Characterization of organic matter fractions in the top layer of soils under different land uses in Central‐Eastern Europe

The objective of this study was to investigate differences in organic matter fractions, such as dissolved organic carbon and humic substances, in soils under different land uses. Soil samples were collected from the upper layer of arable lands and grasslands. Humic substances (HS) were chemically fractionated into fulvic acids (FA), humic acids (HA) and humins (HUM), and based on the separated fractions, the humification index (HI) and the degree of HS transformation (DT) were calculated. Dissolved organic carbon (DOC) was determined by cold (CWE) and hot water (HWE) extractions. Regardless of land use, the results indicated significant differences in soil organic carbon (SOC) and HS composition, with HA and HUM as the dominant fractions. Total SOC was higher in grassland (median = 17.51 g kg^?1) than arable soils (median = 9.98 g kg^?1); the HI and DT indices did not differ significantly between land uses (HI = 0.3–10.3 and DT = 0.2–6.2 for grasslands, p > 0.05; HI = 0.3–3.9 and DT = 0.2–20.1 for arable lands, p > 0.05). This indicates the relatively high stability of organic carbon and efficient humification processes in both land uses. Additionally, in arable soils lower CWE‐C (0.75 g kg^?1) and higher HWE‐C (2.59 g kg^?1) than in grasslands (CWE‐C = 1.13 g kg^?1, HWE‐C = 1.60 g kg^?1) can be related to farming practice and application of soil amendments. The results showed that both labile and humified organic matter are better protected in grassland soils and are consequently less vulnerable to mineralization.     

Factors controlling mineralization of soil organic matter in the Eurasian steppe

To understand the dynamics of soil organic matter (SOM) in the Eurasian steppe, several soil and meteorological properties were tested in order to estimate the amounts of potentially mineralizable organic carbon (PMC) and nitrogen (PMN). Total 41 surface soil samples were collected in Ukraine and Kazakhstan from cropland, forest, grassland, and desert ecosystems. The fresh soils were incubated for 133 days under constant temperature and moisture conditions, and the CO₂ emissions and the mineral N from the soils were monitored. PMC and PMN were determined by fitting models to the cumulative curves of the CO₂ and the mineral N. Tested soil properties included soil pH, sand, silt and clay contents, carbon and nitrogen contents of light fraction (LF, <1.6 g cm^?3) and heavy fraction (HF), and C/N ratio of LF and HF. The meteorological properties considered were mean annual temperature and precipitation. Using multiple regression analysis with the stepwise method, PMC was well estimated by carbon content of LF (LFC) and clay content, compared to the simple correlation with organic carbon (OC). Similarly, PMN was better determined by nitrogen content of LF (LFN) and clay content. These results suggest the partially labile nature of clay-associating OM and of LFC and LFN. The higher PMC and PMN in the forest and grassland sites would be attributed to the higher LFC and LFN, while the lower LFC and LFN in cropland sites would suggest the relatively higher contribution of clay-associating OM to PMC and PMN.     

Structural changes in humic substances after long-term fertilization of a calcareous soil with pig slurries

Pig slurries are widely used on calcareous soils in European rainfed systems. Here we assess their impact on the amount of soil organic carbon (SOC) and on the composition of humic-type substances (HTS). Seven doses of slurry (five from fattening pigs and two from sows) ranging from 1.0 to 4.8 Mg ha⁻¹ yr⁻¹ of organic matter were evaluated after a period of 12 years and compared with mineral fertilizer treatment. At the end of the last annual cropping season (September), SOC was quantified, and HTS were isolated by alkaline extraction followed by acid precipitation, and studied by visible spectroscopy (800–400 nm) and Fourier-transformed infrared spectroscopy (4000–400 cm⁻¹). Following the trend in the slurry organic matter applied rates, SOC increased from 9.5 g C kg⁻¹ (mineral treatment) to 13.8 g C kg⁻¹. This SOC increase was equivalent to c. 25.4% of the slurry organic carbon applied. The incorporation of aliphatic structures, mainly polyalkyl, from slurries into the HTS tends to modify the composition of the soil organic matter (SOM), which is reflected in a decrease in the intensity of FT-IR peaks related to aromatic structures. Despite the trend of significant increase in SOC with fattening slurries, mainly from the organic matter rate of 1.6 Mg ha⁻¹ yr⁻¹ (c. 185 kg N ha⁻¹), the composition of the HTS showed an important aliphatic enhancement. The FTIR results showed that using exclusively the relative intensities of specific peaks (alkyl, carboxyl, aromatic and amide groups) as variables for the discriminant analysis, it is possible to identify HA between different groups of soils treated with progressive levels of slurry. Although the new aliphatic components could be considered important to improve soil physical quality, after the incorporation of additional SOM, the spectroscopic characteristics of HTS in soils treated with slurries suggested a weak effect in long-term C sequestration, as the newly incorporated OC forms are not qualitatively similar to the presumably stable native SOM. These potential changes in SOC and SOM composition at field level are constrained by the maximum allowed N rates from organic origin in some agricultural systems.     

Soil organic carbon stocks,distribution, and composition affected by historic land use changes on adjacent sites

Historic alterations in land use from forest to grassland and cropland to forest were used to determine impacts on carbon (C) stocks and distribution and soil organic matter (SOM) characteristics on adjacent Cambisols in Eastern Germany. We investigated a continuous Norway spruce forest (F-F), a former cropland afforested in 1930 (C-F), and a grassland deforested in 1953 (F-G). For C and N stocks, we sampled the A and B horizons of nine soil pits per site. Additionally, we separated SOM fractions of A and B horizons by physical means from one central soil pit per pedon. To unravel differences of SOM composition, we analyzed SOM fractions by ¹³C-CPMAS NMR spectroscopy and radiocarbon analysis. For the mineral soils, differences in total C stocks between the sites were low (F-F = 8.3 kg m⁻²; C-F = 7.3 kg m⁻²; F-G = 8.2 kg m⁻²). Larger total C stocks (+25%) were found under continuous forest compared with grassland, due to the C stored within the organic horizons. Due to a faster turnover, the contents of free particulate organic matter (POM) were lower under grassland. High alkyl C/O/N-alkyl C ratios of free POM fractions indicated higher decomposition stages under forest (1.16) in relation to former cropland (0.48) and grassland (0.33). Historic management, such as burning of tree residues, was still identifiable in the subsoils by the composition and ¹⁴C activity of occluded POM fractions. The high potential of longer lasting C sequestration within fractions of slower turnover was indicated by the larger amounts of claybound C per square meter found under continuous forest in contrast to grassland.     

Organic carbon stocks and soil erodibility in Canary Islands Andosols

Soil organic carbon (SOC) plays a key role in the structural stability of soils and in their resistance against erosion. However, and as far as andic soils are concerned, these mechanisms and processes, as well as the influence of the different types of SOC on aggregate stability, are not fully understood. The targets of this paper are: (i) to determine the content and forms of SOC in Andosols under evergreen forest vegetation [laurel (Laurus) and heather (Erica) forest] and (ii) to find out the role of soil organic matter (SOM) in the aggregate stability and in the resistance of Andosols to water erosion. Soil samples have been collected in 80 sites in a 40 km² area under udic soil moisture regime. In them, fulvic and humic acids, Walkley–Black SOC, pyrophosphate-extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC, acid oxalate-extractable Fe, Al and Si, USLE K-factor and aggregate stability have been determined. The Andosols over volcanic ash are Aluandic Andosols (non-allophanic Andosols), whereas over basaltic lava flows are Silandic Andosols (allophanic Andosols). The surface (0–30 cm) samples analyzed contain 9.5–30 kg C m^− 2 being significantly higher in allophanic Andosols (p < 0.5). Organic carbon adsorbed onto the mineral fraction (extractable pyrophosphate, C_p) accounts for 35–55% of the total SOC. All samples show a high stability to slaking and raindrop impact, being the first one highly correlated (r = 0.6) with pyrophosphate extractable C (C_p), Fe (Fe_p), and Al (Al_p) in allophanic Andosols, unlike non-allophanic ones. The stability to raindrop impact correlates with pyrophosphate extractable C (C_p) and Fe (Fe_p) in both types of soils (r = 0.3–0.6, p < 0.05). These findings suggest that the high stability to both slaking and water-drop impact is due to the occurrence of allophane–Fe–OC complexes, rather than to the total OC, and the active Fe and Al forms, generated by the weathering of volcanic materials, constitute an essential constituent responsible for C sequestration and resistance to degradation in these soils.     

Soil Na+ concentration controls salt-affected soil organic matter components in Hetao region China

Purpose

There is little knowledge on the organic matter fractions of salt-affected soil aggregates. This study aimed at investigating characteristics of salt-affected soil organic carbon components and the relationships between soil salt concentration and soil organic carbon component content.

Materials and methods

Five typical salt-affected soils in Hetao region China were collected and analyzed for light (LF) and heavy fraction (HF) in different water-stable aggregates. And the soil organic carbon components were measured by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS).

Results and discussion

The results showed that the salt-affected soils were dominant in 53–10-μm water-stable aggregates, 61–80% in the bulk soil, and very low in >?250-μm macro-aggregates, less than 7.06% in the bulk soil. The proportions of >?250-μm macro-aggregates and the mean weight diameter (MWD) were negatively correlated to Na⁺ concentration (p?<?0.05). Furthermore, the macro-aggregates were generally higher in total organic carbon (TOC) and accordingly higher C/N ratio than those in micro-aggregates. Heavy fractions (HF) from both >?53 μm and <?53-μm soil aggregates accounted for 99.30–99.83% of the bulk soil and contained 89.6–98.5% lower TOC and accordingly 49.2–84.8% lower C/N ratio than those in light fractions (LF). The LFs were high in lignin (7.27–34.02% in total pyrolysis products, 19.89% on average) and alkane/alkene-derived compounds (9.51–37.21%, 23.18% on average), but low in N-containing compounds (0–3.64%, 1.71% on average), while HFs were high in both alkane/alkene (4.38–27.46%, 15.06% on average) and N-containing compounds (7.45–26.45%, 13.98% on average), but low in lignin-derived compounds (1.13–8.75%, 3.86% on average).

Conclusions

The tested salt-affected soils were predominant in 53–10-μm micro-aggregates, which was caused by the Na⁺ dispersion effect on soil aggregates. Most SOM was stored in HF that contained high N-containing compounds and low C/N ratios. Our results suggested that the components of SOM were mainly controlled by the soil Na⁺ concentration.

     

Landuse impacts on soil organic carbon fractions in different rainfall areas of a subtropical dryland

Landuse can alter soil organic carbon (SOC) fractions by affecting carbon inflows and outflows. This study evaluated changes in SOC fractions in response to different landuses under variable rainfalls. We compared cropland, grassland and forest soils in high rainfall (Islamabad ~1142 mm) and low rainfall (Chakwal ~667 mm) areas of Pothwar dryland, Pakistan. Forest soils in both rainfall areas had highest SOC (11.32 g kg^?1), particulate organic carbon (POC, 1.70 g kg^?1), mineral-associated organic carbon (MOC, 7.17 g kg^?1) and aggregate-associated organic carbon (AOC, 7.86 g kg^?1). However, in rangeland and cropland soils, these varied with rainfall. Under high rainfall, SOC and MOC were 12% and 17% higher in rangeland than in cropland while POC and AOC were equal. Under low rainfall, SOC and MOC were higher in rangeland than in cropland by 7.21 and 1.79 g kg^?1 at 0–15 cm and equal at 15–30 cm depth. POC and AOC were higher in rangeland than in cropland, in both depths. Averagely, SOC, POC, MOC and AOC were 26%, 68%, 76% and 30% higher in high rainfall than in low rainfall soils. Sensitivity of SOC fractions to landuses observed under different rainfalls could provide useful information for soil management in subtropical drylands.     

Soil organic carbon in density fractions of tropical soils under forest – pasture – secondary forest land use changes

Our knowledge of effects of land use changes and soil types on the storage and stability of different soil organic carbon (SOC) fractions in the tropics is limited. We analysed the effect of land use (natural forest, pasture, secondary forest) on SOC storage (depth 0–0.1 m) in density fractions of soils developed on marine Tertiary sediments and on volcanic ashes in the humid tropics of northwest Ecuador. The origin of organic carbon stored in free light (< 1.6 g cm^?3) fractions, and in two light fractions (LF) occluded within aggregates of different stability, was determined by means of δ¹³C natural abundance. Light occluded organic matter was isolated in a first step after aggregate disruption by shaking aggregates with glass pearls (occluded I LF) and in a subsequent step by manual destruction of the most stable microaggregates that survived the first step (occluded II LF). SOC storage in LFs was greater in volcanic ash soils (7.6 ± 0.6 Mg C ha^?1) than in sedimentary soils (4.3 ± 0.3 Mg C ha^?1). The contribution of the LFs to SOC storage was greater in natural forest (19.2 ± 1.2%) and secondary forest (16.6 ± 1.0%) than in pasture soils (12.8 ± 1.0%), independent of soil parent material. The amount of SOC stored in the occluded I LF material increased with increasing silt + clay content (sedimentary soils, r = 0.73; volcanic ash soils, r = 0.58) and aggregation (sedimentary soils, r = 0.52; volcanic ash soils, r = 0.45). SOC associated with occluded I LF, had the smallest proportion of new, pasture‐derived carbon, indicating the stabilizing effect of aggregation. Fast turnover of the occluded II LF material, which was separated from highly stable microaggregates, strongly suggested that this fraction is important in the initial process of aggregate formation. No pasture‐derived carbon could be detected in any density fractions of volcanic ash soils under secondary forest, indicating fast turnover of these fractions in tropical volcanic ash soils.     

Carbon sequestration and relationship between carbon addition and storage under rainfed soybean–wheat rotation in a sandy loam soil of the Indian Himalayas

Soil organic matter (SOM) contributes to the productivity and physical properties of soils. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, no information is available on the effects of long-term manure addition along with mineral fertilizers on C sequestration and the contribution of total C input towards soil organic C (SOC) storage. We analyzed results of a long-term experiment, initiated in 1973 on a sandy loam soil under rainfed conditions to determine the influence of different combinations of NPK fertilizer and fertilizer + farmyard manure (FYM) at 10 Mg ha⁻¹ on SOC content and its changes in the 0–45 cm soil depth. Concentration of SOC increased 40 and 70% in the NPK + FYM-treated plots as compared to NPK (43.1 Mg C ha⁻¹) and unfertilized control plots (35.5 Mg C ha⁻¹), respectively. Average annual contribution of C input from soybean (Glycine max (L.) Merr.) was 29% and that from wheat (Triticum aestivum L. Emend. Flori and Paol) was 24% of the harvestable above-ground biomass yield. Annual gross C input and annual rate of total SOC enrichment were 4852 and 900 kg C ha⁻¹, respectively, for the plots under NPK + FYM. It was estimated that 19% of the gross C input contributed towards the increase in SOC content. C loss from native SOM during 30 years averaged 61 kg C ha⁻¹ yr⁻¹. The estimated quantity of biomass C required to maintain equilibrium SOM content was 321 kg ha⁻¹ yr⁻¹. The total annual C input by the soybean–wheat rotation in the plots under unfertilized control was 890 kg ha⁻¹ yr⁻¹. Thus, increase in SOC concentration under long-term (30 years) rainfed soybean–wheat cropping was due to the fact that annual C input by the system was higher than the required amount to maintaining equilibrium SOM content.     

Free and intra‐aggregate organic matter as indicators of soil quality change in volcanic soils under contrasting crop rotations

Soil physical fractionation techniques may provide indicators of changing soil organic carbon (SOC) content; however, they have not been widely tested on volcanic soils (Andisols). In this study, we assessed two fractions as potential indicators in volcanic soils, using two sites in Chile converted from natural grassland to arable and mixed crop rotations, 8 and 16 yr previously. In the 8‐yr experiment, SOC had declined under all rotations, with smaller changes where the rotation included 3 or 5 yr of perennial pasture. Whereas the average SOC was only 76% of the level in the preceding natural grassland, the corresponding value after 16 yr for the second site was 98% (and 93% under continuous arable), probably reflecting its high allophane clay content. The fractionation procedure tested proved applicable to both Andisols, but the intra‐aggregate light fraction (IA‐SOM, isolated in sodium iodide solution at 1.80 g/cm³ after ultrasonic dispersion) accounted for a very small proportion of total SOC (<1%). We suggest that in Andisols, the free light fraction (FR‐SOM, isolated in sodium iodide at solution of the same density, but prior to ultrasonic dispersion) is stabilised to a greater extent than in nonvolcanic soils, and the intra‐aggregate fraction plays a more minor role as a pool of intermediate turnover. The relative value of each fraction needs to be confirmed through dynamic experiments, using more sites, and including situations where SOC content is initially low.     

Does the compositional change of soil organic matter in the rhizosphere and bulk soil of tea plants induced by tea polyphenols correlate with Pb bioavailability?

Purpose

Soil organic matter (SOM) plays a vital role in controlling metal bioavailability. However, the relationship between SOM and its fractions, including water-soluble substances (WSS), fulvic acid (FA), humic acid (HA), and soil microbial biomass (SMB), to metal bioavailability in plants has not been thoroughly investigated. This study examined the compositional change of SOM after tea polyphenols (TPs) were added to the soil and its correlation with Pb bioavailability.

Materials and methods

Ultisol samples were collected from Fuyang, spiked with two levels (0 and 300 mg kg^?1 DW) of Pb, and aged for 30 days. Four uniform seedlings were transplanted to each plastic pot, which were filled with 3 kg of air-dried soil. After successful transplantation, three levels (0, 300, and 600 mg kg^?1 DW) of TPs were amended as irrigation solution for the pots. The Pb concentrations in different tissues of the tea plants were determined after 6 months. SOM, WSS, FA, HA, and SMB were extracted and quantified using a Multi N/C Total Organic Carbon Analyser.

Results and discussion

Adding TPs to Pb-polluted soils alleviated Pb toxicity to microorganisms and increased SMB and the rhizosphere effect. The rhizosphere SOM was lower than bulk SOM in Pb-unspiked soils, while the opposite results were observed in Pb-spiked soils. A similar inconsistency for HA in the rhizosphere and bulk soil between Pb-unspiked and Pb-spiked soils might explain the difference in SOM. FA increased with the addition of TPs in both the rhizosphere and bulk soils, which might be the result of TP transformation. Positive correlations are present between the compositions of rhizosphere SOM and Pb in different tissues of the tea plant. SMB correlated negatively with Pb in young leaves and stems. Compared to rhizosphere soil, SOM components in bulk soil were less strongly correlated with Pb in tea plants.

Conclusions

Addition of TPs to soil changes the components of SOM and Pb bioavailability. SOM and its fractions, including WSS, FA, HA, and SMB, show a close relationship to Pb in different tissues of the tea plants.     

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.     

Predicting soil organic matter stability in agricultural fields through carbon and nitrogen stable isotopes

In order to evaluate the sustainability and efficiency of soil carbon sequestration measures and the impact of different management and environmental factors, information on soil organic matter (SOM) stability and mean residence time (MRT) is required. However, this information on SOM stability and MRT is expensive to determine via radiocarbon dating, precluding a wide spread use of stability measurements in soil science. In this paper, we test an alternative method, first developed by Conen et al. (2008) for undisturbed Alpine grassland systems, using C and N stable isotope ratios in more frequently disturbed agricultural soils. Since only information on carbon and nitrogen concentrations and their stable isotope ratios is required, it is possible to estimate the SOM stability at greatly reduced costs compared to radiocarbon dating. Using four different experimental sites located in various climates and soil types, this research proved the effectiveness of using the C/N ratio and δ¹⁵N signature to determine the stability of mOM (mineral associated organic matter) relative to POM (particulate organic matter) in an intensively managed agro-ecological setting. Combining this approach with δ¹³C measurements allowed discriminating between different management (grassland vs cropland) and land use (till vs no till) systems. With increasing depth the stability of mOM relative to POM increases, but less so under tillage compared to no-till practises. Applying this approach to investigate SOM stability in different soil aggregate fractions, it corroborates the aggregate hierarchy theory as proposed by Six et al. (2004) and Segoli et al. (2013). The organic matter in the occluded micro-aggregate and silt & clay fractions is less degraded than the SOM in the free micro-aggregate and silt & clay fractions. The stable isotope approach can be particularly useful for soils with a history of burning and thus containing old charcoal particles, preventing the use of ¹⁴C to determine the SOM stability.     

土壤水分状况对14C标记秸秆的腐解及腐殖质中碳素分布的影响

¹⁴C-tracer technique and closed incubation method were used to study straw ¹⁴C decomposition and distribution in different fractions of newly formed humus under different moisture regimes. Decomposition of straw ¹⁴C was faster during the initial days, and slower thereafter. Decay rate constants of straw ¹⁴C varied from 3.29 × 10^-3 d^-1 to 7.06 × 10^-3 d^-1. After 112 d incubation, the amount of straw ¹⁴C mineralized was 1.17~1.46 times greater in submerged soils than in upland soils. Of the soil residual ¹⁴C, 9.08%~15.73% was present in humic acid (HA) and 31.01%~37.62% in fulvic acid (FA). Submerged condition favored the formation of HA, and HA/FA ratio of newly formed humus (labelled) was greater in submerged soils than in upland soils. Clay minerals affected the distribution of straw ¹⁴C in different humus fractions. Proportion of ¹⁴C present in HA to ¹⁴C remaining in soil was greater in Vertisol than in Ultisol.     

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.     

Chemical structure of humic acids in biosolids-amended soils as revealed by NMR spectroscopy

We used NMR spectroscopy to characterize humid acids extracted from soils that had received long-term application of 2 levels of biosolids to evaluate the soil organic matter (SOM) stability in biosolids-amended soils. The study also quantified fulvic acids (FAs), humic acids (HAs) and Fe/Al oxides. The soils were collected in 2004 from 7 fields, in Fulton County, southwestern Illinois, which received biosolids at a cumulative rate of 0 (control), 554 (low biosolids) and 1,066 (high biosolids) Mg ha⁻¹. The application of biosolids increased both FA and HA contents, but biosolids-amended soil and control soil did not differ in FA/HA ratio. Biosolids application had no effect on water-soluble organic carbon content. Biosolids application increased the presence of Fe/Al in the SOM complex and lowered its C/Fe and C/Al ratios. ¹³C NMR spectra showed increased alkyl C and decreased aromatic C content in soil HAs with the application of biosolids, and the extent of such changes was higher with high than low biosolids treatment. Under biosolids application, the soil HAs’ C structure shifts from O-alkyl-dominant to alkyl-dominant. Biosolids application does not decrease SOM stability but rather increases the stability of soil humic substances.     

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.     

Variations of Soil Organic Carbon Following Land Use Change on Deep‐Loess Hillsopes in China

Land use change is a key factor driving changes in soil organic carbon (SOC) around the world. However, the changes in SOC following land use changes have not been fully elucidated, especially for deep soils (>100 cm). Thus, we investigated the variations of SOC under different land uses (cropland, jujube orchard, 7‐year‐old grassland and 30‐year‐old grassland) on hillslopes in the Yuanzegou watershed of the Loess Plateau in China based on soil datasets related to soils within the 0–100 cm. Furthermore, we quantified the contribution of deep‐layer SOC (200–1,800 cm) to that of whole soil profiles based on soil datasets within the 0–1,800 cm. The results showed that in shallow profiles (0–100 cm), land uses significantly (p  < 0·05) influenced the distribution of SOC contents and stocks in surface layer (0–20 cm) but not subsurface layers (20–100 cm). Pearson correlation analysis indicated that soil texture fractions and total N were significantly (p  < 0·05 or 0·01) correlated with SOC content, which may have masked effects of land use change on SOC. In deep profiles (0–1,800 cm), SOC stock generally decreased with soil depth. But deep soils showed high SOC sequestration capacity. The SOC accumulated in the 100–1,800 m equalled 90·6%, 91·6%, 87·5% and 88·6% of amounts in the top 100 cm under cropland, 7‐year‐old grassland, 30‐year‐old grassland and jujube orchard, respectively. The results provide insights into SOC dynamics following land use changes and stressed the importance of deep‐layer SOC in estimating SOC inventory in deep loess soils. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of cover crop management on soil organic matter

Characterization of soil organic matter (SOM) is important for determining the overall quality of soils, and cover crop system may change SOM characteristics. The purpose of this study was to examine the effect of cover crops on the chemical and structural composition of SOM. We isolated humic substances (HS) from soils with the following cover crop treatments: (a) vetch (Vicia Villosa Roth.)/rye (Sesale cereale L.), (b) rye alone, and (c) check (no cover crops) that were treated with various nitrogen (N) fertilizer rates. CPMAS-TOSS (cross-polarization magic-angle-spinning and total sideband suppression) ¹³C NMR results indicated that humic acids (HA) from soils under rye only were more aromatic and less aliphatic in character than the other two cover crop systems without fertilizer N treatment. Based on the DRIFT (diffuse reflectance Fourier transform infrared) spectra peak O/R ratios, the intensities of oxygen-containing functional groups to aliphatic and aromatic (referred to as recalcitrant) groups, the highest ratio was found in the HA from the vetch/rye system with fertilizer N. The lowest ratio occurred at the vetch/rye system without fertilizer N treatment. The O/R ratio of fulvic acids (FA) can be ranked as: vetch/rye without fertilizer>vetch/rye with fertilizer>no cover crop without fertilizer>rye alone (with or without fertilizer) soils. Both organic carbon (OC) and light fraction (LF) contents were higher in soils under cover crop treatments with and without fertilizer N than soils with no cover crop. These chemical and spectroscopic data show that cover crops had a profound influence on the SOM and LF characteristics.