Soil nitrogen biogeochemistry and its driving mechanisms
Soil nitrogen cycling and regulation
With respect to improving nitrogen (N) fertilizer use efficiency, and promoting the production efficiency of N fertilizer and soil sustainability, the main objectives of our work are to reduce accumulation of excessive soil-available N; to increase immobilization of soil-available N by microbes and minerals so that nitrogen can be stored temporarily in the soil as organic N or fixed ammonium by minerals; and to reduce ammonium volatilization, nitrification, denitrification, and leaching losses. We studied the effects of N fertilizer reduction, the use of nitrification inhibitor, and the regulation of C/N ratios on N transformation processes, and proposed a new index for evaluating soil N-supplying capacity (Yu et al. 2011).
Losses in N can occur during enzymatic hydrolysis, nitrification, and denitrification processes. Thus we should control these soil biogeochemical conversion processes in order to reduce N losses. Our results showed that the use of urease inhibitor can reduce ammonia volatilization; while the application of nitrification inhibitor can prevent oxidation of NH4+, NO3- leaching and gaseous losses (Xu et al., 2002; Zhang et al., 2010). This series of studies demonstrated the synergy of urease and nitrification inhibitors in the regulation of N transformation, thereby alleviating the problems of N loss after fertilization. Experiments in three typical areas in northeastern China, northern China, and the Yangtze River revealed that with application of nitrification inhibitors, N loss was reduced by 50% compared to the conventional fertilization regime. The inhibitors not only retained more N in the soil, but also increased crop N uptake. 15N isotope tracer experiments showed that soil C availability can be regulated by applying organic materials with a high C/N ratio. Soil microbial activity and turnover rates were then increased, thereby enhancing soil N microbial immobilization and mineral N fixation, reducing N losses, and improving N uptake by crops and N retention by soils. The newly formed organic N and fixed ammonium are labile and can be easily released for uptake and utilization, improving soil N for the subsequent crop season (Lu et al., 2010). These studies not only improved the basic theory, but also played an important role in guiding the rational and efficient use of N fertilizer during production and application.
Factors regulating N cycling processes
We explored the factors that regulate soil N mineralization, nitrification, denitrification and leaching processes and the role of soil microbes in these processes. We found that in less fertile soils the amount of ammonia oxidizing archaea (AOA) was positively correlated with nitrification potential, indicating that AOA was the main driver of nitrification, thereby challenging the traditional view of ammonia oxidizing bacteria (AOB) as the dominant microorganism of soil nitrification (Dong et al. 2013). In our studies of denitrification, we found that soil N isotopes constitute the imprint of denitrifiers, creating a record of long-term denitrification processes. This is because the soil N isotope ratio is determined primarily by the isotope fractionation during N deposition, leaching and denitrification. When these fractionation factors are known, the flux of denitrification can be estimated (Bai et al. 2012). The oxygen isotope of nitrate leachate can be used to estimate the contribution of N deposition to soil N leaching (Fang et al. 2012). In the past, due to technical limitations, δ18O of nitrification-derived nitrate has been estimated using a fixed ratio of soil H2O and O2 δ18O (2:1). But this method considered neither the isotope fractionation during nitrification nor the exchange of oxygen between nitrate (the intermediate product of nitrification) and soil water under acidic conditions. We found that the difference between measured δ18O of nitrification-derived nitrate and the estimated value using the above method was affected by elevation. At low elevation, this difference was between 5–9‰, indicating isotope fractionation during oxygen acquisition (Fang et al. 2012). We also found that the difference was negatively correlated with pH, indicating that more oxygen exchange occurred in soils with lower pH. The contribution of N deposition to N leaching may be underestimated by as much as 16% if the traditional calculated value of δ18O is used (Fang et al., 2012).
Responses and feedback of N cycling to human activity and global climate change
One of the key foci of our research has been to examine the ways through which anthropogenic activities and global climate change affect N cycling and the production of N-containing gases (e.g. N2O, NO). Such studies will contribute to predicting the effects of global climate change on ecosystems and the adoption of ecosystem management strategies. Our research has revealed that N deposition has greatly increased ammonia concentration in the soil and consequently reduced the diversity and abundance of soil microbes (Zhang & Han 2012) and nematodes due to ammonia suppression (Wei et al. 2012). Both global warming and increased N deposition will significantly enhance the rate of soil net N mineralization in the temperate steppe in northern China. Furthermore, global warming and N deposition have displayed interactive effects on soil net N mineralization due to changes in the composition of soil microbial communities (Ma et al. 2011). Under the scenario of global warming and increased precipitation, soil N availability would be the major limiting factor for soil C sequestration in the temperate steppe (He et al. 2012). Moreover, increased precipitation may increase ecosystem N stocks in the temperate steppe (Lü et al. 2011). On an overall basis, these studies show that several N cycling processes in ecosystems would be affected by global climate change, with additional consequences for ecosystem structure and function.
Nitrogen cycling is influenced not only by global climate change, but also by ecosystem management strategies. We conducted field experiments to examine the effects of large-animal grazing on N cycling in temperate steppe ecosystems. We found that grazing significantly affected total soil N turnover on an annual and seasonal basis, as well as net N turnover, and reduced emissions of N2O in the temperate steppe (Wolf et al. 2010). Gross N mineralization showed significant seasonal variation, which was controlled primarily by soil temperature. At the same time, the variation of soil net N mineralization occurred in the growing season, with soil water availability the dominant factor (Wu et al. 2012a). Grazing by large animals significantly decreased the utilization of soil inorganic N by plants and soil microorganisms, increased the accumulation of soil nitrates, and consequently increased ecosystem N loss (Wu et al. 2011). Furthermore, high grazing intensity led to greater organic C loss from the large soil aggregate class and thus contributed to increased soil organic C loss (Wu et al. 2012b).
Figure 17: Dynamics of N2O fluxes, soil air concentrations and environmental parameters in an ungrazed steppe site (UG99; a, c), and a winter-grazed site (WG; b, d). From Wolf et al. 2010.