Carbon decomposition in broiler litter-amended soils

Organic carbon (OC) is generally low in Alabama (U.S.A.) soils and varies considerably with cropping systems. Information on decomposition rates of the added C is a prerequisite to designing strategies that improve C sequestration in farming systems. Different models including exponential models have been used to describe OC mineralization in soils as well as to describe its potential as CO₂ to be released into the environment. We investigated the decomposition of broiler litter added to ten non-calcareous soils (Appling, Troup, Cecil, Decatur, Sucarnoochee, Linker, Hartsells, Dothan, Maytag, and Colbert soils). A non-linear regression approach for N mineralization was used to estimate the potentially mineralizable OC pools (C_o) and the first-order rate constant (k) in the soil samples. Results showed that the non-amended soils have distinct differences in their ability to release their native OC as CO₂ and can be divided into four groups depending on their potentially mineralizable C (C_o) and their ability to protect stable organic matter. Sucarnoochee soil represents the first group and contains a moderate amount of OC (11.4 g C kg⁻¹) but had the highest C_o (7.30 g C kg⁻¹ soil). The second distinct group of soils has C_o varying between 5.50 and 5.00 g C kg⁻¹ soil (Decatur, Hartsells, Dothan, and Maytag). The third group has C_o between 5.00 and 4.00 (Appling, Cecil, and Linker). The fourth group has C_o less than 4.00 g C kg⁻¹ soil (Troup and Colbert). Half-life of C remaining in non-amended soils varied from 26 days in Maytag soil to 139 days in Cecil soil. The OC in these non-amended soils represents a very stable form of organic C and thus, not easily decomposed by soil microorganisms. In the broiler litter-amended soils, the C_o varied from 3.82 g C kg⁻¹ in Appling soil amended with broiler litter 1-7.04 g C kg⁻¹ soil in Maytag amended with broiler litter 2. Decomposition of the added OC proceeded in two phases with less than 31% decomposed in 43 days. Potentially mineralizable organic C (C_o) was related to soil organic C (r = 0.661**) and soil C/N ratio (r = 0.819*).     

互花米草入侵潮滩沼泽本土C4植物后土壤有机碳的动态变化

Invasion of an exotic C_4 plant Spartina alterniflora has been shown to increase soil organic carbon(SOC) concentrations in native C_3 plant-dominated coastal wetlands of China. However, little is known about the effects of S. alterniflora invasion on SOC concentrations and fractions in tidal marshes dominated by native C_4 plants. In this study, a field experiment was conducted in a tidal marsh dominated by the native C_4 plant Cyperus malaccensis in the Minjiang River estuary, China. Concentrations of SOC and liable SOC fractions, dissolved organic carbon(DOC), microbial biomass carbon(MBC), and easily oxidizable carbon(EOC),were measured in the top 50-cm soils of the C. malaccensis community, as well as those of three S. alterniflora communities with an invasion duration of 0–4 years(SA-4), 4–8 years(SA-8), and 8–12 years(SA-12), respectively. Results showed that both SOC stocks in the 50-cm soils and mean SOC concentrations in the surface soils(0–10 cm) of the C. malaccensis community increased with the duration of S. alterniflora invasion, whereas SOC concentrations in the 10–50-cm soils decreased slightly during the initial period of S. alterniflora invasion, before increasing again. The pattern of changes in labile SOC fractions(DOC, MBC, and EOC) with invasion duration was generally similar to that of SOC, while the ratios of labile SOC fractions to total SOC(DOC:SOC, MBC:SOC, and EOC:SOC) decreased significantly with the duration of S. alterniflora invasion. The findings of this study suggest that invasion of the exotic C_4 plant S. alterniflora into a marsh dominated by the native C_4 plant C. malaccensis would enhance SOC sequestration owing to the greater amount of biomass and lower proportion of labile SOC fractions present in the S. alterniflora communities.     

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.     

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.     

Toxicological effects of TiO2 and ZnO nanoparticles in soil on earthworm Eisenia fetida

Nanoparticles (NPs) of TiO₂ and ZnO are receiving increasing attention due to their widespread applications. To evaluate their toxicities to the earthworm Eisenia fetida (Savigny, 1826) in soil, artificial soil systems containing distilled water, 0.1, 0.5, 1.0 or 5.0 g kg⁻¹ of NPs were prepared and earthworms were exposed for 7 days. Contents of Zn and Ti in earthworm, activities of antioxidant enzymes, DNA damage to earthworm, activity of cellulase and damage to mitochondria of gut cells were investigated after acute toxicity test. The results from response of the antioxidant system combined with DNA damage endpoint (comet assay) indicated that TiO₂ and ZnO NPs could induce significant damage to earthworms when doses were greater than 1.0 g kg⁻¹. We found that Ti and Zn, especially Zn, were bioaccumulated, and that mitochondria were damaged at the highest dose in soil (5.0 g kg⁻¹). The activity of cellulase was significantly inhibited when organisms were exposed to 5.0 g kg⁻¹ of ZnO NPs. Our study demonstrates that both TiO₂ and ZnO NPs exert harmful effects to E. fetida when their levels are higher than 1.0 g kg⁻¹ in soil and that toxicity of ZnO NPs was higher than TiO₂.     

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.     

Microbial and enzyme properties of apple orchard soil as affected by long-term application of copper fungicide

Copper-based fungicides have been applied in apple orchards for a long time, which has resulted in increasing soil Cu concentration. However, the microbial and enzyme properties of the orchard soils remain poorly understood. This study aimed to evaluate the effect of long-term application of Cu-based fungicides on soil microbial (microbial biomass carbon (C_mic), C mineralization, and specific respiration rate) and enzyme (urease, acid phosphatase, and invertase activities) properties in apple orchards. Soil samples studied were collected from apple orchards 5, 15, 20, 30, and 45 years old, and one adjacent forest soil as for reference. The mean Cu concentrations of orchard soils significantly increased with increasing orchard ages ranging from 21.8 to 141 mg kg⁻¹, and the CaCl₂-extractable soil Cu concentrations varied from 0.00 to 4.26 mg kg⁻¹. The soil mean C_mic values varied from 43.6 to 116 mg kg⁻¹ in the orchard soils, and were lower than the value of the reference soil (144 mg kg⁻¹). The ratio of soil C_mic to total organic C (C_org) increased from 8.10 to 18.3 mg C_mic g⁻¹ C_org with decreasing orchard ages, and was 26.1 mg C_mic g⁻¹ C_org for the reference soil. A significant correlation was observed between total- or CaCl₂-extractable soil Cu and soil C_mic or C_mic/C_org, suggesting that the soil Cu was responsible for the significant reductions in C_mic and C_mic/C_org. The three enzyme activity assays also showed the similar phenomena, and declined with the increasing orchard ages. The mean soil C mineralization rates were elevated from 110 to 150 mg CO₂-C kg⁻¹ soil d⁻¹ compared with the reference soil (80 mg CO₂-C kg⁻¹ soil d⁻¹), and the mean specific respiration rate of the reference soil (0.63 mg CO₂-C mg⁻¹ biomass C d⁻¹) was significantly smaller than the orchard soils from 1.19 to 3.55 mg CO₂-C mg⁻¹ biomass C d⁻¹. The soil C mineralization rate and the specific respiration rate can be well explained by the CaCl₂-extractable soil Cu. Thus, the long-term application of copper-based fungicides has shown adverse effects on soil microbial and enzyme properties.     

Soil organic C stabilization and thresholds in C saturation

When building soil organic matter (SOM) contents in agricultural production systems, stabilization of both pre-existing as well as added C is important. A laboratory mineralization experiment was conducted over 374 days to evaluate the effect of pre-existing SOM on soil C mineralization after addition of organic matter (OM) using sugar cane. The SOM gradient used here stretched from 21 to 106 g C kg⁻¹ soil and was a result of different periods of continuous cultivation of 5, 20, 35 and 105 years in comparison to a forest soil. The rate of organic C mineralization was found to be dependent on the status of pre-existing soil organic C (SOC). Highly degraded soil which had been under continuous cultivation for 35 years and more showed the highest rate of C mineralization per unit SOC (117.9 mg C g⁻¹ C) while forest soil had the lowest amount of C mineralized per unit SOC (73.5 mg C g⁻¹ C). Forest soil had the highest amount of increased C mineralization as a result of organic matter (OM) additions (8.0 mg C g⁻¹ soil) followed by the highly degraded soil that had been under cultivation for 105 years (5.5 mg C g⁻¹ soil). Additional mineralized C as a function of time after forest conversion declined progressively within the first 20 years of continuous soil use. Soil which had been under continuous cultivation for 20 years had the lowest amount of additional mineralized C (4.0 mg C g⁻¹ soil). SOM stabilization efficiency in the studied soils appears to be highest with intermediate cultivation history of about 20 years. These soils that have been recently converted to cultivation also appear to have a greater ability to stabilize added OM than the most degraded soils investigated in this study. It is thus advisable to provide intervention strategies to reverse SOM decline for farming communities at an intermediate stage before the soils are highly depleted of SOC.     

Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland

Woody plant invasion of grasslands is prevalent worldwide. In the Rio Grande Plains of Texas, subtropical thorn woodlands dominated by C₃ trees/shrubs have been replacing C₄ grasslands over the past 150 yr, resulting in increased soil organic carbon (SOC) storage and concomitant increases in soil total nitrogen (STN). To elucidate mechanisms of change in SOC and STN, we separated soil organic matter into specific size/density fractions and determined the concentration of C and N in these fractions. Soils were collected from remnant grasslands (Time 0) and woody plant stands (ages 10-130 yr). Rates of whole-soil C and N accrual in the upper 15 cm of the soil profile averaged 10-30 g C m⁻² yr⁻¹ and 1-3 g N m⁻² yr⁻¹, respectively, over the past 130 yr of woodland development. These rates of accumulation have increased soil C and N stocks in older wooded areas by 100-500% relative to remnant grasslands. Probable causes of these increased pool sizes include higher rates of organic matter production in wooded areas, greater inherent biochemical resistance of woody litter to decomposition, and protection of organic matter by stabilization within soil macro- and microaggregates. The mass proportions of the free light fraction (<1.0 g cm⁻³) and macroaggregate fraction (>250 μm) increased linearly with time following woody plant invasion of grassland. Conversely, the mass proportions of free microaggregate (53-250 μm) and free silt+clay (<53 μm) fractions decreased linearly with time after woody invasion, likely reflecting stabilization of these fractions within macroaggregate structures. Carbon and N concentrations increased in all soil fractions with time following woody invasion. Approximately half of the C and N accumulated in free particulate organic matter (POM) fractions, while the remainder accrued in stable macro- and microaggregate structures. Soil C/N ratios indicated that the organic C associated with POM and macroaggregates was of more recent origin (less decomposed) than C associated with the microaggregate and silt+clay fractions. Because grassland-to-woodland conversion has been geographically extensive in grassland ecosystems worldwide during the past century, changes in soil C and N storage and dynamics documented here could have significance for global cycles of those elements.     

Interactions between residue placement and earthworm ecological strategy affect aggregate turnover and N2O dynamics in agricultural soil

Previous laboratory studies using epigeic and anecic earthworms have shown that earthworm activity can considerably increase nitrous oxide (N₂O) emissions from crop residues in soils. However, the universality of this effect across earthworm functional groups and its underlying mechanisms remain unclear. The aims of this study were (i) to determine whether earthworms with an endogeic strategy also affect N₂O emissions; (ii) to quantify possible interactions with epigeic earthworms; and (iii) to link these effects to earthworm-induced differences in selected soil properties. We initiated a 90-day ¹⁵N-tracer mesocosm study with the endogeic earthworm species Aporrectodea caliginosa (Savigny) and the epigeic species Lumbricus rubellus (Hoffmeister). ¹⁵N-labeled radish (Raphanus sativus cv. Adagio L.) residue was placed on top or incorporated into the loamy (Fluvaquent) soil. When residue was incorporated, only A. caliginosa significantly (p < 0.01) increased cumulative N₂O emissions from 1350 to 2223 μg N₂O-N kg⁻¹ soil, with a corresponding increase in the turnover rate of macroaggregates. When residue was applied on top, L. rubellus significantly (p < 0.001) increased emissions from 524 to 929 μg N₂O-N kg⁻¹, and a significant (p < 0.05) interaction between the two earthworm species increased emissions to 1397 μg N₂O-N kg⁻¹. These effects coincided with an 84% increase in incorporation of residue ¹⁵N into the microaggregate fraction by A. caliginosa (p = 0.003) and an 85% increase in incorporation into the macroaggregate fraction by L. rubellus (p = 0.018). Cumulative CO₂ fluxes were only significantly increased by earthworm activity (from 473.9 to 593.6 mg CO₂-C kg⁻¹ soil; p = 0.037) in the presence of L. rubellus when residue was applied on top. We conclude that earthworm-induced N₂O emissions reflect earthworm feeding strategies: epigeic earthworms can increase N₂O emissions when residue is applied on top; endogeic earthworms when residue is incorporated into the soil by humans (tillage) or by other earthworm species. The effects of residue placement and earthworm addition are accompanied by changes in aggregate and SOM turnover, possibly controlling carbon, nitrogen and oxygen availability and therefore denitrification. Our results contribute to understanding the important but intricate relations between (functional) soil biodiversity and the soil greenhouse gas balance. Further research should focus on elucidating the links between the observed changes in soil aggregation and controls on denitrification, including the microbial community.     

Soil organic carbon changes in particle-size fractions following cultivation of Black soils in China

Soil texture can be an important control on soil organic carbon (SOC) retention and dynamics. The (clay + silt)-sized SOC pool (SOC < 20 μm) in non-cultivated or grassland soils has been proposed to reach an equilibrium or maximum level named protective capacity. Proper knowledge of SOC in this size fraction in non-cultivated and cultivated Black soils is important to evaluate management-induced changes in SOC in NE China. Twenty-seven paired soil samples (non-cultivated vs. cultivated) were collected in the Black soil zone in Heilongjiang and Jilin provinces. Bulk soil was dispersed in water with an ultrasonic probe and then soil size fractions were collected using the pipette technique for SOC analyses. Soil organic carbon in bulk soil and size fractions was measured by dry combustion. Average content of SOC < 20 μm was 23.2 g C kg⁻¹ at the 0–30 cm depth for the non-cultivated soils, accounting for 75.1% of the total SOC at the same depth. There was significant positive relationship between soil clay plus silt content and SOC < 20 μm in non-cultivated soils. Accordingly, a model of the maximum SOC < 20 μm in 0–30 cm depth of non-cultivated Black soils was developed: y = 0.36x where y is the maximum SOC < 20 μm pool (g C kg⁻¹) and x is the percentage of clay + silt (<20 μm) content. The average content of SOC < 20 μm was 18.7 g C kg⁻¹ at 0–30 cm depth for cultivated soils, accounting for 81.5% of total SOC. This average value of SOC was 4.4 g C kg⁻¹ less than the maximum value (23.1 g C kg⁻¹) and accounted for 55.0% of the difference of SOC between non-cultivated and cultivated Black soils. Cultivation resulted in 45.0% loss of sand-sized (>20 μm) SOC concentration relative to SOC < 20 μm. This result indicates that SOC < 20 μm and sand-sized SOC both play important roles in SOC dynamics resulting from management practices. This model can be applied to calculate the actual potential to restore SOC for cultivated Black soils under conservation tillage in NE China.     

Annual soil CO2 effluxes in the High Arctic: The role of snow thickness and vegetation type

Little work has been done to quantify annual soil CO₂ effluxes in the High Arctic region because of the difficulty in taking winter measurements. Since the effects of climate change are expected to be higher in Arctic than in temperate ecosystems, it is important that summer measurements are extended to cover the entire year. This study evaluates the quantity and quality of soil organic C (SOC) and seasonal controls of soil CO₂ effluxes in three soils under three dominating types of vegetation (Dryas, Cassiope, and Salix) at Svalbard. Measurements included soil CO₂ effluxes in the field and the laboratory, temperature, water content, and snow thickness. About 90% of the variation in soil respiration throughout 1 year was due to near-surface soil temperatures which ranged from −12 to +12 °C. Total annual soil CO₂ effluxes varied from 103 g C m⁻² at soils under Cassiope, 152 g C m⁻² under Dryas sites, and 176 g C m⁻² under Salix, with 20%, 14%, and 30%, respectively, being released during a 6-month winter period. The sensitivity of soil respiration with respect to soil temperature was the same year round and differences in winter CO₂ effluxes at the three vegetation types were mainly related to subsurface soil temperatures controlled by snow depth. The quantity and quality of soil organic matter varied under the different vegetation types. Soils under Salix had the largest and most labile pool of SOC and were characterized by a long period of snow cover. In contrast, soils under Cassiope were more nutrient-poor, more acidic and held the smallest amount of total and labile SOC, whereas soils under Dryas remained snow-free most of the winter and therefore had the coldest winter conditions. Thus, winter soil respiration rates under Dryas and Cassiope were significantly lower than those under Salix; under Dryas this was mainly due to snow depth, under Cassiope this was a combination of snow depth and poor litter quality. It is concluded that winter respiration is highly variable across Arctic landscapes and depends on the spatial distribution of snow, which acts as a direct control on soil temperatures and indirect on vegetation types and thereby, the amount and quality of soil organic matter, which serve as additional important drivers of soil respiration.     

Controls over pathways of carbon efflux from soils along climate and black spruce productivity gradients in interior Alaska

Small changes in C cycling in boreal forests can change the sign of their C balance, so it is important to gain an understanding of the factors controlling small exports like water-soluble organic carbon (WSOC) fluxes from the soils in these systems. To examine this, we estimated WSOC fluxes based on measured concentrations along four replicate gradients in upland black spruce (Picea mariana [Mill.] BSP) productivity and soil temperature in interior Alaska and compared them to concurrent rates of soil CO₂ efflux. Concentrations of WSOC in organic and mineral horizons ranged from 4.9 to 22.7 g C m⁻² and from 1.4 to 8.4 g C m⁻², respectively. Annual WSOC fluxes (4.5-12.0 g C m⁻² y⁻¹) increased with annual soil CO₂ effluxes (365-739 g C m⁻² y⁻¹) across all sites (R²=0.55, p=0.02), with higher fluxes occurring in warmer, more productive stands. Although annual WSOC flux was relatively small compared to total soil CO₂ efflux across all sites (<3%), its relative contribution was highest in warmer, more productive stands which harbored less soil organic carbon. The proportions of relatively bioavailable organic fractions (hydrophilic organic matter and low molecular weight acids) were highest in WSOC in colder, low-productivity stands whereas the more degraded products of microbial activity (fulvic acids) were highest in warmer, more productive stands. These data suggest that WSOC mineralization may be a mechanism for increased soil C loss if the climate warms and therefore should be accounted for in order to accurately determine the sensitivity of boreal soil organic C balance to climate change.     

Changes in soil biological and biochemical characteristics in a long-term field trial on a sub-tropical inceptisol

Soil organic carbon (SOC), microbial biomass carbon (MBC), their ratio (MBC/SOC) which is also known as microbial quotient, soil respiration, dehydrogenase and phosphatase activities were evaluated in a long-term (31 years) field experiment involving fertility treatments (manure and inorganic fertilizers) and a maize (Zea mays L.)-wheat (Triticum aestivum L.)-cowpea (Vigna unguiculata L.) rotation at the Indian Agricultural Research Institute near New Delhi, India. Applying farmyard manure (FYM) plus NPK fertilizer significantly increased SOC (4.5-7.5 g kg⁻¹), microbial biomass (124-291 mg kg⁻¹) and microbial quotient from 2.88 to 3.87. Soil respiration, dehydrogenase and phosphatase activities were also increased by FYM applications. The MBC response to FYM+100% NPK compared to 100% NPK (193 vs. 291 mg kg⁻¹) was much greater than that for soil respiration (6.24 vs. 6.93 μl O₂ g⁻¹ h⁻¹) indicating a considerable portion of MBC in FYM plots was inactive. Dehydrogenase activity increased slightly as NPK rates were increased from 50% to 100%, but excessive fertilization (150% NPK) decreased it. Acid phosphatase activity (31.1 vs. 51.8 μg PNP g⁻¹ h⁻¹) was much lower than alkali phosphatase activity (289 vs. 366 μg PNP g⁻¹ h⁻¹) in all treatments. Phosphatase activity was influenced more by season or crop (e.g. tilling wheat residue) than fertilizer treatment, although both MBC and phosphatase activity were increased with optimum or balanced fertilization. SOC, MBC, soil respiration and acid phosphatase activity in control (no NPK, no manure) treatment was lower than uncultivated reference soil, and soil respiration was limiting at N alone or NP alone treatments.     

Soil carbon turnover and sequestration in native subtropical tree plantations

Approximately 30% of global soil organic carbon (SOC) is stored in subtropical and tropical ecosystems but it is being rapidly lost due to continuous deforestation. Tree plantations are advocated as a C sink, however, little is known about rates of C turnover and sequestration into soil organic matter under subtropical and tropical tree plantations. We studied changes in SOC in a chronosequence of hoop pine (Araucaria cunninghamii) plantations established on former rainforest sites in seasonally dry subtropical Australia. SOC, δ¹³C, and light fraction organic C (LF C<1.6 g cm⁻³) were determined in plantations, secondary rainforest and pasture. We calculated loss of rainforest SOC after clearing for pasture using an isotope mixing model, and used the decay rate of rainforest-derived C to predict input of hoop pine-derived C into the soil. Total SOC stocks to 100 cm depth were significantly (P<0.01) higher under rainforest (241 t ha⁻¹) and pasture (254 t ha⁻¹) compared to hoop pine (176-211 t ha⁻¹). We calculated that SOC derived from hoop pine inputs ranged from 32% (25 year plantation) to 61% (63 year plantation) of total SOC in the 0-30 cm soil layer, but below 30 cm all C originated from rainforest. These results were compared to simulations made by the Century soil organic matter model. The Century model simulations showed that lower C stocks under hoop pine plantations were due to reduced C inputs to the slow turnover C pool, such that this pool only recovers to within 45% of the original rainforest C pool after 63 years. This may indicate differences in soil C stabilization mechanisms under hoop pine plantations compared with rainforest and pasture. These results demonstrate that subtropical hoop pine plantations do not rapidly sequester SOC into long-term storage pools, and that alternative plantation systems may need to be investigated to achieve greater soil C sequestration.     

Stability of hydrolytic enzyme activity and microbial phosphorus during storage of tropical rain forest soils

Storage can markedly influence microbial and biochemical properties in soils, yet recommendations for sample storage are based on studies of temperate soils that regularly experience extended cold periods. We assessed the influence of storage conditions on microbial phosphorus and the activity of four hydrolytic enzymes (phosphomonoesterase, phosphodiesterase, β-glucosidase, and N-acetyl-β-d-glucosaminidase) in three lowland tropical forest soils from the Republic of Panama that experience a constant warm temperature. The soils spanned a strong rainfall gradient and contained contrasting physical and chemical properties (pH 3.6-5.9; total carbon 26-50 g C kg⁻¹; clay 33-62%; total phosphorus 0.30-0.60 g P kg⁻¹). Storage treatments were: (i) room temperature (22 °C in the dark), (ii) refrigerated (4 °C in the dark), (iii) air-dried (10 d, 22 °C), and (iv) frozen (−35 °C). There were significant changes in enzyme activities and microbial phosphorus during refrigerated and room temperature storage, although changes were relatively small during the first two weeks. An initial marked decline in enzyme activities for one soil analyzed within 2 h of sampling was attributed to a flush of activity caused by sampling and soil preparation (sieving, etc.). For longer-term storage (>2 weeks), ambient laboratory temperature appeared preferable to freezing and cold storage, because one month of storage caused a marked decline in enzyme activities and microbial phosphorus in one soil. Freezing preserved the activities of some enzymes in some soils at rates comparable to cold or room temperature storage, but caused a marked decline in microbial phosphorus in two soils. Air-drying caused a marked decline in microbial phosphorus and the activity of all enzymes. We therefore conclude that enzyme assays and microbial phosphorus should be determined in tropical forest soils after no more than two weeks storage in the dark at ambient laboratory temperature.     

Response of soil C:N:P stoichiometry,organic carbon stock,and release to wetland grasslandification in Mu Us Desert

Purpose

Wetlands in Mu Us Desert have severely been threatened by grasslandification over the past decades. Therefore, we studied the impacts of grasslandification on soil carbon (C):nitrogen (N):phosphorus (P) stoichiometry, soil organic carbon (SOC) stock, and release in wetland-grassland transitional zone in Mu Us Desert.

Materials and methods

From wetland to grassland, the transition zone was divided into five different successional stages according to plant communities and soil water conditions. At every stage, soil physical and chemical properties were determined and C:N:P ratios were calculated. SOC stock and soil respirations were also determined to assess soil carbon storage and release.

Results and discussion

After grasslandification, SOC contents of top soils (0–10 cm) decreased from 100.2 to 31.79 g kg^?1 in June and from 103.7 to 32.5 g kg^?1 in October; total nitrogen (TN) contents of top soils (0–10 cm) decreased from 3.65 to 1.85 g kg^?1 in June and from 6.43 to 3.36 g kg^?1 in October; and total phosphorus (TP) contents of top soils (0–10 cm) decreased from 179.4 to 117.4 mg kg^?1 in June and from 368.6 to 227.8 mg kg^?1 in October. From stages Typha angustifolia wetland (TAW) to Phalaris arundinacea L. (PAL), in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 19.0 to 11.8 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October. SOC stock decreased and soil respiration increased with grasslandification. The decrease of SOC, TN, and TP contents was attributed to the reduction of aboveground biomass and mineralization of SOM, and the decrease of soil C:N, C:P, and N:P ratios was mainly attributed to the faster decreasing speeds of SOC than TN and TP. The reduction of aboveground biomass and increased SOC release led by enhanced soil respiration were the main reasons of SOC stock decrease.

Conclusions

Grasslandification led to lowers levels of SOC, TN, TP, and soil C:N, C:P, and N:P ratios. Grasslandification also led to higher SOC loss, and increased soil respiration was the main reason. Since it is difficult to restore grassland to original wetland, efficient practices should be conducted to reduce water drainage from wetland to prevent grasslandification.

     

Taxon-specific responses of soil bacteria to the addition of low level C inputs

The addition of small or trace amounts of carbon to soils can result in the release of 2-5 times more C as CO₂ than was added in the original solution. The identity of the microorganisms responsible for these so-called trigger effects remains largely unknown. This paper reports on the response of individual bacterial taxa to the addition of a range of ¹⁴C-glucose concentrations (150, 50 and 15 and 0 μg C g⁻¹ soil) similar to the low levels of labile C found in soil. Taxon-specific responses were identified using a modification of the stable isotope probing (SIP) protocol and the recovery of [¹⁴C] labelled ribosomal RNA using equilibrium density gradient centrifugation. This provided good resolution of the ‘heavy’ fractions ([¹⁴C] labelled RNA) from the ‘light’ fractions ([¹²C] unlabelled RNA). The extent of the separation was verified using autoradiography. The addition of [¹⁴C] glucose at all concentrations was characterised by changes in the relative intensity of particular bands. Canonical correspondence analysis (CCA) showed that the rRNA response in both the ‘heavy’ and ‘light’ fractions differed according to the concentration of glucose added but was most pronounced in soils amended with 150 μg C g⁻¹ soil. In the ‘heavy RNA’ fractions there was a clear separation between soils amended with 150 μg C g⁻¹ soil and those receiving 50 and 15 μg C g⁻¹ soil indicating that at low C inputs the microbial community response is quite distinct from that seen at higher concentrations. To investigate these differences further, bands that changed in relative intensity following amendment were excised from the DGGE gels, reamplified and sequenced. Sequence analysis identified 8 taxa that responded to glucose amendment (Bacillus, Pseudomonas, Burkholderia, Bradyrhizobium, Actinobacteria, Nitrosomonas, Acidobacteria and an uncultured β-proteobacteria). These results show that radioisotope probing (RNA-RIP) can be used successfully to study the fate of labile C substrates, such as glucose, in soil.     

Seasonal changes in the spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils in Indonesia

We evaluated the spatial structures of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N₂O and CO₂ fluxes in the wetter season (1.85 mg N m⁻² d⁻¹ and 4.29 g C m⁻² d⁻¹, respectively) were significantly higher than those in the drier season (0.55 mg N m⁻² d⁻¹ and 2.73 g C m⁻² d⁻¹, respectively) and averaged CH₄ uptake rates in the drier season (−0.62 mg C m⁻² d⁻¹) were higher than those in the wetter season (−0.24 mg C m⁻² d⁻¹). These values of N₂O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO₂ and CH₄ fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N₂O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N₂O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N₂O fluxes and they may have depended on litter amount distribution. CO₂ fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO₂ fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation.     

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.