Spatial characteristics and changes of soil organic matter for cultivated land in suburban area of Xi'an from 1983 to 2009

Abstract: Soil organic matter (SOM) is an important index in estimating the soil carbon stock, soil fertility and quality, and is closely related to soil productivity. Geographical information system technology and geostatistics were used to analyze the spatial variability, impact factors, and soil quality grade changing patterns of soil organic matter (SOM) in Chang'an District, Hu County, and Zhouzhi County in the past 26 years, based on 3 100 topsoil samples using the "S-shape" method of the Cultivated Land Productivity Investigation and Quality Evaluation Project of Shaanxi Province (2007－2009). A global positioning hand-held receiver was used to record sampling site locations. SOM contents satisfied the normal distribution after Box-Cox transformation (p=0.42) with the Kolmogorov-Smironov (K-S) test. The optimal semivariogram model for SOM and its parameters were determined using the geostatistical trend analysis of ArcGIS9.3 and GS+9.0 with cross-validation methods. Subsequently, the unobserved SOM data was estimated by the ordinary Kriging method. The results showed that SOM contents were low, ranged from 5.80 to 34.90g/kg, concentrated at >10-20 g/kg with an average of 15.95 g/kg which was increased by 26.76% compared with that in 1983(12.58 g/kg). More specifically, SOM contents mainly ranged from 15 to 20 g/kg in the Chang'an District and Hu county, and 12 to 15 g/kg in Zhouzhi county, which indicated that it was decreased from east to west. Spatially, SOM contents were significantly lower in the northwest than in the southeast, which was consistent with geographic and geomorphic distribution. Additionally, at the 450 m sampling scale, SOM had moderate spatial correlation, which was both affected by structure factors (such as topography, geomorphology, and soil texture) and random factors (such as fertilization and cropping systems). In 2009, the whole SOM contents of cultivated land was as follows: 93.46% was slightly low (>10-20 g/kg), 6.42% was suitable (>20 g/kg), and 0.12% was deficient (<10 g/kg). Since the 1980s, SOM contents in most of the cultivated land were increased by 5 to 10 g/kg. Chang'an district and Hu county, in closer proximity to Xi'an City, were significantly higher than the outer suburb county of Zhouzhi, which indicated an obvious transition from rural to urban areas. According to the Second Soil Organic Matter Classification Standard of Xi'an, during the past 26 years, 81.75% of SOM grades in cultivated land were improved, and 2.22% were slightly declined. SOM content of cultivated land with formerly levels of 6-9 was increased sharply, while SOM content of cultivated land with formerly levels of 3-5 was decreased. According to the field investigation, SOM contents increasing significantly should be attributed to the widespread practices of straw returning, organic manure, and chemical fertilizer application. At the same time, the main factors affecting the accumulation of SOM in the current arable layer include farmers aging, soil properties and agronomic measures. With yield improvement, soil testing and formulated fertilization that would be to improve farmers' scientific and technological knowledge, and balance organic and chemical fertilizer for improving the SOM level, and fertilizer utilization rate should be popularized. The study can provide a scientific guidance for cultivated land maintenance and regional agricultural production in the suburban area of Xi'an.