The construction of rhizobial strains which increase plant biomass under controlled conditions has been previously reported. However, there is no evidence that these newly constructed strains increase legume yield under agricultural conditions. This work tested the hypothesis that carefully manipulating expression of additional copies of nifA and dctABD in strains of Rhizobium meliloti would increase alfalfa yield in the field. The rationale for this hypothesis is based on the positive regulatory role that nifA plays in the expression of the nifregulon and the fact that a supply of dicarboxylic acids from the plant is required as a carbon and energy source for nitrogen fixation by the Rhizobium bacteroids in the nodule. These recombinant strains, as well as the wild-type strains from which they were derived, are ideal tools to examine the effects of modifying or increasing the expression of these genes on alfalfa biomass. The experimental design comprised seven recombinant strains, two wild-type strains, and an uninoculated control. Each treatment was replicated eight times and was conducted at four field sites in Wisconsin. Recombinant strain RMBPC-2, which has an additional copy of both nifA and dctABD, increased alfalfa biomass by 12.9%o compared with the yield with the wild-type strain RMBPC and 17.9%o over that in the uninoculated control plot at the site where soil nitrogen and organic matter content was lowest. These increases were statistically significant at the 5% confidence interval for each of the three harvests made during the growing season. Strain RMBPC-2 did increase alfalfa biomass at the Hancock site; however, no other significant increases or decreases in alfalfa biomass were observed with the seven other recombinant strains at that site. At three sites where this experiment was conducted, either native rhizobial populations or soil nitrogen concentrations were high. At these sites, none of the recombinant strains affected yield. We conclude that RMBPC-2 can increase alfalfa yields under field conditions of nitrogen limitation, low endogenous rhizobial competitors, and sufficient moisture.
Semi-arid rangelands are important carbon (C) pools at global scales. However, the degree of net C storage or release in water-limited systems is a function of precipitation amount and timing, as well as plant community composition. In northern latitudes of western North America, C storage in cold-desert ecosystems could increase with boosts in wintertime precipitation, in which climate models predict, due to increases in wintertime soil water storage that enhance summertime productivity. However, there are few long-term, manipulative field-based studies investigating how rangelands will respond to altered precipitation amount or timing. We measured aboveground C pools and fluxes at leaf, soil, and ecosystem scales over a single growing season in plots that had 200 mm of supplemental precipitation added in either winter or summer for the past 21 years, in shrub- and exotic-bunchgrass-dominated garden plots. At our cold-desert site (298 mm precipitation during the study year), we hypothesized that increased winter precipitation would stimulate the aboveground C uptake and storage relative to ambient conditions, especially in plots containing shrubs. Our hypotheses were generally supported: ecosystem C uptake and long-term biomass accumulation were greater in winter- and summer-irrigated plots compared to control plots in both vegetation communities. However, substantial increases in the aboveground biomass occurred only in winter-irrigated plots that contained shrubs. Our findings suggest that increases in winter precipitation will enhance C storage of this widespread ecosystem, and moreso in shrub- compared to grass-dominated communities.
Inoculation of alfalfa seeds with any of three recombinant strains of Sinorhizobium meliloti significantly increased overall plant biomass compared with inoculation with the wild-type strains over a 3-year period at three locations. A high proportion of nodules were occupied by the inoculum strains throughout the 3-year period.
Conservation tillage (CT) methods reduce soil erosion, but effects on establishing alfalfa (Medicago saliva L.) are not well known. Oat (Avena saliva L.) is often planted with alfalfa to reduce erosion during establishment, but results are not well documented. We hypothesized that the inclusion of oat during establishment would not be as effective as CT at reducing soil erosion. Moldboard plowing (MB) was compared with two CT practices of disking (DK) and no-till (NT), with alfalfa planted alone (SOLO) or with a companion oat crop (COMP). The study was conducted in 1992 and 1993 on Rozetta silt loam soils (fine-silty, mixed, mesic Typic Hapludalfs) following corn (Zea mays L.) each year. Simulated rainfall, 72 nun h~' for 1 h, was applied twice per growing season, at planting (1 wk) and during oat canopy development (4 wk). Runoff was reduced from 28 mm in MB plots to 13 mm in CT plots at planting in 1992. Four weeks after planting in 1993, runoff was reduced from 47 mm in SOLO to 44 mm in COMP. Averaged across all simulations, sediment concentration was 23.3 g L-' for MB and 3.2 g L-' for CT. The 2-yr mean soil loss dropped from 644 g m" 2 in MB to 94 g m~2 in CT at planting. Mean treatment erosion was reduced 85% by canopy in 1992, a dry year. The wet 1993 growing season had several severe crusting rainstorms that induced large MB soil loss (2173 g m" 2 ) 4 wk after planting. Companion cropping reduced MB soil loss 46% (from 2827 to 1520 g in-2 ) in 1993 and DK and NT further reduced soil loss to 235 and 123 g m" 2 . Forage biomass was unaffected by treatment in 1992. R JNOFF AND EROSION from agricultural lands remain major contributors of nonpoint-source pollution (Carey, 1991). The ability of CT to reduce erosion is well documented. The conversion to CT production practices has been more rapid with continuous row crops than with forage-based systems (Conservation Technology Information Center, 1993). Only 12.5% of Wisconsin's 1.6 million hectares of forages were established using CT systems, as compared with 40% of the 1.8 million hectares of corn and soybean [Glycine max (L.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.