Protection Of Soil Organic Matter Research Articles

High surface area inorganic minerals show promise in protecting carbon from soil organic amendments

Protecting soil organic compounds could assist with global efforts to mitigate climate change, as well as improve soil health. Recent research has indicated that high surface area inorganic minerals (HSAIMs) can protect soil organic matter (OM). We evaluated if adding HSAIMs to soils can protect carbon from common organic inputs added to fallow soils prior to planting. Four added HSAIMs were investigated – two manufactured zeolites, one natural zeolite and bentonite clay – along with a carbon-rich (peat moss) and carbon-poor (composted cow manure) OM source. The added HSAIMs and organic sources were incorporated into a microbially active (clayey Vertisol) and inactive (sandy Arenosol) soil at application rates of 20 tonnes.ha–1 for the minerals and 60 tonnes.ha–1 for the organics. The treatments were incubated for three months under laboratory conditions and were subjected to continuous wetting-drying cycles. Peat moss resulted in higher organic carbon (Corg) and OM losses than cow manure. Moreover, the addition of HSAIMs to the Peat Moss+Arenosol treatments appeared to slightly enhance these losses, albeit not significantly (p>0.05). Carbon/nitrogen (C/N) ratios revealed that the Arenosol treatments were depleting soil N to access the carbon provided by the peat moss. For all other treatment combinations, the addition of HSAIMs promoted greater retention of Corg (p<0.05 in one treatment combination) and OM (p<0.05 in four treatment combinations), relative to the controls. Modelling carbon balances using OM data was more effective than using direct Corg values, due to strong Corg measurement variance. Modelled Corg data revealed the added HSAIMs achieved up to 25 % carbon protection, with the manufactured zeolites particularly effective. The study shows promise for added HSAIMs to protect carbon released from decomposing organic amendments added to fallow soils. However, addition of these minerals to N-limited soils receiving carbon-rich amendments might accelerate carbon losses.

Conversion of Natural Soil to Paddy Promotes Soil Organic Matter Degradation in Small-Particle Fractions: δ13C and Lipid Biomarker Evidence

The stabilization mechanism of soil organic matter (SOM) has received considerable attention. It is widely accepted that mineral sorption/protection is important for SOM stabilization. However, it remains unclear which organic carbon component is beneficial for mineral protection. We collected soil samples from a paddy field (TP) to compare with natural soil (NS). To illustrate the behavior of different SOM pools and their protection by particles, we separated the soils into different particle-size fractions and then removed the active minerals using an acid mixture (1 M HCl/10% HF). The different carbon pools were analyzed using stable carbon isotopes and lipid biomarkers. Our study showed that acid treatment evidently increased the extractability of free lipids, usually over 60%, which confirmed the predominant role of minerals in SOM protection. For NS, the δ13C values increased with decreasing soil particle sizes and soil depths, indicating that 13C-enriched SOM was selectively preserved. However, this trend disappeared after cultivation, which was mainly attributed to the combined effects of the input of 13C-depleted fresh SOM and decomposition of the preserved 13C-enriched SOM. Meanwhile, based on the degradation parameters of the overall lipid biomarkers, SOM showed higher degradation states in clay and silt fractions than in the sand fraction before cultivation. It is possible that the small particle-size fractions could selectively absorb highly degraded SOM. The clay-associated SOM showed a low degradation state, but its carbon content was low after cultivation. We propose that the previously protected SOM was degraded after cultivation and was replaced by relatively fresh SOM, which should be carefully monitored during SOM management.

SAMM version 1.0: a numerical model for microbial- mediated soil aggregate formation

Abstract. Maintaining soil organic matter (SOM) is crucial for healthy and productive agricultural soils and requires understanding at the process level, including the role of SOM protection by soil aggregates and the connection between microbial growth and aggregate formation. We developed the Soil Aggregation through Microbial Mediation (SAMM) model, to represent this important connection. The pools of SAMM are fully measurable, and we calibrated and evaluated it against data from a long-term bare fallow experiment in a tropical sandy soil. This experiment received additions of plant litter of different compositions, which resulted in twice the soil carbon stocks in the best treatment compared to the control (about 8 vs. 4 t C ha−1 in 0–15 cm soil depth) after 25 years. As hypothesized, the SAMM model effectively represented the microbial growth response after the addition of litter and the subsequent formation and later destabilization of aggregates. The low correlations between different calibrated model parameters (r&lt;0.5 for all parameters; r&gt;0.4 for only 4 of 22) showed that SAMM is parsimonious. SAMM was able to capture differences between treatments in soil organic carbon (Nash–Sutcliffe modeling efficiency (EF) of 0.68), microbial nitrogen (EF of 0.24), and litter carbon (EF of 0.80). The amount of carbon within the aggregates (EF of 0.60) and in the free silt and clay fraction (EF of 0.24) was also simulated very well to satisfactorily. Our model results suggested that in spite of the sandy soil, up to 50 % of carbon stocks were stabilized through aggregate protection mechanisms; and that microbial and physical aggregate formation coexists. A version of the SAMM model without aggregate protection (SAMMnoAgg) initially failed to stabilize soil organic carbon (EF decreased to −3.68) and the simulation of microbial nitrogen worsened (EF of 0.13). By recalibrating SAMMnoAgg, it was possible to partially correct for the lack of aggregate protection by reducing the rate of mineral-attached carbon decomposition by about 85 % (EF of 0.68, 0.75, and 0.18 for SOC, litter carbon, and microbial nitrogen, respectively). However, the slightly better evaluation statistics of SAMM (e.g., Akaike information criterion of 5351 vs. 5554) suggest that representing aggregate dynamics in SOM models can be beneficial and necessary to understand the mechanism behind SOM dynamics. Our results indicate that current models without aggregate formation partly compensate for the absence of aggregate protection by lowering the turnover rates of other pools. Thus, they remain suitable options where data on aggregate associated carbon are not available.

The ability of soils to aggregate, more than the state of aggregation, promotes protected soil organic matter formation

Efforts to increase soil organic carbon (SOC) are pursued as a viable climate change mitigation strategy. Boosting SOC stocks requires increasing plant carbon (C) inputs and promoting their persistence in SOC. In well aerated mineral soils, water soluble inputs are expected to stabilize through chemical binding to minerals, forming mineral-associated organic carbon (MAOC) before or after microbial transformation, while structural inputs are expected to stabilize as particulate organic carbon (POC) via protection in soil aggregates. Although ample research is centered on the effects of soil aggregation, its disturbance (e.g., tillage), and microbial processing on SOC cycling, we still lack mechanistic understanding of how plant C input type (i.e., soluble versus structural) and disturbance (i.e., aggregate disruption) independently and in interaction affect POC and MAOC formation and stabilization in soils with inherently different degrees of aggregation.To this end, using 13C enriched structural and soluble plant inputs, we traced SOC formation and stabilization in a lab incubation experiment in soils with differing levels of aggregation, and capacity to form aggregates after disturbance. Our results showed that soluble plant inputs contributed substantially to MAOC and that higher formation and persistence of MAOC occurred in the highly aggregated soil. Moreover, the highly aggregated soil retained more soluble inputs stabilized as MAOC when disturbed. Disturbance in this fine textured, organic carbon rich soil stimulated regeneration of aggregates around structural plant inputs leading to greater persistence of aggregate-occluded POC. Soluble plant inputs, specifically, as well as structural plant inputs in the highly aggregated disturbed soil, compensated for SOC lost due to disturbance alone.Overall, this study provides mechanistic evidence suggesting that management strategies for SOC accrual should consider soil type, aggregation potential, and plant attributes to inform decisions surrounding tillage frequency and cropping regimes. Our mechanistic findings require testing. If confirmed in the field, they would suggest that no disturbance in tandem with high structural plant C inputs would benefit most SOC accrual in systems with soils that have little capacity to form organo-mineral complexes, including large aggregates. On the contrary, with sustained plant inputs, occasional disturbance in systems with soils that have a high capacity to form organo-mineral complexes can promote both POC and MAOC formation and stabilization.

Stabilization of organic matter in soils: drivers, mechanisms, and analytical tools – a literature review

ABSTRACT Soils are the largest terrestrial carbon (C) reservoir, and most of this C is retained as soil organic matter (SOM). Due to its ability to capture, stabilize, and store C for extended periods, soils are considered important allies in decarbonizing the atmosphere. The term ‘C stabilization’ includes a series of mechanisms or processes by which soil C is protected within soils and its losses are reduced through microbial decomposition or leaching. Due to their relevance in the global C cycle, C stabilization mechanisms have received intensive attention from the scientific community. As new analytic technologies push the boundaries of what was previously possible to know, new paradigms emerge. This literature review summarizes the current knowledge of the main mechanisms that may promote SOM stabilization. Factors that govern accumulation of SOM are also addressed. We highlight the role of organo-mineral associations and spatial inaccessibility of SOM due to occlusion within soil aggregates to understand the relative contribution of these mechanisms in different soil conditions (e.g., soil texture, mineralogy, and land- use). In addition, the contribution of cutting-edge approaches and analytical techniques to advance the understanding of SOM protection is presented. Modern techniques to evaluate SOM on a micro, nano, and molecular scale can contribute to the mechanistic understanding of SOM stabilization and the study and adoption of management strategies that maintain and increase C stocks in soils.

Functional groups and mineralization kinetics of soil organic matter under contrasting hydro‐thermal regimes under conservation agriculture‐based rice–wheat system in eastern Indo‐Gangetic Plains

AbstractSoil organic carbon (SOC) sequestration is important to counteract anthropogenic climate change at the global level. Studying the effect of conservation agriculture (CA) on SOC dynamics in the presence of two distinctively different hydro‐thermal regimes across the year in rice–wheat (RW) systems in eastern Indo‐Gangetic Plains (E‐IGP) is of topical interest. The stabilization mechanism of soil organic matter (SOM), and its effect on C mineralization kinetics in these conditions is not well understood. We collected soil samples from six combinations of CA and conventional farming in an ongoing experiment at CIMMYT‐Borlaug Institute for South Asia, Bihar, India. CA enhanced SOC, specifically labile C, while decreasing mineral N, as a result of encapsulation/assimilation in aggregates/microbial biomass. Humic acids registered characteristic Fourier transform infrared (FTIR) peaks at 3200–3600 cm−1, 2920–2930 cm−1, 1645–1655 cm−1 and 1220–1240 cm−1, and displayed lesser degree of humification, aromaticity and redox status under CA. SOC and N mineralization were studied in two different hydro‐thermal regimes pertaining to rice (submergence, 35°C; SM35) and wheat (field capacity, 25°C; FC25) growing periods of E‐IGP. SM35 displayed temperature‐mediated higher decay of SOC. Decay constants of C mineralization were lesser under CA compared with CT. CA promoted higher SOM stability, evidenced by lower decay rates of SOC and N, attributed to (1) better protection of SOM in well‐aggregated soil structure and (2) an excess supply of fresh crop residues ensuring higher rate of SOM formation than its decay. Practising CA in E‐IGP is imperative towards C‐neutral agriculture, especially in the impending global warming scenario.

Soil organic matter stabilization during early stages of Technosol development from Ca, Mg and pyrite-rich parent material

The mechanisms underlying soil organic matter (SOM) accrual in Technosols are still poorly understood. Here we studied a 20-year chronosequence of three Technosols constructed with dolomitic limestone mining waste: two cultivated with sugarcane (for 3 and 7 years) and one under pasture (for ∼20 years). Through a combination of SOM physical fractionation and wet-chemical extractions, we aimed to unravel the processes driving SOM accumulation in these soils and developed a conceptual model for embryonic soils developed from Ca, Mg and pyrite-rich parent material. Our results revealed that the weathering of comminuted waste rocks (e.g., shale, siltstone) provided 2:1 minerals (especially smectite) to participate in mineral-organic associations. The weathering of primary minerals also promoted the formation of secondary minerals, which interacted with SOM and increased the amount of mineral associated organic matter (MAOM) up to 20 g SOC kg−1. Pyrite oxidation produced highly reactive secondary Fe phases, while the dolomite dissolution increased the concentrations of Ca2+ and Mg2+ and maintained near-neutral pH conditions, favoring the formation of cation-bridges. The progressive increase in the different operational Fe fractions (Fe in organomineral complex, poorly crystalline, and crystalline Fe oxides) along the Technosol chronosequence was strongly correlated with MAOM, highlighting the significant role of Fe in SOM accumulation. Furthermore, we observed substantial increase in particulate organic matter (POM), up to 22 g SOC kg−1, which was associated with increased aggregate formation and stability. These processes collectively contribute to the protection of SOM during early stages of soil development from in Ca, Mg, and pyrite-rich parent material, and thus indicate a high potential for carbon sequestration.

Mineralogical control on physically protected soil organic matter in a neotropical moist forest

Context Minerals and organic matter physically associate in many soils, yet the precise nature, either via a hierarchy of aggregate particles or by build-up of organo–mineral associations, remains obscure, especially in tropical forest environments. Aims We investigated physically protected organic matter by comparing soils with contrasting parent material, topography, and pedogenesis, but with similar tropical moist forest on Barro Colorado Island in the lowlands of Panama. Methods Bulk soil from 10 sites was separated by size into free-floating particles, macroaggregates (&gt;250 μm), microaggregates (53–250 μm), and &lt;53-μm particles. A subsample of macroaggregates was disintegrated and separated into coarse particles (&gt;250 μm), occluded microaggregates, and occluded &lt;53-μm particles. Concentrations and natural abundance of stable isotopes for carbon (C) and nitrogen (N) were determined for each fraction, and ratios (C:N, stable isotopes) were used to characterise organic matter for each fraction. Key results Macroaggregates were the largest fraction of bulk soil (71%) and were 15% greater in kaolinite- than smectite-dominated soils. Macroaggregates were composed of coarse particles (14%), occluded microaggregates (62%), and occluded &lt;53-μm particles (24%). Concentrations of C and N widely varied among fractions but the variation was not related to clay mineralogy. The C:N ratio and stable N isotope ratio indicated more decomposed organic matter in kaolinite- than smectite-dominated soils. Conclusions and implications Macroaggregates composed of plant detritus and microaggregates in the Barro Colorado Island soils imply that the aggregate hierarchy route ultimately protects soil organic matter in this tropical forest environment.

Effect of Soil Application of Zeolite-Carbon Composite, Leonardite and Lignite on the Microorganisms

Abstract The aim of the study was to evaluate the effect of mineral-organic mixture on changes in the abundance of selected soil microorganisms. The experiment contained: soil with NPK (nitrogen, potassium, phosphorus) + 3 % or 6 % lignite (MF+CW3 %, MF+CW6 %) and 3 % zeolite-carbon composite (NaX-C); soil with NPK + 3 % or 6 % leonardite (MF+CL3 %, MF+CL6 %) and 3 % NaX-C; soil without fertilisation (C); soil fertilised with mineral NPK fertilisers (MF). Plants participating in the experiment were spring wheat and spring oilseed rape. The presence of the selected microorganisms was determined: Azotobacter spp., actinomycetes, ammonifiers, bacteria and mold fungi. Using Koch’s serial dilution method, the abundance of selected soil microorganisms was performed. The conducted research allows to conclude that the abundance of detected microorganisms depended on both the applied fertilisation and the plant grown. For the spring oilseed rape, the highest abundance of microorganisms was determined in treatments where fertilisation with lignite mixtures was applied, while for spring wheat, with leonardite mixtures. Increasing (from 3 % to 6 %) the share of lignite and leonardite in fertiliser mixtures did not translate into a proportional growth in the abundance of microorganisms, so such a treatment has no economic justification. Given their alkaline pH, the mixtures used can be a substitute for calcium fertilisers to improve soil properties and, consequently, protect soil organic matter from degradation.

5300-Year-old soil carbon is less primed than young soil organic matter.

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the "priming effect" (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO2 efflux cannot be disregarded due to its huge pool size.

Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling

The soil carbon (C) saturation concept suggests an upper limit to the storage of soil organic carbon (SOC). It is set by the mechanisms that protect soil organic matter from mineralization. Biochar has the capacity to protect new C, including rhizodeposits and microbial necromass. However, the decadal-scale mechanisms by which biochar influences the molecular diversity, spatial heterogeneity, and temporal changes in SOC persistence, remain unresolved. Here we show that the soil C storage ceiling of a Ferralsol under subtropical pasture was raised by a second application of Eucalyptus saligna biochar 8.2 years after the first application—the first application raised the soil C storage ceiling by 9.3 Mg new C ha−1 and the second application raised this by another 2.3 Mg new C ha−1. Linking direct visual evidence from one-, two-, and three-dimensional analyses with SOC quantification, we found high spatial heterogeneity of C functional groups that resulted in the retention of rhizodeposits and microbial necromass in microaggregates (53–250 µm) and the mineral fraction (<53 µm). Microbial C-use efficiency was concomitantly increased by lowering specific enzyme activities, contributing to the decreased mineralization of native SOC by 18%. We suggest that the SOC ceiling can be lifted using biochar in (sub)tropical grasslands globally.

Impact of Long-Term Nutrient Supply Options on Soil Aggregate Stability after Nineteen Years of Rice–Wheat Cropping System

Continuing soil degradation remains a serious threat to future food security. Soil aggregation can help protect soil organic matter from biodegradation; it affects soil physical (aeration), chemical (water infiltration), and biological (microbial) activities. The integrated plant nutrition system (IPNS) and organic farming (OF) options have been contemplated as a sustainable strategy to sustain soil aggregate stability under adverse climatic conditions and a possible tool to restore degraded soil systems. Results suggested that the application of plant nutrients based on IPNS and soil test crop response (STCR) including mineral fertilizers and organic manure (farmyard manure: FYM) improved soil aggregate stability and mean weight diameter (MWD) under rice–wheat cropping systems. A long-term (19 year) cropping system (rice–wheat) experiment was examined to identify best nutrient management practices. Seven nutrient supply options were applied: organic, mineral fertilizer in combination with IPNS, IPNS + B/IPNS + C to improve soil aggregate stability and MWD after completing 19 cropping cycles of rice–wheat cropping systems. Results showed that significantly higher (+31%) macroaggregates were dominant in the surface soil layer than in the subsurface soil. The significantly highest macroaggregates were observed under OF (60.12 g 100 g−1 dry soil) management practices followed by IPNS options. The MWD was significantly increased (+17%) between surface and subsurface soil. Maximum MWD was reported with OF (0.93 mm) management practices followed by the IPNS + C (0.78 mm), IPNS + B (0.77 mm), IPNS (0.70 mm), STCR (0.69 mm), NPK (0.67 mm), and unfertilized control (0.66 mm) plots. Overall, results suggest that the adoption of IPNS options, such as organic farming (OF), RDF, STCR, and inclusion of pulses (berseem and cowpea), significantly improved all soil aggregation fractions in the soil system and also offered an additional benefit in terms of soil sustainability.

Low carbon availability in paleosols nonlinearly attenuates temperature sensitivity of soil organic matter decomposition.

Temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition is an important parameter in models of the global carbon (C) cycle. Previous studies have suggested that substrate quality controls the intrinsic Q10 , whereas environmental factors can impose large constraints. For example, physical protection of SOM and its association with minerals attenuate the apparent Q10 through reducing substrate availability and accessibility ([S]). The magnitude of this dampening effect, however, has never been quantified. We simulated theoretical Q10 changes across a wide range of [S] and found that the relationship between Q10 and the log10 -transformed [S] followed a logistic rather than a linear function. Based on the unique Holocene paleosol chronosequence (7 soils from ca. 500 to 6900years old), we demonstrated that the Q10 decreased nonlinearly with soil age up to 1150years, beyond which Q10 remained stable. Hierarchical partitioning analysis indicated that an integrated C availability index, derived from principal component analysis of DOC content and parameters reflecting physical protection and mineral association, was the main explanatory variable for the nonlinear decrease of Q10 with soil age. Microbial inoculation and 13 C-labelled glucose addition showed that low C availability induced by physical protection and minerals association attenuated Q10 along the chronosequence. A separate soil incubation experiment indicated that Q10 increased exponentially with activation energy (Ea ) in the modern soil, suggesting that SOM chemical complexity regulates Q10 only when C availability is high. In conclusion, organic matter availability strongly decreased with soil age, whereas Michelis-Menten kinetics defines the Q10 response depending on C availability, but Arrhenius equation describes the effects of increasing substrate complexity.

Correction to: Facilitated destabilization of physicochemically protected soil organic matter by root-derived low-molecular-weight organic acids

Correction to: Facilitated destabilization of physicochemically protected soil organic matter by root-derived low-molecular-weight organic acids

Facilitated destabilization of physicochemically protected soil organic matter by root-derived low-molecular-weight organic acids

Facilitated destabilization of physicochemically protected soil organic matter by root-derived low-molecular-weight organic acids

Formation efficiency of soil organic matter from plant litter is governed by clay mineral type more than plant litter quality

Plant litters incorporated in soils are decomposed by microorganisms and partially transformed into soil organic matter (SOM) through mineral-organic association and physical protection in soil aggregates. Few studies have linked the effects of clay mineralogy and plant litter quality on controlling the formation efficiency of SOM. Using model soils, the objectives of this study were (1) to determine the effects of clay mineral type and plant litter quality on soil respiration dynamics, and formation efficiency of SOM, physical fractions, and chemical and microbial compositions of SOM at the end of a 120-day incubation; (2) to unravel SOM protection mechanisms and extents by specific clay minerals; and (3) to understand the key role of clay minerals relative to plant litter quality in controlling SOM formation. The changes in X-ray diffraction peak intensity (in terms of peak height) of the clay minerals during incubation and after H2O2 treatment provided evidence for surface adsorption by vermiculite and pore entrapment by kaolinite and illite assemblages. The SOM protection extent parameter, defined based on accumulative soil respiration dynamics, explained well the variation of the formation efficiency of mineral associated SOM (MAOM) and, to a lesser extent, that of occluded particulate SOM (oPOM) in aggregates. 90–96% of plant litter-derived C was protected in the vermiculite material and 33–60% in the pure kaolinite and illite materials. The pure vermiculite material showed the greatest fractions of MAOM and oPOM, the highest relative abundances of O–alkyls and anomeric from carbohydrates in the MAOM fraction. < 1% of plant litter-derived C was transformed into microbial biomass and residues, and fungal residues only were associated with the pure vermiculite material. These results suggested that plant litter incorporated into the soil was decomposed mainly through the ex vivo modification pathway and transformed into SOM through both mineral-organic association and physical protection pathways. Plant litter-derived C was likely protected through mineral-organic association from the earlier stages of decomposition by vermiculite than kaolinite and illite, resulting in more plant carbohydrates associated with vermiculite. Therefore, clay minerals determined SOM formation pathways and played a greater role in controlling SOM formation efficiency than plant litter quality.

Characterizing porous microaggregates and soil organic matter sequestered in allophanic paleosols on Holocene tephras using synchrotron-based X-ray microscopy and spectroscopy

Allophanic tephra-derived soils can sequester sizable quantities of soil organic matter (SOM). However, no studies have visualized the fine internal porous structure of allophanic soil microaggregates, nor studied the carbon structure preserved in such soils or paleosols. We used synchrotron radiation-based transmission X-ray microscopy (TXM) to perform 3D-tomography of the internal porous structure of dominantly allophanic soil microaggregates, and carbon near-edge X-ray absorption fine-structure (C NEXAFS) spectroscopy to characterize SOM in ≤ 12,000-year-old tephra-derived allophane-rich (with minor ferrihydrite) paleosols. The TXM tomography showed a vast network of internal, tortuous nano-pores within an allophanic microaggregate comprising nanoaggregates. SOM in the allophanic paleosols at four sites was dominated by carboxylic/carbonyl functional groups with subordinate quinonic, aromatic, and aliphatic groups. All samples exhibited similar compositions despite differences between the sites. That the SOM does not comprise specific types of functional groups through time implies that the functional groups are relict. The SOM originated at the land/soil surface: ongoing tephra deposition (intermittently or abruptly) then caused the land-surface to rise so that the once-surface horizons were buried more deeply and hence became increasingly isolated from inputs by the surficial/modern organic cycle. The presence of quinonic carbon, from biological processes but vulnerable to oxygen and light, indicates the exceptional protection of SOM and bio-signals in allophanic paleosols, attributable both to the porous allophane (with ferrihydrite) aggregates that occlude the relict SOM from degradation, and to rapid burial by successive tephra-fallout, as well as strong Al-organic chemical bonding. TXM and C NEXAFS spectroscopy help to unravel the fine structure of soils and SOM and are of great potential for soil science studies.

Applying CCB to predict management change affected long‐term SOM turnover of the Extended Static Fertilization Experiment in Bad Lauchstädt

AbstractLong‐term field experiments (LTEs) are invaluable in improving understanding of soil organic matter (SOM) turnover, as some of the involved processes have proceeded over centuries. Prediction of such slow carbon fluxes depends especially on the initialization of slow‐reacting model pools and requires monitoring for a very long time for evaluation. This study reports soil organic matter (SOM) modelling covering more than 100 years for a dataset of 36 treatments of the “Extended Static Experiment” that is based on the classical “Static Experiment Bad Lauchstädt.” The experimental scheme includes different fertilization treatments where the plots are arranged in two field stripes. A special feature of this experiment is the management change for each stripe after seven decades of continuous cultivation that includes a considerable alteration of organic amendments on different SOM levels. The Candy Carbon Balance (CCB) model was calibrated for soil organic carbon (SOC) dynamics regarding initial SOC content and one parameter describing the physical protection of SOM. Calibration was performed for each plot individually, for each stripe, and for the whole experiment, resulting in a relative root mean square error (rRMSE, (−)) of 0.050, 0.058 and 0.064, respectively. Model predictions for soil organic nitrogen (SON) dynamics were satisfactory using the calibration results for SOC and the average C/N ratio from all observations within this dataset (11.8) for model initialization. The model error for SON prediction reached from rRMSE = 0.09 (whole experiment) over rRMSE = 0.085 (each stripe) to rRMSE = 0.084 (each plot). Calibration results suggest some heterogeneity between the two field stripes. Restarting the model after the management change in 1978 using the plot‐specific calculated SOC value from previous simulations for the whole time period provided different results of predicted SOC trends and values, suggesting that the precondition of SOM in steady state was still not fulfilled after seven decades.Highlights SOC and SON were modelled for 36 treatments over more than one century. Model restarts after seven decades indicated that SOM pools were not in steady state. Modelling SON required only an additional input of the average C/N ratio of SOM Effect of serious fertilization changes can be well predicted by the tested model setup

Soil organic matter content and chemical composition under two rotation management systems in a Mediterranean climate

The crop rotation system in organic farming is a determinant factor to accumulate and preserve soil organic matter (SOM), and in depth knowledge on its effects is still lacking. Tillage intensity in particular is crucial to maintain soil aggregates and protect SOM from degradation. The evolution of SOM was tested in two adjacent fields under two different rotation cropping systems (low-intensity tillage and high-intensity tillage), and the effect of a further cultivation of legume in both fields was evaluated using 13 carbon (C)-nuclear magnetic resonance (NMR) and elemental analysis of samples isolated through combined aggregate size and density fractionation. The two adjacent fields had been managed using the following organic farming methods for 13 seasons since 1998: i) alfalfa-based, with nitrogen (N) enrichment and low-frequency tillage with alfalfa (Medicago sativa) (9 seasons), winter wheat (Triticum durum) (3 seasons), and broad bean (Vicia faba) (1 season) and ii) cereal-based, with N depletion and annual tillage with barley (Hordeum vulgare) (7 seasons), sunflower (Helianthus annuus) (2 seasons), broad bean (Vicia faba) (3 seasons), and bare fallow (1 season). Soil sampling was carried out at the end of the 13-year rotation (T0, November 2011) and after winter wheat and chickpea cultivation in both fields over two subsequent years (T1, July 2013). Bulk organic C was significantly higher in the alfalfa-based system than in the cereal-based system at both T0 and T1, with SOM occluded in soil aggregates and associated with mineral particles. In terms of the macroaggregates heavy fraction at T0, the alfalfa-based field contained twice the organic C of that in the cereal-based field, as well as three times the organic C in the occluded particulate organic matter (POM). The occluded POM (oPOM) had a lower aryl/O-alkyl C ratio in the alfalfa-based system than in the cereal-based system, suggesting that oPOM undergoes a lower degree of decomposition during low-intensity management. The aryl/O-alkyl C ratios of the macro- and microaggregate oPOM decreased from T0 to T1 in the cereal-based system, suggesting increased protection of these fractions by soil aggregates. Thus, including legumes in crop rotation appears to positively affect the accumulation of SOM associated with mineral particles and within soil aggregates.

Woody peat addition increases soil organic matter but its mineralization is affected by soil clay in the four degenerated erodible soils

Soil erosion exacerbates the reduction of soil organic matter (SOM) and destruction of soil physical structure in agricultural systems, which seriously threatens agricultural ecological environment and global sustainable agriculture development. Owing to rich SOM and nutrients of woody peat results in it’s widely applied in degraded soil for improving land productivity. But how the impacted of woody peat addition on crop growth and soil organic matter (SOM) in different textured degenerated erodible soils are not well understood. Here, along the latitudinal gradient from north to south of China, four typical regional soils collected in Northeast (black soil), Northwest (loessial soil), Central (lou soil) and South (red soil) of China, where suffered strongly soil erosion. Soil clay content was 37.0%, 16.9%, 29.6%, and 38.5% for the black, loessial, lou, and red soils, respectively. We conducted a plot incubation experiment in a fully ventilated, rainproof outdoor greenhouse for planting wheat (Triticum aestivum L.). The study is aimed to investigate the response of different texture eroded soil to crop growth and physiology, soil evaporation and soil organic matter after woody peat addition. Eight treatments have been designed for four soils with two groups. (i) the control group with no woody peat addition, B0, H0, L0 and R0; (ii) woody peat addition group, BW, HW, LW and RW. Overall, the SOM concentration altered wheat growth characteristic and grain yield in the four typical erosion degraded soils. The highest grain yield of winter wheat is 6.5 ± 1.3 g per plot in B0 and 6.5 ± 1.1 g per plot in BW, and the lowest is 0.4 ± 0.1 g per plot in R0 in red soil. But woody peat addition increased the wheat yield or yield variation, the wheat gain yield increased four times with woody peat addition (RW) than control (R0) in red soil. In addition, the results revealed that the SOM content were increased in black soil by 40.4%, in loessial soil by 40.0%, in lou soil by 101.3%, and in red soil by 94.6% when compared to the control treatments at harvest. From sowing to harvest, SOM decreased by 5.1 ± 1.8 g kg-1 (black soil), 9.9 ± 0.9 g kg-1 (loessial soil), 7.6 ± 1.8 g kg-1 (lou soil), and 6.2 ± 1.0 g kg-1 (red soil) in the soils with woody peat addition. Without woody peat addition, SOM decreased by 5.6 g kg-1 (black soil), 4.7 g kg-1 (lou soil), and 0.6 g kg-1 (red soil), but increased by 0.4 g kg-1 in loessial soil. In conclusion, woody peat addition resulted in wheat yield increasing in the degenerated soils, but it’s significant in the red soil with adequate nutrient supply and increased soil acid buffering capacity. Woody peat addition increased the SOM concentration in the four degenerated soils, but a faster decreasing rate of the ΔSOM in coarse-textured loessial soil than others medium-fine texture soils. Therefore, appropriate woody peat application and clay content is more conducive to the protection of SOM. More deeply research should have been focused on the impact of woody peat addition in coarse-textured soil on the soil environment (such as the clay type, content, and soil priming effect, etc.).