Complex Soil Systems Research Articles

Relative biochemical reactivity of three hexachlorocyclohexane isomers

The rate and mechanisms of degradation of three hexachlorocyclohexane (HCH) isomers were studied with rat liver cytochrome P450, which is known to react with chlorinated compounds in the presence and absence of molecular oxygen. In addition, P450 is produced from HCH metabolites which are found aerobically and anaerobically during microbial biodegradation of this compound in a complex soil system. Conversion assays with P450 were carried out with α-, β-, and γ-HCH. Hexachloroethane served as a reference compound for bioconversion kinetics. Under anaerobic conditions, γ-HCH was readily dechlorinated ( r 0 = 0.31 nmol/min · nmol cyt · P450) to tetrachlorocyclohexene and monochlorobenzene. γ-HCH was also reductively dechlorinated, but at a much slower rate ( r 0 = 0.03 nmol/min · nmol cyt.P450). Both substrates followed Michaelis-Menten kinetics (γ-HCH: V max = 1.87 nmol/min · mg and K m = 47 μmol/liter; γ-HCH: V max = 0.24 nmol/min · mg and K m = 201 μmol/liter). Under aerobic conditions, bioconversions of γ-HCH ( r 0 = 0.05 nmol/min · nmol cyt. P450) and α-HCH ( r 0 smaller than detection limit) were slower. β-HCH proved to be recalcitrant under both aerobic and anaerobic conditions. The differences in bioconversion rates between the three HCH isomers could be related to the differences in spatial chlorine configuration of the three isomers. β-HCH recalcitrance was explained by the absence of axial-orientated chlorines.

Nitrogen Fertilizer Recovery by Corn in Monoculture and Rotation Systems

AbstractCrop rotations including legumes have increased in importance because of their potential to reduce large inorganic N fertilizer needs for corn [Zea mays L.] and other crops. This study was conducted to determine N fertilizer recovery by corn in monoculture and rotational systems. Corn was grown under rainfed conditions on a Sharpsburg silty clay loam (fine, montmorillinitic, mesic, Typic Argiudoll) in four cropping systems: (i) continuous corn monoculture, (ii) a 2‐yr soybean [Glycine max (L.) Merr.]‐corn rotation, (iii) a 4‐yr rotation of oat [Avena sativa (L.)] + clover [80% Melilotus officinalis (L.) and 20% Trifolium pratense]‐grain sorghum [Sorghum bicolor (L.)]‐soybean‐corn, and (iv) a 4‐yr rotation of soybean‐grain sorghum‐oat + clover‐corn at Mead, NE. Broadcast applications of 15N‐depleted NH4NO3 were made at 90 and 180 kg N ha−1 in 1985 and 1986 to evaluate N fertilizer recovery by corn in each cropping system using isotopic methods. Nitrogen recovery determined by isotopic methods was significantly higher for corn in rotation vs. corn in monoculture, averaging 58.6 vs. 52.3% and 49.8 vs. 43.4% at the 90 and 180 kg N ha−1 rates, respectively. In contrast, fertilizer N recovery estimated by the difference method was much greater in continuous corn vs. N recovery in corn following oat + clover in the 4‐yr rotation. These differences indicated that N fertilizer applied to corn in each cropping system appeared to be entering different sizes and types of organic soil N pools, resulting in apparent differences in N immobilization. Our results demonstrate problems exist in estimating fertilizer N recovery with both methods (isotope or difference) and before accurate N recovery estimates by corn or any other crop can be made in complex soil and crop management systems, procedures must be developed to explicitly follow N fertilizer pathways (immobilization, denitrification, volatilization, leaching, etc.). Until that time, correct interpretations with either method can be obtained within a system if all available information is used regarding both soil and crop factors.

Influence of nitrogen fertilisers on the persistence of carbaryl and carbofuran in flooded soils

AbstractAmmonium sulphate and urea, but not potassium sulphate, increased the persistence of carbaryl in a flooded laterite soil with a low native nitrogen content (0.04%), but not in an alluvial soil with a higher nitrogen content (0.11%). Thus, NH4+ but not SO42‐, contributed to the increased persistence of carbaryl. Likewise, ammonium sulphate increased the persistence of carbofuran in the laterite soil, but not in the alluvial soil. Significant accumulations of 1‐naphthol and 2,3‐dihydro‐2, 2‐dimethylbenzofuran‐7‐ol (‘carbofuran phenol’), in soils treated with carbaryl or carbofuran, suggested hydrolysis as the major pathway of degradation. Treatment of the two soils with ammonium sulphate, urea or potassium sulphate led to a decrease in soil‐bound residues and an increase in the respective hydrolysis products, compared with untreated soils. Sorption studies indicated that NH4+ and SO42‐ compete with carbaryl, 1‐naphthol and carbofuran for sorption and exchange sites in the complex soil system. Evolution of [14C]carbon dioxide from ring‐14C in carbaryl and carbofuran was negligible. Consequently, after 40 days, more than 50% of the 14C in [14C]carbaryl and [14C]carbofuran remained in the soils as hydrolysis products (1‐naphthol or 2,3‐dihydro‐2,2‐dimethylbenzofuran‐7‐ol) plus soil‐bound residues.