Nitrogen Cycling In Contour Hedgerow Intercropping System On Sloping Land

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 N₂O and CO₂ 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 CO₂,whereas the opposite trend for the release of soil N₂O was observed.However,soil N₂O and CO₂ emissions enhanced in the presence of A.fruticosa and/or Vetiver in agroforestry systems.Soil N₂O and CO₂ 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 N₂O and CO₂ fluxes were secondary to soil nitrogen content.Prehistory crops residues and species and biomass of vegetation were other important factors for soil N₂O and CO₂ emissions.And they can,in turn,influence soil nitrogen and organic matter and microbial biomass,which can further alter the magnitude of soil N₂O and CO₂ in the present environmental conditions.(5) ¹⁵N isotopes were used to trace the N abosorption and distribution in hedge,crop, and soil profile.¹⁵N 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 ¹⁵N enrichment of A.fruticosa leaf,but ¹⁵N enrichment of vetiver leaf and root were equivalent.Compared to hedge,¹⁵N atom%excess of maize in turn was vetiver intercropping＞A.fruticosa/vetiver intercropping＞A.fruticosa intercropping system＞No hedge,and greater ¹⁵N enrichment of maize grain were found.¹⁵N abundance of surface soil in all plots were higher than those of subsoil.¹⁵N 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 ¹⁵N 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.