Dinitrogen fixation by winter-grown chickpea across scales in a Mediterranean climate

This study was conducted to (1) quantify the spatial variability of N₂ fixation by irrigated chickpea (Cicer arietinum L.) at the field- and micro-scale (0.15 cm spacing), (2) determine the impact of minimum (MT) and standard tillage (ST) on N₂ fixation, and (3) determine which crop and soil parameters controls N₂ fixation. As tillage weakly affected %Ndfa except for yield, the tillage effect on N₂-fixing activity may be indirect through different chickpea growth. For the field study, percent N derived from N₂ fixation (%Ndfa) ranged from 51 to 93% and tended to be related to the amount of soil-derived N in the plant. In contrast, %Ndfa at the micro-scale level, ranging between 0 to 72%, was primarily related to chickpea yield and total plant N whereas available soil N or any of the other measured soil properties were not significant predictors of %Ndfa. Although %Ndfa had a nearly pure nugget variance across the scales, total N derived from N₂ fixation (gNdfa) showed a relatively high level of spatial correlation. At the field scale, total grain N did not increase when N₂ fixation increased and the amount of N derived from the soil was replaced with fixed N. In contrast at the micro plot scale, an increase in N₂ fixation increased total N in the grain but the increase was solely obtained from N₂ fixation as the contribution from soil N remained constant. The range of available soil N pools were likely different at the two scales, leading to differences responses of chickpea to N₂ fixation to available soil N. N₂-fixing activity by irrigated, field grown chickpea remained highly variable and largely unpredictable from any single soil measurement. These findings confirm that predicting and quantifying N input from N₂ fixation by a legume across a field and scales remains a challenge.