C signature of CO2 evolved from incubated maize residues and maize-derived sheep faeces

Analyses of the spatial and temporal variations in the natural abundance of ¹³C are frequently employed to study transformations of plant residues and soil organic matter turnover on sites where long continued vegetation with the C₃-type photosynthesis pathway has been replaced with a C₄-type vegetation (or vice versa). One controversial issue associated with such analyses is the significance of isotopic fractionation during the microbial turnovers of C in complex substrates. To evaluate this issue, C₃-soil and quartz sand were amended with maize residues and with faeces from sheep feed exclusively on maize silage. The samples were incubated at 15 °C for 117 days (maize residues) or 224 days (sheep faeces). CO₂ evolved during incubation was trapped in NaOH and analysed for C isotopic contents. At the end of incubation, 63 and 50% of the maize C was evolved as CO₂ in the soil and sand, respectively, while 32% of the faeces C incubated with soil and with sand was recovered as CO₂. Maize and faeces showed a similar decomposition pattern but maize decomposed twice as fast as faeces. The δ¹³C of faeces was 0.3‰ lower than that of the maize residue (δ¹³C −13.4‰), while the δ¹³C of the C₃-soil used for incubation was −31.6‰. The δ¹³C value of the CO₂ recovered from unamended C₃-soil was similar or slightly lower (up to −1.5‰) than that of the C₃-soil itself except for an initial flush of ¹³C enriched CO₂. The δ¹³C values of the CO₂ from sand-based incubations typically ranged −15‰ to −17‰, i.e. around −3‰ lower than the δ¹³C measured for maize and faeces. Our study clearly demonstrates that the decomposition of complex substrates is associated with isotopic fractionation, causing evolved CO₂ to be depleted in ¹³C relative to substrates. Consequently the microbial products retained in the soil must be enriched in ¹³C.     

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.     

Effects of part and whole straw returning on soil carbon sequestration in C3–C4 rotation cropland

Long‐term no‐tillage management and crop residue amendments to soil were identified as an effective measure to increase soil organic carbon (SOC). The SOC content, SOC stock (SOCs), soil carbon sequestration rate (CSR), and carbon pool management index (CPMI) were measured. A stable isotopic approach was used to evaluate the contributions of wheat and maize residues to SOC at a long‐term experimental site. We hypothesized that under no‐tillage conditions, straw retention quantity would affect soil carbon sequestration differently in surface and deep soil, and the contribution of C3 and C4 crops to soil carbon sequestration would be different. This study involved four maize straw returning treatments, which included no maize straw returning (NT‐0), 0.5 m (from the soil surface) maize straw returning (NT‐0.5), 1 m maize straw returning (NT‐1), and whole maize straw returning (NT‐W). The results showed that in the 0–20 cm soil layer, the SOC content, SOCs, CSR and CPMI of the NT‐W were highest after 14 years of no‐tillage management, and there were obvious differences among the four treatments. However, the SOC, SOCs, and CSR of the NT‐0.5 and NT‐W were the highest and lowest in 20–100 cm, respectively. The value of δ¹³C showed an obviously vertical variability that ranged from –22.01‰ (NT‐1) in the 0–20 cm layer to –18.27‰ (NT‐0.5) in the 60–80 cm layer, with enriched δ¹³C in the 60–80 cm (NT‐0.5 and NT‐1) and 80–100 cm (NT‐0 and NT‐W) layers. The contributions of the wheat and maize‐derived SOC of the NT‐0.5, NT‐1 and NT‐W increased by 11.4, 29.5 and 56.3% and by 10.7, 15.1 and 40.1%, relative to those in the NT‐0 treatment in the 0–20 cm soil layer, respectively. In conclusion, there was no apparent difference in total SOC sequestration between the NT‐0.5, NT‐1, and NT‐W treatments in the 0–100 cm soil layer. The contribution of wheat‐derived SOC was higher than that of maize‐derived SOC.     

Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem

Experimentation with dynamics of soil carbon pools as affected by elevated CO₂ can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ¹³C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO₂ enrichment (FACE) experiment, in which elevated CO₂ (FACE: 560 μmol mol⁻¹) and ambient CO₂ (Control: 360 μmol mol⁻¹) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO₂ transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO₂ was higher than that under ambient CO₂, indicating that elevated CO₂ reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO₂ enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO₂ and mitigating harmful CO₂ under future atmospheric conditions.     

C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies

Natural variations of the ¹³C/¹²C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural-¹³C-labelling approach, i.e. natural δ¹³C variation after C₃-C₄ vegetation changes. In this review, we focus on ¹³C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C₃ and C₄ plants are ¹³C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO₂ released by root respiration was ¹³C depleted by about −2.1 ± 2.2‰ for C₃ plants and −1.3 ± 2.4‰ for C₄ plants compared to root tissue. However, only a very few studies investigated ¹³C fractionation by root respiration. This urgently calls for further research. In soils developed under C₃ vegetation, the microbial biomass was ¹³C enriched by +1.2 ± 2.6‰ and microbial CO₂ was also ¹³C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of ¹³C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ¹³C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO₂, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ¹³C differences between microorganisms and CO₂ by an ‘apparent’ ¹³C discrimination. Preferential consumption of easily decomposable substrates and less negative δ¹³C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C₃ soils because, in C₄ soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (? = +1.1‰) and respired lighter C (? = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the ¹³C fractionation in these processes in C₃ soils. Increasing temperature and decreasing precipitation led, however, to increasing δ¹³C of soil C pools.Based on these ¹³C fractionations we developed a number of consequences for C partitioning studies using ¹³C natural abundance. In the framework of standard isotope mixing models, we calculated CO₂ partitioning using the natural-¹³C-labelling approach at a vegetation change from C₃ to C₄ plants assuming a root-derived fraction between 0% and 100% to total soil CO₂. Disregarding any ¹³C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for ¹³C fractionation in the standard deviations of the C₄ source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO₂ pools. Including ¹³C fractionations directly into the mass balance equations reproduced the assumed CO₂ partitioning exactly. At the end, we therefore give recommendations on how to consider ¹³C fractionations in research on carbon flows between plants, microorganisms, and soil.     

Short-term C4 plant Spartina alterniflora invasions change the soil carbon in C3 plant-dominated tidal wetlands on a growing estuarine Island

Spartina alterniflora is an invasive C₄ perennial grass, native to North America, and has spread rapidly along the east coast of China since its introduction in 1979. Since its intentional introduction to the Jiuduansha Island in the Yangtze River estuary, Spartina alterniflora community has become one of the dominant vegetation types. We investigated the soil carbon in the Spartina alterniflora community and compared it with that of the native C₃Scirpus mariqueter community by measuring total soil carbon (TC), soil organic carbon (SOC), total soil nitrogen (TN), and the stable carbon isotope composition (δ¹³C) of various fractions. TC and SOC were significantly higher in Spartina alterniflora in the top 60 cm of soil. However, there was no significant difference in soil inorganic carbon (IC) between the two communities. Stable carbon isotopic analysis suggests that the fraction of SOC pool contributed by Spartina alterniflora varied from 0.90% to 10.64% at a soil depth of 0-100 cm with a greater percentage between 20 and 40 cm deep soils. The δ¹³C decreased with increasing soil depth in both communities, but the difference in δ¹³C among layers of the top 60 cm soil was significant (p<0.05), while that for the deeper soil layers (>60 cm) was not detected statistically. The changes in δ¹³C with depth appeared to be associated with the small contribution of residues from Spartina alterniflora at greater soil depth that was directly related to the vertical root distribution of the species.     

Effect of conservation agriculture on soil organic and inorganic carbon sequestration and lability: A study from a rice–wheat cropping system on a calcareous soil of the eastern Indo-Gangetic Plains

Increasing soil carbon (C) in arable soils is an important strategy to achieve sustainable yields and mitigate climate change. We investigated changes in soil organic and inorganic carbon (SOC and SIC) under conservation agriculture (CA) in a calcareous soil of the eastern Indo-Gangetic Plains of India. The treatments were as follows: conventional-till rice and wheat (CT-CT), CT rice and zero-till wheat (CT-ZT), ZT direct seeded rice (DSR) and CT wheat (ZT-CT), ZTDSR and ZT wheat without crop residue retention (ZT-ZT), ZT-ZT with residue (ZT-ZT+R), and DSR and wheat both on permanent beds with residue (PB-PB+R). The ZT-ZT+R had the highest total SOC in both 0–15 and 15–30 cm soil layers (20% and 40% higher (p < .05) than CT-CT, respectively), whereas total SIC decreased by 11% and 15% in the respective layers under ZT-ZT+R compared with CT-CT. Non-labile SOC was the largest pool, followed by very labile, labile and less labile SOC. The benefits of ZT and residue retention were greatest for very labile SOC, which showed a significant (p < .05) increase (~50%) under ZT-ZT+R compared with CT-CT. The ZT-ZT+R sequestered ~2 Mg ha⁻¹ total SOC in the 0–15 cm soil layer in 6 years, where CT registered significant losses. Thus, the adoption of CA should be recommended in calcareous soils, for C sequestration, and also as a reclamation technique.     

Fate of nitrogen and carbon in the vadose zone: in situ and laboratory measurements of seasonal variations in aerobic respiratory and denitrifying activities

In situ and laboratory measurements of aerobic respiratory and denitrifying activities were studied in the vadose zone (almost 2.5 m thick) of a fluvic hypercalcaric cambisol characterized by transitory anaerobic conditions. A field experiment was conducted in a bare soil, over a 7-month period starting just after maize harvest and incorporation of maize crop residues. Weather variables (air and soil temperature, rainfall), soil water content, soil solutes (NO₃⁻ and dissolved organic carbon) and soil gases (CO₂ and N₂O), were recorded throughout the experiment. Four soil layers were defined. Bacterial counts were performed in each layer using the most probable number (MPN) method. Aerobic respiratory and denitrifying activities were estimated from laboratory measurements. In situ microbial activity, as revealed by CO₂ and N₂O measurements in the soil atmosphere, was strongly influenced by weather. Laboratory measurements showed that potential aerobic respiratory activity (ARA) occurred throughout the soil profile, whereas semi-potential denitrifying activities SPDA (i.e. measured under organic-C limiting condition) occurred mainly in the top 30 cm soil layer. In the soil profile, the CO₂ concentration gradient was stronger than the N₂O concentration gradient. Seasonal variations in microbial activities increased with depth, whereas DOC concentrations, and variations in those concentrations, decreased with depth, suggesting that DOC quality investigations are necessary in the deep vadose zone to understand microbial activities seasonal variations. Laboratory measurements of potential activities agreed well with in situ microbial activity in natural environmental conditions. NO₃⁻ was a stronger limiting factor for SPDA than was denitrifier density in the soil profile.     

Long-term soil organic carbon dynamics in a subhumid tropical climate: C data in mixed C3/C4 cropping and modeling with ROTHC

Scanty information on long-term soil organic carbon (SOC) dynamics hampers validation of SOC models in the tropics. We observed SOC content changes in a 16-year continuously cropped agroforestry experiment in Ibadan, south-western Nigeria. SOC levels declined in all treatments. The decline was most pronounced in the no-tree control treatments with continuous maize and cowpea cropping, where SOC levels dropped from the initial 15.4 to 7.3-8.0 Mg C ha⁻¹ in the 0-12 cm topsoil in 16 years. In the two continuously cropped alley cropping (AC) systems, one with Leucaena leucocephala and one with Senna siamea trees, SOC levels dropped to 10.7-13.2 Mg C ha⁻¹. Compared to the no-tree control treatments, an annual application of an additional 8.5 Mg ha⁻¹ (dry matter) of plant residues, mainly tree prunings, led to an extra 3.5 Mg C ha⁻¹ (∼0.2% C) in the 0-12 cm top soil after 11 years, and 4.1 Mg C ha⁻¹ after 16 years. The addition of NPK fertilizer had little effect on the quantities of above-ground plant residues returned to the soil, and there was no evidence that the fertilizer affected the rate of SOC decomposition. The fact that both C₃ and C₄ plants returned organic matter to the soil in all cropping systems, but in contrasting proportions, led to clear contrasts in the ¹³C abundance in the SOC. This ¹³C information, together with the measured SOC contents, was used to test the ROTHC model. Decomposition was very fast, illustrated by the fact that we had to double all decomposition rate constants in the model in order to simulate the measured contrasts in SOC contents and δ¹³C between the AC treatments and the no-tree controls. We hypothesized (1) that the pruning materials from the legume trees and/or the extra rhizodeposition from the tree roots in the AC treatments accelerated the decomposition of the SOC present at the start of the experiment (true C-priming), and/or (2) that the physical protection of microbial biomass and metabolites by the clay fraction on this site, having a sandy top soil in which clay minerals are mainly of the 1:1 type, is lower than assumed by the model.     

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.     

Effect of land use types on decomposition of 14C-labelled maize residue (Zea mays L.)

The effect of three land use types on decomposition of ¹⁴C-labelled maize (Zea mays L.) residues and soil organic matter were investigated under laboratory conditions. Samples of three Dystric Cambisols under plow tillage (PT), reduced tillage (RT) and grassland (GL) collected from the upper 5 cm of the soil profile were incubated for 159 days at 20 °C with or without ¹⁴C-labelled maize residue. After 7 days cumulative CO₂ production was highest in GL and lowest in PT, reflecting differences in soil organic C (SOC) concentration among the three land use types and indicating that mineralized C is a sensitive indicator of the effects of land use regime on SOC. ¹⁴CO₂ efflux from maize residue decomposition was higher in GL than in PT, possibly due to higher SOC and microbial biomass C (MBC) in GL than in PT. ¹⁴CO₂ efflux dynamics from RT soil were different from those of PT and GL. RT had the lowest ¹⁴CO₂ efflux from days 2 to 14 and the highest from days 28 to 159. The lowest MBC in RT explained the delayed decomposition of residues at the beginning. A double exponential model gave a good fit to the mineralization of SOC and residue-¹⁴C (R² > 0.99) and allowed estimation of decomposition rates as dependent on land use. Land use affected the decomposition of labile fractions of SOC and of maize residue, but had no effect on the decomposition of recalcitrant fractions. We conclude that land use affected the decomposition dynamics within the first 1.5 months mainly because of differences in soil microbial biomass but had low effect on cumulative decomposition of maize residues within 5 months.     

Changes in soil organic carbon dynamics in an Eastern Chinese coastal wetland following invasion by a C4 plant Spartina alterniflora

The exotic C₄ grass Spartina alterniflora was intentionally introduced to tidal coastal wetlands in Jiangsu province of China in 1982. Since then it has rapidly replaced the native C₃ plant Suaeda salsa, becoming one of the dominant vegetation types in the coastal wetlands of China. Although plant invasion can change soil organic carbon (SOC) storage, little is known about how plant invasion influences C storage within soil fractions. We investigated how S. alterniflora invasion across an 8, 12 and 14-year chronosequence affected SOC and soil nitrogen (N), using soil fractionation and stable δ¹³C isotope analyses. SOC and N concentrations at 0-10 cm depth in S. alterniflora soil increased during the S. alterniflora invasion chronosequence, ranging from 3.67 to 4.90 g C kg⁻¹ soil, and from 0.307 to 0.391 g N kg⁻¹ soil. These were significantly higher than the values in the Suaeda salsa community, by 27.0-69.6% for SOC, and 21.8-55.2% for total N. The S. alterniflora-derived SOC varied from 0.40 to 0.92 g C kg⁻¹ according to mixing calculations, assuming the two possible SOC sources of S. alterniflora and S. salsa, and accounted for 10.8-18.7% of total SOC in the colonized soils. The estimated accumulative rate of SOC from C₄ (S. alterniflora) was 64.1 C kg⁻¹ soil year⁻¹ and from C₃ sources was 78.1 mg C kg⁻¹. The concentration of S. alterniflora-derived SOC significantly decreased from coarse fraction to fine fraction, and linearly increased as the period of S. alterniflora invasion increased. The highest accumulative rate of SOC from a C₄ source occurred in macroaggregates, while the highest rate from C₃ was in microaggregates. The storage of SOC derived from S. alterniflora in the macroaggregates was 0.27-0.44 g C kg⁻¹ soil, accounting for 43.1-49.1% of the total C₄derived SOC in the soil. Our results suggest that S. alterniflora invasion in coastal wetlands could facilitate SOC storage, because of the high potential for accumulation of the C which has been newly derived from S. alterniflora litter and roots.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.     

Decomposition of N-labelled maize leaves in soil affected by endogeic geophagous Aporrectodea caliginosa

A microcosm experiment was carried out for 56 days at 12 °C to evaluate the feeding effects of the endogeic geophagous earthworm species Aporrectodea caliginosa on the microbial use of ¹⁵N-labelled maize leaves (Zea mays) added as 5 mm particles equivalent to 1 mg C and 57 μg N g⁻¹ soil. The dry weight of A. caliginosa biomass decreased in the no-maize treatment by 10% during the incubation and increased in the maize leaf treatments by 18%. Roughly 5% and 10% of the added maize leaf-C and leaf-N, respectively, were incorporated into the biomass of A. caliginosa. About 29% and 33% of the added maize leaf-C were mineralised to CO₂ in the no-earthworm and earthworm treatments, respectively. The presence of A. caliginosa significantly increased soil-derived CO₂ production by 90 μg g⁻¹ soil in the no-maize and maize leaf treatments, but increased the maize-derived CO₂ production only by 40 μg g⁻¹ soil. About 10.5% of maize leaf-C and leaf-N was incorporated into the soil microbial biomass in the absence of earthworms, but only 6% of the maize leaf-C and 3% of the maize leaf-N in the presence of earthworms. A. caliginosa preferentially fed on N rich, maize leaf-colonizing microorganisms to meet its N demand. This led to a significantly increased C/N ratio of the unconsumed microbial biomass in soil. The ergosterol-to-microbial biomass C ratio was not significantly decreased by the presence of earthworms. A. caliginosa did not directly contribute to comminution of plant residues, as indicated by the absence of any effects on the contents of the different particulate organic matter fractions, but mainly to grazing of residue-colonizing microorganisms, increasing their turnover considerably.     

Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm⁻³ (free POM_<1.6), occluded particulate organic matter with densities <1.6 g cm⁻³ (occluded POM_<1.6) and 1.6-2.0 g cm⁻³ (occluded POM_1.6-2.0) and mineral-associated SOM (>2.0 g cm⁻³) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the ¹³C/¹²C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM_<1.6>occluded POM_<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.     

减氮配施有机物质对土壤氮素淋失的调控作用

采用室内土柱模拟试验方法,研究不同氮肥施用下1m土体中氮素的分布和移动特征,揭示土壤氮素动态变化规律。结果表明:FN(农民习惯施无机氮用量)、RN(根据土壤养分供应和作物需求确定的推荐无机氮用量)显著增加了土壤上层NH_4^+-N和NO_3^--N向下层淋失。RN+HA(与推荐无机氮纯养分相等的锌腐酸尿素)和RN40%+OMB(推荐无机氮肥减60%基础上配施自制有机调理物质)可延长上层土壤NH_4^+-N峰值出现时间,降低下层NH_4^+-N。淋溶结束后,等氮量下增施HA较RN降低60cm以下NH_4^+-N残留29.7%～54.2%;降低60—80cm NO_3^--N累积17.4%。RN40%+OMB处理无机氮肥用量最小,0—20cm的NH_4^+-N最高,40—100cm稳定在2.0mg/kg左右;0—20,20—40cm土层NO_3^--N较RN+HA增加12.3%和2.0%,显著降低40cm以下NO_3^--N残留。RN+HA和RN40%+OMB较RN的土壤总无机氮残留分别减少7.4%和20.2%,降低表观淋失率。因此,RN40%+OMB可较好地抑制氮素下移,降低氮素淋失风险,为减少氮素淋失、明确合理氮肥施用方式提供科学依据。     

小麦和玉米秸秆腐解特点及对土壤中碳、氮含量的影响

通过室内模拟培养试验,揭示了不同水分条件下小麦和玉米秸秆在土壤中的腐解特点及对土壤碳、氮含量的影响。结果表明,1)水分条件对有机物质腐解的影响较大,在32 d的培养期间,相对含水量为60%(M60)时,土壤CO2释放速率始终低于含水量80%(M80)的处理。M60条件下释放的CO2-C量占秸秆腐解过程中释放碳总量的40.1%,而M80条件下达到51.5%;M60条件下,添加秸秆土壤中有机碳含量平均提高2.24 g/kg,显著高于M80条件下的1.43 g/kg。2)添加玉米秸秆的土壤,在培养期内CO2释放速率始终高于小麦秸秆处理,CO2-C累积释放量和有机碳净增量分别为408.35 mg/pot和2.12 g/kg;而小麦秸秆处理分别仅为378.94 mg/pot和1.56 g/kg,两种秸秆混合的处理介于二者之间。3)与未添加秸秆相比,土壤中添加小麦或玉米秸秆后,土壤有机碳、微生物量碳、全氮和微生物量氮含量均显著提高,且数量上总体趋势表现为:玉米秸秆两种秸秆混合小麦秸秆。可见,适宜水分条件有利于秸秆腐解过程中秸秆中碳向无机碳方向转化,而不利于向土壤有机碳方向转化;且玉米秸秆比小麦秸秆更易腐解。秸秆在土壤中腐解对补充土壤碳、氮作用很大,可改善土壤微生物生存条件,提高土壤质量。     

Influence of crop residue management, cropping system and N fertilizer on soil N and C dynamics and sustainable wheat (Triticum aestivum L.) production

Management of N is the key for sustainable and profitable wheat production in a low N soil. We report results of irrigated crop rotation experiment, conducted in the North West Frontier Province (NWFP), Pakistan, during 1999–2002 to evaluate effects of residue retention, fertilizer N application and mung bean (Vigna radiata) on crop and N yields of wheat and soil organic fertility in a mung bean–wheat sequence. Treatments were (a) crop residue retained (+residue) or (b) removed (−residue), (c) 120 kg N ha⁻¹ applied to wheat, (d) 160 kg N ha⁻¹ to maize or (e) no nitrogen applied. The cropping system was rotation of wheat with maize or wheat with mung bean. The experiment was laid out in a spit plot design. Postharvest incorporation of crop residues significantly (p < 0.05) increased the grain and straw yields of wheat during both years. On average, crop residues incorporation increased the wheat grain yield by 1.31 times and straw yield by 1.39 times. The wheat crop also responded strongly to the previous legume (mung bean) in terms of enhanced grain yield by 2.09 times and straw yield by 2.16 times over the previous cereal (maize) treatment. Application of fertilizer N to previous maize exerted strong carry over effect on grain (1.32 times) and straw yield (1.38 times) of the following wheat. Application of N fertilizer to current wheat produced on average 1.59 times more grain and 1.77 times more straw yield over the 0 N kg ha⁻¹ treatment. The N uptake in wheat grain and straw was increased 1.31 and 1.64 times by residues treatment, 2.08 and 2.49 times by mung bean and 1.71 and 1.86 times by fertilizer N applied to wheat, respectively. The soil mineral N was increased 1.23 times by residues, 1.34 times by mung bean and 2.49 times by the application of fertilizer N to wheat. Similarly, the soil organic C was increased 1.04-fold by residues, 1.08 times by mung bean and 1.00 times by the application of fertilizer N. We concluded that retention of residues, application of fertilizer N and involvement of legumes in crop rotation greatly improves the N economy of the cropping system and enhances crop productivity in low N soils.     

Savanna-derived organic matter remaining in arable soils of the South African Highveld long-term mixed cropping: Evidence from C and N natural abundance

Sustainable agriculture requires the formation of new humus from the crops. We utilized ¹³C and ¹⁵N signatures of soil organic matter to assess how rapidly wheat/maize cropping contributed to the humus formation in coarse-textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) cropped for lengths of time varying from 0 to 98 years, after conversion from native grassland savanna (C4). We performed natural ¹³C and ¹⁵N abundance measurements on bulk and particle-size fractions. The bulk soil δ¹³C values steadily decreased from −14.6 in (C4 dominated) grassland to −16.5‰ after 90 years of arable cropping. This δ¹³C shift was attributable to increasing replacement of savanna-derived C by wheat crop (C3) C which dominated over maize (C4) inputs. After calculating the annual C input from the crop yields and the output from literature data, by using a stepwise C replacement model, we were able to correct the soil δ¹³C data for the irregular maize inputs for a period of about one century. Within 90 years of cropping 41-89% of the remaining soil organic matter was crop-derived in the three studied agroecosystems. The surface soil C stocks after 90 years of the wheat/maize crop rotation could accurately be described with the Rothamsted Carbon Model, but modelled C inputs to the soil were very low. The coarse sand fraction reflected temporal fluctuations in ¹³C of the last C₃ or C₄ cropping and the silt fraction evidenced selective erosion loss of old savanna-derived C. Bulk soil ¹⁵N did not change with increasing cropping length. Decreasing δ¹⁵N values caused by fertilizer N inputs with prolonged arable cropping were only detected for the coarse sand fraction. This indicated that the present N fertilization was not retained in stable soil C pool. Clearly, conventional cropping practices on the South African highlands neither contribute to the preservation of old savanna C and N, nor the effective humus reformation by the crops.