No abstract

No abstract

No abstract

Humic acids (HAs) isolated from sewage sludge (SS) and control and SS-amended soils were characterized by fluorescence spectroscopy. The main feature of fluorescence spectra was a broad band with the maximum centered at excitation/emission wavelengths that were much shorter for SS-HA (340/438 nm) than for any soil HA (440/510 nm). Titration with Cu(II), Zn(II), Cd(II), and Pb(II) ions decreased fluorescence intensities of HAs. Titration data were fitted to a single-site fluorescence quenching model, and metal ion complexing capacities of each HA sample and stability constants of metal ion-HA complexes were calculated. The binding capacities of HAs and strengths of metal ion-HA complexes followed the order Pb(II) > Cu(II) > Cd(II) > Zn(II). With respect to the control soil HA, SS-HA, which showed the smallest contents of acidic functional groups and the lowest humification degree, was characterized by much smaller binding capacities and stability constants. The binding capacities and affinities of SS-amended soil HA were intermediate between those of native soil HA and SS-HA but closer to the former, thus suggesting a partial incorporation of HA fractions of SS into native soil HAs. These effects are expected to have a great impact on the behavior of metals in SS-amended soils.

No abstract

No abstract

To address recent concerns of impaired nutrient cycling in intensively cropped lowland rice soils of tropical Asia, we have been investigating the effects of continuous rice cropping on the chemical nature of soil organic matter (SOM). In this study, the labile mobile humic acid (MHA) fraction and the more recalcitrant calcium humate (CaHA) fraction were extracted from soils of four long‐term field trials on the International Rice Research Institute (IRRI) farm, which varied in the number of annual irrigated rice crops and hence degree of soil submergence. The two humic acid (HA) fractions were analyzed by Fourier‐transform infrared, fluorescence, and electron spin resonance spectroscopies for elemental composition and acidic functional groups. With increasing soil submergence, the HA fractions became less oxidized or humified, with higher S and H and lower O concentrations, more amide or amino, hydroxyl, and methoxy groups, and fewer carboxyl groups and organic free radicals. The HA extracted from submerged soils appeared to have greater capacity for complexing Cu+2, Fe+3, and VO+2 than did HA from aerated soils. Fertilizer treatments had little effect on HA chemical structure. The MHA was less humified than the CaHA, having fewer acidic functional groups, smaller C:N and C:H ratios, richer concentrations of amides and carbohydrates, lower concentrations of COOH, higher fluorescence intensity, and shorter wavelength of fluorescence emission maximum, and lower concentration of organic free radicals. Free radical concentrations for both HA fractions were highly correlated with other indices of humification reported here and elsewhere.

The effect of three annually consecutive additions of pig slurry at two rates (90 and 150 m3 x ha(-1) x year(-1) on soils and soil humic acids (HAs) was investigated in a field experiment under semiarid conditions. Soils and pig slurries were analyzed by standard methods. The HAs were isolated from soils and pig slurry by a conventional procedure based on alkaline extraction, acidic precipitation to pH 1, purification by repeated alkaline dissolutions and acidic precipitations, water washing, dialysis, and final freeze-drying. The HAs obtained were analyzed for elemental (C, H, N, S, and O) and acidic functional group (carboxylic and phenolic) composition, and by UV-vis, FT-IR, fluorescence, and ESR spectroscopies. With respect to the control soil, the pig slurry amended soils had greater pH and electrical conductivity, slightly larger total N content, and smaller values of C/N ratio. A decrease of total organic C was observed only in soils amended for 2 and 3 years at the higher slurry rate. With respect to control soil HA, pig slurry HA was characterized by larger contents of S- and N-containing groups, smaller acidic functional group and organic free radical contents, a prevalent aliphatic character, extended molecular heterogeneity, and smaller aromatic polycondensation and humification degrees. Amendment with pig slurry HA determines a number of modifications in soil HAs, including increase of C, S, and COOH contents, C/N ratios, and aliphaticity and decrease of extraction yields and N, O, phenolic OH, and organic free radical contents. These effects are generally more evident after the first year of slurry application and tend to disappear with increasing number of treatments. Most probably, over the years the slightly humified slurry HA is mineralized through extended microbial oxidation, whereas only the most recalcitrant components, such as S-containing, phenolic, and aliphatic structures, are partially accumulated by incorporation into soil HA.