Aims High nitrogen (N) fertiliser inputs in intensive sugarcane systems drive productivity but also significant emissions of nitrous oxide (N2O), a potent greenhouse gas. Fertiliser and soil N availability for both plant N uptake and N2O emissions across different N rates remain unknown, hindering efficient N management. This study investigated the contribution of fertiliser and soil N and their interaction to plant N uptake and N2O emissions in two intensively managed tropical sugarcane systems. Methods High temporal resolution N2O measurements were combined with 15N recoveries across four N fertiliser rates, (100, 150, 200 and 250 kg N ha− 1) in soil, plant and N2O emissions. Results Cumulative N2O emissions ranged from 0.3 to 4.1 kg N ha− 1, corresponding to emission factors ranging from 0.7 to 2.4%. Native soil N accounted for > 60% of cumulative N2O emissions and total plant N uptake. Fertiliser N addition increased N2O emissions from native soil N compared to the unfertilised control, highlighting the interaction between fertiliser and soil N, which determined the overall magnitude but also the response of total N2O emissions to N rates dependent on the site conditions. Overall fertiliser 15N loss responded exponentially to N rates with 50% of applied N fertiliser permanently lost even at the recommended N rate. Conclusions The interaction between fertiliser and soil N and its contribution to N uptake and N2O emissions demonstrate the importance of integrating soil fertility management with N fertiliser rate recommendations for sugarcane systems to maintain crop productivity and reduce environmental impacts.

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Contour-levee irrigation system is commonly used for rice cultivation in Latin American and Caribbean countries, but its water dynamics in commercial farm field settings are yet to be fully determined. This study aimed to investigate the water dynamics of the contour-levee irrigation system by analyzing conventional irrigation practices and by quantifying water balance and additionally to examine potential toposequential effects. Field experiments with different irrigation intervals were conducted on three commercial farms in Ibagué, Colombia for two seasons from 2017 to 2018. Irrigation and runoff water flows were constantly measured during the crop cycle using Parshall flumes with water level sensors. Percolation rate and field water table were measured using percolators and piezometers installed along the toposequence. The results showed that conventional irrigation management was highly flexible depending on soil permeability, rainfall, and agronomic factors, not particularly paying attention to ensure the flooded conditions during flowering period. The water balance resulted in the irrigation accounting for 76% of the total water input, whereas the runoff, ET, and percolation accounted for 40%, 21%, and 31% on overall average with considerable variation among the three farms. Percolation rates and duration with standing water did not show a clear and consistent tendency among the toposequential positions, but the percolation rate was significantly different among the farms corresponding to soil permeability. Consequently, clear toposequential effects on the water dynamics or on grain yield were not observed at the study site. To our knowledge, this study is the first to elucidate detailed water dynamics of contour-levee irrigation system in farm fields including toposequential difference.

Aims: High nitrogen (N) fertiliser inputs in intensive sugarcane systems drive productivity but also significant emissions of nitrous oxide (N2O), a potent greenhouse gas. The effects of N fertiliser inputs on native soil N availability for plant N uptake as well as N2O emissions remain unknown, hindering efficient N management. This study investigated the contribution of fertiliser and soil N and their interaction to plant N uptake and N2O emissions in two intensively managed tropical sugarcane systems with different farming practices. Methods: High temporal resolution N2O measurements were combined with 15N recoveries across four N fertiliser rates, (100, 150, 200 and 250 kg N ha-1) in soil, plant and N2O emissions. Results: Cumulative N2O emissions ranged from 0.3 to 4.1 kg N ha-1, corresponding to emission factors ranging from 0.7 to 2.4%. Native soil N accounted for > 60% of cumulative N2O emissions and total plant N uptake. Fertiliser N addition increased N2O emissions from native soil N compared to the unfertilised control. These findings highlight the interaction between fertiliser and soil N, with the latter being the dominant source for N uptake and loss. Overall fertiliser 15N loss responded exponentially to N rates with 50% of applied N fertiliser permanently lost even at the recommended N rate. Conclusions: The response of soil-derived N2O emissions to N rates largely determined the magnitude of total N2O emissions, demonstrating the importance of site and soil specific interaction between soil and fertiliser N for N2O emissions.