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Roots, cereal crowns, and stems growing beneath the soil surface provide impor­ tant resistance to soil erosion. Understanding the amount and distribution of this material in the soil profile could provide insight into resistance to soil erosion by water and improve the performance of soil erosion models, such as the revised universal soil loss equation (RUSLE) and the water erosion prediction project (WEPP). Erosion models use built-in or e>..'ternal crop growth models to populate crop yield and live aboveground and associated belowground biomass databases. We e>..-atnined two data sets from the dryland small grain production region in the Pacific Northwest of the United States to determine root:shoot ratios, the vertical distribution of root and attached belowground biomass, and incorporated residue from previously grown crops. Data were collected in 1993, 1994, 1995, and 2000 from short-term no-till and conventional tillage e.'\."}Jeriments conducted near Pendleton, Oregon, and Pullman, Washington, and in 1999 and 2000 from long-term experiments representative of farming practices near Pendleton, Oregon. T he crops sampled in the short-term data set included soft white winter and spring wheat (Tritiamt aestivum L.;WW and SW, respectively), spring peas (Pisrmr sativum L. ; SP), and "vinter canola (Bmssica napus L. ;WC). Crops sampled in the long-term study included WW, SW, and SP. D ata were collected at harvest in both data sets and during three phenologic stages in each of the crops in the short-term data set. Soil samples were collected to a depth of 60 em (23.6 in) in the short-term and 30 em (11.9 in) in the long-term experiments. In both sets of measurements, we found greater than 70% of root mass is in the top 10 em (3.9 in) of the soil profile "vith the e.'Cception ofSP, which had 70% of root mass in the top 15 em (5.9 in) of the soil profile.WC produced significantly more biomass near the soil surface than WW, SW, or SP. Root-to-shoot biomass ratios in mature wheat ranged from 0. 13 to 0.17 in the top 30 em (11.9 in) of the soil profile, substantially lower than values suggested for use in WEPP (0.25). In the long-term e>..'Periments, soil of the conventionally tilled continuous winter wheat (CWW) plots contained significantly greater biomass than soil of conventionally tilled winter wheat/ fallow (CR ) and no-till "vin­ ter wheat/fallow (NT) treatments. There was no significant difference bet\veen CWW and conventionally tilled winter wheat/spring pea (WP); however, CWW returned more residue to the soil than WP because SP produced less residue and these residues were incorporated with a field cultivator rather than a moldboard plow. More accurate representation of root development, particularly in winter crops, could improve RUSLE and WEPP perfonnance in the Pacific Northwest where winter conditions have proven difficult to model.

Increasing use of N fertilizer for crop production necessitates more precise estimates of N provided by the soil in order to prevent under- or over-fertilization and their adverse effect on plant nutrition and environmental quality. Better laboratory tests and models of N mineralization are needed to better estimate fertilizer need. Long-term changes in N mineralization potential may also identify changes in soil quality that relate to favorable sustainable agricultural practices. We collected samples from the 0–20 cm soil zone of treatments in five long-term (30–60 y old) experiments to determine the effect of crop rotation, tillage, fertilizer and residue management on N mineralized during aerobic incubation, and compared results with N mineralized under field conditions where possible. Soil samples were incubated in glass bottles at 25°C and −0.02 MPa for 0, 7, 14, 28 and 49 d. Soil N mineralization from wheat ( Triticum aestivum L.)-summerfallow, wheat–pea and from wheat–wheat crop rotations were 32, 42 and 51% of that mineralized from non-cultivated pasture soil. Nitrogen mineralized, as a fraction of the total N present, increased with increasing N application, reduction in tillage intensity and higher frequency of cropping. Stubble-mulch soils mineralized 10–20% more N than did moldboard-plowed soils. The fraction of total N mineralized increased with increasing soil organic N content, indicating that organic N added through recent crop management practices is more labile than N in the native soil matrix. Nitrogen mineralization in situ increased linearly as a function of past N fertilizer application, which implies that a substantial portion of previously-applied N may be recovered slowly over time in subsequent crops and that fertilizer N needed for optimum crop yield may not be increasing as rapidly as expected. Nitrogen mineralized during laboratory incubation also increased linearly with increasing N application, but the rate of change differed significantly from that for in situ N mineralization (0.0042 vs 0.0112 kg ha −1 per kg of applied N). The difference in N mineralization rate between laboratory and in situ experiments is not easily explained; perhaps soil processing for incubation altered physical access to organic N pools or caused a shift in microbial communities in soil. The difference in N mineralization rates implies that laboratory incubations do not accurately reflect N mineralization in the field, and strongly suggests that laboratory estimates of N mineralization be interpreted with care. Previous N fertilization, tillage and cropping patterns all affect N mineralization potential, and must be taken into consideration when estimating N fertilizer needs.

Nitrogen fertilizer recommendations could be improved by better estimates of N mineralized ( N min ) from soil organic matter. Application of more or less N than required by the plant can result in detrimental effects on plant nutrition, the environment and producer economics. A range of soils spanning a climosequence ( 700 mm annual precipitation), were sampled, in 20 cm depth increments, to evaluate Nmin along an 80 km transect in northeastern Oregon. Soils were incubated at 25°C with or without addition of wheat residue, for a maximum of 112 d. In 112 d, cropped soils (0–20 cm) mineralized from 28 to 61 mg N kg −1 , depending on the annual precipitation at the sampling site. Uncultivated silt loam soils at paired sampling sites, mineralized twice as much N at moderate and high rainfall sites, as did cultivated silt loam soils. Residue addition resulted in net N immobilization, and decreased the total amount of N min an average of 33% in 112 d for both cropped and native soils. Net mineralization with depth in dryland native soils was greater than in cropped soils.

Rice plants were grown in a temperature-controlled greenhouse in 1.2 L glass fleakers. The rooting media were inoculated with either the parent strain or a nitrogenase-derepressed mutant strain (which excreted NH4 4 + produced by nitrogenase) of the cyanobacterium Anabaena variabilis and exposed to 15 N 2 . Dry matter and total N accumulated in the roots and shoots of plants inoculated with the mutant strain were significantly greater than from plants inoculated with the parent strain. Significantly higher levels of 15 N 2 accumulated in the roots and shoots of plants inoculated with the mutant strain, which indicated that more fixed N was readily available for root uptake and assimilation. Roots and shoots of uninoculated plants exposed to 15 N 2 had a small, consistent but nonsignificant increase in levels of 15 N compared with treatments that were exposed only to the natural atmospheric abundance of the isotope. These results show that the NH 4 + -excreting mutant strain of A. variabilis has the potential to increase N input for plant growth in rice production systems.

An ammonia-excreting mutant strain of Anabaena variabilis (strain SA-1) was studied as a N supplier for the growth of rice (Oryza sativa L., cultivar ‘IR-30’) plants at near ambient (330 μmol mol−1) and elevated (660 μmol mol−1 carbon dioxide (CO2) concentrations. Rice performance with N supplied by the mutant cyanobacterium was compared with that of three other N treatments: rice plants inoculated with the parent strain (A. variabilis, strain SA-0), plants fertilized with 75 Kg N ha−1 as urea, or control plants (uninoculated, with no nitrogen fertilization). Plants at elevated CO2 concentration had a higher dry matter content and accumulated more nitrogen than at near ambient CO2. At either CO2 concentration, the mutant strain enhanced the shoot and grain dry matter accumulation more than did the parent strain. Total N concentration of the shoot and grain fractions was significantly greater (P < 0.05) in plants inoculated with the mutant than with the shoot and grain the cyanobacterium. The benefit of inoculation with the ammonia-excreting mutant strain was equivalent to or greater than the response observed in plants receiving 75 kg N ha−1. A combined response of both the rice plants and the cyanobacterium to high CO2 levels probably accounted for the higher dry matter and N at the elevated than at the ambient CO2 concentration. The potential of the ammonia-excreting mutant cyanobacterium as a N source for sustainable, low-input rice production in the predicted high CO2 atmosphere of the next century is promising.

The Denver Unit CO2 flood, one of the largest in the world, was implemented in 1984. CO2 injection was subsequently expanded in 1989 and again in 1991. By 1994, this 10 year old CO2 flood had a well-established decline in its production function. Through infill drilling and pattern conversion, the mature CO2 flood in the Denver Unit was revived. The redevelopment program depended heavily on the reservoir simulation technology and the multi-discipline team concept to ensure its technical and economical success. Thus far, the redevelopment program has added approximately 6600 barrels of oil per day to the existing production at peak rate, and approximately 14.2 million barrels of oil to the ultimate reserves. This report reviews the infill drilling and pattern reconfiguration simulation studies, the pilot program, which led to full field development, and the results of the project to date.

Most crops require nitrogen fertilizer for peak productivity. However, the increasing production costs for nitrogen fertilizer mandates the exploitation of biological nitrogen fixation. Cyanobacteria make a valuable contribution to the nitrogen content of many soils and nitrogen fixation by cyanobacteria may be responsible for continued rice production in areas of the Orient where little or no fertilizer has ever been added to the soil.

Because of the importance of dinitrogen (N 2 ) fixation and its relation to leaf photosynthesis and dry-matter accumulation in grain legumes, a field study was conducted in 1984 to compare the relative abilities of three grain legumes to maintain N 2 fixation rates under well-defined soil water deficits imposed in field environments. Measurements of leaf water status, root distribution and nitrogenase activity were determined on peanut ( Arachis hypogaea L.), soybean ( Glycine max L. Merr.), and pigeon pea ( Cajanus cajan L.) throughout the growing season. The three crops were grown in a Kendrick fine sand (loamy, siliceous, hyperthermic family of Arenic Paleudults) and subjected to two water management treatments: 1) an irrigated treatment which was well-watered throughout the growing season: and 2) a water-stressed treatment which was subjected to several naturally-occurring water-deficit periods. During periods of mild and moderate water stress, peanut maintained higher leaf water potentials and stomatal conductance than either soybean or pigeon pea. When well-watered, peanut also maintained considerably higher rates of specific nitrogenase activity than either soybean or pigeon pea. However, nitrogenase activity was reduced in all three crops before visible symptoms of water stress (leaf wilting) were apparent. Peanut maintained nitrogenase activity longer after withholding irrigation than either soybean or pigeon pea. Root distribution measured 68 days after emergence suggested that root length density of peanut at soil depths below 30 cm increased in response to the imposed water deficits. Proliferation of roots in those soil layers as a result of water stress was not observed in either soybean or pigeon pea. The maintenance of higher leaf water-status, stomatal conductance and nitrogenase activity by peanut during soil water deficits compared to the other crops was attributed to greater density of roots in the lower depths of the soil profile.

Summary The rate of substrate reduction by partially purefied nitrogenase preparations at different redox potentials has been measured. The applied redox potential required for half maximal rates of reduction at pH 8.0 was found to be −465 mV (±10 mV). The relationship of this potential to the known redox states of nitrogenase is discussed.

Few experiments have been conducted to compare the partitioning and accumulation of nitrogen (N) in plant components of grain legumes grown under different soil water regimes. The objective of this study was to determine the effect of soil water deficit on N accumulation and partitioning in soybean ( Glycine max L. Merr.), pigeon pea ( Cajanus cajan L.), and peanut ( Arachis hypogaea L.). In 1984, the three legumes were subjected in a field environment to either well-watered or water-stressed treatments. Nitrogen concentration, total N accumulation, and N partitioning were determined throughout the growing season by measuring N content and concentration in leaves, stems, pod walls, and seeds. Peanut accumulated more total N than either soybean or pigeon pea under both the well-watered and water-stressed treatments. Water stress decreased both N concentration and total N accumulation, especially in soybean and pigeon pea. Less remobilization of N occurred in soybean leaves and stems during the seed-filling period in the stressed treatment because the water stress limited pod addition and subsequent seed demand for N. Loss of N from leaves during seed growth was greater in the crop with the most senescent growth habit (soybean), and lowest in the non-senescent but determinate crop (pigeon pea). Although peanut does not exhibit rapid leaf senescence during seed maturation as does soybean, considerable loss in leaf N was also observed in peanut leaves during the seed-filling period. Soy-bean, peanut, and pigeon pea differed in accumulation and partitioning of N under water-stressed and non-stressed environments. The partitioning and remobilization of N was dependent on the growth habit of the species and was significantly influenced by soil water deficits.

Soybeans are often subjected to periods of water deficits during ontogeny. Water stress can produce lower leaf water potentials, stomatal closure, decreased photosynthesis, and ultimately result in reduced yields. Although the responses of many physiological processes to water deficits have been extensively studied, the effects on nitrogen fixation, an important process for leguminous plants, have received little attention in field experiments. We conducted an experiment to relate the effects of reduced soil moisture on the nitrogenase activity of soybean nodules to changes in leaf water status, stomatal activity, nodule water content and apparent canopy carbon exchange during an imposed drying cycle. Field-grown soybeans were subjected to a 19-day drying cycle during early seed fill. At chosen intervals, midday measurements of selected physiological proceses were made in stressed and well-watered control plants. Leaf water potential, leaf conductance to water vapor, nodule moisture content, apparent carbon exchange and nitrogenase activity declined as the duration of the stress period increased. All physiological processes, except apparent canopy carbon exchange rate, returned to control levels upon adequate rewatering. The incomplete recovery of apparent carbon exchange was attributed to decreased leaf area index resulting from leaf senescence during the stress period. The data support the hypothesis that decreases in nitrogenase activity are primarily caused by nodule dehydration and tissue damage and not directly related to short-term changes in photosyntheses. It is apparent that nitrogenase activity declined similarly with other physiological processes, however, the observations did not conclusively define the precise relationships of nitrogenase activity to the other physiological processes.

Rhizosphere bacteria influence plant growth through several mechanisms. Beneficial interactions are often difficult to identify and isolate for study, and therefore favorable effects on plant productivity are not easily described in quantitative terms. Major beneficial activities of soil bacteria include solubilization of minerals, fixation of nitrogen, production of growth-promoting hormones and competitive suppression of pathogens. Most recent research to improve crop responses has emphasized the study of nitrogen-fixing bacteria indigenous to rhizospheres of cereal crops and other grasses. The amount of nitrogen available to crops from fixation by these organisms is significant under some circumstances, but efforts to control and increase fixation activity have not been consistently successful. Despite considerable research, inoculation experiments frequently fail to improve rates of nitrogen fixation. However, substantial progress has been achieved in understanding mechanisms involved in plant—bacterial interactions, and in defining conditions which control them. Also, recent studies show clearly that bacterial inoculation can be highly beneficial even when nitrogen fixation is not affected. Further research is needed to develop new and effective methods for suppressing growth of competing microorganisms, and promoting dominance of those most useful to plants. Environmental manipulation may not be sufficient to achieve this objective. Genetic development of both plant and bacterial lines which enhance desired interactions is likely to be the most productive research effort for the future.

There are several reports of genotypic variability in grass species for their ability to achieve associations with N 2 -fixing soil bacteria. These findings imply that plant breeding may be useful in enhancing these associations and thereby making available plant germplasms needed for agronomic and physiological studies of associative N 2 -fixation in grasses. When the N 2 -fixing bacterium Azospirillum brasilense (strain JM 125) was used as inoculum, differences were found among selfed lines of pearl millet ( Pennisetum americanum (L.) K. Shum.) for an ability to support bacterial acetylene reduction activity (ARA) when assayed in enclosed seedling agar tubes. These lines were selfed again or hybridized. Although line to line discrepancies were seen, the ARA of these lines and hybrids when categorized into high or low groups matched the activity of their original parental lines. Also, the selfed and hybrid groups obtained from high ARA lines supported higher numbers of bacteria and lost greater quantities of 14 C from their roots than did those groups obtained from low ARA lines. Two of these lines, one having high ARA and one having low ARA, were grown in soil in the greenhouse and were found to differ in their respective ability to support ARA by the soil N 2 -fixing microbes. However, overall ARA of the high line was quite low unless analyzed in a hydroponic culture system at low O 2 tensions.

Publisher Summary This chapter discusses the studies focusing on cultures of Spirillum lipoferum (Azospirillum). The recent isolation of the N2-fixing bacterium Azospirillum has initiated numerous investigations of this organism. Azospirillum can fix N2 either as a free-living organism or in association with plant roots. This chapter illustrates various applications for Azospirillum cultures and the growth conditions recommended for each specific case. Cells to be used as inoculum in the laboratory or field can be grown under aerobic conditions in NH4Cl-supplemented media or under nitrogen-limiting conditions, respectively. These convenient conditions give the fastest growth rate. Cultures of this type are also useful for producing cells for cytochrome investigations, enzyme extractions, or physiological studies. Growth under microaerophilic conditions on an N‑free semisolid agar is suggested for the isolation of the organism and for studying the substrates and growth conditions for N2 fixation. This method may also be used to prepare small quantities of N2-fixing cells. The cells can be centrifuged down to a pellet and the semisolid agar easily decanted. It is possible to provide large quantities of N2-fixing cells for studies of cell-free nitrogenase when the organism is grown in liquid culture, with N2 as the sole nitrogen source and at a constant optimal pO2.

The midpoint oxidation-reduction potential of the EPR detectable centre of the molybdenum iron protein of Chromatium nitrogenase has been measured. Two centres with identical EPR spectra but different midpoint potentials were detected. The measured midpoint potentials are (1) Em7.5 = −60 mV and (2) Em7.5 = −260 mV. The midpoint potentials were not affected by other components of the nitrogen fixing system.