Effects Of Biochar Application On Water And Nitrogen Utilization And Greenhouse Gas Emissions In Black Soil Paddy Fields

In recent years,the rice market demand in Northeast China has increased,and the rice planting area in black soil of cold region expand rapidly.Then a series of problems appears,such as the rapid increase of water consumption,excessive application of nitrogen fertilizer,straw burning in the open air and the increase of greenhouse gas emissions.Therefore,the purpose of this study is to utilize rice straw as a resource,improve water and nitrogen use efficiency and reduce greenhouse gas emissions.Through the selection of reasonable water,biochar and nitrogen operation research mode,it can provide theoretical and technical basis for the simultaneous realization of water-saving,yield increasing,high efficiency and emission reduction of rice cultivation for the sustainable development of agriculture in northeast cold region.In this study,three factors including irrigation mode,nitrogen application rate and biochar application rate were used.Two water management modes:dry-wet-shallow irrigation(D)and conventional flooding irrigation(F),four levels of biochar application rate:0 t/hm²(B0),2.5 t/hm²(B1),12.5 t/hm²(B2),25 t/hm²(B3),and two levels of nitrogen application:85 kg/hm²(N1),110 kg/hm²(N2)were set.A ¹⁵N tracer micro area was set up in the experimental plot,and one micro area was set up in each experimental plot,that is,three sub processes were set up under the same treatment:M1,M2 and M3,with a total of 48 treatments.Compared with conventional flooding irrigation,¹⁵N tracer technology was used to subdivide the overall nitrogen use efficiency.The absorption and transportation of base fertilizer,tiller fertilizer and panicle fertilizer in rice under two irrigation modes,and the distribution of base fertilizer,tiller fertilizer and panicle fertilizer in rice organs at yellow-ripe stage.Residue of base fertilizer,tiller fertilizer and panicle fertilizer in soil after rice harvest and their distribution in different depth soil layers.The seasonal CH₄and N₂O emissions from paddy fields under different water,biochar and nitrogen managements were studied.The effects of soil temperature,inorganic nitrogen content and other environmental factors on the seasonal CH₄and N₂O emissions were analyzed.The relationships between environmental factors,nitrogen use and loss rate of paddy fields and CH₄and N₂O emissions were clarified,and the global warming potential and greenhouse gas intensity were calculated.Through the fuzzy comprehensive evaluation based on entropy weight method,the yield,water use efficiency,nitrogen absorption and utilization efficiency,nitrogen loss rate,GHGI,profit and other indicators were evaluated,and the water,biochar and nitrogen management mode suitable for water saving,yield increasing and emission reduction of black soil paddy field in cold region was selected.The main results are as follows:(1)The absorption and utilization rates of base fertilizer,tiller fertilizer and panicle fertilizer were 15.55%?23.93%,31.68%?53.24%,48.82%?77.73%,respectively.The results showed that the absorption and utilization efficiency of base fertilizer-¹⁵N under conventional flooding irrigation mode were higher than those under dry-wet-shallow irrigation,and increased with the increase of biochar application rate;the absorption and utilization rate of tiller fertilizer-¹⁵N under dry-wet-shallow irrigation were significantly higher than those under conventional flooding irrigation(P<0.05);when the application rate of biochar was 0?12.5 t/hm²,the uptake and utilization efficiency of fertilizer-¹⁵N increased with the increase of biochar application rate,and the uptake and utilization efficiency of fertilizer-¹⁵N under dry-wet-shallow irrigation were significantly higher than those under conventional flooding irrigation(P<0.05).Only 15.00%?22.06%of the nitrogen in rice grain at yellow-ripe stage came from fertilizer nitrogen accumulated in stems and leaves before heading-flowering.When the application rate of biochar was 0?12.5t/hm²,the contribution rates of base fertilizer-¹⁵N,tiller fertilizer-¹⁵N and panicle fertilizer-¹⁵N to grain increased with the increase of biochar application rate.The accumulation amount of base fertilizer,tiller fertilizer and panicle fertilizer in rice organs at yellow-ripe were panicle,stem and leaf from large to small.The results showed that there was a significant positive correlation between the total nitrogen uptake and utilization,fertilizer absorption and utilization efficiency of tiller and panicle and contribution rate of nitrogen transport of tiller fertilizer and panicle fertilizer to grain(P<0.01).There was a significant positive correlation between the total nitrogen uptake and utilization,the absorption and utilization efficiency of base fertilizer and contribution rate of nitrogen transport of base fertilizer to grain(P<0.05).There was a significant positive correlation between the absorption and utilization efficiency of nitrogen fertilizer at different stages and the contribution rate of nitrogen transport to grain(P<0.01).There was a significant positive correlation between the percentage of panicle fertilizer accumulation to total plant nitrogen accumulation at yellow-ripe stage and the contribution rate of panicle fertilizer absorption and utilization rate and nitrogen transport to grain(P<0.05).(2)When the biochar application rate was 0?12.5 t/hm²,the total residual amount of nitrogen fertilizer in soil increased with the increase of biochar application rate after rice harvest,and the difference between the two irrigation modes was significant(P<0.05)The total residue of nitrogen fertilizer in soil under dry-wet-shallow irrigation varied from 22.54 kg/hm²to 37.86 kg/hm²,26.52%?39.14%of the fertilizer-¹⁵N remained in soil,which was significantly higher than those under conventional flooding irrigation.The residual rates of base fertilizer-¹⁵N in soil were24.59%?52.16%,tiller fertilizer-¹⁵N residue rates were 24.24%?43.50%,and panicle fertilizer-¹⁵N residue rates were 11.58%?25.67%in soil.When the biochar application rate was 0?12.5 t/hm²,the residual amounts of base fertilizer and tiller fertilizer-¹⁵N under two irrigation modes increased with the increase of biochar application rate,while the residual amount of panicle fertilizer-¹⁵N in soil decreased with the increase of biochar application rate.The residual amounts of base fertilizer-¹⁵N,tiller fertilizer-¹⁵N and panicle fertilizer-¹⁵N in soil under dry-wet-shallow irrigation were higher than those under conventional flooding irrigation.25 t/hm²biochar application had a negative effect on the residual amounts of base fertilizer-¹⁵N,tiller fertilizer-¹⁵N and panicle fertilizer-¹⁵N in soil under dry-wet-shallow irrigation.The difference of fertilizer-¹⁵N residue in the same soil depth between the two irrigation modes was significant(P<0.05).When the application rate of biochar was same ranged from 0 t/hm²to 12.5 t/hm²,the residual amount of fertilizer nitrogen in 0-20cm soil layer of rice under dry-wet-shallow irrigation was higher than that under conventional flooding in different stages.Compared with conventional irrigation,dry-wet-shallow irrigation reduced the residue of fertilizer-¹⁵N in 40?60 cm soil layer.(3)There were three peaks of CH₄emissions under dry-wet-shallow irrigation and two peaks under conventional flooding irrigation.The highest peak of CH₄emissions of all treatments appeared at jointing-booting stage.DB0N2 and FB0N2 had the highest CH₄emissions of 23.51mg/(m²·h)and 45.13mg/(m²·h)under dry-wet-shallow irrigation and conventional flooding irrigation,respectively.However,DB3N1 and FB3N2 were the lowest values of CH₄emissions,which were 0.58 mg/(m²·h)and 0.86 mg/(m²·h)respectively.The highest peak of N₂O emission flux occurred at the end of late tillering(sunning stage)under conventional flooding irrigation,while the highest peak of N₂O emissions under dry-wet-shallow irrigation appeared at jointing-booting stage.The treatment of DB0N2 and FB0N2 with the highest N₂O emissions of58.37 ug/(m²·h)and 41.03 mg/(m²·h)during the growth period of rice under dry-wet-shallow irrigation and conventional flooding irrigation,while DB2N1 and FB3N2 were the lowest N₂O emissions,which were 0.57 mg/(m²·h)and-7.75 mg/(m²·h),respectively.The cumulative emissions of CH₄under dry-wet-shallow irrigation was significantly lower than that under conventional irrigation(P<0.05),but the cumulative emissions of N₂O was significantly higher than that under conventional irrigation(P<0.05).Biochar could effectively reduce CH₄and N₂O emissions during the growth period of rice.The CH₄and N₂O emissions under the two irrigation modes were larger at tillering stage and jointing-booting stage,while the CH₄emissions under dry-wet-shallow irrigation mode were lower than that under conventional flooding irrigation during the growth period of rice,while N₂O emissions of each growth stage were higher than those under conventional flooding irrigation.The CH₄and N₂O emissions of each treatment were significantly correlated with the daily minimum temperature(P<0.05),and the CH₄emissions were significantly correlated with the average temperature of the day(P<0.01).There was a significant correlation between CH₄and N₂O emissions and soil temperature under the two irrigation modes(P<0.01).The correlation between CH₄and N₂O emissions and soil temperature in 10 cm soil layer was the highest under dry-wet-shallow irrigation,and the correlation between CH₄and N₂O emissions and 5 cm soil layer temperature was the highest under conventional flooding irrigation.The CH₄emissions of each treatment were negatively correlated with NO₃^--N in paddy soil under the two irrigation modes(P<0.05).Under the two irrigation modes,the total emission of CH₄and N₂O had significantly or extremely significantly negatively correlated with the whole nitrogen fertilizer,base fertilizer,tiller fertilizer and panicle fertilizer absorption and utilization efficiency.Under the dry-wet-shallow irrigation mode,the total CH₄and N₂O emissions were significantly or extremely significantly positively correlated with the loss rate of nitrogen fertilizer,base fertilizer and tiller fertilizer,but not significantly correlated with the loss rate of panicle fertilizer(P>0.05).Under the conventional flooding irrigation mode,the total emission of CH₄and N₂O was significantly or extremely significantly positively correlated with the loss rate of nitrogen,base fertilizer,tiller fertilizer and ear fertilizer.Nitrogen fertilizer,CH₄emissions per unit yield under dry-wet-shallow irrigation were significantly lower than those under conventional flooding irrigation when the application rates of biochar was same(P<0.05),N₂O emissions per unit yield were higher than those under conventional flooding irrigation,the GWP and GHGI under dry-wet-shallow irrigation were significantly lower than those under conventional flooding irrigation(P<0.05).Biochar application could effectively reduce GWP and GHGI.Reasonable water biochar and nitrogen management can effectively solve the problems of straw burning in open air,low water and nitrogen use efficiency and increased greenhouse gas emissions in black soil paddy fields in cold region.At the same time,it can achieve the purpose of increasing yield.Through the selection of fuzzy comprehensive evaluation model based on entropy weight method,it is found that DB2N2 treatment is the best,that is,shallow wet dry irrigation mode+12.5 t/hm²biochar+110 kg/hm²nitrogen fertilizer mode,which can achieve the purpose of water saving,yield increasing,high efficiency and emission reduction.