The importance of P originating from agricultural sources to the nonpoint source pollution of surface waters has been an environmental issue for decades because of the well-known role of P in eutrophication. Most previous research and nonpoint source control efforts have emphasized P losses by surface erosion and runoff because of the relative immobility of P in soils. Consequently, P leaching and losses of P via subsurface runoff have rarely been considered important pathways for the movement of agricultural P to surface waters. However, there are situations where environmentally significant export of P in agricultural drainage has occurred (e.g., deep sandy soils, high organic matter soils, or soils with high soil P concentrations from long-term overfertilization and/or excessive use of organic wastes). In this paper we review research on P leaching and export in subsurface runoff and present overviews of ongoing research in the Atlantic Coastal Plain of the USA (Delaware), the midwestem USA (Indiana), and eastern Canada (Quebec). Our objectives are to illustrate the importance of agricultural drainage to nonpoint source pollution of surface waters and to emphasize the need for soil and water conservation practices that can minimize P losses in subsurface runoff.

Benzoic (BEN) and cinnamic (CIN) acids are commonly found in soils and are considered as strong allelochemicals. Published information suggest that BEN and CIN and other phenolic acids decrease plant growth in part by suppressing nutrient absorption. However, studies on the mechanism of action were not conclusive. We examined the effects of BEN and CIN on the cell plasma membrane in intact soybean (Glycine max L. cv. Maple Bell) seedlings. Treating intact root systems with BEN or CIN rapidly increased electrolyte leakage and ultraviolet absorption of materials into the surrounding solution. After 12 hr of treatment, BEN and CIN lowered the extracellular sulfhydryl group content in roots. The two allelochemicals induced lipid peroxidation, which resulted from free radical formation in plasma membranes, inhibition of catalase and peroxidase activities, and sulfhydryl group depletion. Oxidation or cross-linking of plasma membrane sulfhydryl groups is the first mode of action of both compounds. The BEN- and CIN-induced decrease in soybean nutrient absorption may be a consequence of damage to cell membrane integrity caused by a decrease in sulfhydryl groups followed by lipid peroxidation.

Organic acids are major water-soluble allelochemicals found in soil infested with quackgrass and are involved in several processes that are important in plant growth and development. This study was carried out to gain more information on the effects of benzoic acid (BEN) andtrans-cinnamic acid (CIN) on growth, mineral composition, and chlorophyll content of soybean [Glycine max (L.) Merr. cv. Maple Bell] grown in nutrient solution. The two allelochemicals reduced root and shoot dry biomass of soybean. Treated plants had fewer lateral roots and tended to grow more horizontally compared to the untreated plants. Lateral roots were stunted and less flexible. The amounts of P, K, Mg, Mn, Cl(-), and SO 4 (2-) were lower, and Zn and Fe contents were higher in roots of plants grown with BEN or CIN as compared to untreated plants. Shoots of plants grown with the allelochemical showed greater accumulation of Ca, Mg, and Zn, whereas P and Fe contents were reduced. The BEN and CIN also caused reductions in leaf chlorophyll content. The BEN and CIN may be responsible for negative allelopathic effects of quackgrass on soybean by inhibiting root growth, by altering ion uptake and transport, and by reducing chlorophyll content.

After screening several fungal isolates obtained from Venezuelan soils, the isolate IR‐94MF1 of Penicillium rugulosum was selected for its high mineral phosphate solubilizing activity (Mps+) with hydroxyapatite. Mutants with altered (Mps−) or amplified activity (Mps++) were obtained by UV irradiation of conidia. When glucose was used as the sole C source by the fungus, gluconic acid was associated with the hydroxyapatite activity. In solid media, in comparison to glucose and maltose, sucrose appeared to be the best C source for the solubilization of hydroxy‐apatite and FePO4.

The mechanisms of action of mineral phosphate solubilization (MPS) were studied in the wild‐type Mps+Penicillium rugulosum strain IR94‐MF1 and in negative (Mps−) and superpositive (Mps++) mutants derived from it. MPS activities were measured in liquid media using sucrose as C source, four N (arginine, nitrate, nitrate+ammonium and ammonium) and P sources (KH2PO4, hydroxyapatite, FePO4 and AlPO4). Ammonium significantly (P<0.01) decreased phosphate solubilization, and this activity was 1–66 times higher in the Mps++ mutant than in the wild‐type depending on the P and N sources used. The Mps+ phenotype was strongly associated with the production of gluconic or citric acids. The results also suggest for the MPS− mutant the involvement of the H+ pump mechanism in the solubilization of small amounts of phosphates.