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Sustainable production of lowland rice (Oryza sativa L.) requires minimising undesirable soil nitrogen (N) losses via nitrate (NO₃⁻) leaching and denitrification. However, information is limited on the N transformations that occur between rice crops (fallow and land preparation), which control indigenous N availability for the subsequent crop. In order to redress this knowledge gap, changes in NO₃⁻isotopic composition (δ¹⁵N and δ¹⁸O) in soil and water were measured from harvest through fallow, land preparation, and crop establishment in a 7 year old field trial in the Philippines. During the period between rice crops, plots were maintained either, continuously flooded, dry, or alternately wet and dry from rainfall. Plots were split with addition or removal of residue from the previous rice crop. No N fertilizer was applied during the experimental period. Nitrogen accumulated during the fallow (20 kg NH₄⁺–N ha⁻¹in flooded treatments and 10 kg NO₃⁻–N ha⁻¹in treatments with drying), but did not influence N availability for the subsequent crop. Nitrate isotope fractionation patterns indicated that denitrification drove this homogenisation: during land preparation ~50 % of inorganic N in the soil (top 10 cm) was denitrified, and by 2 weeks after transplanting this increased to >80 % of inorganic N, regardless of fallow management. The 17 days between fallow and crop establishment controlled not only N attenuation (3–7 kg NO₃⁻–N ha⁻¹denitrified), but also N inputs (3–14 kg NO₃⁻–N ha⁻¹from nitrification), meaning denitrification was dependent on soil nitrification rates. While crop residue incorporation delayed the timing of N attenuation, it ultimately did not impact indigenous N supply. By measuring NO₃⁻isotopic composition over depth and time, this study provides unique in situ measurements of the pivotal role of land preparation in determining paddy soil indigenous N supply.
