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Emine Erman Kara Niğde University, Engineering Faculty, Environmental Engineering Department, Niğde-Türkiye ABSTRACT The aim of this experiment was to determine the effect of the decomposition of green manure on available N and nitrate leaching in soil profile under field conditions. The experiment consisted of 5 treatments, Grass, Clover, Bean, Vetch and controls replicated in 4 blocks. The green manure as plant biomass incorporated in the top 20cm soil layer at the begining of flowering. Soil samples were taken once a week from the 20 experimental plots, for a period of 9 weeks. The 1st and 9th samplings were to a depth of 120cm, in 20cm increaments, and the rest to 60cm. The soil was tested for NH4+-N and NO3-N in all soil samples. Ammonium concentrations did not indicate any clear trend with treatment or time, but they were almost negligible compared with nitrate concentrations. The latter were generally very high even in the control, and were greatly affected by incorporation of Vetch and Clover, wheares grass seemed to have immobilized N shortly after its incorporation. Changes with time indicated nitrate leaching to deeper soil layers. INTRODUCTION Degradation of soil organic matter (OM) under continuous cropping is a major reason for decreasing soil productivity. Decomposition of OM is a sequence of microbial processes that is enchanced by increasing temperature, aeration, and optimal moisture content. Such conditions prevail particularly in warm climates under intensive cropping and irrigation. Incorporation of plant residues into the soil is an important means to sustaining soil OM and releated soil properties, such as biological activity, available nutrients reservoir, and soil structure. The source of plant residues can be residues of the previous crop, such as wheat or cotton, green manures, mostly legumes, or wastes from plant industries. The impact of organic matter management on soil has been studied extensively from various points of view, e.g. sustainable agriculture management and soil productivity, no tillvs. Conventional tillage systems, farm residues and green manure in relation to nutrients availability and biological activity in soil (Doran and Smith, 1987; Frazer et. al., 1988; Ladd et al., 1985). The aim of this study is that the decomposition rate constants of green manures depend, apart from environmental conditions, on their composition. Microbial buildup, enzyme activity, and creation of temporal and local anaerobic microsites in soil are affected by the rate of decomposition, and together with residues C/N ratio, determine available N immobilization, net mineralization and denitrification losses. Quantification of these processes is necessary to predict changes in available N in soil following incorporation of plant residues. These changes must be considered under aspects of N fertilization management, in order to assure sufficient available N for the following crop and prevent accumulation of excess nitrate, and the consequent ecological hazards of ground water pollution. MATERIALS AND METHODS A field experiment was carried out in the Bafra Experimental station between September 1994 and July 1995. The aim of the experiment was to determine the effect of decomposition of different green manures on available nitrogen concantrations at different soil layer. The experiment consisted of 5 treatments, control, Vetch, Bean, Clover and Grass, replicated in 4 blocks (Block design) The green manure (plant biomass) was incorporated in the top 20cm soil layer. Soil samples were taken once a week from the 20 experimental plots 9 times. The 1st and 9th samplings were to a depth of 120cm, in 20cm increments, and the rest to 60cm. NH+4-N and NO-3-N were determined in all soil samples.The experimental site, previously used for studying different cropping systems, is located in the Bafra plain about 15-20cm above sea level and between the 41st and 31st degree of north length and 35th degree of east latitude. The mean temperature of the subtropic climate between 1994 and 1995 was 13.30C. The highest temperature with 22.60C could be observed in August, 1994, and the lowest one with 4.70C in March, 1995. The mean monthly precipitation during the above period was 63mm. The mean monthly temperatures and precipitations from April 1994 to March 1995 are shown in Table 1.   Experimental site amended with green manure The experimental field is set up as a block design (4 replicates) and 20 subplots. One block consists of 5 differently treated subplots. After a 6 month growth (15 March 1995) the complete biomass was incorporated by a cultivar. Table 3 shows the carbon and nitrogen contents (%) of the plant material added as green manure to the experimental field.  The experimental field is set up as a block design (4 replicates) and 20 subplots (2,8x6m). One block consists of 5 differently treated subplots . Between the blocks there are 2m broad paths and between the plors 1m broad strips for separating and handling the differently treated plots. After a 6 month growth (15 March 1995) the complete biomass was incorporated by a cultivar. Table 3 shows the carbon and nitrogen contents (%) of the plant material added as green manure to the experimental field. Just before the green manures were sown on 4 November 1994 all plots received 5kgN/da ((NH4)2 SO4 ) and 10kgP/da (Triple super phosphatase) The harvest of the green manuring plants occured on. 15th of March 1995 the upgrown plants were cutted, chopped to 10cm pieces and completely incorporated into the soil by using a cultivator. Soil sampling (two per plot, six or eight per treatment) was carried out with an Pirkhaver auger. The drawn samples were divided and combined in subsamples of 0-20cm, 20-40cm, 40-60cm, 60-80cm, 80-100cm and 100-120cm, freed from roots and organic remains and stored at 40C. In the soil samples the following chemical parameters were determined.NO3- and NH4+ were determined according to Fabig (1980). Plant analysis were carried out according to Schaller, 1988. Carbon was quantified after oxidation potassium dichromate and nitrogen by the Kjeldahl method. To determine the effect of green manuring statistically variance analysis and Duncan's test were caried out. RESULTS AND DISCUSSION NO3--N formation The effect of green manuring on soil NO3--N content was determined significant according to control plots in all applications. The effect of green manuring to the soil NO3--N was seen as Vetch>Clover>Bean>Grass respectively. Soil NO3--N content which was high on the top layer decreased gradually as the depth of the soil increased. The leaching of NO3-Ncontent was determined high according to the control plot on which Clover was applied. (Figure 1).  NH4+-N formation The effect of green manuring on soil NH4+-N content was determined high according to the control plot on which clover was applied. According to the sampling period, the soil NH4+-N content did not clearly change. Soil NH4+-N content which was high on the top layer decreased gradually as the depth of the soil increased gradually as the depth of the soil increased. The effect of green manuring to the soil NH4+-N was seen Clover >Bean> Vetch>Grass respectively. The leaching of NH4+-Ncontent was determined high according to the control plot on which Vetch and Bean was applied. (Figure 2).  Mineral nitrogen formation (N-NH4+N-NO3) The effect of green manuring on soil mineral nitrogen content was determined high according to the control plot on which clover was applied. The effect of green manuring to the soil mineral nitrogen content was seen as Vetch>Clover>Bean>Grass respectively. Soil mineral nitrogen content which was high on the top layer decreased gradually as the depth of the soil increased (Figure 3).  REFERENCES Fabig.(1980). Einfluss des Sauerstoffpartialdruckes auf die eigung von einfachenaromatischen wasserstoff-donatoren (Benzonat, p-Hydroxybenzoat) für die anaerobe Atmung (denitrifikation). Dissertation Universitat Hohenheim. Hochschulsammlung Biologie, Freiburg, 8. Ladd J.N.,Amato M.,Oades J.M.(1985). Decomposition of plant material in Australian soils. III.Residual and microbial biomass C and N from isotope-labelled legume material and soil organic matter decomposition under field conditions. Aust. J. Soil Res. 23: 603-611. Doran J.W.,Smith M.S.(1987). Organic matter management and utilization of soils and fertilizer nutrients. P.53-72. In: J.F. Frazer D.G.,Doran J.W.,Sahs W.W.,Lesoing G.W. (1988). Soil microbial populations and activity under conventional and organic management. J.Environ. Qual. 17: 585-590. Schaller K. (1988). Praktikum zur Bodenkunde und Pflanzenernahrung |