PRODUCTIVITY AND NITROGEN USE OF 3 DIFFERENT WHEAT-BASED ROTATIONS INNORTHWEST SYRIA

Productivity of 3 different 2-year crop rotations: namely continuous w heat, wheat-chickpea, and wheat-fallow, was measured over 4 consecutiv e seasons beginning in 1991-92 at the ICARDA station, Tel Kadya, Syria . Nitrogen (N) fertiliser (30 kg N/ha at sowing) was broadcast every o ther year in the continuous wheat only. N-15-labelled fertiliser was u sed to quantify the amount of nitrogen supplied to the crops through c urrent and past applications of fertiliser and by N-2 fixation. The re maining N in the crop was assumed to come from the soil.In any single season, wheat yields were unaffected by rotation or N level. However: 2-year biomass production was significantly greater (32%, on average) in the continuously cropped plots than in the wheat-fallow rotation. O n average, <10% of the N in the wheat crop came from fertiliser in the season of application, and <1.2 kg N/ha of the residual fertiliser wa s recovered by a subsequent wheat crop. Chickpea fixed 16-48 kg N/ha, depending on the season, but a negative soil N budget was still likely because the amount of N removed in the grain was usually greater than the amount of atmospheric NP fixed. Uptake of soil N was similar in t he cereal phase of all 3 rotations (38 kg N/ha, on average), but over the whole rotation at least 33% more soil N was removed from continuou sly cropped plots than from the wheat-fallow rotation, suggesting that the latter is a more sustainable system. A laboratory study showed th at although wheat and chickpea residues enhanced the gross rate of N m ineralisation by c. 50%, net rates of N mineralisation were usually ne gative. Given the high C/N ratio of the residue, immobilisation, rathe r than loss processes, is the likely cause of the decline in the miner al N content of the soil. Consequently, decomposition of crop residues in the field may in the short term reduce rather than increase the av ailability of N for crop growth.