Factors affecting soil acidification under legumes. III. Acid production by N₂-fixing legumes as influenced by nitrate supply

Abstract

Leaching of nitrate through soil profiles is a major cause of soil acidification. Legume species differ in their ability to take up nitrate (NO₃⁻) and in the degree to which soil NO₃⁻ impairs legume nodulation and N₂ fixation. This pot experiment examined species variation in N₂ fixation and acid production in grain legumes grown for 6 wk on a sandy soil with Ca(NO₃)₂ added at 0–56.7 mg N kg⁻¹ soil. The eight legumes studied were Lupinus angustifolius (‘Gungurru’), Lupinus albus (‘Kiev mutant’), Lupinus luteus (‘Teo’), Cicer arietinum (Selection T1587), Pisum sativum (‘Dundale’ and ‘Wirrega’), Vicia faba (‘Fiord’) and Lathyrus sativus (Selection 453). While number of nodules was not significantly affected by treatment with NO₃⁻, increasing supply of NO₃⁻ decreased nodule mass, with P. sativum ‘Wirrega’ being most sensitive and C. arientinum and V. faba least sensitive. The concentrations of nitrogen in plants generally decreased as the supply of NO₃⁻ increased. Increasing supply of NO₃⁻ decreased the percentage of N₂ fixation, most significantly in L. sativus and least in L. albus. In the absence of added NO₃⁻, the amounts of H⁺ produced per unit biomass (specific acid production) differed by as much as 2.2-fold among the species, with C. arientinum generating most H⁺, followed by L. angustifolius, L. luteus, L. albus, V. faba, L. sativus, P. sativum ‘Dundale’ and P. sativum ‘Wirrega’. This species variation was even greater at higher NO₃⁻ supply. In the presence of NO₃⁻ at up to 14.2 mg N kg⁻¹, acid production by these legumes correlated well with uptake of excess cations but not with the concentration of nitrogen in plants. For all species, release of H⁺ declined with increasing supply of NO₃⁻, decreasing most in L. sativus and least in C. arientinum. The results suggest that there might be scope for selecting grain legume species in order to minimize soil acidification by decreasing leaching of NO₃⁻.