Water Movement And Its Effect On Alfalfa Growth In Layered Soils

The impact of alternating sandy textural layering on the water retention capacity, soil water evaporates and the growth of alfalfa(Medicago Sativa L.) was evaluated through a series of laboratory column tests conducted in 2012-2013. Sandy loess and Sand soils were placed in 90 cm long soil columns with three different layer thicknesses(11.25 cm, 22.5 cm and 45 cm) and two homogeneous columns(Sand or Sandy loess). After being filled, all columns were put into a 90 cm deep plastic water tank for saturation lasting 72 hours. Then, the columns were transferred into a drainage tank which drained with a positive pressure of 2 cm at the bottoms of the soil columns, while the top of the soil column was covered with a filter paper and plastics to prevent surface evaporation. After 96 hours, the water holding capacity for each of soil columns was calculated. Then they were ready for a 56 d evaporation experiment in which the columns were sealed at the bottom to ensure zero flux from the lower end. The above two processes were also simulated using Hydrus-1D software. After that, all columns were saturated again and drained freely for 96 hours with the top of the soil column was enclosed. Then, alfalfas were seeded on the columns. In order to ensure the survival of each treatment, seedlings were allowed to grow up to nearly 12 cm high with adequate water and nutrients, then we started to irrigate according to the rainfall of study area(Yulin). Net photosynthetic rate(PN) for each column was measured using a portable photosynthesis system on sunny days in July, August and September, 2013 respectively. The profile soil water content was measured every week using time domain reflectometry probes(TDR) every 10 cm depth. The shoot, root, total dry biomass and fine root density(FRD) were also measured at the end of this experiment. The results of the measurements and simulations show that for the layered soils, water holding capacity could be significantly increased by increasing the number of layers and decreasing layer thickness. The layered soil column with sand on the surface could dramatically reduce soil evaporation. The FRD gradually decreased with the increasing depth in two homogeneous soils, while the FRD in the sandy loess layers was much higher than that in the sand layers for 3 layered soils. The total FRD was significantly increased by introducing more layers into a profile. Fine root distribution and textural layering affected soil water content and soil water storage in the profiles under the same condition of irrigation. With the increasing layer number of textural breaks and the decreasing layer thickness, the roots grew faster, while alfalfa consumed more water and had higher transpiration. The yield of alfalfa significantly increased with the increasing number of layers for layered soils, and similar phenomenon was observed for net photosynthetic rate.