Nonsymbiotic Dinitrogen Fixation in No-till and Conventional Wheat-fallow Systems1

Abstract

The objective of this study was to determine if differences in nonsymbiotic N, fixation could account for the positive.N budgets in the plant-soil system of no-till soils as compared with balanced or negative budgets under conventional tillage conditions (stubble mulch and plow) in a winter wheat (Triticum aestivum L.) crop-fallow rotation. This study was conducted on a Duroc silt loam (fine silty, mixed, mesic, Pachic Haplustolls) in western Nebraska from May 1982 to September 1983. Nonsymbiotic N2 fixation was estimated using the C2H2 reduction assay on intact soil cores from plow, stubble mulch, no-till, and native sod management practices. Nitrogen fixation potentials of no-till soils were twofold greater than those of plowed soils and were positively correlated with higher soil-water contents for reduced tillage. The amount of N, fixed during one cycle of the wheat-fallow rotation, however, as calculated from the C2H2 reduction assay, was 0.33 kg N ha ' yr~', which falls far short of the positive budget of 26 kg N ha~' yr~' found in the no-till soil. Lower incubation temperatures, which simulated soil temperatures of tillage systems in the field, resulted in an even lower fixation than at 25°C. Additional Index Words: N budget, C2H2 reduction, plow, stubble mulch, Triticum aestivum L. Lamb, J.A., J.W. Doran, and G.A. Peterson. 1987. Nonsymbiotic dinitrogen fixation in no-till and conventional wheat-fallow system. Soil Sci. Soc. Am. J. 51:356-361. A STUDY OF CULTIVATION in relation to soil N losses ./A. from a virgin grassland soil was made at the High Plains Agric. Lab., Sidney, NE (Lamb et al., 1985). A N budget for the soil and plant system was made after 12 yr of cultivation under three different tillage systems. Results revealed that the no-till system had a positive N budget while the plow and stubble mulch budgets were balanced. The objective of this study was 1 Contribution of the Dep. of Agronomy, Univ. of Nebraska-Lincoln. Published as Paper no. 8141, J. Series, Nebr. Agric. Exp. Stn., Lincoln, NE 68583. The research was conducted under Project no. 12-73. Received 6 May 1986. 2 Former Graduate Assistant now Assistant Professor, Northwest Exp. Stn., Univ. of Minnesota, Crookston, MN 56716; Soil Scientist, USDA-ARS, Dep. of Agronomy, Univ. Nebraska-Lincoln; and Professor, Dep. of Agronomy, Colorado State Univ. Fort Collins, respectively. to determine if nonsymbiotic N2 fixation potential would account for the additional N in the no-till system. Moore (1966) reported that many organisms—Azotobacter, Clostridium, other aerobic and anaerobic bacteria, and cyanobacteria—are capable of fixing atmospheric N2 under field conditions. Pederson et al. (1978) reported that only one of 109 western Nebraska soils cropped to winter wheat demonstrated a potential to fix N2. Paul et al. (1971) found that cultivated wheat (Triticum aestivum L.)-fallow soils in Canada fixed only small quantities of N2 nonsymbiotically. The grassland areas they studied only fixed approximately 1 kg N2 ha~' yr~' and were considered systems in equilibrium where N losses and gains were small. Virgin soil fixed a larger amount than cultivated soil subjected to a wide range of cropping conditions. Steyn and Delwiche (1970) studied nonsymbiotic N2 fixation in California soils with the C2H2 reduction technique. They reported N2 fixation values from 2.1 to 4.8 kg ha~' yr^. The lowest N2 fixing soils were in native grassland. Fallowed soil and soil cropped to wheat fixed 3.5 and 40 kg N2 ha~' yr., respectively. All of their measurements were made in the laboratory on disturbed soil cores. Soil moisture and available soluble energy sources were identified as major factors limiting N2 fixation. Jurgensen (1973) concluded that the extent of nonsymbiotic N2 fixation in cultivated soils was small, but long-term N gains may be significant where crop residue was not removed. In a study conducted in the 1979 to 1980 season, Roper (1983) reported a greater amount of nonsymbiotic N2 fixation in a straw-incorporated treatment as compared to incorporation of a treatment where the straw was burned before cultivation. Evidence in the literature (Steyn and Delwiche, 1970; Moore, 1966; Jurgensen, 1973), points to the possibility that a no-till system, where crop residue is left on the soil surface, may have a significant potential for nonsymbiotic N2 fixation. Furthermore, the surface residues under no-till also increase the soil moisture content compared to conventional tillage systems such as stubble mulch and plow. This creates a more favorable environment for N2 fixation. No-till LAMB ET AL.: NONSYMBIOTIC DINITROGEN FIXATION 357 systems could therefore have enough nonsymbiotic N2 fixation to result in positive N budget as compared to N budgets for conventional systems. MATERIALS AND METHODS