Michael P. Russelle

SOIL CARBON ADDITION CONTROLS WEEDS AND FACILITATES PRAIRIE RESTORATION

Soil nitrogen enrichment and consequent vigorous weed growth are thought to hinder the restoration of tallgrass prairie. Adding carbon to the soil may facilitate prairie restoration by inducing immobilization of plant-available nitrogen. Early attempts to use this method, however, have had mixed results. Success of C addition depends on three conditions: weeds must suppress prairie species in the absence of C addition, weeds must be nitrophilic relative to prairie species, and C addition must result in a large enough decrease in N to alter the balance of competition among weeds and prairie species. We examined these conditions by comparing productivity of 10 weeds and 11 tallgrass prairie species under 14 levels of C addition, ranging from 84 to 3346 g C/m 2 . Carbon was tilled into the soil prior to planting. To control for non-N effects of C addition, N was added to a subset of plots. Relative to untreated plots, the highest level of C addition resulted in an 86% decrease in available NO3 -N, a1 43 increase in early season light availability, a 54% decrease in weed biomass, and a sevenfold increase in prairie biomass. Nitrogen addition significantly reduced or reversed all of these effects. Significant species-specific responses to C addition included decreased biomass for six annual weeds and increased biomass for six prairie species, one annual weed, and three perennial weeds. These results suggest that C addition may be a useful tool for restoring N-limited plant communities.

Plant nutrient efficiency : a comparison of definitions and suggested improvement

Selection of plant cultivars tolerant of low nutrient supply may increase productivity on low fertility soils and reduce fertilizer requirements. Considerable effort has been directed towards identifying ‘nutrient efficient’ species and germplasms, but the many different definitions for efficiency make the use of the term ambiguous. The concept of nutrient efficiency was evaluated using data from a study of differences in germplasm response to phosphorus (P) availability in white clover (Trifolium repens L.) and alfalfa (Medicago sativa L.) grown in a sand-alumina culture. Application of various criteria identified in the literature as measures of nutrient efficiency did not clarify differences between purportedly P efficient and inefficient germplasms. Germplasms differed in maximum shoot and total dry mass and in solution P concentration required to achieve 80% maximum yield, but not in tissue P concentration, internal P utilization, or P uptake per unit of fine root dry mass. Differences may have resulted from factors other than efficient use of available P. To reduce the confounding effects that other factors have on nutrient efficiency, we propose that equivalent yields of germplasms be demonstrated where nutrients are not limiting. Mechanisms that enable enhanced nutrient efficiency can be identified less ambiguously using this improved approach.

Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide

Elevated atmospheric CO2 increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO2on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO2 levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO2 in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO2 concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO2 on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO2 on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO2 enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO2, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N2 fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N2-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha−1 and 60 kg N ha−1, with no differences due to N2 fixation capacity or root system architecture.

Defining phosphorus efficiency in plants

The many different definitions for “nutrient efficiency” make the use of the term ambiguous. We evaluated nutrient efficiency using data from a study of response to phosphorus (P) supply in white clover (Trifolium repens L.) and lucerne (Medicago sativa L.). Application of various criteria identified in the literature as measures of nutrient efficiency did not clarify differences between purportedly P efficient and inefficient germplasm. Germplasm differed in maximum shoot and total dry mass and in solution P concentration ([P]s) required to achieve 80% maximum yield, but not in P concentration of tissue ([P]t), internal P utilization, or P uptake per unit of fine root dry mass. Differences in yield may have resulted from factors other than efficient use of P. To reduce the confounding effects that other factors have on nutrient efficiency, it is essential that equivalent yields of germplasm be demonstrated where nutrients are not limiting. Mechanisms that enable enhanced nutrient efficiency can be identified less ambiguously using this approach.

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N2, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO2. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO2 response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO2, and (3) elevated CO2 stimulates symbiotic N2 fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO2 concentration (365 and 700 μmol mol−1) and N addition on whole plant growth and C and N acquisition in an N2-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO2 response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m−2 yr−1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N2 fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO2enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO2 increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO2 regardless of N treatment. Although partial photosynthetic acclimation to CO2 enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO2 (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO2. Elevated CO2 increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N2 fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N2-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO2 that are independent of soil N supply. The enhanced amount of N derived from N2 fixation under elevated CO2 presumably helps meet the increased N demand in N2-fixing species. This response may lead to modified roles of N2-fixers and N2-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO2.

Direct assessment of symbiotically fixed nitrogen in the rhizosphere of alfalfa

Rhizodeposition has been proposed as one mechanism for the accumulation of significant amounts of N in soil during legume growth. The objective of this experiment was to directly quantify losses of symbiotically fixed N from living alfalfa (Medicago sativa L.) roots to the rhizosphere. We used 15N-labeled N2 gas to tag recently fixed N in three alfalfa lines [cv. Saranac, Ineffective Saranac (an ineffectively nodulated line), and an unnamed line in early stages of selection for apparent N excretion] growing in 1-m long polyvinylchloride drainage lysimeters in loamy sand soil in a greenhouse. Plants were in the late vegetative to flowering growth stage during the 2-day labelling period. We determined the fate of this fixed N in various plant organs and soil after a short equilibration period (2 to 4 days) and after one regrowth period (35 to 37 days). Extrapolated N2 fixation rates (46 to 77μg plant−1 h−1) were similar to rates others have measured in the field. Although there was significant accretion of total N in rhizosphere compared to bulk soil, less than 1% was derived from newly fixed N and there were no differences between the ‘excreting’ line and Saranac. Loss of N in percolate water was small. These results provide the first direct evidence that little net loss of symbiotically-fixed N occurs from living alfalfa roots into surrounding soil. In addition, these results confirm our earlier findings, which depended on indirect 15N labelling techniques. Net N accumulation in soil during alfalfa growth is likely due to other processes, such as decomposition of roots, nodules, and above ground litter, rather than to N excretion from living roots and nodules.

Nutrient supply and neutralizing value of alfalfa stem gasification ash

Ash application influences availabilities of plant micronutrients either directly through addition of its miEnergy generation from biomass is an environmentally sound altercronutrients constituents or indirectly through the modinative to other energy producing technologies. Pilot studies have indicated that alfalfa (Medicago sativa L.) is a suitable feedstock for fication of soil pH. Increased availability of B and Mo energy generation via the gasification process. The resulting ash is a has been reported in soils amended with ash from power potential liming agent and a source of plant nutrients. A growth plants (Codling and Wright, 1998; Hammermeister et al., chamber study was conducted with three soils to evaluate the potential 1998). Clapham and Zibilske (1992) reported that wood use of this ash as a soil amendment. Corn (Zea mays L.) received 13 ash application increased acid-extractable (pH 3, 1 M treatments: control, K and/or P fertilizer, seven ash rates (0.6 to 14.6 g ammonium acetate [NH4OAc]) soil Fe, Zn, Cu, and Mn, ash kg 1 soil), and one ash rate with K or P fertilizer. Soil pH increased but in another study, wood ash decreased extractable with ash application on all soils. Ash application increased ammonium Fe and Al (Naylor and Schmidt, 1986). acetate-exchangeable K, Ca, and Mg, and Olsen P in soil and deModification of soil chemical properties by ash applicreased DTPA-extractable soil Fe, Mn, Ni, and Pb. Averaged across cation has resulted in altered elemental composition of the three soils, slopes of the cations recovered in plant and soil vs. cations applied in the ash were 0.48, 0.21, and 0.22 of total ash K, plants. Wood ash increased the K concentration in corn Ca, and Mg, respectively (r 2 0.97). Ash significantly increased plant and winter wheat (Triticum aestivum L.) in greenhouse K and Mo, and decreased Mg, Mn, and Zn concentration. Tissue P studies (Erich, 1991; Etiegni et al., 1991a) and alfalfa in concentrations were not affected by ash, but increased with P fertilizer. field studies (Meyers and Kopecky, 1998). Wood ash at Phosphorus fertilizer increased plant dry mass (DM), but K fertilizer rates below 20 g kg 1 increased P concentration in wheat did not, thus K did not limit yield. Alfalfa stem gasification ash is a (Etiegni et al., 1991a). Other studies have shown that P potential liming agent, a source of K, and would not lead to excessive availability can increase or decrease in ash-amended soils accumulation of trace elements in soil or plants when applied at rates (Erich, 1991; Voundinkana et al., 1998; Moliner and based on lime or K need. Street, 1982; Elseewi et al., 1980). Application of either coal fly ash or wood ash decreased tissue Zn, Fe, and Mn, and increased B and Mo (Elseewi et al., 1980; FranA has been recognized as an excellent soil amendcis et al., 1985; Naylor and Schmidt, 1989). Application ment since well before Jared Eliot wrote in 1748, of wood-fired boiler ash decreased the concentrations “Ashes is allowed on all hands to be some of the best of Mn and Cu in bean (Phaseolus vulgaris L.) plants dressing or manure for land; it enricheth much and lasts (Krejsl and Scanlon, 1996). These effects on plant microlong; but the misery is we can get but little” (Eliot, 1934; nutrient concentration may have implications for human Carman, 1934). Eliot was referring mainly to wood and or animal health. coal ash, which are available today in large amounts from Plant DM production has increased, decreased, or electricity generation. Effects of coal and wood ash on remained unchanged after coal or wood ash application, soil chemical properties, and on plant yield and eledepending on factors such as type and rate of ash applimental composition have been investigated, and recent cation, plant species, and soil properties. Oat (Avena reviews include Hammermeister et al. (1998) on coal sativa L.) grown in soil amended with 30 Mg ha 1 of ash and Mitchell and Black (1997) and Vance (1996) on wood-fired boiler ash produced significantly higher DM wood ash. than plants grown in nonamended soil, but DM declined The major effects of land application of ash are changes at 50 Mg ha 1 ash (Krejsl and Scanlon, 1996). Wood in soil pH and nutrient availability. Ash has been used ash application had no effect on production of spinach historically and primarily as a liming agent or K source. (Spinacia oleracea L.) in a greenhouse study (Clapham The liming potential or calcium carbonate equivalent and Zibliskie, 1992) nor on wheat yield in a field study (CCE) of ash is dependent both on the type of ash and (Huang et al., 1992). soil chemical properties (Clapham and Zibilske, 1992). In recent years, utilization of herbaceous species as Wood ash is generally rich in oxides, hydroxides, and biomass fuel for electricity generation has been viewed carbonates of Ca, K, and Mg, and contains small quantias an environmentally viable option. Preliminary studies ties of micronutrients (Erich and Ohno, 1992; Mitchell have indicated that alfalfa stems are suitable feedstock and Black, 1997). for energy generation via the gasification process (Wilbur et al., 1998). If the ash from alfalfa gasification can M. Mozaffari, C.J. Rosen, and E.A. Nater, Dep. of Soil, Water, and be utilized as a soil amendment, then economic viability Climate, 439 Borlaug Hall, 1991 Upper Buford Circle, Univ. of Minnesota. St. Paul, MN 55108; M.P. Russelle, USDA–ARS U.S. Dairy and public acceptance of this alternative energy source Forage Research Center (Minnesota Cluster) and Dep. of Soil, Water, may increase. Chemical characterization has indicated and Climate, 439 Borlaug Hall, Univ. of Minnesota. St. Paul, MN that alfalfa ash is a potential liming agent, its macronu55108; Joint contribution of the Minnesota Agric. Exp. Stn. and the trient content is equivalent to a 1-1-10 fertilizer, and it USDA-ARS. Received 8 Aug. 2000. *Corresponding author (crosen@ soils.umn.edu). Abbreviations: CCE, calcium carbonate equivalent; DM, dry matter; ICP-AES, inductively coupled plasma atomic emission spectroscopy. Published in Soil Sci. Soc. Am. J. 66:171–178 (2002).

Comment on "Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass"

Tilman et al. (Reports, 8 December 2006, p. 1598) argued that low-input high-diversity grasslands can provide a substantial proportion of global energy needs. We contend that their conclusions are not substantiated by their experimental protocol. The authors understated the management inputs required to establish prairies, extrapolated globally from site-specific results, and presented potentially misleading energy accounting.

Soil nitrogen mineralization indexes and corn response in crop rotations

Predicting N availability from legumes to a subsequent crop has been problematic. We tested the hypothesis that corn (Zea mays L.) grain yield and whole plant N accumulation could be predicted from N mineralization indexes of soil samples containing representative amounts of incorporated residues from the previous crop. Soil samples were taken from a crop rotation study conducted at four locations in Minnesota, in which corn was grown following eight crop treatments, including fallow, alfalfa (Medicago sativa L.), soybeans [Glycine max L. (Merr.)], corn, and wheat (Triticum aestivum L.). Corn received from 0 to 224 kg of fertilizer N/ha. Soil was procured from the plow layer during the 2 weeks before planting and to 1.5 m (for inorganic N) within 1 week after planting. Subsamples were subjected to acid permanganate, autoclave, and glucose extractions, inorganic N determination, and aerobic and anaerobic incubations. With stepwise multiple regression, 1 week of aerobic incubation contributed as much as did incubation times up to 12 weeks to models of grain yield and total N uptake at physiological maturity. Results of acid permanganate, autoclave, and glucose extractions, and of anaerobic incubation did not consistently contribute to the models. Over all locations, topsoil inorganic N and 1 week of aerobic incubation explained between 65 and 81|X% of the variability in grain yield and total N accumulation of nonfertilized corn. For fertilized corn, N application rate alone accounted for the majority of variability in grain yield and total N uptake. Two independent crop rotation experiments provided data used to validate the predictive capability of the regression models. Despite promising relationships derived from the initial experiment, results from validation experiments were not reliably predicted by these equations. Although analyses of soil samples containing crop residues for inorganic soil N and a particular N mineralization index may relate well to yield and N uptake by corn in a given year, variability among years may preclude general use of these models for predictive purposes.