PhD Thesis, 2010 - University College Cork

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Nitrous oxide flux dynamics of grassland undergoing afforestation significantly increase N 2 O emission in the short term (Steudler et al. 1991; Huttunen et al. 2003), the flux from undisturbed mature stands is governed by the tree type and the predominant hydrological situation (Jungkunst et al. 2008; Ullah et al. 2008). However, the parameters affecting N 2 O emission differ significantly between young and mature forests and include: distribution and quality of organic matter (OM); C:N ratio; microbial community composition and hydrological conditions at the site (Inagaki et al. 2004; Gartzia-Bengoetxea et al. 2009; Macdonald et al. 2009). Ball et al. (2002) showed that afforestation of well-drained arable land can lead to a continuous decrease in N 2 O emissions over several years. This result, however, has not been confirmed and no data exist for different types of soil or hydrological conditions. In addition we have found no published data on N 2 O emissions from newly afforested grasslands. While it is likely that similar phenomenon might occur due to reduced fertilisation and soil disturbance, changing soil moisture and temperature could tip the balance towards increased N 2 O emission. Such observations would provide information on the magnitude of background-type emission, intra-annual emission patterns and vital information regarding changes occurring in a changing ecosystem. One of the most remarkable qualities of nitrous oxide emission is the magnitude of variability within similar ecosystems and even the temporal variability within a single site. An overview of N 2 O emission from European grasslands (Flechard et al. 2007) demonstrated the range of possible magnitudes of annual fluxes. While the median values between extreme management practices in grasslands were between 0.17 to 0.74 kg N 2 O–N ha −1 a −1 (mean values range: 0.32 to 1.77 kg N 2 O–N ha −1 a −1 ), the range of all annual fluxes was from −0.50 to 6.48 kg N 2 O–N ha −1 a −1 (negative is uptake). Some of the variability is likely due to climatic differences between sites. However, the 35
Nitrous oxide flux dynamics of grassland undergoing afforestation reported year-to-year differences in any single site often exceed a hundred percent or of order 1–2 kg N 2 O–N ha −1 a −1 . Similar results in a grassland in Ireland were obtained by Hyde et al. (2006). Leahy et al. (2004) have shown that fertilizer-driven emission may be a major factor determining whether managed grasslands is a net global warming potential (GWP) sink or source. In such cases, conversion of the grassland to forestry and associated reduced fertilization is likely to lead to an increased role of CO 2 in the GWP balance and a reduced role of N 2 O. Uptake (negative) fluxes of N 2 O are observed less frequently during forest measurements, but fluxes between −0.02 to 7.0 kg N 2 O–N ha −1 a −1 were reported for deciduous temperate and mixed boreal forests (von Arnold et al. 2005; Matson et al. 2009). This upper limit is close to the emission rate after clear cut at a Puerto Rican sub-tropical wet forest (6.7 kg N 2 O–N ha −1 a −1 ), with the flux of 0.48 kg N 2 –N ha −1 a −1 at an undisturbed reference plot (Steudler et al. 1991). The flux rates characteristic for the wet tropical forests on Australian krasnozems were reported to be in the range of 4.36–7.45 kg N 2 –N ha −1 a −1 (Kiese and Butterbach-Bahl 2002) with particularly intensive emissions during the wet rainy season. The detailed information about these forest N 2 O studies is summarized in Table 5.1. In this paper we present a unique data set of continuous eddy covariance N 2 O flux observations of an ecosystem transitioning from fertilised grassland to a newly planted broadleaf forest over a five year period. The aim of this paper was to study the dynamics of nitrous oxide emission to quantify the change in N 2 O emissions accompanying afforestation of grassland on an impeded-drainage gley soil. 36
Page 1 and 2: National University of Ireland, Cor
Page 3 and 4: 4.5. Discussion 26 4.5.1. Peak trig
Page 5 and 6: DECLARATION I declare that this the
Page 7 and 8: ABSTRACT Nitrous oxide (N 2 O) is a
Page 9 and 10: Introduction little information exi
Page 11 and 12: Literature review 2. LITERATURE REV
Page 13 and 14: Literature review NH 3 → NH 2 OH
Page 15 and 16: Literature review 2.2. Managed gras
Page 17 and 18: Material and methods 3. MATERIAL AN
Page 19 and 20: Material and methods Over the last
Page 21 and 22: Material and methods Applying the R
Page 23 and 24: Material and methods 6. The average
Page 25 and 26: Distinguishing peaks from backgroun
Page 41: Nitrous oxide flux dynamics of gras
Page 45 and 46: Nitrous oxide flux dynamics of gras
Page 57 and 58: Gap-filling techniques for the annu
Page 67 and 68: General discussion 7. GENERAL DISCU
Page 69 and 70: Recommendations for further researc
Page 71 and 72: References REFERENCES Ball, B. C.,
Page 73 and 74: References Flechard, C. R., P. Ambu
Page 75 and 76: References grassland in the South E
Page 77 and 78: References Müller, C., R. J. Steve
Page 79: References Yashiro, Y., W. R. Kadir

PhD Thesis, 2010 - University College Cork

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