Spatial variability of organic matter dynamics in the semi-arid croplands of northern Kazakhstan

Chernozem soils have been recognized as one of the most important soils for food production and for the sink of organic matter on a global scale. Soil degradation or accelerated organic matter decomposition has been reported recently in northern Kazakhstan due to specific agricultural management such as summer fallow to increase the soil moisture for the cropping season. The objectives of the present study were 1) to evaluate the carbonrelated properties of soil and plant in relation to the topography and amount of available water in upland fields, 2) to analyze their spatial variability using geostatistics and 3) to propose a rational system of management for the promotion of sustainable agriculture in this region. Field investigations were carried out in large-scale upland fields in Shortandy, northern Kazakhstan, where a crop rotation system had been developed on Typic Haplustolls or Southern Chernozem soils. The study field (14 km x 5 km) was divided into 70 plots (1 km x 1 km each) and, at the center of each plot, organic carbon (C) content (0-90 cm) and potentially mineralizable C content (0-15 cm) in soil, total, ear and shoot C contents in plant, and crop yield were investigated in addition to the elevation and soil water content at the beginning of the growing season. The total amount of C stored in soil (0-90 cm) was 170.9 t haE-1 with the highest C content of 39.8 t haE-1 being recorded in the surface soil (0-15 cm). Potentially mineralizable C in soil (0-15 cm) amounted to 2.72 t haE-1, equivalent to 6.8% of the total C in the surface soil, suggesting that a considerable part of C in soil could be released as CO2 under favourable conditions for organic matter decomposition. Plant aboveground biomass C amounted to 1.8 t haE-1, of which 1.2 t haE-1 was returned to the field as plant residues and 0.6 t haE-1 was removed as crop (ear). Coefficients of variation in the amount of soil mineralizable C and plant properties exceeded 40%, suggesting a considerable variation in the field. Correlation analysis indicated that the elevation showed a positive relationship with the water content, soil organic C content (p< 0.01), content of potentially mineralizable C and plant yield (p< 0.05). The spatial patterns of the measured properties in the isarithmic maps showed that the content of soil organic C was the highest in the top plateau; water content, plant C content and yield were the highest in the north-facing slope area; whereas the values of all of these parameters were relatively low in the south-facing slope area. These results strongly suggest that the organic matter dynamics in the field was considerably affected by the topography and that the most favourable area for the storage of organic matter was different from that for food production. In conclusion, site-specific agricultural management based on the spatial patterns of organic matter dynamics would be a suitable option for harmonizing sustainable agricultural production with environmental conservation by reducing organic matter decomposition.