| Soil carbon（C）and nitrogen（N）are the quality nutrients of agricultural soil and important indicators representing soil fertility.They are positively correlated with the supply of crops nutrients,soil water holding capacity,soil nutrients availability,soil compaction capacity,erosion resistance and surface litter deposition capacity.Increasing the stock of soil organic carbon（SOC）and total nitrogen（TN）is beneficial to the sustainable utilization of farmland.Nitrogen is one of the most important essential elements for crop growth and development.Nitrogen application increases biomass,crop yield,and improves the carbon input in the soil to enhance the soil carbon and nitrogen stocks.However,excessive nitrogen application and low nitrogen utilization efficiency of crops not only restrict the development of agriculture in China but also cause serious environmental pollution.In the farming-pasture zone of Northeast China,soil organic carbon（SOC）and total nitrogen（TN）have been seriously lost due to the increasing reclamation in recent years.It has been proved that intercropping increases aboveground and belowground biomass and in return increases soil carbon and nitrogen stock through interspecific root interaction.Maize（Zea mays L.）is an important food and feed crop.Alfalfa（Medicago sativa L.）is a high-quality perennial legume forage.At present,most studies on the advantages of maize/alfalfa intercropping mainly focus on the aboveground,but studies on the belowground biomass,soil organic carbon and soil total nitrogen stocks are still very scarce.Therefore,it is of great significance to explore the relationship between nitrogen fertilizer and maize/alfalfa intercropping modes and soil organic carbon and total nitrogen stocks.Furthermore,our study aims to verify whether intercropping and nitrogen application can improve soil quality and promote crop growth.The expected results are helpful to the development of high-quality agriculture,animal husbandry and ecological environment protection.In this experiment,maize/alfalfa intercropping system was used as the research objective crops,and the split-plot design was adopted.There were two nitrogen application levels in the main area:no nitrogen（N0）and nitrogen（N1）.There were five cropping modes in the sub region.Maize monocropping（MM）,alfalfa monocropping（MA）,2 rows of maize and 6 rows of Alfalfa intercropping with spacing of 30 cm（IMA23）,4 rows of maize and 6 rows of Alfalfa intercropping with spacing of 20 cm（IMA42）and 4 rows of maize and 6 rows of Alfalfa intercropping with an interval of 30 cm（IMA43）.All the treatments were repeated 4 times.From2014 to 2018,crop biomass,soil p H,EC,SOC and TN in different soil layers were measured for all treatments and the correlation between these indicators and soil organic carbon and total nitrogen stocks was analyzed.（1）The aboveground and belowground biomass of the 5 cropping modes were significantly increased by applying nitrogen fertilizer.The aboveground biomass from 2015 to 2018 varied from 268.3 to 1323.2 g m-2,and showed an increasing trend and then decreased.The belowground biomass varied from 75.2 to 799.3 g m-2and increased gradually from year to year.Compared with N0,the aboveground biomass and belowground biomass of 5 cropping modes increased by 12-101%and 10.6-99.8%respectively.The aboveground biomass showed a trend of IMA43>MM>IMA42>IMA23>MA,while the belowground biomass showed a trend of IMA43>MA>IMA42>IMA23>MM.Compared with MM,MA and IMA23 decreased aboveground biomass by 3.93-40.1%,while IMA42 and IMA43 increased aboveground biomass by 1.5-52.1%.Compared with MM,MA and all intercropping modes treatments significantly increased the belowground biomass by 98.7-450.1%,and the effect of IMA43 mode was more significant.（2）Compared with the original stocks,IMA42,IMA43 and MA significantly increased SOC and TN stocks with the increase of cropping years.SOC and TN stocks increased by 0.9-5.7%and 0.8-12.9%respectively without nitrogen application.Among the tested cropping modes in 2017 at N0 treatment,the highest increase was observed in the IMA43 mode up to1.845 Mg C,the average carbon fixation rate was 0.369 Mg C ha-1yr-1.SOC and TN stocks increased by 0.5-9.4%and 3.2-16.4%respectively with nitrogen application.The highest increase was observed in IMA43 mode up to 414 kg N at N1 in 2017,and the average nitrogen fixation rate was 82.8 kg N ha-1yr-1.However,MM,MA and IMA23 decreased SOC and TN stocks under N0 treatment.Compared with the original stocks,the SOC stocks loss of five cropping modes was 1.7-12.9%,and the loss of TN stocks range was 2.3-14.6%.The highest decrease was found in MM up to 2.117 Mg C and 0,437 Mg N at N0 treatment.The average annual loss rate of SOC stocks in MM was 0.423 Mg C ha-1yr-1and the average annual loss rate of TN stocks was 86.1 kg N ha-1yr-1.（3）Intercropping,especially without nitrogen application,significantly increased SOC and TN stock.IMA42,IMA43 and MA significantly increased SOC and TN stock compared with MM from 2014 to 2018,and the effect of the IMA43 mode was more significant.The SOC and TN stocks increased by 9-22.9%and 13.4-24.3%respectively without nitrogen application,while SOC and TN stocks increased by 9.8-20.3%and 14.4-21.8%respectively with nitrogen application.（4）In the deep soil layers,SOC and TN stocks gradually decreased.In different soil layers,there were significant differences among planting patterns.Whether nitrogen application or not,SOC and TN stock in 0-40 cm soil layer showed a trend of IMA43>IMA42>MA>IMA23>MM,while in the range of 40-80 cm soil layer,the SOC and TN stock of MA was highest under N0 treatment,whereas IMA43 was highest under N1 treatment.（5）The changes in SOC and TN stocks are mainly caused by biomass,especially belowground biomass.Whether nitrogen application or not,belowground biomass showed a positive correlation with SOC and TN stocks.At the same time,the aboveground and belowground biomass had a significant positive correlation with rainfall,which has caused the interannual variations of biomass.In conclusion,maize/alfalfa intercropping has obvious advantages in improving aboveground and belowground biomass and soil quality.Maize and alfalfa increased the belowground biomass of intercropping system by the application of nitrogen fertilizer,competition and compensation of water and light utilization,and increased the input of root carbon in the soil,thus promoting the accumulation of SOC and TN.Intercropping IMA43 mode could also increase SOC and TN stock without nitrogen application.Therefore,the results of this study proved that fertilization and maize/alfalfa intercropping could increase SOC and TN stocks in the farming-pasture zone of Northeast China,which provided a theoretical basis for SOC and TN sequestration,and provided theoretical support for reducing nitrogen input and reducing environmental risk in maize/alfalfa intercropping. |
