Dinitrogen Fixation by Winter Chickpea Across Scales in Waterlogged Soil in a Mediterranean Climate

Grain legumes convert atmospheric N2 to reactive N through symbiosis with N2-fixing rhizobia. Biological N2 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 N2 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 N2 fixation (Doughton et al., 1993; Wheatley et al., 1995; Horn et al., 1996b, Goss & Varennes 2002; Reiter et al., 2002). Compared to conventional tillage practices, conservation tillage practices can stimulate N2 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 N2 fixation rate (Wheatley et al., 1995). In addition, Ferreira et al. (2000) showed that enhanced N2 fixation in soybean (Glycine max (L.) Merr.) was related to the higher population size and diversity of Bradyrhizobia in no-till compared to tilled systems. Across a landscape under a single tillage management system, however, considerable variation in N2 fixation by