Stable Carbon Pool Research Articles

亚热带红壤区森林土壤剖面微生物残体碳分布及影响因素

土壤剖面中20cm以下土壤有机碳（SOC）储量占土壤剖面总SOC储量50%左右，由于土壤微生物残体碳（MRC）是稳定土壤碳库的重要来源，因此研究土壤剖面中MRC对SOC的贡献对于评估土壤碳储量具有重要意义。然而，目前关于MRC含量及其对SOC贡献的研究多数集中在土壤表层，在土壤剖面和母质中尚不清楚。选取江西省千烟洲亚热带典型森林红壤剖面，通过氨基糖与磷脂脂肪酸（PLFA）微生物标志物分析方法，分析红壤剖面和母质中MRC的影响机制及其对SOC贡献的分布特征。研究结果表明：（1）MRC含量随着土壤剖面深度增加而显著降低（P<0.05），在整个土壤剖面中，细菌MRC对SOC贡献为6%-12%，真菌MRC对SOC贡献为12%-36%，MRC对SOC贡献为18%-46%。从土壤表层至母质，真菌MRC对SOC贡献高于细菌MRC。（2）结构方程模型结果表明，在土壤剖面中，MRC含量主要受到微生物-PLFA含量、容重和溶解态有机碳含量的影响。研究量化了红壤剖面中MRC对SOC的贡献，表明在20cm以下土壤及母质中，微生物残体碳对红壤地区生态系统碳库具有重要贡献。;In soil profiles, soil organic carbon (SOC) storage beneath 20cm accounts for about 50% of the total SOC storage in the terrestrial ecosystem. Microbial residue carbon (MRC) is considered as an important constituent of persistent soil organic carbon. It is important to learn the role of MRC for soil carbon storage in deep soil as the main source of soil stable carbon pool. Until now, most studies involving MRC content and the contribution of MRC to SOC have mainly focused on surface soil, but seldom in deep soil and parent material. In this study, we selected typical red soil profiles in Qianyanzhou, Jiangxi province in subtropical region. We measured the microbial biomarkers of amino sugars and phospholipid fatty acids (PLFAs) which represent microbial residue carbon content and living microbial biomass content, respectively. The objective of the study was to analyze the influence factors of MRC and to quantify the contributions of MRC to SOC along the soil profiles from the surface to parent material. The results showed that (1) bacterial residue carbon (MRC_B), fungal residue carbon (MRC_F) and MRC contents decreased significantly with soil depth (P<0.05), and were about 0.08, 0.18, 0.25 g/kg at the parent material, respectively. The contributions of MRC to SOC along the soil profiles remained unchanged because of the same variation trend of MRC and SOC contents. Along the whole soil profiles, the contributions of MRC_B, MRC_F and MRC to SOC were about 6%-12%, 12%-36% and 18%-46%, respectively. The contribution of MRC_F to SOC was higher than that of MRC_B to SOC from surface soil to parent material. This may be caused by different components of cell walls in bacteria and fungi. The chitin in cell wall of fungi is more difficult to decompose than peptidoglycan in cell wall of bacteria. In addition, MRC_F could have more physical protection than MRC_B by soil aggregates since extracellular mycelia and polysaccharides are involved in the formation of soil aggregates. (2) The structural equation model showed that MRC content was largely determined by microbial biomass content of PLFA biomarkers, soil bulk density (Db) and dissolved organic carbon (DOC) content in the soil profiles, while Db and DOC indirectly influenced but microbial-PLFA content directly influenced the MRC content. Our study quantified the contribution of MRC to SOC along the soil profile and emphasized the contribution of MRC to the formation of ecosystem carbon pool beneath 20cm and in the parent material in red soil region.

Effects of microbial inoculants on soil carbon stock, enzymatic activity, and above ground and belowground biomass in marginal lands of Northern India

AbstractRestoration practices in marginal lands for food production are the most important sustainability challenge for humans at the global level. Nature‐based solutions (NBS) are among the best interventions to improve soil health, which directly enhances the soil productivity and biodiversity for improved sustainability of ecosystems. Our field experiment was conducted to evaluate the effects of three inoculants, two of which were bacterial: Azotobacter chroococcum (T1), Acetobacter diazotrophicus (T2), and the third fungal: Aspergillus awamori (T3) on soil properties, plant biomass, enzymatic activities (phosphatase, dehydrogenase activity), and soil carbon stock in marginal lands. The experimental results clearly revealed that the inoculations significantly enhanced plant growth and biomass (p &lt; 0.01). The soil enzyme (dehydrogenase and phosphatase) activities increased under T3 and T2 treatments in response to inoculation. Interestingly, the significant results were observed in T3 and T1 treatment in the 4th year of observation for soil carbon stock (53.06 Mg ha−1 yr−1 and 51.41 Mg ha−1 yr−1, respectively). The total soil carbon stock was found to be 38.60 Mg ha−1 yr−1 in control plots. The inoculation enhanced (p ≤ 0.05) the soil biological properties like microbial biomass carbon as 267.62 ± 18.73 μgg−1 to 894.24 ± 41.65 μg g−1. We conclude that selected inoculations in marginal lands can provide a suitable option for eco‐restoration and can sequester more atmospheric CO2 in to stable carbon pools to achieve UN Sustainable Development Goals and food security.

Physical and chemical stabilization of soil organic matter in cropland ecosystems under rice–wheat, maize–wheat and cotton–wheat cropping systems in northwestern India

This article examines the impact of rice–wheat, maize–wheat and cotton–wheat cropping systems on total (TOC), labile and stable organic carbon (C) pools, aggregate stability and distribution of organic C in different size aggregates and the mineral fraction (silt + clay) of soil. The rice–wheat soils had significantly (p < .05) lower TOC concentration (4.46 ± 0.15 g C kg−1) compared with the other two cropping systems. Similarly, water-extractable organic C (WEOC, ∼0.48% of TOC), hot-water-soluble C (HWC, ∼5.5% of TOC) and microbial biomass C (MBC, ∼4.2% of TOC) were significantly lower in soils under rice–wheat compared to the other systems. The formation of water-stable aggregates (WSA) was related to the silt + clay (<0.053 mm) fraction of soils. As the silt + clay fraction increased, the proportion of macro-aggregates (>0.25 mm) increased with a concomitant decrease in the proportion of micro-aggregates (<0.25 mm). The carbon preservation capacity (CPC) of macro-aggregates (CPCMacA), micro-aggregates (CPCMicA) and total water-stable aggregates (CPCWSA) was significantly lower in rice–wheat soils compared with the other systems. The CPCMacA, mineral (silt + clay)-associated C (MinAC) and sodium hypochlorite oxidizable C (NaOCl-C) increased with an increase in the proportion of macro-aggregates and the mineral fraction of soils. A linear increase in the stable C pool ((silt + clay)-C and NaOCl-C) and soils’ silt + clay content implies that soils’ fine fraction was the principal mechanism of textural control on soil organic matter (SOM) stabilization.

Opening the black box: Soil microcosm experiments reveal soot black carbon short-term oxidation and influence on soil organic carbon mineralisation

Soils hold three quarters of the total organic carbon (OC) stock in terrestrial ecosystems and yet we fundamentally lack detailed mechanistic understanding of the turnover of major soil OC pools. Black carbon (BC), the product of the incomplete combustion of fossil fuels and biomass, is ubiquitous in soils globally. Although BC is a major soil carbon pool, its effects on the global carbon cycle have not yet been resolved. Soil BC represents a large stable carbon pool turning over on geological timescales, but research suggests it can alter soil biogeochemical cycling including that of soil OC. Here, we established two soil microcosm experiments: experiment one added 13C OC to soil with and without added BC (soot or biochar) to investigate whether it suppresses OC mineralisation; experiment two added 13C BC (soot) to soil to establish whether it is mineralised in soil over a short timescale. Gases were sampled over six-months and analysed using isotope ratio mass spectrometry. In experiment one we found that the efflux of 13C OC from soil decreased over time, but the addition of soot to soil significantly reduced the mineralisation of OC from 32% of the total supplied without soot to 14% of the total supplied with soot. In contrast, there was not a significant difference after the addition of biochar in the flux of 13C from the OC added to the soil. In experiment two, we found that the efflux 13C from soil with added 13C soot significantly differed from the control, but this efflux declined over time. There was a cumulative loss of 0.17% 13C from soot over the experiment. These experimental results represent a step-change in understanding the influence of BC continuum on carbon dynamics, which has major consequences for the way we monitor and manage soils for carbon sequestration in future.

Conversion from double-rice to maize-rice increases iron-bound organic carbon by “iron gate” and “enzyme latch” mechanisms

To assess the impact of the incorporation of maize into double-rice (RR) on soil organic carbon (SOC) sequestration, a two-year experiment was conducted and changes in iron-bound SOC (Fe-SOC), as a proxy for stable carbon pool, were measured. The introduction of a maize-rice (MR) rotation enhanced Fe-SOC by 12–17 % in the maize season, and it stayed 13–36 % higher during rice cultivation compared to RR. Following the “iron gate” theory, Fe oxidation in the maize season was the leading mechanism for improving Fe-SOC content. However, “enzyme latch”, i.e. the suppression of Fe-SOC loss by decreased phenol oxidase activity, was a dominant factor controlling the increase of Fe-SOC in the rice season of MR. Thus, the introduction of maize into the paddy field increased Fe-SOC, which was caused by combination of “iron gate” mechanism in maize season and “enzyme latch” in rice season.

Improved model simulation of soil carbon cycling by representing the microbially derived organic carbon pool.

During the decomposition process of soil organic carbon (SOC), microbial products such as microbial necromass and microbial metabolites may form an important stable carbon (C) pool, called microbially derived C, which has different decomposition patterns from plant-derived C. However, current Earth System Models do not simulate this microbially derived C pool separately. Here, we incorporated the microbial necromass pool to the first-order kinetic model and the Michaelis-Menten model, respectively, and validated model behaviors against previous observation data from the decomposition experiments of 13C-labeled necromass. Our models showed better performance than existing models and the Michaelis-Menten model was better than the first-order kinetic model. Microbial necromass C was estimated to be 10-27% of total SOC in the study soils by our models and therefore should not be ignored. This study provides a novel modification to process-based models for better simulation of soil organic C under the context of global changes.

Topsoil organic matter build-up in glacier forelands around the world.

Since the last glacial maximum, soil formation related to ice‐cover shrinkage has been one major sink of carbon accumulating as soil organic matter (SOM), a phenomenon accelerated by the ongoing global warming. In recently deglacierized forelands, processes of SOM accumulation, including those that control carbon and nitrogen sequestration rates and biogeochemical stability of newly sequestered carbon, remain poorly understood. Here, we investigate the build‐up of SOM during the initial stages (up to 410 years) of topsoil development in 10 glacier forelands distributed on four continents. We test whether the net accumulation of SOM on glacier forelands (i) depends on the time since deglacierization and local climatic conditions (temperature and precipitation); (ii) is accompanied by a decrease in its stability and (iii) is mostly due to an increasing contribution of organic matter from plant origin. We measured total SOM concentration (carbon, nitrogen), its relative hydrogen/oxygen enrichment, stable isotopic (13C, 15N) and carbon functional groups (C‐H, C=O, C=C) compositions, and its distribution in carbon pools of different thermal stability. We show that SOM content increases with time and is faster on forelands experiencing warmer climates. The build‐up of SOM pools shows consistent trends across the studied soil chronosequences. During the first decades of soil development, the low amount of SOM is dominated by a thermally stable carbon pool with a small and highly thermolabile pool. The stability of SOM decreases with soil age at all sites, indicating that SOM storage is dominated by the accumulation of labile SOM during the first centuries of soil development, and suggesting plant carbon inputs to soil (SOM depleted in nitrogen, enriched in hydrogen and in aromatic carbon). Our findings highlight the potential vulnerability of SOM stocks from proglacial areas to decomposition and suggest that their durability largely depends on the relative contribution of carbon inputs from plants.

A New Approach Using Modeling to Interpret Measured Changes in Soil Organic Carbon in Forests; The Case of a 200 Year Pine Chronosequence on a Podzolic Soil in Scotland

Scotland is continuing to afforest land in order to combat climate change, but the long-term capacity for carbon sequestration in forest soils is still uncertain. Soil organic carbon stock changes were studied in a >120 year-old Scots pine chronosequence and adjacent grassland sites on podzolic soils. Significant differences were observed in the top organic soil horizons, while no changes were noted in the deeper mineral soil horizons. Simulations with the RothC-26.3 model revealed that pine forests could lose a significant amount of soil organic carbon through management operations. The lowest modelled stocks of soil organic carbon were not in the young sites (0-25 years old), but at 43 years since reforestation. Using measured data from our study site, simulations of grassland afforestation suggested that accumulation of organic carbon under forest is mainly in the organic horizons, while the deeper mineral soil horizons are likely to become depleted in soil organic carbon compared to grasslands. Our simulations suggest that afforestation of grasslands would increase overall soil carbon stocks but deplete the more stable carbon pools in the deeper mineral soils, which is not a desirable outcome in the context of climate change. The measurements provide comparative estimates of SOC in grassland and forestry sites at steady state; few studies provide these comparative measures. This study presents a new method for simulating soils that are accumulating carbon using the assumption of steady state with respect to the rate of accumulation. The approach has never been presented before and could have important implications for estimates of impacts of land use and climate change on the large SOC stocks held in highly organic soils. Subsequent simulations provide a new method for interpolating measurements that allow the losses due to management operations to be determined.

Contrasting temperature responses of soil respiration derived from soil organic matter and added plant litter

Accurate description of temperature response and sensitivity of different carbon pools within soil is critical for accurately modelling soil carbon stocks and changes. Inconsistent sampling, incubation and fractionation methods highlights the need for new approaches to this area of study. We developed and tested a new protocol which allowed measurement of the temperature response of two carbon pools within soil. A Horotiu silt loam soil, wet up to 60% maximum water holding capacity, was mixed with 13C-enriched plant litter and incubated for 5 or 20 h, at 30 discrete temperatures (~ 2–50 °C). A mixing model was used to separate respired CO2 into litter and soil organic matter sourced carbon pools, which were then fitted using macromolecular rate theory. Overall, litter sourced respiration had a low Topt (the temperature where respiration rate is maximal) and was less temperature sensitive (Q10) than soil organic matter sourced respiration, which was more Arrhenius-like. We attribute these differences in temperature parameters to the factors that control the availability of carbon to microbes from the labile litter (enzyme kinetics with a clear temperature optimum) compared to the relatively stable soil organic matter (desorption and diffusion that exhibit Arrhenius behaviour). The developed method is rapid and reliable and may be suited to exploring temperature response of a variety of 13C-labelled pools in soil and more clearly demonstrates that labile carbon has very different temperature response than more stable carbon pools in soil.

Are humic substances soil microbial residues or unique synthesized compounds? A perspective on their distinctiveness

Humic substances (HS), which are defined as a series of highly acidic, relatively high-molecular-weight, and yellow to black colored substances formed during the decay and transformation of plant and microbial remains, ubiquitously occur in nature. Humic substances represent the largest stable organic carbon pool in terrestrial environments and are the central characteristic of the soil. However, the validity of the HS concept and the justification of their extraction procedure have been recently debated. Here, we argue that the traditional humic paradigm is still relevant. Humic substances are distinctive and complex because the extracted HS formed during the humification are chemically distinct from their precursors and are heterogeneous among soils. By reviewing the concept, formation pathways, and stabilization of HS, we propose that the key question facing soil scientists is whether HS are soil microbial residues or unique synthesized compounds. Without revealing the distinctiveness of HS, it is impossible to address this question, as the structure, composition, and reactivity of HS are still poorly known owing to the heterogeneity and geographical variability of HS and the limits of the currently available analytical techniques. In our view, the distinctiveness of HS is fundamental to the soil, and thus further studies should be focused on revealing the distinctiveness of HS and explaining why HS hold this distinctiveness.

Differences in Mineral Phase Associated Soil Organic Matter Composition due to Varying Tillage Intensity

The volume of soil organic matter (SOM) changes, owing to variations in tillage systems. Conservation tillage (CT) is a useful method for recovering the SOM content of crop fields. However, little is known about the SOM composition of silt- and clay-associated and aggregate-occluded organic matter (OM). The present study aimed at determining the SOM compositions of various SOM fractions in the same Luvisol in a native forest and under ploughing and CT. SOM fractions (silt and clay associated; sand and aggregates associated; restricted OM) were characterized using diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The size of both the aggregate-occluded and resistant SOM pools increased, owing to the shift in the tillage system to CT for 15 years. As a general trend, the soil organic carbon content was inversely proportional to aromaticity under both crop fields, which supported the preferential mineralization of aliphatic components in each fraction. The shift in the tillage system could trigger rapid qualitative changes even in the stable restricted carbon pools; nevertheless, it was difficult to distinguish between the role of OM and the mineral composition in the FTIR spectra. In particular, the clay-related organic-mineral complexes could trigger difficulties in the traditional interpretation methods.

Effect of fly ash on carbon mineralization of biochar and organic manures added to mine spoil

Mine spoil is deprived of organic carbon. Reclamation and re-vegetation of mine spoil initiate photosynthetic carbon fixation and add soil organic matter. Fly ash (FA) generated from coal-fired power stations can be used for reclamation of mine spoil. Our research was aimed to assess the effect on FA on carbon mineralization of organic manures and biochar added to mine spoil. Incubation experiments were conducted to assess the carbon mineralization of farmyard manure (FYM), chicken litter (CL), plant litter (PL), and biochar (BC) added to mine spoil in the presence and absence of FA. After 1 year of incubation, the cumulative CO2 emission was lower for the FA-added mine spoil and it was higher for PL (4.42 vs. 5.09 g CO2–C/kg soil, with and without FA, respectively) and CL (3.75 vs. 4.07 g CO2–C/kg soil) followed by FYM (1.86 vs. 1.97) and BC (0.67 vs. 0.54 g CO2–C/kg soil). The labile C pool was significantly lower for FA-added mine spoil, whereas the stable carbon pool was higher in FA-added soils: FYM (1.4 vs. 0.6 g CO2–C/kg soil) and CL (2.20 vs. 3.17 g CO2–C/kg soil). Substrate-induced respiration, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were significantly higher under PL, CL, and FYM. FA increased MBC, but decreased DOC. Overall, FA decreased CO2 emission from mine spoil probably due to the interaction of organic matter with the surfaces on FA and the resultant physical protection against microbial decomposition.

Tree basal area and conifer abundance predict soil carbon stocks and concentrations in an actively managed forest of northern New Hampshire, USA

Although northern temperate forests account for a substantial portion of global soil carbon stocks, the relationship between soil carbon and management remains unclear. As the active management of forests and associated forest product industries is often cited as a tactic to offset global greenhouse gas emissions, quantifying the effect of management activities on soil carbon pools is paramount to mitigating future climate change. Hence, our goal was to identify commonalities among the spatial variability, potential drivers, and total soil carbon stocks and concentrations at two locations with different management histories in an actively managed northern U.S. mixed wood forest. We measured the carbon pools of two soil profiles, separated into three layers, within 98 0.04-hectare plots in each of the two management areas. We scaled soil and aboveground carbon to estimate total carbon stocks in each area and examined plot-level soil carbon data in relation to landscape and vegetation factors to evaluate potential drivers of soil carbon. Soil carbon represented approximately 40% of the total carbon in both areas. Total soil carbon was similar between the two areas, but the vertical distribution of carbon differed, with more mineral soil carbon in the area with greater coniferous basal area and fewer harvests in the last 80 years. Soil carbon was moderately variable at small spatial scales (<10 m) and showed little or no spatial structure at the scale of hundreds of meters. Aboveground basal area and the proportion of coniferous vegetation were positively related to soil carbon, matching our expectations. Our best models to predict variation in soil carbon among plots also included elevation, composite topographic index, and pH, but not all relationships matched theoretical expectations. Topography and elevation influenced soil horizon depths and bulk density, and therefore had greater importance for total soil carbon than for carbon concentrations. Although many questions remain, the management activities in northern temperate forests appear to be a slow-moving driver of the relatively large but stable soil carbon pool – especially through management influences on site productivity and tree species composition.

Forest conversion to conifers induces a regime shift in soil process domain affecting carbon stability

A substantial part of forests worldwide is located on acidic soils. Acidification processes are typically characterized by non-linear responses of soils to external drivers. Acid buffer ranges and thresholds in soils are widely acknowledged, yet these non-linearities are rarely incorporated into our understanding of soil carbon dynamics. Here, we studied the effect of conversion of broadleaved mixed forest to Norway spruce monocultures on different functional compartments of the belowground carbon cycle, i.e. litter layers, soil fauna and soil micro-organisms, and examined how in turn they affect soil biochemical characteristics and ultimately, soil carbon stability. By studying this effect chain along a soil buffering gradient, we were able to evaluate the relative significance of forest management versus edaphic constraints on soil carbon processing. The effects of conversion are extensive and change trajectories are larger for forests that shifted from one buffering domain to another upon conversion. In the latter, conversion leads to a larger buildup of the litter layer, significantly stronger acidification, and a decrease in microbial functional diversity and earthworm biomass in the mineral soil, mirrored by a collapse of the base saturation. Although topsoil total carbon concentrations increased under spruce, this soil carbon is stored in less stable carbon pools compared to broadleaved forests. These are carbon pools where carbon is not occluded in aggregates or bound to soil minerals and therefore more vulnerable to environmental factors. Our findings show that changes in forest composition can, depending on the initial distance from a threshold in acid buffering, cascade through different compartments of the soil carbon cycle and eventually alter the way carbon is stored.

Differences in pH influence the fate of CO2 in plants.

Soils represent the largest and most stable carbon pools on Earth, exceeding even the carbon aggregate found in the atmosphere and global phytomass. However, our understanding of how CO2 travels from the soil to the atmosphere, and the role of plants in this journey, is not fully understood. An article in this issue of Physiologia Plantarum (Shimono et al. 2019) sheds light on this process and unearths the dramatic effect pH can have on the fate of CO2 in plants.

Simulation of productivity and soil moisture under Marandu palisade grass using the CSM-CROPGRO-Perennial Forage model

Crop models are important tools for assisting farmers and crop consultants to make decisions about fertilisation, irrigation and harvest management, because they allow users to understand productivity from the view of integrated sensitivities of basic plant physiological processes. The first objective of this study was to evaluate the performance of the CSM-CROPGRO-Perennial Forage model (PFM) to simulate regrowth of Urochloa brizantha (Hochst. ex A.Rich.) R.D.Webster cv. Marandu under varying irrigation and nitrogen levels. The second objective was to evaluate the water-balance module of the model under soil and climatic conditions in the Cerrado biome of central-eastern Brazil. The experimental data for model evaluation were obtained from a field experiment conducted during 2015, 2016 and 2017, and included herbage production, plant-part composition and plant nitrogen (N) concentration. The results suggest that the model can be used to simulate growth of Marandu palisade grass adequately under different managements of irrigation and N fertilisation. The findings indicate also that the agreement between simulations and field-observed soil moisture shows good performance of the water-balance module of CSM-CROPGRO-PFM. The most important parameterisation required by the model was the determination and calibration of inputs such as the stable soil carbon pool (SOM3) for N mineralisation, which affected the N response, and the soil water-holding characteristics, which affected the irrigation response. The default parameterisation (species, ecotype, cultivar) of cv. Marandu in CSM-CROPGRO-PFM was sufficient for adequate performance of the model for this new environment and new crop management. However, minor modifications of species parameters were helpful to account for winter-kill of foliage.

Predicting Soil Organic Carbon in Agroecosystems under Climate Change

Predicting Soil Organic Carbon in Agroecosystems under Climate Change

Characteristics of soil organic carbon mineralization and influence factor analysis of natural Larix olgensis forest at different ages

Soil organic carbon (SOC) mineralization is closely related to carbon source or sink of terrestrial ecosystem. Natural stands of Larix olgensis on the Jincang forest farm, Jilin Province were selected to investigate the dynamics of SOC mineralization and its correlations with other soil properties in a young forest and mid-aged forest at soil depths of 0–10, > 10–20, > 20–40 and > 40–60 cm. The results showed that compared with a mid-aged forest, the SOC stock in the young forest was 32% higher. Potentially mineralizable soil carbon (C0) in the young forest was 1.1–2.5 g kg−1, accounting for 5.5–8.1% of total SOC during the 105 days incubation period and 0.3–1.5 g kg−1 in the mid-aged forest at different soil depths, occupying 2.8–3.4% of total SOC. There was a significant difference in C0 among the soil depths. The dynamics of the SOC mineralization was a good fit to a three-pool (labile, intermediate and stable) carbon decomposition kinetic model. The SOC decomposition rate for different stand ages and different soil depths reached high levels for the first 15 days. Correlation analysis revealed that the C0 was significantly positively related with SOC content, soil total N (TN) and readily available K (AK) concentration. The labile soil carbon pool was significantly related to SOC and TN concentration, and significantly negatively correlated with soil bulk density. The intermediate carbon pool was positively associated with TN and AK. The stable carbon pool had negative correlations with SOC, TN and AK.

Effects of N-fixing tree species (Alnus sibirica)on amino sugars in soil aggregates of Larix kaempferi plantation in eastern Liaoning Province, China.

Soil microbial residues are important components of soil stable carbon (C) pools. How N-fixing tree species influence microbial residues in soil aggregates in larch plantations is still unclear. To determine the effects of N-fixing tree species on the distribution of microbial residues in different aggregates, we compared the distribution of amino sugars, biomarker of microbial residues, among aggregates in a pure larch (Larix kaempferi) plantation and a mixed plantation of larch (Larix kaempferi) and alder (Alnus sibirica) in eastern Liaoning Province. The results showed that alder did not affect the distribution of amino sugars, but significantly increased amino sugars content in soil aggregates. The total amino sugars in different soil aggregates were enriched by 130%-170% in the mixed larch plantation compared with those in pure larch plantation. The contributions of glucosamine, galactosamine and muramic acid to the increases of total amino sugars caused by alder introduction were 66.5%-66.9%, 30.0%-30.6% and 2.5%-3.2%, respectively. Alder introduction significantly accelerated the glucosamine/muramic acid ratios in >2000 μm and <250 μm aggregates, but not in 250-2000 μm aggregates. Moreover, alder introduction increased the microbial contribution to soil organic C in all aggregates, but did not influence this contribution among aggregates, indicating that the effects of alder introduction on microbial contribution to aggregates were homogeneous.

Soil organic carbon, macro- and micronutrient changes in soil fractions with different lability in response to crop intensification

Soils under no tillage have experienced unfavorable changes, mainly due to current agricultural practices that consist in monocultures that leave little residue cover. The inclusion of grass as cover crops during the winter season could be a sustainable strategy to increase crop intensification in sequences where soybean predominates, helping to maintain soil fertility, organic matter levels and enhance soil physical properties. The aim of this research was to evaluate the effects of 8 years of sustainable crop intensification (by increasing the proportion of cereals in crop rotations) on soil organic carbon, macro- and micronutrients associated with granulometric fractions of different lability in a Typic Argiudoll of the Rolling Pampa, Argentina. The experiment included two crop sequences commonly used in this area: soybean-soybean (S-S) and maize-soybean-wheat/soybean (M-S-W/S) combined with the inclusion of wheat (Triticum aestivum L.) as cover crop (CC) in winter. The intensification sequence indices (ISI) were 0.39, 0.69, 0.55 and 0.64 for S-S, S-CC-S, M-S-W/S and M-CC-S-W/S, respectively. The carbon measured in the coarse particulate fraction (Pcf) in the 0–5 cm soil depth was 3 times larger in S-CC-S than in S-S. Cropping intensity also modified N, S, P, Ca and Mn in the Pcf with no changes in Mg, K, Zn, Fe and Cu contents. Among the carbon fractions studied, only the carbon measured in the Pcf and the easy mineralizable carbon estimated by the soil respiration in the first soil layer (0–5 cm), were positively correlated with the ISI. In the present study, 8 years under sustainable crop intensification were sufficient to show changes in the mineral associated fraction (Maf). Increases in the C in the Maf in maize legume-based rotation, suggest SOC accumulation in more stable carbon pools.