Water retention and pore size distribution in organic soils from tropical mountain peatlands under forest and grassland

Peatland soils can store large amounts of water and their retention capacity depends upon soil structure, vegetation type and degree of decomposition. In the Serra do Espinhaço Meridional (south-eastern Brazil), numerous peatlands occur in the headwaters of several watercourses that feed rivers of uttermost importance. The present study aimed to characterise the water retention curves for two peatlands under forest and grassland vegetation. Four peat cores were collected, then sampled every 15 cm. Undisturbed samples were used for water retention analysis and disturbed samples for organic matter characterisation. Water retention data fitted well to bimodal sigmoid retention curves, indicating that the porous system is composed of at least two subsystems with different retention behaviours. One subsystem follows a smooth sigmoid trend at higher matric potentials, while the other subsystem accounts for the steep decrease in water content beyond matric potentials around -7,000 cm. The pore size distribution also reflected this bimodal behaviour, with two distinct peaks around 50 and 0.4 µm. Water retention was predominantly affected by bulk density, which reflected the differences in mineral and organic matter content. The degree of decomposition and lignocellulosic composition of the organic matter also significantly affected water retention, especially at lower matric potentials.