Rainfall impact effects on ageing casts of a tropical anecic earthworm

We investigated the erodibility of surface casts produced by an anecic earthworm of the Colombian savannahs by means of indoor rain simulations. The kinetic energy applied to samples, 21.62 J minute⁻¹ m⁻², was estimated to be equivalent to 41% of the energy of the more intense period of a local storm. The erodibility of casts was assessed at different stages of their ageing along with the effects of repeated wetting‐drying cycles. Bare soil cores and soil cores of the same size with a cast on their surface (soil + cast) were used as controls. Saturated hydraulic conductivity (HC) was measured to test whether casts enhance soil water permeability. Fresh, almost liquid, individual casts were completely dispersed by a 2‐hour rainfall simulation at a dispersion rate of 0.9 ± 0.5% of sample minute⁻¹. After 5 hours drying at 32°C and 79% relative humidity, casts were as moist as fresh casts (non‐significant Mann–Whitney U‐test) but had a solid appearance and were no longer dispersed by raindrop impact. Under simulated rainfall, dry casts were very slowly fragmented into large aggregates (> 5 mm). The HC of casts was increased by repeated cycles of wetting and drying but unaffected by length of air‐drying and natural ageing, and averaged 7 (3, n = 6), 20 (3, n = 7) and 32 (1, n = 45) cm hour⁻¹ for bare soil, soil + cast and all casts, respectively (standard error, sample size). Except for bare soil, these values were greater than the greatest rainfall intensities recorded locally. The consequences for soil erosion, nutrient losses and water infiltration are discussed.     

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

Naturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha^–1, 6.2 t ha^–1, and 12.4 t ha^–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha^–1 and 12.4 t ha^–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha^–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.     

Shrinkage Characteristics of Lime Concretion Black Soil as Affected by Biochar Amendment

Studies have reported that biochar is a sustainable amendment that improves the chemical and physical properties of soil.In this study,an incubation experiment was conducted to investigate the effects of different application rates of biochar on the cracking pattern and shrinkage characteristics of lime concretion black soil after three wetting and drying cycles.Biochar derived from the corn straw and peanut shell mixture was applied to the soil at rates of 0,50,100,and 150 g kg~(-1)dry weight,representing the treatments T_(0),T_(50),T_(100),and T_(150),respectively.During the wetting and drying cycles,the cracking pattern and shrinkage characteristics of the unamended and amended soil samples were recorded.Application of biochar significantly increased soil organic carbon content in the samples.During soil desiccation,biochar significantly reduced the rate of water loss.Cracks propagated slowly and stopped due to the relatively higher water content in the soil applied with biochar.The cracking area density(ρ_c),equivalent width,fractal dimension,and cracking connectivity index decreased during the drying process with increasing application rate of biochar.Theρ_(c )value of the T_(50),T_(100),and T_(150) treatments decreased by 33.6%,52.1%,and 56.9%,respectively,after three wetting and drying cycles,whereas the T_(0) treatment exhibited a marginal change.The coefficient of linear extensibility,an index used to describe onedimentional shrinkage,of the unamended soil sample(T_(0))was approximately 0.23.Application of 100 and 150 g kg~(-1)biochar to the soil significantly reduced the shrinkage capacity by 41.45%and 45.54%,respectively.The slope of the shrinkage characteristics curve,which indicates the ralationship between soil void ratio and moisture ratio,decreased with increase in the application rate of biochar.Furthermore,compared with the T_(0) treatment,the proportional shrinkage zone of the shrinkage characteristic curve of the T_(50),T_(100),and T_(150) treatments decreased by 5.8%,13.1%,and 12.1%,respectively.Differences were not observed in the moisture ratio at the maximum curvature of the shrinkage characteristic curve among the treatments.The results indicate that biochar can alter the cracking pattern and shrinkage characteristics of lime concretion black soil.However,the effects of biochar on the shrinkage of lime concretion black soil are dependent on the number of wetting and drying cycles.     

Eluviation of dissolved organic carbon under wetting and drying and its influence on water infiltration in degraded soils restored with vegetation

One mechanism in the restoration of severely degraded soil by vegetation might be the movement of dissolved organic carbon (DOC) to macropore and aggregate surfaces. We propose that this lowers the soil wetting rate and subsequently its slaking resistance by creating a partially hydrophobic surface. In this study, we determined how wetting and drying (w/d) cycles redistribute DOC to soil surfaces, and how DOC affects hydrophobicity where it accumulates, in relation to the soil surface area to volume ratio and to different types of vegetation planted to restore a severely degraded soil. Repacked soil cores that simulate different soil aggregate sizes were tested. The results showed that w/d cycles increase surface DOC concentration through a depletion of DOC in the interior of the soil. Correspondingly, w/d cycles enhanced hydrophobicity, measured as a water repellency index, R, from 1.5–2.3 to 3.6–7.6, the values affected significantly by the type of vegetation. This index (R) did not change for a control soil with no vegetation. The link between the amount of DOC and water repellency was weak (coefficient of determination r² = 0.06–0.26), indicating that DOC quality was probably more important than its quantity. Although increasing the core size resulted in a greater accumulation of DOC on the drying surface of the core, the impact of this on water repellency was minimal. Incubation caused a decrease in the amount of DOC, but had minimal influence on water repellency. This work improves the understanding of changes in soil wetting and soil stabilization under processes of natural weathering and vegetation restoration.     

Enhanced sorption and fixation of radiocaesium in soils amended with K-bentonites, submitted to wetting–drying cycles

The use of bentonites as soil amendment has met with little success in reducing plant uptake of radiocaesium. However, bentonites exchanged with K⁺ have pronounced Cs⁺ binding capacity when subjected to wetting–drying cycles. Fifty‐four different bentonites were collected and characterized for cation exchange capacity and chemical composition. The radiocaesium interception potential (RIP) increased up to 160‐fold (mean 25) when the bentonites were converted to the K‐form and subjected to wetting–drying cycles. This increase in radiocaesium sorption was ascribed to a collapse of the clay sheets into an illite‐like structure, and was most pronounced in bentonites with a high layer charge. The RIP values of K‐bentonites subjected to 25 wetting–drying cycles ranged from 0.22 to 44.3 mol kg^?1. The RIP yields, i.e. the RIP in soil–bentonite mixtures expressed per unit bentonite added, were even higher and ranged up to 99 mol kg^?1. This upper limit is about 10‐fold higher than the RIP value of illite (～ 10 mol kg^?1), the principal ¹³⁷Cs sorbent in soils of temperate climates. Wetting–drying also promoted fixation of radiocaesium in soils amended with K‐bentonites. About 30% of added ¹³⁷Cs could be desorbed with 1 m ammonium acetate (NH₄Ac) from an unamended soil after 25 wetting–drying cycles, while only between 8 and 21% of ¹³⁷Cs could be desorbed from a soil amended with bentonite and a K‐salt. These findings support the proposition that addition of K‐bentonite may be effective in reducing availability of ¹³⁷Cs in soils.     

Repeated drying–rewetting cycles and their effects on the emission of CO2, N2O,NO, and CH4 in a forest soil

Prolonged summer droughts due to climate change are expected for this century, but little is known about the effects of drying and wetting on biogenic trace‐gas fluxes of forest soils. Here, the response of CO₂, N₂O, NO, and CH₄ fluxes from temperate forest soils towards drying–wetting events has been investigated, using undisturbed soil columns from a Norway spruce forest in the “Fichtelgebirge”, Germany. Two different types of soil columns have been used for this study to quantify the contribution of organic and mineral horizons to the total fluxes: (1) organic horizons (O) and (2) organic and mineral soil horizons (O+M). Three drying–wetting treatments with different rewetting intensities (8, 20, and 50 mm of irrigation d^–1) have been compared to a constantly moist control to estimate the influence of rainfall intensity under identical drying conditions and constant temperature (+15°C). Drought significantly reduced CO₂, N₂O, and NO fluxes in most cycles. Following rewetting, CO₂ fluxes quickly recovered back to control level in the O columns but remained significantly reduced in the O+M columns with total CO₂ fluxes from the drying–wetting treatment ranging approx. 80% of control fluxes. Fluxes of N₂O and NO remained significantly reduced in both O and O+M columns even after rewetting, with cumulative fluxes from drying–wetting treatments ranging between 20% and 90% of the control fluxes, depending on gas and cycle. Fluxes of CH₄ were small in all treatments and seem to play no significant role in this soil. No evidence for the release of additional gas fluxes due to drying–wetting was found. The intensity of rewetting had no significant effect on the CO₂, N₂O, NO, and CH₄ fluxes, suggesting that the length of the drought period is more important for the emission of these gases. We can therefore not confirm earlier findings that fluxes of CO₂, N₂O, and NO during wetting of dry soil exceed the fluxes of constantly moist soil.     

Particulate organic matter in water stable aggregates formed after the addition of C-labeled maize residues and wetting and drying cycles in vertisols

Development of soil structure and the dynamics of water stable aggregates (WSA) in many soils are known to be closely related to the cycling of soil organic matter. In some fine and medium textured soils particulate organic matter (POM) has been found to act as a nucleus for macroaggregate formation. However, this role of POM in aggregate formation has not been demonstrated in soils dominated by smectitic clay minerals. This study explored aggregation processes in a Vertisol from a semi-arid region in Northeastern Mexico in relation to the addition of ¹⁴C-labeled maize residues and application of wetting and drying cycles during 105 days of incubation. Fractionation of the WSA formed showed that labeled residues were preferentially accumulated in large macroaggregates (>2000 μm). Treatments with addition of organic residues had three to four times more intra-aggregate particulate organic matter (iPOM) in large macroaggregates than the control after 14 days of incubation. Residue-derived carbon accounted for 53% and 41% of the total carbon stored in the iPOM fraction in amended treatments with and without wetting and drying cycles, respectively. Conversely, residue-derived carbon represented <20% of the total carbon in the iPOM fraction from small macroaggregates (250-2000 μm) and microaggregates (53-250 μm). Results also showed that the amount and concentration of carbon per large macroaggregate did not differ between the large macroaggregates formed under wetting and drying and those formed in continuous moist conditions. However, due to formation of higher number of large macroaggregates per kg of soil, more carbon could be stored in amended soils under wetting and drying than in constantly wet soil: 1.4, 1.8 and 2.7 times more ¹⁴C kg⁻¹ soil after 14, 58 and 105 incubation days, respectively. The results in this study suggest that wetting and drying enhanced protection of the added maize residues inside large macroaggregates by forming more aggregates, rather than by increasing the amount of POM entrapped per aggregate. Therefore, after the addition of organic residues, this soil could accumulate more C than continuous moist soil through the influence that wetting and drying has on soil aggregation.     

Structure properties and pore dynamics in aggregate beds due to wetting–drying cycles

Aggregate hierarchy and porosity changes in aggregate beds as a consequence of wetting–drying cycles were studied in two Andisols and one Mollisol from Chile, collected at two depths. Bulk density and indirect tensile strength were measured in aggregates of different sizes. Aggregate beds were prepared in cylinders with two size classes. Six wetting–drying cycles between 0 and –60 hPa were applied. Bulk density (Db) of soil matrix was controlled after each cycle, and the macroporosity was calculated. A repellency index was measured in one of the Andisols. In addition, also the air permeability was measured after the sixth cycle. It could be proofed, that the aggregate strength is an appropriate parameter to evaluate the aggregate hierarchy, and this parameter is also more sensitive than aggregate Db to discriminate between the effects of land‐use intensity. Aggregate strength is furthermore well correlated with changes in pore water pressure and can be applied to relate strength values with aggregate development level. Only if the predrying exceeds pF > 3.0, aggregate strength correlates with Db. The more pronounced is the land‐use, the higher is the increase of Db values for aggregate beds. The decrease of coarse porosity during wetting–drying cycles can be explained by mass differences between saturated and equilibrated water conditions that considers the water around aggregates and within the contact area. Nevertheless, the relation of relative macroporosity change, calculated by P_exped where D_agg is the Db measured by clod method, and the relative Db change, is useful to explain possible presence of coarse pores inside the aggregates. The newly formed porosity prevents the water repellency, but after six cycles of drying, the repellency index increased in the topsoil while we could detect a decrease in the subsoil samples (under defined conditions in the laboratory) which we assume to be caused by microbial activity. The approaching of aggregates by drying cycles generates in Andisols a reduced area to air fluxes, with low values of air permeability.     

Soil mineral nitrogen concentration within cycles of flood irrigation: Effect of rice stubble and fertilization management

Two field experiments using macro (3.5 × 12m) plots and ¹⁵N-labelled fertilizer on micro (155 mm internal diameter) plots were undertaken to measure the effects of rice (Oryza sativa L.) stubble management and nitrogen fertilizer strategies on N transformations within a series of intermittent flood irrigations. Nitrate concentration fell by 90% during each flooding, and analysis of ¹⁵N micro plots showed the loss was due to denitrification rather than leaching. Over 52% of the ¹⁵N was lost. Apparent loss over four irrigations from macro plots receiving 60 kg urea-N ha⁻¹ was 37 kg N ha⁻¹ while unfertilized plots lost 19 kg N ha⁻. Stubble incorporation reduced nitrate accumulation rate and increased immobilization, thereby reducing denitrification losses by 23%. Nitrate concentration in the 0–100 mm soil layer increased after the soil water content fell below field capacity during the drying portion of each cycle, but the net nitrification rate fell with increasing number of cycles.Ammonium content in the top 100mm of soil fell from 35kgNha⁻¹ to 3kgNha⁻¹ over four irrigations. This fall was ascribed to the combined effects of nitrification and immobilization. Immobilization was greatest on plots where large quantities of rice stubble had been incorporated, and over 50% of the applied ¹⁵N was retained in the soil on these plots compared with 40% (SED = 3.5%) on plots where stubble had been burnt. We conclude that the poor response of rice to fertilization at sowing is due to a combination of denitrification and immobilization of applied nitrogen during cycles of wetting and drying prior to permanent flood.     

Nutrient leaching from dormant trees at an elevated site

Observations are presented on ion concentration in winter deposits to coniferous and deciduous trees at an elevated site (860 m). Such sites, with persistent snow and ice cover and considerable deposition of atmospheric pollutants, are common in Eastern Canada and the North-Eastern U.S. Spot samples at a given sampling date and samples of total deposits accumulated over extended periods of time were taken during the winters of 1986–87 and 1987–88. Samples were analyzed for pH and the major anion and cations. Results of spot samples were similar between different sampling times (and years) for deposits on balsam fir but variable for those on branches and twigs of deciduous trees (birch, choke cherry). The chemistries of snow on the ground and snow on trees not in contact with foliage or branches were found not to be significantly different statistically (p<0.05). However, significant differences exist between concentrations of nutrient ions in snow and ice with and without contact with biological elements of coniferous and deciduous trees. This indicates occurrence of nutrient leaching during winter conditions, leading to possibly non-negligent nutrient stress for K? and Mg?. Based on spot and cumulative samples for balsam fir, and correcting for possible contributions from dry deposition, the contribution from leaching in winter deposits on balsam fir foliage has been estimated at 26, 14 and 9 μeq L^?1, for K⁺, Ca⁺⁺ and Mg⁺⁺, respectively. With approximate estimates of stand characteristics and precipitation loading at the sampling sites, these figures represent 90/c, 1% and 8%, respectively, of the estimated annual growth component of the stand for K⁺, Ca⁺⁺ and Mg⁺⁺ This indicates the possibility of non-negligible nutrient stress for K⁺ and Mg⁺ during winter conditions at the given site.     

Wetting and drying events determine soil N pools in two Mediterranean ecosystems

To improve our knowledge of how nutrient cycling in Mediterranean environments responds to climate change, we evaluated the effects of the continuous changes in soil nitrogen (N) pools during natural wetting and drying events. We measured soil N pools (microbial biomass [MB-N], dissolved organic nitrogen [DON], NH₄⁺ and NO₃⁻) and N ion exchange resins at weekly intervals for one year in two contrasting Mediterranean ecosystems. All soil N fractions in both ecosystems showed high intraseasonal and interseasonal variability that was greater in inorganic soil fractions than in organic N soil fractions. MB-N, DON and resin-NH₄⁺ showed increased concentrations during wetting events. Only the soil NO₃⁻ and resin-NO₃⁻ showed the opposite trend, suggesting a different response to water pulses compared to the other soil variables. Our results show that N pools are continuously changing, and that this high variability is not associated with the total amount of organic matter and labile soil carbon (C) and N soil fractions found in each ecosystem. The highest variability was found for inorganic N forms, which suggests that organic N forms are more buffered in soils exposed to wetting-drying cycles. Our results suggest that the changes in wetting-drying cycles expected with global climate change may have a significant impact on the availability and turnover of organic and inorganic N.     

A preliminary review of weathering rates in relation to their method of calculation for acid sensitive soil parent materials

The release of base cations from chemical weathering is the fundamental process by which base cations lost through leaching or biological uptake are replenished. Soils with a high content of easily weatherable minerals will, in general, be readily able to neutralise incoming acidity and satisfy biological requirements through this supply of base cations from weathering. Conversely those soils with a low content of such minerals will be unable to buffer acid inputs or meet biological demands and will be vulnerable to acidification. It is evident therefore that an accurate estimate of the rate of chemical weathering is a prerequisite for any assessment of ecosystem sensitivity to acidification. The principal methods by which these calculations are made can be grouped into those based on element depletion in soil horizons calculated against a conservative element reference, catchment fluxes, laboratory studies and the use of mathematical models which utilise data from each of these sources. A review of the published data has been undertaken to determine if a systematic variation in weathering rates can be observed for a range of parent materials and/or methods used. Variations of 0.03–0.8 keq ha^?1yr^?1 for K⁺, 0.0–1.0 keq ha^?1yr^?1 for Na⁺, 0.01–1.39 keq ha^?1yr^?1 for Mg²⁺ and 0.0–5.8 keq ha^?1yr^?1 for Ca²⁺ were found. In general individual cation weathering rates determined from catchment fluxes, and also the model MAGIC, are dominated by Ca²⁺, with Mg²⁺ being the next most significant release rate. However it has not been possible to determine any other systematic variations due to the limited number of sites where weathering rate has been determined by more than one method.     

Experimental Study of Factors Affecting Soil Erodibility

The effect of different factors and preparation conditions of monofraction samples from the arable horizon of leached chernozem on soil erodibility and its relationship with soil tensile strength (STS) has been studied. The exposure of samples at 38°C reduces their erodibility by two orders of magnitude. The drying of samples, on the contrary, increases their erodibility. It has been shown that erodibility decreases during the experiment. It has been found that the inoculation of soil with yeast cultures (Naganishia albida, Lipomyces tetrasporus) reliably increases the STS value in 1.5–1.9 times. The sterile soil is eroded more intensively than the unsterile soil: at 4.9 and 0.3 g/(m² s), respectively. The drying of soil followed by wetting to the initial water content (30%) has no significant effect on the STS value in almost all experimental treatments.     

pH change, carbon and nitrogen mineralization in paddy soils as affected by Chinese milk vetch addition and soil water regime

Purpose

The aim of the research was to explore the effect of Chinese milk vetch (CM vetch) addition and different water management practices on soil pH change, C and N mineralization in acid paddy soils.

Materials and methods

Psammaquent and Plinthudult paddy soils amended with Chinese milk vetch at a rate of 12 g?kg^?1 soil were incubated at 25 °C under three different water treatments (45 % field capacity, CW; alternating 1-week wetting and 2-week drying cycles, drying rewetting (DRW) and waterlogging (WL). Soil pH, dissolved organic carbon, dissolved organic nitrogen (DON), CO₂ escaped, microbial biomass carbon, ammonium (NH₄ ⁺) and nitrate (NO₃ ^?) during the incubation period were dynamically determined.

Results and discussion

The addition of CM vetch increased soil microbial biomass concentrations in all treatments. The CM vetch addition also enhanced dissolved organic N concentrations in all treatments. The NO₃–N concentrations were lower than NH₄–N concentrations in DRW and WL. The pH increase after CM vetch addition was 0.2 units greater during WL than DRW, and greater in the low pH Plinthudult (4.59) than higher pH Paleudalfs (6.11) soil. Nitrogen mineralization was higher in the DRW than WL treatment, and frequent DRW cycles favored N mineralization in the Plinthudult soil.

Conclusions

The addition of CM vetch increased soil pH, both under waterlogging and alternating wet–dry conditions. Waterlogging decreased C mineralization in both soils amended with CM vetch. Nitrogen mineralization increased in the soils subjected to DRW, which was associated with the higher DON concentrations in DRW than in WL in the acid soil. Frequent drying–wetting cycles increase N mineralization in acid paddy soils.     

Effect of two soil moisture levels and wetting‐drying cycles on manganese release in NaCl‐amended soils

Abstract

Effect of two moisture levels (22.5 and 13.5%, w/w) and wetting‐drying cycles on manganese solubility was studied in NaCl‐amended soil. During 6 d incubation, higher moisture level released 40‐fold more water‐soluble Mn and 60‐fold more NH₄OAc‐exchangeable‐Mn in non‐salinized soil. In NaCl‐treated soil, 50 to over 200% greater soluble and exchangeable Mn was recovered from samples incubated at 22.5% compared to 13.5% water levels. Wetting‐drying cycles significantly (P≤0.05) decreased water‐soluble Mn, which accounted for 50 to 60% increases in the exchange‐able Mn. Since other non‐oxidizing/reducing cations (Ca, Mg, Na, K) also demonstrated similar behavior, it is proposed that in addition to oxidation upon drying and reduction upon wetting, the increases in exchangeable Mn and simultaneous decreases in soluble Mn concentration are due to sorption processes. These results suggest that under field conditions, the insolubility of Mn due to continued wetting‐drying cycles may eventually lead to Mn deficiency in soils low in Mn.     

Microbial biomass dynamics and soil wettability as affected by the intensity and frequency of wetting and drying during straw decomposition

Water repellency is influenced by soil management and biological process. We carried out a 60‐day laboratory incubation experiment to evaluate the effects of straw amendment, together with the intensity and frequency of wetting and drying (W/D), on microbial processes and water repellency. One W/D cycle consisted of 1.5‐day wetting at −0.03 kPa from the soil core bottom and different drying lengths in a temperature‐controlled laboratory, resulting in different drying intensities. At a regular interval, soil respiration rate (SRR) on drying and wetting, soil microbial biomass C and N (SMB‐C and N), and soil water repellency (SWR) after the wetting were measured simultaneously. Rice straw amendment had a greater effect on SRR, but smaller influences on SMB and SMB‐C : N than W/D frequency and drying intensity. The first W/D caused the largest decrease in soil respiration and the soil respiration recovered partly in the subsequent W/D cycles. The increase in SMB and SMB‐C : N as well as metabolic quotient with W/D frequency and intensity suggested a shift of microbial community from bacterial dominance to fungal dominance. SWR was significantly related to SMB‐C (R²= 0.689, P < 0.001). However, this study was limited to these indirect measurements. Direct measurements of fungal biomass and microbial community are needed in the future. The results suggest that rice straw amendment in dry season may increase C sequestration due to reduced decomposition and stabilize soil structure due to the enhancement of microbial induced water repellency.     

Influences of starch and Zn-EDTA on Zn-desorption and rice ( Oryza sativa L.) nutrition under different moisture regimes

Desorption of Zn from sorbed Zn in soils is controlled by different moisture regimes in rice fields and thus control the Zn availability to rice. A laboratory experiment was conducted at different moisture regimes viz. flooded-dried, alternate wetting and drying and preflooding condition in two Inceptisols and two Alfisols of West Bengal, India, with (50?g kg^???1) and without starch as control to investigate the desorption of adsorbed Zn from Zn-EDTA (0.0, 0.1, and 0.2?mg Zn g^???1 soil). Percent desorption of Zn was found significantly higher on alternate wetting and drying condition in Alfisols. Starch applications enhanced desorption under flooded-dried and alternate wetting and drying, but decreased it under preflooding conditions. The variations in Zn desorption among soils and moisture treatments are the result of changes in soil pH, Fe-oxides, bonding energy constants, and free energies for Zn adsorption. After desorption experiment a pot trial was conducted with rice (cv. IET-4094) under alternate wetting and drying moisture conditions. The results show that the dry matter yield and 1000 grain weight of rice are appreciably higher when alternate flooding and drying moisture condition was maintained. This finding was in close agreement with the Zn desorption studies under the alternate flooding and drying moisture regimes.     

Effect of potassium fertilizer composition on the potassium status of soddy-podzolic soils under model experimental conditions

A model experiment was conducted with samples from arable horizons of loamy sandy and loamy soddy-podzolic soils under laboratory conditions. The samples were stored with KCl, K₂SO₄, and Potassium-E was added at rates of 240 and 960 kg/ha under alternating moistening and drying for 90 days. The contents of nonexchangeable, exchangeable, and easily exchangeable potassium and the K⁺ activity in 1: 2.5 water suspensions were measured in the original samples and after the end of the experiment. The contents of nonexchangeable, exchangeable, and easily exchangeable potassium were found to not be reliably affected by the composition of potassium fertilizers; the K⁺ activity was lower when potassium was added as K₂SO₄ than in the cases of KCl and Potassium-E.     

Root system response of selected lowland Thai rice varieties as affected by cultivation method and potassium rate under alternate wetting and drying irrigation

ABSTRACT

Root system is an important factor for crop productivity under water- and nutrient-limited environments. A pot study was conducted to evaluate root system response of three Thai rice varieties (Pathumthani 1, RD57, RD41) under three cultivation methods (dry direct seeding [DDS], wet direct seeding [WDS], transplanting [TP]) and three levels of alternate wetting and drying irrigation (–5, –15, –30 kPa). A second pot experiment examined the effect of potassium (K) rates (0, 80, 120, 160 kg ha^–1) on root system response of the same varieties under DDS and TP subjected to alternate wetting and drying at –5 kPa. Pathumthani 1 was more tolerant to moisture stress; RD57 and RD41 showed an inconsistent response to moisture-deficit conditions. Rice plant under TP was more sensitive to moisture stress; rice plant performed better under DDS even at the highest soil moisture stress of –30 kPa. K application at the rate of 120 kg ha^–1 as basal under DDS was optimum for root system development of Pathumthani 1, while RD57 and RD41 had higher actual root length at the same K rate regardless of cultivation methods. Alternate wetting and drying up to –30 kPa could be safely applied for the three tested varieties. The performance of root system of Pathumthani 1 was better under DDS method of cultivation. K fertilization at 120 kg ha^–1 can be recommended as optimum rate under water-saving cultivation techniques for the three tested varieties.     

The effects of wetting and drying cycles,temperature and extracting solutions on measured potassium fixation in soils of two regions of morocco

Abstract

Five representative soils with contrasting physical, chemical, and mineralogical characteristics from the Chaouia and Gharb regions of northwestern Morocco were selected for a study of the effects of wetting and drying cycles (W‐D), temperature, amount of K⁺ added, and extracting solution on the determination of K‐fixation. With drying at 40°C and the use of of 0.2 M CaCl₂ for K⁺ extractions, the measured amount of K⁺ fixed increased with increased number of W‐D cycles when high quantities of K were added. The drying, however, caused release of K⁺ when no or small amounts of K⁺ were added even for soils not containing mica (illite). With 2 W‐D cycles, measured K⁺ fixation decreased with increasing drying temperature from 40^oC to 100^oC regardless of the extracting solution when the calculated fixation was based on the initial extractable K⁺ rather than the quantity extracted from the zero treatment after wetting and drying. Also, significant differences in K⁺ fixation existed between extracting solutions at any given temperature. Because of the different drying temperatures and extracting salts used it is difficult to compare results of K⁺ fixation reported in different studies. Frequent changes in temperature and soil water content during the growing season in Mediterranean climates may have an important influence on K⁺ availability.