Soil Nitrogen Transformation Mechanisms In Forests And Peatlands And Their Biogeochemical Implications

Nitrogen（N）is a major limiting nutrient for plant growth in most terrestrial ecosystems.However,over the recent decades,nitrogen deposition has been rapidly increasing in most regions of the world.Increased N deposition may alleviate N limitation and increase terrestrial ecosystems productivity,it usually causes N saturation,soil acidification,nutrient imbalance,biodiversity losses,and so on.In addition,climate changes（i.e.,global warming and changing precipitation）also have shown the impacts on the ecosystem function（e.g.,productivity and carbon（C）sequestration）,ecosystem structure（e.g.,plant and microbial diversity）,then caused N-induced stresses（e.g.,soil acidification）,through changing N cycling of terrestrial ecosystems,especially for some as important C,N stocks,such as forests and boreal peatlands.Soil is one of the most important N pools of forests and boreal peatlands with complex transformations among many N forms.Soil N transformation processes between labile N forms（extractable organic N[EON],ammonium[NH₄⁺],and nitrate[NO₃^-]）regulate soil N availability and interact with many ecosystem functions.Our understanding is not sufficient in terms of quantitatively analyzing soil N transformations,monitoring and observing their responses to increasing N deposition and climate change and tracing the N sources of accumulation soil N pools,despite an increasing concern around such issues,which,in turn,influence the ecological function of forests and boreal peatlands,even generate climate feedback.Natural abundances of the rare stable isotope of nitrogen,¹⁵N,are now being used widely in research on N cycling in organisms and ecosystems by integrating N cycle processes via N isotope fractionations and the mixing of various N-containing pools.In this dissertation,surrounding the soil N transformation mechanisms in forests and peatlands and their biogeochemical implications,three parts of work have been organized depending on the application of N stable isotope techniques.First,it has long been difficult to quantify the transformation processes of total soil organic and labile N forms in soils,which has left large uncertainties in evaluating atmospheric N deposition effects on soil N dynamics.Based on concentrations and natural abundances of N isotopes of soil organic N,EON,NH₄⁺,and NO₃^-,we established a new set of isotopic frameworks to constrain the fractions（f）and fluxes（F）of soil N depolymerization,mineralization,nitrification,NO3^-losses via denitrification and leaching.Since the establishment of the bacterial and chemical methods for measuring N isotopes of different soil N forms in the 2000s,this development of the quantitive isotopic elucidation of soil microbial N transformations provides new and straightforward insights into the present soil N transformation processes and allows us to evaluate the soil N status.We analyzed the concentrations andδ¹⁵N of different soil N forms from six subtropical forests,combined with the previously published databases of other five forests,to form a soil N dataset across 11 forests under a wide range of mean average temperature（MAT,8.7–21.0°C）,mean average precipitation（MAP,850-1927 mm）,and N deposition（2.4–51.7 kg-N/ha/y）in East Asia.Based on this new method,the relative fractions（f）and the cumulative product fluxes（F）of soil N depolymerization（D）,mineralization（M）,nitrification（N）,and of NO₃^-losses（L）via denitrification and leaching in forests was obtained for the 1^st time.The calculated results showed that mean f _D,f _M,f _N,and f _L of the forests were 1.4±0.0%,33.4±0.5%,67.9±1.0%,and63.9±1.4%,respectively;while the mean F_D,F_M,F_N,and F_L of the forests were62.0±2.6%,23.4±1.1%,15.5±1.0%,and 10.3±0.9%,respectively.Then,the responses of soil N transformation processes to atmospheric N deposition and other environmental variables were checked.We found that N deposition is one of the most important factors that significantly affected all four transformations.When the N deposition increased,f _D,f _M,and f _N increased,while f _L decreased among the study forests.Besides,soil C:N showed significant effects on depolymerization,mineralization,and NO₃^-losses.Mineralization was controlled by MAP,and also affected by soil p H.NO₃^-losses were also affected by MAT and soil p H.Denitrification is the main pathway of NO₃^-losses in forests.The contribution of denitrification（relative to the NO₃^-leaching）to total NO₃^-losses also increased with increasing N deposition.Thus,increased N deposition promotes the intensity of microbial transformations in forest soils.These findings are highly useful for assessing and modeling forest N cycles under different N deposition regimes.Second,due to the soil environments unfavorable for microbial activities,such as acidic water conditions,low soil temperature,frequent waterlogging,etc.,boreal peatlands are generally considered strongly N limited.However,the evidence from dissolved inorganic N（DIN）as the main source of plant N utilization and observed gross nitrification rates also proved that the strength of peat microbial N transformations might be largely overlooked in boreal peatlands.We evaluated the peat N status of three boreal peatlands of northeastern China,the concentrations of peat different N forms showed as the following:SON>DON>NH₄⁺>NO₃^-.Based on the concentrations andδ¹⁵N of peat organic N,EON,NH₄⁺,and NO₃^-,we estimate the relative fractions（f）and fluxes（F）of peat N depolymerization（D）,mineralization（M）,nitrification（N）,and of NO₃^-losses（L）via denitrification and leaching through the established quantitative isotopic method.The calculated results showed that the fractions(mean f _D,f _M,f _N,and f _L of the study peatlands were 2.9±0.5%,32.3±3.2%,46.9±1.9%,and 97.2±0.3%,respectively)and fluxes（mean F_D,F_M,F_N,and F_L were370.3±49.4 mg-N/kg,124.8±27.3 mg-N/kg,60.5±11.6 mg-N/kg,and 59.3±11.3 mg-N/kg,respectively）of peat microbial N transformations were pretty high in boreal peatlands.In addition,by analyzing the effects of soil and environmental variables on the variance in f values,we found that f _D was controlled by peat C:N,while f _M was controlled by the MAP among the study peatlands.Denitrification was the major pathway of peat NO₃^-losses.The contribution of denitrification to total NO₃^-losses was as high as 79%.These results and findings provide new geochemical evidence for understanding the peatland N cycle and its response mechanisms to environmental changes.Third,despite the existing paleontological methods,such as fossil pollen and macrofossil,which can reflect the historical vegetation variation of some specific plant species,we still lack detailed and quantitative records of historical variations of vegetation compositions,which hinders the understanding of C,N accumulated rates and predicting the role of boreal peatlands under climate changes.In chapter 6,we considered the concentrations and isotopes of C and N of nine peat profiles across seven boreal peatlands at the millennium scale and established a quantitative isotopic method to decipher the relative contributions（f）of moss(f _SM),shrub(f _SS),and graminoid(f _SG)to peats.Relative contributions of mosses,shrubs and graminoids to C,N accumulation（average 51±16%,32±13%,and 17±8%,respectively）and the corresponding C,N input rates（C:40.9±13.8,32.4±31.7,15.9±12.9 g-C/m²/y;N:1.3±0.5,0.9±0.6,0.5±0.4 g-N/m²/y）were estimated.It was found that mosses contributed as the dominant plants during the Medieval Warm Period（MWP）and the Little Ice Age（LIA）,while the moss was significant down by the graminoids and shrubs during the global warming period（GWP）.There was no significant difference in C,N input rates of mosses among the MWP,LIA,and GWP,while C,N input rates of shrubs and graminoids significantly increased which led to a significant increase in total C,N input rates during GWP.This study provides a new method for quantitatively assessing historical productivity changes of major vegetation types and reveals that vascular plant invasion significantly increased C,N sequestration in boreal peatlands.Through multiple regression analysis,it was found that the changes of temperature and precipitation in the past millennium showed no significant effect on the C,N input rates of mosses,but significant impacts on the C,N input rates of shrubs and graminoids,indicating that continuous climate warming and drying might lead to more severe vascular plants expansion of boreal peatlands in the future.These results provide the new quantitative basis for accurately assessing changes of vegetation types and the C,N sequestration capacity of boreal peatlands.