Enabling Biological Nitrogen Fixation for Cereal Crops in Fertilized Fields

Agricultural productivity relies on synthetic nitrogen fertilizers, yet half of that reactive nitrogen is lost to the environment. There is an urgent need for alternative nitrogen solutions to reduce the water pollution, ozone depletion, atmospheric particulate formation, and global greenhouse gas emissions associated with synthetic nitrogen fertilizer use. One such solution is biological nitrogen fixation (BNF), a component of the complex natural nitrogen cycle. BNF application to commercial agriculture is currently limited by fertilizer use and plant type. This paper describes the identification, development, and deployment of the first microbial product optimized using synthetic biology tools to enable BNF for corn (Zea mays) in fertilized fields, demonstrating the successful, safe commercialization of root-associated diazotrophs and realizing the potential of BNF to replace and reduce synthetic nitrogen fertilizer use in production agriculture. Derived from a wild nitrogen-fixing microbe isolated from agricultural soils, Klebsiella variicola 137-1036 ("Kv137-1036") retains the capacity of the parent strain to colonize corn roots while increasing nitrogen fixation activity 122-fold in nitrogen-rich environments. This technical milestone was then commercialized in less than half of the time of a traditional biological product, with robust biosafety evaluations and product formulations contributing to consumer confidence and ease of use. Tested in multi-year, multi-site field trial experiments throughout the U.S. Corn Belt, fields grown with Kv137-1036 exhibited both higher yields (0.35 ± 0.092 t/ha ± SE or 5.2 ± 1.4 bushels/acre ± SE) and reduced within-field yield variance by 25% in 2018 and 8% in 2019 compared to fields fertilized with synthetic nitrogen fertilizers alone. These results demonstrate the capacity of a broad-acre BNF product to fix nitrogen for corn in field conditions with reliable agronomic benefits.

Biological nitrogen fixation in maize: optimizing nitrogenase expression in a root-associated diazotroph

Plants depend upon beneficial interactions between roots and root-associated microorganisms for growth promotion, disease suppression, and nutrient availability. This includes the ability of free-living diazotrophic bacteria to supply nitrogen, an ecological role that has been long underappreciated in modern agriculture for efficient crop production systems. Long-term ecological studies in legume-rhizobia interactions have shown that elevated nitrogen inputs can lead to the evolution of less cooperative nitrogen-fixing mutualists. Here we describe how reprogramming the genetic regulation of nitrogen fixation and assimilation in a novel root-associated diazotroph can restore ammonium production in the presence of exogenous nitrogen inputs. We isolated a strain of the plant-associated proteobacterium Kosakonia sacchari from corn roots, characterized its nitrogen regulatory network, and targeted key nodes for gene editing to optimize nitrogen fixation in corn. While the wild-type strain exhibits repression of nitrogen fixation in conditions replete with bioavailable nitrogen, such as fertilized greenhouse and field experiments, remodeled strains show elevated levels in the rhizosphere of corn in the greenhouse and field even in the presence of exogenous nitrogen. Such strains could be used in commercial applications to supply fixed nitrogen to cereal crops.

Refraction and its components in Melanesian schoolchildren in Vanuatu

Refraction and its components were measured on 788 Melanesian children and 39 children of other races, aged 6 through 19 years, in the South Pacific island nation of Vanuatu. Of the 788 Melanesian children, 766 (97.2%) were found to have uncorrected visual acuity of 6/6 or better, and 763 (96.8%) were found to have a spherical equivalent refraction between -0.25 and +1.00 D. Only 23 (2.9%) Melanesian children were found to have myopia greater than 0.25 D, only 2 (0.3%) were found to have hyperopia greater than 1.00 D, and only 2 (0.3%) were found to have refractive astigmatism greater than 1.00 D. Mean corneal refracting power was greater for girls than for boys, at all ages, but did not appear to change in any regular manner with age. Mean axial length and mean vitreous length were greater for boys than for girls, at all ages, each increasing approximately 1.0 mm from age 6 and 7 years to age 18 and 19 years. Although the low prevalence of ametropia may be considered to be due mainly to genetic factors, the possibility of environmental factors cannot be excluded.

A Method of Making Two-dimensional Glass Micromodels of Pore Systems

Abstract

Two-dimensional micro models of pore systems can be made in glass by use of a photo-imaging technique followed by chemical etching of the glass. The method is a modification of one used for making printed circuits in the electronics industry.

Pore systems with many thousands of Individual elements of specified geometry and arrangement can be fabricated. Such models are useful for observing the effects of pore and fluid variables on the trapping and subsequent mobilization of residual phases during secondary and tertiary recovery processes.

Introduction

This paper describes a method for making transparent, two-dimensional micro models of pore systems in glass.

The displacement of one fluid by another in a porous medium, say water displacing oil in a reservoir rock, is affected by the geometry and surface properties of the pores in the medium, the properties of the fluids and their interactions with each other and the solid surfaces, as well as on the forces acting on the system.

The complex interactions among moving multiphase fluids and porous media can be observed conveniently in transparent, two-dimensional micro models and this can provide a better understanding of the processes.

The advantage of two-dimensional models over other idealized porous media, such as packs of glass beads, is that the geometry and topology of the system can be specified precisely. A disadvantage is that three-dimensional networks have topologic properties not shared by two-dimensional networks. For example, bicontinua cannot exist in two dimensions(1).

Attempts have been made in the past to mechanically scribe a pattern on wax-coated glass and to etch this pattern with hydrofluoric acid(2). However, such a process relies on the manual dexterity of the model maker and does not lend itself to making complex or repetitive patterns. Others have used mono-layer packs of glass beads successfully(3.4), but such systems do not allow precise control of pore geometry nor wide variability of pore geometry within a single system.

Bonnet and Lenormand(5) and Mahers et al.(6) (1981) produced micro models in nylon film using a photo-etching procedure similar to one commonly used for making printing plates. The advantage over previous methods is that large and complex models composed of many thousands of pores of specified geometry and arrangements can be photo-reduced to give pores of sizes approaching those commonly found in reservoir rocks.

The technique described in this paper uses photo imaging followed by chemical etching in glass and has the limitation that pores less than about 20 μm in radius are difficult to etch with precision. However, in the case of pores bounded by glass, the roughness and wettability are more easily controlled than in models with surfaces of nylon or resin.

The method is similar to one used for making printed circuits in the electronics industry, but modification-is necessary because the available photo resists cannot withstand exposure to hydrofluoric acid and the resultant etch in the glass is shallow and commonly imperfect. Davis and Jones(10) used similar techniques with photo-sensitive resists and acid water.