| dc.description.abstracteng | Silicon (Si) is the second most abundant element in the Earth’s crust. In rocks, Si primarily occurs as primary silicates. In soils, Si is present mostly as secondary silicates or amorphous silica of biogenic or pedogenic origin. For plants, Si can be an essential element due to its numerous beneficial functions: in soils, Si can mobilize phosphorous (P) by occupying anion adsorption sites. Si also mitigates plant toxicity by binding toxic cations that become mobile at low soil pH. In plants, Si can increase drought resistance by precipitating in various cell components of leaves which reduces transpiration. In recent years, assessing the Si status in arable soils has received more attention because well-balanced Si levels in soils may increase crop yields (economic interest) and mitigate severe droughts (climate change). In SE Asia, Si research is particularly relevant because three parameters come together in this region: highly weathered tropical soils (i.e., desilicated soils), drought risk due to shorter rainy seasons and crops, which are Si accumulators such as rice (Oryza sativa), sugarcane (Saccharum officinarum) and oil palm (Elaeis guineensis). Si-accumulating plants require well-balanced Si levels in soils in addition to common plant nutrients (e.g., N, P, K, Ca, and Mg).
Indonesia is the second largest palm-oil producer in the world. In 2022, about 16 million ha land was under oil-palm cultivation. Oil palms are still commonly planted as monocultures, whereby four management zones can be distinguished: (1) palm circles refer to the immediate circulate area around a palm stem that are fertilized; (2) oil-palm rows refer to rows of planted oil palms that contain cover crop (understorey vegetation); (3) interrows are interim spaces between planted oil-palm rows that are sprayed regularly with herbicides and usually serve as harvesting paths; (4) frond piles refer to interrows where pruned palm fronds are stacked in piles to serve as litter decomposition sites. Additionally, cover crop is left in place.
Within Indonesia, Sumatra has been greatly affected by land conversion, i.e., from lowland rainforests and agroforestry systems into oil-palm monocultures. Palm oil is a tropical cash crop with high demand on the global market. The monetary value of palm oil continues to encourage smallholder farmers (≤ 2 ha) and private- and state-owned companies (≥ 2 ha) to cultivate oil palms. Currently, research is identifying ways of improving oil-palm management practices with the objective of reusing the same plantation sites. This is of relevance because many oil-palm plantations in Sumatra are on the verge of being replanted. Furthermore, this could also reduce the need to convert more pristine forests.
The aim of this thesis was to investigate the impact of rainforest conversion into oil-palm plantations on stocks of mobile Si and its interacting Si phases in soils and further, to identify measures to sustain plant-soil Si-cycling in this land-use system. The study was conducted in smallholder oil-palm plantations established in two different water regimes (well-drained and riparian areas) in Jambi Province, Indonesia. Four objectives were investigated: i) assessing the current state of soil Si pools under oil-palm plantations, ii) examining, whether oil-palm management practices have caused differing topsoil Si levels within an oil-palm plantation, iii) identifying processes (e.g., erosion or harvest) potentially altering Si cycling under oil-palm cultivation and iv) estimating Si storage, return and losses within oil-palm plantations to present a first Si balance. The objectives were analyzed in three independent studies. The results are as follows:
Si availability and Si fluxes in two water regimes: our data could not provide statistical evidence that Si fluxes differed significantly between well-drained and riparian areas. In fact, soil Si pools and plant Si contents in various oil-palm components were similar or only showed a tendency of higher Si availability in riparian areas. This suggests that an additional influx of dissolved Si by stream water or flooding could be negligible in the soil-plant system under oil-palm cultivation. Alternatively, it could also imply that Si uptake by oil-palm roots is similar in both water regimes, thereby offsetting a potentially larger Si supply. As Si uptake by oil-palm roots is poorly researched, further analysis would be needed to verify either theory.
Principal drivers of Si cycling under smallholder oil-palm plantations: Si cycling under oil-palm plantations could be mainly driven by biogenic-amorphous silica (i.e., phytoliths alongside silicious microorganisms in topsoils) and mobile Si (i.e., Si in soil solution) at the soil-plant interface. This can be explained by the presence of easily soluble phytoliths occurring in topsoils, litter, and oil-palm biomass. If topsoils were maintained well and a cover crop left in every interrow, Si cycling under oil-palm plantations may potentially be self-sufficient. Nevertheless, Si in soil solution is also replenished by less soluble soil Si pools in minor quantities – in topsoils, mainly by Si bound to organic matter and in subsoils mainly by Si occluded in pedogenic oxides and hydroxides.
Si balance: the data from all three studies enabled us to propose a Si balance for smallholder oil-palm plantations established in well-drained areas: a mature oil palm could store 4 – 5 kg of Si, a smallholder oil-palm plantation in our study area about 570 – 680 kg of Si per ha. Roughly 0.06 kg of Si could be returned to soil by a pruned palm frond. In one year, pruning and subsequent stacking was estimated to return 110 – 130 kg of Si per ha to soil under frond piles. In contrast, a single fruit bunch could store 0.02 – 0.07 kg of Si. In 2015 and 2018, annual fruit-bunch harvest (1 ha smallholder plantation) resulted in Si losses of 30 – 70 kg of Si per ha. Topsoil erosion from vegetation-scarce interrows involved additional Si losses in the range of 5 – 9 kg Si per ha. A Si balance was only proposed for well-drained areas as Si concentrations were similar in both water regimes and estimating Si storage, return and losses involved aboveground biomass data only from well-drained sites.
Recommended measures: based on differing topsoil Si concentrations observed in four different management zones (palm circles, oil-palm rows, interrows and frond piles) of an oil-palm plantation, the following measures could maintain or even increase Si levels in soils under smallholder oil-palm plantations in our study area: i) preventing surface sealing (study 1); ii) maintaining a cover crop (e.g., grasses and sedges) in vegetation-scarce interrows and returning empty fruit bunches to the palm circle to serve as an organic fertilizer (study 2); iii) suggesting to distribute chipped oil-palm stem parts prior to replanting the same plantation sites (study 3); and iv) ensuring a spatially more even Si return from decomposing palm fronds to soils, e.g., by changing the position of frond-piles every 5 – 10 years (studies 2, 3). | de |