Effects Of Phosphorus Application And Maize/alfalfa Intercropping On Soil Organic Carbon And Total Nitrogen Stocks In Northeast Farming-pasture Zone,China

Soil carbon and nitrogen are key parameters of agricultural soil and important indicators of soil fertility.Traditional land tillage reduces soil fertility,which severely limits the sustainable development of agriculture.Therefore,fertilization and intercropping methods to improve soil fertility have gradually become one of the research hotspots of agro-ecosystem.At present,most researches focus on the effects of intercropping systems dominated by Gramineae to explore the effect of intercropping on soil carbon and nitrogen stock,however,studies involving the effect of intercropping system dominated by perennial Leguminosae on soil carbon and nitrogen stock are rare.In addition,phosphorus fertilizer not only plays a vital role in agricultural production,but also in soil carbon and nitrogen sequestration.Under different phosphorus levels,the effects of intercropping system on soil carbon and nitrogen stock have not been reported.In this experiment,maize/alfalfa intercropping system was conducted in a randomized split block design,with two P levels:without P application（P0）and with P application（P1）in the main plot.There were five cropping patterns in the subplot:monoculture maize（MM）,monoculture alfalfa（MA）,two-row maize intercropped with six-row alfalfa,with 30 cm inter-row spacing between maize and alfalfa（IMA23）,four-row maize intercropped with six-row alfalfa,with 20 cm inter-row spacing（IMA42）,four-row maize intercropped with six-row alfalfa,with 30 cm inter-row spacing（IMA43）.From 2015 to 2018,the biomass,soil pH,EC,SOC and TN content in different cropping patterns of different soil layers under different P levels were measured,and the influencing factors of soil organic carbon and total nitrogen stocks were analyzed.The main results were obtained as follows:（1）From 2015 to 2018,phosphorous application increased the root biomass of MM,MA,IMA23,IMA42 and IMA43 significantly.Root biomass varied between 65and 799 g m^-2,and the overall trend was IMA43>MA>IMA42>IMA23>MM.Compared with MM,IMA43 significantly increased root biomass by 2-4.6 times,indicating that IMA43 showed obvious advantages in biomass.（2）Compared with the initial stocks,IMA42,IMA43 and MA could increase SOC and TN stocks with the increase of cropping years with P application,among which IMA43 had the largest increase range.IMA43 increased SOC and TN stock by 5-8.9% and 5.6-15.9%respectively,carbon and nitrogen sequestration were at rate of 685 kg C ha^-1 yr^-1 and 96 kg N ha^-1 yr^-1.Compared with MM,IMA43 increased SOC and TN stocks the most,by 19.8-20.3%and 14.4-21.9%,respectively,with increases of 3.49-6.59 Mg C ha^-1 and 489-680 kg N ha^-1.IMA42 and IMA43 could increase SOC and TN stocks with the increase of cropping years without P application,among which IMA43had the largest increase range.IMA43 increased SOC and TN stock by 1.9-5.7%and5.3-12.8%respectively,carbon and nitrogen sequestration were at rate of 202 kg C ha^-1 yr^-1 and 64 kg N ha^-1 yr^-1.Compared with MM,IMA43 increased SOC and TN stocks the most,by 13.2-18.1%and 8.5-19.5%,respectively,with increases of 4.39-5.97 Mg C ha^-1 and 293-655 kg N ha^-1.（3）In 40-80 cm soil layer,the SOC and TN stocks of IMA43 were relatively high,ranging from 15.3-17.4 Mg C ha^-1 to 1.48-1.70 Mg N ha^-1 with P application.While,the SOC and TN stocks of MA were higher than that of intercropping patterns between14.1-15.4 Mg C ha^-1 and 1.35-1.48 Mg N ha^-1 without P application.The results indicated that MA played an important role in soil C and N sequestration in low P soil,and IMA43 was more advantageous in deep soil with P application.（4）There were significant positive correlations between above and root biomass and growth season precipitation.At the same time,there were significant positive correlations between root biomass and SOC and TN stocks.Therefore,root biomass promoted the SOC and TN sequestration,at the same time,precipitation causes inter-annual variation of biomass and inter-annual variation of soil stocks.In conclusion,IMA43 is the optimal pattern of maize/alfalfa intercropping system,which can increase soil SOC and TN stocks with the increase of cropping years by increasing root biomass.IMA43 could increase soil carbon and nitrogen sequestration in low phosphorus saline alkali degraded soil without P application,but the effect of P application is better.Therefore,it is of great significance to choose the best intercropping pattern for improving soil fertility and sustainable development of degraded grassland in Songnen.The results of this study provide a theoretical basis for exploring the long-term effects of phosphorus application and intercropping on soil organic carbon and total nitrogen stocks in northeast farming-pasture zone,China.