Mechanistic Insights Into The Role Of Soil Nitrogen Transformation Processes In Regulating Soil Nitrogen Fate

Soil plays very important role in nitrogen (N) biogeochemical cycle. The investigation on soil N transformation is of important significances to understand the temporal-spatial variability in N biogeochemical cycle, to optimize N fertilization,and to make N pollution control countermeasures and so on. The project will measure N gross transformation rates in the humid subtropical soils in China using the paired 15N labelling method to investigate the relationship between soil N transformation and inorganic N form, fate, the transferred N form in soil ecosystems, to research the effects of the coupling degree between soil inorganic N form and corp’s favorite N form on N use efficiency, crop growth, and ecological environment effects, to study the relationship between nitrification rate and N loss caused by denitrification in the humid subtropical region. The research would be helpful to elucidate the mechanisms of the temporal-spatial variability in N biogeochemical cycle, to understand inorganic N conservation mechanisms and the key factors,and provide basic knowledge for N fertilization and evaluation of its environmental impacts in the areas.It is important to clarify the quantity and composition of hydrologic N export from terrestrial ecosystem and its primary controlling factors, because it affected N availability,productivity and C storage in natural ecosystems. The most previous investigations were focused on the effects of N deposition and human disturbance on the composition of hydrologic N export. However, few studies were aware of whether there were significant differences in the concentrations and composition of hydrologic N export from natural ecosystems in different climate zones, and what is the primary controlling factor. In the present study, three natural forest ecosystems and one natural grassland ecosystem that were located in different climate zones and with different soil pH range were selected. The concentrations of total dissolved N, DON, NH4+,NO3- in soil solution and stream water,soil properties,and soil gross N transformation rates were measured to answer above questions. Our results showed that NO3- concentrations and the composition pattern of hydrologic N export from natural ecosystems varied greatly in the different climate zones. The NO3-concentrations in stream water varied largely,ranging from 0. 1 mg N L-1 to 1 .6 mg N L-1. While, DON concentration in stream water did not differ significantly and the concentrations of NH4+ were uniformly low (average 0.1 mg N L-1) in all studied areas. There was a trade-off relationship between the proportions of NO3- and DON to total dissolved N in stream water. In subtropical strongly acidic forests soil area, DON was the dominance in total dissolved N in stream water. While, NO3--N became dominance in temperate acidic forests soil area, subtropical alkaline forests soil region, and the alpine meadow areas on the Tibetan Plateau. The proportions of NO3-to total dissolved N in both soil solution and stream water significantly increased with the increasing of the gross autotrophic nitrification rates (p < 0.01). Our results indicated that the characteristics of soil N transformations were the most primary factor regulating the composition of hydrologic N losses from ecosystems. The nitrification was the central soil N transformation processes regulating N composition in soil solution and hydrologic Nlosses. These results provided important information on understanding easily the composition of hydrologic N export from terrestrial ecosystem.Nitrogen (N) transformation dynamics are often seen to adapt to the prevailing climatic conditions to optimize the N usage in response to the specific plant species N uptake characteristics in natural ecosystems. Thus,the adjustment of plant species preferential N uptake to soil N transformation characteristics is key to an optimized N use efficiency (NUE) and the understanding how N losses via denitrification,leaching or runoff can be minimized. However, despite the intimate connection between plant and soil N characteristics is well known, only a few quantitative studies are available that address these internal ecosystem connections on a mechanistic level. In this study,cucumber, potato and rice which are characterized by differing optimal preferential N-uptake, and two soils with different pH (pH 4.9 and 7.8, respectively) were investigated. N recoveries of applied 15N either as nitrate or ammonium in plant and soil were determined and N losses were calculated by 15N balance. The results indicate that not only the match of applied dominant N form with the optimal preferential N-uptake of crop species, but also soil N transformation characteristics could significantly affect the recoveries and loss of applied 15N. Crops preferring ammonium took up more of the applied ammonium-N in the soil characterized by low N/M (nitrification rate/mineralization rate) ratios than in the soil with high N/M ratios.In contrast, crops preferring nitrate took up more applied ammonium-N in the soil with high N/M ratio than in the soil with low N/M ratio. A match between the applied N form with crop species preferential N uptake exists and that soil gross N transformation dynamics play an important role in providing an essential support for the specific plant associated N use. It is the intimate connection between plant and soil N dynamics that is critical for an enhanced N use efficiency with reduced N losses in monoculture agricultural systems. These observations can serve as a blueprint for the introduction of new crop species which should take into account site-specific soil and climatic conditions as well as known plant N-uptake preferences and characteristics.Nitrogen (N) fertilizer use efficiency (NUE) in flooded paddy fields is relatively low. Many N fertilizer management options have been proposed to enhance NUE andminimize environmental damage. However, few investigations are focusing on the role of the characteristics of soil N transformations in regulating NUE and N losses in paddy fields. In this study, we determined the N recoveries of applied 15N either as nitrate or ammonium in plant and soil and N losses by 15N balance in rice potexperiments under contrasting pH conditions (pH 4.9 and 7.8, respectively) to test the role of soil N transformations on NUE and N losses under rice growth conditions.Results showed that gross nitrification rates in the acidic soil (JX) were much lower than those in the alkaline soil (SC) either under 60% water holding capacity (WHC)or rice growth (flooding) conditions. The NH4+-N concentration in soil solution was maintained at a relatively high level for a long time period after N fertilizer application in JX compared to SC soil, caused by different nitrification rates. The 15N uptake by rice in JX soil (29～78%) was always significantly higher than in the alkaline soil (22～54%), while N losses from the plant-soil system in JX soil (17～21%)were always significantly lower than from the SC soil (20～34%) at the same rice growth stage in the labeled 15N ammonium treatment. However, there were no significant differences in 15N uptake by rice and N losses in applied 15NO3-treatment between the two studied soils. These results indicated that nitrification, not denitrification, was the key process determining NUE and N losses in paddy soils.The results of the N application gradient experiment also indicated that higher amounts of N fertilizer should be applied for the same amount of N uptake, however,this caused higher N losses, in soils characterized by high nitrification rate (e.g. SC).Results highlighted that soil N transformations in particular nitrification rate provided a very good guideline for an optimized N management.In this study,a 15N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a longterm repeated N application experiment in purple soil and red soil. The field studies were located at Yanting Agro-Ecological Station of Purple Soil, a member station of the Chinese Ecosystem Research Network(CERN), Chinese Academy of Sciences, in the center of Sichuan Province in southwestern China initiated in 2003 in the rain-fed purple soil region of China, and the Yingtan National Agroecosystem Field Experiment Station located at 38 m mean above sea level in Yujiang County,Jiangxi Province initiated in 2002, respectively, in the subtropical zone of China. The experiment in the purple soil included six fertilization treatments applied on an equivalent N basis (280 kg N ha-1), except for the residue only treatment which received 112 kg N ha-1: (1) UC, unfertilized control;(2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure(40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6)RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The field experiment in the red soil included four treatments: CK, no manure; Low rate manure (LM), receiving 150 kg N ha-1 y-1; High-rate manure (HM), receiving 600 kg N ha-1 y-1; High-rate manure and lime (HML), receiving 600 kg N ha-1 y-1 and 3000 kg Ca(OH)2 ha-1 (3y)-1. The results in the purple soil showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following longterm applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH4+)immobilization rates and caused a strong competition with nitrifiers for NH4+, thus preventing a build-up of nitrate (NO3-). Overall, in this study,we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM,OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China. Fourteen years of repeated manure application caused an increase in soil gross N mineralization and NH4+immobilization rates, although the increase was only significant following high rate of pig manure application. Both rates were further enhanced by the combined application of pig manure and lime compared with the application of pig manure alone. Gross autotrophic nitrification rates increased with increasing application rate of manure,and further increased following the combined application of pig manure and lime. In contrast, both dissimilatory NO3- reduction to NH4+ (DNRA) and NO3-immobilization rates were negligible irrespective of the manure application rate. Thusthe NO3- produced via autotrophic nitrification could not be effectively converted to NH4+ and microbial biomass N, leading to built-up of NO3- in soils. There was a positive relationship between gross autotrophic nitrification rates and N2O emissions and NO3- leaching, demonstrating that autotrophic nitrification play an important role in controlling N losses. Autotrophic nitrification is the key mechanism controlling N losses in red soils under repeated manure applications. Thus any reduction of gaseous N emission and/or N leaching should aim to control the rate of gross autotrophic nitrification.