Weed control is one of the biggest problems with low input sustainable systems and organic systems. In this project, we explore the possibility of controlling weeds through limiting N transfer from N2-fixing soybean plants. Soybean varieties have different capabilities for N transfer, but this does not appear related to differences in mycorrhizal colonization – mycorrhizae act as the ‘bridge’ for N transfer into weeds plants. Low transfer seems to be an inherent trait of individual soybean genotypes. The results show that weed competitiveness and reproduction decrease greatly when N availability is limited.
1. Critically evaluate the role of N transfer in the development of weed populations.
a. Determine N transfer distances and penetration into weed patches, and the impact of transfer on weed population dynamics.
b. Determine the influence of N on weed competitiveness and reproduction.
c. Determine the impact of N on the N nutritional status of weed seeds and the competitiveness of offspring.
2. Compare weed growth, patch development, and seed production in a traditional organic production system with the new, modified system that uses a “low N transfer” soybean variety in rotation with crops that have low N requirements and high N use efficiencies.
Weed control has remained as one of the most imposing obstacles in the development of sustainable and organic agricultural systems. Losses in crop yield and quality make weed control a major financial constraint, especially in the South. Substantial weed problems persist despite the development of many important non-chemical management strategies (e.g. cultivation, mowing, row spacing, variety selection, and cover crops). A 1997 AERO publication gave voice to the “many little hammers” approach to weed management in which a suite of low- or non-toxic weed management tactics are used in an integrated manner to collectively reduce weed pests to acceptable levels.
Much of the agricultural acreage in the southeastern U.S is on sandy soils with very low N fertility. Soybean and peanut are two important crops in the region, and they are commonly grown without additions of N fertilizer. Sufficient N is acquired through N2-fixation to maximize yields. The results from our previous field experiments on these soils had suggested that substantial amounts of N could be transferred from N2-fixing soybean and peanut to competing weeds. Furthermore, transfer appeared dependent on mycorrhizae. Thus, the results provided evidence of inter-species N-transfer through mycorrhizal hyphae.
In this project, we further investigated the extent that N transfer influenced weed populations. The intent is to test a new farming system paradigm for highly weathered, sandy soils that relaxes the need for soil N building to support the high N requirements of crop plants and reduces weed competitiveness by limiting N availability. The project included examination of N transfer from different soybean genotypes to determine if transfer might be controlled by N ‘donor’ characteristics.
The field experiments were conducted at the Center for Environmental Farming Systems in Goldsboro, NC. From past experiments, soils are known to be sandy, have very low nitrogen status with organic matter levels below 0.5%. The fields were fertilized with all nutrients except nitrogen. Fields were drilled to soybean with high (N93-1264) and low (cv. Young) N-transfer genotypes (see Fig.1) in separate areas. The genotypes are quite similar in terms of growth rate, height, branching, and reproductive characteristics.
A series of circular plots were established in the soybean fields. Soybean occurring inside the plot were removed. Scarified sicklepod or prickly sida seeds were planted into separate, replicated plots. The weeds were planted in three concentric rings plus a center grouping, which will provide a range of separation distances from the crop (see adjacent figure). Treatments were constructed to test weed response as a function of weed: (1) density, (2) distance from soybean (0.25 to 1.00 m from plot edge), (3) N transfer into a weed patch, and (4) low versus high N-transfer soybean. The treatments were : 1) the complete set of rings filled with weed plants, 2) weeds in the outer ring removed, 3) weeds in the outer and inner ring removed, 4) weeds in the outer two rings removed, and 5) all weeds removed except in the center. Interspecific and intraspecific interference for plants at different positions in the patch were quantified. The basic model describing interference among neighboring plants is written:
P(W)=Y – XWNA(N)
where P(W) is the performance (e.g., fecundity, total or reproductive biomass) of the weed plants, Y is the performance of individuals with neighbors removed, XWN- is the slope of the regression of performance, and A(N) is the performance (e.g. biomass) of neighbors. The model can be enhanced by substituting a spatially explicit term for A(N) to account for the differences in proximity to neighboring plants.
Non-destructive measurements of plants in the patches were made throughout the growing season, until maturity. Observations included height, stem and canopy diameter, leaf number, SPAD meter readings [Spectrum Technology’s “chlorophyll meter” (Plainfield, IL)], etc. for individual plants within a ring. Periodically, individual leaves from separate weed plants in a ring, and from randomly selected soybean plants in the field, were sampled to quantify N2-fixation, N transfer, and N uptake from the soil over time. During maturation, senescent fruits on the weeds will be collected and catalogued to maintain records for individual plants in different positions within the plot.
The amounts of N originating from soil N and from the N2-fixing soybean will be determined using the 15N natural abundance technique. The 15N abundance in soybean and weed tissues indicate the relative contributions of N from the soil and N2-fixation by the soybean plants using the equation:
Amount of fixed N = ( 1 – atom % 15N excess of weed with soybean ) x weed total N
atom % 15N excess of weed alone
We know from the previous experiments that atom % 15N abundance is sufficiently different in nodulating N2-fixing plants and non-nodulating plants to clearly distinguish contributions from the N2-fixation process. Individual leaves from many plants in a ring will be sampled during the growing season, and seeds will be harvested at maturity from plants in a ring, and the tissues combined to provide the large sample size necessary to optimize the accuracy of the estimates. Separate patches established without surrounding soybean provided the baseline 15N profiles for soil N. Total N and 15N enrichments were determined using a Finnegan MAT DELTAplus Isotope Ratio Mass Spectrometer.
To verify that the mycorrhizal network was in place for all experiments, soil cores were taken periodically during the growing season. Modified standard methods were used to stain roots for the detection of mycorrhizal colonization and to assess colonized root length using the grid-line intersect method.
Two weed mixtures of one mycorrhizal host (sicklepod) and one non-mycorrhizal host (Palmer amaranth) species were established in plots (patches) that have been drilled to high (N93-1264) or low (cv.‘Young’) N transfer soybean genotypes. Patches developed at two different density levels, either 5 or 10 plants m-2, in 2 m2 microplots to give total weed numbers of 10 or 20 plants per patch. Proportions of the two species were varied in a replacement series design. Patches with Palmer amaranth alone indicated the degree of competition from the soybean genotypes without the involvement of transferred N. Based upon previous observations, we anticipate that the competitive effects of these two soybean genotypes will be similar. The weed species assemblages will be non-destructively monitored throughout the season, and plants will be harvested late in reproduction.
The experiment will also include sets of plots within the soybean fields that receive split rate N fertilizer additions for a total application rate of ~140 kg ha-1. That will, potentially, remove the beneficial effects of N transfer from the N2-fixing crop into a low N environment, allowing examination of inter-species interactions with only the detrimental competitive effect of soybean, as well as assessment of the extent that inorganic N interferes with the N transfer process. Additional plots will be fumigated or treated with fungicide to allow assessment of inter-species competition with suppressed mycorrhizal populations and, presumably, N-transfer. Also, weed mixture plots will be established without surrounding soybean to allow evaluation of the mycorrhizal and non-mycorrhizal weed interactions on low N soils without influence from soybean, N2-fixation, or N-transfer.
The replacement series method is indicated for experiments examining the degree of interference that may occur between two species. In this type of experiment, total density of the assemblage is held constant while the proportion of competing species is varied. The resulting biomass is then expressed as a fraction of the yield of each respective species when grown alone.
To determine how limitations in N impact weed competitiveness and reproductive output, experiments examined nitrogen fertilization effects on sicklepod growing in an old-field successional area with a Tarboro sandy loam soil (Typic Udipsamment, pH of 5.5, 0.3% humic matter, 5-22 ppm inorganic nitrogen). The experiments were conducted in 2000 and 2001 at the Center for Environmental Farming Systems in Goldsboro, North Carolina, USA. A randomized complete block design was used, with four nitrogen treatments and five replicates. Individual plots had an area of 1 m2 and were surrounded by a 1 m buffer strip within blocks and a 2 m buffer strip between blocks.
Sicklepod seeds were obtained from Azlin Seed Company (Leland, MS, USA), who had grown the seed-yielding parental plants at an application rate of 67 kg N ha-1. Nitrogen treatments were established by applying ammonium nitrate fertilizer (NH4NO3) at four levels: 0, 112, 224, and 448 kg N ha-1. In each plot, sicklepod seeds and ammonium nitrate fertilizer were broadcast concurrently (on June 9, 2000 and June 16, 2001). At time of emergence (five days, on average), plots were thinned to four seedlings of uniform size per plot, with one in each 0.25 m2 section of the plot.
During the summer growing season, weed control was maintained by hand within plots and by mowing within the buffer areas. Fruits (legumes) were harvested at maturity, but prior to dehiscence. One plant per plot was selected for fruit collection. Fruit removal continued until all mature legumes had been collected. Harvests were completed on December 5 in 2000 and November 20 in 2001. Following final fruit collection, plants were harvested, oven dried at 60 C for 48 h and then weighed.
After threshing seeds from fruits by hand, a mesh sieve was used to separate seeds into two seed-size classes: small (<2.4 mm) and large (>2.4 mm). Seeds of each category were counted and weighed on a per plant basis. One-way analysis of variance was performed on untransformed data to test for the effects of nitrogen fertilizer treatment on measures of growth and reproduction. Nitrogen concentrations in the seed were determined using an Elemental Analyzer (Model Flash EA 1112, ThermoQuest, Rodano, Milano, Italy). Results and discussion below often focus on nitrogen content, which is calculated from nitrogen concentration x seed mass.
Seed Mass Distributions
A total of 700 seeds per nitrogen treatment were selected randomly and weighed to the nearest 0.01 mg using an electronic micro-balance. The distributions of seed masses were compared using two-sided pairwise Kolmogorov-Smirnov non-parametric tests. Skewness (g1) and kurtosis (g2) were determined. Skewness values indicate the direction and degree of the tail of the distribution curve, if different from a normal distribution, while kurtosis values represent the peakedness.
A time-course study was used to evaluate the effects of seed size and nitrogen treatment on growth of seedlings using a two-way factorial design. Seeds from the small and large seed-size classes (described above) were germinated in paper rolls in a germination chamber for 72 h in the dark. The paper rolls were placed vertically into 4-L beakers and kept moist by capillary action from a 0.1 mM CaSO4 solution. Upon emergence of the radicle, seedlings from each seed size class were transferred into eight 50-L continuous flow, solution culture systems. The continuous-flow solution culture systems were located in a controlled-environment growth room in the Southeastern Plant Environment Laboratory at North Carolina State University, Raleigh, N.C. A combination of cool-white fluorescent and incandescent lamps provided a photosynthetic photon flux (PPF) of 700 ± 50 _mol m-2 sec-1 at wavelengths of 400 to 700 nm and a photomorphogenic radiation (PR) of 12 W m-2 between wavelengths of 700 to 850 nm. The seedlings were grown in a modified Hoagland’s solution containing low nitrogen (0.3 mM KNO3) or high nitrogen (1.0 mM KNO3). Solution pH was automatically monitored and maintained at 5.8 ± 0.2, and temperature was maintained at 30 ± 0.5 ºC. Seedlings were harvested every five days until day 40 (after transplant). Shoot and root tissues were separated at harvest, oven dried at 60 ºC for 48 h, and then weighed. Linear regression analysis of total biomass over time was performed on square-root transformed data. The square-root transformation was selected because it increased homogeneity of variance across harvest dates and gave a more linear biomass response. Using the General Linear Model Procedure (SAS, 2000), multiple contrast procedures were performed on the transformed data to test for equality of slopes of linear regressions (representing relative growth rate) for all combinations of nitrogen and seed factors.
The effects of seed mass on seedling competitiveness were examined in a replacement series experiment. The two-factor experiments included densities of 10 or 20 plants pot-1 exposed to a low or high nitrogen treatment with four replications per treatment. Small seeds from the low-N treatment and large seeds from the high-N treatment in the field study were used. The small:large seed-size class ratios for the 10 plants pot-1 density were 10:0, 7:3, 5:5, 3:7, and 0:10. The ratios for the 20 plants pot-1 density were 20:0, 15:5, 10:10, 5:15, and 0:20.
Seeds were germinated in paper rolls in a germination chamber for 72 h in the dark, as before. Upon radicle emergence, seedlings of each seed-size class were placed in 19 L pots (with drain holes) containing river bottom sand, and placed in controlled environment chambers. Seedlings from each seed size were labeled with small colored stakes. The chambers provided a 12-h photoperiod plus a 2-h night interruption to suppress flowering. A combination of metal halide, high-pressure sodium and incandescent lamps supplied a PPF of 900-1100 µmol m-2 s-1 for the 12-h photoperiod. Only incandescent lamps were used for the night interruption.
Pots were watered once daily with one of two nutrient solutions. The nutrient solutions were a modified Hoagland’s solution (Downs and Thomas 1991) with or without 1.0 mM KNO3 and were applied to saturation (approximately 2 L). In addition, pots in the low nutrient treatment were saturated with the nutrient solution containing 1.0 mM KNO3 once per week. All treatments were harvested 50 days after transplanting. Shoots were removed and placed in a drying oven at 60 ºC for 48 h. Relative per plant yield (dry biomass) was calculated for plants originating from each seed size class:
Relative yield of plants from small seeds (s) RYs = Ysl / Yss
Relative yield of plants from large seeds (l) RYl = Yls / Yll
Where: Ysl = mean per-pot shoot biomass of seedlings from small seeds when grown with seedlings from large seeds; Yls = mean per-pot shoot biomass of seedlings from large seeds when grown with seedlings from small seeds; Yss = mean per-pot shoot biomass of seedlings from small seeds in monoculture; Yll = mean per-pot shoot biomass of seedlings from large seeds in monoculture. Actual RY was compared to expected relative yield (EY, a theoretical value calculated assuming plants were uniformly competitive in mixture and in monoculture) for each ratio of small: large seed size proportions using paired t-tests.
When the experiments began, it became evident that a number of important questions had to be answered about the system before large scale experiments and the related interpretations could be successful. In this and previous field studies, we have found that differences exist in amounts of N transferred from soybean to weed species. As shown in Fig.1 below, soybean cv Young transferred little N, while N93-1264 transferred the most. Obviously, cv Young would be the candidate soybean genotype that should be considered for organic systems.
In several experiments, we have examined mycorrhizal colonization of roots. All of the soybean genotypes we have examined are readily colonized. Thus, there is no evidence that mycorrhizal colonization was responsible for a genotype’s ability to supply N to surrounding weed species.
In attempting to understand mycorrhizal-mediated transfer, a key question was whether diverse mycorrhizal species were present in the soils being used on experiment stations. As shown in Table 1 below, a large number of species was present, and this was found to be true at all field sites where N transfer was detected.
Table 1. AMF species contained in the mixed inoculum
Acaulospora aff. mellea Spain & Schenck
Acaulospora aff. scrobiculata Trappe
Acaulospora sp. brown with a rough outer wall
Gigaspora aff. margarita Becker & Hall
Gigaspora aff. rosea Nicolson & Schenck
Glomus aff. botryoides Rothwell & Victor
Glomus aff. clarum Nicolson & Schenck
Glomus aff. etunicatum Becker & Gerdemann
Glomus aff. intraradices Schenck & Smith
Glomus aff. mosseae (Nicolson & Gerdemann) Gerdemann & Trappe
Paraglomus occultum (?) (Walker) Morton & Redecker
Scutellospora aff. heterogama (Nicolson & Gerdemann) Walker & Sanders
Scutellospora aff. pellucida (Nicolson & Schenck) Walker & Sanders
Scutellospora aff. nigra (Redhead) Walker & Sanders
Scutellospora sp. heterogama variant
To answer the question of ‘if N transfer occurs, would it influences reproductive output of weeds’, we conducted several experiments with the sicklepod. As shown in Table 2 below, N had a strong effect on reproduction. Numbers of fruits and seeds, along with seed mass all were decreased as N nutrition decreased.
Another key question was whether maternal N nutrition influenced germination of the seed being produced. As shown in Table 3, N had no impact. Germination was effected by seed size, so any N induced change in seed size (or mass) might well impact germination rate.
To test whether seed size and the associated nitrogen content affected growth of offspring, a series of hydroponics studies was conducted over a 40-day period, with seedlings grown in the presence of low or high nitrogen levels. Seed size had no detectable effect on seedling relative growth rate in the high nitrogen treatment solutions (t=0.73, P=0.47). But in the low nitrogen solutions, comparisons of slopes via orthogonal contrasts revealed that small seeds had a lower relative growth rate (t=2.40, P=0.025) and less biomass at 40 days after transplanting. As expected, slopes for large or small seeds under high vs. low fertility also differed (large seeds: t=4.98, P<0.001; small seeds: t=6.65, P<0.001). A few previous studies have shown that smaller seed size, and presumably lower nutrient content, can lead to reduced seedling biomass or height, especially in conditions of low nitrogen fertility.
To test whether smaller seed size and lower nitrogen content affected competitive interactions, sicklepod seeds from small and large size classes were grown in a replacement series experiment. Seeds were grown in monocultures and in mixtures at two plant densities, and exposed to either low or high nitrogen fertility for 50 days.
Visual evaluation of the replacement series plots of shoot biomass data indicates that competitive ability was not affected by seed size class under high nitrogen nutritional conditions at either density level. However, plants from small seeds were less competitive in the low nitrogen treatment. This was most apparent from the concavity of the biomass curve for plants from small seeds and convexity of the curve for plants from large seeds at the high plant density. The competition experiment results are not surprising, as others have observed reduced growth of seedlings from smaller seeds under low fertility conditions.
The field experiments to examine whether limited N transfer could be used to control weed populations in crop rotations have been largely unsuccessful. Interruptions of the experimental cycle by adverse climate conditions, especially periods of drought, and have led to highly variable data sets that offer no insights into whether the approach could work. The problems have been outlined in yearly reports. It is evident from the many field studies that N transfer is inhibited when soybean donor plants are stressed. This effect was particularly strong when the crop system was under drought stress – a condition known to strongly inhibit N2 fixation. Another problem in the field studies was that we were unsuccessful in establishing weed populations, especially those for pigweed, with enough vigor to allow assessment of the N transfer distances. The intent was to conduct experiments in conditions that simulated the field environment. But, without continuous irrigation, it was not possible to have the concentric weed population ‘rings’ that were intended. It also was not possible to assess weed population competitiveness. Thus, the idea that weed competitiveness would be increased by substantial N transfer must rest on observations that N nutrition in general leads to large increases in reproductive output, as demonstrated by the sicklepod experiments with more controlled conditions.
Educational & Outreach Activities
Moyer-Henry KA, JM Burton, DW Israel, T.W. Rufty. 2006. Nitrogen transfer between plants: A 15N natural abundance study with crop and weed species. Plant and Soil. 282:7-20.
Tungate KD, MG Burton , DJ Susko, SM Sermons, TW Rufty. 2006. Altered weed reproduction and maternal effects under low N fertility. Weed Science 54:847-853.
Tungate KD, DW Israel , DWH Watson, TW Rufty. 2007. Potential shifts in weed interference in an agroecological system with elevated temperatures. Environmental and Experimental Botany 60: 42-49.
The results clearly show that nitrogen management strongly affects weed competitiveness and reproduction. Careful use and placement of nitrogen can be used by farmers as they pursue weed control in organic systems. It is premature to know the exact impact of controlling N transfer by using low-transfer soybean genotypes. Early indications are that soybean cv ‘Young’ may allow lower N transfer and thus assist with controlling weed development.
It is premature for any economic analysis.
While the concept of controlling nitrogen availability is a sound one, the specifics for following this path will require individual decisions with different systems. The use of crops that are highly efficient in using N in a rotation should, in itself, help to control weeds. More information on soybean N transfer ability will be needed before specific cultivar recommendations can be made.
Areas needing additional study
A soybean plant breeding program (Joseph Burton) is conduction field evaluations of N transfer capabilities of different soybean genotypes and intending to develop the low N-transfer trait.