Soil Organic Matter Accumulation Research Articles

Dynamics and underlying mechanisms of N2O and NO emissions in response to a transient land-use conversion of Masson pine forest to tea field

Nowadays, there has been a rapid expansion of tea field converted from forestry for pursuing higher economic benefits. However, few researches focus on the effects of transient land-use conversion from Masson pine forest to artificial tea fields on soil N2O and NO emissions and the underlying mechanisms. A parallel field experiment was conducted from Masson pine forest and a newly converted tea plantation from Masson pine forest from 2013 to 2017 in subtropical central China. Masson pine forest conversion to tea field dramatically increased soil N2O and NO emissions (up to 4.00 ± 0.43 and 1.93 ± 0.45 kg N ha−1 yr−1, respectively) in the first year possibly due to enhanced soil organic N mineralization. With the extension of tea planting age, N2O and NO emissions showed an upward trend (ranged from 1.19 to 5.28, and 0.15 to 1.78 kg N ha−1 yr−1, respectively) influenced by fertilization and soil organic matter accumulation. The direct emission factors for N2O and NO in the newly converted tea fields were the largest in the first year (2.64 and 1.07%, respectively) after land-use conversion, and higher than the default value recommended by IPCC. The NO/N2O ratio was mainly lower than 1 in the fertilized tea field, and soil N2O and NO emission peaks mainly occurred in tea-growing season (wet season) with higher soil moisture and NH4+-N concentrations, and dominated by amoA-containing bacteria (AOB), suggesting nitrifier-denitrification could be the dominant process involved in soil nitrogenous gases emissions in tea field. These results can be summarized as dramatically increased soil N2O and NO emissions during the transient land-use conversion from Masson pine forest to tea field were possibly due to the substantial net soil organic N mineralization and the enhanced abundance of nitrification functional genes (AOB).

Tree species affect soil organic matter stocks and stoichiometry in interaction with soil microbiota

The purpose of this research was to evaluate tree species effects on quantitative and qualitative soil organic matter (SOM) properties of forest floors and mineral soil layers. Additionally, the contribution of soil microbial biomass to SOM was studied in five forest stands with different dominant tree species.The study was conducted at the afforested spoil heap ‘Sophienhöhe’ located at the lignite open-cast mine Hambach near Jülich, Germany. The 35 year-old afforested sites consisted of monocultural stands of Douglas fir (Pseudotsuga mienziesii), pine (Pinus nigra), beech (Fagus sylvatica) and red oak (Quercus rubra) as well as a mixed deciduous stand site planted mainly with hornbeam (Carpinus betulus), lime (Tilia cordata) and common oak (Quercus robur). There, boundary conditions regarding soil, climate, topography and management were highly similar, equivalent to a common garden experiment but on landscape level. Because the parent material used for site recultivation was free from organic matter or coal material, the SOM accumulation is a result of in situ soil development.Tree species had a significant effect on soil organic carbon (SOC) stocks, stoichiometric patterns of C, hydrogen (H), nitrogen (N), oxygen (O) and sulfur (S) and the microbial biomass carbon (MBC) content in the forest floor and the top mineral soil layers (0–5 cm, 5–10 cm, 10–30 cm). In general, forest floor SOC stocks were significantly higher in coniferous forest stands compared to deciduous tree species. Differences in SOM quantity became less pronounced with increasing depth, while stoichiometric molar ratios of SOM as indices of litter turnover and SOM composition differed also in deeper layers. Differences in H:C and O:C ratios among tree species clearly increased along the depth gradient in mineral soils, indicating that SOM turnover by oxidative processes depends on tree species. Differences in depth gradients of the microbial quotient (MBC to SOC ratio) among tree species emphasized differences in the microbial C turnover. Furthermore, the relationship between the microbial quotient and SOM stoichiometry (C:N and C:S ratio) became stronger with increasing soil depth. This suggests that N and especially S limitation determined the microbial turnover of SOM in deeper mineral soil layers.

Post-agricultural restoration: Implications for dynamics of soil organic matter pools

In order to bridge the gap of knowledge in respect of soil organic matter (SOM) dynamics of post-agricultural Luvisols during restoration, the study focused on alteration processes of functionally different SOM pools, 14C ages, and C turnover rates in a chronosequential approach. The study comprises three Luvisol chronosequences of 37 years (North), 120 years (Middle), and 42 years (South) across a climate gradient of the temperate broad-leaved forests of European Russia. Restoration led to SOM accumulation within both active (free particulate organic matter (POM) and occluded POM and passive (clay fraction) pools, and as a result, in gains of total organic carbon (C). Accumulation of new C was confirmed by increasing ∆14C signatures of occluded POM and clay fractions. After 22 years of restoration, ∆14C signatures progressed from negative to positive values for the occluded POM fraction, whereas they showed a similar development but remained negative for the clay faction. This highlights the fact that the incorporation of new C into the passive pool takes place, but that it occurs at a slower rate compared to that of the active pool. The 14C age of occluded POM fraction declined from 600 to 1200 years before present (BP) in arable sites to about 200 years BP after 22 years of restoration and thereafter. The 14C age of the clay fraction decreased from 1670 to 2660 years BP in arable soils to 750 years BP in 22 year abandoned site, 450–310 years BP in 32–66 year abandoned sites, and 200–370 years BP in near-natural forests. This decline of the 14C age reveals a change of the proportions between old and young C within the passive pool caused by management and the development of an ecosystem towards a steady state. Recovery of C was accompanied by initially increasing and then decreasing turnover rates of SOM, both again, much more pronounced in the active pool. These dynamics trace an initial ecosystem disturbance after abandonment, which slows down with time. Although C recovery and adjusted turnover rates of SOM progressed towards a new steady state of ecosystem; full restoration was not achieved within the chronosequential time scale of 120 years.

Effect of land use and carbonates on organic matter stabilization and microbial communities in Mediterranean soils

Soil organic matter (SOM) is sensitive to land use and to physico-chemical soil properties as well soil microbial communities controlling SOM stabilization. Our study aimed at exploring how carbonates of Mediterranean soils – known to stabilize SOM by enhancing soil aggregation and binding SOM by calcium – are affecting SOM and microbial communities under different land uses. In soils with and without carbonates sampled in forests, cultivated and abandoned fields, we have fractionated SOM-pools according to particle sizes and determined C mineralization rates and polyvalent cations in these fractions. In agreement with larger scale assessments in the Mediterranean, our results show that SOM contents are greater in forest than in agricultural soils, but that the differences among land use types depend on carbonate contents. While SOM contents are greater in carbonated soils of forests, cultivated soils have low SOM levels independent of the carbonate content, presumably due to a reduced protection of particulate SOM by century-long intense tillage. Forest soils contain particularly large amounts of SOM associated with the coarser soil fraction. In this fraction, C mineralization was smallest in the carbonated forest soil, possibly due to a low SOM quality indicated by high C/N ratios and low δ15N values. Greater contents of Ca, Fe and Al in carbonated soils also suggests that stabilization through Ca and phyllosilicates was important. Mineralization of sand-associated SOM was highest in cultivated and abandoned soils, which is indicative for a small SOM stabilization, which in turn might restrict SOM accumulation after land abandonment. Microbial communities assessed by phospho-lipid fatty acid signatures and microbial carbon use efficiency were more closely associated with land use than with carbonate content – a pattern that reflects the distribution of C pools among size fractions which differed more strongly among land use types than between soils with and without carbonates.

Evolution of soil surface charge in a chronosequence of paddy soil derived from Alfisol

Limited information is available on the changes in surface electro–chemical properties of paddy soils during pedogenesis. This study evaluated the category, evolution and origin of soil surface charge along an Alfisol–derived paddy soil chronosequence. Ammonium acetate adsorption, as well as cesium (Cs+) adsorption and sequential desorption methods were applied to reveal the surface charge evolution along a paddy soil chronosequence. Results indicated that organic matter increased by 52.79%, 84.90%, 144.35%, and 126.52% in the surface paddy soils after 3, 8, 30, and 70 years of rice cultivation, respectively. The main phyllosilicates in Alfisol and Alfisol–derived paddy soils were montmorillonite and illite, which increased with paddy cultivation over time. Soil negative charge, as determined by ammonia (NH4+) adsorption, was strongly correlated with data from Cs+ adsorption methods. The negative charge on the surface paddy soil, as estimated by NH4+ adsorption, increased by 16.41%, 33.21%, 56.49%, and 31.68% after 3, 8, 30, and 70 years of rice cultivation, respectively. Further investigations revealed that an increasing variable surface charge is mainly attributed to soil organic matter accumulation. Also, hydroponics cultivation was shown to prompt the destruction and weathering of primary minerals and formation of soil secondary phyllosilicate minerals, which are mainly responsible for increasing permanent surface charge along the paddy soil chronosequence. Thus, the nutrients holding capacities of paddy soils are maintained or even improved, thereby increasing agricultural productivity, when compared with upland soils.

Monitoring the bovine activity in grazing by an electronic sensing device based on GPS

The joint analysis about cattle positioning and activity in pasture associated to soil attributes, become important for making decisions and forage management. In this work, the prototype of an electronic sensing device based on global positioning system (GPS) was developed with the objective of monitoring dairy cattle position in pasture and associating animal preference areas to possible changes for soil penetration resistance and organic matter accumulation. Our study was performed at Rocadinho Farm, Agreste region of Pernambuco, Brazil, in 40 x 40 m area, managed under continuous stocking method. The GPS positioning sensors were fixed by collars on the animal neck and programmed to record the animal location during grazing. The mean error observed was 0.65 m. Before and after grazing, 36 deformed samples were collected in the soil surface layer for determination of organic matter (from 0.00 to 0.05 m) and penetration resistance (from 0.00 to 0.10 m). The developed electronic device had adequate accuracy and autonomy for cattle monitoring, which allowed to determine preference sites of grazing. After grazing, there was increase of 6.5% for organic matter and 79.4% for soil penetration resistance in the sites of greater pasture exploitation.

Substitution of mineral fertilizers with biogas digestate plus biochar increases physically stabilized soil carbon but not crop biomass in a field trial

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B0), with 1 Mg biochar ha−1season−1 for two growing seasons (B2), or with 40 Mg biochar ha−1 application (B40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 μm aggregate-occluded and mineral-associated OM. CO2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha−1), while biochar had no effect. B40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 μm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha−1 with digestate resulted in 20% more SOC in the <20 μm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B0 and B2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates.

Sustainability of Impacts of Poplar Growth on Soil Organic Matter in Eutric Cambisols

Short rotation coppices (SRC) with poplar on arable soils constitute no-till management in combination with a changed litter quality compared to annual crops. Both tillage and litter quality impact soil organic matter (SOM) composition, but little is known on the sustainability of this impact at the molecular level. We compared the microbial colonization and SOM quantity and quality of a young (4 years), old (17 years) and a former SRC with hybrid poplar (Populus maximoviczii × Populus nigra cv. Max) to adjacent arable sites with annual crops or grass. Total fungal and arbsucular mycorrhizal fungal phospholipid fatty acid (PLFA) markers were increased under no-till treatments with permanent crops (SRC and grass) compared to tilled cereals. Enrichments in fungal biomass coincided with C accumulation close to the soil surface (0–5 cm) but was abolished under former SRC after return to annual tillage. This management change altered the spatial distribution but not the accumulation of SOM within the topsoil (0–30 cm). However, lasting qualitative changes in SOM with increased proportions of lignin, lipids and sterols were found under current and former SRC. Increased colonization by arbuscular mycorrhizal fungi was correlated with increased invertase activity (R = 0.64; p &lt; 0.05), carbohydrate consumption and a corresponding accumulation of lignins and lipids in the SOM. This link indicates a regulatory impact of mycorrhizal fungi on soil C dynamics by changing the quality of SOM. Increased stability of SOM to microbial degradation by higher portions of lipids and sterols in the SOM were assumed to be a sustainable effect of poplar growth at Eutric Cambisols.

Changes in dissolved organic matter composition and dynamics in a subtropical mangrove river driven by rainfall

Dissolved organic matter (DOM) plays an important role in sustaining ecosystem services of mangrove forests through well-described biogeochemical and ecological functions. This study was conducted in the Fukido River (Ishigaki Island, Japan) to better understand the seasonal and episodic changes in DOM concentration and composition in a subtropical mangrove system. Water samples were collected seasonally along a headwater–mangrove–sea transect on 10 occasions from September 2014 through June 2016. DOM was fractionated based on hydrophobicity into two fractions (hydrophobic and hydrophilic) and also analyzed by excitation-emission matrix spectroscopy combined with parallel factor analysis (PARAFAC). Although seasonal changes in DOM concentration and composition were not observed, both hydrophobic and hydrophilic DOM concentrations and levels of the identified three PARAFAC components clearly increased during a typhoon event. It is suggested that episodic increases in freshwater input due to a typhoon caused enhanced leaching of DOM from mangrove litter and dissolution of mangrove soil organic matter (SOM), which was otherwise retained in the mangrove soil by salinity-induced aggregation. The aggregation–dissolution properties of SOM are crucial in determining the magnitude of DOM outwelling and possibly SOM accumulation rate by enhancing advective DOM exchanges. Future studies are needed to evaluate the size of the carbon pool and outwelling of DOM after classifying mangrove forests based on the hydrological regime that influences biogeochemical conditions in the forests. Regional Index TermsJapan, Okinawa, Ishigaki, Fukido.

Impact of andosolization on pedogenic Fe oxides in ferrallitic soils

The accumulation of soil organic matter (SOM), poorly crystalline Fe oxides and metal-humus complexes is a trait of non-allophanic Andosols. The process of andosolization, which may occur in ferrallitic soils with high organic matter content, can involve transformation from well crystallized Fe oxides to poorly crystallized Fe oxides and Fe-humus complexes. This study investigates such changes in pedogenic Fe mineral associations for a soil toposequence between 1500 and 2260 m altitude along the southern flank of the volcanic Bambouto Mountains, Western Cameroon. The soils consist of highly weathered material, dominated by kaolinite, gibbsite and Fe oxides, grading to Protoandic Umbrisols at high altitude, recording an increase in SOM content with increasing altitude. As revealed by selective extraction analysis, the relative amount of poorly crystalline Fe oxides is low in the Bt and Bo horizons of low-altitude pedons, as well as in deep subsurface horizons of the high-altitude pedons. In contrast, it is significantly higher in the A and Bw horizons of the high-altitude pedons, with a clear increase with increasing altitude. Mössbauer spectroscopy analysis of B horizon samples identifies goethite as the dominant Fe oxide phase in nearly all pedons, with higher hematite contents in a mid-altitude zone marked by lower annual rainfall than in other parts of the toposequence. The Mössbauer spectra also reveal the presence of dissolved organic matter (DOM)-ferrihydrite, whose abundance is greatest in the Bw horizon of the high-altitude pedons, with an increase in relative abundance with increasing altitude. The observed patterns are attributed to dissolution-reprecipitation of Fe oxides that initially formed through ferrallitic weathering of volcanic parent materials that were roughly uniform along the toposequence. At high altitude, coupled hematite dissolution and DOM-ferrihydrite formation are favoured by high organic matter contents and low pH, related to cool humid environmental conditions and their effect on the vegetation and organic matter cycling.

Long-term manure application increases soil organic matter and aggregation, and alters microbial community structure and keystone taxa

Microbes play pivotal roles in soil organic matter (SOM) turnover: formation and decomposition. Organic fertilizers play crucial role for SOM accumulation, aggregate formation and influence microbial community composition and co-occurrence networks in microhabitats. Here, we investigated prokaryotic and fungal communities and their co-occurrence networks in four aggregate size classes in upland Ultisol following 27 years of mineral and/or organic fertilizer (rice straw, peanut straw, radish, or pig manure) application. Organic fertilizers and aggregate size classes have main and interactive effects on SOM content in aggregates (p < 0.001). Aggregate size classes accounted for most of the variance (43%) of SOM content, with more SOM accumulated in macroaggregates (>250 μm) than microaggregates (<250 μm). Increased aggregate size affected prokaryotic and fungal community structure by increasing Rhizobiales and decreasing Eurotiales. Solibacterales and Mortierellales were particularly abundant in small microaggregates (<53 μm) due to substrate preferences. Organic fertilizers regulate microbial community structure more than aggregate size, accounting for 41% and 29% of variance in prokaryotic and fungal communities, respectively. Pig manure exerted the strongest effect on SOM content and aggregation, and influenced microbial community structure more strongly than plant residues, primarily by increasing Bacillales, Gaiellales and Pezizales, and decreasing Thermogemmatisporales. This effect of pig manure was related with efficient increase of SOM content and pH. Co-occurrence network analysis revealed more positive or negative linear relationships among microbial groups in microaggregates than in macroaggregates, indicating stronger synergistic and antagonistic microbial interactions in microaggregates with fewer favorable niches (higher recalcitrant SOM and less labile SOM). Thermogemmatisporales was identified as the most influential keystone taxon (relative abundance ∼4.9%) in soil, and its abundance rapidly diminished with increasing SOM content in macro- and microaggregates. Thus, microbial community structure is dependent on aggregate size, and this should be considered during sampling. Overall, long-term pig manure amendment increased the SOM content and aggregation, altering prokaryotic and fungal community structure and keystone taxa.

Manure over crop residues increases soil organic matter but decreases microbial necromass relative contribution in upland Ultisols: Results of a 27-year field experiment

Organic fertilizers increase soil organic matter (SOM) stocks, but the underlying processes depend on the fertilizer type and remain largely unknown. To evaluate the predominant C stabilization mechanisms, upland Ultisols subjected to 27 years of mineral and organic fertilization were analyzed for SOM content, aggregate size classes, and amino sugar composition. The long-term field experiment had seven treatments: no fertilization (Control), mineral NPK fertilizers (NPK), NPK plus lime (NPK + Lime), NPK plus peanut straw (NPK + PeanutStraw), NPK plus rice straw (NPK + RiceStraw), NPK plus radish residue (NPK + RadishResidue), and NPK plus pig manure (NPK + PigManure). The 27-year application of mineral fertilizers (NPK and NPK + Lime), NPK + crop residues, and NPK + PigManure increased SOM content by 11.0–13.2%, 16.3–25.3%, and 44.3%, respectively, compared with the Control. The aliphaticity and recalcitrance indices based on 13C nuclear magnetic resonance spectra of organic fertilizers were higher for pig manure than for crop residues. Both indices were closely correlated with SOM content after 27 years, so higher proportions of recalcitrant C in manure facilitated SOM accumulation. NPK + PigManure increased the mass proportion of large macroaggregates 2.9-fold compared with the Control, and reduced the effective diffusion coefficient of oxygen in the soil. Consequently, NPK + PigManure limited the activity and abundance of aerobes and the accessibility of SOM to microorganisms, in turn facilitating SOM accumulation. The application of mineral fertilizers, NPK + crop residues, and NPK + PigManure increased microbial necromass to 2.85–3.03, 3.21–3.45, and 3.62 g C kg−1, respectively, from 2.63 g C kg−1 in the Control. Compared with crop residues, pig manure did not affect bacterial necromass but increased fungal necromass from 2.19 to 2.39 g C kg−1 to 2.58 g C kg−1, which might associate with increased SOM stability. However, the relative contribution of microbial necromass to SOM was lower under NPK + PigManure than under NPK + crop residues, since more added C was protected in the NPK + PigManure soil. Our results suggest that manure may contribute to SOM accumulation and stabilization in three ways: directly through the input of recalcitrant organic C, indirectly through the stabilization of aggregates and physical protection of C, and to a lesser extent through increasing fungal necromass.

Mississippi river sediment diversions and coastal wetland sustainability: Synthesis of responses to freshwater, sediment, and nutrient inputs

Management and restoration of coastal wetlands require insight into how inundation, salinity, and the availability of mineral sediment and nutrients interact to influence ecosystem functions that control sustainability. The Mississippi River Delta, which ranks among the world's largest and most productive coastal wetland complexes, has experienced extensive deterioration over the last century due, in large part, to enhanced vulnerability to relative sea-level rise and lateral erosion caused by a combination of natural processes and anthropogenic modifications of hydrology. This land loss crisis has prompted the State of Louisiana to develop a comprehensive restoration plan that includes constructing and implementing a series of large-scale sediment diversions that will reconnect sediment- and nutrient-rich Mississippi River water to adjacent bays, estuaries, and wetlands. Sediment loading through diversions is predicted to enhance the long-term sustainability of coastal wetlands; however, the additive effects of increased inundation, abrubt changes in the salinity regime, and high nutrient loads on wetland plant growth and organic matter (SOM) decomposition rates, which help regulate accretion and elevation change, is uncertain. Therefore, this review attempts to synthesize existing information to inform predictions of the interactive effects of diversions on these drivers of coastal wetland sustainability. The data suggest that sediment deposition within an optimal elevation range will increase the overall productivity of existing wetlands where prolonged flooding does not counter this effect by limiting plant growth. A reduction in salinity may increase plant productivity and cause vegetation shifts to less salt tolerant species, but seasonal swings in salinity may have unforeseen consequences. Nutrient-loading is predicted to lead to greater aboveground productivity, which, in turn, can facilitate additional sediment trapping; however, belowground productivity may decline, particularly in areas where sediment deposition is limited. In areas experiencing net deposition, nutrient-enrichment is predicted to enhance belowground growth into new sediment and contribute to positive effects on soil organic matter accumulation, accretion, and elevation change. Thus, we contend that sediment input is essential for limiting the negative effects of flooding and nutrient-enrichment on wetland processes. These conclusions are generally supported by the biophysical feedbacks occurring in existing prograding deltas of the Mississippi River Delta complex.

Microtopography-mediated hydrologic environment controls elemental migration and mineral weathering in subalpine surface soils of subtropical monsoonal China

Local topography and elevation gradients can exert important influences on soil formation processes such as elemental migration, mineral weathering, and soil organic matter (SOM) accumulation, yet these influences remain insufficiently investigated to date, particularly in surface soils of subtropical monsoonal regions. Here, we report on an investigation of a series of surface soils collected from four different topographic locations across the subalpine Dajiuhu Critical Zone Observatory (CZO), representing hillslope (planar), swale and river channel (convergent), and bulge (divergent) microtopographic sites. Evidence provided by rare-earth element (REE) patterns, immobile element ratios, clay-mineral compositions, and particle-size distributions suggests that these soils have rather uniform parent materials. X-ray diffraction (XRD) analysis revealed that secondary clay minerals in these soils are complex, being dominated by various interstratified clays. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy shows similar patterns among the swale, bulge, and river-channel soils that differ from those of the hillslope soils. Most soils at convergent sites with poor drainage contain more smectitic clays (interstratified illite/smectite and chlorite/smectite) and less vermiculitic clays (hydroxyl-interlayered vermiculite and interstratified illite/hydroxyl-interlayered vermiculite), and exhibit weaker chemical weathering and fewer elemental losses than those from non-convergent sites. The diversity of clay types can be ascribed to the complexity and heterogeneity, in particular of pH and hydrology, in these soil environments. Across the range of microtopographic sites investigated here, elemental migration and chemical weathering are generally coupled, with greater elemental losses associated with more intense chemical weathering. Soil organic carbon (SOC) content generally increases at higher elevations, which is attributable to lower temperatures and a consequent reduction of microbial remineralization, and under more reducing soil water conditions. Though influenced by eolian dust, variations in Fe/Mn, Ce anomaly, and Corg/P can reflect redox conditions of different soils. SOC retention is also closely associated with soil redox status, with more reducing conditions being more conducive to SOC preservation.

Understory ferns alter soil carbon chemistry and increase carbon storage during reforestation with native pine on previously degraded sites

Reforestation with native species and resulting understory succession can exert important influences on soil organic matter (SOM) storage and chemistry, but a mechanistic understanding of these effects is lacking. We studied different aged Masson pine (Pinus massoniana L.) plantations with and without the understory fern, Dicranopteris dichotoma (Thunb.) Berhn., in subtropical China to assess how SOM over a 30 year sequence of pine growth and fern expansion. To do this, we measured total SOM, lignin-derived phenols, soil carbon (total C and 13C), soil nitrogen (total N and 15N), and soil microbial community composition via phospholipid fatty acid (PLFA) analyses. We found that the accumulation of newly-formed SOM outweighed decomposition of old SOM, with the majority of this increase being derived from fern detrital inputs. Where ferns were present, ferns contributed 54–61% of total soil C storage in surface (0–10 cm depth) soils, which was 62–91% higher than pre-reforestation soil C storage. We found that the abundance of lignin-derived compounds was lower in fern dominated soils, perhaps because soils under ferns supported more soil fungi, the primary decomposers of the lignin in soil. Fern soils also showed higher ratios of syringyls to vanillyls and decreased δ13C values, an indicator that ferns altered the composition of SOM at the molecular level while contributing significantly to SOM accumulation. Reforestation especially when accompanied by fern expansion also improved soil N and phosphorus (P) status, with observed declines in soil δ15N in fern dominated soils aligning with increased nutrient retention and observed increases in soil C storage. Our study highlights the potentially important role of understory ferns in mediating SOM chemistry and soil C storage during ecosystem recovery.

When drought meets forest management: Effects on the soil microbial community of a Holm oak forest ecosystem

The growth and survival of plants in semiarid Mediterranean forests can be improved through the benefits conferred by thinning, a forest management practice that removes trees and reduces the competition between the remaining ones. Here, we evaluate the impacts of induced drought (the exclusion of 25% of the natural rainfall for 5 years) and thinning, and their interaction, with the objective of determining whether the thinning of Holm oak (Quercus ilex L.) modulates the resistance of the soil microbial community to drought. Sequencing of 16S rRNA and ITS amplicons revealed that drought, thinning, and their interaction influenced the composition of the bacterial community, while the fungal community was exclusively affected by thinning. Thinning consisted of the removal of the aboveground parts of the Holm oak trees, which were thereafter left in forest stand. Thinning contributed to the C and N contents, with parallel increases in microbial biomass, particularly in summer. Drought increased the amounts of total organic C and total N, likely due to the reduced enzyme activities. Indeed, the composition of the bacterial community was modulated primarily by the indirect and long-term effects of drought - the accumulation of soil organic matter - rather than by the direct effect of the lower water content imposed by the drought treatments. Thinning under drought conditions did not increase soil organic C (SOC) content. However, the resistance of the soil microbial community to drought was fostered by thinning, particularly at the functional level, as indicated by the enzyme activities related to C, N and P cycles. These responses were associated to variations in the composition of the microbial communities in thinned, drought-exposed plots, in comparison to unthinned, drought-exposed plots. In conclusion, the interaction between forest management and drought influenced the soil microbial community of a Holm oak-dominated Mediterranean ecosystem.

Soil physical attributes and organic matter accumulation under no-tillage systems in the Cerrado

Soil management has a major effect on soil physical characteristics, and consequently on soil organic matter (SOM) content, which are important for the success of crop production. The aim of this study was to evaluate the soil physical attributes and the accumulation of SOM in no-tillage systems (NTS) with different periods of implantation in a conventional tillage area and to compare them with native forest (NF) in the Cerrado biome. The experiment was planned in a 3 × 4 factorial scheme, consisting of three soil treatments (NTS for 17 years (NTS17), NTS for 5 years (NTS5) and NF) and four soil depths (0–0.1, 0.1–0.2, 0.2–0.3 and 0.3–0.4 m), with a completely randomised design and four replicates. At deep soil layers (0.2–0.4 m) the NTS17 area had a greater soil density than the NTS5 and NF areas, and greater SOM compared with the NTS5 area. Soil macroporosity in the NTS5 area was below 10% at all soil depths evaluated. The NF area had the greatest total organic carbon content (1.39 dag kg–1), stock of carbon (16.63 Mg ha--1), amount of soil organic matter (28.66 Mg ha--1) and equivalent carbon credits (60.96 Mg ha–1). Carbon stocks were similar in the NTS areas in all soil depths evaluated. The results indicate that conventional tillage areas can be successfully recovered under the Cerrado edaphoclimatic conditions with the implantation of an NTS.

Vpliv suše na drobne korenine dreves in ektomikorizo v gozdnih ekosistemih

Drought stress elicits many changes in tree fine roots and ectomycorrhizal fungi. Trees cope with drought through avoidance mechanisms or tolerance. Drought can result in changes in colonization by ectomycorrhizal fungi and in the structure of ectomycorrhizal communities. Survival of tree seedlings is supported through common mycelium networks. In moderate drought, there is greater colonization by ectomycorrhizal fungi compared to severe drought, resulting in several beneficial effects to the tree. Under drought, the frequency of ectomycorrhizal fungus Cenococcum geophilum Fr. often increases. C. geophilum sustains tree fine roots function and therefore roots are able to absorb water as soon as the drought period is over. Under drought, synthesis of recalcitrant organic compounds in roots, e.g. lignine, is increased. Recalcitrant compounds such as melanine are also found in C. geophilum, contributing to the accumulation of recalcitrant soil organic matter.

Effect of waterlogging on soil biochemical properties and organic matter quality in different salt marsh systems

This study investigated the effects of hydroperiod on soil organic matter quality in three different salt marshes in the Baiona lagoon (N Italy) representing terrestrial, intertidal and subaqueous ecosystems in the area. The study specifically aimed to gain some insight into how soil waterlogging (hydroperiod) affects the chemical and biological properties of soils as well as the quality and structure of the soil organic matter (SOM). Total contents of selected nutrients, total organic carbon and carbon stable isotope (δ13C) were measured in all soil profiles. The results of these analyses enabled us to define the different origin of the SOM by discriminating between terrestrial and aquatic SOM sources. The findings also show that accumulation of nutrients and SOM is significantly magnified in intertidal systems, in which pedoturbation effects induced by water movements are particularly strong. In addition, DRIFT spectra of humic acids revealed the changes in the main functional groups in relation to increased waterlogging, highlighting the lower aromaticity and complexity in subaqueous soils (SASs), which is possibly due to the effect of the soil water saturation on the chemical and biological SOM transformation processes. Microbial biomass carbon (MBC), microbial quotient (Qmic) and the activities of some soil enzymes were measured to estimate soil metabolic activity in the systems and to evaluate how the microbial pool contributes to transforming the SOM. In all systems, the enzymatic activities were generally higher in subsurface horizons than in the surface horizon. This unexpected behaviour can be explained by the combined effect of water movement, erosion processes and preservation of SOM under anaerobic conditions. This study represents an attempt to investigate and understand the ongoing degradation processes in salt marsh ecosystems. The findings emphasize the strong influence of water flow and erosional processes associated with soil waterlogging on chemical and biological reactions in intertidal and subaqueous systems.

Soil carbon sequestration to depth in response to long-term phosphorus fertilization of grazed pasture

The impact of phosphorus (P) fertilizer application on the sequestration of soil organic carbon (C) was investigated in an acidic permanent pasture soil grazed by sheep. Data were collected after 20 years of P fertilization (average 19 kg P ha−1 year−1) compared to a non-P fertilized control, on a P deficient soil. The high P treatment had higher productivity and stocking rate (hereafter termed fertilization treatment). Soils to a depth of 110 cm were analysed for soil bulk density, whole soil C and nitrogen (N), and for C, N, P and sulphur (S) that was associated with a soil pool of fine fraction C (FFC). Fertilization and higher stocking rate had no effect on soil pH but led to a decrease in soil bulk density to depth, and an increase in total soil C that was associated with an increase in both the whole soil C (WSC) and the FFC throughout much of the soil profile. The increase in soil C was evident in both the C concentration (mg C kg−1 soil) and C stocks (Mg C ha−1) to a depth of at least 60 cm, with the largest difference between fertilizer treatments occurring in the top 10 cm soil layer. In the 0–10 cm layer, P fertilization increased soil organic matter (SOM) by ~1% from 3% SOM in the unfertilized treatment to 4% SOM in the fertilized treatment. Over the top 60 cm of soil profile, P fertilization increased total C stock by 12 Mg C ha−1, with 92% of the C stock associated with the FFC. The higher soil C in the fertilized treatment was also correlated with a concomitant increase in N, P and S content in the FFC pool (R2 = 0.97, 0.89 to 0.82 for N, P and S, respectively), whereas in the unfertilized treatment nutrient correlations with C in the FFS were observed for N and S only. Although C:N, C:P and C:S nutrient mass ratios in the FFC pool differed markedly with soil depth, no significant difference in nutrient ratios were observed due to fertilizer treatment for any of the nutrients. Collectively our data indicate that sustained P input to a texture contrast pasture soil (Yellow Chromosol) and its associated increase in stocking rate and productivity in a temperate climate had significant long-term benefit for C sequestration and accumulation of soil organic matter (SOM) throughout the soil profile.