M. T. Pardo

Sorption of lead, copper, zinc, and cadmium by soils: effect of nitriloacetic acid on metal retention

Abstract Laboratory experiments were carried out to evaluate lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) sorption‐desorption by three soils of contrasting characteristics. Talamanca (silt loam, montmorillonite, Calcic Haploxeralfs), Mazowe (clay, kaolinite, Rhodic Kandiustalf), and Realejos (sandy silt loam, allophane, Typic Hapludands). A second objective was to study the effect of nitriloacetic acid (NTA) on the sorption process. The Talamanca soil, which had a native pH of 6.4 and presented the highest effective cation exchange capacity (ECEC), sorbed more of each of the metal tested than did the other two soils. When the other two soils were compared metal sorption was also related to pH and ECEC. The very low sorption capacity showed by Realejos may be attributed to the low net surface negative charge density of this soil, arising from its allophanic nature. A common feature of the three soils was the relative strong sorption of both Pb and Cu relative to Cd and Zn with Pb showing the highest sorption levels. The selectivity sequences of metals retention were Pb>Cu>Zn>Cd for Talamanca soil, Pb>Cu>Zn≈Cd for Mazowe, and Pb>Cu>Cd>Zn for Realejos. Metal desorption values were low. The order of metal desorption (Cd≈Zn>Cu>Pb) was the same for the three soils studied. Quantitative differences observed in the extractability of the sorbed metals between the soils (Realejos>Mazowe>Talamanca) indicated that soil properties which enhanced metal sorption contributed at the same time to slow down the backward reaction. The addition of NTA to the soil suspension significantly depressed metal sorption by the three soils investigated. Compared with the free ligand system Pb, Cu, Zn, and Cd sorption in the presence of NTA decreased roughly 50%.

Laboratory appraisal of carbon sequestration and nutrient availability after different organic matter inputs in virgin and cultivated zimbabwean soils

The effects of adding different organic amendments (maize straw, sunflower straw, and two types of manure) to Rhodic Kandiustalf from North Zimbabwe were evaluated in laboratory experiments using soil samples from a large commercial farm and from the neighboring virgin ecosystem. The study focused on a) assessing the changes in soil respiratory activity, b) comparing the accumulation patterns of stable humus substances after a 55-day incubation period, c) checking the differences in availability of plant nutrients and d) comparing the response to organic matter addition of virgin and cleared sites to evaluate the extent to which the response to organic input depends on the soil degradation status. By adding external organic matter sources the soil respiratory activity increased in the following order: sunflower straw ∼ fresh manure > maize straw > old manure. The sequestration in soil of the organic matter added was higher with lignocellulosic wastes than with old manure. Irrespective of the organic input, the mineralization coefficients evidenced the higher biodegradability of organic matter accumulated in the cultivated rather than in virgin soil. The four types of organic matter input compensated the selective accumulation of humic colloids of a low molecular weight (fulvic acids), a natural tendency of most tropical soils, and to a large extent (mainly when sunflower straw was applied) increased the humic acid/fulvic acid ratios. However, cultivation induces changes in the soil physico-chemical status and, per unit of C added, the addition of lignocellulosic wastes to cultivated soil was ca. 50% less effective in accumulating humic acid than in virgin soil. The chemical fertilizer performance of the different amendments used (i.e., the percentage of nutrients at zero time still available after the incubation experiment) only provides values above 100% (mobilizing effect) in the case of some microelements [manganese (Mn), zinc (Zn)] in soils treated with lignocellulosic wastes, but the opposite trend (microbial or physico-chemical immobilization) occurs with some macroelements [phosphorus (P), calcium (Ca), and magnesium (Mg)]. These results can be interpreted in the sense that cultivated soil displays an increasing biogeochemical activity compared with virgin soil, as corresponds to its higher mineralization coefficients of exogenous organic matter.

Influence of phosphate on zinc reaction in variable charge soils

Abstract The influence of phosphate (P) on the adsorption‐desorption of zinc (Zn) and on Zn fractions was examined for four variable charge soils. The soils were two kaolinitic Alfisols and two Andisols containing high amount of amorphous materials. Zinc adsorption‐desorption isotherms were produced using a batch protocol involving sequences of increasingly concentrated Zn solutions. The P concentrations were varied in the preparation of these isotherms to asses the effect of P on the isotherms shape. Subsequently, Zn fractions were extracted sequentially with KNO3, NaOH, and HNO3+H2O. The results showed that both soil type and pH influence the effect of P on Zn reactions. In the kaolinitic Alfisols, P and Zn were adsorbed in similar amounts and the effect of increasing P rates on Zn adsorption‐desorption reactions was negligible. On the contrary, in the allophanic Andisols, whose capacity to bind P exceeds more than 30 times their capacity to retain Zn, zinc adsorption was clearly enhanced with increasi...

Effect of cultivation on physical speciation of humic substances and plant nutrients in aggregate fractions of crusting soil from Zimbabwe

Abstract The effects of clearing and cultivation of Rhodic Kandiustalf from Mazowe (Zimbabwe) were studied by quantifying humic substances and plant nutrients (available and exchangeable cations) in aggregate fractions. It was found that cultivation was associated with a relative depletion of plant nutrients in coarse aggregate fractions (i.e. a virtual migration of soil fertility towards microaggregate soil compartments). This effect was connected with the organic matter distribution patterns in the physical fractions: cultivation has led to a relative concentration of the colloidal fractions (humic acids and fulvic acids) in the microaggregates, accompanied by a selective depletion of the latter organic fraction. It is suggested that the increased physicochemical importance of the microaggregate fractions after cultivation may be an index for the disruption of the original soil structure leading to crust formation and it may also be related to the effect of ploughing on the humification processes and the mobility of the colloidal organic fractions.

Cultivation-Induced Effects on the Organic Matter in Degraded Southern African Soils

We studied quantitative and qualitative changes in soil organic matter (SOM) due to different land uses (reference woodland versus cultivated) on six soils from Tanzania (Mkindo and Mafiga), Zimbabwe (Domboshawa and Chickwaka), and South Africa (Hertzog and Guquka). Structural characteristics of the humic acids (HAs) were measured by Curie-point pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and solid-state 13C nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy. Significant changes in concentration and composition of SOM were observed between land uses. Losses of organic carbon after cultivation ranged from 35% to 50%. Virgin soils showed large proportions of colloidal humus fractions: humic acids (HAs) and fulvic acids (FAs) but negligible amounts of not-yet decomposed organic residues. The change in land use produced a contrasting effect on the composition of the HAs: a noteworthy “alkyl enhancement” in Mkindo soil and “alkyl depletion” in Chikwaka and to a lesser extent in Domwoshawa. The remaining soils displayed only minor alterations.

Biofertilization of Degraded Southern African Soils with Cyanobacteria Affects Organic Matter Content and Quality

We studied, under laboratory conditions, the effects of Nostoc 9v (cyanobacteria) on the soil organic matter (SOM) content and quality of different soils from Tanzania, Zimbabwe, and South Africa. Soils were inoculated with Nostoc 9v at a rate of 5 mg dry biomass g−1 soil and incubated for 3 months at 27°C under 22 W m−2 illumination for 16 h and 8 h under darkness. Moisture content was maintained at 60% of field capacity. The composition of the SOM and the structural features of the humic acid (HA) fraction were studied by visible and infrared (IR) spectroscopies. Nostoc 9v proliferated and colonized the surfaces of all soils very quickly. The results obtained showed significant changes in the quantitative and qualitative characteristics of the SOM due to cyanobacterial growth. Inoculation and further incubation with Nostoc 9v promoted increases in organic carbon (OC) that ranged from 0.4 g C kg−1 soil to 9.0 g C kg−1 soil. Cyanobacterial growth also affected SOM characteristics through the incorporation of free, extractable, or particulate biomass with a predominantly aliphatic character. Important descriptors of the extent to which cyanobacterial metabolism modified the characteristics of the native SOM were the changes in the observed amounts of the two humin types, the increase in lipid concentration, and the changes in the optical density of the HAs.

Effect of cultivation on chemical characteristics and respiratory activity of crusting soil from Mazowe (Zimbabwe)

Abstract Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico‐chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter. The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low‐performance organo‐mineral interactions: in these soils the a...

Cadmium sorption by two acid soils as affected by clearing and cultivation

Abstract The objective of this study was to determine the effect of clearing and cultivation on the sorption of cadmium (Cd) by two acid soils from Zimbabwe with differing cultivation stories. In their original state, not cleared‐not cultivated (virgin soils), the two soils exhibited noticeable and similar capacities to sorb Cd. The Mazowe soil contains the highest level of organic matter (40 g kg‐1) and a effective cation exchange capacity (ECEC) of 144 mmolc kg‐1. Yet, Bulawayo soil (23.5 g kg‐1 organic matter and ECEC of 146 mmolc kg‐1) has higher pH and Mn and Fe oxide content and these characteristics seemed to counteract the effect of lower organic matter. After 50 years of cultivation, The Mazowe soil has lost 60% of its organic matter and ECEC, and consequently the ability of its soil matrix to bind Cd has proportionally decreased. In Bulawayo (cleared in 1983 and first ploughed in 1984), on the contrary, the organic matter and ECEC of the cultivated soil remains over 95% of the values on its virg...

Laboratory Assessment of Nostoc 9v (Cyanobacteria) Effects on N2 Fixation and Chemical Fertility of Degraded African Soils

The potential of Nostoc 9v for improving the nitrogen (N)2–fixing capacity and nutrient status of semi‐arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g−1 soil and 5 mg dry biomass g−1 soil. The soils were incubated for 3 months at 27 °C under 22 W m2 illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N2 fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30–100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20 mg phosphorus (P) kg−1 soil, from 5 to 58 mg potassium (K) kg−1 soil, from 4 to 285 mg calcium (Ca) kg−1, and from 12 to 90 mg magnesium (Mg) kg−1 soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed.

Cadmium sorption-desorption by soils in the absence and presence of phosphate

Abstract Batch experiments were carried out to determine the effect of phosphate anion (P) on the sorption–desorption of cadmium (Cd). The soils selected for the study were two kaolinitic Alfisols and two Andisols, which differ in their variable charge components. At the natural soil pH, Cd sorption in the absence of P was related to the net negative surface charge of the soils. Sorption of P increased the negative charge of the soils but the increase per unit amount of P added decreased with increasing P sorption. Both pH and soil characteristics influence the effect of P on Cd reactions. In the Alfisols, P and Cd were sorbed in similar amounts and the effect of increasing P rates on Cd sorption–desorption reactions was negligible. In the allophanic Andisols, whose capacity to bind P exceeds several times their capacity to retain Cd, the sorption of Cd was clearly enhanced with increasing P rates. In these soils, two distinct effects of P addition on Cd retention were observed. The first one arose from variations in the soil-suspension pH and the concomitant changes in soil surface charge. The second effect was directly related to the amount of sorbed P. Most of the Cd sorbed in the four soils was strongly bonded. In the Andisols, increasing P sorption on the surfaces restricted Cd desorption.