东北黑土有机碳的分布及其损失量研究

为了分析东北黑土土壤有机碳(SOC)的分布特征及其开垦以来黑土SOC的损失程度,我们于2004～2005年在黑龙江和吉林两省采集了32个自然黑土剖面样品,在每个自然黑土样品附近对应采集32个景观条件相似的耕作黑土样品。结果表明,自然黑土样品0～30cm土层SOC含量平均为32.20 g kg-1,最高可达63.46 g kg-1,黑龙江省自然黑土SOC含量(34.55 g kg-1)高于吉林省(23.80 g kg-1)。耕作土壤SOC平均含量为22.71 g kg-1,远低于自然土壤。受温度的影响,随着纬度的增加,自然黑土与耕作黑土SOC含量逐渐递增。由于土壤侵蚀以及耕垦和去除作物残留物等农业管理措施的综合作用,使得耕作黑土表层SOC含量小于自然黑土。与自然黑土相比,耕作黑土0～10cm土层SOC损失量在26.84%～46.57%之间,亚表层损失相对较少。黑土SOC含量下降也是土壤水土流失致使黑土层变薄的一个直接表现。耕作黑土表层流失厚度可以通过自然与耕作黑土剖面SOC含量的分异差值来估算。通过对土壤剖面上SOC的分布进行校正剔除土壤侵蚀的影响后得到的同等深度SOC含量的差值才可视为由耕作以及有机质输入量差异等因素造成的SOC损失量。未经校正而进行的自然黑土和耕作黑土同一深度SOC含量的比较可能过高估计了农业管理措施对土壤SOC损失量的影响。     

Distribution of soil organic carbon and phosphorus on an eroded hillslope of the rangeland in the northern Tibet Plateau, China

Some studies on the relationship between soil erosion and subsequent redeposition of eroded soils in the same field and soil quality have been conducted in croplands, yet few studies have revealed this relationship in rangelands. We selected a toposequence with a slope of 30% and a horizontal length of 342 m from the rangeland in the northern Tibet Autonomous Region, China (31°16′N, 92°09′E) to determine the relationship between soil erosion, soil organic carbon (SOC) content and available P patterns within a hillslope landscape. Soil samples for the determination of ¹³⁷Cs as well as SOC, available P and particle‐size fractions were collected at 20 m intervals along a transect of this hillslope. Soil redistribution was caused primarily by wind erosion at toe‐slope positions, but primarily by water erosion at the hillslope positions above the toe‐slope. In upper‐ and mid‐slope portions (0 m to 244 m horizontal length), SOC content is closely correlated to ¹³⁷Cs concentration (r = 0.74, P < 0.01, n= 15), suggesting that SOC distribution along the slope was similar to ¹³⁷Cs distribution, which itself was dependent on topographic changes. However, SOC contents in toe‐slope portions are less than those above the toe‐slope (i.e. upper‐ and mid‐slope portions), and the correlation between ¹³⁷Cs and SOC in the toe‐slope portion is weaker than that above the toe‐slope. A highly significant correlation (r = 0.72, P < 0.001, n= 20) between ¹³⁷Cs concentration and available P was found within the whole hillslope landscape, implying the distribution pattern of available P was somewhat different from that of SOC. We suggest that the distribution of SOC within the hillslope landscape is also affected by factors such as assimilation rates due to difference in grassland productivity at different points and different biological oxidation rates of carbon related to patterns of moisture distribution.     

Carbon sequestration in secondary pasture soils: a chronosequence study in the South African Highveld

Soil restoration is a means of combating desertification in semi‐arid and arid parts of the world. There, vast areas of the cropped soil degrade, particularly because of the loss of organic matter. One approach to reverse this loss is the conversion of cropland into permanent grassland for use as pasture. This study was designed to evaluate how fast and to what degree degraded cropland may re‐sequester soil organic carbon (SOC) when converted into permanent secondary pasture. Topsoil samples (0–5, 5–10 and 10–20 cm) were taken from chronosequences of secondary pastures (1 to 31 years old) at three agro‐ecosystems in the semi‐arid Highveld of South Africa. Long‐term croplands and primary grassland used as pastures served as the controls. In bulk soil samples (<2 mm) and their clay (<2 µm), silt (2–20 µm), fine sand (20–250 µm) and coarse sand (250–2000 µm) fractions, the contents of carbon (C) and nitrogen were determined. In all three agro‐ecosystems, using a mono‐exponential model, the SOC stocks increased exponentially until a maximum was reached 10–95 years after land conversion. This gain in SOC was clearly pronounced for the top 0–5 cm of soil, but hardly detectable at 10–20‐cm depth. The sand fractions recovered organic C more rapidly but less completely than did the finer size separates. Overall, between 9.0 and 15.3 t of SOC were sequestered in the 0–20 cm of surface soil by this land conversion. Thus, the SOC recovery in the secondary pastures resulted in SOC stocks that were 29.6–93.9% greater than those in the arable land. Yet, in no agro‐ecosystem, at any soil depth, nor in any soil fraction, did the measured SOC content reach that of the primary grassland. In part this can be attributed to a slightly finer texture of the primary grassland that had not lost silt through wind erosion or had never been used as arable land because of slightly elevated clay contents. Overall it appears, however, that previous losses of SOM cannot easily be rectified, suggesting that the native primary grassland soils are only partially resilient to land‐use change.     

Organic carbon mineralization responses to temperature increases in subtropical paddy soils

Purpose

Understanding organic carbon mineralization and its temperature response in subtropical paddy soils is important for the regional carbon balance. There is a growing interest in factors controlling soil organic carbon (SOC) mineralization because of the potential for climate change. This study aims to test the hypothesis that soil clay content impedes SOC mineralization in subtropical paddy soils.

Materials and methods

A 160-day laboratory incubation at temperatures from 10 to 30 °C and 90% water content was conducted to examine the dynamics of SOC mineralization and its temperature response in three subtropical paddy soils with different clay contents (sandy loam, clay loam, and silty clay soils). A three-pool SOC model (active, slow, and resistant) was used to fit SOC mineralization.

Results and discussion

Total CO₂ evolved during incubation following the order of clay loam > silty clay > sandy loam. The temperature response coefficients (Q ₁₀) were 1.92?±?0.39, 2.36?±?0.22, and 2.10?±?0.70, respectively, for the sandy loam soil, clay loam soil, and silty clay soil. But the soil clay content followed the order of silty clay > clay loam > sandy loam. The sandy loam soil neither released larger amounts of CO₂ nor showed higher temperature sensitivity, as expected, even though it contains lower soil clay content among the three soils. It seems that soil clay content did not have a dominant effect which results in the difference in SOC mineralization and its temperature response in the selected three paddy soils. However, dissolved organic carbon (DOC; representing substrate availability) had a great effect. The size of the active C pool ranged from 0.11 to 3.55% of initial SOC, and it increased with increasing temperature. The silty clay soil had the smallest active C pool (1.40%) and the largest Q ₁₀ value (6.33) in the active C pool as compared with the other two soils. The mineralizable SOC protected in the silty clay soil, therefore, had even greater temperature sensitivity than the other two soils that had less SOC stabilization.

Conclusions

Our study suggests that SOC mineralization and its temperature response in subtropical paddy soils were probably not dominantly controlled by soil clay content, but the substrate availability (represented as DOC) and the specific stabilization mechanisms of SOC may have great effects.     

Potential aerobic C mineralization of a red earth paddy soil and its temperature dependence under long‐term fertilizer treatments

Topsoil samples from a long‐term fertilizer trial on a red earth rice paddy from Jiangxi Province, China, were used to investigate soil organic carbon (SOC) mineralization using aerobic incubation for 58 days at 20 °C and 25 °C. SOC mineralization rates varied between 0.62 and 0.76 mg C/g SOC/h at 20 °C, and between 0.65 and 0.97 mg C/g SOC/h at 25 °C. There was no significant correlation between the mineralization potential and SOC content in treated soil samples. However, a close correlation was found between total C mineralization and the carbon stability index. This suggests that the potential C release from the soil is controlled by C lability rather than by total SOC. The calculated Q₁₀ quotient was negatively correlated with dithionate‐citrate‐bicarbonate‐extracted Fe. It is suggested that the free Fe‐oxyhydrates that are prevalent in red earth paddy soils provide physico‐chemical protection and control biological decomposition rates under warming and these are modified in the long‐term fertilizer treatments. The enhancement of physico‐chemical protection of labile SOC by free Fe‐oxyhydrates is a potential mechanism for soil C stabilization under warming conditions. The interaction with fertilizers in the red earth‐derived paddies of South China deserves further study.     

Thermal oxidation does not fractionate soil organic carbon with differing biological stabilities

Thermal analysis techniques have been used to differentiate soil organic carbon (SOC) pools with differing thermal stability. A correlation between thermal and biological stability has been indicated in some studies, while others reported inconsistent relationships. Despite these controversial findings and no standardized method, several recently published studies used thermal analysis techniques to determine the biological stability and quality of SOC in mineral soils. This study examined whether thermal oxidation at temperature levels between 200°C and 400°C, combined with evolving gas analysis and isotope ratio mass spectrometry, is capable of identifying SOC pools with differing biological stability in mineral soils. Soil samples from three sites being under Miscanthus (C₄‐plant) cultivation for more than 17 years following former agricultural cropland (only C₃‐plant) cultivation were used. Due to natural shifts in ¹³C content, young and labile Miscanthus‐derived SOC could be distinguished from stable and old C₃‐plant‐derived SOC. The proportion of Miscanthus‐derived SOC increased significantly with increasing temperatures up to 350°C in bulk soil samples, indicating increasing oxidation of labile and young SOC with increasing temperatures. Use of density fractions to validate the thermally oxidized SOC from bulk soil samples revealed that the thermal oxidation patterns did not reflect the biological stability of SOC. The suggested biologically labile particulate organic carbon (light fraction from density fractionation) was clearly enriched in Miscanthus‐derived young SOC. The thermal oxidation patterns, however, revealed preferential oxidation of these biologically labile fractions not at low temperatures, but rather at higher temperatures. The reverse was found for the biologically stable mineral‐associated density fraction (heavy fraction). Based on different soil types, it was concluded that the thermal stability of SOC between 200°C and 400°C is not a suitable indicator of the biological stability of SOC and, thus, thermal oxidation is not capable of fractionating SOC pools with differing biological stability.     

The sensitivity of water extractable soil organic carbon fractions to land use in three soil types

Soil organic carbon (SOC) has a high impact on the sustainability of ecosystems, global environmental processes, soil quality and agriculture. Long-term tillage usually leads to SOC depletion. The purpose of this study was to determine the impact of different land uses on water extractable organic carbon (WEOC) fractions and to evaluate the interaction between the WEOC fractions and other soil properties. Using an extraction procedure at 20°C and 80°C, two fractions were obtained: a cold water extractable organic carbon (CWEOC) and a hot water extractable organic carbon (HWEOC). The results suggest that there is a significant impact from different land uses on WEOC. A lower relative contribution of WEOC in SOC and a lower concentration of labile WEOC fractions are contained in arable soil compared to forestlands. Chernozem soil was characterized by a lower relative contribution of WEOC to the SOC and thus higher SOC stability in contrast to Solonetz and Vertisol soils. Both CWEOC and HWEOC are highly associated with SOC in the silt and clay fraction (<53 µm) and were slightly associated with SOC in the macroaggregate classes. The WEOC fractions were highly and positively correlated with the SOC and mean weight diameter.     

伊朗西部不同土地利用方式下钙质土壤疏水性研究

Soil wettability and water repellency, two important soil physical properties, play an important role in water retention and water conductivity in arid and semi-arid regions. To date, there is a lack of information on soil water repellency in calcareous soils of western lran. In this study, soil water repellency and its affecting factors were studied using 20 soil series collected from Hamadan Province~ western Iran. The effects of soil properties including organic carbon content （SOC）, total nitrogen （TN）, C：N ratio, texture, CaCO3 content, and both fungal and bacterial activities on water repellency were investigated using air-dried, oven-dried and heated soil samples. Water repellency index （WRI） was determined using the short-time sorptivity （water/ethanol） method. To distinguish the actual effects of SOC, a set of soil samples were heated at 300 ~C to remove SOC and then WRI was measured on the heated samples. Relative water repellency index （RWRI） was defined as the change of WRI due to heating relative to the oven-dry WRI value. Results of the WRI values showed that the soils were sub-critically water-repellent. Pasture soils had higher WRI values compared to tilled soils, resulting from high SOC and TN, and high activities of bacteria and fungi. It was observed that SOC, TN, fungal activity, and SOC：clay ratio had significant positive impacts on WRI. Strong positive correlations of RWRI with SOC, TN and fungal activity were also observed. Pedotransfer functions derived for predicting WRI showed that the WRI values had an increasing trend with the increases in fungal activity, salinity, alkalinity and fine clay content, but showed a decreasing trend with increasing bacterial activity.     

Nitrogen leaching and soil organic carbon sequestration of a Barley crop with improved N use efficiency – A regional case study

The potential of modified spring barley crops with improved nitrogen (N) use efficiency to reduce nitrogen (N) leaching and to increase soil organic carbon (SOC) storage was assessed at the regional scale. This was done using simulation model applications designed for reporting according to the Helcom (Helsinki Commission) and Kyoto protocols. Using model simulations based on modified crops N dynamics and SOC were assessed for three agro-ecological regions (latitudes ranging 55°20′–60°40′ N) in Sweden over a 20-year period. The modified N use properties of spring barley were implemented in the SOILNDB model (simulating soil C, N, water and heat, and plant N dynamics) by changing the parameters for root N uptake efficiency and plant N demand within a range given by previous model applications to different crops. A doubling of the daily N uptake efficiency and increased N demand (by ca 30%) reduced N leaching by 24%–31%, increased plant N content by 9%–12%, depending on region. The effects of the modified crop on SOC was simulated with the ICBM model, resulting in an increased SOC content (0–25 cm depth) by 57–79 kg?C?ha^?1?y^?1. The results suggest that a modified crop might reduce N leaching from spring barley area, in a range similar to the targets of relevant environmental protection directives, a result which held more in the northern than southern regions. The simulated SOC increase on a hectare basis was highest for the central region and least for the most northern region. For the total agricultural area the share of spring barley area was small and more crops would need to be modified to reach the emission reduction targets.     

Decomposition of soil organic carbon influenced by soil temperature and moisture in Andisol and Inceptisol paddy soils in a cold temperate region of Japan

Purpose

Understanding the effects of temperature and moisture on soil organic carbon (SOC) dynamics is crucial to predict the cycling of C in terrestrial ecosystems under a changing climate. For single rice cropping system, there are two contrasting phases of SOC decomposition in rice paddy soils: mineralization under aerobic conditions during the off-rice season and fermentation under anaerobic conditions during the growth season. This study aimed to investigate the effects of soil temperature and moisture on SOC decomposition under the aerobic and subsequently anaerobic conditions.

Materials and methods

Two Japanese paddy soils (Andisol and Inceptisol) were firstly incubated under four temperatures (±5, 5, 15, and 25°C) and two moisture levels (60 and 100% water-filled pore space (WFPS)) under aerobic conditions for 24 weeks. Then, these samples were incubated for 4 weeks at 30°C and under anaerobic conditions. Carbon dioxide (CO₂) and methane (CH₄) productions were measured during the two incubation stages to monitor the SOC decomposition dynamics. The temperature sensitivity of SOC was estimated by calculation of the Q₁₀ parameter.

Results and discussion

The total CO₂ production after the 24-week aerobic incubation was significantly higher in both soils for increasing soil temperature and moisture (P < 0.01). During the subsequent anaerobic incubation, total decomposed C (sum of CO₂ and CH₄ productions) was significantly lower in samples that had been aerobically incubated at higher temperatures (15 and 25°C). Moreover, CH₄ production was extremely low in all soil samples. Total decomposed C after the two incubation stages ranged from 256.8 to 1146.1 mg C kg^?1 in the Andisol and from 301.3 to 668.8 mg C kg^?1 in the Inceptisol. However, the ratios of total decomposed C to SOC ranged from 0.29 to 1.29% in the Andisol and from 2.21 to 4.91% in the Inceptisol.

Conclusions

Both aerobic and anaerobic decompositions of SOC in two paddy soils were significantly affected by soil temperature and moisture. Maintaining optimal soil temperature and medium moisture during the off-rice season might be an appropriate agricultural management to mitigate CH₄ emission in the following rice growth season. Although it is high in SOC content, Andisol has less biodegradable components compared to Inceptisol and this could be a probable reason for the distinct difference in temperature sensitivity of SOC decomposition between two paddy soils.

     

Effect of temperature on biochar priming effects and its stability in soils

Recent studies have shown both increased (positive priming) and decreased (negative priming) mineralisation of native soil organic carbon (SOC) with biochar addition. However, there is only limited understanding of biochar priming effects and its C mineralisation in contrasting soils at different temperatures, particularly over a longer period. To address this knowledge gap, two wood biochars (450 and 550 °C; δ¹³C −36.4‰) were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol; δ¹³C −17.3 to −28.2‰) at 20, 40 and 60 °C in the laboratory. The proportions of biochar- and soil-derived CO₂–C were quantified using a two-pool C-isotopic model.Both biochars caused mainly positive priming of native SOC (up to +47 mg CO₂–C g⁻¹ SOC) in the Inceptisol and negative priming (up to −22 mg CO₂–C g⁻¹ SOC) in the other soils, which increased with increasing temperature from 20 to 40 °C. In general, positive or no priming occurred during the first few months, which remained positive in the Inceptisol, but shifted to negative priming with time in the other soils. The 550 °C biochar (cf. 450 °C) caused smaller positive priming in the Inceptisol or greater negative priming in the Entisol, Oxisol and Vertisol at 20 and 40 °C. At 60 °C, biochar caused positive priming of native SOC only in the first 6 months in the Inceptisol. Whereas, in the other soils, the native SOC mineralisation was increased (Entisol and Oxisol) and decreased (Vertisol) only after 6 months, relative to the control. At 20 °C, the mean residence time (MRT) of 450 °C and 550 °C biochars in the four soils ranged from 341 to 454 and 732−1061 years, respectively. At 40 and 60 °C, the MRT of both 450 °C biochar (25−134 years) and 550 °C biochar (93−451 years) decreased substantially across the four soils. Our results show that biochar causes positive priming in the clay-poor soil (Inceptisol) and negative priming in the clay-rich soils, particularly with biochar ageing at a higher incubation temperature (e.g. 40 °C) and for a high-temperature (550 °C) biochar. Furthermore, the 550 °C wood biochar has been shown to persist in soil over a century or more even at elevated temperatures (40 or 60 °C).     

Comparison of Three Methods for Measurement of Soil Organic Carbon

Quantification of soil organic carbon (SOC) is an important element in the assessment of the carbon sequestration potential of soils in tree-based intercropping (TBI) systems. The organic carbon (OC) concentrations of soils in TBI systems often differ from those in conventional agricultural systems due to the additional C inputs from litter fall and roots. However, the presence of soil inorganic carbon (SIC) can confound the measurements of SOC. This study compared three methods of measuring SOC: (i) measurement of the total soil C (TC) in one subsample and, after treatment in a muffle furnace (575 °C) for 24 h, measurement of SIC in another subsample; (ii) SOC measured after fumigation with 12 M hydrochloric acid (HCl) to remove SIC; and (iii) SOC measured after digestion with 0.73 M H₂SO₃ to remove SIC. The TC, SOC, and SIC concentrations were determined by combustion. A correction factor was applied to express SIC and SOC concentrations on an original, untreated soil basis. Measurement of SOC by the muffle furnace method resulted in the greatest SOC concentrations for Populus spp. (hybrid poplar) for samples from two of the three depths (0–10 and 20–40 cm). Measurement of SOC by the HCl fumigation and H₂SO₃ digestion methods were highly correlated, suggesting complete removal of SIC with minimal oxidation of SOC. These results have implications for the method of measuring SOC in calcareous soils under coniferous and deciduous tree species to a depth of 40 cm.     

On farm assessment of tillage impact on soil carbon and associated soil quality parameters

No-tillage (NT) farming offers innumerable benefits to soil and water conservation, however, its potential to sequester soil organic carbon (SOC) and related soil properties varies widely. Thus, the impact of long-term (>4 yr) NT-based cropping systems on SOC sequestration and selected soil physical and chemical parameters were assessed across soils within five Major land Resource Areas (MLRAs: 99 and 111 in Michigan; 124 and 139 in Ohio; and 127 in Pennsylvania) in eastern U.S.A. Soil samples were collected from paired fields of NT and plow tillage (PT) based cropping systems and an adjacent woodlot (WL). The SOC concentration, bulk density (ρ_b), texture, pH, electrical conductivity (EC), soil N, coarse particulate organic matter (CPOM) C and N, and nitrate N (NO₃-N) concentrations were determined. Conversion from NT to PT practice increased surface soil pH from 5.97, 6.56 and 6.02 to 6.62, 6.91 and 7.09 under MLRAs 127, 111 and 99, respectively. NT soils had higher SOC concentration soils by 30, 50 and 67% over PT soils at 0–5 cm depth under MLRAs 99, 111 and 127, respectively. Considering the whole soil profile SOC, WL had higher SOC pool than NT and PT practices under MLRAs 99, 111 and 124, however, there was no significant difference (P < 0.05) between NT and PT practices across five soils. Almost the same trend was observed in the case of depthwise soil N content. NT soil had higher N content than PT soils by 27, 44 and 54% under MLRAs 99, 127 and 111, respectively. However, whole soil profile N content of NT soil was significantly higher by 12% than PT soil under MLRA 99. Concentrations of CPOM associated C and N of NT soil was higher than PT soil under MLRAs 99, 111 and 127 at 0–5 soil depth. These results indicated that impact of tillage on soil C and associated soil quality parameters is confined within specific soil types.     

Evaluation of two methods of soil quality assessment as influenced by slash and burn in tropical rainforest ecology of Nigeria

The sustainability of slash-and-burn agriculture for sustainable crop production has been a subject of controversy. The objective of this study was to quantify the effect of slash and burn on soil quality. Two sites, Ibadan (7° 23′ N; 3° 51′ E) and Akure (7° 17′ N; 5° 14′ E), within the tropical rainforest of Nigeria were selected for the study. Burnt and unburnt soils were cropped with maize, melon, and cowpea (in sole and intercrops). Soil and earthworm cast samples were collected and analyzed for physical, chemical, and biological indicators. Integration into soil processes and quality indices involved the transformation of analyzed indicators using Soil Management Assessment Framework (SMAF) and Multiple Variable Indicator Transform (MVIT) techniques. Organic matter, water-stable aggregates, pH, cation exchange capacity, macroporosity, and water infiltration were reduced after burning in both sites. Active carbon and potentially mineralizable nitrogen increased after burning at Ibadan but decreased after burning at Akure. Soil quality decreased after burning by a range of 11.3–24.8% using SMAF and MVIT, respectively, although only MVIT showed significant differences (p ≤ 0.05). Test crops increased in yields on burnt soils due to prompt release of nutrients to the crops; the benefit was dwarfed by the adverse effects of burning on soil quality indices.     

Degradation of [carboxyl-14C] 2,4-D and [ring-U-14C] 2,4-D in 114 agricultural soils as affected by soil organic carbon content

This study compared the degradation of [carboxyl-¹⁴C] 2,4-dichlorophenoxyacetic acid (2,4-D) (C2,4-D) and [ring-U-¹⁴C] 2,4-D (R2,4-D) in 114 agricultural soils (0–15 cm) as affected by 2,4-D sorption and soil properties (organic carbon content, pH, clay content, carbonate content, cation exchange capacity, total microbial activity). The sample area was confined to Alberta, Canada, located 49–60° north longitude and 110–120° west latitude and soils were grouped by soil organic carbon content (SOC) (0–0.99%, 1–1.99%, 2–2.99%, 3–3.99% and >4% SOC). Degradation rates of C2,4-D and R2,4-D followed first-order kinetics in all soils. Although total microbial activity increased with increasing SOC, degradation rates and total degradation of C2,4-D and R2,4-D decreased with increasing SOC because of increased sorption of 2,4-D by soil and reduced bioavailability of 2,4-D and its metabolites. Rates of R2,4-D degradation were more limited by sorption than rates of C2,4-D degradation, possibly because of greater sorption and formation of bound residues of 2,4-D metabolites relative to the 2,4-D parent molecule. Based on the sorption and degradation parameters quantified, there were two distinct groups of soils, those with less than 1% SOC and those with greater than 1% SOC. Specifically, soils with less than 1% SOC had, on average, 2.4 times smaller soil organic carbon sorption coefficients and 1.4 times smaller 2,4-D half-lives than soils with more than 1% SOC. In regional scale model simulations of pesticide leaching to groundwater, covering many soils, input parameters for each pesticide include a single soil organic carbon sorption coefficient and single half-life value. Our results imply, however, that the approach to these regional scale assessments could be improved by adjusting the values of these two input parameters according to SOC. Specifically, this study indicates that for 2,4-D and Alberta soils containing less than 1% SOC, the 2,4-D pesticide parameters obtained from generic databases should be divided by 2.5 (soil organic carbon sorption coefficient) and 1.5 (half-life value).     

Soil properties influencing erodibility of soils in the Ntabelanga area,Eastern Cape Province,South Africa

Soil erosion has serious off-site impacts caused by increased mobilization of sediment and delivery to water bodies causing siltation and pollution. To evaluate factors influencing soil erodibility at a proposed dam site, 21 soil samples collected were characterized. The soils were analyzed for soil organic carbon (SOC), exchangeable bases, exchangeable acidity, pH, electrical conductivities, mean weight diameter and soil particles’ size distribution. Cation exchange capacity, exchangeable sodium percentage, sodium adsorption ratio, dispersion ratio (DR), clay flocculation index (CFI), clay dispersion ratio (CDR) and Ca:Mg ratio were then calculated. Soil erodibility (K-factor) estimates were determined using SOC content and surface soil properties. Soil loss rates by splashing were determined under rainfall simulations at 360?mmh^?1 rainfall intensity. Soil loss was correlated to the measured chemical and physical soil properties. There were variations in soil form properties and erodibility indices showing influence on soil loss. The average soil erodibility and SOC values were 0.0734?t?MJ^?1?mm^?1 and 0.81%, respectively. SOC decreased with depth and soil loss increased with a decrease in SOC content. SOC significantly influenced soil loss, CDR, CFI and DR (P??1. Addition of organic matter stabilize the soils against erosion.     

Analysis of indigenous zinc in alkaline sodic soil solutions by graphite furnace atomic absorption spectrometry

Abstract

Analytical determination of ultra low levels of micronutrients in soil solutions is one of the major problems limiting trace element studies in alkaline sodic soils. The low concentrations of trace metals in environmental samples are often measured by the graphite furnace atomic absorption spectrometry (GFAAS) technique. However, considerable difficulty was found with zinc (Zn) determination in alkaline sodic soil solutions. The effects of various chemical modifiers (phosphoric acid, ammonium phosphate, nitric acid, and palladium (Pd)] on GFAAS determination of ultra trace levels of Zn in soil solutions of four alkaline sodic soils were investigated in this study. The concentration of Zn in the soil solution was significantly (P<0.01) higher in the presence of modifiers relative to that determined in the absence of modifiers. In addition, coefficient of variance [(CV) 21.5%] in the absence of chemical modifiers was up to three times higher than those determined with chemical modifiers (7 to 10.2%). Chemical modifiers also decreased the characteristic concentration (increased sensitivity). The decrease ranged from 22.1 to 34.6% for Pd and phosphoric acid, respectively, compared to that in the absence of chemical modifiers. The maximum ashing temperature for standard Zn solution increased from 300°C in the absence of modifiers to 800°C with phosphoric acid and ammonium phosphate, indicating increased stability of Zn at high temperatures. Based on the recovery test data, characteristic concentration, atomization temperature, and the shape of atomic signal peak, analysis of Zn in the presence of phosphoric acid was recommended for soil solution Zn.     

Carbon quality and the temperature sensitivity of soil organic carbon decomposition in a tallgrass prairie

The temperature sensitivity of soil organic carbon (SOC) decomposition will influence the accuracy of the quantitative prediction of carbon (C) balance between ecosystem C fixation and decomposition in a warmer world. However, a consensus has not yet been reached on the temperature sensitivity of SOC decomposition with respect to SOC quality. The fundamental principles of enzyme kinetics suggest that temperature sensitivity of decomposition is inversely related to the C quality of the SOC. This “C quality-temperature” hypothesis was tested in a 170-day laboratory experiment by incubating soil samples with changing temperature (low-high-low) at a ±5 °C step every 24 h. Soil samples were collected from a long-term warming experiment in a tallgrass prairie. There were four treatments of soil samples before lab incubation: control (C), warmed (W), field incubation (FI, litter exclusion), and warmed plus field incubation (WFI). Results showed that SOC decomposition rates were influenced by labile organic C (LOC) content, which were low in the soils under field incubation and decreased with increasing lab incubation time. Field warming and field incubation increased the temperature sensitivity of SOC decomposition in the 1st two lab incubation cycles but the treatment effects diminished as decomposition proceeded, probably due to increased contribution of recalcitrant C. In line with the hypothesis, we found that the lower the SOC quality, the higher the Q₁₀ values. This relationship held across treatments and lab incubation cycles, regardless of whether the differences in SOC quality resulted from inherent differences in SOC chemistry or from differences in the extent of SOC decomposition. Treatment effects of field warming and field incubation on SOC quality and Q₁₀ values also negatively correlated with each other. Our results suggest that dynamics of low-quality SOC have the highest potential to impact long-term C stocks in soils. Potential decreases in SOC quality in response to warming and consequent shifting species composition may result in a positive feedback of SOC to climate change in the future.     

Aggregate-associated soil organic carbon and total nitrogen following amendment of puddled and sawah-managed rice soils in southeastern Nigeria

Puddling during sawah rice cultivation destabilizes the soil structure. The re-formation of soil water-stable aggregates (WSA) following puddling and amendments, and their associated organic carbon (SOC) and total N were studied at Akaeze and Ikwo in south-eastern Nigeria. The amendments, which were randomized in triplicate, include control, NPK fertilizer, poultry dropping, rice husk powder and rice husk ashes (RHA). Soil samples from 0 to 15 cm depth were taken from the field after 2 years of cultivation. Most of the SOC were found in the very fine aggregates. There was no consistent trend in the treatment effects. However, the NPK-amended soils showed the lowest values of WSA > 2 mm in both locations, whereas the poultry dropping-amended soils showed the least and the highest mean-weight diameter (MWD) values at Akaeze and Ikwo, respectively. The SOC of the whole soil in Akaeze correlated positively with MWD (r = 0.92*). Irrespective of location, SOC in soils and WSA > 2.00 mm correlated positively with MWD (r = 0.56*; 0.65*, respectively) while SOC in WSA 0.50–0.25 mm accounted for low MWD values. More carbon was sequestered at Akaeze than at Ikwo, with the RHA-amended soils being the highest at both locations.     

Invertase and amylase activities in ryegrass and white clover plants and their relationships with activities in soils under pasture

Invertase and amylase activities were determined in the herbage and roots of ryegrass and white clover grown under aseptic and field conditions, and in the herbage, litter, roots, and soil of three grass-legume pastures. These activities were appreciably greater in all plant materials than in soil, on either a dry weight or an organic C basis. Invertase-amylase activity ratios in plant materials and in soil were not directly related at the three pasture sites. The activities of aseptically grown plants were not consistently influenced by harvest conditions under light or darkness. They did differ in different tissues and plant species, with amylase activity greatest in white clover. Plant roots did not appear to excrete enzymes with these activities.Chromatographic data suggest that β-amylases contributed most to the amylase activity of herbage and roots, whereas α-amylases and possibly other hydrolytic enzymes, were most important in litter and soil. Other evidence suggests that β-amylases, as well as α-amylases, would also have been active in the soils.Under field conditions, the invertase and amylase activities of the herbage and roots appear to be due mainly to plant-derived enzymes, rather than to associated micro-organisms. In the soils, microorganisms and plants appear to contribute to these activities.