Differential response of mineral-associated organic matter in tropical soils formed in volcanic ashes and marine Tertiary sediment to treatment with HCl,NaOCl, and Na4P2O7

Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using ¹³C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na₄P₂O₇ under pasture (C₄) and secondary forest (C₃). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm^?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na₄P₂O₇ extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.     

In search of stable soil organic carbon fractions: a comparison of methods applied to soils labelled with 14C for 40 days or 40 years

A reliable method for the isolation of a stable fraction of soil organic carbon (SOC) would be very helpful for improving our understanding of the mechanisms responsible for stabilization of SOC and the dynamics of SOC turnover. We tested acid hydrolysis, physical fractionation (particle density/size), photo‐oxidation, treatment with chemical oxidants (NaOCl and NaS₂O₈) and thermal treatment on two soils incubated with ¹⁴C‐labelled barley straw for either 40 days or 40 years. Different intensities of the treatments were included. Acid hydrolysis, photo‐oxidation and treatment with a chemical oxidant consistently removed more 40‐year‐old C than 40‐day‐old C, which suggests that the isolated fractions contained a large proportion of material with a relatively rapid turnover. The clay + silt associated SOC fraction contained a small proportion of 40‐day‐old C and a large proportion of 40‐year‐old C. This is consistent with a SOC fraction with medium turnover. The thermal treatment removed more 40‐year‐old C than 40‐day‐old C. At 400°C there was still a small proportion of the 40‐year‐old C remaining, whereas almost all the 40‐day‐old C was removed. This is consistent with a stable SOC fraction. However, because only 2–3% of the C remained after this treatment, the isolated SOC fraction may be of little quantitative importance. Furthermore, the thermally resistant fraction is likely to be heavily altered by the treatment, and therefore unsuitable for further studies of the chemical nature of stable SOC.     

Chemical stabilization of organic carbon pools in particle size fractions in no-till and meadow soils

The knowledge about the relevance of physical and chemical fractionation methods to soil organic carbon (SOC) stabilization mechanisms is fragmentary but needed to manage the SOC pool. Therefore, our objective was to compare the C contents of the particle size fractions coarse and fine sand, silt, and clay of the two uppermost horizons of a soil under three different management systems (meadow; no-till corn, NT; no-till corn with manure, NTm). The mineral composition was dominated by silt (48–60%). However, coarse sand and clay showed the highest enrichment of C compared to the bulk soil. In spite of an enrichment factor below 1, the high proportion of silt made this fraction the main C store. In the upper 30 cm, this fraction amounted to 27.1 Mg C ha⁻¹ in NTm and progressively less in NT (15.5 Mg C ha⁻¹), and meadow (14.9 Mg C ha⁻¹), representing 44%, 39%, and 39% of the total SOC pool, respectively. The C in the isolated particle size fractions was further investigated by an oxidizing treatment with Na₂S₂O₈ and a treatment with HF to solubilize the mineral phases. The pools of oxidizable C were comparable among particle size fractions and pedons, as indicated by Na₂S₂O₈ treatment. The pools of C preferentially associated with soil minerals were also comparable among pedons, as indicated by HF treatment. However, NTm stored the largest pool (12.6 Mg ha⁻¹) of mineral-associated C in 0–30 cm depth. The silt-associated and mineral-bound SOC pool in NTm was greater compared to NT due to increased organic matter (OM) input. Thus, the silt particle size fraction at the North Appalachian Experimental Watershed (NAEW) has the potential for SOC sequestration by stabilizing OM inputs. Mineralogical and molecular level analyses on a larger set of fractions obtained from entire rooted soil profiles are required, however, to compare the SOC sequestration capacity of the land uses.     

Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Fe_o; Al_o), and dithionite‐extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.     

Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils

After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root‐cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra‐microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation‐resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF‐res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N/ha) from undisturbed soil and coarse fPOM N (kg N/ha), silt and clay N (kg N/ha) and its C:N ratio (R² = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF‐res OC (% of soil organic carbon) and its C:N ratio (R² = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio‐available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils.     

Soil‐organic‐matter stability in sandy cropland soils is related to land‐use history

Sandy cropland soils in NW Europe were found to contain unusually high organic‐carbon (OC) levels, and a link with their land‐use history has been suggested. This study's aim was to assess the discriminating power of physical and chemical fractionation procedures to yield information on soil‐organic‐matter (OM) stability for these soils. In relict‐ and cultivated‐heathland soils, much higher proportions of 6% NaOCl treatment–resistant but 10% HF–soluble OC (MOC) and N (32.2% and 29.9%) were measured compared to a set of “permanent"‐cropland soils without a history of heathland land use (11.9% and 8.5%). Also, the proportions of 6% NaOCl– and 10% HF treatment–resistant OC and N in the relict and cultivated heathlands (19.2% and 12.0%) were higher than in the permanent‐cropland soils (17.7% and 5.7%). Stepwise multiple linear‐regression yielded a significant relationship between the annual mineralization (g C [100 g OC]^–1), soil OC (g C kg^–1) content, and %MOC: Annual mineralization = 4.347 – 0.087 soil OC – 0.032 %MOC (R² = 0.65). Combinations of incubation experiments for quantification of the labile soil OM pool with chemical fractionation may thus yield meaningful data for development of soil‐organic‐matter models with measurable pools, but their applicability will be limited to specific combinations of former land use with soil, climate, and current management.     

Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents

Oxidative treatment can isolate a stable organic matter pool in soils for process studies of organic matter stabilization. Wet oxidation methods using hydrogen peroxide are widely used for that purpose, but are said to modify poorly crystalline soil constituents. We investigated the effect of a modified NaOCl oxidation (pH 8) on the mineral composition of 12 subsoils (4.9–38.2 g organic C kg^?1) containing varying amounts of poorly crystalline mineral phases, i.e. 1.1–20.5 g oxalate‐extractable Fe kg^?1, and of different phyllosilicate mineralogy. Post‐oxidative changes in mineral composition were estimated by (i) the determination of elements released into the NaOCl solution, (ii) the difference in dithionite‐ and oxalate‐extractable Si, Al and Fe, and (iii) the specific surface areas (SSAs) of the soils. The NaOCl procedure reduced the organic C concentrations by 12–72%. The amounts of elements released into the NaOCl extracts were small (≤ 0.14 g kg^?1 for Si, ≤ 0.13 g kg^?1 for Al, and ≤ 0.03 g kg^?1 for Fe). The SSA data and the amounts of dithionite‐ and oxalate‐extractable elements suggest that the NaOCl oxidation at pH 8 does not attack pedogenic oxides and hydroxides and only slightly dissolves Al from the poorly crystalline minerals. Therefore, we recommend NaOCl oxidation at pH 8 for the purpose of isolating a stable organic matter pool in soils for process studies of organic matter stabilization.     

Depletion of soil organic carbon and nitrogen under Pinus taeda plantations in Southern Brazilian grasslands (Campos)

Establishment of pine (Pinus spp.) plantations on grasslands could increase carbon (C) sequestration to counteract increased atmospheric carbon dioxide concentrations. In the grasslands of the southern Brazilian highland (Campos), large areas have been converted to Pinus plantations over the last 30 years. In order to assess the impact of this land‐use change on the amount and composition of soil organic matter (SOM), we investigated a grassland pasture site (G), and both an 8‐year‐old (P8) and a 30‐year‐old (P30) plantation with Pinus taeda. Soil samples down to 45 cm were analysed for texture, pH, soil organic carbon (SOC) and total nitrogen (N_tot) concentrations. Chemical composition of SOM was determined by using cross‐polarization magic angle spinning (CPMAS) ¹³C NMR spectroscopy. We analysed for stable C isotope (δ¹³C) and assessed the lignin composition by CuO oxidation. Additionally, contents of pyrogenic organic material (PyOM) were determined because the Campos is regularly burnt. Both pine plantations revealed relatively small SOC concentrations in the mineral soil of 72.6 mg g^?1 (P8) and 56.8 mg g^?1 (P30) and N_tot concentrations of 4.0 mg g^?1 (P8) and 2.9 mg g^?1 (P30) for the A horizon, while grassland showed significantly (P < 0.01) larger contents of 100.2 mg g^?1 for SOC and 5.9 mg g^?1 for N_tot. Accumulation of litter layers suggests decreased input of organic material into the mineral soil under pine, which was confirmed by the δ¹³C values and lignin composition. Smaller contents of vanillyl‐ (V), syringyl‐ (S), and cinnamyl (C)‐phenols, smaller ratios of S/V and C/V, and smaller ratios of acidic to aldehydic forms of V and S phenols indicated a high degree of decomposition of residual grass‐derived SOM in the upper part of the mineral soil (0–10 cm) under pine plantations. This was confirmed by CPMAS ¹³C NMR spectroscopy, showing an increasing Alkyl C/O‐Alkyl C ratio at the same depth. No significant changes in the contents of PyOM could be detected, but all sites tended to show the greatest concentrations at deeper soil depths > 15 cm, indicating a vertical relocation of PyOM. The results suggest that decomposition of residual SOM originating from grassland species contributes to the decrease of SOC and N_tot and to an acidification in the topsoil under pine plantations. We also suggest that slow litter decomposition and incorporation and the absence of fires at the plantations are additional reasons for the reduced amount of SOM. Depletion of SOM and the acidification of the topsoil may reduce the availability and supply of nutrients and diminish the C sequestration potential of the mineral soil.     

Using soil organic matter fractions as indicators of soil physical quality

The objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM ), rather than SOM per se , as indicators of soil physical quality (SPQ ) based on their effect on aggregate stability (AS ). Chemically extracted humic and fulvic acids (HA and FA ) were used as chemical fractions, and heavy and light fractions (HF and LF ) obtained by density separation as physical fractions. The analyses were conducted on medium‐textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC ), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se . In both geographical regions, soils under cropland showed the smallest content of SOC , HA and carbon concentration in LF and HF , and the largest HF /LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA /SOC (r  = −0.83*^*), HA /FA (r  = 0.58*^*), carbon concentration of LF (r  = 0.69*^*) and HF (r  = 0.70*^*) and HF /LF ratio (r  = 0.80*^*), cropland showed lowest AS . These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.     

Is thermal oxidation at different temperatures suitable to isolate soil organic carbon fractions with different turnover?

Findings of previous studies suggest that there are relations between thermal stability of soil organic matter (SOM), organo‐mineral associations, and stability of SOM against microbial decay. We aimed to test whether thermal oxidation at various temperatures (200°C, 225°C, 275°C, 300°C, 400°C, or 500°C) is capable of isolating SOM fractions with increasing stability against microbial degradation. The investigation was carried out on soils (Phaeozem and Luvisol) under different land‐use regimes (field, grassland, forest). The stability of the obtained soil organic carbon (SOC) fractions was determined using the natural‐¹³C approach for continuously maize‐cropped soils and radiocarbon dating. In the Luvisol, thermal oxidation with increasing temperatures did not yield residual SOC fractions of increasing microbial stability. Even the SOC fraction resistant to thermal oxidation at 300°C contained considerable amounts of young, maize‐derived C. In the Phaeozem, the mean ¹⁴C age increased considerably (from 3473 y BP in the mineral‐associated SOC fraction to 9116 y BP in the residual SOC fraction after thermal oxidation at 300°C). An increasing proportion of fossil C (calculated based on ¹⁴C data) in residual SOC fractions after thermal oxidation with increasing temperatures indicated that this was mainly due to the relative accumulation of thermally stable fossil C. We conclude that thermal oxidation with increasing temperature was not generally suitable to isolate mineral‐associated SOC fractions of increasing microbial stability.     

A rapid method for the determination of fixed ammonium in soil

Abstract

Five different procedures for the oxidation of organic carbon were compared in order to determine the most suitable method for use as a pretreatment before the determination of fixed NH₄‐N in soils. Three of the procedures involved a method based on the use of a solution containing 0.1M Na₄P₂O₇, 0.1M NaCl, and 6% H₂O₂, applied for three different periods of time, 10 days, 20 days, and whatever number of days are required until frothing no longer occurs. This third period of time is designated as NF (no frothing). The remaining two procedures involved a method based on the use of a 3M NaOCl solution applied for two different periods of time, 3‐cycles and 5‐cycles.

Two of the procedures, the NF treatment with H₂O₂+NaCl+Na₄P₂O₇ and the 5‐cycle treatment with NaOCl, were found to give the must efficient removal of organic C. The amount of residual N in the soil after the NF treatment was highly correlated with the amount of residual N in the soil after the 5‐cycle treatment. It is concluded that the 5‐cycle treatment is as effective as the NF treatment for the oxidation of organic C in the soil before carrying out the determination of fixed NH₄‐N. The 5‐cycle treatment has the advantage of being a faster and simpler analytical procedure than the NF treatment.     

Impact of Soil Management on Organic Carbon Content and Aggregate Stability

Abstract

Soil cultivation influences organic carbon storage and soil structures. To evaluate the impact of different soil‐management practices on soil organic carbon (SOC) pools and aggregate stability in black soils, SOC in whole soil, various size aggregates, and density‐separated fractions from three long‐term experiments (20 years) was examined. The three soil‐management systems were grassland (GL), bare land (BL), and croplands. The croplands had two treatments: nitrogen and phosphorus fertilizer application (NP) and NP together with organic manure (NPM). The SOC in the 0‐ to 10‐cm layer decreased in the order NPM>GL>NP>BL and also declined with the soil depth. The SOC of GL increased by 9.7% as compared to NP after 20 years of natural vegetation restoration. The SOC of NPM increased by 11% over NP after 13 years of organic manure application. The percentages of water‐stable aggregate (>0.25 mm) (WSA_>0.25mm) decreased in the order GL>BL>NPM>NP in the top 0‐ to 20‐cm horizon. WSA_>2mm, the most important fraction for carbon (C) storage in GL and NPM, accounted for 33 and 45% of the whole soil for GL in the depths of 0–10 and 10–20 cm, respectively, and 25 and 18% for NPM in the same soil layers. A significant positive correlation was found between the C stored in WSA_>2mm and total SOC (r=0.81, P<0.05) and between the mean weight diameters (MWD) of aggregates and total SOC (r=0.78, P<0.05). Water‐stable aggregate_0.25–2mm was the largest fraction of WSA_>0.25mm, ranging from 54 to 72% for the 0‐ to 10‐cm layer and 46 to 71% for the 10‐ to 20‐cm layer; thus these aggregates would play a major role in soil sustainability as well as the resistance to soil erosion. The organic carbon (OC) of heavy fraction (HF) accounted for 94–99% of the OC in the WSA_0.25–2mm, whereas free particulate organic matter (fPOM) and occluded particulate organic matter (oPOM) contributed a minor fraction of the OC in the WSA_0.25–2mm, suggesting that C sequestration in HF could enhance the stability of aggregates and C pools in black soil.     

Characterization of recently 14C pulse-labelled carbon from roots by fractionation of soil organic matter

The inability of physical and chemical techniques to separate soil organic matter into fractions that have distinct turnover rates has hampered our understanding of carbon (C) and nutrient dynamics in soil. A series of soil organic matter fractionation techniques (chemical and physical) were evaluated for their ability to distinguish a potentially labile C pool, that is ‘recent’ root and root‐derived soil C. ‘Recent’ root and root‐derived C was operationally defined as root and soil C labelled by ¹⁴CO₂ pulse labelling of rye grass–clover pasture growing on undisturbed cores of soil. Most (50–94%) of total soil + root ¹⁴C activity was recovered in roots. Sequential extraction of the soil + roots with resin, 0.1 m NaOH and 1 m NaOH allocated ‘recent’ soil + root ¹⁴C to all fractions including the alkali‐insoluble residual fraction. Approximately 50% was measured in the alkali‐insoluble residue but specific activity was greater in the resin and 1 m NaOH fractions. Hot 0.5 m H₂SO₄ hydrolysed 80% of the ¹⁴C in the alkali‐insoluble residue of soil + roots but this diminished specific activity by recovering much non‐¹⁴C organic matter. Pre‐alkali extraction treatment with 30% H₂O₂ and post‐alkali treatment extractions with hot 1 m HNO₃ removed organic matter with a large ¹⁴C specific activity from the alkali‐insoluble residue. Density separation failed to isolate a significant pool of ‘recent’ root‐derived ¹⁴C. The density separation of ¹⁴C‐labelled roots, and roots remixed with non‐radioactive soil, showed that the adhesion of soil particles to young ¹⁴C‐labelled roots was the likely cause of the greater proportion of ¹⁴C in the heavy fraction. Simple chemical or density fractionations of C appear unsuitable for characterizing ‘recent’ root‐derived C into fractions that can be designated labile C (short turnover time).     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

The effects of long-term superphosphate application on soil organic matter content and composition from an intensively managed New Zealand pasture

Changes in quantity and composition of soil organic matter (SOM) in pasture receiving annual superphosphate (SP) applications for 41 years at 0 (control), 188 and 376 kg SP ha^?1a^?1 were investigated in soil samples collected from 0–75 and 75–150 mm depths by determining total carbon (TC), total nitrogen (TN), biomass C (BC), biomass N (BN) and subjecting the soils to sequential extraction using cold water, hot water, a mixture of hydrochloric (0.1 M HC1) and hydrofluoric (0.3 M HF) acids (HCl/HF), and sodium pyrophosphate (Na₄P₂O₇) followed by sodium hydroxide (NaOH) for extracting labile and stable SOM fractions. There were significant differences in some SOM fractions between control (0) and SP treatments (188 and 376), especially in the topsoil (0–75 mm) but these were not observed between the two SP treatments. Soil TN (0–75 mm), BN and BN: TN ratio (0–75 and 75–150 mm depths) and the proportion of hot–water–extractable C (HC) in soil TC (HC:TC) (0–75 mm) were significantly greater in the SP treatments than in the control. HC1/HF extractable C and the proportions of soil TC as HC1/HF extractable C (HC1/HF extractable C: TC) were smaller in the topsoil of SP treatments than in the control. Similar results were observed in humin N: TN ratio and the proportions of soil TC as cold–water–extractable carbohydrate (CWcC: TC) and of soil HC as hot–water–extractable carbohydrate (HWcC : HC). Increases in the proportion of labile fraction in SOM were reflected in values of BN, BC: BN, BN: TN, HWcC : HC and HC: TC whereas decreases in the proportion of stable fraction in SOM were found in humin N: TN and HCl/HF–extractable C: TC ratios. Increases in labile SOM (BN and N–containing compounds such as amino acids and amino sugars, which were extractable by hot water but were not present as carbohydrate) and decreases in stable SOM (HC1/ HF–extractable C and humin fraction) in soils under pastures treated with annual SP applications compared with the control were attributed to pasture improvement and the amelioration of P and S deficiency, resulting in a greater return of plant residues and animal excreta and also an increase in clover growth and associated biological N₂ fixation. The additional labile SOM in SP treatments compared with that of the control was not associated with the soil mineral Al and Fe components.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Carbon input from 13C-labeled crops in four soil organic matter fractions

This study quantified the fate of new carbon (C) in four crop sequences (lentil–wheat, canola–wheat, pea–wheat, and continuous wheat). Lentil–wheat and continuous wheat were grown in intact soil cores from a Brown Chernozem (BCz) and canola–wheat, pea–wheat, and continuous wheat in cores from a Dark Brown Chernozem (DBCz). In the first growing cycle, plants were pulse-labeled with ¹³C-CO₂. Soil ¹³C pools were measured once after the labeled growing cycle to quantify root biomass contribution to soil organic matter (SOM) in a single cycle and again after a second growing cycle to quantify the fate of labeled root and shoot residues. ¹³C was quantified in four SOM fractions: very light (VLF), light (LF), heavy (HF), and water extractable organic matter (WEOM). For BCz lentil, BCz wheat, DBCz canola, DBCz pea, and DBCz wheat in the labeling year, root-derived C estimates were 838, 572, 512, 397, and 418 mg of C per kg soil, respectively. At the end of the second growing cycle, decreases in root-derived C were greater in the VLF, which lost 62 to 95 % of its labeled ¹³C, than the LF (lost 21 to 56 %) or HF (lost 20 to 47 %). Root-derived C in WEOM increased 38 to 319 %. On DBCz, even though wheat and pea produced less aboveground biomass than canola, they generated similar amounts of SOC by fraction indicating that their residues were more efficiently stabilized into the soil than canola residues. Combining ¹³C repeat-pulse labeling and SOM fractionation methods allowed new insights into C dynamics under different crop sequences and soil types. This combination of methods has great potential to improve our understanding of soil fertility and SOM stabilization.     

Biogeochemical factors related with organic matter degradation and C storage in agricultural volcanic ash soils

Laboratory incubation experiments in addition to physicochemical analyses of volcanic ash soils were carried out in order to identify biogeochemical factors related with soil organic C (SOC) stabilization in the long term and with the potential for C sequestration of agroecosystems. Up to 24 vineyard plots under similar subtropical conditions in Tenerife Island (Spain) were sampled. Soil samples were incubated for 30 days in laboratory conditions (27 °C and 66% water holding capacity) and the CO₂ released was periodically measured to plot C mineralization curves. Soil organic matter (SOM) with special emphasis paid on the humic acid (HA) was characterized by elemental composition, spectroscopic techniques: visible, infrared (IR) and ¹³C nuclear magnetic resonance (¹³C NMR) and analytical pyrolysis–gas chromatography/mass spectrometry (GC/MS). The dependent variables examined were either the total mineralization coefficient (TMC, g C · kg C soil^?1 day^?1) in laboratory incubations, or the SOC. A very significant negative correlation was found between SOC and TMC, i.e., in our soils, the higher the biodegradation rates under laboratory conditions, the lower the soil C sequestered in the corresponding plots. In it was also observed that the concentration of amorphous minerals (Al_o + ½ Fe_o index) and the water holding capacity at 0.033 MPa were associated with lower CO₂ release; the latter could suggest microanaerobic conditions hampering biodegradation in these thixotropic soils. Conversely, no correlation was found between SOC or TMC and typical soil physical and chemical factors, such as granulometric fractions or exchangeable calcium. The molecular characteristics of the HAs showed also predictive potential as regards SOC resilience, reflecting the comparatively fast biodegradation of SOM composed mainly of biomass constituents (prominent lignin signature and O-alkyl ¹³C NMR region). The poor correlation between total aromaticity of the HAs and SOM resistance against biodegradation could be explained by a dual origin of aromatic structures in HAs, either consisting of methoxyl-containing non-decomposed lignin structures or condensed black carbon-like polyaromatic structures. The results suggested the possibility of predicting the vulnerability of SOC to biodegradation from laboratory incubation experiments, which results of interest for modeling global change scenarios.     

A simplified modelling approach for quantifying tillage effects on soil carbon stocks

Soil tillage has been shown to affect long‐term changes in soil organic carbon (SOC) content in a number of field experiments. This paper presents a simplified approach for including effects of tillage in models of soil C turnover in the tilled‐soil layer. We used an existing soil organic matter (SOM) model (CN‐SIM) with standard SOC data for a homogeneous tilled layer from four long‐term field experiments with conventionally tilled (CT) and no‐till (NT) treatments. The SOM model was tested on data from long‐term (>10 years) field trials differing in climatic conditions, soil properties, residue management and crop rotations in Australia, Brazil, the USA and Switzerland. The C input for the treatments was estimated using data on crop rotation and residue management. The SOM model was applied for both CT and NT trials without recalibration, but incorporated a ‘tillage factor’ (TF) to scale all decomposition and maintenance parameters in the model. An initial value of TF = 0.57 (parameter uncertainty, PU = 0.15) for NT (with TF set to 1.0 for CT) was used on the basis of a previous study with observations of soil CO₂ respiration. The simulated and observed changes in SOC were then compared using slopes of linear regressions of SOC changes over time. Results showed that the SOM model captured observed changes in SOC content from differences in rotations, N application and crop residue management for conventional tillage. On the basis of SOC change data a mean TF of 0.48 (standard deviation, SD = 0.12) was estimated for NT. The results indicate that (i) the estimated uncertainty of tillage effects on SOC turnover may be smaller than previously thought and (ii) simple scaling of SOM model parameters may be sufficient to capture the effects of soil tillage on SOM turnover in the tilled layer. Scenario analyses showed that the average extra C input needed to compensate for soil tillage was 762 (SD = 351) kg C ha⁻¹ year⁻¹. Climatic conditions (temperature and precipitation) also affected how much extra C was needed, with substantially larger inputs being required for wetter and warmer climates.     

Effects of different quality plant residues on soil carbon accumulation and aggregate formation in a tropical sandy soil in Northeast Thailand as revealed by a 10‐year field experiment

Particulate organic matter (POM) plays important role in soil organic carbon (SOC) retention and soil aggregation. This paper assesses how quality (chemical composition) of four different‐quality organic residues applied annually to a tropical sandy loam soil for 10 years has affected POM pools and the development of soil aggregates. Water‐stable aggregate size distribution (>2, 0·25–2, 0·106–0·25 mm) was determined through wet sieving. Density fractionation was employed to determine POM (light—LF, and heavy—HF fractions, 0·05–1 mm). Tamarind leaf litter showed the highest SOC (<1 mm) accumulation, while rice straw showed the lowest. LF‐C contents had positive correlations with high contents of C and recalcitrant constituents, (i.e. lignin and polyphenols) of the residues. Dipterocarp, a resistant residue, showed the highest LF‐C, followed by the intermediate residues, tamarind, and groundnut, whereas HF was higher in groundnut and tamarind than dipterocarp residues. Rice straw had the lowest LF‐ and HF‐C contents. Tamarind had the highest quantity (51 per cent) of small macroaggregates (0·25–2 mm), while dipterocarp had the most (2·1 per cent) large macroaggregates (>2 mm). Rice straw had the lowest quantities of both macroaggregates. Similar to small‐sized HF (0·05–0·25 mm), small macroaggregates had positive correlation with N and negative correlation with C/N ratios, while large macroaggregates had positive correlations with C and recalcitrant constituents of the residues. Tamarind, with intermediate contents of N and recalcitrant compounds, appears to best promote small macroaggregate formation. Carbon stabilized in small macroaggregates accounted for the tamarind treatment showing the largest SOC accumulation. Copyright © 2010 John Wiley & Sons, Ltd.