponding biochar (350 °C) were used in an 84-d controlled environment study. A three-pool isotope mixing model, utilising two levels of 13C enrichment in residue (16.6‰ and 23.8‰) and biochar (16.8‰ and 24.1‰), was also applied to partition the C from three sources: 1) root respiration, 2) organic amendment mineralisation, and 3) SOC priming. The SOC mineralisation was increased following both surface-applied and incorporated residues, over the nil organic amendment (control) by 72.3 and 78.3 CO2–C m−2 respectively over 84 days. In contrast, biochar lowered the mineralisation of SOC by 62.9 g CO2–C m−2 compared to the control. The cumulative mineralisation of sugarcane residue biochar (18.9 g CO2–C m−2) was lower (P = 0.03) than surface applied residue (50.1 g CO2–C m−2) and incorporated residue (71.9 g CO2–C m−2) over the study period. While there were no differences in total crop N uptake between the organic-amended soils and the control, the source of N was significantly different. The sugarcane plants utilised 31.0% and 29.4% of the supplied urea N in the nil organic-amended control and biochar treatment, respectively. This was significantly reduced to 24.8% and 20.6% in the surface residue and incorporated residue treatments, respectively. In comparison, the plant uptake of N derived from the organic amendments was 27.8%, 15.4% and 6.4% from incorporated residues, surface-applied residues and biochar, respectively (P < 0.001). Results suggest that the increased mineralisation of SOC, partly driven by the high C:N ratio (73:1) and the unbalanced nutrient stoichiometry may lead to low SOC accumulation from surface residue application and that sugarcane residue biochar results in SOC stabilisation and an increase in the use efficiency of fertiliser N in sugarcane systems.

Tillage history and crop residue input enhanced native carbon mineralisation and nutrient supply in contrasting soils under long-term farming systems

Understanding the legacy effect of tillage-based farming systems on soil organic carbon (SOC) mineralisation and nutrient [nitrogen (N), phosphorus (P) and sulphur (S)] supply after crop residue input is critical to appropriately manage plant available nutrients at the farm scale. To enhance this understanding, crop residues [canola (Brassica napus: δ13C 124‰) or wheat (Triticum aestivum: δ13C 461‰)] were added to Luvisol and Vertisol from two long-term (16–46 years) field experiments and incubated under a controlled environment for 126 days. The practices in the Luvisol were conventional tillage (CT) and reduced tillage (RT) under mixed crop-pasture rotation, and no-till (NT) under continuous cereal-cover crop rotation. The practices in the Vertisol were CT and NT under wheat-wheat rotation. The residue input significantly stimulated SOC mineralisation via “positive priming”, which was greater (p < 0.05) in the CT than RT/NT in the Luvisol only. The SOC mineralised after 126 days was 3.1–4.2 and 1.6–2.5 times higher in the canola and wheat residue-amended soils, respectively, than the unamended soils. Although the CT or RT versus NT had higher net N availability in the Luvisol only, the residue input did not increase plant available N in both soils, possibly due to stronger residue-induced N immobilisation than mineralisation. The results showed a significant release of available P and S in both residue-amended soils (canola > wheat) after 26–50% of residue-C was mineralised over 126 days, and the Vertisol had greater net available P than Luvisol. Our results suggest that considerable quantities of available P and S may release from the soil reserves via SOC priming, and possibly via dissolution/desorption reactions in the soils, in addition to their direct release from the residues. In conclusion, crop residue input to historical farming systems enhanced the supply of available P and S, which varied with tillage, crop residue and soil type.

Effect of crop residue addition on soil organic carbon priming as influenced by temperature and soil properties

Priming of soil organic carbon (SOC) is a crucial factor in ecosystem carbon balance. Despite its increasing importance in the changing global climate, the extent of influence of temperature and soil properties on the priming effect remains unclear. Here, soil priming was investigated using 13C labeled wheat residues in two cultivated, subtropical (Vertisol) and semi-arid (Luvisol), soils of Australia at four incubation temperatures (13, 23, 33 and 43 °C). The priming effect was computed from respired CO2 and associated δ13C, which were measured periodically over the 52-day incubation period. Wheat residue addition resulted in greater priming effect in the Luvisol (1.17 to 2.37% of SOC) than the Vertisol (0.02 to 1.56% of SOC). The priming of SOC was the highest at 23 °C in the Luvisol, and at 43 °C in the Vertsiol, which indicates a variable positive priming effect of temperature in different soil types. Wheat residue addition significantly increased the temperature sensitivity (Q10) of SOC mineralization in the Vertisol at temperature ranges below 33 °C (i.e., 13–23 and 23–33 °C) and had no significant effect in the Luvisol. A negative correlation was observed between temperature and the Q10 values. Across soils, the Q10 of residue C was lower than SOC suggesting that soil C is more vulnerable to climatic warming. This work demonstrates that the magnitude of SOC priming by wheat residue and Q10 of SOC mineralization varied significantly with soil type (Luvsiol > Vertisol) and incubation conditions (temperature and time). Given the current trend towards increasing atmospheric temperatures, future studies should evaluate temperature effects on the priming of different pools of SOC induced by crop residue in different agro-ecosystems.

Shifts in priming partly explain impacts of long‐term nitrogen input in different chemical forms on soil organic carbon storage

Input of labile organic carbon can enhance decomposition of extant soil organic carbon (SOC) through priming. We hypothesized that long-term nitrogen (N) input in different chemical forms alters SOC pools by altering priming effects associated with N-mediated changes in plants and soil microbes. The hypothesis was tested by integrating field experimental data of plants, soil microbes and two incubation experiments with soils that had experienced 10years of N enrichment with three chemical forms (ammonium, nitrate and both ammonium and nitrate) in an alpine meadow on the Tibetan Plateau. Incubations with glucose-13 C addition at three rates were used to quantify effects of exogenous organic carbon input on the priming of SOC. Incubations with microbial inocula extracted from soils that had experienced different long-term N treatments were conducted to detect effects of N-mediated changes in soil microbes on priming effects. We found strong evidence and a mechanistic explanation for alteration of SOC pools following 10years of N enrichment with different chemical forms. We detected significant negative priming effects both in soils collected from ammonium-addition plots and in sterilized soils inoculated with soil microbes extracted from ammonium-addition plots. In contrast, significant positive priming effects were found both in soils collected from nitrate-addition plots and in sterilized soils inoculated with soil microbes extracted from nitrate-addition plots. Meanwhile, the abundance and richness of graminoids were higher and the abundance of soil microbes was lower in ammonium-addition than in nitrate-addition plots. Our findings provide evidence that shifts toward higher graminoid abundance and changes in soil microbial abundance mediated by N chemical forms are key drivers for priming effects and SOC pool changes, thereby linking human interference with the N cycle to climate change.

Interactive effects of initial pH and nitrogen status on soil organic carbon priming by glucose and lignocellulose

Soil pH and the availability of carbon (C) substrate and nutrients to microorganisms are well recognized to influence C priming. However, the mechanisms underpinning such interplay so far remain elusive. Given that liming acid soils, residue retention and fertilization are common agricultural practices, small changes in SOC content by these practices could have a big impact on the global C budget. This study aimed to gain insight into the impact of initial pH and mineral N availability on the priming effect of two C substrates with contrasting biodegradability. Stable 13C-labelled substrates, glucose and lignocellulose, were applied at the rate of 0.5 mg C g−1 soil with or without NH4NO3 to the same soil matrix with three different initial pH levels; pH 4.1 (strongly acidic), 4.7 (moderately acidic) and 6.6 (slightly acidic). The N treatment was based on a C:N of 10 of the added substrate (0.05 mg N g−1 soil) to ensure N was non-limiting. Interestingly, the priming effect was not linearly related to soil pH; greatest at pH 4.1, followed by pH 6.6 and lowest at pH 4.7. The greater net increase in microbial biomass upon C supply in strongly acidic soils compared to the moderately and slightly acidic soils would have enhanced co-metabolic decomposition of native soil organic C (SOC). The cumulative amount of primed SOC during the 30-day incubation period was greater in glucose- (21 μg C g−1) than lignocellulose-amended soils (13 μg C g−1). Nitrogen application reduced the C priming effect of both C substrates at all pH levels. This reduction was more prominent with lignocellulose and in the moderately acidic soils. The results suggest that maintaining optimal soil pH for nutrient availability and N application that exceeds the microbial N requirements in agricultural fields may minimize SOC loss via the priming effect in the short term.