[Spatial and temporal variability of soil C/N ratio in Songnen Plain maize belt]

The C/N ratio of soils is a sensitive indicator of soil quality, and an indicator for assessing carbon and nitrogen nutrition balance of soils. Its variation is significant in reflecting the carbon and nitrogen cycling of soils. Based on field investigation, sample collection and analysis, and application of geostatistics and GIS technology, spatial and temporal variation of C/N ratio was analyzed and studied from 1980 to 2005 in Songnen Plain maize belt. The results indicated that the mean value of C/N ratio is 10.56 and 12.30 in 1980 and 2005, respectively. Spatial correlation distance of soil C/N ratio in two periods is 196.3 km and 51.1 km, showing a decreasing trend, which indicated that farming management factors were enhancing. In the past 25 years, 84.88% of soil C/N ratio was on rise with the highest value in the west of the study area, but parts of Dehui County and Jiutai County decreased. As for different land use types, soil C/N ratios in the upland, paddy land, forest and woodland and grassland showed upward trends, with the highest increase from 10.03 +/- 1.12 in 1980 to 12.61 +/- 0.87 in 2005 in grassland and higher in upland and paddy land than the national average. The increasing soil C/N ratio illustrated that soil carbon increased faster than nitrogen. To maintain the steady growth of soil C/N ratio, it is suggested that the return of carbon be paid more attention when the input of nitrogen, such as incorporating crop residues into soil and inputting more organic fertilizers into soils for future farming practices.