Long-Term Effects of Continuous Rice Cultivation on Soil Chemical and Biological Properties in Niger State Floodplains, North Central Nigeria

This study investigates the long-term effects of continuous irrigated rice cultivation on soil chemical and biological properties in the floodplain farming areas of Niger State, located in Nigeria's North Central geopolitical zone. The Badeggi and Mokwa rice schemes in Niger State have been under continuous rice production for 20 to 35 years, yet the cumulative effects of prolonged submergence, intensive nutrient removal, and repetitive tillage on soil health have not been systematically documented, representing a significant research gap for paddy soil sustainability. Using a chronosequence design, soils from rice paddocks in cultivation for 5, 10, 20, and 35 years, and one uncultivated floodplain reference site, were sampled and analysed for pH, organic carbon, total nitrogen, available phosphorus, cation exchange capacity, iron and manganese reduction status, electrical conductivity, microbial biomass carbon, and rice root-associated methanogen populations by qPCR. Findings revealed that soils under 35 years of continuous rice showed significant phosphorus accumulation (mean Bray P of 68 mg/kg), elevated electrical conductivity approaching marginal salinity (1.8 dS/m), and increased reducible iron and manganese, reflecting prolonged anaerobic chemistry. Microbial biomass carbon declined with cultivation duration despite organic carbon accumulation, attributed to shifts in microbial community toward methanogen-dominated anaerobic communities. Methane-producing Archaea populations increased 4.8-fold in the 35-year compared to 5-year paddocks. The study provides the first chronosequence dataset for paddy soils in the North Central zone and recommends adoption of alternate wetting and drying irrigation to reduce methanogenesis and slow the soil chemical deterioration trajectory. Keywords: continuous rice cultivation, paddy soils, soil chemical properties, Niger State, methane.