Introduction 
Sulfur (S) is the fourth most required element for rapeseed growth and production. The need for S in rapeseed is significantly higher than that of other crops, and due to the increased use of S-free fertilizers, the deficiency of this element in agricultural soils under cultivation of this species is aggravating. The application of nitrogen (N) in soils deficient in S exacerbates the deficiency of S absorption by rapeseed and subsequently reduces the efficiency of N fertilizer used. Conversely, with the combined application of N and S at appropriate levels, grain yield of different crops, particularly rapeseed, can be significantly increased. This highlights the importance of a balanced application of N- and S-containing fertilizers in rapeseed production. The aim of this study was to shed light on the interaction of S and N supply in rapeseed growth and productivity.

Materials and Methods
In order to investigate the effects of S and N application on growth characteristics and grain yield of rapeseed (Hayola 50 cultivar) under rainfed conditions, this experiment was conducted as a factorial based on a randomized complete block design with three replications at Gachsaran Agricultural Research Station during the two cropping seasons: 2019-2020 and 2020-2021. Experimental treatments consisted of: 1- S fertilizer (S) in three forms including S0: no sulfur consumption, SC: consumption of 40 kg of pure sulfur in the form of ammonium sulfate SB: bio sulfur at the rate of 40 kg ha-1 of pure sulfur, and 2- nitrogen fertilizer (N) at three levels: N0: no nitrogen application, N50: 50 Kg ha-1 pure nitrogen (in urea form) and N100: 100 kg ha-1 nitrogen. The seeds were planted in a wheat-rapeseed rotation within a conservation tillage system. In the cropping season 2018-2019, the amount of rainfall was 764 mm, which was significantly higher than the long-term average. Of this amount, 332 mm occurred in autumn, 256 mm in winter, and 176 mm in spring; thus, most of the precipitation occurred in autumn, i.e. prior to planting. In the cropping season 2020-2021, the amount of rainfall was 435 mm, which was approximately equal to the long-term average. The distribution of rainfall was 175 mm in autumn, 135 mm in winter, and 124 mm in spring.
Results and Discussion
Results showed that the application of S in both chemical and biological forms caused a significant increase in grain and dry matter yield. Yield components, thousand kernel weight, pods/plant and grains/pod were significantly affected by S and N application. The application of S and N was associated with a significant increase in dry weight at flowering and subsequent remobilization of dry matter. In fact, in the conditions of no application of S, application of N at the levels of 50 and 100 caused an increase of 16 and 33% in the accumulation of dry matter at the time of flowering, respectively. Meanwhile these increases were 17 and 50% with the use of chemical S and 29 and 52% with the use of biological S, respectively. Also, the S and N fertilizers indicated synergistic effects in relation to their effects on dry matter accumulation, grains/pod and grain yield. The effect of S on the one thousand grain weight was not significant. Meanwhile, with increase of N fertilizer level, this yield component increased significantly, as the one thousand grain weights were 3.27, 3.30, and 3.34 g in the presence of 0, 50, and 150 kg ha-1 N, respectively. 

Conclusions 
In general, application of 100 kg ha-1 of N along with 40 kg ha-1 of S in the biological form was the most appropriate treatment. The use of S in both forms had a positive effect on plant growth and also increased N effectiveness. The advantage of bio-sulfur over ammonium sulfate for rainfed cultivation, in addition to improving the microbial and chemical balance of the soil, is its application in one step and gradual availability to the rapeseed plants. Given the slow release of S, the amount of leaching is reduced and the availability of bio-sulfur is better ensured at sensitive stages of plant growth.