The impacts of soil properties and urea fertigation on nitrous oxide (N2O) emissions from uncropped areas of furrow‐irrigated Vertisol paddocks are unknown. We sampled soils from the head‐ditch end (upslope) and sediments from the tail‐drain end (downslope) of 10 Vertisols under irrigated cotton (Gossypium hirsutum L.) production in northeastern Australia. Four replicates of each sample were incubated in open‐top polyvinyl chloride (PVC) chambers at 25 ± 2°C for 25 d. Nitrous oxide emissions were measured periodically after simulated irrigations on Days 0 and 15 with either water or, for soils, urea solution. Compared with the soils, sediments were enriched in silt, total organic carbon (TOC), total nitrogen (TN), ammonium N, and dissolved organic C (DOC) but had lower pH and sand content. Sediments emitted more N2O than soils from the same paddocks after water irrigations. Nitrous oxide fluxes varied by two orders of magnitude between paddocks, with most variation explained by baseline nitrate N, TOC, pH (inversely), and sand content. Urea solution applied to soils at 30 kg N ha−1 irrigation−1 increased N2O emitted, but more so after the second irrigation. In irrigated cotton systems, tail‐drain sediments are a potential hotspot for N2O emissions that has not previously been documented.
Nitrogen (N) fertiliser inputs for irrigated cotton production are rapidly increasing to support ever-increasing yields, but much of the applied N may be lost as N gases, including nitrous oxide (N2O), via denitrification in medium–heavy clay soils. The addition of a nitrification inhibitor can reduce overall N loss and N2O emissions. Currently, nitrapyrin (2-chloro-6-trichloro methyl pyridine) is the only inhibitor used with anhydrous ammonia (AA), whereas 3,4-dimethyl pyrazole phosphate (DMPP) has potentially greater stability and longevity in soil, but is not compatible with AA.
A newly-developed formulation based on DMPP, 3,4-dimethyl pyrazole tetra-methylene sulfone (DMPS), can be direct-injected with AA. We compared N2O emissions from DMPS- and nitrapyrin-treated AA from two Vertosols used for irrigated cotton. At Emerald (Queensland), both inhibitors reduced N2O emitted by 77% over 2 months. At Gunnedah (New South Wales), DMPS was active in the soil for 3 months, reducing N2O by 86%, whereas nitrapyrin activity lasted for 2 months and reduced N2O by 65%. Realising the potential for improved environmental benefits from directly injecting DMPS with AA requires an agronomic benefit justifying its additional cost to the farmer. Future research needs to investigate the potential for reduced N rates when using these inhibitors – without compromising high yields.
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