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Conventional management of oil palm plantations, involving high fertilization rate and herbicide application, results in high yield but with large soil greenhouse gas (GHG) emissions. This study aimed to assess a practical alternative to conventional management, namely reduced fertilization with mechanical weeding, to decrease soil GHG emissions without sacrificing production. We established a full factorial experiment with two fertilization rates (conventional and reduced fertilization, equal to nutrients exported via fruit harvest) and two weeding methods (herbicide and mechanical), each with four replicate plots, since 2016 in a ≥ 15-year-old, large-scale oil palm plantation in Indonesia. Soil CO2 , N2O , and CH4 fluxes were measured during 2019–2020, and yield was measured during 2017–2020. Fresh fruit yield (30 ± 1 Mgha-1yr-1) and soil GHG fluxes did not differ among treatments (P≥ 0.11), implying legacy effects of over a decade of conventional management prior to the start of the experiment. Annual soil GHG fluxes were 5.5 ± 0.2 Mg CO 2 -C ha -1 yr -1 , 3.6 ± 0.7 kg N 2 O-N ha -1 yr -1 , and - 1.5 ± 0.1 kg CH 4 -C ha -1 yr -1 across treatments. The palm circle, where fertilizers are commonly applied, covered 18 % of the plantation area but accounted for 79 % of soil N2O emission. The net primary production of this oil palm plantation was 17 150 ± 260 kgCha-1yr-1 , but 62 % of this was removed by fruit harvest. The global warming potential of this planation was 3010 ± 750 kgCO2eqha-1yr-1 , of which 55 % was contributed by soil N2O emission and only < 2 % offset by the soil CH4 sink. [ABSTRACT FROM AUTHOR]

The rapidly growing areal extent of oil palm (Elaeis guineensis Jacq.) plantations and their high fertilizer input raises concerns about their role as substantial N2O sources. In this study, we present the first eddy covariance (EC) measurements of ecosystem‐scale N2O fluxes in an oil palm plantation and combine them with vented soil chamber measurements of point‐scale soil N2O fluxes. Based on EC measurements during the period August 2017 to April 2019, the studied oil palm plantation in the tropical lowlands of Jambi Province (Sumatra, Indonesia) is a high source of N2O, with average emission of 0.32 ± 0.003 g N2O‐N m−2 year−1 (149.85 ± 1.40 g CO2‐equivalent m−2 year−1). Compared to the EC‐based N2O flux, average chamber‐based soil N2O fluxes (0.16 ± 0.047 g N2O‐N m−2 year−1, 74.93 ± 23.41 g CO2‐equivalent m−2 year−1) are significantly (~49%, p < 0.05) lower, suggesting that important N2O pathways are not covered by the chamber measurements. Conventional chamber‐based N2O emission estimates from oil palm up‐scaled to ecosystem level might therefore be substantially underestimated. We show that the dynamic gas exchange of the oil palm canopy with the atmosphere and the oil palms' response to meteorological and soil conditions may play an important but yet widely unexplored role in the N2O budget of oil palm plantations. Diel pattern of N2O fluxes showed strong causal relationships with photosynthesis‐related variables, i.e. latent heat flux, incoming photosynthetically active radiation and gross primary productivity during day time, and ecosystem respiration and soil temperature during night time. At longer time scales (>2 days), soil temperature and water‐filled pore space gained importance on N2O flux variation. These results suggest a plant‐mediated N2O transport, providing important input for modelling approaches and strategies to mitigate the negative impact of N2O emissions from oil palm cultivation through appropriate site selection and management. [ABSTRACT FROM AUTHOR]

Conventional management of oil palm plantations, involving high fertilization rate and herbicide application, result in high yield but with large soil greenhouse gas (GHG) emissions. This study aimed to assess a practical alternative to conventional management, namely reduced fertilization with mechanical weeding, to decrease soil GHG emissions without sacrificing production. We established a full factorial experiment with two fertilization rates (conventional and reduced fertilization, equal to nutrients exported via fruit harvest) and two weeding methods (herbicide and mechanical), each with four replicate plots, since 2016 in a = 15-year old, large-scale oil palm plantation in Indonesia. Soil CO2, N2O, and CH4 fluxes were measured during 2019 - 2020 and yield was measured during 2017 - 2020. Fresh fruit yield (30 ± 1 Mg ha-1 yr-1) and soil GHG fluxes did not differ among treatments (P = 0.11), implying legacy effects of over a decade of conventional management prior to the start of experiment. Annual soil GHG fluxes were 5.5 ± 0.2 Mg CO2-C ha-1 yr-1, 3.6 ± 0.7 kg N2O-N ha-1 yr-1, and -1.5 ± 0.1 kg CH4-C ha-1 yr-1 across treatments. The palm circle, where fertilizers are commonly applied, covered 18% of the plantation area but accounted 79% of soil N2O emission. The net primary production of this oil palm plantation was 17150 ± 260 kg C ha-1 yr-1 but 62% of this was removed by fruit harvest. The global warming potential of this planation was 3010 ± 750 kg CO2-eq ha-1 yr-1 of which 55% was contributed by soil N2O emission and only < 2% offset by soil CH4 sink. [ABSTRACT FROM AUTHOR]

Silicon (Si) is known to have multiple beneficial effects on crops. Most plant-available Si in soils is provided through litter decomposition and subsequent phytolith dissolution, especially in strongly desilicated tropical soils. The importance of Si cycling in tropical soil-plant systems raised the question if oil-palm cultivation, the oil palm being a Si-accumulating crop, alters Si cycling. As Si accumulates in plant tissue, we hypothesized that i) Si is stored in the aboveground biomass of oil palms with time, and that ii) the system might lose considerable amounts of Si every year through fruit-bunch harvest. To test these hypotheses, we sampled leaflets, the rachis, fruit-bunch stalk, fruit pulp, kernels and frond bases from mature oil palms planted on well-drained and temporarily flooded riparian smallholder oil-palm plantations (n = 4 each) in lowland Sumatra, Indonesia. We quantified Si concentrations of these oil-palm parts by NaCO3 extraction. We further estimated Si storage in the total above-ground biomass of the oil palms, Si return to soils through decomposing pruned palm fronds, and Si losses from the system through harvest, to assess if Si return to soils via pruned palm fronds sufficed for maintaining Si cycling in the system, or if any measures are needed to compensate for Si export through fruit-bunch harvest. At all sites, leaflets of oil-palm fronds had a significantly higher (p ≤ 0.05) mean Si concentration (≥ 1 wt. %) than the rachis, frond base, fruit-bunch stalk, fruit pulp and kernel (≤ 0.5 wt. %). All analysed oil-palm parts had a Si/Ca weight ratio ≥ 1, except for the rachis. At well-drained sites, mean Si concentrations in leaflets increased with palm-frond age (R² = 0.98). Estimates of Si storage in the total above-ground biomass of oil palms, Si return to soils through decomposing pruned palm fronds, and Si losses through harvest were similar at well-drained and riparian sites: a single palm tree could store about 4–5 kg of Si in its total above-ground biomass, a smallholder oil-palm plantation of 1 hectare could store about 550 kg of Si in the palm trees' above-ground biomass. Pruned palm fronds were estimated to return 110–131 kg of Si per hectare to topsoils each year. Fruit-bunch harvest corresponded to an annual Si export of 32–72 kg Si per hectare in 2015 and 2018. Thus, on smallholder plantations in our study area, more Si can be returned to soils through pruned palm fronds than is lost through fruit-bunch harvest. Greater Si losses would occur if oil-palm stems were removed from plantations prior to replanting. Therefore, it is advisable to leave oil-palm stems on the plantations e.g., by distributing chipped stem parts across the plantation at the end of a plantation cycle (~25 years). This would return about 550 kg ha-1 Si stored in the palm trees' above-ground biomass to the soils. [ABSTRACT FROM AUTHOR]

Effects of oil-palm (Elaeis guineensis Jacq.) management on silicon (Si) cycling under smallholder oilpalm plantations have hardly been investigated. As oil palms are Si accumulators, we hypothesized that management practices and topsoil erosion may cause Si losses and changes in spatial Si concentration pattems in topsoils under oil-palm cultivation. To test this hypothesis, we took topsoil samples under mature oil-palm plantations in well-drained and riparian areas of Jambi Province, Indonesia. The samples were taken from four different management zones within each oil-palm plot: palm circles, oil-palm rows, interrows and below frond piles. We quantified mobile Si (Si,) and Si in amorphous silica (Si) by CaCl and NaCO extraction, respectively. Both fractions are important Si pools in soils and are essential for plant-soil Si cycling. We further installed sediment traps on sloping, well-drained oil-palm plantations to estimate the annual loss of soil and Si caused by erosion. In well-drained areas, mean topsoil SiAm concentrations were significantly higher below frond piles (3.97 ±1.54 mg g') compared to palm circles (1.71± 0.35 mg g'), oil-palm rows (1.87 ± 0.51 mg g') and interrows (1.88 ± 0.39 mg g'). In riparian areas, highest mean topsoil Siconcentrations were also found below frond piles (2.96 ± 0.36 mg g') and in grass-covered interrows (2.71 ± 0.13 mg g'), whereas topsoil Si concentrations of palm circles were much lower (1.44 ± 0.55 mg g'). We attributed the high Siconcentrations in topsoils under frond piles andin grass-covered interrows to phytolith release from decaying oil-palm fronds, grasses, and sedges. The significantly lower SiAm concentrations in palm circles (in both well-drained and riparian areas), oil-palm rows and unvegetated interrows (only in well-drained areas) were explained by a lack of litter return to these management zones. Mean topsoil Si; concentrations were in a range of10 20 g g'. They tended to be higher in riparian areas, but the differences between well-drained and riparian sites were not statistically significant. Soil-loss calculations based on erosion traps confirmed that topsoil erosion was considerable in oil-palm interrows on slopes. Erosion estimates were in a range of 4 6Mg ha' yr', involving Si losses in a range of 5 9 kg' ha' yr'. Based on the observed spatial Si patterns, we concluded that smallholders could efficiently reduce erosion and support Si cycling within the system by (1) maintaining a vegetation cover in oil-palm rows and interrows, (2) incorporating oil-palm litter into farm management and (3) preventing soil compaction and surface-crust formation. [ABSTRACT FROM AUTHOR]