The Phosphorus Nutrition of Two Grain Legumes as Affected by Mode of Nitrogen Nutrition

Date
1979
Authors
Cassman, Kenneth G. Kumar
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Two sand culture experiments were conducted to examine the effects of P stress, nodulation, and N source on the growth, dry matter distribution (DMD), and root development of soybean (Glycine~ (L.) Merr.). In both experiments two levels of nitrogen (O and 5.0 mM N) were employed to establish two contrasting modes of N nutrition: plants were either (1) solely dependent upon symbiotic N fixation, or (2) primarily dependent upon uptake of combined N from the nutrient solution. Total dry weight of N-fixing plants grown at optimal P levels was approximately 60% that of N-supplied plants. Mode of N nutrition had no effect upon the total dry weight of plants grown at deficient P levels. The DMD within the plant differed depending upon the P and N supply. Nodule dry weight of N-fixing plants grown at optimal P levels comprised 9% of the total plant dry weight and 61% of the root dry weight of 35 day old soybean. A decrease in the P supply inhibited nodule growth relatively more than either root or shoot growth. Nodule dry weight of N-supplied plants grown at optimal P levels comprised 2% of total plant weight and 15% of root dry weight and a decrease in the P supply affected shoot growth relatively more than either nodule or root growth. The higher yields of N-supplied plants resulted from increased early growth during the time when the plants not provided combined N were establishing an N-fixing, root-nodule system. When grown at suboptimal P levels, there was a similar partitioning of dry matter between the underground portion of the plant and the shoot in both N-fixing and N-supplied plants. However, the root:total plant dry weight ratio of N-fixing plants was significantly less than that of N-supplied plants. This difference was attributed to the larger nodule mass on N-fixing plants. There was an inverse relationship between nodule mass and total root length although the number of first-order lateral roots on nodulated and non-nodulated plants was the same. The data suggest that two functional equilibriums are operative in the N-fixing plant, namely, the partitioning of dry matter between (1) the underground portion of the plant and the shoot and, (2) the root and nodules. Phosphorus stress affected the root-nodule equilibrium relatively more than the partitioning of dry matter between the belowground and above-ground parts of the plant. In N-supplied plants, P stress primarily affected the partitioning of dry matter between the root and shoot. An N x P field experiment with a split-plot randomized block design was conducted to identify the critical external and internal P requirements of both soybean and cowpea (Vigna unguiculata (L.) Walp.) as affected by mode of nitrogen nutrition. Six P treatments were established in main plots on a Humoxic Tropohumult soil. Within each main plot an N deficient and N-luxuriant subplot was established: sugarcane bagasse was incorporated into the entire field at a rate of 16,000 kg/ha; N-deficient subplots received no urea-N applications while N-luxuriant subplots received applications of urea-N before planting and during crop growth totaling 1000 kg/ha. In vitro monitoring of the net N mineralization of incubated soil samples collected from N-deficient subplots during crop growth indicated that the bagasse immobilized most of the available soil N during the first seven weeks of crop growth and that plants in these treatments were predominantly dependent upon symbiotic N fixation to meet their N requirement for growth. Acetylene reduction activity and nodule dry weight of soybean and cowpea plants from N-luxuriant subplots sampled 32 and 46 days after emergence were less than 11% of their counterparts from N-deficient subplots grown at comparable P levels. This indicated that plants from the N-luxuriant treatments were primarily dependent upon uptake of combined N from the soil to meet their N requirement for growth. Results from the field experiment showed that soybean was more sensitive to low soil P than was cowpea. When grown without P or N fertilizer, soybean yielded 28% of the maximum yield obtained from optimal P treatments on N-deficient subplots while the comparable relative yield for cowpea was 72%. The sensitivity of N-fixing soybean to low soil P levels when grown on N-deficient soil could be characterized by: (1) a relative growth rate (RGR) which declined progressively throughout the crop growth period, (2) lower index tissue and seed P concentrations than the N-supplied soybean plants grown at comparable P levels although the critical internal P concentration required for 90% maximum yield of N-fixing and N-supplied plants was the same, (3) an external P requirement approximately 60% higher than N-supplied soybean plants and, (4) a relatively larger difference between the yield potentials of N-fixing and N-supplied plants than for cowpea. The tolerance of N-fixing cowpea to low soil P when grown on N-deficient soil could be characterized by: (1) a high RGR during the later stages of growth, (2) tissue P concentrations which were similar to those of N-supplied cowpea plants grown at comparable P-levels, (3) an external P requirement which was not affected by soil N availability and, (4) a relatively smaller difference between the yield potentials of N-fixing and N-supplied plants than for soybean. It was concluded that screening of grain legumes for tolerance to low soil fertility levels should be conducted on N-deficient soils to insure that nutrient requirements are assessed for the symbiotic, N-fixing plant.
Description
Keywords
Citation
Rights
Access Rights
Email libraryada-l@lists.hawaii.edu if you need this content in ADA-compliant format.