Influences Of Moisture And Temperature On Soil Nitrogen Transformation And N₂O Emission And Guiding Significance On Regulating Measures In Subtropical And Tropical Regions

Soil moisture and temperature are the major factor regulate soil nitrogen （N） transformation, which affects soil N transformation through both instantaneously and persistently. The instantaneous effects on N transformation in soils observed immediately after altering soil moisture and temperature and the persistent effects refer to those regulated by the inherent soil properties. Understanding the persistent effects and instantaneous effect of soil moisture and temperature on N transformation and nitrous oxide （N₂O） emissions in soils provides insights into nitrogen （N） transformation and in mitigating greenhouse effects. Hainan Island is covers only 35,100 square kilometers and the annual mean temperature ranged from 24-25℃ but the humidity was varies from humid to semi-humid and further to semiarid zones from the East to the West, which is excellent for exploring the effects of instantaneous and persistent effects of humidity on N transformation and N₂O emissions. Therefore, in this study, six forest soils were collected in each climate zone and a 15N dilution method associated with a MCMC model were conducted to study the effect of persistent effects of soil moisture on gross N transformation and N₂O emission. In addition, laboratory experiments were employed to study the instantaneous effects of soil temperature and moisture on soil inorganic N and N₂O emission and the effects of litters and nitrification inhibotr on N transformation and N₂O emissions.A laboratory study was conducted to study the gross N transformation rates in different soils by employing 15N tracing methods under conditions of 60% WHC and 25℃. The gross rate of mineralization and turnover rate of soil organic N increased significantly （P<0.05 for both）, but net mineralization rate did not differ significantly with the increase of aridity index （Di） （P>0.05）. Gross and net rate of nitrification increased significantly （P<0.05 for both）, while cumulative N₂O emissions over the incubation period did not differ significantly along the aridity gradient （P>0.05）. Statistical analysis showed that soil pH, which was correlated significantly with Di, was correlated significantly with the gross rate of N mineralization, turnover rate of soil organic N, gross nitrification（P<0.01）. In addition, the caly contents was positively correlated with gross nitrification rate（P<0.Ol）. N₂O emissions did not vary significantly from the humid soils to the semiarid soils along the aridity gradient. Balanced relationships between the gross rates of NH4+ nitrification and the ratio of N₂O in the products responding to the change in soil pH are most likely the main reason that N₂O emissions did not vary with the significant variations of gross and net nitrification rates among the climate regions. The investigation indicated that the climate-induced soil properties had significant effects on the gross and net N transformation rates, but not on net mineralization and N₂O emissions in the forests in the Hainan Island.To study the instantaneous effects of soil moisture and temperature on N₂O emissions, laboratory studies were conducted to study the effects of soil moisture and temperature on soil net transformation rates and N₂O emissions in the soils collected from the humid and semi-arid zones in tropical forest in Hainan Island. The differences were observed in the responses of soil net mineralization and nitrification to soil moisture in the soils collected from different climate zones. The net mineralization rate in the humid soil was all below zero and was largest in 45% WHC. However, the net mineralization rates in the semiarid soil were all above zero except at 25% WHC and was the largest in 65% WHC. The N₂O in both soils increased with the increasing of soil moisture. Below 85% WHC, the N₂O emissions were not significantly different between two soils, indicating that the responses of N₂O emissions to soil moisture were not affected by soil properties, such as pH or clay contents.The net mineralization rates increased with increasing of temperature from 20℃ to 40℃ in humid and semiarid soils. The net nitrification rate also increased with increasing of temperature as well. The net nitrification rates were significantly lower at 20℃ and 30℃ than at 40℃, but there was not significant difference between 20℃ and 30℃ in humid soil. However, the net nitrification rates significantly increased from 20℃ to 40℃。 Accumulated N₂O emission increased with increasing of temperature, suggesting that the N₂O emission was mainly affected by instantaneous effects.Litter management was an important measure regulating nitrogen transformation in the forest soils and the effects was affected by soil properties and the climate. Therefore, an laboratory experiment was conducted to study the effect of litter addition on nitrogen transformation. The results indicate that the addition of litter with a C/N of 33 collected from the broad-leaved forest decreased the net mineralization rates in both soils. Addition of litter altered the responses of net mineralization to temperature as well. The net nitrification decreased after addition of litter at various temperatures in humid soil, but increased of net nitrification in semi-arid soil. At the 20℃-30℃ the addition of litter significantly simulated the N₂O emission in humid soil （p<0.05）. However, at 40℃, the addition of litter significantly decreased N₂O emission. The addition of litter significantly increase the N₂O emission in semiarid soil （P<0.05）.Laboratory experiment was conducted to examine the relationship between inhibitory efficiency of DCD and Nitrapyrin on soil nitrification in paddy soils. Six soils with various pH and textures were collected in China. The study was conducted at 25℃ and 60%WHC. The results showed that the relative nitrification was positively correlated with soil pH. In addition, the accumulative N₂O emission were positively correlated with soil relative nitrification, which suggested that N₂O was mainly produced through nitrification. The inhibitory efficiency of Nitrapyrin on soil nitrification and N₂O emission were all higher than DCD and the inhibitory efficiency of Nitrapyrin on N₂O emission increased with increasing soil nitrification capacity, but had no significant relationship. The absolute reduction of N₂O emission by nitrification inhibitors increased with increasing soil nitrification capacity. Both DCD and Nitrapyrin had a low inhibitory efficiency on N₂O emission in acid soils which had a relative low nitrification capacity, might be due to that the N₂O was mainly produced through heterotrophic nitrification in these acid paddy soils.