Sustainability of nitrogen use in two dryland farming systems

Abstract

In the drier environments of rainfed agricultural systems both water and nitrogen are major limitations to crop productivity. In the Mediterranean region the traditional cereal-fallow system is being replaced by a cereal- legume rotation. An experiment to measure crop N uptake and biological nitrogen fixation in wheat/lentil and wheat/chickpea rotations was conducted in 1991/92. Recovery of ¹⁵N-labelled urea fertilizer by wheat was poor. Significant amounts of crop nitrogen came from biological nitrogen fixation in lentil (>50%) and chickpea (ca. 30%).

In Kenya farmers are attempting to grow intercrops of maize and bean in semi-arid regions. Experiments in four seasons measured crop N uptake and biological nitrogen fixation by maize/bean and maize/cowpea intercrops. Recovery of ¹⁵N-labelled ammonium sulphate fertilizer was poor in all crops. There was no measurable N fixation by bean. In contrast, cowpea fixed 50% of their N, and so provided a net input of N into the system.

These two dryland soils are characterized by rapid rates of mineralization-immobilization turnover and poor recoveries of N fertilizer. In the long-term the N status of these cropping systems can only be maintained or increased either by the use of N fertilizer, or by the inclusion of a legume. Lentil and cowpea appear well suited to replace the need for N fertilizer in the 2 areas described here.