Effect of ammonium and phosphorus supply of H+ production in gel by two tropical forage grasses

The effect of the supply of ammonium (NH4+) and phosphorus (P) in gel on the amounts of hydrogen ion (H+) excreted from plant roots was studies with Brachiaria humidicola (a highly acid-soil tolerant tropical grass) and B. brizantha (less acid-soil tolerant) grown in soil in a glasshouse. The H+ production was measured over 24 h in agar gel containing full nutrient solution with a range of NH4+ -N levels (0, 0.25, 0.5, and 5.0 mM NH4+ -N). Highly soluble P, K2HPO4, or relatively insoluble P, rock P, was supplied at four concentrations (0, 11.5, 34.5 or 115 micromolar P) in the gel. Increasing NH4+ concentration in the gel increased H+ production for both grasses, but there was some inhibition of growth for B. brizantha at the highest N concentration. For B. humidicola, but not B. brizantha H+ production was greater with 34.5 micromolar K2HPO4 than 11.5 micromolar K2HPO4. At 34.5 micromolar P for both grasses there was no difference in H+ production when P was supplied as rock P or K2HPO4. With 11.5 micromolar P both grasses produced less acid in the gel with the rock P compared with K2HPO4. The reduced H+ production is probably due to a lower availability of P in the rock P compared with K2HPO4. This effect was greater with B. brizantha than B. humidicola, implying that 11.5 micromolar rock P was not able to supply sufficient P for the growth of B. brizantha. Brachiaria humidicola was able to dissolve more rock P than B. brizantha or alternatively, the growth of B. humidicola was less adversely affected by the low P supply from rock P than B. brizantha. Plant-induced acidity does not seem to occur as a response to a lack of available P, but rather these grasses only produce acid if there are enough nutrients for growth, i.e., both NH4+ and P. If either N or P is limiting, growth is limited as is NH4+ uptake, so that H+ production is curtailed.