Carbon and nitrogen mineralization after maize harvest between and within maize rows: a microcosm study using 13C natural abundance

The sequestration of carbon in soil is not completely understood, and quantitative information about the rates of soil organic carbon (SOC) turnover could improve understanding. We analyzed the effects of the uneven distribution of crop residues after harvest of silage maize on C and N losses (CO₂‐C, dissolved organic carbon (DOC) and nitrogen (DON), and NO₃^–) from a Haplic Phaeozem and on the occurrence of priming effects induced by the decomposition of accumulated maize residues. Soil columns were taken from a continuous maize (since 1961) field after harvest i) between maize stalk rows (M_bare), ii) within the maize rows including a standing maize stalk (M_stalk), and iii) from a continuous rye (since 1878) field after tillage (rye stalk and roots were mixed into the Ap horizon). The soil columns were incubated for 230 days at 8 °C with an irrigation rate of 2 mm 10^–2 M CaCl₂ per day. Natural ¹³C abundance was used to distinguish between maize‐derived C (in SOC and maize residues) and older C originating from former C₃ vegetation. The uneven distribution of maize residues resulted in a considerably increased heterotrophic activity within the maize rows as compared with soil between seed rows. Cumulative CO₂ production was 53.1 g CO₂‐C m^–2 for M_stalk and 23.3 g CO₂‐C m^–2 for M_bare. The contribution of maize‐derived C to the total CO₂ emission was 83 % (M_stalk) and 67 % (M_bare). Calculated as difference between CO₂‐C release from M_stalk and M_bare, 19 % of the maize residues (roots and stalk) in M_stalk were mineralized during the incubation period. There was no or only a marginal effect of the accumulation of maize residues in M_stalk on leaching of DOC, DON, and NO₃^–. Total DOC and DON leaching amounted to 2.5 g C m^–2 and 0.16 g N m^–2 for M_stalk and to 2.1 g C m^–2 and 0.12 g N m^–2 for M_bare. The contribution of maize‐derived C to DOC leaching was about 25 % for M_stalk and M_bare. Nitrate leaching amounted to 3.9 g NO₃^–‐N m^–2 for M_stalk and to 3.5 g NO₃^–‐N m^–2 for M_bare. There was no priming effect induced by the decomposition of fresh maize residues with respect to CO₂ or DOC production from indigenous soil organic carbon derived from C₃ vegetation.     

Carbon dynamics determined by natural C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-plant) since 1961 with and without NPK fertilization from the long-term experiment ‘Ewiger Roggen’ in Halle, Germany, were incubated for 230 days at 8 °C and irrigated with 2 mm 10⁻² M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance. The percentage of maize-derived carbon was highest in CO₂ (42-79%), followed by C_mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO₂-C and C_mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO₂-C and 1.9-2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO₂-C and 0.3-0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r≥0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r≤0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil.     

Organic carbon and stable C isotope in conservation agriculture and conventional systems

Conservation agriculture might have the potential to increase soil organic C content compared to conventional tillage based systems. The present study quantified soil organic carbon (SOC) and soil C derived from C₃ (wheat) and C₄ (maize) plant species using δ¹³C stable isotope. Soil with 16 y of continuous application of zero tillage (ZT) or conventional tillage (CT), monoculture (M) or rotation (R) of wheat and maize, and with (+r) and without retention (−r) in the field of crop residues were studied in the central highlands of Mexico. The highest SOC content was found in the 0-5 cm layer under ZTM and ZTR with residues retention. The soil cultivated with maize showed a higher SOC content in the 0-10 cm layer with residue retention than without residue. In the 10-20 cm layer, the highest SOC content was found in the CT treatment with residue retention. The SOC stock expressed as equivalent soil mass was greatest in the 0-20 cm layer of the ZTM (wheat and maize) and ZTR cultivated treatments with residue retention. After 16 y, the highest content of soil δ¹³C was found in ZTM + r and CTM + r treated soil cultivated with maize; −16.56‰ and −18.08‰ in the 0-5 cm layer, −18.41‰ and −18.02‰ in the 5-10 cm layer and −18.59‰ and −18.72‰ in the 10-20 cm layer respectively. All treatments had a higher percentages of C-C₃ (derived from wheat residues or the earlier forest) than C-C₄ (derived from maize residues). The highest percentages of C-C₄, was found in ZTM + r and CTM + r treated soil cultivated with maize, i.e. 33.0% and 13.0% in 0-5 cm layer, 9.1% and 14.3% in the 5-10 cm layer and 5.0% and 6.8% in 10-20 cm layer, respectively. The gross SOC turnover was lower in soil with residue retention than without residues. It was found that the ZT system with residue retention and rotation with wheat is a practice with a potential to retain organic carbon in soil.     

Energetic eco‐physiology of the soil microbiota in two landscapes of southern and northern Germany

Interactions between microbial communities and organic matter were analyzed for soils from the project regions ’︁Ecosystem Research in the Agricultural Landscape/FAM, Munich’ in southern Germany and ’︁Ecosystem Research in the Bornhöved Lake district’ from northern Germany using ratios between microbial biomass content (C_mic), microbial metabolic quotient (qCO₂) and organic carbon content (C_org). In the agricultural soils in southern Germany, the qCO₂/C_org ratio differed significantly with respect to agricultural management in contrast to ecophysiological C_mic/C_org ratio. In addition, C_mic/C_org ratio decreased from 39 to 21 mg C_mic g^—1 C_org and qCO₂/C_org ratio increased from 72 to 180 mg CO₂‐C g^—1 C_mic h^—1 (g C_org g^—1 soil)^—1 with increasing soil depth. For the upper soil horizons from the landscape in northern Germany the two quotients differed significantly with reference to land use showing highest microbial colonization under grassland and lowest under beech forest. In contrast, C use efficiency was lowest in arable field under maize monoculture and highest in a wet grassland having a high organic C content.     

Natural 13C abundance and soil carbon dynamics under long‐term residue retention in a no‐till maize system

Residue retention and reduced tillage are both conservation agricultural practices that may enhance soil organic carbon (SOC) stabilization in soil. We evaluated the long‐term effects of no‐till (NT) and stover retention from maize on SOC dynamics in a Rayne silt loam Typic Hapludults in Ohio. The six treatments consisted of retaining 0, 25, 50, 75, 100 and 200% of maize residues on each 3 × 3 m plot from the crop of previous year. Soil samples were obtained after 9 yrs of establishing the experiment. The whole soil (0–10 and 10–20 cm of soil depths) samples under different treatments were analysed for total C, total N, recalcitrant C (NaOCl treated sample) and ¹³C isotopic abundance (0–10 cm soil depth). Complete removal of stover for a period of 9 yrs significantly (P < 0.01) decreased soil C content (15.5 g/kg), whereas 200% of stover retention had the maximum soil C concentration (23.1 g/kg). Relative distribution of C for all the treatments in different fractions comprised of 55–58% as labile and 42–45% as recalcitrant. Retention of residue did not significantly affect total C and N concentration in 10–20 cm depth. ¹³C isotopic signature data indicated that C₄‐C (maize‐derived C) was the dominant fraction of C in the top 0–10 cm of soil layer under NT with maize‐derived C accounting for as high as 80% of the total SOC concentration. Contribution of C₄‐C or maize‐derived C was 71–84% in recalcitrant fraction in different residue retained plots. Residue management is imperative to increase SOC concentrations and long‐term agro‐ecosystem necessitates residue retention for stabilizing C in light‐textured soils.     

Organic Farming Affects C and N in Soils Under Olive Groves in Mediterranean Areas

Under semiarid climatic conditions, intensive tillage increases soil organic matter losses, reduces soil quality, and contributes to climate change due to increased CO₂ emissions. There is a need for an agricultural management increasing soil organic matter. This paper presents the organic carbon (OC) and nitrogen (N) stocks, C:N ratio and stratification ratios (SRs) of these properties for olive groves soils under long‐term organic farming (OF), and conventional tillage (CT) in Los Pedroches valley, southern Spain. The results show that OF increased C and N stocks. The soil organic carbon (SOC) stock was 73·6 Mg ha⁻¹ in OF and 54·4 Mg ha⁻¹ in CT; and the total nitrogen (TN) stock was 7·1 Mg ha⁻¹ and 5·8 Mg ha⁻¹ for OF and CT, respectively. In the surface horizon (A: 0–16·9 cm in OF and Ap: 0–21·8 cm in CT) and Bw horizon (16·9–49·6 cm in OF and 21·8–56 cm in CT), SOC and TN concentrations and C:N ratios were higher in OF than in CT. Soil properties stratification in depth, expressed as a ratio, indicates the soil quality under different soil management systems. The SR of SOC ranged from 2·2 to 3·1 in OF and from 2·1 to 2·2 in CT. However, only SR2 (defined by Ap‐A/C) showed significant differences between CT and OF. The SR of TN showed similar trends to that of the SR of SOC. Organic farming contributes to a better soil quality and to increased carbon sequestration. Copyright © 2013 John Wiley & Sons, Ltd.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

Plant roots are more important than temperature in modulating carbon release in a limed acidic soil

Lime is a common amendment to overcome soil acidity in agricultural production systems. However, plant root effects on lime and soil carbon (C) dynamics in acidic soils under varied temperature remain largely unknown. We monitored root effects of soybean on the fate of lime applied to an acidic soil at 20 and 30°C in growth chambers. Soil respired CO₂ was continuously trapped in columns without and with plants until the final stage of vegetative growth. Lime‐derived CO₂ was separated from total respired CO₂ based on δ¹³C measurements in CO₂. Leaching was induced at early and late vegetative growth stages, and the leachates were analysed for dissolved organic (DOC) and inorganic C (DIC) concentrations. Soil respiration significantly increased with lime addition at both temperatures (p < 0.001). The presence of soybean doubled the recovery of lime‐derived CO₂‐C at 20°C at the early growth stage; however, by the end of the experiment, the contribution of lime‐derived CO₂‐C to soil respiration was negligible in all treatments, indicating that the contribution of lime to soil respiration was shortlived. In contrast, DIC and DOC concentrations in leachates remained elevated with liming and were greater in the presence of soybean. We observed no main temperature effects and no interactive effects of temperature and soybean presence on lime‐derived CO₂‐C, DIC and DOC. These results highlight the role of plant‐modulated processes in CO₂ release and C leaching from lime in acidic soils, whereas an increase in temperature may be less important. Temperature and plant roots alter the rate of key processes controlling C dynamics in a limed acidic soil. Lime‐derived CO₂‐C, DIC and DOC increased more in the presence of plants than with increased temperature. Root effects are more important than temperature for inorganic and organic carbon dynamics in limed acidic soils.     

Carbon sequestration in soils of central Asia

Problems of frequent drought stress, low soil organic carbon (SOC) concentration, low aggregation, susceptibility to compaction, salinization and accelerated soil erosion in dry regions are accentuated by removal of crop residues, mechanical methods of seedbed preparation, summer clean fallowing and overgrazing, and excessive irrigation. The attendant soil degradation and desertification lead to depletion of SOC, decline in biomass production, eutrophication/pollution of waters and emission of greenhouse gases. Adoption of conservation agriculture, based on the use of crop residue mulch and no till farming, can conserve water, reduce soil erosion, improve soil structure, enhance SOC concentration, and reduce the rate of enrichment of atmospheric CO₂. The rate of SOC sequestration with conversion to conservation agriculture, elimination of summer fallowing and growing forages/cover crops may be 100 to 200 kg ha⁻¹ y⁻¹ in coarse‐textured soils of semiarid regions and 150 to 300 kg ha⁻¹ y⁻¹ in heavy‐textured soils of the subhumid regions. The potential of soil C sequestration in central Asia is 10 to 22 Tg C y⁻¹ (16±8 Tg C y⁻¹) for about 50 years, and it represents 20 per cent of the CO₂ emissions by fossil fuel combustion. Copyright © 2004 John Wiley & Sons, Ltd.     

Responses of Soil Microbial Community Structure and Activity to Incorporation of Straws and Straw Biochars and Their Effects on Soil Respiration and Soil Organic Carbon Turnover

Like straw, biochar incorporation can influence soil microorganisms and enzyme activities and soil carbon(C) responses; however,few studies have compared the various effects of straw and biochar and the underlying mechanisms. An experiment was performed to study the changes in soil respiration(SR) and soil organic C(SOC) fluxes in response to the incorporation of three kinds of straw(reed, smooth cordgrass, and rice) and their pyrolyzed products(biochars) at Chongming Island, China. In addition, the microbial activity and community structure of some amended soils were also analyzed to clarify the mechanisms of these responses. The results showed that all biochar incorporation(BC) induced lower SR than the corresponding unpyrolyzed straw incorporation(ST), and the average SR in the soils following BC and ST during the experimental periods was 21.69 and 65.32 μmol CO_2 m~(-2)s~(-1), respectively.Furthermore, the average SOC content was 16.97 g kg~(-1) following BC, which was higher than that(13.71 g kg~(-1)) following ST,indicating that compared to ST, BC was a low-C strategy, even after accounting for the C loss during biochar production. Among the BC treatments, reed-BC induced the lowest SR(17.04 μmol CO_2 m~(-2)s~(-1)), whereas smooth cordgrass-BC induced the highest SR(27.02 μmol CO_2 m~(-2)s~(-1)). Furthermore, in contrast with ST, BC significantly increased the abundance of some bacteria with poorer mineralization or better humification ability, which led to lower SR. The lower easily oxidizable C(EOC) and higher total C contents of biochars induced lower SR and higher SOC in the soil following BC compared to that following ST. Among the BC treatments,the higher total nitrogen content of rice biochar led to significantly higher soil microbial biomass, and the lower EOC content of reed biochar led to lower soil microbial activity and SR.     

Mulching Affects Soil Properties and Greenhouse Gas Emissions Under Long‐Term No‐Till and Plough‐Till Systems in Alfisol of Central Ohio

Agricultural activities emit greenhouse gases (GHGs) and contribute to global warming. Intensive plough tillage (PT), use of agricultural chemicals and the burning of crop residues are major farm activities emitting GHGs. Intensive PT also degrades soil properties by reducing soil organic carbon (SOC) pool. In this scenario, adoption of no‐till (NT) systems offers a pragmatic option to improve soil properties and reduce GHG emission. We evaluated the impacts of tillage systems (NT and PT) and wheat residue mulch on soil properties and GHG emission. This experiment was started in 1989 on a Crosby silt loam soil at Waterman Farm, The Ohio State University, Columbus, Ohio, USA. Mulching reduced soil bulk density and improved total soil porosity. More total carbon (16.16 g kg⁻¹), SOC (8.36 mg L⁻¹) and soil microbial biomass carbon (152 µg g⁻¹) were recorded in soil under NT than PT. Mulch application also decreased soil temperature (0–5 cm) and penetration resistance (0–60 cm). Adoption of long‐term NT reduced the GHG emission. Average fluxes of GHGs under NT were 1.84 g CO₂‐C m⁻² day⁻¹ for carbon dioxide, 0.07 mg CH₄‐C m⁻² day⁻¹ for methane and 0.73 mg N₂O‐N m⁻² day⁻¹ for nitrous oxide compared with 2.05 g CO₂‐C m⁻² day⁻¹, 0.74 mg CH₄‐C m⁻² day⁻¹ and 1.41 mg N₂O‐N m⁻² day⁻¹, respectively, for PT. Emission of nitrous oxide was substantially increased by mulch application. In conclusion, long‐term NT reduced the GHG emission by improving the soil properties. Copyright © 2016 John Wiley & Sons, Ltd.     

Soil carbon sequestration in sub‐Saharan Africa: a review

Restoration of degraded soils is a development strategy to reduce desertification, soil erosion and environmental degradation, and alleviate chronic food shortages with great potential in sub‐Saharan Africa (SSA). Further, it has the potential to provide terrestrial sinks of carbon (C) and reduce the rate of enrichment of atmospheric CO₂. Soil organic carbon (SOC) contents decrease by 0 to 63 per cent following deforestation. There exists a high potential for increasing SOC through establishment of natural or improved fallow systems (agroforestry) with attainable rates of C sequestration in the range of 0·1 to 5·3 Mg C ha⁻¹ yr⁻¹. Biomass burning significantly reduces SOC in the upper few centimeters of soil, but has little impact below 10 to 20 cm depth. The timing of burning is also important, and periods with large amounts of biomass available generally have the largest losses of SOC. In cultivated areas, the addition of manure in combination with crop residues and no‐till show similar rates of attainable C sequestration (0 to 0·36 Mg C ha⁻¹ yr⁻¹). Attainable rates of SOC sequestration on permanent cropland in SSA under improved cultivation systems (e.g. no‐till) range from 0·2 to 1·5 Tg C yr⁻¹, while attainable rates under fallow systems are 0·4 to 18·5 Tg C yr⁻¹. Fallow systems generally have the highest potential for SOC sequestration in SSA with rates up to 28·5 Tg C yr⁻¹. Copyright © 2005 John Wiley & Sons, Ltd.     

Environmental Controls on Fallow Carbon Dioxide Flux in a Single‐Crop Rice Paddy,Japan

Soil organic carbon (SOC) is one of the important measures of soil fertility and sustainability in arable lands. With continuous CO₂ flux measurements, this study assessed the SOC decomposition and its environmental controls at both half‐hourly and season‐long scales in a single‐crop rice (Oryza sativa L.) paddy during three fallow periods between 2004 and 2007. Measurements were made on a gray lowland soil sited in eastern Japan using the eddy covariance method. Ecosystem respiration was strongly affected by soil water content measured at 0–0·1 m depth. At 0·5 m³ m^{− 3} or more of soil water content, the baseline of ecosystem respiration decreased by 50% compared with that at 0·2 m³ m^{− 3} . The effect was quantified at half‐hourly scale using an empirical multiple regression model, together with the soil surface temperature and the time after residue incorporation. At season‐long scale, net biome production, which is equivalent to the change in the SOC pool during the fallow period, was estimated from the flux and ancillary data at 150 g C m^{− 2} in 2004–2005, 70 g m^{− 2} in 2005–2006, and 270 g C m^{− 2} in 2006–2007. Apparently, as much as 46 to 79% of the soil organic matter incorporated (crop residues, ratoon, and stable manure) was decomposed during the fallow period. Precipitation, or associated soil water content, was important for the carbon balance of the field at season‐long scale because of its large interannual variability and relatively low permeability of the paddy soil. Copyright © 2013 John Wiley & Sons, Ltd.     

Carbon Life Cycle Assessment for Prairie as a Crop in Reclaimed Mine Land

A life cycle assessment with carbon (C) as the reference unit was used to balance the benefits of land preparation practices of establishing tall‐grass prairies as a crop for reclaimed mine land with reduced environmental damage. Land preparation and management practices included disking with sub‐soiling (DK‐S), disking only (DK), no tillage (NT), and no tillage with grazing (NT‐G). To evaluate the C balance and energy use of each of the land preparations, an index of sustainability (I_s = C_O/C_I, Where: C_O is the sum of all outputs and C_I is the sum of all inputs) was used to assess temporal changes in C. Of the four land preparation and management practices, DK had the highest I_s at 8·53. This was due to it having the least degradation of soil organic carbon (SOC) during land‐use change (−730 kg ha⁻¹ y⁻¹) and second highest aboveground biomass production (9,881 kg ha⁻¹). The highest aboveground biomass production occurred with NT (11,130 kg ha⁻¹), although SOC losses were similar to DK‐S, which on average was 2,899 kg ha⁻¹ y⁻¹. The I_s values for NT and DK‐S were 2·50 and 1·44, respectively. Grazing from bison reduced the aboveground biomass to 8,971 kg ha⁻¹ compared with NT with no grazing, although stocking density was low enough that I_s was still 1·94. This study has shown that converting from cool‐season forage grasses to tall‐grass prairie results in a significant net sink for atmospheric CO₂ 3 years after establishment in reclaimed mine land, because of high biomass yields compensating for SOC losses from land‐use change. Copyright © 2014 John Wiley & Sons, Ltd.     

A mass balance approach to assess carbon dioxide evolution during erosional events

The accelerated greenhouse effect and the degradation of land resources by water and wind erosion are two major, yet interrelated global environmental challenges. Accelerated decomposition of soil organic carbon (SOC) in cultivated soils results in decline in SOC stocks over time and also contributes to increased levels of CO₂ in the atmosphere. Off‐site transport of SOC in runoff waters during erosional events also contributes to SOC depletion, but there is a paucity of data in the literature documenting erosional SOC losses and the fate of eroded SOC. In this paper, we present a mass balance approach to compute CO₂ evolved from mineralization of SOC during transport and deposition of eroded soils. Erosion‐induced CO₂ emission rates ranging between 6 and 52 g C m⁻² yr⁻¹ were computed using data on SOC stocks and dynamics from a series of long‐term experiments conducted across a range of ecological regions. For the cropland of the world, we estimated an annual flux of 0.37 Pg CO₂‐C to the atmosphere due to water erosion. This flux is significant and suggests that water erosion must be taken into consideration when constructing global and regional C budgets. Through its contribution to atmospheric CO₂ increase, water erosion can have a positive feedback on the accelerated greenhouse effect. Copyright © 2001 John Wiley & Sons, Ltd.     

吉林西部盐碱地区稻田土壤有机碳矿化特征

以吉林西部盐碱地区(前郭灌区)土壤为研究对象,选取不同盐碱程度的4块水田(P1、P2、P3和P4),采用野外实地调研采样与室内模拟试验相结合的方法,分别在培养期的第1,4,7,10,14,21,28,35,70天测定土壤CO_2气体的排放通量,结合土壤基本理化性质,分析盐碱稻田矿化模拟培养过程中CO_2通量的动态变化,研究土壤盐碱化程度对有机碳矿化过程的影响。结果表明:P1、P2、P3为弱碱化土,P4为强碱化土;各样地土壤有机碳(SOC)含量差异显著,并存在表层富集现象,与碱化度(ESP)呈显著负相关关系(r=-0.945);SOC矿化量累积过程与培养时间符合一级动力学模型C_t=C_0(1-e^-kt),各样地土壤在矿化培养初期CO_2释放量较大,释放强度降低较快,矿化速率随时间延长呈缓慢平稳下降,在培养期结束时降至最低。SOC矿化过程受多种因子共同作用,ESP是该过程的主要影响因子。土壤的盐碱化抑制了土壤碳循环的速度,相对于碳源过程而言,对碳汇的影响更大。伴随SOC含量增加,SOC矿化反应强度和矿化反应的完全程度加强,矿化反应累积量增加,反之,随ESP程度增加而减弱。     

Contribution of crop residue carbon to soil respiration at a northern Prairie site using stable isotope flux measurements

Heterotrophic respiration from agricultural soils can be differentiated as originating from microbial decomposition of recent litter inputs or crop residue carbon (CRC) and resident soil organic carbon (SOC) pools of varying age and stages of decomposition. Our objective was to determine the relative contributions of these pools to respiration in a northern agroecosystem where the non-growing season is long. A tunable diode laser trace gas analyzer was used to determine atmospheric stable C isotope ratio (δ¹³C) values and ¹²CO₂ and ¹³CO₂ fluxes over an agricultural field in the Red River Valley of southern Manitoba, Canada. Measurement campaigns were conducted in the fall of 2006 and spring of 2007 following harvest of a maize (C₄) crop from soil having SOC derived from previous C₃ crops. Stable CO₂ isotopologue gradients were measured from the center of four 200 × 200 m experimental plots, and fluxes were calculated using the aerodynamic flux gradient method. The soil in two of the experimental plots underwent intensive tillage, while the other two plots were managed using a form of reduced tillage. Approximately 70% and 20-30% of the total respiration flux originated from the maize C₄-CRC during the fall of 2006 and spring of 2007, respectively. At least 25% of the maize residue was lost to respiration during this non-growing period. No difference in the partitioning of heterotrophic respiration into that derived from CRC and SOC was detected between the intensive tillage and recently established reduced tillage treatments at the site.     

Carbon Depletion by Plowing and its Restoration by No‐Till Cropping Systems in Oxisols of Subtropical and Tropical Agro‐Ecoregions in Brazil

The continuous use of plowing for grain production has been the principal cause of soil degradation. This project was formulated on the hypothesis that the intensification of cropping systems by increasing biomass‐C input and its biodiversity under no‐till (NT) drives soil restoration of degraded agro‐ecosystem. The present study conducted at subtropical [Ponta Grossa (PG) site] and tropical regions [Lucas do Rio Verde, MT (LRV) site] in Brazil aimed to (i) assess the impact of the continuous plow‐based conventional tillage (CT) on soil organic carbon (SOC) stock vis‐à‐vis native vegetation (NV) as baseline; (ii) compare SOC balance among CT, NT cropping systems, and NV; and (iii) evaluate the redistribution of SOC stock in soil profile in relation to soil resilience. The continuous CT decreased the SOC stock by 0·58 and 0·67 Mg C ha⁻¹ y⁻¹ in the 0‐ to 20‐cm depth at the PG and LRV sites, respectively, and the rate of SOC sequestration was 0·59 for the PG site and ranged from 0·48 to 1·30 Mg C ha⁻¹ y⁻¹ for the LRV site. The fraction of C input by crop residues converted into SOC stock was ~14·2% at the PG site and ~20·5% at the LRV site. The SOC resilience index ranged from 0·29 to 0·79, and it increased with the increase in the C input among the NT systems and the SOC sequestration rates at the LRV site. These data support the hypothesis that NT cropping systems with high C input have a large potential to reverse the process of soil degradation and SOC decline. Copyright © 2013 John Wiley & Sons, Ltd.     

添加玉米秸秆对黄棕壤有机质的激发效应

玉米秸秆还田是培肥地力的一项重要措施,但是玉米秸秆添加后会改变土壤原有有机质的矿化过程,即引起激发效应,从而影响土壤碳平衡和周转。因此,适量秸秆还田将是高效且环境友好的提升土壤生产潜力的关键。本试验以黄棕壤为研究对象,设不添加玉米秸秆对照(CK)和添加占干土重的1%、5%和9%的粉碎玉米秸秆处理进行室内培养,分析土壤CO_2释放动态及激发效应。试验结果表明,添加不同量玉米秸秆后,土壤CO_2释放速率和累积量呈现出抛物线型变化趋势。在培养前期,各处理土壤CO_2释放速率表现为9%5%1%CK,到培养的第8天左右,添加5%玉米秸秆的土壤CO_2释放速率超过了添加9%玉米秸秆的土壤,在培养后期,所有处理的土壤CO_2释放速率慢慢地趋于一致。从累积CO_2释放量来看,添加5%玉米秸秆的处理比9%玉米秸秆的处理土壤总CO_2释放量高,表明添加5%秸秆的处理对微生物群落和微生物活性的作用最大。在整个培养阶段,玉米秸秆添加对土壤有机质的激发效应均为负值,而加入的玉米秸秆并没有完全矿化,从而使土壤有机碳含量因添加玉米秸秆而升高。这些结果表明,添加玉米秸秆有利于提高黄棕壤土壤有机碳含量,在本试验的短期培养过程中以土壤干重9%的添加量增加最多。     

Soil Organic Carbon Molecular Properties: Effects of Time Since Reclamation in a Minesoil Chronosequence

Studies on molecular characterization of soil organic carbon (SOC), although critical to understanding SOC dynamics, are relatively scarce for reclaimed minesoils. The aim of this study was to assess the effects of time since reclamation on SOC molecular properties in a reclaimed minesoil chronosequence using different spectroscopic indices. The chronosequence consisted of four minesoils, with similar soil‐forming conditions, reclaimed to pasture ecosystem, and distinguished only by time since reclamation varying from 1 to 22 years. Results indicated that the SOC molecules in the older minesoils were comprised of highly humified polyaromatic and polycondensed species with higher proportions of O‐containing and N‐containing functional groups. In each minesoil, resistant and labile SOC fractions were identified. The resistant SOC fraction constituted >20 and <10 per cent of total SOC (g kg⁻¹) in the oldest and youngest minesoil, respectively. Among the spectroscopic indices, the ϵ₂₈₅ ultraviolet‐visible index, diagnostic of aromatic C, was the most efficient in identifying the intrasite short‐term (1‐year) changes in SOC molecular properties. Strong positive relationships found between different spectroscopic indices and total SOC (g kg⁻¹), with consistently higher r² values observed in older minesoils, indicated that SOC molecular characteristics played a key role in overall SOC dynamics, becoming more influential with increasing time since reclamation. In addition, significant relationships between the spectroscopic indices and different soil quality parameters indicated that SOC molecular properties influenced soil quality as well. Overall, the results indicated that SOC molecular properties were useful indicators of both SOC dynamics and soil quality in this minesoil chronosequence. Copyright © 2013 John Wiley & Sons, Ltd.