FLOODING STRESS Dinitrogen Fixation by Winter Chickpea Across Scales in Waterlogged Soil in a Mediterranean Climate J. Lee, J. Six & C. van Kessel Department of Plant Sciences, University of California-Davis, Davis, CA, USA Introduction Grain legumes convert atmospheric N 2 to reactive N through symbiosis with N 2 -ﬁxing rhizobia. Biological N 2 ﬁxation associated with legumes and the subsequent decomposition of legume residues are the primary processes that replenish N removed by harvest in agroecosystems without the addition of fertilizer N (Galloway et al., 1995, Peoples et al., 1995). Therefore, legumes are often included in crop rotations to increase soil N fertility and reduce N fertilizer inputs for a succeeding cereal crop, leading to a general increase in grain yield of the crop. Legumes can also contribute to the reduction of crop disease and weed occurrence (Stevenson & van Kessel, 1996), and increase soil organic C content (Aslam et al., 2003; Kong et al., 2005). Biological N 2 ﬁxation in legumes and its potential to supply soil N and improve soil N balances can vary greatly depending on different agricultural management practices. Tillage has been shown to inﬂuence legume growth and the potential of N input from N 2 ﬁxation (Doughton et al., 1993; Wheatley et al., 1995; Horn et al., 1996b, Goss & Varennes 2002; Reiter et al., 2002). Com- pared to conventional tillage practices, conservation tillage practices can stimulate N 2 ﬁxation in legumes via enhanced N demand and nodulation (Hughes & Herridge, 1989; van Kessel & Hartley 2000; Reiter et al., 2002). Reduced residue inputs into soil by conservation tillage inhibit soil N mineralization, which can lead to a greater N 2 ﬁxation rate (Wheatley et al., 1995). In addition, Ferreira et al. (2000) showed that enhanced N 2 ﬁxation in soybean (Glycine max (L.) Merr.) was related to the higher population size and diversity of Bradyrhizo- bia in no-till compared to tilled systems. Across a landscape under a single tillage management system, however, considerable variation in N 2 ﬁxation by Keywords chickpea; Mediterranean; nitrogen ﬁxation; scale; tillage; waterlogging Correspondence J. Lee Department of Plant Sciences University of California One Shields Avenue, Davis CA 95616, USA Tel.: +1 530 754 7537 Fax: +1 530 752 4361 Email: ecolee@ucdavis.edu Accepted November 8, 2010 doi:10.1111/j.1439-037X.2010.00452.x Abstract Studies on N 2 ﬁxation by grain legumes during periods of winter waterlogging prone Mediterranean regions have rarely been performed across scales. Here, we quantiﬁed the spatial variability of N 2 ﬁxation by rain-fed chickpea (Cicer arietinum L.) at the ﬁeld- and micro-scales (0.15 m spacing) after waterlogging during the vegetative growth phase in the winter. We also determined effects of tillage (standard and minimum) and crop and soil variables on N 2 ﬁxation in water stressed conditions. After waterlogging, yield was greatly reduced but there were no visible signs of water stress or tillage effects on N 2 ﬁxation. At the ﬁeld scale, percent N derived from N 2 ﬁxation (%Ndfa) ranged from 51 to 93 % and was related to the amount of soil-derived N in the plant. Total grain N did not increase when N 2 ﬁxation increased and the amount of N derived from the soil was replaced with ﬁxed N. In contrast, %Ndfa at the micro-scale, ranging between 0 to 72 %, was primarily related to yield and total plant N whereas available soil N or any of the other measured soil properties were not signiﬁcant predictors of %Ndfa. Total N in the grain increased solely due to N 2 ﬁxation as the contribution from soil N remained constant. Although %Ndfa had a nearly pure nugget variance across the scales, total N derived from N 2 ﬁxation (gNdfa) showed a relatively high level of spatial correlation. The range of available soil N pools was likely different at the two scales, leading to differences in the responses of chickpea N 2 ﬁxation to available soil N. J. Agronomy & Crop Science (2011) ISSN 0931-2250 ª 2010 Blackwell Verlag GmbH, 197 (2011) 135–145 135