How do the heterotrophic and the total soil respiration of an oil palm plantation on peat respond to nitrogen fertilizer application?
Introduction
Worldwide the area under oil palm is currently estimated as 10.7 million hectares. Demand for edible or biofuel palm oil along with other derivatives (e.g. soap and makeup) has increased the planted area at an average annual rate of 6% since 2003 (FAO, 2011). This expansion is often made to the detriment of tropical rain forest and peat swamp forest in particular (Carlson et al., 2012, Carlson et al., 2013) as oil palm (Elaeis guineensis, OP) is one of the few crops that can produce high yields on tropical peatlands (Boehm et al., 2013, Miettinen et al., 2012, Ramdani and Hino, 2013). If land-use change, chiefly driven by industrial enterprises, is maintained at the current rate, all the undisturbed peat swamp forest may vanish by 2030 (Koh et al., 2011, Lee et al., 2014, Miettinen et al., 2011, Rudel et al., 2009). Peatlands of Southeast Asia store a significant amount of carbon (C) (> 50 Gt) in their soil. However, these peatlands are concentrated in a few areas of mainly Borneo, Sumatra and Papua (total 247.700 km2) (Page et al., 2011, Yu et al., 2010). Contrasting with peatlands of the temperate and boreal zone, peat in Southeast Asia accumulates under tall rainforest and can reach depth up to 20 m (Posa et al., 2011).
The conversion of tropical peat swamp forest into OP plantation requires drainage which typically accelerates the rate of peat mineralization (Hergoualc'h and Verchot, 2011, Hergoualc'h and Verchot, 2014) and enhances soil CO2 emissions to the atmosphere. The large carbon dioxide (CO2) fluxes from tropical peatlands play an important role in global climate change (Frolking et al., 2011); therefore promoting policies and strategies to manage peatlands more sustainably is of global concern (Murdiyarso et al., 2010). For climate change mitigation mechanisms such as REDD + (reducing emissions from deforestation and forest degradation) or for national greenhouse gas accounting, accurate emission factors of C dynamics are essential. A number of recent studies have evaluated the effect of nitrogen (N) fertilizer on CO2 emissions in tropical peatlands. Of those, Sakata et al. (2015) and Watanabe et al. (2009) did not find any significant effect of fertilizer on emissions, whereas Jauhiainen et al. (2014) observed an increase in CO2 flux from agricultural land and a decrease from degraded land after N fertilizer application. No study has assessed the short-term changes in peat-derived CO2 fluxes following N fertilizer application (priming effect) in oil palm plantations.
Recommended fertilizer application rates in oil palm plantations vary according to climatic conditions, soil type, age of palms, and palm yield potential (Comte et al., 2012). Malaysian recommendations for peat soil range from 50 to 100 kg N ha− 1 y− 1 for immature (less than 3 years old) palms and from 120 to 160 kg N ha− 1 y− 1 for mature ones (Mutert et al., 1999). Present application in major growing areas on peat for 4–10 year old palms amounts to 0.45 kg N per palm (68 kg N ha− 1 y− 1 for a density of 150 palms ha− 1; ZZ von Uexkull, 2014). Fertilizer is typically spread in a circle which radius is defined by the longest fronds or in a circular area with a radius of ~ 2.5 m from the palm trunk (FAO, 2012, Lim et al., 2012). Timing of fertilizer application is site-specific, based on plant demand and is made at any moment during the year except during high rainfall periods (Lim, 2005).
Several long-term studies, carried out on mineral soils, have shown that an adequate fertilizer application rate leads to increased crop yields and that crop residues can enhance soil organic matter (SOM) levels without affecting the turnover of native SOM (Snyder et al., 2009). It has also been reported that application of N fertilizer can chemically stabilize soil C, which limits the rate of soil C decline or can even increase the levels of C in the soil (Lemke et al., 2010, Minasny et al., 2012, Paustian et al., 1997, Wilts et al., 2004). However, drained tropical peat soils cropped to OP, display high mineralization rates that greatly exceed C increases through residue inputs (Hergoualc'h and Verchot, 2014). This can be attributed to the fact that SOM with a C:N ratio greater than 20 typically requires additional N for decomposition to occur (Elser et al., 2007, Snyder et al., 2009). Because tropical peats commonly have a C:N ratio greater than 30, N fertilizer application is expected to enhance organic matter mineralization in these soils and increase CO2 emissions.
Little is known about the impact of applying N fertilizer to tropical peat soils. Simulation models of peat C dynamics such as ECOSSE require information about the effect of fertilizer application on SOM mineralization and CO2 fluxes (Smith et al., 2010). In turn, the Roundtable on Sustainable Palm Oil (RSPO) seeks reliable scientific data to issue N fertilizer recommendations for tropical peats to achieve optimal production with reduced environmental impacts (Lim et al., 2012). The aim of this research was to quantify total and heterotrophic soil CO2 emissions from an OP plantation on a tropical peat soil after two customary doses of N fertilizer.
Section snippets
Study site
The research site was a 7 year old OP plantation located on a deep peat coastal plain in the province of Jambi, Sumatra, Indonesia (Fig. 1). The climate in the region is tropical humid. The average annual rainfall is 2466 mm with a season drier than the rest of the year from June to August. The minimum and maximum values of the mean monthly temperatures are 22 °C and 33 °C, respectively (NOAA, 2011, Siderius, 2004). The OP site was situated in the Bakrie Sumatera Plantation of SNP (Sumber-Tama Nusa
Soil properties and environmental parameters
The peat profiles contained about 20% (by volume) of recognizable plant tissue within the upper 100 cm of the soil profile and the top 10 cm layer was not saturated with water. Therefore, the peat was classified as Hemic Histosol (IUSS-Working-Group-WRB, 2006). The soil displayed a low pH and a high C content typical of ombrogenic peats (Table 1). It had a relatively high proportion of very light fraction plus light fraction (VLF + LF) (18.7% ± 13.3) with a high C:N ratio (i.e. 45.5 ± 6.6). The C:N
Carbon dioxide fluxes in response to nitrogen addition
To date there is no consensus in the literature about the direction and timing of N addition effect on soil emissions of CO2 (Lemke et al., 2010, Minasny et al., 2012, Paustian et al., 1997). Urea application is known to release CO2 since C is a main component of it. The IPCC guidelines (IPCC, 2006), for instance, suggest the use of an emission factor from urea fertilizer application in forestry and agriculture of 0.2 g CO2–C g− 1 urea. This value does not consider any potential increase in SOM
Conclusion
The management of oil palm plantations on peatlands, especially the application of N fertilizer, has implications on the C budget that to date remain unassessed. This research evaluated total and heterotrophic soil CO2 emissions after two doses of N fertilizer and confirmed the short timing of emission enhancement which occurred in the few days following fertilizer application. One recommendation for further studies on the topic would therefore be to intensively monitor CO2 fluxes immediately
Acknowledgments
This research is part of a large research program on REDD + run by the Center for International Forestry Research with financial support from NORAD (Grant Agreement # QZA-10/0468), AusAID (Grant Agreement # 46167) and the European Community's Seventh Framework Program [FP7/2007–2013] (Grant Agreement # 226310). The work was conducted as part of the CGIAR research programs on Climate Change, Agriculture and Food Security CCAFS). We would like to thank PT. Bakrie Sumatera for providing the
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