The impact of nitrogen fertilizer use on greenhouse gas emissions in an oil palm plantation associated with land use change

Resumen

Se estudian las emisiones de gases de efecto invernadero relacionadas con el cambio de uso del suelo en una plantación de palma de aceite espescífica. Se analizaron las emisiones de óxido de nitrógeno (N₂O) procedentes de la aplicación de fertilizantes nitrogenados durante la etapa de cultivo en palmeras de diferentes edades dentro de la plantación. La emisión de N₂O varía de 19.11 a 22.17 kg de N₂O-N/ha, lo que resulta en la emisión de 1052.26-1209.51 kg de CO₂-eq/ha. Sin embargo, no se encontró una relación evidente entre las emisiones de N₂O o CO₂-eq y la edad de las palmeras. Por otra parte, también se evaluó el impacto del cambio de uso del suelo en el desarrollo de la plantación mediante la evaluación de variaciones en las existencias de carbono dentro de la plantación. La conversión de finca cauchera a plantación de palma de aceite libera el contenido de carbono en el suelo (i.e., produce emisión de carbono), pero dicho fenómeno está previsto en la literatura. En conjunto, las emisiones relacionadas con el fertilizante y las vinculadas con el combustible durante la etapa de cultivo contribuyen con alrededor de 79 y 21%, respectivamente, de la emisión total de gases de efecto invernadero de la plantación. Por lo tanto es probable que la aplicación de fertilizantes nitrogenados incremente las emisiones resultantes de la transformación de finca cauchera a plantación de palma de aceite, pero los valores se encuentran dentro de los límites estimados para una plantación de palma de aceite en Malasia.

Abstract

The emissions of greenhouse gases (GHGs) in an oil palm plantation associated with land use change have been evaluated on a site-specific basis. Nitrous oxide (N₂O) emissions from the application of nitrogen fertilizers during the growth stages of the palm oil were analyzed for palms of different ages within the plantation. The N₂O release ranges between 19.11-22.17 kg of N₂O-N/ha, resulting in the emission of 1052.26-1209.51 kg of CO₂-eq/ha. However, there is no clear relationship between the emissions of N₂O or CO₂-eq and the age of the oil palms. On the other hand, the impact from land use change for the development of the site was also evaluated by assessing the emissions from carbon stock changes within the plantation. The transformation of a rubber estate into an oil palm plantation loses the soil carbon content (i.e., release of carbon emissions). However, this phenomenon has been anticipated in literature. Overall, fertilizer-related emissions and fuel emissions during the growth stages contribute to about 79 and 21%, respectively, of the total GHG emissions from the plantation. Therefore, it is likely that the application of nitrogen fertilizer may increase the existing carbon emission from the conversion of rubber to oil palm plantation, but the values are within the estimated for a Malaysian oil palm plantation.

1

Introduction

The establishment of oil palm plantations in Malaysia has rapidly expanded in the past 25 years, especially in the west coast of the Malaysian Peninsula, where soil is most fertile and productive (Henson, 2005). Oil palm has been extensively planted in parts of East Malaysia on newly explored forest land. Generally, oil palm plantations in this country have been developed from logged-over, degraded forest and also as replacement of other crops such as rubber, coconut and cocoa, since these crops have become less profitable than oil palm (MPOB, 2001; Henson, 2004). Greenhouse gases (GHG) emissions from land use change are regularly debated, particularly in relation to biofuels (e.g., establishing new plantations on agricultural land). Emissions are in particular related to changes in aboveground and belowground biomass, as well as soil organic matter (Brinkmann Consultancy, 2009). Specifically, the establishment and operation of a new plantation lead to the removal of the original aboveground and belowground carbon stocks (e.g., forest, grassland, etc.). On the other hand, a plantation stores carbon through the growth of oil palms.

Oil palm is a perennial crop. There are few important phases in its life cycle, beginning from the agricultural phase (establishment and growth of the plantation) through the oil extraction phase (Castanheira et al., 2014). The growth stages of oil palm include activities related to the development of this plant, whereby typically three stages are considered throughout its lifetime, i.e. nursery, immature plantation and mature plantation (Schmidt, 2007). It is believed that the most significant contribution to global warming from an oil palm plantation is from the agricultural stage, i.e. cultivating (15%), clearing (17%) and replanting (18%) (Schmidt, 2010). The emissions arising from operations during oil palm growth and fresh fruit bunches (FFBs) processing are in particular related to the use of fossil fuels for internal transport and machinery, fertilizers, fuels for the palm oil mill, and emissions from the palm oil mill effluent (POME) (Brinkmann Consultancy, 2009).

Common inputs of oil palm fertilizers comprise nitrogen fertilizers (i.e., ammonium nitrate, ammonium sulphate, urea and ammonium chloride), phosphate rock, potassium chloride and kieserite. The emissions released may vary between the type of fertilizers and the mode of production. Specifically, GHG emissions related to the use of fertilizers in an oil palm plantation include nitrous oxide (N₂O) emissions from the application of nitrogen fertilizers. According to the Intergovernmental Panel on Climate Change (IPCC) guidelines, 1% of N₂O-N is emitted from the total N applied during fertilizer application. Emissions from fertilizer application represent more than 50% of the overall plantation emissions (Schmidt, 2010; Castanheira et al., 2014). The amount of fertilizer used in an oil palm plantation may result in high N₂O emission into the atmosphere, ultimately leading to significant global warming (Corley and Tinker, 2003).

N₂O is a potent greenhouse gas with a global warming potential over a 100-yr. period 298 times higher than carbon dioxide (CO₂) (IPCC, 2007). Agriculture contributes about 42% ofthe increasing N₂O emission into the atmosphere. N₂O is produced in agricultural soils by microbial transformation of compounds that contain nitrogen, such as fertilizer and animal dung and urine (Giltrap et al., 2014). Artificial fertilizers are applied to boost the crop's growth (De Datta, 1995). The inputs of N-fertilizer can occur through either direct or indirect pathways. Direct N₂O emission occurs from direct addition of N-fertilizer on the soil whereas indirect N₂O emission may results from processes such as N-deposition from the atmosphere, N-fixation by legumes, and decomposition of biomass residues (Schmidt, 2007; Millar et al., 2010). The increase in available mineral N in soil may enhance the formation of N₂O through the process of nitrification and denitrification and its emission to the atmosphere (Mosquera et al., 2007; Hewitta et al., 2009; Vandermeer et al., 2009). Generally, operations during the growth stages, such as transportation, FFB processing and use of palm oil-derived products may increase N₂O emissions to the atmosphere (Klaarenbeeksingel, 2009). The focus of this study is mainly on the growth stages of oil palm, in which the GHG emissions of agricultural-related activities were estimated. Additionally, the impact on GHG emissions of land use change resulting from the establishment of an oil palm plantation was evaluated. Therefore, in this study we attempt to evaluate the impact of nitrogen fertilizers on the direct emission of GHG and to incorporate the indirect emission due to the expansion (conversion) of the oil palm plantation from the previous land use on a site-specific basis.

2

Materials and methods

2.1

Study site and data

The study was conducted at Kempas Estate, an oil palm plantation located in the north of Malacca/Johor, in the state of Malacca, covering an area of 1700 ha (Fig. 1). The average production of the oil palm plantation, managed and operated by Sime Darby Plantation Malaysia, is 24.11 Mt/ha of FFBs. Kempas Estate land use changed from a previous rubber plantation into an oil palm plantation.

Fig. 1.

Kempas Estate. (a) Location of. (b) Distribution: 1, mature palm block (> 20 yrs.; 2, mature palms (15-20 yrs.); 3, mature palms (10-15 yrs.); 4, immature palms (< 5 yrs.).

Data of nitrous oxide emissions were estimated from the amount of applied N-fertilizer, obtained from the scheduled fertilizing scheme between 1986-2009. Other relevant data for estimating N-related emissions were obtained from Schmidt (2007) for average Malaysian oil palm-related emissions. For assessing emissions from oil palms of different ages, data from the year 2012 comparing immature and mature plants was used. Oil palms were classified as immature (< 5 years) and mature (> 5 years). The typical life cycle of an oil palm is about 25-30 yrs., then new palms should be replanted (Chase and Henson, 2010). Organic carbon, soil organic matter and other relevant values for estimating the carbon stock were determined experimentally in Mejan (2013). Emissions from fuel use (e.g., for internal transportation within the estate) were determined in Abdullah (2013).

2.2

Estimation of GHG emissions and carbon stock changes

The amount of direct N₂O emissions from nitrogen fertilizer application in Kempas Estate was estimated by incorporating the type of peat soil, based on the model described by the IPCC. The equation for estimating N₂O emissions is as follows (IPCC, 2006):

where F_SN is the annual amount of synthetic N-fertilizer applied to the soils (kg N ha^–1); F_ON is the annual amount of organic N-fertilizer applied (kg N ha^–1); F_cr is the amount of nitrogen in crop residues (aboveground and belowground) returned to soils annually (kg N ha¹); F_som is the annual amount of mineralized N in mineral soils, in association with loss of soil C from soil organic matter as a result of changes to land use or management (kg N ha^–1); F_SOMis the area of organic soils cultivated annually (ha); EF₁ is the emission factor for emissions from N inputs (kg N₂O-N/kg N input); 0.0125 is the default emission factor used in this study; EF₂ is the emission factor for emissions from organic soil cultivation (kg N₂O-N/ha-yr); and 16 was used as default emission factor.

CO₂-eq is the conversion of other gases such as nitrogen to the equivalent amount of carbon dioxide based on its global warming potential (GWP). In order to calculate CO₂ equivalents, standard ratios are used to convert other gases into equivalent amounts of CO₂, which describes their total warming impact relative to CO₂ over a set period. Emissions were converted into CO₂-eqs using the GWP of the gases. According to the IPCC (2006), the equation is as follows:

where F_SN represents the amount of synthetic N-fertilizer applied to soils (kg N yr^–1); F_E1 is the emission factor for N₂O emissions from N inputs (kg N₂O-N kg N input^–1); 44/28 is a conversion factor of N₂O-N emissions to N₂O emissions; and GWP represents the GWP of N₂O (t CO₂-eq).

The amount of carbon stocks was estimated according to the IPCC guidelines as follows:

where CS are the carbon stocks; SOC is the soil organic carbon (t C ha^–1); C_veg is the above and belowground vegetation carbon stock (tC ha^–1); C_BM is the carbon stock in living biomass (above and belowground biomass) (t C ha^–1); C_DOM is the carbon stock in dead organic matter (t C ha^–1); F_lu is the factor reflecting the difference in SOC associated with land use; F_mgis the factor associated with management practice; and F_I is the factor associated with different levels of carbon input to soil. The SOC and C_DOM values for an oil palm plantation were determined from field experiments. The value of C_BM was taken from Brinkmann Consultancy (2009), which is relevant for the Malaysian oil palm industry. The values of F_lu, F_mg and were taken from the IPCC guidelines as 1, 1.22 and 1, respectively.

The calculation of carbon stock changes and the resulting CO₂-eq emissions were determined according to the IPCC guidelines as:

where CO₂-eq is the annualized CO₂-eq emission from carbon stock change (t CO₂-eq t^–1 PO); ΔCS are the changes in carbon stocks (t C ha^–1); CS_R is the carbon stock of the reference (previous) land use (t C ha^–1); CS_A is the carbon stock of the actual land use (oil palm plantation) (t C ha^–1); and P is the palm oil productivity (t PO ha^–1).

3

Results and discussion

The emissions from carbon stock changes within Kempas Estate were estimated for assessing the impact of land use change in the plantation development. The estate was transformed into an oil palm plantation from a previous rubber plantation area because rubber is no longer a profitable crop compared to oil palm (Henson, 2005). The estimated emissions are presented in Figure 2a,b.

Fig. 2.

Emissions from carbon stock changes. (a) t CO₂-eq/ha, (b) kg CO₂-eq/MJ palm oil.

As noted in Figure 2a, b, emissions show positive values regardless of the age of the palms. This indicates that the transformation of land use into an oil palm development increased GHG emissions (although at a relatively low amount). In other words, the transformation into an oil palm plantation from rubber estate looses its carbon content (indicated by carbon release), often referred to as carbon debts of the plantation. The emissions of GHG from carbon stock changes range between 1.62-3.48 t CO₂-eq/ha (Fig. 2a), and between 5.43-11.62 kg CO₂-eq/MJ palm oil (Fig. 2b). This corresponds to the carbon stocks of the oil plantation, which are estimated between 77-87 t C/ha. Notwithstanding this carbon loss due to land conversion, this phenomenon has been anticipated in literature. In fact, the replacement of a rubber estate into an oil palm plantation may result in the production of synthetic rubber from fossil oil elsewhere to meet the world rubber demand, which is the industrial process that produces highest carbon emissions (Hansen et al., 2014). Nevertheless, carbon emissions from the conversion of a rubber estate are much lesser than the conversion of a tropical forest (high carbon stock land) into an oil palm estate (Henson, 2005; Brinkmann Consultancy, 2009; Castanheira, 2014; Hansen et al., 2014). Some studies state that there is no difference in the estimated soil carbon content in rubber and oil palm plantations (e.g., Lai, 2004; Siangjaeo et al., 2011).

Variations (yearly average) in nitrous oxide and the resulting CO₂-eq emissions in the oil palm estate from 1986-2009 are shown in Figure 3, which shows that there is no significant difference in the yearly variation of emissions. On average, about 22.3 kg N₂O-N/ha and 1270.5 kg CO₂-eq/ha were released during the period.

Fig. 3.

Nitrous oxide and CO₂-eq emissions from 1986-2009.

The emissions from N-fertilizer application during the plantation stage of the palm oil production were analyzed for oil palms of different ages at Kempas Estate. Table I shows the amount of N-fertilizer applied and the calculated N₂O and CO₂-equivalent (CO₂-eq) emissions according to the age of the palms. Overall, the mean values of applied N-fertilizer range between 108-134 kg N/ha, with the release of 19.11-22.17 kg of N₂O-N/ha, resulting in an emission of 1052.26-1209.51 kg of CO₂-eq /ha. A slightly greater amount of N-fertilizer was applied for oil palms aged between 5-20 years compared to other categories (i.e., < 5 years and > 20 years) (Fig. 4a). For immature palms, empty fruit bunches were typically used as an additional source of fertilizer in addition to N-fertilizer, for assisting the growth of the newly planted palms. Generally, the amounts of N-fertilizer used in the estate are slightly higher than the average in Malaysia, which are of 90 and 105 kg N/ha for immature and mature oil palms, respectively, as reported by Schmidt (2007).

Table I.

Amount of N-fertilizer applied and calculated N₂O and CO₂-eq emissions according to the age of the oil palm (immture and mature).

	Age of palm (year)
	< 5 (immature)	5-20 (mature)	20 (mature)
N-fertilizer applied (kg N/ha)	108	134	118
N2O emissions (kg N2O-N/ha)	22.17	19.31	19.11
CO2-eq (kg CO2-eq/ha)	1123.15	1209.51	1052.26

Fig. 4.

Amount of (a) N-fertilizer applied (kg); (b) nitrous oxide emission (kg N₂O-N/ha); and (c) CO₂-eq emission (kg CO₂-eq/ha) in Kempas Estate, according to the age of oil palms.

Despite little variations in the amount of N₂O released, the amount of the emission is relatively higher for immature palms < 5 years, i.e. 22 kg N₂O-N/ha compared to ~19 kg N₂O-N/ha for mature palms (Fig. 4b), suggesting that N₂O emissions may decrease as oil palms develop maturely to more than 20 years. Note that the relatively higher N₂O emission during the immature stage may be attributed to several factors such as N-fixation processes, return of nitrogen in crop residues and decomposition of biomass (Schmidt, 2007). The high content of N supply during the early stage of palm development is essential to ensure favorable growth conditions and to catalyze the rapid growth of the palm (Sugiyama et al., 1984; Greef, 1994; Sawan et al., 2001). On the other hand, the resulting CO₂-eq emission is relatively higher for oil palms aged between 5-20 years (Fig. 4c), which is mainly due to the greater amount of N-fertilizer applied, coupled with the fraction of N₂O contribution, as discussed earlier. Therefore, it can be seen that the contribution from the N-fertilizer applied is very important, since it reflects the overall CO₂-eq emissions resulting from N-related emissions. Despite this, there is no clear relationship between N₂O or CO₂-eq emissions and the age of the oil palms.

Aside from the N-related emissions, contributions from P-related emissions are also significant to reflect the impact from the use of fertilizers. Similarly, emissions from fuel use (e.g., for internal transportation) are another major input of GHG during the growing stage of the palm oil plantation. Contributions from major inputs of GHG in the estate are illustrated in Figure 5. N- and P-related emissions, as well as fuel emissions contribute to about 79% and 21% of total GHG emissions, respectively. Clearly, nitrous oxide has the largest contribution to GHG compared to other major emission inputs during the growing stage. N-related emissions come from various sources including direct and indirect N emissions, whereby the use of synthetic fertilizer may directly enhance the concentration of nitrogen in soil leading to the release of nitrous oxide through the soil nitrogen cycle prosesses, i.e. nitrification and denitrification (Kusin et al., 2014). Notwithstanding this, N₂O emissions in the estate (i.e. 19.11-22.17 kg N₂O-N/ ha) are in agreement with the values reported by Schmidt (2007) for Malaysian oil palm plantations in general. On the other hand, the resulting CO₂-eq emission (i.e., 1052.26-1209.51 kg of CO₂-eq/ha) is far below the reported values elsewhere (e.g. Germer and Sauerborn, 2008; Fargione et al., 2008; Wicke et al., 2008; Koh et al., 2011; Page et al., 2011, among others). Brinkmann Consultancy (2009) estimated that the GHG emissions related to the use of fertilizers of between 1000-1500 kg CO₂-eq. ha^–1 yr^–1.

Fig. 5.

GHG emissions during tha plantation stage of oil palm production.

4

Conclusions

The growing stage is considered an important phase throughout the life cycle of palm oil production. This paper has highlighted the significant contribution of this stage, particularly from the use of synthetic fertilizer, to the emission of GHGs. In addition, the impact from land use change on the oil palm plantation development has also been assessed. The findings from this study are twofold:The transformation from a rubber plantation into an oil palm plantation slightly decreases the carbon stocks of the soil (and hence positive carbon emissions).

NO₂ is the largest contributor of GHGs during the plantation stage but the amount is within the estimated values for a Malaysian oil palm plantation

Regarding these findings, it is noted that the use of nitrogen fertilizers may be a significant contributor to the emission of GHGs and the loss of carbon content from the plantation during the establishment of the oil palm crop. Therefore, it is likely that the application of nitrogen fertilizer may increase carbon emissions from the conversion of a rubber plantation to an oil palm plantation. However, the emission of CO₂-eq in the estate is still low compared to some other reported values elsewhere.

Acknowledgments

This study was funded through the Long-Term Research Grant Scheme (LRGS) of the Ministry of Higher Education Malaysia (MOHE), project number LRGS/6375401. The second author wishes to thank University Putra Malaysia for the support from Graduate Research Fellowship Scholarship awarded. Data from Kempas Estate, Sime Darby Plantation Malaysia is greatly appreciated.