Tünde Nyilas

Surface charge heterogeneity and aggregation of clay lamellae in aqueous suspensions

Montmorillonite (2 1 layer silicate) is the most often studied swelling clay mineral. The faces of the lamellae have permanent negative charges owing to isomorphic substitutions, and pH-dependent charges develop on the surface hydroxyls at the edges. The acid–base titration of sodium montmorillonite suspensions at 0.01, 0.1 and 1 M NaCl was evaluated by fitting of potentiometric data measured between pH 4 and 9 using FITEQL. A surface complexation model assuming ion exchange on faces, and protonation–deprotonation processes at edges in the negative electrostatic field emanating from the particle face were introduced. This model provides excellent fitting of the measured points. The calculated number of edge sites was 3–4 mmol/100 g besides a permanent charge of 100±2 mmol/100 g on the faces. The intrinsic equilibrium constants of the protonation and deprotonation processes were logK a 1 i nt>= 5.1±0.1 and logK a 2 i nt>=–7.9±0.1, respectively. The point of zero charge for edge sites was at pH∼6.5, i.e., positive charges can develop on the Al–OH sites of edges only at pHs below around 6.5. Study of the pH-dependent particle aggregation showed the formation of stable suspensions at low (below 0.01 M NaCl) ionic strength even at low pH, where the edges are positively charged. However, an increase in salt concentration induces edge-to-face aggregation below pH∼6.5. X-ray diffraction patterns of montmorillonite films prepared from slightly acidic suspensions proved that the formation of well-ordered lamella packages is hindered by the attraction between edges and faces. Characteristic changes in gel formation and in the plastic, elastic and viscous properties induced by decreasing pH in dense suspensions containing 0.01 M NaCl provided experimental evidence for the structure of the particle network. The significant increase in thixotropy and yield values and also the formation of viscoelastic gels only below pH∼6.5 verify that attractive interaction exists between oppositely charged parts of lamellar particles.

Quantification of soil organic matter degradation by Rock-Eval pyrolysis

One of the biggest challenges for the safe maize production is the yield stability in a wide range of environments with different soil fertility, weather conditions, prevailing pests and diseases as well as cultural practices. In order to achieve it, new hybrids with higher tolerance to biotic and abiotic stress are continuously being created. In order to compare performance of maize hybrids in environments with different levels and types of stress, and relate it to the stalk lodging incidence, we compared performance of 64 maize hybrids obtained by crossing 16 inbred lines with four inbred testers in three different environments. Two environments at same location (Rugvica) differed in crop rotation, and the third environment at location Botinec was considered as dry because of low water capacity of the soil. Mixed models that included all design elements and genetic background of hybrids were used to analyze the effect of lodging on yield in different environments. The results show that stalk lodging had a significant effect on yield, but significant lodging x environment interaction indicates that this effect was rather environment specific.Sweet pepper Istra F1 hybrid was grown in a greenhouse on K rich soil. For evaluation of K antagonism, two K rates (55 and 85 kg ha-1) were applied in a randomized complete block design with four replications. Total number of fruits, total number of BER affected fruits as well as average fruit mass were recorded for each harvest. Fruit and leaf K and Ca content were determined at three plant growth stages (at the stage of the first, third and fifth fruit cluster). Higher K rate gave higher : average fruit mass (169.45 g), total number of fruits per plant (7.95), number of BER affected fruits per plant (3.82), K in leaves (5.44% DM), K in fruits (6.35% DM), but negatively correlated with Ca concentration in fruits (0.57% DM) and leaves (3.24% DM). This resulted in decreased marketable yield of sweet pepper fruits.

Investigation of the effect of soil mineral composition on soil organic matter stability

Czirbus N., Nyilas T., Raucsik B., Hetényi M. (2016): Investigation of the effect of soil mineral composition on soil organic matter stability. Soil & Water Res., 11: 147−154. The soil organic matter (SOM) pools with different stability play different roles in the agricultural and environmental processes, e.g. in the global carbon cycle. In this work Rock-Eval (RE) pyrolysis measurements and mathematical deconvolution of RE pyrograms were used for determining the abundance, quality, and main biological sources of SOM, as well as for evaluating the relative proportion of the labile (fresh plant and litter) and resistant (lignin and cellulose) bio-macromolecules, immature (humic substances) and mature refractory (naturally stable biological compounds, organic matter stabilized by physico-chemical processes and black carbon) geo-macromolecules. The samples were taken from the A-horizons of Leptosol, Luvisol, and Acrisol formed under different soil conditions (mineral composition, pH, and leaching). In agreement with the present vegetation, bulk RE data together with the results from the mathematical deconvolution of pyrograms display terrigeneous plant remnants as precursors of SOM for all the three samples. The presence of the more stable components, indicated by our results in the A-horizon only of Luvisol and Acrisol, could be a consequence of the intense leaching. In contrast, due to the limited leaching, the soluble components also remained in the Ahorizon of Leptosol, for this reason SOM seems to be relatively well preserved. The comparison of the results measured and calculated on whole soil samples and their mineral-free hydrolysis-resistant macromolecular fractions (ROM) reveals that the stabilization of SOM is mainly controlled by organo-mineral association in Leptosol and Luvisol. These results suggest the importance of iron-oxides and -hydroxides and clay minerals in the stabilization of SOM for Leptosol and Luvisol, respectively, whereas in Acrisol the stability of SOM is due to the high relative contribution of naturally stable organic compounds.