BACKGROUND As an important component of plant nitrogenase, nitrate reductase and human enzyme prosthetic groups, Mo plays an important role in maintaining plant growth and development, and human health. A previous study showed that total molybdenum in soil was mainly restricted by soil parent material and soil type, and effective molybdenum was mainly affected by total molybdenum content, soil texture, pH value and organic matter (Liu Peng, 2001; Wei Minghui, 2020; Xia Yan, 2021). The Huaibei Plain in Anhui Province is located in the molybdenum deficient soil area of the North China Plain. At present, there is a lack of systematic research on the molybdenum content and its driving factors in the soil in this area. Therefore, it is of great practical significance to study the distribution of soil molybdenum content and its influencing factors in agricultural land for scientific fertilization and improvement of soil molybdenum availability.

OBJECTIVES To study the content and spatial distribution characteristics of total molybdenum and available molybdenum in different types of soil, and deeply analyze the main factors restricting the distribution characteristics of total molybdenum and available molybdenum in surface soil and put forward scientific suggestions for scientific fertilization of soil and improvement of soil molybdenum availability.

METHODS When the crops were mature, surface soil samples with a depth of 0-20cm were collected, and the sampling density was one sample/km². The contents of total molybdenum, available molybdenum, iron oxide, aluminum oxide, silicon oxide, phosphorus, manganese, organic matter and pH value in soil were determined by inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and other methods. The content and spatial distribution characteristics of total molybdenum and available molybdenum in soil were studied by using statistics, Pearson correlation analysis and other methods, and the main factors restricting the distribution characteristics of total molybdenum and available molybdenum in soil were analyzed and revealed.

RESULTS The contents of total molybdenum and available molybdenum in lime concretion black soil were mainly deficient, and the proportion of total molybdenum and available molybdenum deficiency in soil was 93.3% and 87.3%, respectively. The correlation coefficients of total molybdenum content with pH value, manganese, iron oxide, phosphorus, and silicon and aluminum ratio (i.e. the ratio of silicon oxide and aluminum oxide content in the soil) in lime concretion black soil were 0.268, 0.213, 0.189, 0.153 and -0.199 respectively, and the correlation coefficients of available molybdenum in lime concretion black soil with pH value and organic matter were -0.310 and 0.117, respectively. The proportion of slightly deficient, moderate, and slightly enriched total molybdenum content in fluvo-aquic soil was 31.2%, 28.4% and 21.1% respectively, and the correlation coefficients of total molybdenum content with manganese, phosphorus and organic matter in fluvo-aquic soil were 0.611, 0.330 and 0.205 respectively. The proportion of soil area lacking available molybdenum content in fluvo-aquic soil was 86.2%, and the correlation coefficient between total molybdenum and available molybdenum content in fluvo-aquic soil was 0.166.

CONCLUSIONS The low content of total molybdenum in lime concretion black soil was mainly affected by soil pH, manganese, iron oxide, phosphorus, and silicon and aluminum ratio. The pH value and organic matter are important factors that restrict the low content of available molybdenum in lime concretion black soil. The content of total molybdenum in fluvo-aquic soil was significantly related to manganese, phosphorus and organic matter. The low content of total molybdenum was an important factor for the deficiency of available molybdenum in fluvo-aquic soil. It is suggested that scientific fertilization should be carried out by comprehensively considering the content of available molybdenum in soil and its main limiting factors, in which organic fertilizer should be applied to the lime concretion black, and molybdenum fertilizer should be applied reasonably to the fluvo-aquic soil.