Effects of continuous replacing equal amount of chemical fertilizer nutrients with rice straw and straw biochar on rice yield and nitrogen use efficiency in cold region

Objectives Straw returning is one of the important measures to improve soil quality, crop productivity and reduce chemical fertilizer input simultaneously. We studied the growth, yield of rice and nitrogen fertilizer use efficiency when replacing partial fertilizer with rice straw and straw biochar in the long run, to provide scientific support for the optimal management of rice straw resources.

Methods The long-term field experiment, located in Shenyang Agricultural University, was established in 2013. The five treatments were: merely chemical NPK fertilizer (NPK), and replacing equal amount of chemical fertilizer nutrients with 1.5 t/hm² rice straw biochar (low-input rate, LB), 3.0 t/hm² rice straw biochar (high-input rate, HB), 4.5 t/hm² rice straw (low-input rate, LS), and 9.0 t/hm² rice straw (high-input rate, HS). The tiller number, chlorophyll content, plant height, grain yield and yield components of rice were investigated. The partial factor productivity of nitrogen fertilizer (PFP_N) were calculated.

Results Compared with the high rate treatment, LB and LS treatment increased the total tiller number by 27.1% on average. And LB increased the tiller number by 13.9% than LS treatment at the end of tillering stage (P<0.05). At the tillering stage, the chlorophyll content (CCI) and plant height of LB and LS treatment were significantly higher than those of HB and HS, with increases of 25.7% and 10.4%, respectively. The CCI and plant height in LB treatment were 9.9% and 5.9% higher than in LS treatment at tillering stage, however, the CCI and plant height in LS treatment were 11.5% and 4.0% higher than in LB treatment from the jointing to ripening stage. Compared with NPK treatment, LB and LS significantly increased grain yield by 6.5% and 6.2%, HS had similar grain yield, while HB significantly decreased grain yield by 20.3%. As a result, LB, LS and HS significantly improved PFP_N by 11.7%, 26.7% and 49.0%, while HB decreased PFP_N by 11.7%. According to the correlation analysis, the contribution to yield increase in LB treatment was contributed by tiller number (25.2%), effective spike number (2.1%), and seed-setting rate (0.8%); that in LS treatment was by plant height (5.2%), and 1000-grain weight (2.7%).

Conclusions Continuous straw or biochar replacement of chemical fertilizer at low rate could stimulate the tillering and growth of rice plants, increase rice yield and the partial factor productivity of nitrogen fertilizer. Replacing chemical fertilizer with high rate of straw could maintain rice yield and enhance the partial factor productivity of nitrogen fertilizer greatly, while high rate of biochar will bring about yield decline risk in the long run.