Nitrification of a tropical peat soil cultivated with oil palm (Elaeis guineensis Jacq.)

As knowledge on nitrogen (N) cycle is evolving rapidly, soil nitrification has become the centre of research interest because it is the single most important process in the N-cycle that leads to N losses in broad range of environments. With further expansion of oil palm on tropical peat lands, N cycling is predicted to be modified. However, very few current literatures reported nitrification in tropical peat soils cultivated with oil palm. Therefore, there is a need to understand the nitrification process in cultivated tropical peat soils with oil palm. Soil sampling were carried out up to 60 cm depth at weeded circle (WC), harvesting path (HP) and frond heap (FH). General chemical analyses were carried out in the laboratory. The results showed that WC contained the highest ammonium (8.48 μg g-1 soil), nitrate (6.22 μg g-1 soil) and inorganic N (14.70 μg g-1 soil) contents compared with FH and HP in 0-10 cm soil depth. PNR indicated, most of the nitrification activity happened at 0-10 cm in WC area. In 20-30 cm WC area, 10-20 cm, 20-30 cm of FH area and 10-20 cm HP area indicated negative value of PNR. This suggests that, nitrification were restricted at these depths and the NO3- availability in these areas were predicted to originate from the topsoil through vertical down movement. Laboratory incubation study also indicated, nitrification were affected not only by the availability of substrate (NH4+), but also by the type of N fertilizers. Nitrification was inhibited by addition of AS, and higher input of AS led to greater inhibition of nitrification. Net nitrification in urea reached its highest value in 2 kg N palm-1 treatment, which is higher (21.33 μg g-1 soil day-1) than in AS (3.41 μg g-1 soil day-1). In the next laboratory incubation study, the dynamics of NO3- did not show significant change with increased soil moisture without addition of urea. This results indicated, nitrification in peat soil needed reactive N supply (urea) regardless of soil moisture conditions. However, increasing soil moisture resulted in lower NO3- and PNR which reflect that soil moisture controlled the magnitude of PNR when urea was added. Aside from laboratory incubation experiments, field soil sampling were also collected in dry and wet season from research plots consisted of ground cover treatments with three N rates. Data from the study suggested that installation of legume cover crops (LCC) in combination with N fertilizers resulted in higher mineralization and nitrification in wet season compared to natural ground cover. Nitrification rate was stimulated by the availability of substrates (NH4+) regardless of the moisture condition. LCC can be the contributing factors in higher availabity of NH4+ in the wet season that could be loss to the environments. Therefore, LCC may be unnecessary in peat soil where N fertilizers were applied. Finally, selected soil samples from the study mentioned above were used to determine the microbial composition and to identify potential nitrifiers existed in tropical peat soil cultivated with oil palm using 16S metagenomics. The results indicated that conventional ammonium and/or ammonia oxidizers (Nitrosomonas and Nitrosococcus) were almost non-existent in all the soil samples. This study observed that soil samples from the surface area of WC, FH and HP area were abundant with Bacilli and Pseudomonales suggesting heterotrophic pathways of ammonia oxidation in this area. However, pronounced shift in ammonium oxidizing archaea (AOA) number was observed (Thaumarchaeota) with inclusion of conventional legume sytems compared with natural ground cover. Planctomycetaceae order from the phylum Planctomycetes (anammox bacteria) were also abundant in number suggesting anaerobic ammonium oxidation is possible in tropical peat soil cultivated with oil palm. In conclusion, nitrification was prominent in tropical peat soil cultivated with oil palm disregarding low pH condition and high moisture content. The nitrification rate varies among the operational zones and depth and the process was influenced by N fertilizer addition and also regulated by moisture condition. However, the major pathways of ammonia oxidation (heterotrophic bacteria or by AOA) and the extention of anaerobic ammonium oxidation still remained elusive and require further studies.

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