INFLUENCE OF COMPETITION FOR NITROGEN IN SOIL ON NET MINERALIZATION OF NITROGEN

We tested the hypothesis that plants only stimulate net mineralization of N when intense competition for N exists between plants and heterot rophs. Nitrogen mineralization in the soil used was insensitive to the range of moisture fluctuations that were inevitable during plant grow th. Pots were planted to wheat (Triticum aestivum L.) or left unplante d and received no straw, straw added in one central layer, or straw ad ded uniformly through the whole soil volume. Through the addition of N -15-labelled nitrate, initial soil inorganic N was increased to 17 mu g g(-1) in unplanted treatments and to 17 mu g g(-1) and 72 mu g g(-1) in planted treatments. Straw addition increased microbial immobilizat ion of labelled N (soil inorganic N at planting), but did not reduce n et mineralization of unlabelled soil N (soil organic N at planting), i ndicating that straw decomposers immobilized N early in the growth per iod. Plant growth did not reduce immobilization of N by straw decompos ers. Net mineralization of N was not affected by plant growth at the l ow rate of N addition, but was reduced at the high rate of N addition. We conclude that the influence of wheat growth on net mineralization of N depends on soil N availability, with reductions in net mineraliza tion at high N levels due to increased immobilization.