| Biodiversity increases ecosystem productivity and stability.These biodiversity effects are thought to be generated by two ecological mechanisms:complementarity(CE)and selection effects(SE)among species.Intercropping is one of the increasing plant diversity in agro-ecosystem,with significant overyielding and nutrient acquisition advantages.However,relatively few studies have evaluated the role of complementarity and/or selection effects in yield and nutrient advantages by intercropping systems.Meanwhile,the maintenance or improvement of productivity temporal stability and soil fertility have become key issue towards sustainability of intercropping development as continuous overyielding generally removes more nutrients from soils.Therefore,we specifically focused on(1)the contribution of complementarity and selection effects to productivity and nutrient acquisition,(2)temporal stability of productivity,and(3)soil fertility parameters including physical,chemical and biological properties of soils collected from 2012 to 2014(the 4th to 6th year)in two long-term experiments that were initiated in 2009 in Gansu(long-term experiment 1)and Ningxia(long-term experiment 2)Provinces,Northwest China,respectively.The main results were as follows:(1)Faba bean/maize and chickpea/maize continuous intercropping systems still had yield advantage.In long-term experiment 1,compared with their respective expected from monocultures,grain yield and biomass were increased by 29.5%and 15.8%for faba bean/maize intercropping,and by 33.5%and 26.9%for chickpea/maize intercropping.Average land equivalent ratios(LERs)based on grain yield and above-ground biomass from 2012 to 2014 were 1.35 and 1.22 for faba bean/maize intercropping,and 1.15 and 1.18 for chickpea/maize intercropping,respectively.However,there were less significant grain yield and biomass advantages of soybean/maize and oilseed rape/maize intercropping systems.Moreover,grain yield and biomass of faba bean/maize and biomass of chickpea/maize were enhanced by P applications relative to no P application.In long-term experiment 2,grain yield and biomass of faba bean/maize intercropping were increased by 10.9%and 7.1%compared with the expected from monoculture,respectively.Average LER based on grain yield and biomass was 1.40 and 1.17 for faba bean/maize intercropping in the experiment.Furthermore,grain yield and biomass by faba bean/maize receiving N fertilization were 17.0%-42.6%and 16.8%-42.5%greater than those of no N application.Faba bean with rhizobium inoculation did not affect system productivity.(2)The altered productivity derived mainly from CE and SE among species in intercropping systems,which was dependent on combinations of crop species.In long-term experiment 1,on the productivity,there existed positive CE only for faba bean/maize,positive CE and SE for chickpea/maize,positive SE only for soybean/maize,and positive CE and negative SE for oilseed rape/maize.Moreover,CE of faba bean/maize intercropping system was higher than that of other maize-based intercropping systems.On average,SE by chickpea/maize was greater than that by soybean/maize,and SE by soybean/maize was greater than that by oilseed rape/maize intercropping in which SE was negative.In the long-term experiment 2,the productivity advantage by faba bean/maize system resulted from the interaction between positive CE and negative SE.(3)Not only did faba bean/maize and chickpea/maize intercropping systems have yield advantage,but they had nutrient acquisition advantage.In long-term experiment 1,nutrient accumulations by faba bean/maize and chickpea/maize intercropping systems were enhanced by 23.4%-29.4%and 22.1%-30.6%compared with the expected(weighted means of monoculture),respectively.Also,P fertilizer recovery efficiency(PRE)was higher 19.3 and 12.9 percentage point in faba bean/maize and chickpea/maize intercropping than that of the respective expected from monocultures.However,there was no significant difference between monoculture and intercropping for nutrient accumulations and PREs in soybean/maize and oilseed rape/maize systems.Compared with no P application,moreover,P fertilization significantly improved P accumulation of all four crop combinations,N accumulation of faba bean/maize and chickpea/maize,and K accumulation of chickpea/maize system.In long-term experiment 2,nutrient accumulations of faba bean/maize intercropping were increased by on average 12.6%ranging from 1.9%to 22.4%.N fertilizer recovery efficiency(NRE)by faba bean/maize intercropping system was also 14.0 to 61.9 percentage point greater than that of monoculture.Furthermore,N fertilization enhanced total nutrient accumulations by 14.2%-44.6%relative to no N fertilization.But there was adverse effect on NRE by 92.0%when received higher N application rate compared with lower N application rate(75 kg N ha-1).(4)The difference in nutrient accumulations between monoculture and intercropping system was a function of CE and SE interactions.In long-term experiment 1,enhanced N and K accumulations derived from positive CE and negative SE in faba bean/maize,and positive CE only resulted in enhanced P accumulation in faba bean/maize.The altered nutrient accumulations derived from positive SE only in soybean/maize.Both positive CE and SE lead to the increased nutrient accumulations in chickpea/maize,and no enhanced nutrient accumulation was due to offset of CE to SE in oilseed rape/maize.In long-term experiment 2,nutrient acquisition advantage of faba bean/maize derived from the interaction between positive CE and negative SE,where CE was higher than SE.(5)The temporal stability of productivity could be either maintained or enhanced by continuous intercropping systems relative to corresponding monoculture systems.Clearly,in long-term experiment 1,intercropping maintained the productivity temporal stability in the majority of faba bean/maize,soybean/maize and chickpea/maize systems compared with their monocultures.In contrast,the productivity temporal stability by faba bean,chickpea and oilseed rape crop species was to some extent decreased by intercropping with receiving P fertilization over the corresponding monocultured crop species.In long-term experiment 2,grain yield temporal stability in faba bean/maize intercropping was remarkably enhanced compared to the expected from monoculture.Nevertheless,temporal stability of grain yield was always decreased for intercropped faba bean and increased for intercropped maize,compared with their monocultured crop species.(6)The majority of soil fertility properties were generally maintained or even enhanced by intercropping compared with monoculture systems,regardless of fertilization application.In long-term experiment 1,continuous intercropping and P fertilization not only maintained soil organic matter and Olsen P content in a longer time scale,but increased total N by on average 1.0%.In long-term experiment 2,it was noted that soil organic matter and total N were markedly increased by 10.3%and 8.0%under intercropping compared with the weighted means of monoculture,respectively.No significant difference in soil Olsen P content was detected between monoculture and intercropping.There was highly improvement of soil water stable aggregate and enzyme activities under intercropping in both long-term experiments.In detail,soil water stable large macroaggregates content(>2 mm)was increased by on average 10.1%in long-term experiment 1 and by 9.1%in long-term experiment 2 at the expense of reduction in small macroaggregates(0.25-2 mm),microaggregates(0.106-0.25 mm)and the free silt and clay particles(<0.106 mm)by 13.9%-24.2%,compared with the weighted means of monoculture across three years.In addition,soil acid phosphatase,urease,nitrate reductase and sucrase activities were 6.9%-105.0%higher in intercropping than in monoculture system.In conclusion,intercropping system had greater production and higher yield stability than monoculture system when selected rational combinations of crop species.Intercropping advantages derived from the interaction between CE and SE among species,and interspecific facilitation may play an important role in yield stability in intercropping system.Moreover,the majority of soil fertility properties were maintained or even enhanced by intercropping in spite of continuous overyielding and more nutrient removal from soils,which may further support benefits of intercropping in the long time. |
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