Aims Fine root surface area is important for nutrient and water uptake, but its seasonal dynamics remain poorly understood. Our objective was to investigate the seasonal dynamics of fine root surface area and its relation with the soil water content in Robinia pseudoacacia, Platycladus orientalis and Pinus tabulaeformis plantations.
Methods The study was conducted in Ansai County (36°30′ N, 108°51′ E), Shaanxi Province, China. Fine root surface area and soil water content were investigated by soil cores (300.0 cm depth, 6.8 cm diameter) taken in April, June, August and October 2007, at different horizontal distances (50, 100 and 150 cm) from the stem. Live root surface area and diameter were measured using the WinRHIZO root morphology and architecture measurement system. Fine root (≤2 mm) area density (surface area of root per unit soil volume, FRAD) was determined for every soil sample. The soil samples were oven-dried at 105 °C for 8 h and weighed to determine the soil water content by dry weight.
Important findings Throughout the study period, large variations in soil water content occurred in the 0-200 cm soil layer where 86.5%, 82.4% and 87.7% of the total fine root surface area were concentrated in the R. pseudoacacia, P. orientalis and P. tabulaeformis plantations, respectively. There was a significant positive correlation (p < 0.05) between vertical distribution of fine root surface area and soil water distribution in the profile. The model S = AhB(C + Dh + Eh2 + Fh3) describes the fine root surface area vertical distribution of the plantations with R2 > 0.85. The mean soil water content in all plantations changed in order of October > April > June > August. The highest value of fine root surface area was in June for R. pseudoacacia and P. tabulaeformis and in June and October for P. orientalis. There were differences in the seasonal dynamics of fine root surface area and soil water content. Of the water that the three plantations needed for growth, 87% came from precipitation, but there was no significant correlation (p < 0.05) between fine root surface area and soil water content. The study indicates that combined and integrated soil moisture, temperature, available nitrogen and genetic characteristics of tree species should be considered in research on seasonal dynamics of fine roots.