The Transformation Of Nitrite Nitrogen In Acid Forest Soil And Its Main Influencing Factors

Nitrite?NO₂^--N?is generally regarded as in very low concentration comparing to the other various nitrogen?N?forms in soils.The transformation of nitrite was often overlooked in the researches on N cycling.Recent studies found that nitrite could not be underestimated as it plays as reaction intermediate in the varieties of N transformation processes,such as autotrophic nitrification,heterotrophic nitrification,denitrification et al.Therefore,it is critical to understand the mechanism of nitrite accumulation and the factors controlling nitrite transformatioa.The investigation of the nitrite transformation processes could make great contributions to improve the global N cycle.By using the paired 15N labelling technique,this thesis focused on the estimation of nitrite sources and sinks,the quantification of nitrite production and consumption rates and the influence factors of nitrite dynamic in acidic soils.To improve the unreliable extraction of nitrite in acidic soil,this study optimized the conditions for the extraction of soil nitrite.The results showed that,to maximize the extraction efficiency of inorganic N in acidic soil,the pH of KCl extracting solution should be adjusted to between 5.0 and 6.0 by the phosphate buffer,and the extraction time should be 30 min.For the soil with the pH less than 6.0,the extracting solution should be the mixture of 3 M KCI and the pH 8.4 buffer with a ratio of 4:1.For the soil with:the pH more than 7.5,the extracting solution should be a mixture of 3 M KCl and the pH 7.5 buffer with a ratio of 4:1.For the soil with the pH betrween 6.0 and 7.5,it is better to use 2 M KCI as the extracting solution.The results showed that the improved extracting method enhanced the nitrite extraction efficiency from around 21%-63%to more than 90%,which can also ensure the extraction efficiency of ammonium.To improve the accuracy of 15N abundance measurement resulting from low nitrite concentration in soil ecosystem,this study optimized the pretreatment method before the 15N abundance measurement of NO₂^--N,NO₃^--N and NH₄⁺-N.The isotopic composition of inorganic N in extracts was determined using the method based on conversion to N₂O.This method can avoid the shortage in inaccuracy of the traditional distillation and diffusion separation method.The isotopic composition of inorganic N from soil samples had good linear relationship with reference samples.The R2 values of the linear relationships were more than 0.99.The results showed that the isotopic composition measurement of soil samples and 15N tracing experimental samples got good repeatability and accuracy.The method is not only easy and rapid,but also accurate and sensitive to determine the isotopic composition with small quantity of samples.This improved method ensured the accuracy and veracity of the isotopic composition measurement.This improvement will provide technical support for N cycling research in a more accurate and scientific level.The nitrite dynamic during the inorganic N extraction process by KCl solution was investigated by 15N tracing technique basing on the improved method in this study.The potential influence on soil N cycling by the transient nature of nitrite was also estimated during the extraction of inorganic N.The results showed that rapid and vehement chemical reactions of the nitrite could lead to the fast production and consumption of nitrite during the extraction process if the adjustment of the pH of extraction solution was absent.The results showed that soil organic N is the dominant resource of nitrite during the extraction process.The pH value is the main influence factor of the accumulation of nitrite in extraction process.The rate of nitrite production is higher than its consumption rate.Meanwhile,soil organic N is the sink of nitrite during the extraction process.It is found that the ammonization is not relevant to the nitrite production and consumption during the extraction process.Thus,the unimproved method is still accurate and scientific for the estimation on soil gross N transformation by using 15N paired-labelling technique related with the ammonium and nitrate in soil ecosystem.The decrease in nitrite concentration and 15N isotopic composition could affect the accuracy of the soil nitrite transformation,so the pH adjustment of KCl solution is recommended in future studies.The nitrite dynamic in acidic forest soils at the temperate and subtropical areas was investigated under lab conditions by using the improved method in this study.The results showed that the rapid nitrite transformation could be the key driver of the N cycling in the studied acidic soils.Soil organic N is the main resource of nitrite in acidic soils.The results indicated that nitrite is not a discounted intermediate during the heterotrophic nitrification process.The pH value is the dominate influence factor of the accumulation of nitrite in acidic soils.The average rate of gross nitrite consumption is higher than that of gross nitrite production.The main sink of nitrite mainly transformed into the nitrate and organic N.It is found that dissolved organic N could be another fate of nitrite in temperate forest soils,while dissolved organic N may not be a fate of nitrite in subtropical forest soils.The nitrite could chemically transform into nitric oxide and nitrous oxide in subtropical acidic forest soils.The study on the nitrite dynamic and the influence factors could be significant to the further understanding of N cycling in soil ecosystem.Thus,the investigation on nitrite transformation process cannot be ignored in the future N cycling studies.Since pH and soil organic matter could be the most important impact factors in acidic soils,five pH gradients and four C/N gradients were set up to investigate the impact of different pH and C/N conditions on the contribution of ammonium and nitrate to the nitrite accumulation in two different acidic soils.The results showed that C/N could affect the pathway of nitrite production.With the increase of C/N,the contribution of nitrate to nitrite production reduced from 10%to 4%in farmland soils,and from 14%to 2%in forest soils.The contribution of soil organic N to nitrite production increased with the increase of C/N,which is in keep with the former studies.The contribution of nitrate to nitrite production at pH 4.5 and pH 5.5 was higher than that of other pH gradients.Under the pH 3.5 condition,the contribution of soil organic N to nitrite production was over 97%in farmland soils,and almost 100%in forest soils.The contribution of ammonium to nitrite production in all soil treatments was almost negligible in this study.In this thesis,the efficiency of nitrite extraction was improved by adjusting extracting conditions,the accuracy and precision of 15N isotopic composition were optimized by converting inorganic N to nitrous oxide.This improvement effectively solves the problems of nitrite measurement due to the low concentration and transient existence in acidic soils.The nitrite dynamic processes were studied in six patterns of acidic forest soils by using the improved method for the first time.The rates of gross nitrite production and consumption,the resource and fate of nitrite as well as the influence factors were quantitatively identified in acidic soils.This thesis provided the technique and theoretical base for the further research on soil N cycling.