Co-limitation of nitrogen and water, and yield and resource-use efficiencies of wheat and barley

In semiarid Mediterranean environments, low nitrogen (N) and water availabilities are key constraints to cereal productivity. Theoretically, for a given level of N or water stress, crops perform better when co-limitation occurs. Empirical evidence of this theoretical concept with field crops is rather scarce. Using data from field experiments we evaluated whether N-use efficiency (NUE) and water-use efficiency (WUE) in small grain cereals increases with the degree of co-limitation. Four field experiments were carried out during three growing seasons including factorial combinations of bread wheat, durum wheat and barley, grown under different N fertiliser rates and water regimes. Yield gap was calculated as the difference between maximum attainable yield and actual yield while stress indices for N (NSI) or water (WSI) were calculated as the ratios between actual N uptake or water use and those required to achieve maximum yields, respectively. Water and N co-limitation was calculated as C WN = 1 – |NSI–WSI|. The relationships of yield gap, NUE and WUE with the different co-limitation indices were evaluated. Yield gap (range from –3.8 to –8.1 Mg ha–1) enlarged (was more negative) with the highest levels of stress and, as expected from theory, it was reduced with the degree of co-limitation. WUE ranged from 6.3 to 21.8 kg ha–1 mm–1 with the maximum values observed under conditions in which co-limitation increased. Reduction in yield gap with increased degree of co-limitation was mainly due to a positive effect of this variable on WUE.