| Soil nitrogen（N）is a major constraint to crop production around the world.Chemical N fertilizer applied in crop production has increased dramatically in last decades,but half of chemical fertilizer applied is lost from agricultural fields through erosion,leaching,and gaseous emissions,leading to soil and environmental degradation.Biotic stresses threaten crop growth.The application of pesticides in conventional agroecosystems also carries large risks of environmental degradation and ecological disruption.Therefore,conventional chemical-based production systems are not a sustainable solution for development of agriculture.Accordingly,exploiting the biological function potential of crops through crop rotation is increasingly necessary to supplement synthetic chemical use in agriculture to achieve environmentally favorable agricultural systems.N fixed by legumes can be used directly by the crops to promote the crop growth and is less susceptible to loss to the environment compared to chemical N fertilizers.Increasing the frequency of legumes in crop rotation can improve soil N.Crop rotation is an agronomic way in pest management including weed,disease,and insect control.Therefore,inclusion of legumes in crop rotation has potential to reduce the need for inorganic fertilizers,improve N use efficiency and pest alleviation.However,high frequency of the same legume in crop rotation increases risk of biotic stresses including weeds and plant diseases.There is an urgent need to know how legume species and frequency should be arranged in crop rotations to most effectively alleviate biotic stresses and improve overall productivity and sustainability.Therefore,it is increasingly necessary to develop rotation systems to determine the effects on integrated improvement on biotic stress management and overall productivity.In order to clarify above knowledge gaps,a field crop rotation experiment was conducted in west Canadian prairie,a semiarid area from 2010 to 2017.The rotation experiment consisted of three typical annual legumes,including pea（Pisum sativum L.）（P）,chickpea（Cicer arietinum L.）（C）,and lentil（Lens culinaris Medik.）（L）,and two non-legume crops wheat（Triticum aestivum L.）（W）and mustard（Brassica juncea L.）（M）.Each legume was included in the following levels of 4-Year rotations:（i）legume-intensified rotation systems,consisting of pea-pea-pea-wheat（PPP）,chickpea-chickpea-chickpea-wheat（CCC）,lentil-lentil-lentil-wheat（LLL）,and lentil-chickpea-pea-wheat（LCP）rotation systems;（ii）legume-moderate rotation systems,consisting of pea-wheat-pea-wheat（PWP）,chickpea-wheat-chickpea-wheat（CWC）,lentil-wheat-lentil-wheat（LWL）,lentil-wheat-chickpea-wheat（LWC）,pea-wheat-lentil-wheat（PWL）,and pea-mustard-chickpea-wheat（PMC）;（iii）legume-least rotation systems,consisting of pea-wheat-wheat-wheat（PWW）and chickpea-wheat-wheat-wheat（CWW）;and（iv）legume-excluded（or wheat-continued）wheat-wheat-wheat-wheat（WWW）.The combination of two cycles of rotation can be considered together as a longer rotation with8-Year history.We determined the effect of different rotation systems on soil N content,plant growth of legumes and wheat,N balance,and overall productivity in agroecosystem.Results showed:（i）Diversified rotation system is beneficial for the growth of legumes.LCP improved root biomass by 44%for pea compared with PPP systems,respectively.LWC system increased root biomass of chickpea by 87 and 191%,and chickpea nodule mass by 156 and 242%,compared with CWC and CCC systems,respectively.LCP reduced root rot severity of pea by 3-fold compared with PPP.LWC alleviated root rot severity and nodule damage of chickpea by 8-and4-fold compared with CCC,respectively.LWC improved the seed yield of chickpea by 96 and95%compared with CCC and CWC,respectively.LLL improved yield of lentil by 17 and 14%compared with LWL and PWL.In chickpea production,LWC and PMC improved the systems’resistance to or resilience from weed and disease stresses,and averagely presented a 14% advance on systems robustness.（ii）Appropriately increasing the frequency of legumes prior to wheat could be beneficial for wheat production.Legume-intensified,legume-moderated,and legume-least rotation systems improved soil N in wheat field by averagely 37,26 and 13%compared with legume-excluded rotation system WWW.Legume-included rotation systems improved wheat yield by averagely 19%,seed N content by 25%,and NUE by 28%compared with legume-excluded rotation system WWW.Inclusion of legumes in rotation improved wheat yield,and wheat-continued rotation system WWW produced minus net income in wheat production.（iii）Appropriately increasing the frequency of legumes could improve the overall soil N and N use efficiency.Increasing the frequency of legumes could significantly improve N2fixation in rotation system.Legume-intensified,legume-moderated and legume-least rotation systems averagely improved soil N content by 45,24,and 11%compared with legume-excluded rotation systems during the 8-Year rotation systems,respectively.Legume-intensified,legume-moderated and legume-least rotation systems lost 181,133 and 72%less N to the environment compared with WWW,respectively.Legume-intensified and legume-moderated rotation systems displayed a positive N budget（N input>N output）;while legume-least and legume-excluded rotation systems presented a negative N budget（N input<N output）.The output:input ratio of commercial N was improved in legume-intensified,legume-moderated and legume-least rotation systems by averagely 5.1-,2.7-,and 1.6-fold compared with legume-excluded rotation systems,respectively.（iv）Appropriately increasing the frequency of pea and lentil in rotation could improve the overall productivity and comprehensive benefits in agroecosystems.Pea-included rotation systems improved the total seed yield by 27 and 19%compared with chickpea-included and lentil-included rotation systems.Chickpea-intensified rotation CCC reduced the total yield by51%,compared with other rotations.Lentil-included rotation systems improved the net income by 33,24,and 374%,compared with pea-included,chickpea-included,and wheat-continued rotation systems.Results indicate that（i）the rotational effect on legume productivity varies with legume species;（ii）diversified rotation systems incorporated with legume and non-legumes are more resistant to biotic stresses for legumes;（iii）appropriately increasing the frequency of pea and lentil in rotation,and diversified legume intensification could improve N fixation and soil N content,reduce N loss and dependence on N fertilizer,and are sustainable to enhance the comprehensive benefits and development of agroecosystem. |
