Inhibition of phosphorus sorption to goethite, gibbsite, and kaolin by fresh and decomposed organic matter

The direct effects of dissolved organic matter (DOM) on the sorption of orthophosphate onto gibbsite, goethite, and kaolin were examined using a one-point phosphorus sorption index and the linear Tempkin isotherm model. DOM extracted from fresh and decomposed agricultural residues, as well as model organic and humic acids, were used. Changes in the chemical and sorptive characteristics of the DOM in the absence and presence of added orthophosphate (50 mg l⁻¹) were also determined. For residue-derived materials, DOM sorption to all minerals correlated well with percent hydrophobicity, apparent molecular weight, and phenolic acidity in the absence of added orthophosphate. Sorption of DOM to goethite and gibbsite was significantly decreased in the presence of added P. The correlation coefficient values of percent hydrophobicity, apparent molecular weight, and phenolic acidity to sorption also declined in the presence of added P. Thus, the addition of P substantially lowered fractionation of DOM after sorption to goethite and gibbsite. In contrast, few significant P sorption-induced differences were observed in the kaolin system. According to one-point P sorption results, DOM in the form of Aldrich humic acid, oxalate, and decomposed clover and corn residue, significantly inhibited P sorption to goethite at concentrations of 50 and 200 mg total soluble carbon (C_TS l⁻¹). Phosphorus sorption to gibbsite was significantly inhibited by 50 mg C_TS l⁻¹ derived from decomposed corn residue, fresh dairy manure residue, and oxalate solution. At 200 mg C_TS l⁻¹, all DOM solutions were found to inhibit P sorption to gibbsite. This study suggests that DOM inhibition of P sorption depends on the chemical properties of both the sorbent and the DOM itself. In general, DOM from decomposed organic materials inhibited P sorption to a greater extent than did DOM derived from fresh materials. This stronger inhibition highlights the importance of microbial processes in the release of soluble soil P, a key determinant of P availability to plants.     

Soil water effects on the use of heat units to predict crop residue carbon and nitrogen mineralization

Soil heat units (degree days) have previously been shown to predict net N mineralization from crop residues and papermil sludge. The present study was designed to identity the effects of soil water potential on predictions of mineralization with heat units and to compare field and laboratory results of white lupin (Lupinus albus L. cv. Ultra) N mineralization. Lupin-amended soil and unamended controls were incubated at factorial combinations of temperature (15, 20, and 25°C) and soil moisture (-0.30,-0.03, and-0.01 MPa) for 198 days. Incorporation of the lupin residue resulted in net N immobilization. No net N mineralization had been observed for any temperature at a soil moisture level of-0.30 MPa by the close of the incubation study. The number of heat units that accumulated until commencement of net N mineralization did not differ for five of the six remaining temperature x water treatment combinations.The number of heat units that accumulated until net N mineralization began (2058–2814 degree-days) in the present study were similar to those reported in a complementary field study (1990–2360 degree-days). Temperature and moisture interactively affected lupin-residue C mineralization. The cumulative substrate C that had evolved by the time of net N mineralization did not differ for a given temperature between soil moisture levels of-0.03 and-0.01 MPa. Heat units were not useful for describing crop-residue C mineralization in this study. Heat units appear to adequately predict net N mineralization from organic residues at soil water potentials within the-0.03 to-0.01 MPa range, but may not be valid for prolonged drier conditions.     

Crop rotation impacts on potato protein

The impacts of nitrogen (N) fertilization on potato ( Solanum tuberosum L.) protein yield and nutritional quality are well documented but of little benefit to growers with limited access to fertilizer or capital (e.g. in lesser developed countries). This study was conducted 1) to evaluate the extent which crude protein yield in potatoes can be influenced by crop rotation with no N fertilizer and 2) to determine if crop rotation and minimal application of N fertilizer can meet the total protein yield of potatoes achieved with recommended quantities of N fertilizer. A field study was conducted in which potatoes followed previous crops of alfalfa (Medicago sativa L. Nitro), hairy vetch ( Vicia villosa Roth), white lupin ( Lupinus albus L. Ultra), oats ( Avena sativa Astro), and potatoes. Tuber protein yield following alfalfa with no N fertilizer was about 50 kg/ha greater than when following potatoes or oats in one study year. In another year, tuber protein yield was greatest following vetch, achieving 149 kg protein/ha with no N fertilizer. These results were directly linked to the N contributions of rotation crop residues and were reflected in the plant-available N levels measured in soil. Applying one-half the recommended rate of N fertilizer resulted in protein yields comparable to a well-fertilized potato-potato rotation in a relatively dry year, but not in a year with more favorable precipitation. Particular crops grown in rotation with potatoes can make significant contributions to the total protein harvested in tubers.