Changes in surface (un)saturated soil hydraulic conductivities along a soil degradation – regeneration gradient in upland Central México

Abstract:

Changes in top-soil infiltration capacity as a function of soil degradation, agricultural activity and vegetation regrowth in the tropics are increasingly well-documented. However, most studies to date have employed the constant-head approach that involves ponding of water on the surface that may over-estimate surface saturated soil hydraulic conductivity (Ksat) due to the activation of macro-pore water fluxes that only contribute to overall Ksat during highly intense or prolonged rainfall. Tension infiltrometry allows a distinction to be made between the relative contributions to total Ksat by preferential and matrix flow by quantifying rates of infiltration under increasingly wetter conditions. This study presents values of Ksat and relative contributions by preferential flow for a forest disturbance and regeneration gradient in a volcanic upland setting in Central Mexico. A total of 128 infiltration tests were carried out in mature secondary pine-oak forest (SPOF) and in a denuded gully (G) marking the two extremes of the studied land-cover gradient, as well as in freshly cleared and one-year-old cropped fields (AG0 and AG1), 5-year-old fallow land (FL5), and 13-year-old fallow (FL13) during rainy periods when the soil was reasonably moist. Mean Ksat values for the respective land covers were 38, 91, 284, 265, 71 and 86 mm h-1 for SPOF, G, AG0, AG1, FL5 and FL13, respectively. Mean relative contributions by preferential flow ranged from 16% in FL13 to 58% in FL5 with intermediate values in G, AG0, AG1, SPOF (20, 27, 29 and 33%). A pattern of “infiltration islands” was detected in FL13 where mean Ksat close to tree trunks was higher compared to mid-crown and outer crown positions (121, 77 and 60 mm h-1). The soil under SPOF showed medium to strong hydrophobicity leading to considerable underestimation of Ksat when measured by tension infiltrometry, while ponded single–ring infiltrometry gave an average Ksat of 1579 ± 1577 mm h-1 for this site.