Spatio-temporal variability of available sulphur and micronutrients (Zn, Fe, Cu, Mn, B and Mo) in soils of a hilly region of northern India

Knowledge about spatio-temporal variabilities of plant available (hereafter called available) nutrients in soils is important for undertaking revised and efficient site-specific soil nutrient management practices in order to achieve higher crop production and nutrient use efficiency in different parts of the world. Despite emerging deficiencies of available sulphur (AS) and micronutrients in various soil-crop situations, the information per-taining to temporal changes in spatial variability of AS and available micronutrients in soils of different parts of the world is limited due to scarce studies in the said area. The present study was carried out to assess and compare the status and spatial as well as temporal distribution pattern of AS, available micronutrients, and some selected soil properties [soil pH, electrical conductivity (EC), and soil organic carbon (SOC)] in cultivated soils of a hilly region of India, in 6 years (2015-2021). We also visualized correlations among soil properties, AS, and available micronutrients, and studied spatial structures of the soil parameters. We collected altogether 2, 871 (1, 127 in 2015 and 1, 774 in 2021) georeferenced representative topsoil (0-15 cm depth) samples from cultivated areas of Uttarakhand state of India and analysed. Soil properties, AS and available micronutrients exhibited moderate variability with coefficient of variation (CV) values ranging from 10 to 100%. The mean values of AS and available micronutrients in 2015 and 2021 differed significantly, as revealed by t-test. The mean concen-trations of AS (13.2 mg kg -1), available iron (AFe) (36.5 mg kg -1), available boron (AB) (0.92 mg kg -1) and available molybdenum (AMo) (0.40 mg kg -1) in 2021 were significantly lower than the concentrations of AS (44.7 mg kg -1), AFe (42.8 mg kg -1), AB (1.46 mg kg -1), and Amo (0.94 mg kg -1) in 2015. The AS concentration was positively and significantly correlated (r = 0.064 p < 0.05 in 2015, r = 0.261 p < 0.01 in 2021) with soil pH and negatively and significantly correlated (r =-0.082 p < 0.01 in 2015, r =-0.125 p < 0.01 in 2021) with SOC in both the years. There were positive and significant correlations of SOC with available zinc (AZn) (r = 0.111 p < 0.01), available copper (ACu) (r = 0.062 p < 0.05) and AB (r = 0.159 p < 0.01) in 2015 and with AZn (r = 0.101 p < 0.01), AFe (r = 0.187 p < 0.01), ACu (r = 0.164 p < 0.01), available manganese (AMn) (r = 0.169 p < 0.01) and AMo (r = 0.266 p < 0.01) in 2021. The exponential model was found to be best fitted for all of soil parameters with lower MSE values (-0.007 to 0.006 in 2015 and-0.010 to 0.012 in 2021) in both the years of estimation. The nugget/sill ratios (varied from 0.32 to 0.74 in 2015 and from 0.29 to 0.57 in 2021) indicated the moderate spatial dependence for all the soil parameters except AFe (nugget/sill ratio 0.14, strong spatial dependence) in 2015 and EC (nugget/sill ratio 0.24, strong spatial dependence) in 2021.There were varied distribution patterns of soil properties, AS and available micronutrients in the study area for both the years of estimation. The distribution patterns of AS, AB and AMo changed to a greater extent over the period of time. Thus, the study highlighted the change in spatial variability of AS and available micronutrients in the study area over a period of 6 years.