Biomass estimations in tropical peat swamp forests are quite complex when hollow trees are frequently found due to the unavailability of data on hollow size and the limited data on accurate measures of biomass. Destructive samplings were done for both above-(AGB) and belowground biomass (BGB) and hollow sizes of remained trees at a logged-over peat swamp forest in Sarawak, Malaysia. Subsequently, allometric equations taking hollows into account for both the above-and belowground biomass of tropical peat swamp forests were also being developed. It was observed that these were hollows in Shorea albida and Combretocarpus rotundatus trees with diameters at breast height (DBH) exceeding 40 cm; S. albida is a dominant or co-dominant species, and C. rotundatus grows in peat swamp forests throughout Sarawak. The hollow volumes ranged from 0.23 to 1.08 m 3 , and occupied 42.3% of stem volume on average. The larger biomass produced by previous allometric models were partially due to the presence of hollows. Thus, new models for estimating both AGB and BGB were developed that included one (only DBH), two (DBH and height [H]

No abstract

(1) Background: Nitrogen (N) fertilization on drained tropical peatland will likely stimulate peat decomposition and mineralization, enhancing N2O emission from the peat soil. (2) Methods: A field experiment was conducted to quantify the N2O emissions from soil in an oil palm plantation (Elaeis guineensis Jacq.) located in a tropical peatland in Sarawak, Malaysia, under different rates of N fertilizers. The study was conducted from January 2010 to December 2013 and resumed from January 2016 to December 2017. Nitrous oxide (N2O) flux was measured every month using a closed chamber method for four different N rates; control—without N (T1), 31.1 kg N ha−1 yr−1 (T2), 62.2 kg N ha−1 yr−1 (T3), and 124.3 kg N ha−1 yr−1 (T4); (3) Results: Application of the N fertilizer significantly increased annual cumulative N2O emissions for T4 only in the years 2010 (p = 0.017), 2011 (p = 0.012), 2012 (p = 0.007), and 2016 (p = 0.048). The highest average annual cumulative N2O emissions were recorded for T4 (41.5 ± 28.7 kg N ha−1 yr−1), followed by T3 (35.1 ± 25.7 kg N ha−1 yr−1), T1 (25.2 ± 17.8 kg N ha−1 yr−1), and T2 (25.1 ± 15.4 kg N ha−1 yr−1), indicating that the N rates of 62.2 kg N ha−1 yr−1 and 124.3 kg N ha−1 yr−1 increased the average annual cumulative N2O emissions by 39% and 65%, respectively, as compared to the control. The N fertilization had no significant effect on annual oil palm yield (p = 0.994). Alternating between low (deeper than −60 cm) and high groundwater level (GWL) (shallower than −60 cm) enhanced nitrification during low GWL, further supplying NO3− for denitrification in the high GWL, and contributing to higher N2O emissions in high GWL. The emissions of N2O ranged from 17 µg N m−2 hr−1 to 2447 µg N m−2 hr−1 and decreased when the water-filled pore space (WFPS) was between 70% and 96%, suggesting the occurrence of complete denitrification. A positive correlation between N2O emissions and NO3− at 70–96% WFPS indicated that denitrification increased with increased NO3− availability. Based on their standardized regression coefficients, the effect of GWL on N2O emissions increased with increased N rate (p < 0.001). Furthermore, it was found that annual oil palm yields negatively correlated with annual N2O emission and NO3− for all treatments. Both nitrification and denitrification increased with increased N availability, making both processes important sources of N2O in oil palm cultivation on tropical peatland.; and (4) Conclusions: To improve understanding of N2O mitigation strategies, further studies should consider plant N uptake on N2O emissions, at least until the completion of the planting.

No abstract

Key message Shorea albida trees, which grow in oligotrophic tropical peat swamp forests, invest less in defense than growth, and therefore develop tall hollow stems that regrow after breakage. Abstract The tropical peat swamp forests of Sarawak, Malaysia are low-nutrient soils, but nonetheless have tall Shorea albida trees. Most of these large trees have hollow stems and broken crowns. We examined tree morphology to determine how this species develops and maintains tall above-ground parts in oligotrophic peat soils. We measured hollow diameter at breast height (D hollow ), tree diameter at breast height (DBH), tree height, and height of breakage of 81 trees. Destructive sampling was also conducted for seven trees, and these data were used to determine wood density and both hollow diameter and stem diameter per meter height. All sampled trees developed hollow trunks before they reach the canopy layer. Linear regression of D hollow on DBH indicated that the radial expansion rate of the hollow was slightly less than the thickening growth. Stem breakage and crown breakage reduced tree height, but most broken trees regrew from pre-existing upper branches or by epicormic branching. These results suggest that S. albida trees devote more resources to growth than defense, and therefore become large with hollow stems.

No abstract

A long-term study on the effect of nitrogen (N) fertilization on soil carbon dioxide (CO2) fluxes in tropical peatland was conducted to (1) quantify the annual CO2 emissions from an oil palm plantation under different N application rates and (2) evaluate the temporal effects of groundwater level (GWL) and water-filled pore space (WFPS) on soil organic carbon (SOC) and CO2 fluxes. Monthly measurement of soil CO2 fluxes using a closed chamber method was carried out from January 2010 until December 2013 and from January 2016 to December 2017 in an oil palm plantation on tropical peat in Sarawak, Malaysia. Besides the control (T1, without N fertilization), there were three N treatments: low N (T2, 31.1 kg N ha−1 year−1), moderate N (T3, 62.2 kg N ha−1 year−1), and high N (T4, 124.3 kg N ha−1 year−1). The annual CO2 emissions ranged from 7.7 ± 1.2 (mean ± SE) to 16.6 ± 1.0 t C ha−1 year−1, 9.8 ± 0.5 to 14.8 ± 1.4 t C ha−1 year−1, 10.5 ± 1.8 to 16.8 ± 0.6 t C ha−1 year−1, and 10.4 ± 1.8 to 17.1 ± 3.9 t C ha−1 year−1 for T1, T2, T3, and T4, respectively. Application of N fertilizer had no significant effect on annual cumulative CO2 emissions in each year (p = 0.448), which was probably due to the formation of large quantities of inorganic N when GWL was temporarily lowered from January 2010 to June 2010 (−80.9 to −103.4 cm below the peat surface), and partly due to low soil organic matter (SOM) quality. A negative relationship between GWL and CO2 fluxes (p < 0.05) and a positive relationship between GWL and WFPS (p < 0.001) were found only when the oil palm was young (2010 and 2011) (p < 0.05), indicating that lowering of GWL increased CO2 fluxes and decreased WFPS when the oil palm was young. This was possibly due to the fact that parameters such as root activity might be more predominant than GWL in governing soil respiration in older oil palm plantations when GWL was maintained near or within the rooting zone (0–50 cm). This study highlights the importance of roots and WFPS over GWL in governing soil respiration in older oil palm plantations. A proper understanding of the interaction between the direct or indirect effect of root activity on CO2 fluxes and balancing its roles in nutrient and water management strategies is critical for sustainable use of tropical peatland.

&lt;p&gt;Anthropogenic activities, and in particular the use of synthetic nitrogen (N) fertilizer, have a significant influence on soil nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) emission from oil palm plantation on tropical peatland. Finding a suitable N rate for optimum N uptake efficiency and yield with low environmental impact and production cost is crucial for the economic growth of Malaysia&amp;#8217;s oil palm sector. However, studies on the impact of N fertilizers on N&lt;sub&gt;2&lt;/sub&gt;O emissions&amp;#160;from&amp;#160;tropical peatland are limited. Thus, long-term monitoring was conducted to investigate the effects of N fertilization on soil N&lt;sub&gt;2&lt;/sub&gt;O emissions. This study was conducted in an oil palm (&lt;em&gt;Elaeis guineensis Jacq&lt;/em&gt;.) plantation located in a tropical peatland in Sarawak, Malaysia. Monthly soil N&lt;sub&gt;2&lt;/sub&gt;O fluxes were measured using the closed-chamber method in a control (T1, without N fertilization), and under three different N treatments: low N (T2, 31.1 kg N ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;), moderate N (recommended rate) (T3, 62.2 kg N ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;), and high N (T4, 124.3 kg N ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;), from January 2010 to December 2013 and from January 2016 to December 2017. The only N fertiliser rate to significantly increase (p&lt;0.05) annual cumulative N&lt;sub&gt;2&lt;/sub&gt;O emissions was 124.3 kg N ha&lt;sup&gt;-1&lt;/sup&gt; (T4). Increased&amp;#160;in water-filled pore space (WFPS) (&gt;70%) with a decrease in both N&lt;sub&gt;2&lt;/sub&gt;O flux and nitrate (NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;) implies that complete denitrification has taken place.&amp;#160;Increased in NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;&amp;#160;uptake by oil palm with an increase in WFPS decreased NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; concentration in soil, resulting in the reduction of N&lt;sub&gt;2&lt;/sub&gt;O emission. This study highlights the importance of WFPS on denitrification and &lt;span&gt;N uptake &lt;/span&gt;&lt;span&gt;by oil palm in tropical peatland. This needs to be taken into account for the accurate assessment of N dynamics in oil palm plantations on tropical peatland in order to enhance N fertilization management strategies and counteract anthropogenic activities that produce greenhouse gases.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Keywords: WFPS, oil palm yield, NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, N uptake&lt;/p&gt;