| Nitrogen is one of the largest macronutrients necessary for plant growth. Nitrogen is mainly present in organic forms in soils even though most plants absorb inorganic nitrogen, which accounts proximately 1% of total N in soils. Organic nitrogen in soils is transformed into inorganic one through mineralization and then absorbed by plants. It is thus necessary for us to realize the processes of mineralization in soils that is required to make recommendations for fertilization and to minimize nitrogen losses into environments.There are many factors affecting nitrogen mineralization, including temperature, moisture, agricultural practices, crop types and so on. Much research work has been carried out on nitrogen mineralization in laboratory and field and quite different results were obtained. Purple soil widespread Southwest China is agriculture utilized. In recent decade, a great change in nitrogen mineralization in the soil and lost into environment has been taken place due to intensive agriculture utilization and fertilization especially in the soils grown for vegetables around the cities. It could be thus helpful for us to understand the nitrogen transformation and losses into water in order to make recommendations for reasonable fertilization and environmental protection.Nitrogen mineralization and losses into environment have been simulated in laboratory for quite a long time. First-order dynamic model was brought forward by Stanford and used to quantify soil nitrogen mineralization through simulation experiments (1972). But it is difficult to extrapolate nitrogen mineralization in the field, since it is impossible to reflect the real situation in field. Hatch et al. (1992) developed a technique to quantify nitrogen mineralization and leachate, termed as field soil core incubation. This method has been widely used and proved effectively in undisturbed forests and grasslands with constant nitrogen mineralization. Sha Li-qing and Zhou Cai-ping also used this technique to study nitrogen mineralizatyion in forest soils in Xishuangbanna and Changbai Mountain (2000, 2001). However, application of this technique is limited to study nitrogen mineralization and losses in arable land that is intensively disturbed.Nitrate produce from mineralization and nitrification is movable in soils and easily lost in ground and surface water. In general, nitrate -N loss enhances with mineralization increased if excessively present in soils. In recent years, a great attention has been paid to nitrogen losses from arable lands and waterpollution it resulted in. as well as the contribution to water eutrophication. Due to lack of the effective technique for measurement of nitrogen mineralization and loss in field conditions, it is only possible for us to roughly estimate the gross amount of nitrogen lost into water and to evaluate the role of soil and fertilizer nitrogen from arable lands in water eutrophication.In our present experiments, we improved Hatch's technique to suit the measurement of nitrogen mineralization and nitrogen leaching in arable purple soils. They included typical representative fertile soil for vegetable cultivation and barren ones on slopes. Nitrogen mineralization and losses were measured by soil core incubation technique in the fields. A resin bag was placed at the base of soil core to catch nitrate moved downward from it. At the same time, laboratory simulation experiments were also carried out to determine nitrogen mineralization and leaching from soils so as to reveal mutual relationship between field and laboratory simulation experiments . The main objectives are (i) to develop technique suitable for the measurement of nitrogen mineralization and losses in arable lands, (ii) to determine nitrogen mineralization and losses in fields, (iii) to explore the factors influencing nitrogen mineralization and losses. The following are the main achievements obtained in our experiments:Field experiment Larger changes in NO3'-N, NHt+-N, available N (hydrolyzed by IN NaOH), N mineralization and nitrate leaching were found in acid purple soil for vegetable cultivation, whilst their variation coefficients were less compared to the other soils. NlV-N and available N changed more greatly in contrast to NO3'-N and their variation coefficients which varied less in yellow soil spread in the slop. Similarly, NO3"-N, NH/-N and available N varied less in alkali purple soil on slope. In details, variation coefficients of NH/-N ranged from 3.6% to 29.2% which were higher than those of NO3"-N, ranging from 2.0% to 15.3% in the three soils. No obvious changes in the variation coefficients of NlV-N and NCV-N were detected seasonally. 4 to 30 replications are necessary to reach 10% of precision for relate measurements. 14 (acidic purple soil), 10 (yellow soil) and 18 (alkaline purple soil) of replications were adopted in our experiments, respectively, indicating that relative accuracy in the experiments.Statistic analyses indicated that the contents of NO3'-N and NH4+-N significantly influenced the amount of NO3'-N losses in three experimental soils. Significant correlations were observed between NO3'-N losses and the contents of the two inorganic nitrogen in the soils. In acidic purple soil significant correlation was found between NO3'-N leachate and available nitrogen while there did not exist correlation in the other two soil.Nitrification rate, ammonification rate, mineralization rate, nitrification ratio, ammonification ratio, mineralization ratio and nitrate leachate ratio varied most greatly in the acidic purple soil in contrast to the alkaline purple soil in which they did less compared to the other soils experimented. Larger variations of ammonification rate and ratio were detected in the yellow soil. Taking into account of high content of NO3-N acidic purple soil grown for vegetables, which was ten times over than that in other soils, due tolarge amount of nitrogen fertilizers applied, it seems reasonable to suggest the great influences of fertilization and NO3 -N in soils on nitrogen mineralization and losses.There existed significant positive correlations between nitrification ratio via the ratios of mineralization and nitrification in the acid purple soil. Similarly, mineralization ratio correlated positively with nitrogen leachate ratios and ammonification ratio correlated negatively with the ratios of nitrification, nitrogen mineralization and leaching in the same soil. In the yellow soils, significant positive correlations were found between nitrification ratio and leachate ratios, whilst significant negative ones between mineralization and nitrification ratios. In the alkaline purple soil, no significant correlation was observed with ammonification via nitrification ratios in relation to nitrogen loss ratio. On the other hand, significant negative correlations were found with mineralization ratio via nitrogen loss ratio and ammonification ratio via nitrification ratio. The results indicated thus quite complex relations between mineralization and nitrogen loss in soils and they were influenced by many soil factors.Laboratory experiment Laboratory experiments were carried out to investigate nitrogen transformation in soils in variable temperatures and soil moisture in order to simulate it in fieldsWhen temperature kept at 20 "C, they did not change with the increase of soil water content in acidic purple soil. In yellow soil nitrification ratio increased while ammonification ratio decreased. In contrast, mineralization and loss did not change. In alkali purple soil Changes of nitrification ratio, ammonification ratio and leaching have not been taken up except for increased mineralization ratio. At 25*0, there existed a tendency of increased nitrification, decrease ammonification and constant mineralization and nitrogen loss as soil moisture increased in acidic purple soil. Similar changes were also observed in yellow soil except nitrogen loss which was decreased. In contrast, nitrification, ammonification, mineralization and nitrogen loss changed little if any as soil moisture increased. At 30*C, nitrification magnified in contrast to ammonification (decreased), mineralization and nitrogen loss (unchanged) with moisture increased in three experimental soils.When soil water content is 40% of field capacity, the different temperature had different effect on nitrification ratio of acid purple soil in the sequence: 20°C, 30°C>25*C, and ammonification ratio is 25"C >20°C, 30'C. Mineralization ratio and leaching ratio increased with the increase of soil temperature. Nitrification ratio increased with the increase of temperature, however ammonification ratio did not exist difference between at 20°C and at 25X: and it decreased when temperature increased to 30°C in yellow soil. Mineralization ratio is contrary to the rule of ammonification ratio and leaching ratio increased with the increase of temperature in yellow soil. In alkali purple soil nitrification ratio increased when 20*C increased to 25*C, and it did not change when temperature increased to 30'C. Ammonification ratio did not change with the change of temperature and mineralization ratio is different at different temperature in the sequence: 20TC>30"C>25"C. Leaching ratio did not change ranging from 20°C to 25*C, but it increasedwhen temperature reached to 30°C.Results of different soils were different under the same temperature. At 20°C nitrification ratio and mineralization ratio in purple soil were higher, but ammonification ratio was higher in yellow soil. Leaching ratio did not exist difference among three soils. At 25 'C nitrification ratio was higher in purple soil, ammonification ratio and mineralization ratio were higher in yellow soil. At 30'C nitrification ratio was higher in purple soil, but ammonification ratio and leaching ratio in yellow soil.When soil water content is 80% of field capacity, the soil under different temperature represented different results. Nitrification ratio and ammonification ratio did not change with the increase of temperature, but mineralization ratio increased in acid purple soil. Leaching ratio increased when temperature increased from 20"C to 25"C, but it did not change when temperature increased to 30'C. In yellow soil nitrification ratio increased and ammonification ratio decreased with the increase of temperature. Mineralization ratio did not change and leaching ratio increased when temperature reached to 30'C. In alkali purple soil nitrification ratio and ammonification ratio did not change with the increase of temperature. Mineralization ratio represent different under different temperature in the sequence: 20*C>30 "025X1:. Leaching ratio did not change when temperature increased from 20"C to 25*C, but it increased when temperature increased to 30'C.Results of three soils under the same temperature represent difference. At 20 *C the ratio of nitrification in purple soil was higher while ammonification ratio was higher in yellow soil. Mineralization ratio in alkali purple soil was higher. At 25 "C the ratio of nitrification in purple soil was higher while ammonification ratio and mineralization ratio were higher in yellow soil. Leaching ratio of acid purple soil was higher at 25'C. At 30'C nitrification ratio was higher in purple soil, but ammonification ratio and leaching ratio were higher in yellow soil.Three soils under the same simulated rainfall describe different results. Nitrification ratio in purple soil was higher while ammonification ratio of yellow soil was higher under different rainfall. Mineralization ratio of yellow soil was higher under three rainfalls (300mm, 400mm and 500mm) while mineralization ratio of alkali purple soil was higher under rainfall was 200mm. Leaching ratio of three soils did not exist difference when rainfalls were 400mm and 500mm. Leaching ratio of yellow soil was higher under 300mm rainfall while that of brown purple soil was higher under 200mm rainfall.Results of three soils represent difference under different rainfall. In acidic purple soil nitrification ratio was the lowest under the 400mm rainfall while ammonification ratio was the highest. Mineralization ratio did not exist under different rainfall and it was negative. Leaching ratio was higher under the rainfall (200mm and 300mm). In yellow soil nitrification ratio and ammonification ratio were lower under the 300mm rainfall while mineralization ratio and leaching ratio were higher. Nitrification ratio did not exist difference under the different rainfall in alkali purple soil. Ammonification ratio was the highest under the...
|
PDF Full Text Request