Soil Organic Matter Fractions Research Articles

Impact of deforestation on the soil physical and chemical attributes, and humic fraction of organic matter in dry environments in Brazil

Deforestation of Caatinga and inadequate land use of these dry environments have impacted soil quality in Northeastern Brazil. The objectives of this study were: (a) to evaluate the effect of deforestation and different agricultural uses on the physical and chemical properties of soil, and humic fractions of soil organic matter in dry environments; and (b) to detect the soil properties that were most affected by anthropic actions. We evaluated four dry areas in Chapada do Araripe, NE Brazil: preserved native vegetation; degraded native vegetation; cassava conventional cultivation; and eucalyptus agro-energy cultivation. Soil fertility, total organic carbon and humic fractions of soil organic matter were lower in the degraded native vegetation area. The best indicators for soil quality evaluation were: macroporosity; bulk density; soil resistance penetration; sum of bases (mainly Ca2+); available P; and saturation by Al3+. Total organic carbon and humic acid fractions of soil organic matter were important in improving soil quality. These properties were influenced by deforestation and agricultural uses, suggesting that the deforestation of native vegetation in dry environments has high capacity to degrade the soil, preventing its regeneration.

Soil aggregation and associated organic matter under management systems in sandy-textured soils, subtropical region of Brazil.

Increasing the diversity of plant species in agricultural production areas favors the maintenance or improvement of soil quality, particularly for soils with a sandy texture. This beneficial effect is related to the formation of aggregates of different origins. This study aimed to (i) verify whether soil use and management affect the proportion of biogenic (Bio) and physicogenic (Phy) aggregates and (ii) verify whether biogenic aggregation is more likely to lead to soil improvement than physicogenic aggregation. Three management systems were evaluated (permanent pasture, PP; no-tillage system, NT; and no-tillage + Brachiaria system, NT + B) as well as a reference area (Atlantic Forest biome vegetation, NF). According to their origin or formation pathway, the aggregates were separated, identified, and classified as Bio (formed by biological processes) and Phy (resulting from chemical and physical actions). The differentiation between Bio and Phy aggregates was performed based on the visualization of morphological features, such as shape, size, presence of roots, porosity, and subunit arrangements, and junctions. Only the PP area was able to promote greater aggregate formation of biological origin, with greater amounts of Bio aggregates. The highest total organic carbon (TOC) contents and the least negative δ13C values were also quantified in the aggregates of the PP area. The NT + B system provided an increase in the TOC content of its aggregates in comparison with aggregates in the NT and NF areas. Among the formation pathways, the Bio aggregates had the highest TOC and soil organic matter fractions contents and the most negative δ13C values. Perennial forage grasses vegetation was more important than the plant species diversity in favoring Bio aggregate formation. The beneficial effect of Brachiaria can be observed when incorporated as part of intercropping with corn in grain production systems. The biogenic aggregates favored the concentration of more labile soil organic matter fractions. The results of this study can provide important theoretical information for future studies focused on the combination of different plant species in agricultural food production areas on sandy-textured soils.

Soil Strength and Structural Stability Are Mediated by Soil Organic Matter Composition in Agricultural Expansion Areas of the Brazilian Cerrado Biome

A growing demand for resources has led to the expansion of agricultural areas worldwide. However, land conversion associated with poor soil management might lead to soil physical degradation. We investigated the effects of land conversion on soil physical properties in the Brazilian Cerrado region, under native Cerrado vegetation (NV)—pasture (PA) and NV—cropland (CL) conversion scenarios. Soil physical properties related to compaction, pore size distribution, and structure stability were assessed up to a 30 cm depth. Additionally, carbon levels of soil organic matter fractions (particulate and mineral-associated organic matter) were determined. Our results indicate that the compaction process equivalently reduced the soil porosity in PA and CL. However, soil penetration resistance was higher in PA (~2.5 MPa) than in CL (~1.5 MPa), as well as the stable mean weight diameter of soil aggregates. The highest total and labile organic carbon levels were observed in CL, while the lowest levels of total and labile organic carbon occurred in PA (smaller than in CL). These results suggest that the higher structural stability found in PA was mediated by the predominance of stabilized carbon (a decrease in the proportion of soil labile carbon), causing the gaining of soil strength under negligible soil volume variation (in comparison with CL). Our results suggest that the reduction in the soil porosity by compaction due to PA and CL uses can equivalently reduce macropore space and soil hydraulic functioning, and that soil carbon quality alterations (i.e., labile vs. stabilized fractions) are responsible for the gain in soil strength in long-term degraded PA areas. Future research should focus on understanding the magnitude in which soil organic matter controls soil physical attributes, such as soil strength in these expansion areas, and whether this gain in soil strength limits plant development and compromises productivity in the long term.

Variations in the concentration, composition, and sources of WEOM in erosion and deposition landscapes over an erosion intensity gradient on the Loess Plateau of China

Water-extractable organic matter (WEOM) is the most active and mobile fraction of soil organic matter and responds sensitively to soil erosion. However, previous studies on the chemical nature of WEOM in the erosion and deposition landscapes were controversial at site-level studies and mainly focused on surface soils, limiting our assessment of WEOM’s environmental fate at larger and deeper scales. Here, we collected eroded soils and deposited sediments from a depth of 0–200 cm in five eroding catchments with an erosion intensity gradient across the Loess Plateau. We examined the concentration, composition, and potential sources of WEOM using UV–visible absorption and fluorescence excitation-emission matrix-parallel factor analysis. Soil WEOM quantity and quality varied significantly in the erosion and deposition landscapes, with greater variations in soils below 20 cm, where WEOM in the deposition zone was characterized by higher concentration, aromaticity, hydrophobicity, and UVA and UVC humic-like components derived from terrestrial and soil organic matter. WEOM variations in deep soils increased with increasing erosion modulus among sites, and were mainly driven by changes in soil organic carbon concentration, aggregate stability, and carbon to nitrogen ratio. However, the pattern of WEOM parameters between erosion and deposition zones at 0–20 cm was opposite to that in soils below 20 cm. Vegetation inputs compensated the loss of organic matter in sites with suitable climatic status and weak erosion, and led to higher concentrations and exogenous features of WEOM in the surface erosion area. Concludingly, the WEOM concentration, components, and sources differed significantly between erosion and deposition zones, and these differences, as well as their response to erosion intensity, revealed an opposing pattern in surface and deep soils.

A rapid and sensitive assay to quantify amino sugars, neutral sugars and uronic acid necromass biomarkers using pre-column derivatization, ultra-high-performance liquid chromatography and high-resolution mass spectrometry

Microbial necromass comprises a large fraction of soil organic matter (SOM) due to the accumulation and stabilization of microbial residues from dead archaea, bacteria and fungi. Amino sugars, neutral sugars and uronic acids have been used as microbial necromass biomarkers to trace the origin and composition of microbial residues in the SOM pool. Due to the structural complexity of sugars, derivatization reactions and high-throughput analytical methods are required to separate and quantify these sugar-related compounds. Our aim was to develop a rapid and sensitive assay to measure amino sugar, neutral sugar and uronic acid compounds using pre-column 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatization. PMP-derivatives were separated and quantified via reversed phase (RP) ultra-high-performance liquid chromatography (UPLC) coupled to high-resolution Orbitrap mass spectrometry (MS). The method was validated and applied on hydrolyzed peptidoglycans and the biomass of archaeal, bacterial, fungal and plant species, as well as with soils. This developed PMP method allowed the separation and quantification of 18 sugar-related compounds, including four amino sugars, three N-acetyl amino sugars, eight neutral sugars, and three uronic acids within 20 min. This PMP method showed a precision for isotope enrichment detection of 0.03–0.05 atom % 13C for D-glucose and D-glucosamine. This is the first time talosaminuronic acid (deriving from archaeal pseudopeptidoglycan) was identified and quantified using PMP derivatization. The application of this novel PMP method on pure hydrolyzed biomass and soils, showed the successful chromatographic and mass spectrometric separation and quantification of amino sugar, neutral sugar and uronic acid compounds. A multivariate analysis using these sugar-related PMP derivatives showed a clustering of the species according to their respective taxonomic group (archaea, gram-positive bacteria, gram-negative bacteria, fungi and plants). The modified PMP method can be applied to identify and quantify soil microbial necromass biomarkers, as well as their contribution to SOM. The sensitive isotope tracer detection allows tracing isotopically labeled materials into necromass biomarkers in SOM pools.

Agricultural practices influence biological soil quality indicators in an irrigated semiarid agro-ecosystem

A study was conducted to investigate how soil biological measurements can be used to indicate differences in soil quality in arid and semiarid agroecosystems. The objectives were to assess soil biological quality indicators under different land management practices and at different depths and to explore the relationships between soil microbial community and organic matter fractions. Soil samples were collected from three replicates of six land management systems at 0–5 cm and 5–15 cm soil depths. Four management systems had medium to fine soil texture including a permanent grass field with tall fescue (Lolium arundinaceum [Schreb]; TFC), peach orchard with clover understory (Prunus persica with Trifolium repens L.; PCC), alfalfa field (Medicago sativa; ALF) and conventionally tilled soil with an annual crop rotation (CTC). Two management systems, including a young cottonwood tree orchard (Populus fremontii and Populus deltoides; CWS) and conventionally tilled soil in annual crop rotation (CTS), had coarse soil texture. Management systems had significant impacts on most of the soil biological indicators assessed at the two depths with both depths responding similarly to management practices. For the soil biological indicators that showed differences, the TFC generally had the most favorable indicator measurements compared to other management systems. The principal component analysis showed a clear separation between the management systems and microbial communities at both soil depths. The ordination pattern was structured by soil textural differences between medium to fine-textured soils from TFC, PCC, ALF, and CTC fields and coarse-textured soils (CWS, CTS). Spearman’s correlation coefficients (rs) showed that the soil organic matter (rs = 0.85, P = 0.001) and the permanganate oxidizable carbon (rs = 0.93, P = 0.001) were significantly correlated with the total microbial biomass at 0–5 cm soil depth. Also, the soil organic matter (rs = 0.84, P = 0.001) was significantly correlated with the total microbial biomass at 5–15 cm soil depth. This study suggests that in semiarid agroecosystems, management practices with permanent covers have the potential to improve soil quality, and soil texture may also influence the directional changes in soil quality within this region.

Biological quality and organic matter dynamics in minesoil at 18 years reclamation with perennial grasses

Mining is one of the most destructive economic activities on natural ecosystems. There are around 750 ha impacted by open-pit coal mining in Candiota Mine, the largest in Brazil, with 1.2 billion tons subject to mining. In soils impacted by mining, litter decomposition, always linked to a more extended period of restoration and maintenance of the vegetation, presents positive correlations with the diversity of the edaphic fauna and the enzymatic activity of these areas, promoting improvements in the ecological functions of the new ecosystem. The objective of the present study was to analyze the impact of revegetation with different summer perennial grasses subjected to different mowing managements on the biological and chemical quality of soils after coal mining in an experimental area at the Candiota Mine. Analyzes were performed to identify edaphic organisms using modified Berlese-Tüllgren funnels and microbiological analyzes to quantify Microbial Biomass Carbon (MBC), Microbial Respiration (MR), and urease enzymatic activity. The different fractions of Soil Organic Matter (fulvic, humic acids, and humin) were also quantified to understand the degree of humification in the area. All results were compared in mean tests by Tuckey's test (p < 0.05) and by Principal Component Analysis. After 18 years of revegetation and 11.6 years of mowing management in the area, there was an improvement in MBC and MR compared to the Control, evidencing the positive effect of the perennial grasses used in the experimental area. However, mowing management cannot yet be considered the most promising management for improving edaphic conditions.

Soil organic matter fractions and carbon distribution under different management in Lesotho, southern Africa

AbstractSoil organic matter (SOM) is a complex mixture of multiple fractions of soil organic C (SOC) that can be influenced by management decisions. This study evaluate the effects of tillage (moldboard plow [MP] and no‐tillage [NT]) and commercial fertilizer as limestone ammonium nitrate (28–0–0) at three rates (0 [0‐N], 100 [100‐N], and 200 [200‐N] kg N ha–1) on SOC, soil total N (STN), and SOM fractions (total particulate organic matter‐C [POM‐C] and mineral‐associated organic matter‐C [MAOM‐C]). The study was established in 2008 on the National University of Lesotho Campus Farm, Roma Valley of the Maseru District in Lesotho, southern Africa. The soil classified as Berea series. Soil samples were collected from 0‐to‐5‐, 5‐to‐10‐, 10‐to‐15‐, and 15‐to‐30‐cm depths. Under NT, the SOC, STN, and POM at 0‐to‐15‐cm were 54, 47, and 40% higher than 15‐to‐30‐cm depth, respectively. Differently, 0‐to‐15‐cm the MP had 17, 17, 35% higher SOC, STN, and POM than 15‐to‐30‐cm depth. The SOC in the 0‐to‐15‐cm increased by 27% with MP and by 36% with NT at the 200‐N compared with the 100‐N rate. The highest N‐rate (200‐N) increased total POM by 28.8% for MP and 22.6% for NT than the100‐N rate. The greater increase in POM under MP with high N rate was probably related to the low initial total POM, where small changes in POM will be pronounced, whereas the high total POM in NT could mask the effect the high N rate addition. The coarse and fine POM accounted for 39 and 61%, respectively, of the changes in total POM at both NT and MP. The high percentage of SOC was observed within fine POM fraction (48.4%) in MP, which makes it susceptible to wind erosion, and within MAOM fraction (48.9%) in NT practice. The C/N ratio was highest with coarse POM (C/N ≈ 25) and lowest with MAOM (C/N ≈ 7.2) at both tillage practices. The low C/N ratio made MAOMC vulnerable to microbial decomposition. These findings suggest the need for conservation efforts where conservation tillage and N rates could be used to reduce SOC and POM losses and contribute to improve land sustainability and SOM conservation.

Seeing the forest for the fractions – Comparing soil organic matter fractionation methods using molecular features after forest stand conversion

The molecular composition of soil organic matter (SOM) contains key information on the persistence of soil carbon (C) in relation to changes in vegetation and environmental factors. Depending on the ecosystem, analytical method and specific objectives, many SOM fractions and numerous fractionation schemes have been proposed to study soil C. However, the molecular composition and environmental significance of those different SOM fractions have not yet been compared systematically. We use a reverse fit approach to fill this knowledge gap: i.e. we chose a study area with a well-known land history to assess which information is stored in the most frequently analysed SOM fractions. The Gaume forest (Belgium) is an ancient deciduous forest (>200 years) in which small stands were converted to Norway spruce (Picea abies) 40–50 years ago. Those stands are located along a lithological gradient in soil buffering capacity. We investigated the molecular composition of bulk mineral soil samples and five organic SOM fractions by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The SOM fractions included particulate OM (POM), water extractable OM (WEOM), and fractions obtained by alkaline extraction, including the traditionally used humic acid (HA) and fulvic acid (FA) and the total base extract from which they are obtained (BE). Our results indicate that pyrolysates of bulk mineral soil did not prove useful to reflect environmental changes after forest stand conversion. Principal Component Analysis indicated that within each organic fraction similar changes occurred when comparing soil depths, degree of SOM decomposition, litter inputs, and soil buffering capacity. However, only the HA and BE appeared successful in capturing all these processes: the degree of SOM decomposition was not expressed in pyrolysates from the POM, differences in litter input between forest stand types were not evidenced in the WEOM, and effects of buffering capacity were not demonstrated in the WEOM and FA fractions. Thus, the molecular composition of different SOM fractions can be used complementary to each other to study environmental and ecological effects of forest stand conversion on soil C dynamics.

Assessing Soil Organic Carbon Stocks and Particle-Size Fractions across Cropping Systems in the Kiti Sub-Watershed in Central Benin

Soil organic carbon storage in agricultural soil constitutes a crucial potential for sustainable agricultural productivity and climate change mitigation. This paper aimed at assessing soil organic carbon stock and its distribution in three particle size fractions across five cropping systems located in Kiti sub-watershed in Benin. Soil samples were collected using a grid sampling method on four soil depth layers: 0–10, 10–20, 20–30 and 30–40 cm in five cropping systems maize–cotton relay cropping (MCRC), yam–maize intercropping (YMI), teak plantation (TP), 5-year fallow (5YF) and above 10-year fallow (Ab10YF) from July to August 2017. Soil organic carbon stock (C stock) was estimated for the different soil layers and particle-size fractionation of soil organic matter was performed considering three fractions. The fractions coarse particulate organic matter (cPOM: 250–2000 µm), fine particulate organic matter (fPOM: 53–250 µm) and non-particulate organic matter (NOM: &lt;53 µm) were separated from two soil depth layers: 0–10 and 10–20 cm. The results showed that fallow lands Ab10YF and 5YF exhibited the highest C stock, 22.20 and 17.74 Mg C·ha−1, while cultivated land under tillage MCRC depicted the lowest, C stock 11.48 Mg C·ha−1. The three organic carbon fractions showed a significant variation across the cropping systems with the NOM fraction holding the largest contribution to total soil organic carbon for all the cropping systems, ranging between 3.40 and 7.99 g/kg. The cPOM and fPOM were the most influenced by cropping systems with the highest concentration observed in Ab10YF and 5YF. The findings provide insights for upscaling farm management practices towards sustainable agricultural systems with substantial potential for carbon sequestration and climate change mitigation.

Soil Carbon and Nitrogen Stocks under Agrosilvopastoral Systems with Different Arrangements in a Transition Area between Cerrado and Caatinga Biomes in Brazil

Production systems that promote the accumulation of soil organic matter (SOM) must be implemented to maintain the sustainability of agriculture, livestock, and forestry. Since increases in MOS content contribute to improving the chemical, physical, and biological quality of the soil, as well as helping to reduce carbon emissions to mitigate climate change. Therefore, the objective of this study was to evaluate soil organic carbon (SOC) and nitrogen (N) stocks after the implementation of agrosilvopastoral (ASP) systems in a Cerrado-Caatinga transition zone in Brazil. Native vegetation of Cerrado-Caatinga (NV), regenerating stratum of Cerrado-Caatinga (RS), two arrangements of ASP systems cultivating Cenchrus ciliaris L. intercropped with Eucalyptus camaldulensis Dehnh. × Eucalyptus tereticornis Sm. hybrid (ASP1 and ASP2), and intercropped with Eucalyptus urophylla S.T. Blake × Eucalyptus grandis W. Mill ex Maiden hybrid (ASP3 and ASP4) were evaluated. Soil C and N stocks and the C content in the humic fractions of SOM were evaluated at 0–10, 10–20, and 20–30 cm soil depths. The introduction of ASP2, ASP3, and ASP4 systems in an area previously occupied by low productivity pasture increased and restored SOC stocks to levels found in NV, at a depth of 0–30 cm. N stocks were higher in ASP systems, regardless of the arrangement studied. As a result, the ASP systems provided accumulations that ranged from 1.0 to 4.31 Mg SOC ha−1 yr−1 and from 0.33 to 0.36 Mg N ha−1 yr−1. The carbon contents in humic fractions remained higher in NV. The hierarchical grouping and principal component analysis showed that the implementation of the ASP systems was efficient in increasing soil C and N stocks over time. In conclusion, the present study identified that integrated production systems can support land use intensification strategies based on sustainable and low-carbon agriculture in a transition area between the Cerrado and Caatinga biomes in Brazil.

Native multispecies and fast-growing forest root biomass increase C and N stocks in a reclaimed bauxite mining area.

This study is aimed at evaluating C and N stocks in fractions of soil organic matter (SOM) in an area of bauxite mining under recovery with tree species. We have analyzed the long-term recovery of C and N stocks of organic matter fractions from five types of forest cover (Eucalyptus, Anadenanthera peregrina, mixed plantation of 16 native species, a mined area without vegetation cover as a control site, and a natural forest cover as a reference site). The total organic C (TOC) and N (TN) stocks and also organic matter fractions, particulate organic matter (POM), mineral-associated organic matter (MAOM), microbial biomass (MB), and labile C (LC), were determined, as well as the C/N ratio and the carbon management index (CMI). Although the stocks of TOC and LC, CMI, and MB did not differ between the types of forest cover in the 0-60 cm layer, they were lower than the values in the native forest. Forest cover increases the stocks of TOC, LC, MB, and CMI in the area of bauxite mining compared to the control site. In addition, we found that the TOC C and TN stocks and also SOM fractions (LC, C-MAOM, C-POM, N-MAOM, and N-POM) are positively correlated (r ≥ 0.71 for all cases) with volume of roots larger than 2 mm. Therefore, Eucalyptus, A. peregrina, and a mixed of 16 native trees contribute for restoring stocks of soil C and N following bauxite mining in the Brazilian Atlantic Forest.

Distribution of Organic Carbon Fractions in Soil Aggregates and Their Contribution to Soil Aggregate Formation of Paddy Soils

ABSTRACT Different soil organic matter fractions play different roles in governing soil aggregation. Therefore, this study determined the distribution of labile organic carbon (OC) and stable OC fractions in soil aggregates and their contribution to the soil aggregation of paddy soils in the Central Plain of Thailand. Quantitative analysis of the OC fractions in soil aggregates was determined using wet oxidation method and their changes in the chemical composition of organic compounds were investigated using FTIR spectroscopy. The results showed that the large macroaggregate (LMA: 2–8 mm) content of studied soils generally increased with increasing soil organic carbon, clay, polyvalent cations and aluminum (Al) oxides contents. Generally, both the labile OC and stable OC fractions increased with increasing aggregate size classes, suggesting a hierarchical order of soil aggregation and physical protection of organic matter against microbial decomposition. The labile OC fraction in all aggregate size classes was positively correlated with the content of LMA (r = 0.54*–0.77**), suggesting that the labile OC fraction represented a major organic cementing agent that contributed to LMA formation. FTIR analysis revealed that the labile OC fraction mainly consisted of plant-derived and microbial-derived polysaccharides. Therefore, paddy soils with higher organic cementing agent (labile OC fraction) and inorganic cementing/flocculating agents may provide a stronger intimate association of these transient binding agents, particularly the microbial-derived polysaccharides with clay mineral surfaces and metal cations/Al oxides. In turn, the greater amounts of stabilized, microbial-derived polysaccharides also efficiently promoted the formation of LMA from a combination of microaggregates.

Soil microbial communities under wheat and maize straw incorporation are closely associated with soil organic carbon fractions and chemical structure

The incorporation of different types of crop straw may lead to distinct forms and chemical structures of soil organic matter (SOM), which are closely associated with the communities of soil microorganisms in agricultural ecosystems. However, the relationships between the physiochemical composition of SOM and soil microbial communities in response to the incorporation of different types of crop straw remain largely unknown. In this study, we sought to gain insights into these relationships based on a long-term (10 year) field experiment, in which we examined the effects of five treatments [no fertilizer (control), mineral fertilizer (F), mineral fertilizer and wheat straw return (WF), mineral fertilizer and maize straw return (MF), and mineral fertilizer with wheat and maize straw (WMF)]. Using advanced solid-state 13carbon (C) nuclear magnetic resonance and high-throughput sequencing techniques, we investigated the associations between SOM physical fractions and chemical composition and the compositions of soil fungal and bacterial communities Our observations indicated that compared with treatment MF, the long-term incorporation of wheat straw increased the proportions of carbohydrate carbons in light-fraction and coarse-particulate organic matter (cPOM), which were significantly correlated with soil bacterial community composition. Long-term maize straw incorporation was found to increase the proportions of course and fine-POM fractions relative to the WF treatment, as well as the proportions of their aromatic carbons, which were significantly correlated with soil fungal community composition. Collectively, the findings of this study revealed that bacterial and fungal communities are characterized by two distinct responses to wheat and/or maize straw incorporation, which are strongly associated with the physical fractions and chemical structure of SOM, particularly in the case of POM. These findings provide valuable insights for understanding the associations between soil microbial communities and SOM fractions following crop straw incorporation, which will contribute to the development of optimal strategies for crop straw management.

Intensification and diversification of pasturelands in Brazil: Patterns and driving factors in the soil carbon stocks

The extensive system of pasture management in Brazil can reduce soil C stocks and increase CO2 emissions to the atmosphere. However, the adoption of sustainable management systems can change these conditions and enact modern practices of natural resource management as defined by the concept of ecosystem services. A field study was therefore carried out to assess the effects of adopting more intensive and diversified systems of pasture management on the quality and stocks of soil C. Treatments included fertilized pasture (FP), integrated crop-livestock (ICL), and integrated livestock-forest (ILF) and were compared to conventional management systems (CS) under different soil and climate conditions (tropical humid, subtropical dry and subtropical humid). Soil carbon stocks and soil organic matter (SOM) fractions were determined in the top one-meter layer. With the SOM fractions, we calculated the C lability (LI), pool (CPI) and management (CMI) indices. The results showed that adopting ICL systems under the conditions of a tropical humid and subtropical humid climate produced increases in soil C stocks of 0.75 Mg ha−1 yr−1 (P < 0.01) and 0.15 Mg ha−1 yr−1 (P < 0.05), respectively, relative to the CS. Similarly, the conversion of a CS to FP (tropical humid climate) and ILF (subtropical dry climate) led to increases in soil C stocks of 2 and 0.55 Mg ha−1 yr−1 (P < 0.05), respectively. In general, the adoption of more intensive and diversified systems of pasture management can also afford increases in the LI, CPI and CMI indices. These results can serve as a scientific basis for government initiatives that focus on recovering degraded pastures in Brazil through the use of more sustainable management systems and can also help to sequester more C in soils.

Forest-Fruticulture Conversion Alters Soil Traits and Soil Organic Matter Compartments.

Fruticulture in the Amazonian Rainforest is one of the main causes of deforestation, biodiversity loss, and soil erosion. Fruticulture plays a key role in the soil traits and soil organic matter (SOM) compartments by altering the soil ecosystem. Our aim was to assess the influence of Forest-Fruticulture conversion on soil traits, and SOM fractions in Brazil's Legal Amazon. The experiment was carried out in field conditions using four land uses as main treatments: Bixa orellana, Theobroma grandiflorum, Paullinia cupana, and the Amazon Rainforest. The soil physicochemical traits were analyzed using samples that were collected from 0-5, 5-10, and 10-20 cm soil depth by using grids (10 × 10 m) with 36 sampling points. Our results showed that the Fruticulture promoted an increase in bulk density, GMD, aggregate diameter, soil porosity, gravimetric moisture, sand, clay, carbon associated with humic acid, and, the sum of bases (K+, Ca2+, and Mg2+), while the Amazon Rainforest showed the highest values of silt, soil P content, SOC, p-SOC, m-SOC, carbon associated with fulvic acid, humine, and soil C stock. Overall, the fruticulture farming systems have negative effects on SOM compartments. The results of our study highlight the importance of considering fruticulture with endemic plant species by promoting soil fertility and soil aggregation.

Phosphorus species in sequentially extracted soil organic matter fractions

The majority of organic P (Porg) in soil is considered to be part of soil organic matter (SOM) associations, but its chemical nature is largely ‘unresolved’. In this study, we investigated the Porg composition in different SOM fractions of a Gleysol soil using the Humeomics sequential chemical fractionation (SCF) procedure combined with nuclear magnetic resonance (NMR) spectroscopy.In summary, SCF procedure with subsequent NaOH-EDTA extraction of the soil residue extracted a total of 1769 mgP/kgsoil compared to 1682 mgP/kgsoil of a single-step NaOH-EDTA extraction. Approximately 38 % of the extracted Porg was present in the form of the unresolved Porg pool, which was represented by one or two underlying broad signals in the phosphomonoester region of solution 31P NMR spectra. The SCF revealed that phosphomonoesters were recovered in each fraction: 47 % of the unresolved phosphomonoesters were associated with the SOM fraction released by breaking ester bonds (40 %) and ether bonds (7 %), whereas about 30 % of this unresolved Porg pool appeared in the SOM fraction closely associated with the soil mineral phase. Furthermore, the extractability of inositol phosphates (IP) was increased from 312 mgP/kgsoil to 534 mgP/kgsoil (factor 1.7) using the SCF procedure compared to a single-step NaOH-EDTA extraction. Previous studies have reported the presence of IP in molecular size fractions greater than 10 kDa. Our findings on the removal of IP with the fractionation of the SOM could explain the presence of IP in these large associations.We demonstrate that major pools of Porg are closely associated with SOM structures, comprising a diverse array of chemical species and bonding types. These results forward our understanding of Porg stabilisation, P transformation, and P cycling in terrestrial ecosystems towards an association point of view.

Multiple Effects of Humic Components on Microbially Mediated Iron Redox Processes and Production of Hydroxyl Radicals.

Microbially mediated iron redox processes are of great significance in the biogeochemical cycles of elements, which are often coupled with soil organic matter (SOM) in the environment. Although the influences of SOM fractions on individual reduction or oxidation processes have been studied extensively, a comprehensive understanding is still lacking. Here, using ferrihydrite, Shewanella oneidensis MR-1, and operationally defined SOM components including fulvic acid (FA), humic acid (HA), and humin (HM) extracted from black soil and peat, we explored the SOM-mediated microbial iron reduction and hydroxyl radical (•OH) production processes. The results showed that the addition of SOM inhibited the transformation of ferrihydrite to highly crystalline iron oxides. Although FA and HA increased Fe(II) production over four times on average due to complexation and their high electron exchange capacities, HA inhibited 30-43% of the •OH yield, while FA had no significant influence on it. Superoxide (O2•-) was the predominant intermediate in •OH production in the FA-containing system, while one- and two-electron transfer processes were concurrent in HA- and HM-containing systems. These findings provide deep insights into the multiple mechanisms of SOM in regulating microbially mediated iron redox processes and •OH production.

Soil Organic Matter Fractions in Relation to Root Characteristics of Different Tree Species in Altitude Gradient of Temperate Forest in Carpathian Mountains

The roots are a key functional component of belowground systems and one of the main factors influencing the quality and quantity of soil organic matter. Our research aimed to determine the fractional composition of the soil organic matter (SOM) in soils under various tree species on an altitude gradient. In our research, we related the SOM fractions with the root characteristics. There is a lack of information on the relationship between the SOM fractions and the root properties. We assessed labile and heavy fractions of SOM content in forest mountain soils with a climosequence approach. The study plots were established at 600, 800, and 1000 m above sea level in a beech stand (Fagus sylvatica L.) and a fir stand (Abies alba Mill.). In this case, three research plots with beech and fir were designated in each altitude variant. Forest stands growing in the same soil conditions were selected for the study. The research used stands of similar age with the same tree canopy density. The basic physicochemical properties (pH, hydrolytic acidity, carbon and nitrogen content, base cations content) and the fractional composition of the SOM were determined from soil samples. In addition, we determined the basic characteristics of the roots (diameter, length, biomass, decomposition, production). The correlation between soil organic matter fractions and root characteristics was recorded. This study confirmed the importance of climatic conditions in shaping the fractional composition of forest soils. In the highest locations, characterized by lower temperatures, the light fraction of the SOM exhibited the highest C and N content, which is the effect of slower decomposition processes. Apart from climatic conditions, the stabilization of SOM is influenced by the tree species composition of a forest stand. Beech forest stands lead to a larger accumulation of a heavy fraction of SOM. This study indicates a positive correlation between the light fraction of SOM, root biomass, and decomposition rate of roots. Our research shows that avoiding single-species coniferous stands and introducing admixtures of deciduous species, which increase the heavy SOM fraction, is justified in forest management.

Enhancing Carbon Sequestration in Mediterranean Agroforestry Systems: A Review

The agroforestry systems with a high potential for C sequestration are those degraded by poor management strategies. Studies on changes in soil C status in these ecosystems mostly take into account labile C pools. Labile and stable soil organic matter (SOM) fractions are affected by soil management and land-use changes. Stable C pools are essential to understanding effects of land-use on soil C storage in the long term. The SOM stability is partly enhanced by the interaction of SOM with minerals and its inclusion into soil aggregates. Recalcitrant substances (e.g., lignin and chitin) also contribute to the passive SOM fraction. Macroaggregates mostly reflect the influence of plant roots and coarse intra-aggregate particulate SOM (POM), whereas microaggregates reflect the influence of fine interaggregate POM, clay concentration and humified SOM fraction. Often, POM is more sensitive to soil management changes than total SOM. Glomalin is a recalcitrant protein consisting of chitin produced by mycorrhizal fungi to protect hyphae. Glomalin has implications on C sequestration in agroforestry soils, but further research is needed before any prediction can be made. One challenge is reducing the CO2 emission from roots, and increasing the recalcitrant root C.