| With the intensive production of agriculture in China,people blindly rely on nitrogen fertilizer,resulting in excessive use of nitrogen fertilizer in various provinces and cities in China,which not only does not bring continuous increase in crop yield,but reduces the nitrogen fertilizer utilization efficiency and soil quality,but also intensifies N loss and volatilization resulted to environmental pollution and seriously threaten the sustainable development of agricultural ecosystems.Biochar can not only maximize the effective utilization of straw nutrients,but also as a soil amendment improve the soil quality,increase the crop yield and nutrient utilization efficiency,so it is an important technical means toachieve nitrogen-reducing and efficiency-increasing.Therefore,the purple soil and rapeseed-corn rotation system in the southwest China dryland were taken as the research objects in our study.Through two consecutive years（2017-2019）field positioning experiments,the four biochar levels(0,10,20 and 40 t·hm-2,named B0,B10,B20 and B40)and three nitrogen levels(conventional nitrogen application 180 kg·hm-2,80%conventional nitrogen application 144 kg·hm-2 and 60%conventional nitrogen application 108 kg·hm-2,named N100,N80 and N60)were designed to discuss the effects of combined application on soil environment,crop yield,nitrogen absorption and utilization,and comprehensively evaluate the effect of fertilization of each treatment to find out the main soil environmental factors affecting crop growth and screen out suitable fertilization modes of soil fertilization and crop high-yield and high-efficiency,which provided a theoretical basis for the improvement of farmland soil productivity,green sustainable development and crop high-yield and high-efficiency in the purple land area of southwestern dry land.The main research results are as follows:（1）Biochar significantly increased the farmland soil temperature at seedling stage and mature stage of rapeseed,and also significantly increased the soil water content at flowering stage of rapeseed in the second year.However,nitrogen fertilizer has little effect on soil temperature and humidity.Besides,10 t·hm-2biochar combined with 144 kg·hm-2 pure nitrogen increased soil temperature and humidity in rapeseed farmland,compared with conventional nitrogen management.Except for the increase of soil organic matter with the increase of biochar,the soil total nitrogen,total phosphorus,total potassium,available nitrogen,available phosphorus and available potassium content increased first and then decreased with the increase of biochar,and the application effect of10 t·hm-2 biochar was the best,nevertheless the effect of nitrogen fertilizer on soil nutrients was basically insignificant.Nitrogen fertilizer reduction was not conducive to the comprehensive evaluation of farmland soil nutrients,but the addition of biochar was beneficial to improve the comprehensive evaluation value of farmland soil nutrients in rapeseed and corn seasons,and 10t·hm-2 biochar combined with 180 kg·hm-2 and 144 kg·hm-2 pure nitrogen had the highest comprehensive evaluation value.（2）The application of biochar improved the soil water stability aggregates（WR0.25,W-MWD and W-GMD）,and the effect of 10 t·hm-2 biochar was the best,with the improvement effect was significant at the level of 180 kg·hm-2 and 144 kg·hm-2 pure nitrogen,and the destruction rate of soil aggregates was also reduced,because of significant increase of the content of soil water-stable aggregates in the 2～5 mm particle size range.Compared with conventional nitrogen application management,10 t·hm-2 biochar combined with 144 kg·hm-2 pure nitrogen significantly increased the content of water-stable aggregates in rapeseed farmland soil.In addition,biochar also increased the content of organic carbon in the aggregate of soil mechanical stability,because of the increase of the organic carbon content in the particle size range of 1～2,0.25～1 and<0.25 mm,and the increase effect was remarkable at the 20 and 40 t·hm-2biochar level.Biochar and nitrogen fertilizer had great influence on the soil total organic carbon and activated carbon components.For soil total organic carbon,10 t·hm-2 biochar combined with 180 kg·hm-2 pure nitrogen could significantly increase the soil total organic carbon content in a short period of time.With the increase of biochar,soil particulate organic carbon increased,while soil water-soluble organic carbon,soil oxidizable organic carbon,and soil microbial biomass carbon all increased first and then decreased,and the content of 10 t·hm-2 biochar treatment was highest,which significantly increased the content of water-soluble organic carbon in the late growth stage of rapeseed,the microbial biomass carbon content in the early stage of rapeseed,and the oxidizable organic carbon content in each growth period of rapeseed.Compared with conventional nitrogen application management,10 t·hm-2 biochar combined with 144 kg·hm-2 pure nitrogen increased the content of soil activated carbon components in each growth period of rape.Except that soil water-soluble organic carbon was not greatly affected by nitrogen fertilizer,soil particulate organic carbon,soil oxidizable organic carbon and soil microbial biomass carbon all decreased with the nitrogen fertilizer reduction,especially 108 kg·hm-2 pure nitrogen treatment.Correlation analysis showed that the content of soil particulate organic carbon was significantly positively correlated with soil total organic carbon content,followed by the content of soil water-soluble organic carbon,and the soil oxidizable organic carbon content was significantly negatively correlated.（3）Compared with nitrogen fertilizer,biochar was more likely to change the soil total microbial biomass in dry land farmland.The application of 10 t·hm-2 biochar significantly increased the content of farmland soil bacteria,fungi and actinomycetes,while nitrogen fertilizer reduction(108kg·hm-2 pure nitrogen treatment)decreased the content of fungi and actinomycetes.Biochar and nitrogen fertilizer also changed the soil microbial community structure in farmland.The application of 10 t·hm-2 biochar was more beneficial to enrich the soil microbial community structure,while nitrogen fertilizer reduction(108 kg·hm-2 pure nitrogen treatment)reduced the richness of soil microbial community structure.Compared with conventional nitrogen application management,10 t·hm-2 biochar combined with 144 kg·hm-2 pure nitrogen significantly increased the total soil microbial biomass and actinomycete content.The application of 10 t·hm-2 and 20 t·hm-2 biochar also increased the functional metabolic activity of farmland soil microbial communities,which significantly increased the utilization intensity of sugar,polymer,amino acid and carboxylic acid by microorganisms,while nitrogen fertilizer reduction(108 kg·hm-2 and 144 kg·hm-2 pure nitrogen treatment)decreased the metabolic activity of soil microbial communities,which reduced the utilization intensity of sugar and polymer by microorganisms.After applying the appropriate amount of biochar(10 t·hm-2 and 20 t·hm-2biochar),the nitrogen fertilizer reduction(108 kg·hm-2 and 144 kg·hm-2 pure nitrogen treatment)also significantly reduced the utilization intensity of amino acids,carboxylic acids and amines by microorganisms;The application of 10 t·hm-2 and 20 t·hm-2 biochar was more conducive to promoting the utilization of 31 carbon sources by soil microorganisms,which significantly increased the utilization intensity of D-xylose,D-cellobiose,N-acetyl-D glucosamine,β-methyl-D-glucoside in sugar,γ-hydroxybutyric acid,D-malic acid,methyl pyruvate in carboxylic acids,tween 40,tween 80,Α-cyclodextrin,glycogen in polymers,glycyl-L-glutamic acid in amino acids by microorganisms.However,Nitrogen reduction(108 kg·hm-2 and 144 kg·hm-2 pure nitrogen treatment)significantly reduced the utilization intensity of D-xylose,D-cellobiose,D-mannitol,glucose 1-phosphate in sugar,phenethylamine in amine,Tween 80 and glycogen in polymers by microorganisms.The application of 10 t·hm-2 and 20 t·hm-2 biochar could also significantly increase the soil microorganisms Shannon index,Simpson index and evenness index,while the nitrogen fertilizer reduction(108 kg·hm-2 and144 kg·hm-2 pure nitrogen treatment)decreased the diversity of soil microbial community functions.Besides,biochar significantly changed the functional characteristics of farmland soil microbial communities.Sugars,carboxylic acids,phenolic acids,amino acids and polymers could be used as sensitive carbon sources for distinguishing 10 t·hm-2 biochar treatment from other biochar treatment.Compared with conventional nitrogen application management,10 t·hm-2 biochar combined with 144 kg·hm-2 pure nitrogen significantly improved the utilization intensity of soil microorganisms to sugars,polymers,amino acids and carboxylic acids.As a result,the Shannon index and uniformity index also significantly improved.With the increase of biochar,the soil invertase and urease in each growth period of rapeseed during the two growing seasons increased first and then decreased.The application of 10 t·hm-2biochar could significantly increase soil urease and invertase activities,while 40 t·hm-2 biochar and nitrogen fertilizer reduction(108 kg·hm-2 and 144 kg·hm-2 pure nitrogen treatment)all inhibited soil urease and sucrase activities.The regression model showed that AWCD and PLFA increased gradually with the increase of soil urease and AWCD also increased gradually with the increase of soil invertase.（4）The application of 10 t·hm-2 biochar significantly improved the stem diameter,average root diameter and total root surface area to improve the root morphology of rapeseed,and thus significantly improved the nitrogen absorption and utilization of rapeseed.It also improved the main agronomic traits such as sequence length,effective branches number and effective pods number,and significantly increased the chlorophyll a,chlorophyll b,total chlorophyll content at bud,flowering stages and dry matter in each growth period of rapeseed,and thus significantly increased rapeseed yield.However,nitrogen fertilizer reduction(108 kg·hm-2 and 144 kg·hm-2 pure nitrogen treatments)had a significant inhibitory effect on it.Between them,the inhibition of 144 kg·hm-2 pure nitrogen treatment was mainly reflected in chlorophyll,nitrogen accumulation,agronomic traits and yield.Biochar had also significant effects on the dry matter accumulation,nitrogen accumulation,agronomic traits and yield of corn,and the effect of 10 t·hm-2 biochar treatment was the best.In both rapeseed growing seasons,10 t·hm-2 biochar combined with 180 kg·hm-2 and 144kg·hm-2 pure nitrogen treatment had the higher nitrogen utilization efficiency,which were 10.02 and 10.33%higher than conventional nitrogen application management.The yield of rapeseed with 10t·hm-2 biochar combined with 180 kg·hm-2 pure nitrogen treatment was the highest,with an average of 22.36%higher than that of conventional nitrogen application management.Besides,10 t·hm-2biochar combined with 144 kg·hm-2 pure nitrogen treatment was also 32.23%higher than that of conventional nitrogen application management in 2018-2019.Correlation analysis and path analysis showed that the increase of main sequence length,effective branches and effective pods number were the main factors to increase the rapeseed yield.Multivariate nonlinear regression model shows that,compared with the highest yield of rapeseed(2.67 t·hm-2)achieved with 30 t·hm-2biochar combined with 223 kg·hm-2 pure nitrogen,10 t·hm-2biochar combined with 144 kg·hm-2 pure nitrogen treatment（yield is 2.45 t·hm-2）could reduce nitrogen fertilizer by 35.43%with the yield decreased by only 8.24%.The annual output value and net profit of rapeseed and corn of 10 t·hm-2,20 t·hm-2 biochar combined with 180 kg·hm-2 pure nitrogen and 10 t·hm-2biochar combined with 144kg·hm-2 pure nitrogen was the higher,which were 13.88%,11.62%,13.21%,27.28%,22.83%,and 27.64%higher than the conventional nitrogen application management respectively,which showed that 10 t·hm-2 biochar combined with 144 kg·hm-2 pure nitrogen could improve the yield and nitrogen use efficiency of rapeseed in the purple land area of southwestern dry land by promoting the development of rapeseed root system and yield components.Besides,it could also improve the nitrogen accumulation and yield of corn.Therefore,it would eventually achieve the dual purpose of high yield value and high profit of rapeseed-corn,and nitrogen reduction and efficiency-increasing.This positive residual effect of biochar in the rapeseed-corn rotation system could be maintained at least 2 year.（5）Correlation analysis was conducted on 34 soil factors and yield,which showed 27evaluation indicators were significantly correlated with yield.The 27 indicators included soil physics,chemistry,and biology,which were significant and extremely significant positive correlations.Comprehensive evaluation of 27 indicators through gray correlation analysis,principal component analysis and cluster analysis found that the comprehensive evaluation value,comprehensive score and cluster analysis results were more consistent,and were also in line with the change trend of rapeseed yield,and available phosphorus,geometric mean diameter of water stability,fungi number,utilization of sugars,amino acids,polymers and carboxylic acids by microorganisms could be as the main soil factors affecting rapeseed yield under the combined application of biochar and nitrogen fertilizer.At the same time,the comprehensive analysis of soil fertility effect found that 10 t·hm-2biochar combined with 144 and 180 kg·hm-2 pure nitrogen were the optimal fertilization model in the purple land area of southwest dry land.Combining economic and environmental benefits,10 t·hm-2biochar combined with 144 kg·hm-2 pure nitrogen not only helped to improve the soil physical,chemical and microbiological characteristics,thereby improving soil fertility and crop yield,but also achieved nitrogen reduction and efficiency increasing.To sum up,the application of appropriate biochar to the purple land area of southwest dry land could fertilize the soil and increase the rapeseed-corn yield,nervelessness nitrogen fertilizer reduction had an adverse effect on it.Compared with conventional nitrogen application management,10 t·hm-2 biochar combined with 144 kg·hm-2 nitrogen fertilizer improved the temperature and humidity of farmland soil,soil aggregates structure,soil fertility,farmland soil microbial communities structure,farmland soil microbial biomass,soil microbial community functional activity and soil microbial diversity,thereby improving crop yield and nitrogen absorption and utilization.It was an effective measure to achieve nitrogen fertilizer reduction in the rapeseed-corn rotation system and increase farmland soil productivity,and had a guiding significance for scientific management and reasonable fertilization of biochar combined with nitrogen fertilizer reduction in rapeseed-corn rotation mode of the southwest dryland purple soil farmland,while promoting a green economy and sustainable development. |
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