Temporal change in soil macropores measured using tension infiltrometer under different land uses and slope positions in subtropical China

Soil macropores play a principal role in water infiltration but they are highly variable. The objectives of this study were (1) to investigate the temporal change in macropores of an Ultisol as affected by land use and slope position and (2) to analyze contribution of macropores to water infiltration. Water infiltration was measured at upper and lower slopes in citrus orchard and watermelon field once every 2 months for 1 year using tension infiltrometers at a successive pressure head from -12, -6, -3, to 0 hPa. Hydraulic conductivity (K) was significantly affected by land use and slope position except at 0 hPa pressure head, showing a significant temporal variation. Effective macroporosity, derived from the increment of hydraulic conductivity between -3 and 0 hPa, showed a significant temporal variation. Such temporal variation was land use (P < 0.05) and slope position (P < 0.001) dependent. Despite of low proportion in total soil volume (averaged 3.5 cm(3) m(-3)), the macropores contributed 47 % of water flux on average. The macroporosity was more stable and higher in the citrus orchard (2.43 cm(3) m(-3), coefficient of variance (CV) = 75 %) than in the watermelon field (1.72 cm(3) m(-3), CV = 117 %) and contributed more to infiltration in the citrus orchard (60 %, CV = 16 %) than in the watermelon field (33 %, CV = 43 %) as well, because tillage was operated only in the watermelon field. No-tillage increased water conducting macropores but did not increase hydraulic conductivity irrespective of slope position.