A Meta Analysis on Nitrogen Fertilizer Experiments on Cereal Crops in Iran

Though chemical fertilizers increase crop production; their overuse has hardened the soil, decreased fertility, strengthened pesticides, polluted air and water, and released greenhouse gases, thereby bringing hazards to human health and environment as well. Using of chemical fertilizer in agriculture has a history of more than fifty years in Iran. Recently, nitrogen fertilizers consume more than 61 percent of the chemical fertilizer in our country. Globally, the role of chemical fertilizers especially nitrogen fertilizers in agricultural production has been widely studied over the past 50 years, and in our country a considerable amount of research in universities has been dedicated to studying in this field. Meta-analysis is a method for analyzing the results of various studies on a subject. In fact, meta-analysis is a type of research on other research to re-examine the various studies carried out on a particular topic, compare them statistically and, using specific statistical techniques, the results of all those studies combine into a single result. Experiments on the effects of nitrogen fertilizers on cereals yield have a long history in Iran. However, because of high variation in the results, a final conclusion is not readily achieved.
In this study, the researches of the effect of different levels of nitrogen fertilizers on yield and yield components of cereals (wheat, corn and rice) over the past 20 years have been investigated. These studies included a variety of scientific-research articles. So, 46 papers were selected and the information was extracted from them. To overcome such a difficulty meta-analysis was used to combine and re-analyze the data of independent experiments. For this, 46 published papers related to nitrogen application on cereals including 23, 14 and 9 papers, respectively on wheat, corn and rice were selected based on criteria to satisfy the required data for meta-analysis. Fertilizer application rates for wheat, corn and rice varied in the range of 20-250, 25-275 and 10-90 kg.ha-1 N, respectively.
About 86% of the experiments used treatments between 25 and 150 kg.ha-1 nitrogen, and the frequency of nitrogen levels above 200 kg.ha-1 was very low in corn experiments. In the case of wheat, the range of nitrogen levels in the experiments was between 20 and 250 kg.ha-1. In rice, the range of nitrogen uptake was far more limited than wheat and corn, it is between 10 and 90 kg ha-1. On average in all nitrogen levels, grain yield of wheat, corn and rice compared to control were increased by 2477, 4699 and 1509 kg.ha-1, respectively. Meta-analysis results showed that nitrogen fertilizers significantly increased both grain yield and Biological yield of the studied cereals. However, harvest index was not statistically affected. Maximum grain and biological yields were attained by 50-100 and 100-150 kg.ha-1 N, respectively. Meta-analysis was also conducted for each crop separately. The results indicated that optimal nitrogen levels for the grain and biological yields were 75-100 and 100 kg.ha-1 N except corn. For corn maximum amount could be achieved from 50-100 kg.ha-1 N for grain yield and from 50-100 kg.ha-1 N for biological yield. For rice, maximum of grain and biological yields were reported by 75-90 kg.ha-1 N. The studied cereals harvest index showed no significant response to nitrogen fertilizers.
The results of this study showed that despite the long history of using this inputs in the agricultural sector, the optimal use is still unclear. In this study, statistical comparisons between fertilizer levels were performed, while the ecological dimensions of nitrogen application were also significant. Further studies can compare types of efficacy, soil health and environmental aspects of nitrogen use and other chemical fertilizers, provide a wider range of possibilities for sustainable field management and sustainable management of the country.