Effects of tillage and nitrogen addition rate on nitrogen metabolism,grain yield and protein content in wheat in lime concretion black soil region

Aims The purpose of this study was to determine a suitable combination of tillage method and nitrogen rate to improve wheat (Triticum aestivum) yield and protein content in lime concretion black soil.
Methods Under the field experimental conditions, three tillage methods (subsoiling and rotary tillage, rotary tillage, and conventional tillage) were used as the main treatments, and four nitrogen application rates (0, 120, 225 and 330 kg·hm^–2) were used as sub-treatments. Nitrogen assimilation after jointing stage, grain yield, and protein content were determined in wheat plants to study the effects of different tillage methods and nitrogen application rate on these variables.
Important findings Results showed that the glutamine synthetase (GS) activity, free amino acid content, and soluble protein content in wheat plants initially increased and then decreased during growth. The peaks of GS activity, free amino acid content, and soluble protein content occurred 10 days after flowering in the subsoiling treatment with 225 or 330 kg·hm^–2 nitrogen application rate, and at the flowering stage for other treatment combinations. Compared with the conventional tillage and rotary tillage, the bulk density of 10 to 40 cm soil in the subsoiling treatment was significantly reduced, and the soil total porosity and root dry weight were significantly increased. Tillage method and nitrogen application rate had a significant impact on grain yield and protein content in wheat plants. Grain yield and protein content were highest in the subsoiling treatment. Regardless of the tillage method, the grain yield and protein content both increased with increasing nitrogen application rate. The grain yield in the subsoiling treatment was highest with nitrogen application rate at 330 kg·hm^–2, whereas the outputs of conventional tillage and rotary tillage were peaked at nitrogen application rate of 225 kg·hm^–2. The grain proteincontent was highest at nitrogen application rate of 225 kg·hm^–2 under the three tillage methods. Thus, subsoiling with optimum nitrogen rate should be promoted in lime concretion black soil. Subsoiling increased grain yield and protein quality by improving soil conditions and the absorption of root systems for soil nitrogen.