Effect of ammonium and phosphorus supply on h+ production in gel by two tropical forage grasses |
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Authors: | Katherine A.B. Logan Richard J. Thomas John A. Raven |
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Affiliation: | 1. Centro Internacional de Agricultura Tropical , Apartado Aereo 6713, Cali, Colombia;2. Department of Biological Sciences , University of Dundee , Dundee, DD1 4HN, UK;3. Centro Internacional de Agricultura Tropical , Apartado Aereo 6713, Cali, Colombia;4. Department of Biological Sciences , University of Dundee , Dundee, DD1 4HN, UK |
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Abstract: | 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 studied 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 NH/‐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 μM 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 μM K2HPO4 than 11.5 μM K2HPO4. At 34.5 μM P for both grasses there was no difference in H+ production when P was supplied as rock P or K2HPO4. With 11.5 μM 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 μM 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. |
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