Spatial and temporal variability of soil nutrients in the black soil area of northeast China

A total of 1400 soil samples from the plow layer (0-20 cm) at an approximate interval of 5 km were collected in the autumn of 2002 over the entire black soil area of Northeast China to determine the spatial and temporal variability of seven variables, such as soil organic matter (C-org), total nitrogen (N-tot), total phosphorus (P-tot), total potassium (K-tot), alkali-dissolvable nitrogen (N-Page), available phosphorus (P-Olsen), and available potassium (K-Standford), with classical statistics and geostatistical analysis. From 1979 to 2002, the content of C-org in black soil had dropped 1.75-2.40 g/kg, about 0.08-0.10 g/kg each year. In contrary, P-Olsen content had increased more than 2 times than that in 1979, from 12.66 mg/kg to 14.66 mg/kg during the last 23 years, about 0.55-0.64 mg/kg annually. Therefore, P-Olsen was recognized as the most affected soil nutrient property by human activities during the past 23 years. Most soil nutrients (except P-Olsen) had moderate spatial variability, and were correlated with geographic latitude. Especially, C-org, N-tot and N-Page were highly and significantly correlated with latitude, which indicated that these soil nutrient properties were easily affected by natural factors such as climate, temperature and so on.