Biological nitrogen fixation is a very valuable alternative to nitrogen fertilizer. This process will be discussed in the “Biological Nitrogen Fixation” book. A wide array of free-living and associative nitrogen fix...Biological nitrogen fixation is a very valuable alternative to nitrogen fertilizer. This process will be discussed in the “Biological Nitrogen Fixation” book. A wide array of free-living and associative nitrogen fixing organisms (diazotrophs) will be covered. The most extensively studied and applied example of biological nitrogen fixation is the symbiotic interaction between nitrogen fixing “rhizobia” and legume plants. While legumes are important as major food and feed crops, cereals such as wheat, maize and rice are the primary food crops, but do not have this symbiotic nitrogen fixing interaction with rhizobia. It has thus been a “holy grail” to transfer the ability to fix nitrogen to the cereals and this topic will be also addressed in these books.展开更多
Nitrogen is abundant in the atmosphere but is generally the most limiting nutrient for plants.The inability of many crop plants,such as cereals,to directly utilize freely available atmospheric nitrogen gas means that ...Nitrogen is abundant in the atmosphere but is generally the most limiting nutrient for plants.The inability of many crop plants,such as cereals,to directly utilize freely available atmospheric nitrogen gas means that their growth and production often rely heavily on the application of chemical fertilizers,which leads to greenhouse gas emissions and the eutrophication of water.By contrast,legumes gain access to nitrogen through symbiotic association with rhizobia.These bacteria convert nitrogen gas into biologically available ammonia in nodules through a process termed symbiotic biological nitrogen fixation,which plays a decisive role in ecosystem functioning.Engineering cereal crops that can fix nitrogen like legumes or associate with nitrogen-fixing microbiomes could help to avoid the problems caused by the overuse of synthetic nitrogen fertilizer.With the development of synthetic biology,various efforts have been undertaken with the aim of creating so-called‘‘N-self-fertilizing’’crops capable of performing autonomous nitrogen fixation to avoid the need for chemical fertilizers.In this review,we briefly summarize the history and current status of engineering N-self-fertilizing crops.We also propose several potential biotechnological approaches for incorporating biological nitrogen fixation capacity into non-legume plants.展开更多
This presentation introduces the advances inbiological nitrogen fixation research abroad, in particular,describes the great progress and achievements on itsresearch in China as follows: collection of rhizobial resourc...This presentation introduces the advances inbiological nitrogen fixation research abroad, in particular,describes the great progress and achievements on itsresearch in China as follows: collection of rhizobial resources and establishment of the largest database of Rhizobium inChina, correction and development of Rhizobium taxonomy in international; discovery of a couple of nif genes,identification and unification of linkage among the nif gene operons of Klebsiella pneumoniae, finding of regulative mechanism of positive regulation nif gene and its sensitivity to oxygen, temperature; finding of the activity of nodulation gene nodD3 product in Sinorhizobium meliloti which is notcontrolled by flavonoid produced from its host alfalfa;finding of the association between expression of genes coding the products for carbon utilization and nitrogen metabolism and their regulations; chemical synthesis of nodulationfactor of Sinorhizobium meliloti; constructions of engineered nitrogen fixers and utilization in practice based on theresearch of gene expression and regulation; chemicalsimulation of the structure and function of nitrogenase and bringing forward the model of nitrogenase active center for the first time in international and synthesis of modelcompounds which were paid attention by colleagues abroad. Finally, the development of nitrogen fixation research inChina in future has been put forward, suggesting that the nifgene regulation and its role in providing crops with nitrogen element, signal transduction and molecular interactions between Rhizobium and legume, coupling between carbonand nitrogen metabolisms, nitrogen fixation andphotosynthesis, and functional genomics of nitrogen-fixing nodule symbiosis, etc., would be actively worked on.展开更多
Many insects feed on a low‐nitrogen diet,and the origin of their nitrogen supply is poorly understood.It has been hypothesized that some insects rely on nitrogen‐fixing bacteria(diazotrophs)to supplement their diets...Many insects feed on a low‐nitrogen diet,and the origin of their nitrogen supply is poorly understood.It has been hypothesized that some insects rely on nitrogen‐fixing bacteria(diazotrophs)to supplement their diets.Nitrogen fixation by diazotrophs has been extensively studied and convincingly demonstrated in termites,while evidence for the occurrence and role of nitrogen fixation in the diet of other insects is less conclusive.Here,we summarize the methods to detect nitrogen fixation in insects and review the available evidence for its occurrence(focusing on insects other than termites).We distinguish between three aspects of nitrogen fixation investigations:(i)detecting the presence of potential diazotrophs;(ii)detecting the activity of the nitrogen‐fixing enzyme;and(iii)detecting the assimilation of fixed nitrogen into the insect tissues.We show that although evidence from investigations of the first aspect reveals ample opportunities for interactions with potential diazotrophs in a variety of insects,demonstrations of actual biological nitrogen fixation and the assimilation of fixed nitrogen are restricted to very few insect groups,including wood‐feeding beetles,fruit flies,leafcutter ants,and a wood wasp.We then discuss potential implications for the insect's fitness and for the ecosystem as a whole.We suggest that combining these multiple approaches is crucial for the study of nitrogen fixation in insects,and argue that further demonstrations are desperately needed in order to determine the relative importance of diazotrophs for insect diet and fitness,as well as to evaluate their overall impact on the ecosystem.展开更多
Despite its enormous benefits,mining is respon-sible for intense changes to vegetation and soil properties.Thus,after extraction,it is necessary to rehabilitate the mined areas,creating better conditions for the estab...Despite its enormous benefits,mining is respon-sible for intense changes to vegetation and soil properties.Thus,after extraction,it is necessary to rehabilitate the mined areas,creating better conditions for the establishment of plant species which is challenging.This study evaluated mineral and organic fertilization on the growth,and carbon and nitrogen(N)metabolism of two Crotalaria species[Cro-talaria spectabilis(exotic species)and Crotalaria maypu-rensis(native species from Carajás Mineral Province(CMP)]established on a waste pile from an iron mine in CMP.A control(without fertilizer application)and six fertilization mixtures were tested(i=NPK;ii=NPK+micronutrients;iii=NPK+micronutrients+organic compost;iv=PK;v=PK+micronutrients;vi=PK+micronutrients+organic compost).Fertilization contributed to increased growth of both species,and treatments with NPK and micronutrients had the best results(up to 257%cf.controls),while organic fertilization did not show differences.Exotic Crotalaria had a greater number of nodules,higher nodule dry mass,chlorophyll a and b contents and showed free ammonium as the predominant N form,reflecting greater increments in biomass compared to native species.Although having lower growth,the use of this native species in the rehabilitation of mining areas should be considered,mainly because it has good development and meets current government legislation as an opportunity to restore local biodiversity.展开更多
Optimized nitrogen(N)management can increase N-use efficiency in intercropping systems.Legume-nonlegume intercropping systems can reduce N input by exploiting biological N fixation by legumes.Measurement of N utilizat...Optimized nitrogen(N)management can increase N-use efficiency in intercropping systems.Legume-nonlegume intercropping systems can reduce N input by exploiting biological N fixation by legumes.Measurement of N utilization can help in dissecting the mechanisms underlying N uptake and utilization in legume-nonlegume intercropping systems.An experiment was performed with three planting patterns:monoculture maize(MM),monoculture soybean(SS),and maize-soybean relay intercropping(IMS),and three N application levels:zero N(NN),reduced N(RN),and conventional N(CN)to investigate crop N uptake and utilization characteristics.N recovery efficiency and 15N recovery rate of crops were higher under RN than under CN,and those under RN were higher under intercropping than under the corresponding monocultures.Compared with MM,IMS showed a lower soil N-dependent rate(SNDR)in 2012.However,the SNDR of MM rapidly declined from 86.8%in 2012 to 49.4%in 2014,whereas that of IMS declined slowly from 75.4%in 2012 to 69.4%in 2014.The interspecific N competition rate(NCRms)was higher under RN than under CN,and increased yearly.Soybean nodule dry weight and nitrogenase activities were respectively 34.2%and 12.5%higher under intercropping than in monoculture at the beginning seed stage.The amount(Ndfa)and ratio(%Ndfa)of soybean N2 fixation were significantly greater under IS than under SS.In conclusion,N fertilizer was more efficiently used under RN than under CN;in particular,the relay intercropping system promoted N fertilizer utilization in comparison with the corresponding monocultures.An intercropping system helps to maintain soil fertility because interspecific N competition promotes biological N fixation by soybean by reducing N input.Thus,a maize-soybean relay intercropping system with reduced N application is sustainable and environmentally friendly.展开更多
Three assays were developed from April 3, 1995 to October 10, 2005. The work with corn was conducted in a greenhouse, using velvet bean (Mucuna aterrima) and sunn hemp (Crotalaria juncea) as green manure with 15N labe...Three assays were developed from April 3, 1995 to October 10, 2005. The work with corn was conducted in a greenhouse, using velvet bean (Mucuna aterrima) and sunn hemp (Crotalaria juncea) as green manure with 15N labeling of either shoots or roots, in two soils with contrasting textural classes. The mineralization of N from legume plants incorporated into the two soils was investigated too. This work included two green manures: velvet bean and sunn hemp, and the common bean (Phaseolus vulgaris) residues. Nitrogen from the velvet bean accounted for a greater proportion of the soil inorganic N;shoots were responsible for most of N accumulated. Common bean residues caused immobilization of inorganic N. The leguminous species added were intensively and promptly mineralized, preserving the soil native nitrogen. One hundred days after emergence of the corn, velvet bean provided higher accumulation of nitrogen in the soil, higher absorption by corn plants and accumulation in the aerial part. The green manure decomposition was more intense in the medium textured soil. In this soil, highest nitrogen losses were also observed. The sugarcane (Saccharum spp.) was cultivated for five years in the field and was harvested three times;15N recovery was evaluated in the first two harvests. The combination of inorganic fertilizer and green manure resulted in higher sugarcane yields than either N source applied separately;however, in the second cutting the yields were higher where sunn hemp was used than in plots with ammonium sulfate. The recovery of N by the first two consecutive harvests accounted for 19% to 21% of the N applied as sunn hemp and 46% to 49% of the N applied as ammonium sulfate. Very little inorganic N was present in the 0-40 cm soil layer with both N sources.展开更多
To ascertain the possibility of cultivating maize using biological nitrogen fixation(BNF)by leguminous green manure crops in maize/leguminous green manure intercropping systems,BNF and nitrogen(N)transfer were studied...To ascertain the possibility of cultivating maize using biological nitrogen fixation(BNF)by leguminous green manure crops in maize/leguminous green manure intercropping systems,BNF and nitrogen(N)transfer were studied in Xining and Wuwei,two typical northwestern Chinese cities.The experimental treatments included monocultured maize,monocultured green manures(hairy vetch and common vetch),and their intercropping systems.The proportions of N derived from the atmosphere(%N_(dfa))in intercropping systems were not significantly different from that in monocultured green manure systems at either experimental site,except for that in hairy vetch(HV)in Xining.The amount of N derived from the atmosphere(N_(dfa))of common vetch(CV)significantly decreased from 1.16 and 1.10 g/pot in monoculture to 0.77 and 0.55 g/pot when intercropped with maize,in Xining and Wuwei,respectively,and the N_(dfa) of HV when intercropped significantly decreased from 1.02 to 0.48 g/pot in Xining.In the intercropping systems in Xining and Wuwei,the amounts of N transferred(N_(transfer))from CV to maize were 21.54 and 26.81 mg/pot,accounting for 32.9 and 5.9%respectively of the N accumulation in maize,and the values of N_(transfer) from HV to maize were 39.61 and 46.22 mg/pot,accounting for 37.0 and 23.3%,respectively,of the N accumulation in maize.Path analysis showed that soil nutrient and green manure biomass were mainly related to N_(dfa),and thatδ^(15) N had a primary relationship with N_(transfer).We found that 5.9-37.0%of N accumulation in maize was transferred from green manures,and that the N transfer ability to maize of HV was higher than that of CV.In conclusion,intercropping with leguminous green manures provided a feasible way for maize to effectively utilize biologicallyfixed N.展开更多
Introduction:Nitrogen fixation by microorganisms within biological soil crust(“biocrust”)communities provides an important pathway for N inputs in cool desert environments where soil nutrients are low and symbiotic ...Introduction:Nitrogen fixation by microorganisms within biological soil crust(“biocrust”)communities provides an important pathway for N inputs in cool desert environments where soil nutrients are low and symbiotic N-fixing plants may be rare.Estimates of N fixation in biocrusts often greatly exceed that of N accretion rates leading to uncertainty regarding N loss pathways.Methods:In this study we examined nitrogen fixation and denitrification rates in biocrust communities that differed in N fixation potential(low N fixation=light cyanobacterial biocrust,high N fixation=dark cyanolichen crust)at four temperature levels(10,20,30,40°C)and four simulated rainfall levels(0.05,0.2,0.6,1 cm rain events)under controlled laboratory conditions.Results:Acetylene reduction rates(AR,an index of N fixation activity)were over six-fold higher in dark crusts relative to light crusts.Dark biocrusts also exhibited eight-fold higher denitrification rates.There was no consistent effect of temperature on denitrification rates,but there was an interactive effect of water addition and crust type.In light crusts,denitrification rates increased with increasing water addition,whereas the highest denitrification rates in dark crusts were observed at the lowest level of water addition.Conclusions:These results suggest that there are no clear and consistent environmental controls on short-term denitrification rates in these biologically crusted soils.Taken together,estimates of denitrification from light and dark biocrusts constituted 3 and 4%of N fixation rates,respectively suggesting that losses as denitrification are not significant relative to N inputs via fixation.This estimate is based on a previously published conversion ratio of ethylene produced to N fixed that is low(0.295),resulting in high estimates of N fixation.If future N fixation studies in biologically crusted soils show that these ratios are closer to the theoretical 3:1 ratio,denitrification may constitute a more significant loss pathway relative to N fixed.展开更多
文摘Biological nitrogen fixation is a very valuable alternative to nitrogen fertilizer. This process will be discussed in the “Biological Nitrogen Fixation” book. A wide array of free-living and associative nitrogen fixing organisms (diazotrophs) will be covered. The most extensively studied and applied example of biological nitrogen fixation is the symbiotic interaction between nitrogen fixing “rhizobia” and legume plants. While legumes are important as major food and feed crops, cereals such as wheat, maize and rice are the primary food crops, but do not have this symbiotic nitrogen fixing interaction with rhizobia. It has thus been a “holy grail” to transfer the ability to fix nitrogen to the cereals and this topic will be also addressed in these books.
基金supported by the National Natural Science Foundation of China(32070270,32050081,32088102,and 31825003)the Chinese Academy of Sciences Project for Young Scientists in Basic Research(YSBR-011).
文摘Nitrogen is abundant in the atmosphere but is generally the most limiting nutrient for plants.The inability of many crop plants,such as cereals,to directly utilize freely available atmospheric nitrogen gas means that their growth and production often rely heavily on the application of chemical fertilizers,which leads to greenhouse gas emissions and the eutrophication of water.By contrast,legumes gain access to nitrogen through symbiotic association with rhizobia.These bacteria convert nitrogen gas into biologically available ammonia in nodules through a process termed symbiotic biological nitrogen fixation,which plays a decisive role in ecosystem functioning.Engineering cereal crops that can fix nitrogen like legumes or associate with nitrogen-fixing microbiomes could help to avoid the problems caused by the overuse of synthetic nitrogen fertilizer.With the development of synthetic biology,various efforts have been undertaken with the aim of creating so-called‘‘N-self-fertilizing’’crops capable of performing autonomous nitrogen fixation to avoid the need for chemical fertilizers.In this review,we briefly summarize the history and current status of engineering N-self-fertilizing crops.We also propose several potential biotechnological approaches for incorporating biological nitrogen fixation capacity into non-legume plants.
基金supported by the National Key Basic Research Project(Grant No.001CB108904).
文摘This presentation introduces the advances inbiological nitrogen fixation research abroad, in particular,describes the great progress and achievements on itsresearch in China as follows: collection of rhizobial resources and establishment of the largest database of Rhizobium inChina, correction and development of Rhizobium taxonomy in international; discovery of a couple of nif genes,identification and unification of linkage among the nif gene operons of Klebsiella pneumoniae, finding of regulative mechanism of positive regulation nif gene and its sensitivity to oxygen, temperature; finding of the activity of nodulation gene nodD3 product in Sinorhizobium meliloti which is notcontrolled by flavonoid produced from its host alfalfa;finding of the association between expression of genes coding the products for carbon utilization and nitrogen metabolism and their regulations; chemical synthesis of nodulationfactor of Sinorhizobium meliloti; constructions of engineered nitrogen fixers and utilization in practice based on theresearch of gene expression and regulation; chemicalsimulation of the structure and function of nitrogenase and bringing forward the model of nitrogenase active center for the first time in international and synthesis of modelcompounds which were paid attention by colleagues abroad. Finally, the development of nitrogen fixation research inChina in future has been put forward, suggesting that the nifgene regulation and its role in providing crops with nitrogen element, signal transduction and molecular interactions between Rhizobium and legume, coupling between carbonand nitrogen metabolisms, nitrogen fixation andphotosynthesis, and functional genomics of nitrogen-fixing nodule symbiosis, etc., would be actively worked on.
基金We thank Tamir Rosenberg for technical support.We thank Shimon Rachmilevitch,Martin Kaltenpoth,and Itamar Gilady for discussions and comments on earlier versions of the manuscript.We thank the Daniel E.Koshland Fund,the Sol Leshin Program for BGU-UCLA Academic Cooperation,and the Israel Science Foundation(Award No.364/16)for supporting this work.This is publication number 1032 of the Mitrani Department of Desert Ecology。
文摘Many insects feed on a low‐nitrogen diet,and the origin of their nitrogen supply is poorly understood.It has been hypothesized that some insects rely on nitrogen‐fixing bacteria(diazotrophs)to supplement their diets.Nitrogen fixation by diazotrophs has been extensively studied and convincingly demonstrated in termites,while evidence for the occurrence and role of nitrogen fixation in the diet of other insects is less conclusive.Here,we summarize the methods to detect nitrogen fixation in insects and review the available evidence for its occurrence(focusing on insects other than termites).We distinguish between three aspects of nitrogen fixation investigations:(i)detecting the presence of potential diazotrophs;(ii)detecting the activity of the nitrogen‐fixing enzyme;and(iii)detecting the assimilation of fixed nitrogen into the insect tissues.We show that although evidence from investigations of the first aspect reveals ample opportunities for interactions with potential diazotrophs in a variety of insects,demonstrations of actual biological nitrogen fixation and the assimilation of fixed nitrogen are restricted to very few insect groups,including wood‐feeding beetles,fruit flies,leafcutter ants,and a wood wasp.We then discuss potential implications for the insect's fitness and for the ecosystem as a whole.We suggest that combining these multiple approaches is crucial for the study of nitrogen fixation in insects,and argue that further demonstrations are desperately needed in order to determine the relative importance of diazotrophs for insect diet and fitness,as well as to evaluate their overall impact on the ecosystem.
基金This research was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq)the Instituto Tecnológico Vale(ITV),Fundação de Desenvolvimento da Pesquisa(FUNDEP)Fundação de Amparo e Desenvolvimento da Pesquisa(FADESP).
文摘Despite its enormous benefits,mining is respon-sible for intense changes to vegetation and soil properties.Thus,after extraction,it is necessary to rehabilitate the mined areas,creating better conditions for the establishment of plant species which is challenging.This study evaluated mineral and organic fertilization on the growth,and carbon and nitrogen(N)metabolism of two Crotalaria species[Cro-talaria spectabilis(exotic species)and Crotalaria maypu-rensis(native species from Carajás Mineral Province(CMP)]established on a waste pile from an iron mine in CMP.A control(without fertilizer application)and six fertilization mixtures were tested(i=NPK;ii=NPK+micronutrients;iii=NPK+micronutrients+organic compost;iv=PK;v=PK+micronutrients;vi=PK+micronutrients+organic compost).Fertilization contributed to increased growth of both species,and treatments with NPK and micronutrients had the best results(up to 257%cf.controls),while organic fertilization did not show differences.Exotic Crotalaria had a greater number of nodules,higher nodule dry mass,chlorophyll a and b contents and showed free ammonium as the predominant N form,reflecting greater increments in biomass compared to native species.Although having lower growth,the use of this native species in the rehabilitation of mining areas should be considered,mainly because it has good development and meets current government legislation as an opportunity to restore local biodiversity.
基金supported by the National Natural Science Foundation of China(31671625,31271669)the National Key Research and Development Program of China(2016YFD0300202)
文摘Optimized nitrogen(N)management can increase N-use efficiency in intercropping systems.Legume-nonlegume intercropping systems can reduce N input by exploiting biological N fixation by legumes.Measurement of N utilization can help in dissecting the mechanisms underlying N uptake and utilization in legume-nonlegume intercropping systems.An experiment was performed with three planting patterns:monoculture maize(MM),monoculture soybean(SS),and maize-soybean relay intercropping(IMS),and three N application levels:zero N(NN),reduced N(RN),and conventional N(CN)to investigate crop N uptake and utilization characteristics.N recovery efficiency and 15N recovery rate of crops were higher under RN than under CN,and those under RN were higher under intercropping than under the corresponding monocultures.Compared with MM,IMS showed a lower soil N-dependent rate(SNDR)in 2012.However,the SNDR of MM rapidly declined from 86.8%in 2012 to 49.4%in 2014,whereas that of IMS declined slowly from 75.4%in 2012 to 69.4%in 2014.The interspecific N competition rate(NCRms)was higher under RN than under CN,and increased yearly.Soybean nodule dry weight and nitrogenase activities were respectively 34.2%and 12.5%higher under intercropping than in monoculture at the beginning seed stage.The amount(Ndfa)and ratio(%Ndfa)of soybean N2 fixation were significantly greater under IS than under SS.In conclusion,N fertilizer was more efficiently used under RN than under CN;in particular,the relay intercropping system promoted N fertilizer utilization in comparison with the corresponding monocultures.An intercropping system helps to maintain soil fertility because interspecific N competition promotes biological N fixation by soybean by reducing N input.Thus,a maize-soybean relay intercropping system with reduced N application is sustainable and environmentally friendly.
基金To the technical research support of Gilberto Farias,Benedito Mota,and Maria Aparecida C.de GodoyTo FAPESP and CNPq for the grants.Piraíseeds for green manure and cover crops and Fundag for the support.
文摘Three assays were developed from April 3, 1995 to October 10, 2005. The work with corn was conducted in a greenhouse, using velvet bean (Mucuna aterrima) and sunn hemp (Crotalaria juncea) as green manure with 15N labeling of either shoots or roots, in two soils with contrasting textural classes. The mineralization of N from legume plants incorporated into the two soils was investigated too. This work included two green manures: velvet bean and sunn hemp, and the common bean (Phaseolus vulgaris) residues. Nitrogen from the velvet bean accounted for a greater proportion of the soil inorganic N;shoots were responsible for most of N accumulated. Common bean residues caused immobilization of inorganic N. The leguminous species added were intensively and promptly mineralized, preserving the soil native nitrogen. One hundred days after emergence of the corn, velvet bean provided higher accumulation of nitrogen in the soil, higher absorption by corn plants and accumulation in the aerial part. The green manure decomposition was more intense in the medium textured soil. In this soil, highest nitrogen losses were also observed. The sugarcane (Saccharum spp.) was cultivated for five years in the field and was harvested three times;15N recovery was evaluated in the first two harvests. The combination of inorganic fertilizer and green manure resulted in higher sugarcane yields than either N source applied separately;however, in the second cutting the yields were higher where sunn hemp was used than in plots with ammonium sulfate. The recovery of N by the first two consecutive harvests accounted for 19% to 21% of the N applied as sunn hemp and 46% to 49% of the N applied as ammonium sulfate. Very little inorganic N was present in the 0-40 cm soil layer with both N sources.
基金financially supported by the China Agriculture Research System of MOF and MARA(CARS22)the National Natural Science Foundation of China(32072678)+3 种基金the Agricultural Science and Technology Innovation Program of Chinese Academy of Agricultural Sciences(CAAS-ASTIP-2020)the Chinese Outstanding Talents Program in Agricultural Sciencethe Protection and Utilization of Crop Germplasm Resources of China Green Manure(19200393)the Fund Project of Qinghai Academy of Agricultural Sciences(2019-NKY-06)。
文摘To ascertain the possibility of cultivating maize using biological nitrogen fixation(BNF)by leguminous green manure crops in maize/leguminous green manure intercropping systems,BNF and nitrogen(N)transfer were studied in Xining and Wuwei,two typical northwestern Chinese cities.The experimental treatments included monocultured maize,monocultured green manures(hairy vetch and common vetch),and their intercropping systems.The proportions of N derived from the atmosphere(%N_(dfa))in intercropping systems were not significantly different from that in monocultured green manure systems at either experimental site,except for that in hairy vetch(HV)in Xining.The amount of N derived from the atmosphere(N_(dfa))of common vetch(CV)significantly decreased from 1.16 and 1.10 g/pot in monoculture to 0.77 and 0.55 g/pot when intercropped with maize,in Xining and Wuwei,respectively,and the N_(dfa) of HV when intercropped significantly decreased from 1.02 to 0.48 g/pot in Xining.In the intercropping systems in Xining and Wuwei,the amounts of N transferred(N_(transfer))from CV to maize were 21.54 and 26.81 mg/pot,accounting for 32.9 and 5.9%respectively of the N accumulation in maize,and the values of N_(transfer) from HV to maize were 39.61 and 46.22 mg/pot,accounting for 37.0 and 23.3%,respectively,of the N accumulation in maize.Path analysis showed that soil nutrient and green manure biomass were mainly related to N_(dfa),and thatδ^(15) N had a primary relationship with N_(transfer).We found that 5.9-37.0%of N accumulation in maize was transferred from green manures,and that the N transfer ability to maize of HV was higher than that of CV.In conclusion,intercropping with leguminous green manures provided a feasible way for maize to effectively utilize biologicallyfixed N.
基金We would like to thank Heidi Guenther,Matt Ross,and Conor Morrison,who all helped conduct the laboratory experiment.In addition,we would like to thank Will Wieder and the Townsend Lab at the University of Colorado for assistance analyzing gas samples and Dr.William Adams for providing use of laboratory equipment.Finally,we would like to acknowledge the two anonymous reviewers and Dr.Bettina Weber for reviewing the manuscript.
文摘Introduction:Nitrogen fixation by microorganisms within biological soil crust(“biocrust”)communities provides an important pathway for N inputs in cool desert environments where soil nutrients are low and symbiotic N-fixing plants may be rare.Estimates of N fixation in biocrusts often greatly exceed that of N accretion rates leading to uncertainty regarding N loss pathways.Methods:In this study we examined nitrogen fixation and denitrification rates in biocrust communities that differed in N fixation potential(low N fixation=light cyanobacterial biocrust,high N fixation=dark cyanolichen crust)at four temperature levels(10,20,30,40°C)and four simulated rainfall levels(0.05,0.2,0.6,1 cm rain events)under controlled laboratory conditions.Results:Acetylene reduction rates(AR,an index of N fixation activity)were over six-fold higher in dark crusts relative to light crusts.Dark biocrusts also exhibited eight-fold higher denitrification rates.There was no consistent effect of temperature on denitrification rates,but there was an interactive effect of water addition and crust type.In light crusts,denitrification rates increased with increasing water addition,whereas the highest denitrification rates in dark crusts were observed at the lowest level of water addition.Conclusions:These results suggest that there are no clear and consistent environmental controls on short-term denitrification rates in these biologically crusted soils.Taken together,estimates of denitrification from light and dark biocrusts constituted 3 and 4%of N fixation rates,respectively suggesting that losses as denitrification are not significant relative to N inputs via fixation.This estimate is based on a previously published conversion ratio of ethylene produced to N fixed that is low(0.295),resulting in high estimates of N fixation.If future N fixation studies in biologically crusted soils show that these ratios are closer to the theoretical 3:1 ratio,denitrification may constitute a more significant loss pathway relative to N fixed.
