Understanding the response of weed community composition to different fertilization regimes is of vital importance for designing judicious nutrient management strategies within integrated weed control programs.We investigated the effects of diverse fertilization regimes on weed and its seedbank community composition in winter wheat-summer maize and winter wheat-summer fallow cropping systems subjected to25years of field experiments.The field experiments were carried out under irrigation(winter wheat-summer maize)and rain-fed(winter wheat-summer fallow)conditions in north-western China.The two systems were subjected to ten and eight different fertilization regimes,respectively,including no artificial disturbance(Set-aside),no fertilizer control(CK)and various combinations of inorganic nitrogen(N),phosphorus(P)and potassium(K)fertilizers,i.e.N,NK,PK,NP,NPK,NPK plus crop straw(SNPK)or dairy manure(M1NPK and M2NPK,where M refers to dairy manure and the following number indicates the rate of manure application).We hypothesized that the different systems of long-term fertilization and cropping would have a significant impact on the diversity of the weed and its seedbank communities.The following results were obtained.
　　The results revealed that17weed species representing11families under irrigated and21weed species in13families under rain-fed in winter wheat growing season;while21weed species in total,representing10families and21genera in summer maize/fallow season.Higher weed densities and richness indices were observed on treatments with no artificial disturbance(Set-aside)and those involving unbalanced fertilizer applications(CK,N,PK and NK),whereas lower densities and richness indices of weed and its seed bank were found on plots receiving balanced fertilizer applications and NPK with organic supplements,under both cropping systems.These results suggest that a rich soil nutrient content provides a higher crop growth,which curbs light radiation,thus resulting in a suppressed density of weeds.The results also demonstrated a consistently higher weed density on plots with wheat-fallow cropping than in corresponding plots with wheat-maize cropping.The observed differences in relative density of weed communities between irrigated and rain-fed experiments might be attributed to the effects of different cropping systems.A total of26weed species belonging to13families under winter wheat-summer maize and20weed species representing10families under winter wheat-summer fallow cropping systems were observed in the seed bank study.the results showed that the long-term application of balanced fertilizers and NPK with crop straw treatment significantly reduced the density of soil weed seed bank;none and unbalanced fertilizers had significantly increased the number of species and density of seed bank.The reason may be that poor and unbalanced nutrient supply limits the crop growth;as a result,more water,light and other resources are readily available to weeds.We also found a new weed species in soil seed bank as compared with aboveground investigation on weed community,which may suggest that many environmentally vulnerable weed species had been replaced by new invaders.
　　Conclusion:Balanced fertilizers application(N and P)showed the lowest weed density and reduced weed species diversity,as compared with unbalanced fertilizers application.Lower weed density and reduced weed species in crop field would further help to decrease the competition to soil nutrients and light from weeds,which in tum may further increase the crop production.The present work also suggested that rain-fed condition encouraged the growth of the weed species as compared with irrigated condition where the more intensive land use exacerbated the survival environments for weed species.Therefore,this thesis provides experimental evidence that rationale fertilizer management strategies can control the weed density and diversity,which can be incorporated into integrated weed management programs in local region.