calibration of soil available nitrogen and water content with grain yield of dry land wheat

introduction: nitrogen (n) is one of the most important growth-limiting nutrients for dryland wheat. mineral nitrogen or ammonium (nh4+) and nitrate (no3−) are two common forms of inorganic nitrogen that can serve as limiting factors for plant growth. nitrogen fertilization in dryland area can increase the use of soil moisture, and improve wheat yields to some extent. many researchers have been confirmed interactions between water stress and nitrogen fertilizers on wheat, especially under field conditions. because of water stress affects forms of nitrogen uptake that leads to disorder in plant metabolism, reduction in grain yield and crop quality in dryland condition. on the other hand, use of suitable methods for determining nitrogen requirement can increase dryland wheat production. however, nitrogen recommendations should be based on soil profile content or precipitation. an efficient method for nitrogen fertilizer recommendation involves choosing an effective soil extractant and calibrating soil nitrogen (total n, no3− andnh4+) tests against yield responses to applied nitrogen in field experiments. soil testing enables initial n supply to be measured and n supply throughout the season due to mineralization to be estimated. this study was carried out to establish relationship between nitrogen forms (total n, no3− andnh4+) in soil and soil profile water content with plant response for recommendation of nitrogen fertilizer. materials and methods: this study was carried out in split-split plot in a rcbd in dryland agricultural research institute (dari), maragheh, iranwhere n application times (fall, 2/3 in fall and 1/3 in spring) were assigned to the main plots, n rates to sub plot (0, 30, 60 and 90 kg/ha), and 7 dryland wheat genotypes to sub-sub plots (azar2, ohadi, rasad and 1-4 other genotypes) in three replications in 2010-2011. soil samples were collected from 0-20, 20-40, 40-60 and 60-80 cm in sub-sub plots in shooting stage (zgs32). ammonium measurement in the soil kcl extracts was down by spectrophotometry method and colorimetric reaction at 655 nm. also, absorption spectrophotometry method was used for determination of nitrate in soil extract based on its uv absorbance at 210 nm. in this method two measurements were carried out; one before (by zn coated by cu) and second after reduction of nitrate). using the difference between these two measurements, concentration of nitrate in the extracts was determined. soil water content was also measured with diviner 2000 after calibration in 0-20, 20-40, 40-60 and 60-80 cm soil profile in sub-sub plots. after wheat harvest, the most suitable regression model between soil mineral nitrogen (nm) and soil moisture (θ) was fitted with wheat grain yield by datafit version 9.0 software. results and discussion: the best model between soil n forms (nitrate, ammonium and mineral nitrogen) was calibrated between mineral nitrogen (nm) and soil moisture (θ) with crop response (y=a+bn_m+c ln⁡〖(θ)〗+dn_m^2+eln⁡〖(θ)〗^2+fn_m ln⁡〖(θ)〗) that explained 80% of dryland wheat yield variations. in this model, the contributions of mineral nitrogen (no3− +nh4+) were 26%, soil moisture 50% and their interactions 24%. according to this model, the effect of soil moisture on production of grain yield was 2.3 folds greater than the mineral n. these results are most suitable for sampling and calibration of mineral nitrogen in 0-40 cm in dryland wheat stem elongation (zgs32). critical value of soil mineral n was 41 kg/ha, equal to 18.0 mg nm/kg in this layer for obtaining higher grain yield (over 2500 kg/ha). according to regression model, application of 50 kg n/ha in autumn was able to provide nm critical level in 0-40 cm layer for dryland wheat genotypes under experimental conditions. also simulation model showed that nitrogen fertilizer increased grain yield and it is more than the soil mineral nitrogen. if the soil mineral nitrogen is 20 kg/ha or less in 0-40 cm soil layer, there may be increase of grain yield up to 4000 kg/ha through the application of nitrogen fertilizers. therefore, increasing of mineral nitrogen in the soil profile up to 20 kg/ha is not appropriate for wheat production in northwest of iran drylands. conclusion: it can be concluded that, there is a relationship between soil nitrogen and moisture content with dryland wheat response and suggested model can be used for nitrogen recommendations for dryland wheat. according to the model, the effects of nitrogen fertilizer application on grain yield were much more than the effect of soil mineral nitrogen. therefore, the increasing of soil nitrogen storage is not recommended in dryland conditions.