Effects Of Nitrogen Fertilizer Reduction And Biochar Application On Soil Properties And Rice Productivity And Nitrogen Use Efficiency

Nitrogen is one of the most important nutrients affecting rice growth and development.At present,the application rate of nitrogen fertilizer in China is high and the utilization rate is low.Excessive application of nitrogen fertilizer has caused a series of problems such as resource waste,environmental pollution,imbalance of soil nutrients,and reduction in crop yield and quality.As a soil conditioning agent,biochar has significant effects on carbon fixation,soil improvement,crop growth and fertilizer utilization.Field experiments were conducted to explore the impacts of nitrogen（N）reduction and biochar application on soil nutrient and rice productivity and nitrogen use efficiency in four seasons during 2018-2019.There were six treatments,including control（no N application,CK）,conventional N application(180 kg·hm^-2,N100),20%N reduction(144 kg·hm^-2,N80),20%N reduction plus biochar application（N80+BC）,40%N reduction(108 kg·hm^-2,N60),40%N reduction plus biochar application（N60+BC）.The main results are as follows:（1）In short term,nitrogen fertilizer reduction has non-significant effects on soil chemical properties.However,with the increase in application time nitrogen reduction combined with biochar can improve soil properties.Such as at mature stage in the later season of 2019,nitrogen reduction combined with biochar improved soil p H by 6.90%–7.66%,CEC by 69.88%,and organic matter by 24.57%–29.70%,over nitrogen reduction alone.Soil total nitrogen,total phosphorus,and total potassium were non-significantly influenced by nitrogen reduction combined with biochar application over nitrogen reduction alone,whereas the available nutrients were slightly influenced.At mature stage in the early and late seasons of 2018,20%nitrogen reduction combined with biochar application resulted in higher soil total nitrogen content than nitrogen reduction alone.At mature stage in the early season of 2019,20%nitrogen reduction combined with biochar substantially influenced nitrate nitrogen and available phosphorus content,over nitrogen reduction alone.（2）Nitrogen reduction combined with biochar substantially increased the effective tiller number and improved the maximal PSⅡefficiency（Fv/Fm）of rice at tillering stage.20%nitrogen reduction combined with biochar increased the accumulation of plant dry matter by 20.43%compared with nitrogen reduction alone at mature stage in late season of2018.40%nitrogen reduction combined with biochar significantly increased rice yield of early seasons by 15.78%and 9.98%in 2018 and 2019,respectively.（3）Nitrogen reduction combined with biochar can promote nutrient uptake in above ground rice plant parts.20%nitrogen reduction combined with biochar significantly increased nitrogen and potassium content in aboveground parts,over nitrogen reduction alone.At mature stage in late season of 2018 and early and late season of 2019,nitrogen content in aboveground parts increased by 42.30%,29.84%,and 22.05%,respectively,of which the nitrogen content in stem increased significantly by 52.25%,19.96%,and 47.03%,respectively.Potassium content in aboveground parts increased by 62.26%,52.35%,and38.42%,of which the potassium content in the panicle increased significantly by 45.73%,39.89%,and 55.48%,respectively.Phosphorous content in aboveground plant parts were non-significantly influenced by both nitrogen reduction alone and 20%nitrogen reduction combined with biochar.However,40%nitrogen reduction combined with biochar has significantly increased phosphorous content at the mature stage in both early and late seasons of 2019.（4）Compared with conventional nitrogen application,nitrogen reduction alone has non-significantly influenced the agronomic efficiency of nitrogen fertilizer.However,nitrogen reduction combined with biochar substantially increased the agronomic efficiency of nitrogen fertilizer.40%and 20%nitrogen reduction combined with biochar increased the agronomic efficiency of nitrogen fertilizer by 71.70%–309.39%and 57.17%–142.07%in all four seasons,respectively.Nitrogen reduction combined with biochar substantially influenced agronomic efficiency,over nitrogen reduction alone.In the early season of 2018,40%nitrogen reduction combined with biochar increased agronomic efficiency by 118.89%,whereas,in the late season of 2018,20%nitrogen reduction combined with biochar increased agronomic efficiency by 115.92%in the late season of 2018.The effect of nitrogen reduction combined with biochar on the nitrogen apparent recovery was increased over time.In the late season of 2019,20%and 40%nitrogen reduction combined with biochar increased nitrogen apparent recovery by 186.23%and 85.49%over nitrogen reduction alone and increased significantly by 146.07%and 136.61%over conventional nitrogen application.Compared with conventional nitrogen application,nitrogen reduction and nitrogen reduction combined with biochar significantly increased the nitrogen partial factor productivity,and the effect of combined application of biochar was better than that of simple nitrogen reduction.40%nitrogen reduction combined with biochar increased nitrogen partial factor productivity by 69.88%–96.78%.In short,40%nitrogen reduction combined with biochar could be an appropriate option in improving soil health and crop productivity along with environmental protection.This study shows that nitrogen reduction combined with biochar can improve soil nutrient status,and under the premise of ensuring yield,to ensure fertilizer efficiency in southern China,which shows that a long-term application of biochar is a feasible management for reducing nitrogen and increasing efficiency in rice fields.