Landscape-scale assessment of soil properties, water quality and related nutrient fluxes under oil palm cultivation: A study case in Sumatra, Indonesia

The rapid expansion of oil palm cultivation in Southeast Asia raises environmental concerns, necessitating a critical evaluation of the impacts of this production system on ecological health. Oil palm growers in Indonesia are faced with the challenge of sustaining high yields to keep pace with the growing global demand for oil and fats, while reducing the environmental impacts of oil palm cultivation. Environmental impacts associated with the deforestation at the initial phase of an oil palm plantation establishment are well documented, however the impacts of mature oil palm plantation on water quality remain poorly investigated. Oil palm is a perennial crop (25-30 years) cultivated predominantly on weathered tropical soils, so high fertilizer input is necessary to sustain high yields, which is expected to endanger neighboring aquatic ecosystems when excess nutrients are carried out to waterways. In Indonesia, 39 % of oil palm planted area is owned by smallholder farmers, who rely on mineral fertilizers to support oil palm production, and 52 % are large private plantations operated by private industries. In addition to mineral fertilizers, industrial plantations also apply mill byproducts as organic fertilizers in the plots surroundings the mill (due to transportation costs). Soil characteristics and fertilizer management in oil palm plantations (i.e. mineral vs. organic fertilizer applications) were expected to alter the soil fertility status and nutrient loads to waterways. Due to the fact that oil palm plantations generally extend over thousands of contiguous hectares, crossing several watersheds and covering different soil types, the effect of fertilizer management on the soil response and nutrient loads to waterways requires landscape-scale studies accounting for soil variability and long-term fertilization sequences across the plantation. The first objective of the thesis was to (i) perform a literature review that provides an overview of the agricultural practices in oil palm plantations as well as hydrological processes involved in the nutrient transfers from those agroecosystems to waterways. Then I aimed to (ii) assess the effect of long term mineral and organic fertilizer sequences on the soil response, considering different soil types, (iii) characterize the dominant hydrological processes involved in the nutrient fluxes to waterways in oil palm plantations, and (iv) assess the effect of fertilizer management and soil characteristics on groundwater quality and nutrient fluxes to streams. The study area was located in the Petapahan area, Central Sumatra, Indonesia, which has a tropical humid climate (annual rainfall > 2000 mm) and weathered soils (Ferralsols). The study area was a landscape (100 km²) including a 4000 ha industrial plantation and a 1500 ha smallholder plantation using rational fertilizer programs. Low-fertility Ferralsols responded significantly to continuous applications of organic fertilizers, with greater improvement in loamy-sand uplands than in loamy-lowlands, compared to repeated applications of mineral fertilizers. I proposed that spatial fertilizer management at the landscape-scale should complement the current plotscale fertilizer management to get higher nutrient use efficiency and improve soil fertility in an oil palm plantation. One year (2009-2010) multi-site monitoring of stream water quality showed nutrient concentrations below Indonesian standards for water quality. In this case study, mature oil palm cultivation did not contribute to the eutrophication of aquatic ecosystems. This was ascribed to nutrient dilution in streams from the high rainfall as well as high nutrient demand by oil palm that was met with a rational fertilizer program. Assessment of nutrient fluxes from baseflow showed that loamy-sand uplands were more sensitive to nutrient losses than loamy lowlands, and organic fertilization helped to reduce nutrient losses to streams. The study also showed high dissolved organic matter content in streams, likely from natural sources. Oil palm agroecosystems in the study area are characterized by fast groundwater renewal indicating the potential for inputs to be quickly transported from soils to the streams. This may be of concern when unbalanced fertilizer management leads to over-application of nutrients or persistent agrochemicals like pesticides bind to dissolved organic matter, since they will be susceptible to contribute to nonpoint source pollution in streams.La rapide expansion de la culture du palmier à huile en Asie du Sud-Est soulève maintes interrogations sur ses impacts environnementaux. Les planteurs indonésiens doivent désormais assurer de hauts rendements pour répondre à une demande mondiale croissante d’huile de palme, tout en minimisant leurs impacts. Les impacts environnementaux associés à la déforestation lors de la phase initiale d’établissement d’une plantation sont déjà bien documentés. En revanche, les impacts d’une plantation mature sur la qualité de l’eau a été très peu étudiée. Le palmier à huile est une culture pérenne (25-30 ans) généralement cultivée sur des sols tropicaux peu fertiles d’où la nécessité de forts apports de fertilisants, apports susceptibles de menacer les écosystèmes aquatiques quand les nutriments en excès sont transportés vers les rivières. En Indonésie, les petits planteurs villageois détiennent 39 % des surfaces plantées en palmier à huile et n’utilisent que des fertilisants minéraux pour assurer leur production. Les industriels privés possèdent 52 % des surfaces, et appliquent, en plus des fertilisants minéraux, des fertilisants organiques issus des rejets de leurs usines. Ces fertilisants organiques sont généralement appliqués dans les parcelles à proximité de l’usine pour réduire les coûts de transport. Les caractéristiques du sol et la gestion de la fertilisation (i.e. fertilisants minéraux vs. organiques) des palmeraies sont susceptibles d’influer sur la fertilité du sol et sur les transferts de nutriments vers les rivières. Etant donné que les plantations s’étendent généralement sur plusieurs milliers d’hectares d’un seul tenant, couvrant plusieurs bassins versants et différents types de sol, l’effet de la gestion de la fertilisation sur la réponse du sol et les transferts de nutriments vers les rivières nécessite des études à l’échelle du paysage. Celles-ci doivent tenir compte tant de la variabilité du sol au sein de la plantation que de la variabilité des séquences de fertilisation pluriannuelles. Le premier objectif de cette étude est (i) de réaliser une revue de littérature sur les pratiques agricoles utilisées dans les palmeraies ainsi que sur les processus hydrologiques impliqués dans les transferts de nutriments dans ce type de contexte, (ii) d’évaluer l’effet de séquences pluriannuelles de fertilisation minérale et organique sur la réponse du sol, tenant compte de la variabilité des sols au sein de la plantation, (iii) de caractériser et quantifier les processus hydrologiques dominants impliqués dans le transfert de nutriments depuis la palmeraie vers les rivières, (iv) et enfin d’évaluer l’effet de la gestion de la fertilisation et des caractéristiques du sol sur la qualité des eaux souterraines et sur les flux de nutriments vers les rivières. La zone d’étude est située dans la région de Petapahan, dans le centre de Sumatra, en Indonésie. Le climat y est tropical humide (précipitations annuelles > 2000 mm) et les sols peu fertiles (Ferralsols). Il s’agit d’un paysage de 100 km² incluant une plantation villageoise de 1500 ha et une plantation industrielle de 4 000 ha, pratiquant une gestion raisonnée de la fertilisation. Cette étude a montré une amélioration significative des propriétés chimiques des sols suite à des applications continues de fertilisants organiques, avec une amélioration encore plus sensible sur les sols sablo-limoneux que sur les sols limoneux. Une gestion spatiale de la fertilisation à l’échelle de la plantation serait plus efficace et devrait compléter la gestion à la parcelle pour une meilleure stratégie d’application des fertilisants adaptée à la variabilité des sols sur les milliers d’hectares de la plantation. Le suivi multi-site sur un an de la qualité des eaux de surface dans le paysage a montré des niveaux de concentrations de nutriments en deçà des limites maximales recommandées par les standards indonésiens. Dans cette étude de cas, la culture d’une palmeraie mature ne semble pas avoir contribué à l’eutrophisation des cours d’eaux. Les raisons en seraient la dilution du système par la forte pluviosité locale, et la pratique d’une fertilisation raisonnée. L’évaluation des flux de nutriments a montré que les sols sablo-limoneux étaient plus sensibles que les sols limoneux aux pertes de nutriments et que la fertilisation organique pouvait réduire significativement ces pertes. De fortes teneur en matières organiques ont été observées dans les rivières, mais probablement dues à des causes naturelles. Le renouvellement rapide des eaux souterraines induit une grande réactivité du système aux intrants qui peuvent être rapidement drainés vers les cours d’eau. Des apports massifs de nutriments (fertilisation non raisonnée) ou des pesticides liés à la matière organique dissoute pourraient donc entraîner un risque de pollution en aval de l’agrosystème

Agricultural practices in oil palm plantations and their impact on hydrological changes, nutrient fluxes and water quality in Indonesia: a review

Rapid expansion of oil palm (Elaeis guineensis Jacq.) cultivation in Southeast Asia raises environmental concerns about deforestation and greenhouse gas emissions. However, less attention was paid to the possible perturbation of hydrological functions and water quality degradation. This work aimed to review (i) the agricultural practices commonly used in oil palm plantations, which potentially impact hydrological processes and water quality and (ii) the hydrological changes and associated nutrient fluxes from plantations. Although many experimental trials provide clear recommendations for water and fertilizer management, we found that few studies investigated the agricultural practices actually followed by planters. Our review of hydrological studies in oil palm plantations showed that the main hydrological changes occurred during the first years after land clearing and seemed to dissipate with plant growth, as low nutrient losses were generally reported from plantations. However, most of those studies were carried out at the plot scale and often focus on one hydrological process at a single plantation age. So, there is insufficient information to evaluate the spatiotemporal fluctuations in nutrient losses throughout the entire lifespan of a plantation. Furthermore, few studies provided an integrated view at the watershed scale of the agricultural practices and hydrological processes that contribute to nutrient losses from oil palm plantations and the consequences for surface and groundwater quality. Future research efforts need to understand and assess the potential of oil palm plantations to change hydrological functions and related nutrient fluxes, considering agricultural practices and assessing water quality at the watershed scale

Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: New insights and improvement of previous predictions

International audienceChlordecone (CLD), was widely applied in banana fields in the French West Indies from 1972 to 1993. The WISORCH model was constructed to assess soil contamination by CLD and estimated that it lasts from 100 to 600 years, depending on leaching intensity and assuming no degradation. However, recent studies demonstrated that CLD is degraded in the environment, hence questioning the reliability of previous estimations. This paper shows how to improve the model and provides insights into the long-term dissipation of CLD. In-situ observations were made in nearly 2545 plots between 2001 and 2020, and 17 plots were sampled at two dates. Results of soil analyses showed an unexpected 4-fold decrease in CLD concentrations in the soil, in contrast to simulations made using the first version of WISORCH at the time. Neither erosion, nor CLD leaching explained these discrepancies. In a top-down modeling approach, these new observations of CLD concentrations led us to implement a new dissipation process in the WISORCH model that corresponds to a DT50 dissipation half-life of 5 years. The new version of the improved model allowed us to update the prediction of the persistence of soil pollution, with soil decontamination estimated for the 2070s. This development calls for re-evaluation of soil pollution status. Further validation of the new version of WISORCH is needed so it can contribute to crop management on contaminated soil