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

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.