Long-term changes in macronutrients, micronutrients, and potentially toxic elements under 33 years of continuous chemical fertilizer and organic matter application

This study examined the effects of chemical fertilizer and organic material [rice straw (RS) and RS compost] applications over 33 years (began in 1982) in a rice paddy on soil properties and nutrients. From 1995 to 2013, soil samples from the 0–12 cm layers were collected every 3 years from five fertilizer treatments: (1) PK chemical fertilizer only, (2) NPK fertilizer (NPK), (3) NPK + RS, (4) NPK + RS compost, and (5) NPK + high-dose RS compost. This analysis assessed changes in soil macro- (C, N, Ca, Mg, K, and P) and micronutrient elements (Fe, Mn, Zn, Ni, Cu, and Mo) and other potentially toxic elements (Al, Na, As, Cr, and Pb). Results revealed that organic matter additions increased the content of C and N over the soil sampling period. RS and RS compost treatments enhanced the annual SOC increase (0.089 − 0.477 g kg⁻¹ year⁻¹) compared with N treatment alone (0.010 − 0.039 g kg⁻¹ year⁻¹), suggesting that composting markedly improved N uptake and soil microbial activity. The content of macronutrient elements (Ca, Mg, P, and K) remained stable with organic matter added treatments, with Ca and P significantly correlating with C and N (<i>p</i> &lt; 0.05). However, micronutrient element levels of Fe, Mn, and Zn decreased, likely due to redox reactions and runoff; meanwhile, potentially toxic elements showed weaker association with organic matter addition and different accumulation patterns in the soil. Principal component analysis confirmed that C, N, P, and Ca exhibited strong interrelationships and were closely associated with compost addition. Overall, long-term applications of organic matter in single-crop paddy rice systems in Japan tended to enhance the accumulation of macronutrient elements.