Trend Analysis of Nitrogen Use and Productivity in Wheat (Triticum aestivum L.) Production Systems of Iran

At global level nitrogen (N) fertilizers had drastic effects on crop yields increment during the last century. However, high application rates of this input have resulted to environmental pollution all around the world in addition decreased yields per unit of applied N is also reported in some countries. To fulfill increasing demands for agricultural crops with conservative application of N fertilizers, increasing N use efficiencies is recognized as a sustainable management. This calls for systematic studies on N use efficiency and its components at crop, field and regional levels. However, N efficiencies of agricultural crops at national level are not fully analyzed in Iran. In this research, forty years (1960-2010) data on yield and N application rate were analyzed for yield trend, N efficiencies and its related components for wheat (Triticum aestivum L.) production systems of Iran.
Required data of wheat yield and nitrogen fertilizer application rates during the 40 years study period was obtained from official web sites of national agricultural statistics as well as Ministry of Jihad Agriculture. Using these data partial nitrogen productivity (kg yield kg N-1); nitrogen use efficiency (kg yield kg-1 N, ignoring soil N), nitrogen uptake efficiency (%); nitrogen utilization efficiency (kg yield kg-1 absorbed N); and relative contribution of Nitrogen to grain yield (%) was estimated based on previously reported methods. Yield and N fertilizer application rate were subjected to time series analysis and fertilizer rates were predicted for the next decade over the studied period.
The results indicated that during the studied period mean annual growth rate of wheat yield and nitrogen application were 2.9 and 6.9%, respectively leading to 3.4 fold increase in yield and 9.5 fold increase in N fertilizers so that fertilize application rate was changed from 25 to 240 kg ha-1. However, N fertilizer application rate in wheat production systems of Iran is reducing since 80’s and prediction based on time series indicted that this rate will not exceed 250 kg ha-1 by 2020. Partial N productivity was estimated as 22.5 kg grain .kg-1 applied N which is reduced linearly over the studied period with the slope of -0.19 kg ha-1.y-1. Mean N use efficiency (NUE) in wheat production systems over the country averaged over four studied decades was 14.5 kg grain kg-1 applied N. Separation of NUE to its components showed that on average 35.5% of applied N was recovered in wheat agroecosystems of Iran and due decreasing trend of recovery N uptake efficiency was reached to 30% in the late 80’s. However, mean N utilization efficiency was estimated as 40 kg grain kg-1 absorbed N and increased slightly during the last four decades. Relative contribution of N fertilizers to wheat yield was increased with a positive slope from 24% in the first decade of the study period to 32% after 40 years. Based on the results it seems that the decreasing trend in NUE of wheat production systems of Iran is mainly due to low uptake efficiency which was not compensated for by slight improvement of N utilization efficiency.
Nitrogen application rate in wheat production systems of Iran has increased expansively during the last four decades. However, nitrogen uptake efficiency in wheat production systems of the country is considerably low and has resulted in a decreasing trend in nitrogen use efficiency over the last 40 years despite relative increase of N utilization efficiency over this period. Increasing N recovery through protecting N losses at field level, genetic improvement of N utilization efficiency and regulation of N fertilizers price compared to the price of wheat grain are urgent priorities to sustain wheat yield at national level with lower N application rates.