等高绿篱—坡地农业复合系统氮素循环研究
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摘要
等高绿篱—坡地农业复合系统(Contour hedgerow intercropping system)作为一种有效的、可持续发展的土地利用和综合生产途径,已被推荐为我国红壤丘陵区防治水土流失、充分利用水肥光热资源、提高系统生产力的重要措施。但是这一经营方式也有一些弱点和限制因素,在不合理的经营管理下就会表现出来,影响了这一有效措施的推广应用。为此,本研究从氮素入手,采用田间监测和室内分析相结合的研究方法,对红壤丘陵区等高绿篱—坡地农业复合经营模式氮素吸收、分布和损失进行了研究,探讨了绿篱复合经营对温室效应的影响,并基于种间相互作用机理,研究了绿篱对作物的化感效应。旨在深入了解氮素在绿篱复合系统中的吸收、分布和损失特征及种间相关关系,进而寻求优化配置模式,以期为等高绿篱复合经营技术的推广应用和山地丘陵区水土保持及生态环境建设提供理论依据。经过为期2年研究,取得主要结果如下:
(1)通过观测2年四季绿篱和作物的生长,发现坡耕地种植紫穗槐和香根草后,与玉米、小麦和蚕豆复合种植,系统土地当量比(LER)均大于1,但紫穗槐、紫穗槐/香根草、香根草3种复合经营模式之间有差异,残茬不同,LER也有所差异;与大豆复合种植,LER均小于1,尤其是大豆与紫穗槐组成的系统。
(2)在可比面积上,紫穗槐、紫穗槐/香根草、香根草与小麦、蚕豆、玉米复合时作物吸氮量均高于单作,而与大豆复合时大豆吸氮量却低于单作。小麦相对于香根草的氮营养竞争比率(CR)略大于小麦相对于紫穗槐的CR,且CR均大于1,而小麦相对于紫穗槐/香根草的CR小于1;蚕豆相对于香根草的CR大于1,且蚕豆-大豆轮作略大于蚕豆-玉米轮作,蚕豆相对于紫穗槐、紫穗槐/香根草的CR接近于1或大于1,生长季节间有差异;玉米相对于绿篱的CR均大于1;大豆相对绿篱的CR均小于1。
(3)不同绿篱复合及作物轮作类型差异引起农田全氮、铵态氮和硝态氮各组分时空变化显著。与试验前相比,土壤剖面全氮、铵态氮和硝态氮含量均有不同程度地降低。对于土壤铵态氮,0-100cm土壤剖面累积量减少量由大到小顺序依次为:香根草复合体系＞无绿篱种植体系＞紫穗槐复合体系＞紫穗槐/香根草复合体系;对于小麦-玉米、蚕豆-玉米和蚕豆-大豆3种轮作方式,紫穗槐和紫穗槐/香根草复合体系及无绿篱种植体系中均以蚕豆-大豆处理减少量最多,而香根草复合体系中蚕豆-大豆处理减少量最少,其中土壤剖面铵态氮含量夏季作物收获时降低幅度明显高于冬季作物收获时。对于土壤硝态氮,土壤剖面含量在4个生长季节呈先降低后增高,尔后又降低再升高的趋势,紫穗槐、紫穗槐/香根草复合体系及无绿篱种植体系,0-100cm土壤剖面硝态氮减少量均以蚕豆-玉米处理最低,而香根草复合体系以蚕豆-大豆处理最低;土壤剖面全氮含量变化趋势与土壤剖面硝态氮含量变化趋势一致。在土壤剖面垂直方向上,土壤全氮、铵态氮和硝态氮含量均以0-40cm土层变化最大。
(4)坡耕地单作种植紫穗槐和香根草后,一定程度上增加了土壤N_2O排放,表现为温室气体N_2O的排放源,但潜在地减小了土壤CO_2排放,表现为是温室气体CO_2的汇;与作物复合经营下土壤均表现为N_2O和CO_2的源,尤其在紫穗槐生长下效果更显著。整个玉米(大豆)季节,3种复合类型中,以紫穗槐复合体系土壤N_2O排放量最大;4种种植体系中土壤N_2O排放总量,小麦-玉米轮作均表现为最大,该结果与传统观点相左,尚需进一步研究;紫穗槐、紫穗槐/香根草和香根草复合体系及无绿篱种植体系,小麦-玉米处理土壤N_2O排放总量依次为494.81、382.27、449.73和328.84 g N hm~(-2)。坡耕地种植绿篱及轮作种植残茬,通过改变土壤氮、有机质含量,土壤温度、土壤湿度等环境因子,影响了土壤N_2O和CO_2排放,从而影响到土壤N_2O和CO_2排放通量之间的相关关系。
(5)利用~(15)N同位素对不同复合经营模式组分氮素吸收利用、分布及土壤剖面分布的示踪研究表明,对于绿篱,紫穗槐叶、杆和根体内~(15)N原子百分超依次为:单作紫穗槐＞紫穗槐复合体系＞紫穗槐/香根草复合体系,香根草茎叶和根则依次为:单作香根草＞紫穗槐/香根草复合体系＞香根草复合体系,其中紫穗槐体内~(15)N浓度均表现为叶最大,香根草体内~(15)N浓度茎叶和根系相当。对于玉米,玉米秸秆、籽粒和根系~(15)N原子百分超基本呈现香根草复合体系＞紫穗槐/香根草复合体系＞紫穗槐复合体系＞无绿篱种植的趋势,玉米体内~(15)N浓度以籽粒最大。
对于土壤,所有样地表层(0-20cm)土壤~(15)N丰度均高于下层,表层土壤~(15)N丰度从高到低依次为:无绿篱种植＞紫穗槐/香根草复合体系＞单作香根草＞香根草复合体系＞单作紫穗槐＞紫穗槐复合体系;整个剖面来看,紫穗槐为基础的系统,土壤~(15)N丰度依次为:紫穗槐/香根草复合体系＞紫穗槐复合体系＞单作紫穗槐,香根草为基础的系统,香根草复合体系＞紫穗槐/香根草复合体系＞单作香根草;3种复合系统土壤~(15)N丰度均低于无绿篱种植,且紫穗槐复合体系＜香根草复合体系＜紫穗槐/香根草复合体系。紫穗槐单作体系、香根草单作体系、紫穗槐复合体系、紫穗槐/香根草复合体系、香根草复合体系和无绿篱种植体系氮素表观回收率依次为:61.97%、66.97%、72.05%、90.53%、73.15%和77.04%;肥料氮利用率依次为:44.00%、37.72%、41.09%、51.99%、36.89%和6.03%;土壤残留率依次为17.97%、29.25%、30.96%、38.54%、36.26%和71.01%。
(6)大豆和蚕豆为受体的模拟实验表明,紫穗槐与香根草地上部水浸液显著抑制了大豆和蚕豆种子的萌发,且随着浓度升高抑制作用加强;而根际土水浸液对种子萌发无明显影响。紫穗槐地上部水浸液对大豆幼苗和根生长有显著抑制作用,对蚕豆幼苗生长表现出一定促进作用,对蚕豆根生长具有抑制作用;香根草地上部水浸液对大豆幼苗生长无显著影响,对大豆根生长在较低浓度时有一定的促进作用,在较高浓度时具有抑制效果;对蚕豆幼苗生长有一定促进作用,对蚕豆根生长有一定抑制作用。紫穗槐和香根草根际土水浸液对大豆幼苗生长无显著影响,对蚕豆幼苗生长具有一定的促进作用。
紫穗槐和香根草水浸液所得成分主要包括酸、酮、醛、酚、烯、醇和芳香族化合物等,其中苯甲酸及其衍生物、肉桂酸及其衍生物、低分子脂肪酸、苯甲醛、酚等为已研究的化感物质,可能在绿篱化感作用中起主要作用,但该试验在理想状况下进行的,所得结果只能供参考。此外,绿篱浸提液通过改变土壤水溶性总酚、铵态氮、有效磷、有机质、电导率等性质影响了作物生长。
(7)综合以上研究,紫穗槐和香根草可以作为红壤丘陵区坡耕地利用的绿篱品种,而大豆不是合适的复合作物。基于系统经济效益、生态环境效益和种间相关关系,紫穗槐复合经营下,蚕豆-玉米轮作是较好的种植方式,香根草复合经营下,小麦-玉米和蚕豆-玉米2种轮作方式差异不大;紫穗槐与大豆复合种植是最差的组合模式。
Contour hedgerow agroforestry systems are important patterns of agroforestry on the slopes in hilly region,widly popularized all over the world,which shows hedgerow intercropping is suitable for soil and water conservation,soil fertility amelioration,land productivity improvement.In the surface soil,however,competition for nitrogen and water,and above the ground the competition for sunlight between the component may occur.Thus,a field experiment was conducted in the terraces based on contour hedgerow agroforestry as the over objective of the study,discusses nitrogen abosorption,ultilization, gases emissions in time and space under field conditions,and estimates the total N loss and effect of atmosphere environment.A pot experiment was also carried out to determine the allelopathic effects of A.fruticosa and Vetiver shoots and/or rhizosphere soil.The main conclusions were as follows:
(1) The aboveground biomasses and yields of A.fruticosa and Vetiver,and wheat, fababean,maize and soybean crops were determined for 4 growing seasons in two successive years in agroforestry and cropping systems in Xianning,Hubei province, central China to analyze the effect of hedge on the growth of crops.Based on the two using hedges,land equivalent ratio(LER) was greater than 1.0 for wheat/hedge, fababean/hedge and maize/hedges intercropping regardless of hedge species,but LER was varied with hedge speices.However,in soybean/hedge intercropping was always smaller than 1.0 among three hedge intercropping systems,especially in A.fruticosa intercropping systems.
(2) The total nitrogen uptake in A.fruticosa,Vetiver,wheat,fababean,maize and soybean was also determined in agroforestry and intercropping systems.The results showed that the nitrogen acquisition in crops decreased with time.In equivalent area, nitrogen uptake of wheat was always greater in intercropping than that in corresponding cropping system in two successive years.The similar trends were observed in faba bean/hedge and maize/hedge intercropping systems.In contrast,intercropped soybean was less than that by sole soybean,whereas nitrogen uptake by intercropped A.fruticosa was also less than that sole A.fruticosa.Based on the nitrogen uptake,the competition ratio of crop to hedge was used to assess the interspecific nitrogen competition.The competition ratio of wheat to A.fruticosa(CR_(WA)) is less than CR_(WV),both CR_(WA) and CR_(WV) are grater than 1.0,which indicates that a better utilization of the nitrogen resources by the sole wheat crops compared to the intercrop wheat crops in intercropping systems above mentioned.However,in A.fruticosa/Vetiver intercropping system,the CR is smaller than 1.0.In conformity with wheat/hedge intercropping,the competition ratio of fababean to Vetiver is always greater than 1.0,and the relatively greater CR observed in faba bean-soybean rotations,yet CR in A.fruticosa and/or A.fruticosa/Vetiver is varied in two successive years.The competition ratio of maize to hedge is greater than 1.0 in the present intercropping systems,wheareas the competition ratio of soybean to hedge is smaller than 1.0,particularly in A.fruticosa intercropping system.
(3) Soil total nitrogen,ammonium-N and nitrate-N were significantly influenced by the present hedges and crop rotations.Total N and available N in soil decreased comparing with soil N content in the absence of hedges.Ammonium-N content in soil were followed in turn by vetiver intercropping system＞No hedges＞A.fruticosa intercropping system＞A.fruticosa/vetvier intercropping system;among wheat-maize, faba bean-maize and faba bean-soybean rotations,the reductions of ammonium-N in soil profile were larger in A.fruticosa intercropping system,A.fruticosa/vetvier intercropping system and no hedges,whereas the opposite trend was observed in vetiver intercropping system.Moreover,ammonium-N content decreased slowly during the growth of winter wheat(faba bean) than that of summer maize(soybean).Nitrate-N in soil decreased during the the growth of winter wheat but it increased slightly during the growth of summer maize。
(4)Soil N_2O and CO_2 fluxes were measured using the closed chamber.The results showed that conversion from cropland to A.fruticosa forestland and/or Vetiver grassland might potentially decrease the release of soil CO_2,whereas the opposite trend for the release of soil N_2O was observed.However,soil N_2O and CO_2 emissions enhanced in the presence of A.fruticosa and/or Vetiver in agroforestry systems.Soil N_2O and CO_2 fluxes were the highest in summer,and they were correlated with soil temperature in 5cm depth and moisture,but there is no significant.The effects of soil temperature in 5cm depth and moisture on soil N_2O and CO_2 fluxes were secondary to soil nitrogen content.Prehistory crops residues and species and biomass of vegetation were other important factors for soil N_2O and CO_2 emissions.And they can,in turn,influence soil nitrogen and organic matter and microbial biomass,which can further alter the magnitude of soil N_2O and CO_2 in the present environmental conditions.
(5) ~(15)N isotopes were used to trace the N abosorption and distribution in hedge,crop, and soil profile.~(15)N atom%excess in A.fruticosa was followed in turn by sole A.fruticosa＞A.fruticosa intercropping system＞A.fruticosa/vetiver intercropping,for vetiver,the following in turn were sole vetiver＞A.fruticosa/vetiver intercropping＞vetiver intercropping system.Greater ~(15)N enrichment of A.fruticosa leaf,but ~(15)N enrichment of vetiver leaf and root were equivalent.Compared to hedge,~(15)N atom%excess of maize in turn was vetiver intercropping＞A.fruticosa/vetiver intercropping＞A.fruticosa intercropping system＞No hedge,and greater ~(15)N enrichment of maize grain were found.
~(15)N abundance of surface soil in all plots were higher than those of subsoil.~(15)N abundance of surface soil in turn was no hedge＞A.fruticosa/vetiver intercropping＞sole vetiver＞vetiver intercropping＞sole A.fruticosa＞A.fruticosa intercropping.Fertilizer use efficiency of sole A.fruticosa,sole vetiver,A.fruticosa intercropping,A.fruticosa/vetiver intercropping,vetiver intercropping and no hedge in turn were 44.00%,37.72%,41.09%, 51.99%,36.89%and 6.03%,respectively;for percentage of ~(15)N fertilizer,they were in turn 17.97%,29.25%,30.96%,38.54%,36.26%and71.01%.
(6)The water extracts of the shoots of A.fruticosa and/or Vetiver affect the seed germination of soybean and faba bean and simplified vigor index(SVI),and the suppression increased with the increased concentration.The water extracts of the shoots of A.fruticosa significantly decreased seedling and root growth of soybean and root growh of fababean,wheareas enhanced seedling of faba bean to what extent.The water extracts of the shoots of Vetiver did not affect seedling of soybean and increased seedling of faba bean.The water extracts of the shoots of Vetiver in lower concentration enhanced root growth of soybean and in higher concentration suppressed root growth of soybean,and reduced root growth of faba bean to a great extent.The water extracts of the rhizosphere soil of A.fruticosa and/or Vetiver did not affect seed germination of soybean and delayed seed germination of faba bean,and did not affect seedling of soybean and improved seedling of faba bean.The water extracts of the rhizosphere soil of A.fruticosa in lower concentration enhanced root growth of soybean strongly and in higher concentration decreased root growth of soybean,and enhanced root growth of faba bean. Allelochemicals in the shoots and rhizosphere soil of hedge were analysed by gas chromatography-mass spectrometry(GC-MS).The compounds of A.fruticosa and vetiver mainly include acid,phenol,dehyde,ketone,ester and aromatic compound,and so on. The phytotoxic effects of S1-and/or R1-amended soil to the shoot length of crop might be slightly eliminated after the addition of N fertilization,but not all.
(7) Based on estimating the environmental(greenhouse gases emission) and economic (crop yields) benefits,A.fruticosa and Vetiver should be popularized in sloplands in hilly red region,whereas soybean should be excluded in the present sytems.A combination of A.fruticosa and Vetiver improved soil N utilization,but it is not sustainable due to soil N defecicy.Based on the yields of crops and soil nutrients,faba bean-maize rotation in A.fruticosa intercropping system may be a preferable cropping manner,in Vetiver intercropping system,both wheat-maize and faba bean-maize are equivalent.However, the responses to hedge species and crop rotations reported here need be verified in longer term experiment before they can be used to facilitate police and management decisions.
(1)通过观测2年四季绿篱和作物的生长,发现坡耕地种植紫穗槐和香根草后,与玉米、小麦和蚕豆复合种植,系统土地当量比(LER)均大于1,但紫穗槐、紫穗槐/香根草、香根草3种复合经营模式之间有差异,残茬不同,LER也有所差异;与大豆复合种植,LER均小于1,尤其是大豆与紫穗槐组成的系统。
(2)在可比面积上,紫穗槐、紫穗槐/香根草、香根草与小麦、蚕豆、玉米复合时作物吸氮量均高于单作,而与大豆复合时大豆吸氮量却低于单作。小麦相对于香根草的氮营养竞争比率(CR)略大于小麦相对于紫穗槐的CR,且CR均大于1,而小麦相对于紫穗槐/香根草的CR小于1;蚕豆相对于香根草的CR大于1,且蚕豆-大豆轮作略大于蚕豆-玉米轮作,蚕豆相对于紫穗槐、紫穗槐/香根草的CR接近于1或大于1,生长季节间有差异;玉米相对于绿篱的CR均大于1;大豆相对绿篱的CR均小于1。
(3)不同绿篱复合及作物轮作类型差异引起农田全氮、铵态氮和硝态氮各组分时空变化显著。与试验前相比,土壤剖面全氮、铵态氮和硝态氮含量均有不同程度地降低。对于土壤铵态氮,0-100cm土壤剖面累积量减少量由大到小顺序依次为:香根草复合体系＞无绿篱种植体系＞紫穗槐复合体系＞紫穗槐/香根草复合体系;对于小麦-玉米、蚕豆-玉米和蚕豆-大豆3种轮作方式,紫穗槐和紫穗槐/香根草复合体系及无绿篱种植体系中均以蚕豆-大豆处理减少量最多,而香根草复合体系中蚕豆-大豆处理减少量最少,其中土壤剖面铵态氮含量夏季作物收获时降低幅度明显高于冬季作物收获时。对于土壤硝态氮,土壤剖面含量在4个生长季节呈先降低后增高,尔后又降低再升高的趋势,紫穗槐、紫穗槐/香根草复合体系及无绿篱种植体系,0-100cm土壤剖面硝态氮减少量均以蚕豆-玉米处理最低,而香根草复合体系以蚕豆-大豆处理最低;土壤剖面全氮含量变化趋势与土壤剖面硝态氮含量变化趋势一致。在土壤剖面垂直方向上,土壤全氮、铵态氮和硝态氮含量均以0-40cm土层变化最大。
(4)坡耕地单作种植紫穗槐和香根草后,一定程度上增加了土壤N_2O排放,表现为温室气体N_2O的排放源,但潜在地减小了土壤CO_2排放,表现为是温室气体CO_2的汇;与作物复合经营下土壤均表现为N_2O和CO_2的源,尤其在紫穗槐生长下效果更显著。整个玉米(大豆)季节,3种复合类型中,以紫穗槐复合体系土壤N_2O排放量最大;4种种植体系中土壤N_2O排放总量,小麦-玉米轮作均表现为最大,该结果与传统观点相左,尚需进一步研究;紫穗槐、紫穗槐/香根草和香根草复合体系及无绿篱种植体系,小麦-玉米处理土壤N_2O排放总量依次为494.81、382.27、449.73和328.84 g N hm~(-2)。坡耕地种植绿篱及轮作种植残茬,通过改变土壤氮、有机质含量,土壤温度、土壤湿度等环境因子,影响了土壤N_2O和CO_2排放,从而影响到土壤N_2O和CO_2排放通量之间的相关关系。
(5)利用~(15)N同位素对不同复合经营模式组分氮素吸收利用、分布及土壤剖面分布的示踪研究表明,对于绿篱,紫穗槐叶、杆和根体内~(15)N原子百分超依次为:单作紫穗槐＞紫穗槐复合体系＞紫穗槐/香根草复合体系,香根草茎叶和根则依次为:单作香根草＞紫穗槐/香根草复合体系＞香根草复合体系,其中紫穗槐体内~(15)N浓度均表现为叶最大,香根草体内~(15)N浓度茎叶和根系相当。对于玉米,玉米秸秆、籽粒和根系~(15)N原子百分超基本呈现香根草复合体系＞紫穗槐/香根草复合体系＞紫穗槐复合体系＞无绿篱种植的趋势,玉米体内~(15)N浓度以籽粒最大。
对于土壤,所有样地表层(0-20cm)土壤~(15)N丰度均高于下层,表层土壤~(15)N丰度从高到低依次为:无绿篱种植＞紫穗槐/香根草复合体系＞单作香根草＞香根草复合体系＞单作紫穗槐＞紫穗槐复合体系;整个剖面来看,紫穗槐为基础的系统,土壤~(15)N丰度依次为:紫穗槐/香根草复合体系＞紫穗槐复合体系＞单作紫穗槐,香根草为基础的系统,香根草复合体系＞紫穗槐/香根草复合体系＞单作香根草;3种复合系统土壤~(15)N丰度均低于无绿篱种植,且紫穗槐复合体系＜香根草复合体系＜紫穗槐/香根草复合体系。紫穗槐单作体系、香根草单作体系、紫穗槐复合体系、紫穗槐/香根草复合体系、香根草复合体系和无绿篱种植体系氮素表观回收率依次为:61.97%、66.97%、72.05%、90.53%、73.15%和77.04%;肥料氮利用率依次为:44.00%、37.72%、41.09%、51.99%、36.89%和6.03%;土壤残留率依次为17.97%、29.25%、30.96%、38.54%、36.26%和71.01%。
(6)大豆和蚕豆为受体的模拟实验表明,紫穗槐与香根草地上部水浸液显著抑制了大豆和蚕豆种子的萌发,且随着浓度升高抑制作用加强;而根际土水浸液对种子萌发无明显影响。紫穗槐地上部水浸液对大豆幼苗和根生长有显著抑制作用,对蚕豆幼苗生长表现出一定促进作用,对蚕豆根生长具有抑制作用;香根草地上部水浸液对大豆幼苗生长无显著影响,对大豆根生长在较低浓度时有一定的促进作用,在较高浓度时具有抑制效果;对蚕豆幼苗生长有一定促进作用,对蚕豆根生长有一定抑制作用。紫穗槐和香根草根际土水浸液对大豆幼苗生长无显著影响,对蚕豆幼苗生长具有一定的促进作用。
紫穗槐和香根草水浸液所得成分主要包括酸、酮、醛、酚、烯、醇和芳香族化合物等,其中苯甲酸及其衍生物、肉桂酸及其衍生物、低分子脂肪酸、苯甲醛、酚等为已研究的化感物质,可能在绿篱化感作用中起主要作用,但该试验在理想状况下进行的,所得结果只能供参考。此外,绿篱浸提液通过改变土壤水溶性总酚、铵态氮、有效磷、有机质、电导率等性质影响了作物生长。
(7)综合以上研究,紫穗槐和香根草可以作为红壤丘陵区坡耕地利用的绿篱品种,而大豆不是合适的复合作物。基于系统经济效益、生态环境效益和种间相关关系,紫穗槐复合经营下,蚕豆-玉米轮作是较好的种植方式,香根草复合经营下,小麦-玉米和蚕豆-玉米2种轮作方式差异不大;紫穗槐与大豆复合种植是最差的组合模式。
Contour hedgerow agroforestry systems are important patterns of agroforestry on the slopes in hilly region,widly popularized all over the world,which shows hedgerow intercropping is suitable for soil and water conservation,soil fertility amelioration,land productivity improvement.In the surface soil,however,competition for nitrogen and water,and above the ground the competition for sunlight between the component may occur.Thus,a field experiment was conducted in the terraces based on contour hedgerow agroforestry as the over objective of the study,discusses nitrogen abosorption,ultilization, gases emissions in time and space under field conditions,and estimates the total N loss and effect of atmosphere environment.A pot experiment was also carried out to determine the allelopathic effects of A.fruticosa and Vetiver shoots and/or rhizosphere soil.The main conclusions were as follows:
(1) The aboveground biomasses and yields of A.fruticosa and Vetiver,and wheat, fababean,maize and soybean crops were determined for 4 growing seasons in two successive years in agroforestry and cropping systems in Xianning,Hubei province, central China to analyze the effect of hedge on the growth of crops.Based on the two using hedges,land equivalent ratio(LER) was greater than 1.0 for wheat/hedge, fababean/hedge and maize/hedges intercropping regardless of hedge species,but LER was varied with hedge speices.However,in soybean/hedge intercropping was always smaller than 1.0 among three hedge intercropping systems,especially in A.fruticosa intercropping systems.
(2) The total nitrogen uptake in A.fruticosa,Vetiver,wheat,fababean,maize and soybean was also determined in agroforestry and intercropping systems.The results showed that the nitrogen acquisition in crops decreased with time.In equivalent area, nitrogen uptake of wheat was always greater in intercropping than that in corresponding cropping system in two successive years.The similar trends were observed in faba bean/hedge and maize/hedge intercropping systems.In contrast,intercropped soybean was less than that by sole soybean,whereas nitrogen uptake by intercropped A.fruticosa was also less than that sole A.fruticosa.Based on the nitrogen uptake,the competition ratio of crop to hedge was used to assess the interspecific nitrogen competition.The competition ratio of wheat to A.fruticosa(CR_(WA)) is less than CR_(WV),both CR_(WA) and CR_(WV) are grater than 1.0,which indicates that a better utilization of the nitrogen resources by the sole wheat crops compared to the intercrop wheat crops in intercropping systems above mentioned.However,in A.fruticosa/Vetiver intercropping system,the CR is smaller than 1.0.In conformity with wheat/hedge intercropping,the competition ratio of fababean to Vetiver is always greater than 1.0,and the relatively greater CR observed in faba bean-soybean rotations,yet CR in A.fruticosa and/or A.fruticosa/Vetiver is varied in two successive years.The competition ratio of maize to hedge is greater than 1.0 in the present intercropping systems,wheareas the competition ratio of soybean to hedge is smaller than 1.0,particularly in A.fruticosa intercropping system.
(3) Soil total nitrogen,ammonium-N and nitrate-N were significantly influenced by the present hedges and crop rotations.Total N and available N in soil decreased comparing with soil N content in the absence of hedges.Ammonium-N content in soil were followed in turn by vetiver intercropping system＞No hedges＞A.fruticosa intercropping system＞A.fruticosa/vetvier intercropping system;among wheat-maize, faba bean-maize and faba bean-soybean rotations,the reductions of ammonium-N in soil profile were larger in A.fruticosa intercropping system,A.fruticosa/vetvier intercropping system and no hedges,whereas the opposite trend was observed in vetiver intercropping system.Moreover,ammonium-N content decreased slowly during the growth of winter wheat(faba bean) than that of summer maize(soybean).Nitrate-N in soil decreased during the the growth of winter wheat but it increased slightly during the growth of summer maize。
(4)Soil N_2O and CO_2 fluxes were measured using the closed chamber.The results showed that conversion from cropland to A.fruticosa forestland and/or Vetiver grassland might potentially decrease the release of soil CO_2,whereas the opposite trend for the release of soil N_2O was observed.However,soil N_2O and CO_2 emissions enhanced in the presence of A.fruticosa and/or Vetiver in agroforestry systems.Soil N_2O and CO_2 fluxes were the highest in summer,and they were correlated with soil temperature in 5cm depth and moisture,but there is no significant.The effects of soil temperature in 5cm depth and moisture on soil N_2O and CO_2 fluxes were secondary to soil nitrogen content.Prehistory crops residues and species and biomass of vegetation were other important factors for soil N_2O and CO_2 emissions.And they can,in turn,influence soil nitrogen and organic matter and microbial biomass,which can further alter the magnitude of soil N_2O and CO_2 in the present environmental conditions.
(5) ~(15)N isotopes were used to trace the N abosorption and distribution in hedge,crop, and soil profile.~(15)N atom%excess in A.fruticosa was followed in turn by sole A.fruticosa＞A.fruticosa intercropping system＞A.fruticosa/vetiver intercropping,for vetiver,the following in turn were sole vetiver＞A.fruticosa/vetiver intercropping＞vetiver intercropping system.Greater ~(15)N enrichment of A.fruticosa leaf,but ~(15)N enrichment of vetiver leaf and root were equivalent.Compared to hedge,~(15)N atom%excess of maize in turn was vetiver intercropping＞A.fruticosa/vetiver intercropping＞A.fruticosa intercropping system＞No hedge,and greater ~(15)N enrichment of maize grain were found.
~(15)N abundance of surface soil in all plots were higher than those of subsoil.~(15)N abundance of surface soil in turn was no hedge＞A.fruticosa/vetiver intercropping＞sole vetiver＞vetiver intercropping＞sole A.fruticosa＞A.fruticosa intercropping.Fertilizer use efficiency of sole A.fruticosa,sole vetiver,A.fruticosa intercropping,A.fruticosa/vetiver intercropping,vetiver intercropping and no hedge in turn were 44.00%,37.72%,41.09%, 51.99%,36.89%and 6.03%,respectively;for percentage of ~(15)N fertilizer,they were in turn 17.97%,29.25%,30.96%,38.54%,36.26%and71.01%.
(6)The water extracts of the shoots of A.fruticosa and/or Vetiver affect the seed germination of soybean and faba bean and simplified vigor index(SVI),and the suppression increased with the increased concentration.The water extracts of the shoots of A.fruticosa significantly decreased seedling and root growth of soybean and root growh of fababean,wheareas enhanced seedling of faba bean to what extent.The water extracts of the shoots of Vetiver did not affect seedling of soybean and increased seedling of faba bean.The water extracts of the shoots of Vetiver in lower concentration enhanced root growth of soybean and in higher concentration suppressed root growth of soybean,and reduced root growth of faba bean to a great extent.The water extracts of the rhizosphere soil of A.fruticosa and/or Vetiver did not affect seed germination of soybean and delayed seed germination of faba bean,and did not affect seedling of soybean and improved seedling of faba bean.The water extracts of the rhizosphere soil of A.fruticosa in lower concentration enhanced root growth of soybean strongly and in higher concentration decreased root growth of soybean,and enhanced root growth of faba bean. Allelochemicals in the shoots and rhizosphere soil of hedge were analysed by gas chromatography-mass spectrometry(GC-MS).The compounds of A.fruticosa and vetiver mainly include acid,phenol,dehyde,ketone,ester and aromatic compound,and so on. The phytotoxic effects of S1-and/or R1-amended soil to the shoot length of crop might be slightly eliminated after the addition of N fertilization,but not all.
(7) Based on estimating the environmental(greenhouse gases emission) and economic (crop yields) benefits,A.fruticosa and Vetiver should be popularized in sloplands in hilly red region,whereas soybean should be excluded in the present sytems.A combination of A.fruticosa and Vetiver improved soil N utilization,but it is not sustainable due to soil N defecicy.Based on the yields of crops and soil nutrients,faba bean-maize rotation in A.fruticosa intercropping system may be a preferable cropping manner,in Vetiver intercropping system,both wheat-maize and faba bean-maize are equivalent.However, the responses to hedge species and crop rotations reported here need be verified in longer term experiment before they can be used to facilitate police and management decisions.
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