Study area
The study area is located in the Yanhe watershed of the Loess Plateau at N36°23′-37°17′ and E108°45′-110°28′ in northern Shaanxi Province. An on-site ecology station has comprehensively managed this area for 25 years. The area is 287 km in length, and 7687 km2 in size. Ninety percent of the land is hilly, 3% is villages, rivers, and lakes, and only 7% is suitable for intensive agriculture. The study area has a sub-arid climate characterized by heavy seasonal rainfall with periodic local flooding and drought; the average annual rainfall at the experimental site is 497 mm (1970-2000, CV22%) with distinct wet and dry seasons. The rainy season starts in July and ends in October, and rainfall in August accounts for 23% of the annual precipitation. The annual reference evapotranspiration is approximately 1,000 mm. Most of the lands are located at 900-1500 m altitude, closely dissected, and sharp-edged with very steep slopes (the slopes are 40%). The topography, soil type, soil and land-use patterns of Yanhe watershed are very typical in the Loess Plateau. Land-use types include sloping land, terraces, orchards, woodland, shrubland, natural grassland, wasteland and others [23].
Study approach and sampling design
The chronosequence method was used because of a conversion history in this area. The management was similar to previous practices according to known cultivation climate, topography, and soil type. Soil samples were collected in August 2006. The soil samples were taken with a corer and dried at 105°C for 12 h. Soil moisture was measured using the oven-drying method (Nanjing Institute of Soil Research, Chinese Academy of Sciences, 1980). Soil samples were taken at 10 different depths: 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 cm below ground with 5 replicates per vegetation community. At each location, the soil bulk densities of samples taken from 0-20, 20-40 and 40-60 cm below ground were measured using a 100-cm3 cylinder, respectively.
Soil rehabilitation in relation to vegetative cover is commonly studied by monitoring changes in plants and soil along a vegetative chronosequence developed on similar soil under similar climates [16]. This chronological approach has been widely used in applied ecosystem researches [24] and is considered retrospective, because existing conditions are compared with known original conditions and treatments. Retrospective approach was used in this study because of the availability of adjacent vegetation communities established 5, 10, 15, 20 and 25 years ago on eroded soils with similar properties. These vegetation communities provided a time gradient of grass occupancy on similar sites. Changes in soil properties were measured by comparing sites of different ages. Five age series (5-, 10-, 15-, 20- and 25-year-old vegetation community) were found adjacent to the study area, and have undergone light livestock grazing in recent years. Within each community (5, 10, 15, 20 and 25-year-old vegetation), five sampling sites were selected and samples were collected with five replicates. Also, five non-vegetated lands near the planted sites (farmland) were chosen as controls for the chronosequence.
Data analysis
Soil water storage (soil water quantity within a certain depth) was calculated based on soil moisture and soil bulk density. Soil water storage has been used as the key factor to evaluate soil moisture, because calculations based on soil volume eliminated the influence of soil depth [25].
Soil moisture analysis of variance (ANOVA) and correlation were carried out using the SPSS11.0 procedures for sites in different succession stages. Duncan's test (p < 0.05) was used to compare means of soil variables when the results of ANOVA were significant at p < 0.05.
Calculation of soil water storage
Soil water storage (soil water quantity within a certain depth) was calculated based on soil moisture and soil bulk density. Soil water storage has been used as the key factor to evaluate soil moisture, because calculations based on soil volume eliminated the influence of soil depth (Hu, 1992). Soil water storage [(mm)] for a given soil layer was calculated using the following formula:
(1)
Where is soil moisture (%), is soil bulk density (g cm-3), and is the depth of the layer (cm).
In this research, soil water storage at 0-60 cm, 60-120 cm and 120-200 cm below ground were calculated according to formula (1). Total soil water storage at 0 to 200 cm below ground was calculated as the sum of the soil water storage at 0-60 cm, 60-120 cm and 120-200 cm below ground.