Effect of Different Phosphorus and Sulfur Levels on Nitrogen Fixation and Uptake by Chickpea (Cicer arietinum L.)
Nitrogen fixing capacity of leguminous plants can be increased by the supply of adequate amounts of nutrients, especially phosphorus and sulfur. Some nutrients have direct involvement in the process of nitrogen fixation while others affect by improving growth of host plant. Field experiments were conducted at two different locations in northern rainfed Punjab, Pakistan, to assess the amount of nitrogen fixation and nitrogen uptake by chickpea (Cicer arietinum L.) during crop growing seasons 2006-2007 and 2007-2008. Treatments were: phosphorus levels (0, 40 and 80 kg P2O5 ha-1); sulfur sources (gypsum and ammonium sulfate) and levels (0, 15 and 30 kg S ha-1). The experimental design was randomized complete block with split-split plot arrangement: phosphorus levels in main plots, sulfur sources in sub-plots and sulfur levels in sub-sub-plots. Phosphorus (80 kg P2O5 ha-1) increased nitrogen fixed and uptake by 33 and 31 % over control. However, effect of phosphorus on nitrogen derived from atmosphere was not significant while that of sulfur was significant. Sulfur (30 kg S ha--1) increased nitrogen derived from atmosphere, amount of nitrogen fixed and nitrogen uptake by 6, 25 and 17 % over control. Ammonium sulfate treatments caused significantly higher amount of nitrogen fixed and higher nitrogen uptake as compared to gypsum treatments, although both sulfur sources were similar regarding percent nitrogen derived from atmosphere. A strong positive correlation (R= 0.98**) between amount of nitrogen fixed and nitrogen uptake at both locations was observed. There was a direct effect of sulfur on the process of nitrogen fixation, whereas phosphorus mainly affected growth of chickpea.