Soil carbon sequestration can be estimated from field to global scale using numerical 9 soil/ecosystem models. In this chapter we describe the structure and development of models 10 that have been widely used at international level, from simple models that include carbon 11 only to model that include descriptions of the dynamics of a range of nutrients. We also 12 present examples of the application from field to global scale of different models to answer a 13 range of different questions on the impact of land use and climate changes on soil carbon sequestration. A full discussion of the impact of soil carbon modelling on political and socio-economical aspects is included to emphasise the need of a close interaction between model developers, researchers, land owners/users and policy makers to ensure the development of robust approaches to climate change, food security and soil protection. Whatever type of models are used to meet future challenges, it is important that they continue to be tested using appropriate data, and that they are used in regions and for land uses where they have been developed and validated.
& Key message Permanent sampling plots (PSPs) are a powerful and reliable methodology to help our understanding of the diversity and dynamics of tropical forests. Based on the current inventory of PSPs in Indonesia, there is high potential to establish a long-term collaborative forest monitoring network. Whilst there are challenges to initiating such a network, there are also innumerable benefits to help us understand and better conserve these exceptionally diverse ecosystems.
Replacement of forest by agricultural systems is a major factor accelerating the emissions of greenhouse gases; however, related field studies in the tropics are very scarce. To evaluate the impact of forest transition to plantations on soil methane (CH 4) and respiration (CO 2) fluxes, we conducted measurements in an undisturbed forest, a disturbed forest, young and old rubber plantations, and an oil palm plantation on mineral soil in Jambi, Sumatra, Indonesia. Methane fluxes and their controlling variables were monitored monthly over fourteen months; soil respiration was measured less frequently. All of the plantations were managed by smallholders and had never been fertilized. To assess the effect of common management practices in oil palm plantations, we added urea at a rate of 33.3 kg N/ha and thereafter monitored intensively soil CH 4 fluxes. The soil acted as a sink for CH 4 (kg CH 4-CÁha −1 Áyr −1) in the undisturbed forest (−1.4 AE 1.0) and young rubber plantation (−1.7 AE 0.7). This was not the case in the other land-use systems which had fluxes similar to fluxes in the undisturbed forest, with 0.4 AE 0.9, −0.2 AE 0.3, and 0.2 AE 0.7 kgÁha −1 Áyr −1 in the disturbed forest, old rubber plantation, and oil palm plantation, respectively. In the oil palm plantation, there was no inhibitory effect of nitrogenous fertilizer on methanotrophy. Annual soil respiration (Mg CO 2-CÁha −1 Áyr −1) was higher in the oil palm plantation (17.1 AE 1.9) than in the undisturbed forest (13.9 AE 1.2) while other land-use systems respired at a similar level to the undisturbed forest (13.1 AE 1.4, 15.9 AE 1.7, and 14.1 AE 1.0 in the disturbed forest, young, and old rubber plantations, respectively). Substrate (litterfall and soil) availability and quality exerted a strong control over annual fluxes of both gases along the land-use gradient. Temporal variation in CH 4 was extremely high and in respiration fluxes was moderate, but was not specifically linked to seasonal variation. Further comprehensive and long-term research is critically needed to determine more thoroughly the direction and magnitude of changes in soil trace gas emissions as affected by forest-to-plantation conversion in the tropics.
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