Controlling nitrate leaching in irrigated agriculture.

The impact of improved irrigation and nutrient practices on ground water quality was assessed at the Nebraska Management System Evaluation Area using ground water quality data collected from 16 depths at 31 strategically located multilevel samplers three times annually from 1991 to 1996. The site was sectioned into four 13.4-ha management fields: (i) a conventional furrow-irrigated corn (Zea mays L.) field; (ii) a surge-irrigated corn field, which received 60% less water and 31% less N fertilizer than the conventional field; (iii) a center pivot-irrigated corn field, which received 66% less water and 37% less N fertilizer than the conventional field; and (iv) a center pivot-irrigated alfalfa (Medicago sativa L.) field. Dating (3H/3He) indicated that the uppermost ground water was <1 to 2 yr old and that the aquifer water was stratified with the deepest water approximately 20 yr old. Recharge during the wet growing season in 1993 reduced the average NO3-N concentration in the top 3 m 20 mg L(-1), effectively diluting and replacing the NO3-contaminated water. Nitrate concentrations in the shallow zone of the aquifer increased with depth to water. Beneath the conventional and surge-irrigated fields, shallow ground water concentrations returned to the initial 30 mg NO3-N L(-1) level by fall 1995; however, beneath the center pivot-irrigated corn field, concentrations remained at approximately 13 mg NO3-N L(-1) until fall 1996. A combination of sprinkler irrigation and N fertigation significantly reduced N leaching with only minor reductions (6%) in crop yield.

Purification and enzymatic properties of endo-alpha 1, 2-mannosidase from pig liver involved in oligosaccharide processing.

An endo-alpha 1,2-mannosidase, which is involved in N-linked oligosaccharide processing, has been purified to homogeneity from crude pig liver microsomes using conventional techniques. Two catalytically active polypeptides, of 48 kDa, have been isolated which degrade [14C]Glc3-1-Man9,-GlcNAc2 to [14C]Glc3-1-Man and a specific Man8-GlcNAc2 isomer. They are not, however, active on synthetic alpha-mannosides. [14C]Glc1-Man9-GlcNAc2 was found to be approximately sevenfold more rapidly hydrolyzed than the [14C]Glc2- and [14C]Glc3-homologues. The 48 kDa and 50 kDa proteins are not N-glycosylated and ran on Superdex 75 as monomers. Kinetic studies showed that these proteins had similar catalytic properties: (i) the pH optima were found to be close to 6.5; (ii) neither activity was metal ion dependent; (iii) hydrolysis of [14C]Glc3-Man9-GlcNAc2 was inhibited strongly by Glc-alpha 1,3-Man (app. Ki approximately 120 microM), but not by 1-deoxymannojirimycin or swainsonine. Other evidence, including immunological data, strongly suggests that the 48 kDa and 50 kDa polypeptides are proteolytic degradation products of a single endo-alpha 1,2-mannosidase, rather than distinct subunits of an oligomeric complex. Possible functions of the endo-alpha 1,2-mannosidase in N-linked oligosaccharide processing are discussed.