Temporal and spatial variability of soil organic matter and total nitrogen in a typical oasis cropland ecosystem in arid region of Northwest China

Soil degradation resulted from unreasonable land use has been a serious problem in the arid region of Northwest China. This paper seeks to understand the relationship between topsoil properties and farming practices for land management targeting at improving soil quality in this region. The temporal and spatial variation of soil organic matter (SOM), total nitrogen (STN) were analyzed with classical statistics and geostatistics methods in Linze County, Gansu Province, an typical oasis agricultural area in arid region of northwest China, as affected by farming practices using the data from 1982 to 2008. The results of classical statistics indicated that the average SOM and STN concentrations were 12.78 ± 4.38 g kg⁻¹ and 0.72 ± 0.21 g kg⁻¹ in 1982, 13.76 ± 4.02 g kg⁻¹ and 0.81 ± 0.20 g kg⁻¹ in 2008, respectively. The net carbon and nitrogen increased at an average rate of 0.104 Mg C ha^-1 year⁻¹ (1 Mg = 10⁶g) and 0.014 Mg N ha⁻¹ year⁻¹ in the past 26 years. Geostatistical analyses showed that the spatially related areas of SOM and STN were expanded by effect of farming practices. The spatial distributions of SOM and STN contents were influenced by regional soil parent materials. Temporal variation maps of SOM revealed a change trend was closely related to the amount of the application of organic manures. It was concluded that the applications of chemical fertilizers were effective for increasing of STN, and the continuous applications of chemical fertilizers combining with applications of organic manures (approximately 45 m³ ha⁻¹) every other year were helpful in accumulating soil organic carbon.