Anthropogenic and Natural Disturbances of the Nitrogen Cycle at Multiple Scales from Local to Global: A Modeling Investigation of Nitrous Oxide and Ammonia Emissions

Abstract

Nitrogen (N) is an essential element that affects natural vegetation and crop growth in terrestrial and aquatic ecosystems. Meanwhile, N is tightly coupled with carbon and other nutrients (e.g., phosphorous) that influence the structure and functioning of ecosystems in the long-term run. However, “Nitrogen Cascade” has been raised by previous scientists as a substantial amount of reactive N has been introduced into almost all ecosystems across land and oceans. Anthropogenic perturbation of the global N cycle contributed approximately two-thirds of the annual flux of reactive N (Nr) into the atmosphere in the early 21st century. The addition of excess reactive N compounds to terrestrial ecosystems plays a significant role in the enormous emissions of N-containing gases, including oxides of nitrogen (NOx), nitrous oxide (N2O), and ammonia (NH3) in the atmosphere. The high output of these N gases remains a matter of great concern to human health and the environment. Thus, it is essential to understand and quantify how natural and human disturbances have affected the N cycle including these N gas emissions at multiple scales from local to global. A growing amount of N fertilizer and manure has been applied to agricultural systems since the 1960s. This dissertation focuses on three cutting-edge research issues in the field of global N cycle as follows: First, I investigated how much N fertilizer and manure have been introduced into world’s grasslands. Much attention has been paid to N fertilizer and manure N applications to global cropland, however, there is still a lack of spatially-explicit estimates of continuous time-series datasets of manure and fertilizer N inputs in global grasslands. My research has filled a gap in global N data sets, which provided a critical data set for global land modeling. Second, this dissertation examined how Nr together with other environmental factors have affected the emissions of N-containing gases (N2O and NH3) from agricultural systems at multiple scales during the historical period. Finally, this dissertation has projected NH3 emissions under future climate change scenarios. More specifically, three global N input datasets I developed at a resolution of 0.5 degree by 0.5 degree during 1860-2016 (i.e., annual manure N deposition (by grazing animals) rate, synthetic N fertilizer and N manure application rates) showed that total N inputs, sum of manure N deposition, manure and fertilizer N application, to pastures and rangelands increased from 15 to 101 Tg N yr-1 from 1860 to 2016. Manure N deposition accounted for 83% of the total N inputs, whereas manure and fertilizer N application accounted for 9% and 8%, respectively, during 2000-2016. A coupled Dynamic Land Ecosystem Model (DLEM) with the bi-directional NH3 exchange module in the Community Multiscale Air-Quality (CMAQ) model (DLEM-Bi-NH3) was used to estimate and predict NH3 emissions from N fertilizer/manure application at multiple scales. At the global scale, results showed that the annual increase of NH3 emissions showed large spatial variations. Southern Asia, including China and India, accounted for more than 50% of total global NH3 emissions since the 1980s, followed by North America and Europe. In addition, results showed that not considering environmental factors in the empirical methods (constant emission factor in the IPCC Tier 1 guideline) could underestimate NH3 emissions in the context of global warming, with the highest difference (i.e., 6.9 Tg N yr-1) occurring in 2010. Then, we addressed how global warming would affect NH3 emissions over the 21st century based on the set of Representative Concentration Pathway (RCP) emission scenarios. The results show that compared with the period 1986-2005, NH3 emissions due to global warming would increase by 15%~31% by 2099. The Northern Hemisphere would experience a substantial increase of NH3 emissions due to the future global warming. At the regional scale, results indicated that NH3 emissions were 21.3±3.9 Tg N yr-1 from southern Asia agricultural systems with a rapidly increasing rate of 0.3 Tg N yr-1 per year during 1961-2014. Among the emission sources, 10.8 Tg N yr-1 was released from synthetic N fertilizer use, and 10.4±3.9 Tg N yr-1 was released from manure production in 2014. Ammonia emissions from China and India together accounted for 64% of the total amount in SA during 2000-2014. Based on DLEM simulations, the pre-industrial N2O emissions from terrestrial ecosystems were estimated to be 6.20 Tg N yr-1, with an uncertainty range of 4.76 to 8.13 Tg N yr-1. Among that, global croplands contributed to 0.41 (0.32-0.55) Tg N yr-1. However, due to agricultural activities, the total emissions increased by 195%, from 1.1±0.2 to 3.3±0.14 Tg N yr-1, during 1961-2014. Fertilizer N applications accounted for 70% of the total emissions during 2000-2014. At the regional scale, Europe and North America were two leading regions for N2O emissions in the 1960s. However, East Asia became the largest emitter amongst all regions after the 1990s. This dissertation has also identified knowledge gaps and limitations in existing information that need to be investigated in the future to improve our understanding of N dynamics and our ability to evaluate and predict impacts of reactive N enrichment on N-containing gas emissions and ecosystem health.