Soil chemical indicators and nutrient cycling variations across sequential years of rice cultivation: A case study of floodplain conditions of the Amazon, Brazil

To meet the rising food demand for next generations, soils are needed to stand biological yield and promote animal, plant and human welfare. In this logic, the aim of the study was to determine the effects of different soil uses; (a) successive irrigated rice planting and (b) different soil management practices onto soil chemical properties and nutrient cycling. The study was carried out in areas with dissimilar years of irrigated rice farming, managed with conventional tillage (CT) and minimum cultivation (MC). Having as a reference a native vegetation plot, areas with 2, 4, 9, 14 and 26 years of different management such as conventional tillage or minimum cultivation were evaluated. Soil samplings were performed in the 0 - 0.10, 0.10 - 0.20 and 0.20 - 0.30 m layers. Three sub samples per layer were collected within each replicate and then condensed to the combined sample; straw sampling was also performed. Total nitrogen (TN) and total organic carbon (TOC) were determined by dry digestion in a Vario El III elemental. Highest TOC results were obtained in Ar9 and Ar14 areas in 0-0.10 and 0.10-0.20 m layers; straw production varied from 7.61 to 8.94 Mg ha-1. Ar2 presented higher value in relation to others; nevertheless these variations were not significant. Still, it was observed that the Ar9 and Ar14 areas presented a higher total biomass production (grains and straw) than those found in Ar4, Ar2 and Ar26; so, these variations did not differ. Conversion of native ecosystems to irrigated rice production areas significantly affects soil fertility. Key words: Total organic carbon, soil management, irrigated rice.