The Yunwu Mountains are located in the typical steppe of the Loess Plateau, which is also the prairie fire pilosity area in the semiarid region of the plateau. Because of low rainfall, soil and water loss, and drought, the restoration of vegetation is difficult. Artificial grassland construction and pasture enclosure were used as restoration measures in this region by previous researchers, and achieved many positive effects. However, in recent years, because of long-term enclosure and lack of appropriate management, the grassland has apparently degenerated. This degeneration of vegetation cover is accompanied by accumulation of much plant litter. Because of the limited water, litter decomposition is limited, which seriously impacts on nutrient recycling in the ecosystems. After fire, organic matter is transformed into inorganic components that the vegetation can readily absorb. These inorganic components undergo a series of chemical changes in the soil. Fire may also affect the transport of water to be absorbed by plants.
In the present study, we mainly focused on the effect of fire on soil nutrient characteristics. Three Stipa bungeana communities were selected as experimental stands that had been burned in 2011 (new burning), 2009 (three years after burning) and 2000 (eleven years after burning). A nearby unburnt stand was selected as a control stand. In September 2011, soil nutrients were determined. The aim of this study was to investigate the characteristics of selected soil chemical properties in the different burning periods.
Changes in soil organic C (SOC), total N, total P and available K concentrations in the unburnt stand were contrasted with those in the stands which had been previously burnt in different years, including new burning, three years after burning, and eleven years after burning. The soils were sampled in 10cm layers to a depth of 50cm, and changes in soil nutrient concentrations were compared in the four stands in each of the 0-10cm, 10-20cm, 20-30cm, 30-40cm, and 40-50cm layers. The results were as follows: (1) SOC, total N, total P and available K concentrations at all depths in the newly burnt stand were significantly higher than in the unburnt stand (P<0.05). (2) SOC, total N, total P and available K concentrations at all depths three years after burning stand showed no significant differences from concentrations in the unburnt stand (P>0.05). (3) Total N concentrations at all depths eleven years after burning were significantly higher than in the unburnt stand (P<0.05), and SOC, total P and available K concentrations at all depths, except for 0-10cm, were significantly higher than concentrations in the unburnt stand (P<0.05).(4) In these four stands, SOC, total N, total P and available K concentrations decreased in the soil profile from the top to the bottom sampled layer. (5) The soil surface layer (0-10cm) had pH values of 7.9, 8.5,and 8.0 in the newly burnt stand, three years after burning and eleven years after burning respectively, which were not significantly different from the pH in the unburnt stand (pH=8.2) (P>0.05). Overall, new burning and previous burning (up to eleven years previously) compared to no burning led to significant improvement in the measured soil nutrient concentrations compared with the unburnt stand, but had not changed significantly three years after burning, which may be related to rainfall after fire.