Nitrous Oxide Emission from Crop Fields and Its Role in Atmospheric Radiative Forcing

Deepanjan Majumdar

8.1 Introduction

Nitrous oxide, popularly known as a laughing gas has emerged as an important gas for environmental sustainability. In the troposphere, N2O is a chemically inert gas, but acts as a potential greenhouse gas. The greenhouse effect of N2O was first reported by Yung et al. (1976). Global warming potential of N2O, over a time horizon of 100 years, is measured 296 times that of CO2 (Ramaswamy et al. 2001). Therefore, an increasing trend of atmospheric N2O is a serious concern, although N2O emission is very low as compared to CO2, the most abundant greenhouse gas in the atmosphere (IPCC 1996a). Nitrous oxide emission from 1750 to 2000 has caused an atmospheric radiative forcing of 0.15 Wm-2 or 6% of the enhanced radiative forcing by well-mixed greenhouse gases during this time, equivalent to 2.43 Wm-2. While other greenhouse gases, like CO2, CH4 and halocarbons, have contributed 1.46, 0.48 and 0.34 Wm-2, respectively. Thus, global average surface temperature (the average of near surface air temperature over land and sea surface temperature) has increased by 0.6 ± 0.2°C over 20th century (IPCC 2001).

Before the effect of N2O on radiative balance of atmosphere could be recognized, Crutzen (1970) had pointed out its vital role in the destruction of stratospheric ozone (O3), which protects the biosphere from harmful UV-B radiation (280 nm < \ < 320 nm) (Baird 1995). Photooxidation of N2O is the major source of stratospheric NOx ("active nitrogen"), which is the main catalyst of gas-phase O3 destruction at altitudes between 25 and 40 km above earth's surface (Brasseur et al. 1999). A doubling of the atmospheric N2O might increase the UV-B entry to atmosphere by about 20% causing a 10% decrease in the O3 layer (Crutzen and Ehhalt 1977). Shea (1988) estimated that there would be 4-6% increase in skin cancer with each 1% drop in O3 layer. Because N2O is photolysed more rapidly in the stratosphere by the enhanced UV flux, the increase in N2O mixing ratio is diminished (Prather 1998).

Air Pollution Control Division, National Environmental Engineering Research Institute, Nehru Marg, Nagpur - 440020, India e-mail: [email protected]/[email protected]

S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.8. © Springer-Verlag Berlin Heidelberg 2009

Measurements on entrapped N2O in polar ice indicate that global concentration of N2O is highest in recent times during the past 45,000 years (Leuenberger and Siegenthaler 1992). After the last ice age, N2O concentration increased and then remained constant at ~275ppbv for about 10,000 years until the 19th century. Then after, globally averaged surface-mixing ratio increased from about 270nmolmol-1 (ppbV) in pre-industrial times (1750) to about 314nmolmol-1 in 1998 and continued to grow at a current rate of 0.8±0.2nmolmol-1 yr-1 or +0.25 ± 0.05% yr-1 (Prather et al. 2001).

Emission of N2O from soil is termed as a loss of added and residual soil N, which is indeed an economic loss. However, this loss is marginal in terms of nitrogen. The deleterious effects of N2O in atmosphere assume a greater significance due to its high atmospheric lifetime which is estimated between 110 and 168 years (Cicerone 1989; Prinn et al. 1990). Due to its high residence time in the atmosphere, atmospheric burden of N2O will persist for many years, even if its emission is completely stopped now. Even a low rate of N2O increase results in its atmospheric accumulation and any concentration perturbation will last for centuries. So, role of N2O, both as a greenhouse and O3 depleting gas combined with its long atmospheric lifetime, calls for its mitigation. It is one of the targetted greenhouse gases under 1997 Kyoto Protocol, which warrants emission limitations and reduction commitments for greenhouse gases, not controlled by the Montreal Protocol.

8.2 Global N2O Budget

Among the atmospheric nitrogen species, N2O abundance comes second to molecular nitrogen (N2). Nitrous oxide budget is still not well quantified today due to uncertainties in the estimation of individual sources. Several researchers have come out with budgetary estimates at different times using available literature and data (McElroy and Woofsy 1986; Davidson 1991). Frequent updating of estimates is necessary since individual source strengths vary widely with time due to wide heterogeneity of the systems emitting N2O and the complex interactions of physical, chemical and biological variables. Table 8.1 shows a compilation of recent estimates on global N2O emissions from different sources (IPCC 1996b).

8.3 Guidelines for Greenhouse Gas Inventorization from Crop Fields vis-a-vis Agriculture

IPCC has provided a detailed methodology to estimate national greenhouse gas inventories from agriculture, which has been published in documents, like "Revised 1996IPCC Guidelines for National Greenhouse Gas Inventories" (http://www.ipcc-nggip.iges.or.jp/public/gl/invs1.htm) and "Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories" (http://www.ipcc-nggip.iges.or.jp/public/gp/english/). The methodology attempts to relate N2O

Table 8.1 Estimates of global N20 budget (Tg N year-1) (IPCC 1996b)

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