Lower Soil Organic Carbon Concentrations Research Articles

Shifting relationships between SOC and molecular diversity in soils of varied carbon concentrations: Evidence from drained wetlands

The molecular diversity of soil organic matter (SOM) is considered to be a critical factor influencing soil organic carbon (SOC) persistence, and is found to show a positive relationship with SOC in many upland soils with relatively low SOC concentrations. However, the SOM diversity-SOC relationship remains under-investigated in high-SOC soils such as wetlands, which exhibit divergent SOC accumulation mechanisms compared to upland soils and may induce contrasting shifts in SOM composition during drainage-induced SOC changes. Here we utilized three contrasting wetlands (i.e., two fens and a bog) that encompassed a wide SOC range and experienced decades of artificial drainage to investigate shifts in SOM diversity after drainage. The molecular diversity of SOM was assessed based on SOM pyrolysis products using pyrolysis–gas chromatography-mass spectrometry, and its influencing factors (including plant inputs, soil minerals, and extracellular enzyme activity, etc.) were evaluated. We observed divergent responses of SOM diversity to long-term drainage in the investigated wetlands. While SOM diversity increased with SOC concentration in the low-SOC (i.e., ≤ 60 mg g−1 soil) fen after drainage, it did not change in high-SOC (i.e., > 60 mg g−1 soil) wetlands after drainage regardless of SOC variations. While plant inputs (e.g., root mass) drove SOC accumulation in these wetlands, microbial processing rather than plant inputs dominated increases in SOM diversity, thereby resulting in the decoupling of SOC concentration and SOM diversity in high-SOC soils. These findings suggest that SOC and SOM diversity had a non-linear relationship in divergent soils. In organic-rich soils such as peatlands, SOM diversity did not increase with SOC accumulation after drainage, in contrast to low-SOC soils. Our study provides a novel, molecular diversity-based perspective on understanding wetland SOC stability in the context of drainage caused by increasing human activity and/or climate changes.

Negative responses of terrestrial nitrogen fixation to nitrogen addition weaken across increased soil organic carbon levels

The traditional view holds that biological nitrogen (N) fixation is energetically expensive and thus, facultative N fixers reduce N fixation rates while obligate N fixers are excluded by non-N fixers as soil N becomes rich. This view, however, contradicts the phenomenon that N fixation does not decline in many terrestrial ecosystems under N enrichment. To address this paradoxical phenomenon, we conducted a meta-analysis of N fixation and diazotroph (N-fixing microorganism) community structure in response to N addition across terrestrial ecosystems. N addition inhibited N fixation, but the inhibitory effect weakened across increased soil organic carbon (SOC) concentrations. The response ratios of N fixation (including free-living, plant-associated, and symbiotic types) to N addition were lower in the ecosystems with low SOC concentrations (<10 mg/g) than in those with medium or high SOC concentrations (10–20 and > 20 mg/g, respectively). The negative N-addition effects on diazotroph abundance and diversity also weakened across increased SOC levels. Among the climatic and soil factors, SOC was the most important predictor regarding the responses of N fixation and diazotroph community structure to N addition. Overall, our study reveals the role of SOC in affecting the responses of N fixation to N addition, which helps understand the relationships of biological N fixation and N enrichment as well as the mechanisms of terrestrial C and N coupling.

Soil organic carbon and nitrogen responses to occasional tillage in a continuous no-tillage system

No-tillage (NT) practice is increasingly adopted to improve soil health and sustainability. However, long-term NT may increase soil organic carbon (SOC) and nutrient stratification, soil acidification, compaction, and herbicide resistance in weeds. Targeted strategic disturbance of continuous no-tillage is occasionally practiced to overcome challenges of long-term no-tillage, yet limited information is available on their impacts on SOC and nitrogen (N) components. We evaluated the response of SOC and N fractions to different tillage practices after the imposition of one-time strategic minimum tillage operation (i.e., stubble mulch tillage: SMT) in a continuous NT system in semi-arid drylands. Tillage treatments included conventional tillage (CT), NT, SMT, and strip-tillage (ST). The CT, NT, and ST plots were established in 2013, and SMT plots were established with only one pass of stubble mulch tillage in september of 2019 in long-term NT plots. Soil samples were collected from 0 to 15 and 15–30 cm depth of each plot before SMT and 2-days, 7-months, 14-months, 19-months, and 26-months after SMT implementation. The CT management resulted in 12–27% and 11–16% lower SOC concentration than under NT, SMT, and ST in 0–15 and 15–30 cm depth, respectively. The CT and ST had 22–53%, 44–79%, and 43% greater soil inorganic N than NT and SMT after 2-days, 7-months, and 19-months, respectively, in 0–15 cm depth. The 3d-carbon dioxide-carbon (CO2-C) was 32–65%, 48–65%, 62–102%, and 122–195% greater under CT and ST than under NT and SMT after 2-days, 7-months, 19-months, and 26-months, respectively, in 0–15 cm depth. The microbial biomass carbon (MBC) showed varying responses, with 31–64% lower MBC after 2-days and 26-months, but it was 35–39% greater after 7-months under CT than under NT and SMT. There was no difference between NT and SMT systems on SOC and N fractions for most of the samplings. Further research may reveal how often SMT can be utilized in a long-term NT system without negative impacts on soil properties in semi-arid dryland cropping systems. This study showed one stubble mulch tillage after six years of continuous NT did not affect SOC and N concentrations in 0–30 cm depth.

Effects of decadal nitrogen and phosphorus fertilization on microbial taxonomic and functional attributes associated with soil organic carbon decomposition and concentration in an alpine meadow

Microorganisms drive many important soil processes and ecosystem functioning; however, the soil microbial community can respond to environmental change, which can alter decomposition and storage of soil organic carbon (SOC). We assessed soil microbial taxonomic and functional attributes linked to SOC decomposition and concentration based on a grassland experiment fertilized with nitrogen (N) and phosphorus (P). There were divergent responses at both the taxonomic and functional level to fertilization, with a greater quantitative response in terms of the taxonomic attributes of bacteria and fungi than the abundance of SOC degradation genes. There was a negative correlation between SOC concentration and the abundance of SOC degradation genes. This indicated that N and P fertilization had the potential to stimulate SOC decomposition through modification of the soil microbial community, particularly by increasing the abundance of SOC degradation genes. Redundancy and variation partitioning analyses, as well as structural equation models revealed that soil total P was a pivotal factor shaping the microbial community both for taxonomic and functional gene attributes in the alpine meadow. The abundance of SOC degrading genes had stronger effects on SOC concentrations than did taxonomic attributes. Quantification of the abundance of SOC degradation genes can provide more direct and effective understanding of SOC decomposition processes and prediction of shifts in soil function as organic C pool in grasslands than a focus on the taxonomy of microbial communities. Given the important role of microbial community in regulating SOC decomposition and lower SOC concentrations under N and P fertilization, we recommend that the alpine meadow should be protected from mineral fertilizers, particularly P alone fertilization, to sustain high grassland ecosystem function as SOC sink in this region.

What Influence Does Conventional Tillage Have on the Ability of Soils to Sequester Carbon, Stabilise It and Become Saturated in the Medium Term? A Case Study in a Traditional Rainfed Olive Grove

Soils have the capacity to store three times more carbon (C) than the atmosphere. This fact has focused scientific and governmental attention because it is one way to mitigate climate change. However, there comes a time when the capacity of soils to store C reaches a limit, considering soil organic carbon (SOC) saturation. In the Mediterranean area, agricultural soils are traditionally exposed to conventional tillage (CT), causing soil properties and quality degradation. Therefore, this study aimed to determine whether CT modifies the carbon storage capacity (carbon saturation), linked to soil mineral fractions &lt;20 µm in olive grove soil in a Mediterranean area over 15 years. The results showed losses of SOC and soil organic carbon stock (SOC-S) over the period studied. Moreover, CT significantly affected aggregate grain size, reducing the percentage of small macro-aggregates (2000–250 µm) by 51.1%, 32.9%, 46.6%, and 50.6% for the Ap, Bw, BC, and C horizons, respectively, and promoting an increase in fine fractions (large micro-aggregates (250–53 µm), silt + clay fraction (53–20 µm) and fine silt + clay (&lt;20 µm)). After 15 years, SOC fractionation showed a decrease in SOC concentration within the large macro-aggregate fraction (&gt;2000 µm) of 38.6% in the Bw horizon; however, in the small macro-aggregates (2000–250 µm), an increase in SOC concentration over time, of 33.5%, was observed in the Ap and Bw horizons. This increasing trend continued in the fine soil fractions. Concerning SOC bound to the fine mineral fraction (&lt;20 µm), evolution over time with CT led to an increase in soil sequestration capacity in the first horizons of 44.7% (Ap horizon) and 42.9% (Bw horizon), and a decrease in depth (BC horizon) of 31.3%. Finally, the total saturated soil organic carbon stock (T-SOC-Ssat), after 15 years, experienced an increase of 30.5 Mg ha−1, and these results conditioned the soil organic carbon stock deficit (SOC-Sdef), causing a potential increase in the capacity of soils to sequester carbon, of 15.2 Mg ha−1 in 15 years. With these results, we can affirm that the effect of CT in the medium term has conditioned the degradation of these soils and the low SOC concentrations, and has therefore made it possible for these soils, with the application of sustainable management practices, to have a high carbon storage capacity and become carbon sinks.

Organic carbon is mostly stored in deep soil and only affected by land use in its superficial layers: A case study

AbstractSoils are the largest terrestrial C pool on Earth. However, most studies only investigated soil organic carbon (SOC) storage on surface soils layers. This has resulted in lack of knowledge on SOC dynamics in deep soil layers despite the fact that they contain a significant portion of the stored organic C. This study assessed how contrasting land uses and agronomic practices affect SOC stocks up to a 5‐m depth, and investigated superficial SOC concentration, and particle and density fractionation in a case study in the U.S. Southeastern Plains. On average, deep (90–500 cm) SOC accounted for 80% of total organic C across the investigated land‐use systems. A close relation between SOC concentration and Mehlich‐3 extractable Al and Fe concentration in deep soil samples suggested that non‐crystalline metals play a key role stabilizing deep SOC in this system. Land use had a significant effect only on superficial SOC concentration, and light, heavy, and mineral associated organic C within micro and macroaggregates. Soils from native vegetation, and non‐irrigated sod‐based rotation plots showed greater concentrations of SOC in almost all assessed micro and macroaggregates fractions, whereas those from the conventional crop rotation and irrigated sod‐based rotation plots showed lower SOC concentration in all assessed fractions.

Comparing deep soil organic carbon stocks under kiwifruit and pasture land uses in New Zealand

Soil organic carbon (SOC) is important natural capital for agricultural production, as it affects soil physical, chemical and biological functions and the provision of ecosystem services. Measures of land-use effects on SOC stocks generally focus on the top 0.3 m of soil, as the topsoil has the highest SOC concentration. However, while subsoil horizons have low SOC concentrations, they contain a greater absolute amount of SOC with longer mean residence times than topsoil layers. Perennial horticultural crops offer potential to store SOC deep in the soil profile because of their long-lived and deep rooting systems. To investigate the hypothesis that kiwifruit (Actinidia chinensis Planch.) can increase subsoil SOC stocks, we sampled soils from 19 paired kiwifruit and pasture sites in New Zealand in 2018. Pasture was selected for comparison as it was the antecedent land use before establishment of the kiwifruit orchards. Paired land uses were located within 100 m of each other on the same soil type. Kiwifruit vines were at least 15 years old and the pasture was not cultivated during that time. Total SOC and nitrogen (N), and labile soil SOC stocks were assessed to a depth of 2 m. Kiwifruit production resulted in a modest increase in SOC and N stocks at a depth of 1.5–2.0 m (1.6 Mg C ha−1 and 0.52 Mg N ha−1), averaging to increases of 0.06 Mg C ha−1 y−1 and 0.02 Mg N ha−1 y−1. However, cumulative SOC and N stocks to 2-m depth were not different between land uses. The labile water extractable pools of SOC were lower under kiwifruit in the topsoil (0–0.1 m) and corresponded with lower N stocks and a higher soil C:N ratio at this depth. Further work on the dynamics of subsoil SOC pools is needed to understand the contribution of perennial horticulture crops to subsoil SOC storage.

Salinity Affects Topsoil Organic Carbon Concentrations Through Regulating Vegetation Structure and Productivity

AbstractEstuarine salt marshes have been recognized as one of the most efficient carbon sinks in the biosphere, with considerable potential for climate change mitigation. However, there are still uncertainties about the response of soil carbon stocks to enhanced soil salinization caused by accelerated sea‐level rises and aggravated saltwater intrusion. We therefore conducted both field investigations in the Chongming Dongtan salt marsh of the Yangtze River Estuary, China, and manipulative experiments on marsh soils occupied, respectively, by the invasive Spartina alterniflora, and the native Phragmites australis and Scirpus mariqueter, to identify the effects of elevated soil salinity on top soil organic carbon (SOC) concentration. Our field data showed that SOC concentrations were significantly positively associated with soil salinity concentrations, annual net primary productivity, and marsh surface elevation but showed a significant negative relationship with median grain size. Compared with the two native species, S. alterniflora preferred more saline conditions and had a higher SOC concentration. Although raised flooding salinities (0–35 ppt) did not strongly affect SOC concentrations, elevated soil salinities significantly corresponded with low SOC concentrations and plant biomass in manipulative experiments. These findings indicated that soil salinity, plant species, and soil texture were key factors controlling SOC concentrations in the studied salt marsh. Moreover, soil salinity could affect SOC concentrations through regulating vegetation spatial structure and plant biomass production. The further invasion of the S. alterniflora community will exert a positive influence on SOC concentrations in the Chongming Dongtan salt marsh.

Soil properties on sub-Antarctic Macquarie Island: Fundamental indicators of ecosystem function and potential change

We examined the nature and properties of soils on Australian sub-Antarctic Macquarie Island to determine key environmental factors driving their distribution, development and change. We provide the first classification of these soils using Australian and international (WRB) systems while combining elemental, stable and radio-isotope analysis to interpret processes of soil formation and key controlling environmental factors. Soil organic carbon (SOC) and total nitrogen (TN) concentrations across the island were influenced largely by elevation and topographic position with coastal soils and wetter depressions containing more SOC and TN compared with drier and higher elevation locations. Soils on the high, exposed plateau of the island contained low SOC and TN concentrations by comparison. Results suggested that soils of the coastal zone are subject to ongoing aggradation with significant inputs of nutrient, particularly extractable P (Ext P), from oceanic and especially avifauna sources. Nutrient subsidy was concentrated on coastal margins and the more sheltered eastern side of the island, diminishing significantly with increasing elevation and distance from the coast. Soils of the central plateau contained very low Ext P concentrations throughout the profile and appear to be relic if not degrading. Further comprehensive soil mapping, classification and monitoring across Macquarie Island will elucidate the important role that soils serve for healthy ecosystem function in these sub-Antarctic environments and provide early warning indicators of significant environmental change.

Soil organic carbon depletion and degradation in surface soil after long-term non-growing season warming in High Arctic Svalbard

Arctic tundra active-layer soils are at risk of soil organic carbon (SOC) depletion and degradation upon global climate warming because they are in a stage of relatively early decomposition. Non-growing season (NGS) warming is particularly pronounced, and observed increases of CO2 emissions during experimentally warmed NGSs give concern for great SOC losses to the atmosphere. Here, we used snow fences in Arctic Spitsbergen dwarf shrub tundra to simulate 1.86 °C NGS warming for 9 consecutive years, while growing season temperatures remained unchanged. In the snow fence treatment, the 4-11 cm thick A-horizon had a 2% lower SOC concentration and a 0.48 kg C m−2 smaller pool size than the controls, indicating SOC pool depletion. The snow fence treatment's A-horizon's alkyl/O-alkyl ratio was also significantly increased, indicating an advance of SOC degradation. The underlying 5 cm of B/C-horizon did not show these effects. Our results support the hypothesis that SOC depletion and degradation are connected to the long-term transience of observed ecosystem respiration (ER) increases upon soil warming. We suggest that the bulk of warming induced ER increases may originate from surface and not deep active layer or permafrost horizons. The observed losses of SOC might be significant for the ecosystem in question, but are in magnitude comparatively small relative to anthropogenic greenhouse gas enrichment of the atmosphere. We conclude that a positive feedback of carbon losses from surface soils of Arctic dwarf shrub tundra to anthropogenic forcing will be minor, but not negligible.

Soil organic carbon changes following degradation and conversion to cypress and tea plantations in a tropical mountain forest in Kenya

This study investigates, in a montane forest in Kenya, the changes in amount and stability of soil organic carbon (SOC) as a consequence of: a) forest degradation, by comparing primary and degraded forests; b) the replacement of degraded forests with cypress and tea plantations, by considering sites installed at different time in the past. The SOC concentrations and stocks were determined in different layers to 1 m depth, and the SOC turnover time (TT) derived by measuring the 14C concentration in the layers within the 0–30 cm depth. A significant SOC decline was evident in the 0–5 and 5–15 cm layers of degraded forest while, on the long term, both plantations induced a significant SOC increase in the 0–30 cm depth. The longer TT’s and lower SOC concentrations in the upper layers of degraded rather than primary forests imply an impact of forest degradation on the decomposition of the fast cycling SOC. Similarly, the shorter TT with increasing plantations age implies differences in SOC stabilization mechanisms between plantations and forests. Cypress and tea plantations established on degraded forests stimulate a long term SOC accrual but at the same time decrease the stability of the SOC pool.

Long-term no-till as a means to maintain soil surface structure in an agroecosystem transformed into irrigation

The aim of this study was to determine the most appropriate soil management to reduce the structural degradation of soils susceptible to crusting in Mediterranean areas recently transformed into irrigation. A long-term field experiment (LTE) under rainfed conditions was established in 1996 in NE Spain to compare three tillage systems (no-tillage, NT; reduced tillage, RT; conventional tillage, CT). The experiment was transformed to irrigated corn in 2015. In 2015, an adjacent experiment with the same layout was created (short-term experiment, STE) in an area previously managed under long-term NT. The study was carried out during the second corn growing season (i.e. year 2016). Soil samples were collected from 0 to 5cm depth at different dates during corn season. Dry and water-stable macroaggregates and their C concentration, soil organic carbon (SOC) and labile C concentration, soil respiration, bulk density, penetration resistance (PR), water infiltration, macroporosity, microporosity, amount of crop residues and ground cover, corn development, aerial biomass, and grain yield were measured. In LTE and STE tillage led to a breakdown of dry sieved aggregates (of 2–4 and 4–8mm size) in RT and CT, being slowly reconsolidated throughout the corn growing season. However, macroaggregate water-stability did not increase in CT and RT compared to NT due to a lower SOC concentration, making the soil more susceptible to its degradation by the action of water. SOC differences between treatments were more pronounced in LTE than STE given the long-term differential management in the first, which allowed greater accumulation of SOC under NT. In LTE, PR between corn rows was greater under NT than CT and RT and non-significantly different between treatments within the row. In the case of STE, PR increased over time after tillage (CT and RT) to match NT in the last sampling. Crop establishment was slower in CT than NT in LTE highlighting the impact of soil surface degradation on crop development. However, contrarily to the differences in corn yield in 2015, a careful planting in 2016 led to a lack of differences between tillage systems on corn yield. Our results indicate that in areas transformed into irrigation intensive tillage leads to greater susceptibility to soil structural degradation. Thus, in these areas the adoption of conservation agriculture practices such RT and NT enhances soil resilience to degradation processes and ensures an adequate development of the crop.

Distribution of Soil Organic Carbon in Riparian Forest Soils Affected by Frequent Floods (Southern Québec, Canada)

Measuring soil organic carbon (SOC) in riparian forest soils affected by floods is crucial for evaluating their concentration and distribution along hydrological gradients (longitudinal and transversal). Hydromorphological factors (e.g., sedimentation vs. erosion, size of floodplain, flood recurrence) may be the cause of major variations in the concentration of organic matter and SOC in soils and could have a direct impact on C levels in soil profiles. For this study, SOC concentrations were assessed in riparian soils collected along transects perpendicular to the riverbanks which cross through inundated and non-inundated zones. Other soil properties (e.g., acidity, nitrogen, texture, bulk density) that may affect the concentration of SOC were also considered. The main purpose of this study was to assess SOC concentrations in soils subjected to flooding with those outside the flood zones, and also measure various soil properties (in surface soils and at various depths ranging from 0 to 100 cm) for each selected area. Across the various areas, SOC shows marked differences in concentration and spatial distribution, with the lowest values found in mineral soils affected by successive floods (recurrence of 0–20 years). SOC at 0–20 cm in depth was significantly lower in active floodplains (Tukey HSD test), with average values of 2.29 ± 1.64% compared to non-inundated soils (3.83 ± 2.22%). The proportion of C stocks calculated in soils (inundated vs. non-inundated zones) was significantly different, with average values of 38.22 ± 10.40 and 79.75 ± 29.47 t·ha−1, respectively. Flood frequency appears to be a key factor in understanding the low SOC concentrations in floodplain soils subjected to high flood recurrence (0–20 years).

Deep ploughing increases agricultural soil organic matter stocks.

Subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC concentrations. However, measures for enhancing SOC storage commonly focus on topsoils. This study assessed the long-term storage and stability of SOC in topsoils buried in arable subsoils by deep ploughing, a globally applied method for breaking up hard pans and improving soil structure to optimize crop growing conditions. One effect of deep ploughing is translocation of SOC formed near the surface into the subsoil, with concomitant mixing of SOC-poor subsoil material into the 'new' topsoil. Deep-ploughed croplands represent unique long-term insitu incubations of SOC-rich material in subsoils. In this study, we sampled five loamy and five sandy soils that were ploughed to 55-90cm depth 35-50years ago. Adjacent, similarly managed but conventionally ploughed subplots were sampled as reference. The deep-ploughed soils contained on average 42±13% more SOC than the reference subplots. On average, 45years after deep ploughing, the 'new' topsoil still contained 15% less SOC than the reference topsoil, indicating long-term SOC accumulation potential in the topsoil. In vitro incubation experiments on the buried sandy soils revealed 63±6% lower potential SOC mineralisation rates and also 67±2% lower SOC mineralisation per unit SOC in the buried topsoils than in the reference topsoils. Wider C/N ratio in the buried sandy topsoils than in the reference topsoils indicates that deep ploughing preserved SOC. The SOC mineralisation per unit SOC in the buried loamy topsoils was not significantly different from that in the reference topsoils. However, 56±4% of the initial SOC was preserved in the buried topsoils. It can be concluded that deep ploughing contributes to SOC sequestration by enlarging the storage space for SOC-rich material.

High emissions of greenhouse gases from grasslands on peat and other organic soils.

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site-averaged GHG budgets were surprisingly variable (29.2±17.4 t CO2 -eq. ha-1 yr-1 ) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO2 (27.7±17.3 t CO2 ha-1 yr-1 ) dominated the GHG budget. Nitrous oxide (2.3±2.4kg N2 O-N ha-1 yr-1 ) and methane emissions (30.8±69.8kg CH4 -C ha-1 yr-1 ) were lower than expected except for CH4 emissions from nutrient-poor acidic sites. At single peatlands, CO2 emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO2 on WTD for the complete data set. Thus, regionalization of CO2 emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (Nair ) as a variable combining soil nitrogen stocks with WTD. CO2 increased with Nair across peatlands. Soils with comparatively low SOC concentrations showed as high CO2 emissions as true peat soils because Nair was similar. N2 O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH4 emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.

Soil respiration under three different land use types in a tropical mountain region of China

Soil respiration (SR) is one of the largest contributors of terrestrial CO2 to the atmosphere. Environmental as well as physicochemical parameters influence SR and thus, different land use practices impact the emissions of soil CO2. In this study, we measured SR, bi-monthly, over a one-year period in a terrace tea plantation, a forest tea plantation and a secondary forest, in a subtropical mountain area in Xishuangbanna, China. Along with the measurement of SR rates, soil characteristics for each of the land use systems were investigated. Soil respiration rates in the different land use systems did not differ significantly during the dry season, ranging from 2.7±0.2 µmol m-2 s-1 to 2.8±0.2 µmol m-2 s-1. During the wet season, however, SR rates were significantly larger in the terrace tea plantation (5.4±0.5 µmol m-2 s-1) and secondary forest (4.9±0.4 µmol m-2 s-1) than in the forest tea plantation (3.7±0.2 µmol m-2 s-1). This resulted in significantly larger annual soil CO2 emissions from the terrace tea and secondary forest, than from the forest tea plantation. It is likely that these differences in the SR rates are due to the 0.5 times lower soil organic carbon concentrations in the top mineral soil in the forest tea plantation, compared to the terrace tea plantation and secondary forest. Furthermore, we suggest that the lower sensitivity to temperature variation in the forest tea soil is a result of the lower soil organic carbon concentrations. The higher SR rates in the terrace tea plantation were partly due to weeding events, which caused CO2 emission peaks that contributed almost 10% to the annual CO2 flux. Our findings suggest that moving away from heavily managed tea plantations towards low-input forest tea can reduce the soil CO2 emissions from these systems. However, our study is a case-study and further investigations and upscaling are necessary to show if these findings hold true at a landscape level.

Microorganisms and their substrate utilization patterns in topsoil and subsoil layers of two silt loams, differing in soil organic C accumulation due to colluvial processes

In order to gain more knowledge regarding the microbial community and its influence on carbon sequestration in subsoil, two depth profiles with different natural soil organic carbon (SOC) concentrations were sampled. This makes it possible to investigate the extent to which natural SOC availability or other subsoil specific conditions influence the composition and the functional diversity of the microbial community and in return how the microbial community composition affects SOC sequestration under these conditions. Soil samples were taken at four different depths on two neighbouring arable sites; one Colluvic Cambisol with high SOC concentrations (8–12 g kg−1) throughout the profile and one Haplic Luvisol with low SOC concentrations (3–4 g kg−1) below 30 cm depth. Multi-substrate-induced-respiration (MSIR) was used to identify shifts in functional diversity of the microbial community along the profiles. The amino sugars muramic acid and glucosamine were measured as indicators for bacterial and fungal residues and ergosterol was determined as a marker for saprotrophic fungi. Discriminant analysis of respiration values obtained from the 17 substrates used in the MSIR revealed that substrate use in subsoil differed significantly from that in topsoil and also differed highly between the two subsoils. Amino sugar analysis and the ratio of ergosterol to microbial biomass C showed that fungal dominance decreased with depth. MSIR clearly demonstrated that not only the fungi to bacteria ratio but also substrate use of the microbial community changed with depth according to substrate availability.

Chemical stabilization of organic carbon in agricultural soils in a semi-arid region (SE Spain)

SUMMARYLand use and management, together with soil properties, determine soil organic carbon (SOC) concentration and its stabilization mechanisms. Four soils (0–30 cm depth) were studied in a semi-arid region with different uses and management regimes: two soils with olive cultivation, both under a non-tillage regime and one with a cover crop (OCC) and the other without (ONT); a fluvial terrace soil (FT) with cereal–sunflower–fallow rotation; and an unaltered soil under natural vegetation (oak trees; OT). The OT soil had a higher SOC concentration than the agricultural soils (OCC, ONT and FT), followed by the FT soil without significant differences. The olive grove soils had a lower SOC concentration but the two types of management differed significantly, with higher concentrations due to the cover crop. Hydrofluoric acid (HF)-soluble, hydrochloric acid (HCl)-resistant, and non-oxidizable (sodium peroxodisulphate; Na2S2O8) SOC fractions were determined at different depths (0–5, 5–10, 10–20 and 20–30 cm). The relative HCl-resistant and non-oxidizable SOC fractions increased with depth, whereas the relative HF-soluble SOC fraction varied slightly among the four soils considered. Differences in the SOC-stabilization mechanism were found according to the chemical SOC fractionation. In the FT and OT soils, where HF-soluble SOC and soil respiration rates were higher, the intense biological activity rapidly degraded the plant debris, being partially fixed and stabilized by the fine mineral-soil fraction as the principal stabilization mechanism of SOC. The olive grove soils had lower biological activity but higher SOC resistance to oxidation with Na2S2O8, thus suggesting that chemical recalcitrance of soil organic matter was a relevant stabilization mechanism in these soils.

Spatial and Temporal Variability of Soil Organic Carbon in the Corn Belt of Northeastern China, 1980s–2005: A Case Study in Four Counties

Estimates of changes and possible causes of regional soil organic carbon (SOC) are critical for evaluation of potential responses of terrestrial biosphere to global changes. A total of 382 soil samples, collected in the 1980s from four counties in the Corn Belt of northeastern China, and of 1,514 samples collected in 2005 from the same area, were examined for SOC concentrations. Spatial and temporal SOC concentrations were evaluated after the 25-year interval using geostatistics and kriging interpolation method. Results indicated that the average topsoil SOC concentrations of the study area increased from 1.24% in the 1980s to 1.47% in 2005. Spatial distributions of SOC in the two separate evaluations showed greater SOC concentrations in the middle part of the study area and lower SOC concentrations in the remaining areas. From the 1980s to 2005, SOC concentrations in the central part increased, whereas those in the northeastern and southwestern parts decreased. The increasing trend of SOC concentrations might be attributed to the land-use and land cover changes, crop productivity increase, and agricultural management.

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ13C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ13C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of 13CO2 to maize (Zea mays L.). Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested. The δ13C abundance before and after the continuous labeling was −24.20 ± 0.05 ‰ and −23.80 ± 0.05 ‰ in the rooted bulk soil. The labeling caused increases in δ13C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ13C abundance was enriched in the >2 mm and 1–2 mm aggregates (−23.17 ± 0.12 ‰ and −23.26 ± 0.05 ‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ13C abundance in pore water (−24.0 ± 0.01 ‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ13C abundance was in general greater in the horizontal cell than in the vertical cell. The δ13C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (−23.66 ± 0.11 ‰ and −23.5 ± 0.10 ‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ13C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell. Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.