Distribution characteristics of effective medium and micronutrient element contents in paddy soils of China

Abstract: The objective of this study was to investigate the spatial variability and the degree of abundance and deficiency of medium and micronutrient element contents in typical paddy soils of China, and to develop the best fertilization management for improving rice yields. Based on 289 monitoring points of paddy soils distributed across the northeast, Yangtze River delta, the middle of the Yangtze River, the southwest and south of China, soil samples collected in 2016 were determined for the contents of available medium and micronutrient elements (calcium, magnesium, sulfur, iron, manganese, copper, zinc, boron and molybdenum) in paddy soils. The spatial variability of these elements across the five regions was assessed. Combined with the critical values for limiting rice growth due to deficiency of available medium and micronutrient elements, the abundance and deficiency degree of available medium and micronutrient elements in the paddy soils were evaluated. The results showed that the contents of exchangeable magnesium and available boron in the northeast of China were much higher than those in the other regions, while the contents of available zinc in paddy soils were the lowest among the five regions. The highest average content of available manganese and the lowest average content of available sulfur in the paddy soils were both found in Yangtze River delta. The highest average content of available copper but the lowest average content of available iron, manganese and molybdenum among paddy soils was observed in middle of the Yangtze River. As compared with the other areas, the exchangeable calcium content of the paddy soils in the southwest of China was significantly higher (P<0.05), while the available copper content was significantly lower (P<0.05). Compared with the other areas, the contents of available iron and molybdenum in the south of China were significantly higher (P<0.05), while the contents of exchangeable calcium and magnesium were significantly lower (P<0.05). Critical values for evaluation of these elements were summarized based on practices and previous studies. The critical values for exchangeable calcium, exchangeable magnesium and available sulfur were 400, 50 and 16 mg/kg, respectively. The critical values for available iron, manganese, copper, zinc, boron and molybdenum were 4.5, 10, 2, 1.5, 0.5 and 0.15 mg/kg, respectively. The soils would be lack of the corresponding element if the contents of these elements were below the critical values and thereby the rice growth may be limited. Based on the critical values, the exchangeable calcium and magnesium contents in paddy soils of the five regions were rich in China. The contents of available sulfur in paddy soils were abundance for rice growth in the northeast, the middle of the Yangtze River, and the southwest of China, while 42.2% and 41.8% of monitoring points in Yangtze River delta and south of China were lack of available sulfur. The contents of available iron, manganese and copper were abundance in all the paddy soils. The proportions of monitoring points with lower contents of available zinc than its critical value were 75.0%, 52.3%, 31.9%, 53.2% and 10.4% in the northeast, Yangtze river delta, the middle of the Yangtze River, the southwest, and south of China, respectively. The proportions of monitoring points with lower contents of available boron than its critical value were 38.5%, 65.2%, 92.2%, 88.6% and 78.3% in the northeast, Yangtze river delta, the middle of the Yangtze River, the southwest, and south of China, respectively. The proportions of monitoring points with lower contents of available molybdenum than its critical value were 28.6%, 60.4%, 82.6%, 42.0% and 33.4% in the northeast, Yangtze river delta, the middle of the Yangtze River, the southwest, and south of China, respectively. It indicated that the contents of calcium, magnesium, iron, manganese and copper in the paddy soils of typical areas in China were abundance for rice growth. Sulfur fertilizer was needed in some areas of Yangtze River delta and southwest of China. The available zinc, boron and molybdenum in most of paddy soils were deficiency, and additional micronutrient element fertilizers were needed for rice growth, especially for zinc in the northeast, zinc, boron and molybdenum in Yangtze River delta, boron and molybdenum in middle of the Yangtze River, boron in southwest, and boron in south of China. And the medium and micronutrient element fertilizers should be included as part of fertilization management in practice for improving rice yields.