Thursday, August 9, 2007 - 9:20 AM

COS 99-5: Dinitrogen fixation by winter-grown chickpea across scales in a Mediterranean climate

Juhwan Lee, Johan Six, and Chris van Kessel. University of California - Davis

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.