Modeling the relationship of soil water repellency with soil moisture for Pinus massoniana and Schima superb secondary forests

Abstract

In China, secondary forests occupy a large area of forested lands. However, the relationship of soil water repellency with soil moisture, and the threshold value of soil moisture at which soil water repellency disappears in the forest soils are unknown. In this study, the relationship between soil water repellency and soil moisture was investigated and analyzed by water drop penetration time with soil samples. The soil samples were collected in 10 kinds of stands that were three pure Pinus massoniana stands (PPM [P1 to P3]); three pure Schima superb stands (PSS [S1 to S3]); and four mixed forest stands (PSM [M1 to M4]) with sandy clay, silt clay, and clay soil, respectively, from the state-owned forest farm of Qingshigang, Hunan, China. The obtained relationships could be characterized by nonlinear curves with one peak and a long tail to the right for all the stands. Three nonlinear models, including Gaussian, Lorentzian, and Lognormal models, were applied to fit the data with regression analysis. The results showed that (1) at the beginning, soil water repellency increased with the increasing soil moisture until reaching to its maximum value, then decreased and gradually became zero at which the threshold value of soil moisture was yielded for each of the stands. However, the maximum value of soil water repellency and the threshold value of soil moisture varied depending on the soil types, soil properties, and forest types. (2) The smaller the initial soil moisture contents, the smaller the peak values of the water repellency. (3) The Lorentzian model more accurately modeled the relationship than the Gaussian and Lognormal models and thus could be used to simulate the response of soil water repellency to soil moisture and to predict the maximum water repellency. It is expected that this study can provide useful information and guidelines for improvement of soils and comparison of different repellent soils in the secondary forests. This study can also offer corresponding suggestions for other secondary forests.