Nodulation and nitrogen fixation by arrowleaf clover: Effects of phosphorus and potassium

“Yuchi” arrowleaf clover ( Trifolium vesiculosum Sav.) has a potential for high forage productivity with desirable symbiotic nitrogen fixation within most temperate regions. Our objective was to determine the effects of soil fertility on growth, nodulation, nitrogenase and associated enzyme activities of arrowleaf clover. In greenhouse experiments top growth increased with additions of 300 mg K kg −1 soil with and without 100 mg P kg −1 soil to a Cumulic Haplustoll (Port silt loam, pH 6.1). Nodule mass without P fertilizer additions increased linearly up to 400 mg K kg −1 soil. When both P and K fertilizer additions were combined nodule mass increased significantly only up to the 300 mg K addition. However, nodule weight, increased 4-fold with the PK combination treatments. Nitrogenase activity, as measured by C 2 H 2 reduction, more than doubled with P additions and increased linearly up to 400 mg K kg − soil, with and without the P additions. Aspartate amino-transferase (AST) activity of nodule cytosol more than doubled with P additions but increased only with up to 300 mg K without P. Highest AST activities were recorded with the 400 mg K addition when combined with P. Glutamate synthetase (GS) activity increased with up to 300 mg K without P addition, but when combined with P was approximately 3 times higher, increasing linearly to 400 mg K. Differences in glutamate synthetase (GOGAT) activity were not significant with K additions without P, but when combined with P treatments were almost doubled up to the 400 mg K concentrations. Multiple regression for nitrogenase (C 2 H 2 red.) as the dependent variable = 5.89 (AST) + 12.79 (GS) + 21.52 (GOGAT) + 13.53 (GDH); R 2 = 0.92 and C.V. = 15.6%. Nodule cytosol P and K compositions reflected soil treatment levels and combinations. Reciprocal effects of monovalent cations were highly significant, with increased K concentrations reducing Na content; nitrogenase = 0.12 (P) + 0.01(K) + 0.14(Ca) −0.34 (Na); R 2 = 0.86 and C.V. = 21.9%.