Interactions of water,mulch and nitrogen on sorghum in Niger

We tested the hypothesis that plants only stimulate net mineralization of N when intense competition for N exists between plants and heterotrophs. Nitrogen mineralization in the soil used was insensitive to the range of moisture fluctuations that were inevitable during plant growth. Pots were planted to wheat (Triticum aestivum L.) or left unplanted and received no straw, straw added in one central layer, or straw added uniformly through the whole soil volume. Through the addition of¹⁵ N-labelled nitrate, initial soil inorganic N was increased to 17 g g^–1 in unplanted treatments and to 17 g g^–1 and 72 g g^–1 in planted treatments. Straw addition increased microbial immobilization of labelled N (soil inorganic N at planting), but did not reduce net mineralization of unlabelled soil N (soil organic N at planting), indicating that straw decomposers immobilized N early in the growth period. Plant growth did not reduce immobilization of N by straw decomposers. Net mineralization of N was not affected by plant growth at the low 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 mineralization at high N levels due to increased immobilization.