Influence of pH and aluminium on nitrogen partitioning in defoliated grasses

Reduced inputs of fertilizer and lime to grasslands can result in decreased soil pH, increased aluminium concentration in the soil solution and, consequent upon these, inhinition of nitrogen acquisition by plannts. This study tested the hypothesis that under conditions of low pH and high aluminium concentrations a grass species more tolerant of acid conditions ( Deschampsia flexuosa ) maintains a greater total supply of nitrogen to growing leaves after defoliation from both root uptake and mobilization of stores than a less tolerant species ( Lolium perenne ). The grasses were grown without alumimium at pH values 3.5, 4.0, 5.0 or 5.5 and at a range of aluminium concentrations (0, 20, 100, 200 or 1000 mmol m −3 ) at pH 4.5. Labelling with 15 NH 4 15 NO 3 after defoliation allowed estimation of both root uptake and mobilization of stores in supplying nitrogen to growing leaves. The total amount of nitrogen supplied to the growing the leaves after defoliation by the combination of current root uptake and mobilization was maximal when aluminium was in the range 20–20 mmol m‐3. The total nitrogen supplied to growing leaves the both species after defoliation was greater when the grasses were grwon at pH 5.0 and 5.5 than when grown at pH 4.0. L. perenned and D. flexuosa were affected similarly by both aluminium and pH with respect to the total nitrogen supplied to the leaves. The effects of aluminium an dpH upon the supply of nitrogen to growing leaves after defoliation my not be a dominant factor in the vegetation dynamics of grazed swards.