The effects of agricultural management on selected soil properties of the arable soils in Tibet, China

Lhasa is a crucial agricultural region of the Tibetan plateau for local grain and vegetable supplies. Therefore, to sustain soil productivity, it is important to understand how agricultural management practices can cause changes in soil properties. Based on the information from the soil survey conducted in the late 1980s, we selected and sampled the following sites in the summer of 2007: 17 sites of the tillage (A) layer soils and 13 sites of soil profiles, including the tillage and subsoil layers from three types of arable land soils in Lhasa (alluvial soil, steppe soil, and meadow soil). At the same time, another 55 composite samples and core samples were taken from the grain-crop land, open vegetable land and greenhouse vegetable land of the alluvial soil. The selected soil properties were measured and compared to the soil survey data from the 1980s. The results showed that because of wind erosion and irrigation, the arable soils in the investigated area have become significantly more sandy (P < 0.05) since the late 1980s. Moreover, because of fertiliser application and acid precipitation, the soil pH and cation exchange capacity of the study soils are significantly lower (P < 0.05) than in the late 1980s, thus leading to soil acidification and lower soil fertility. Soil organic matter and the total nitrogen contents in the cultivated steppe soils and meadow soils increased, possibly because of manure addition and fertiliser use in the region. The soil organic matter and the total nitrogen content decreased in the alluvial soils, possibly due to an intensified cultivation; however, the available nitrogen and phosphorus increased significantly (P < 0.01), whereas potassium decreased significantly (P < 0.05). These changes were mainly attributed to the heavy use of nitrogen and phosphorus fertilisers and the infrequent use of potassium fertiliser. The changes in the A layer (tillage layer) were more apparent than in the other layers. This finding was especially evident in the vegetable land, where the changes are attributed to the agricultural management activities that often occur in this layer. The soil organic matter in the B layer increased significantly (P < 0.05) due to the accumulation of plant roots and the deposition of organic matter from the A horizon. For the same soil under different land use, the rank of the soil fertility was cropland < open vegetable land < greenhouse vegetable land, which further suggests that the changes in the soil properties were mainly due to the application of manure and the intensity of cultivation.