Search

Summer fallow results in inefficient precipitation use and could potentially be replaced with a green manure (GM) crop that reduces fertilizer N application. A 12‐yr study on a Williams loam (fine‐loamy, mixed, superactive, frigid Typic Argiustoll) was used to determine GM impacts on soil‐N fertility, soil organic carbon (SOC), water use, yield, and water productivity (WP) of spring wheat (Triticum aestivumL.). Lentil (Lens culinarisMedikus cultivar Indianhead) was grown in rotation with nonfertilized spring wheat and killed either mechanically (GMMF) or chemically (GMCF). Inorganic N fertilizer treatments included annually cropped wheat (ACW), and wheat‐fallow rotations (WFR) with mechanical (MF) or chemical (CF) fallow management. Low soil nitrate during the first 5 yr of the study resulted in a 33% decrease (P< 0.05) in wheat yield in GM compared to WFR. During the latter 6 yr, wheat yield differed by 2%, due to 26% greater spring soil nitrate (0–0.6 m) in GM than WFR, and growing season change in nitrate N showed a 2.2‐fold N‐cycling advantage of GM over WFR. Water use during nonwheat periods was similar in GM and WFR when lentil was killed at full bloom, but when grown to lower pod set GM used 20% more water than WFR. After 11 yr, SOC in the surface 15 cm was 9.1% greater in treatments with chemical management than those with mechanical management. Green manure, with proper management, maintains WP and offsets fertilizer N needs after about three cropping cycles compared with traditional wheat–fallow rotations.

There are conflicting reports on the amount of N transferred from legumes to intercropped nonlegumes and on the need for N fertilizer to obtain high total dry matter yields on soil with low N availability. This study evaluates (i) the ability of fababean (Vicia fabaL.) to meet its N requirements from biological N2fixation: (ii) the amount of N transferred to intercropped oat (Avena sativaL.); and (iii) N fertilizer management to maximize dry matter production and N2fixation in a cold soil. A randomized complete block experimental design with four replications was used with two rows of fababean alternating with two rows of oat. Urea was applied in bands between fababean or oat rows at 90 kg N ha−1to get the following treatments: (i) oat with added N (+N), fababean +N, inoculated with Rhizobium leguminosarumbv. viceae(+I); (ii) oat +N, fababean without added N, inoculated (−N, +I); (iii) oat −N, fababean −N,+I; (iv) oat −N, fababean +N,−I; and (v) oat −N, fababean −N, + I. A second study compared uptake of 15N‐depleted fertilizer by oat and fababean, where the fertilizer was broadcast over the plot or band‐applied between rows of oat and fababean planted in alternate rows. Less than 1% of the uninoculated fababean plants nodulated in either experiment; thus, they were a good check for estimating N2fixation using the difference method. Nitrogen fertilizer increased dry matter production of fababean in some years, but did not increase N uptake. There was little evidence of N transfer from fababean to oat. Oat depended on N fertilizer for maximum dry matter production 3 of the 4 yr. Maximum total dry matter production was achieved by fertilizing the oat and not the fababean in 3 of the 4 yr. The amount of N2fixed by fababean averaged 50 kg N ha−1. Fababean plants obtained >50% of their N from broadcast urea, but <10% when urea was banded between oat and fababean rows. Configuring a commercial drill to band fertilizer between pairs of oat rows alternating with fababean is a practical way to maximize both N2fixation and total dry matter production.

Information on amounts of N2fixed by legumes in subarctic regions is lacking. We determined the N2‐fixation potential of seven legume species under field conditions in subarctic Alaska. Seasonal N2fixation was estimated for alfalfa (Medicago sativaL.), red clover (Trifolium pratenseL.), yellow sweetclover (Melilotus officinalisLam.), fababean (Vicia fabaL.), lentil (Lens culinarisMedik.), pea (Pisum sativumL.), and white lupin (Lupinus albaL.) on a neutral (pH 7.2) and an acid (pH 5.4) soil during two field seasons. A lime variable was included at the acid soil location. Dinitrogen fixation was estimated by the 15N isotope dilution method and by the total‐plant N difference method. Three non–N2‐fixing crops and uninoculated analogs at one location were used as reference crops. Agreement among reference crops and between methods of estimating N2fixation was adequate for most applications, but differences of >50 kg ha−1were sometimes observed. Maximum N2 fixation estimates by the isotope dilution method were 58 kg ha−1for alfalfa, 121 for red clover, 109 for sweetclover, 204 for fababean, 72 for lentil, 121 for pea, and 162 for white lupin. Liming usually did not significantly affect N2fixation. Nitrogenase activity, as measured by the acetylene reduction assay, peaked at or soon after flowering in some legumes; in others, it either plateaued or continued to increase after flowering until the end of the growing season.

Green manures (GM) may offset inorganic N needs and improve soil quality. Study objectives were to determine effects of green manure on soil‐N fertility, water use, soil quality, and yield of spring wheat (Triticum aestivumL.). On two treatments, lentil (Lens culinaris Medikuscv. Indianhead) was green manured in a green manure‐spring wheat rotation. Lentil was killed by disking (GMMF) or chemicals (GMCF). Additional treatments were annually cropped wheat (AW) in a mechanical fallow (MF) or chemical fallow (CF) sequence. No inorganic N was used on GMMF and GMCF. Experiments were started in 1991 on a Williams loam (fine‐loamy, mixed Typic Argiboroll) near Culbertson, MT. Green‐manure treatments used 56 mm more water than fallow treatments when lentil was grown to lowerpod set. When lentil was killed at full bloom, there were no differences in water use among GM and fallow treatments. There were no differences among treatments in soil water at wheat planting. Wheat yield was 25% less on GM than on MF and CF. Soil NO3‐N (0–0.6 m) was 35% less on GM than MF and CF rotations. There were no differences in soil quality indicators of bulk density, organic C, pH, electrical conductivity, and deep NO3‐N (0.6–1.8 m) among treatments after two cycles of GM. Potentially mineralizable N was 66% greater on GM treatments than on fallow treatments. Short‐term results (5 yr) show that available N limited wheat production more than did soil water on the GM treatments. Soil improvement using green manures may require many additional cropping cycles.

An understanding of the C and N dynamics of crop residues is important for efficient nutrient management. The present experiment was conducted to determine the rate of mass and N loss from alfalfa, faba bean, barley, and rape crop residues in a subarctic agricultural soil. Mass, C, and N losses were measured from residues contained in mesh bags and placed on the soil surface or buried 15 cm below the surface. The mass loss from October, 1988, to May, 1989, was the same for surface and buried alfalfa, barley, and rape residues, averaging 40, 20, and 61%, respectively, while surface and buried faba bean residue sustained 30 and 40% mass loss, respectively. The mass loss of the buried residues continued over the summer but not of those placed on the soil surface, resulting in an average 23% greater mass loss of the buried materials after 1 year. The N loss from October to May was similar from the surface and from the buried placements for the alfalfa, faba bean, and rape residues, averaging 11.3, 10.3 and 38.4 g N kg-1 residue, respectively, while the surface and buried barley lost 2.9 and 4.2 g N kg-1, respectively. The C:N ratio of all of the residues increased during the winter. These data indicate that the rate of decomposition and N mineralization from crop residues in subarctic environments can equal that measured in temperate climates. Furthermore, the concurrent loss of mass and N combined with an increase in the C:N ratio of the residues suggests that physical rather than biological processes were functioning during the winter. Most of the mass and N loss from these residues occurred during the winter, out of phase with crop demand, thereby creating the potential for N loss from the system and inefficient use of crop residue N.

Large-scale argicultural development in high latitude regions could lead to large losses of soil C due to accelerated decomposition. Changes in decomposition rates of forest floor material upon land clearing in interior Alaska were simulated by measuring, over a 2-year period, changes in mass, cellulose, lignin, and N of forest floor materials and in mass of filter papers and wood in a forest floor and a fallowed field. All materials decomposed slowly at the surface, with about 90% of the original weight remaining after 2 years. Decomposition rates were higher for materials buried in the field than the forest. Cellulose loss in forest floor materials closely followed mass loss, whereas lignin loss was not significant. However, weight loss of wood was rapid when buried in the field, with about 20% of the initial mass remaining after 2 years. Relationships between mass loss of buried forest floor materials and soil degree days were significant (r=70%–80%). Temperature was a major, but not the only factor, controlling decomposition rates. Forest floor materials showed significant N losses, indicating net N mineralization and that N deficiency was not a factor affecting decomposition. C loss to the atmosphere due to decomposition of forest floor materials after forest clearing will be minimal and similar to that in the undisturbed forest if left on the soil surface, but will be substantial if incorportated into the soil. Incorporation is necessary for cropping; thus some accelerated decomposition is unavoidable in clearing subarctic forests for cultivation.