Impact of irrigation management on paddy soil N supply and depth distribution of abiotic drivers

In rice production, water-saving irrigation management is expanding and likely alters depth profiles of soil moisture, redox potential (Eh) and microbial activity. It is, however, unclear how such conditions then impact net soil N-release and availability to the rice crop, because we do not know well enough how water-saving irrigation management shapes depth-distribution of Eh and reductive processes, and microbial activity. A field experiment with rice was laid out on a typical young floodplain paddy soil of Bangladesh with three irrigation schemes, viz. continuous flooding (CF), safe alternate wetting and drying (AWD) and direct seeded rice (DSR), with 120 kg N ha−1 (N120) or without (N0) urea application. We evaluated changes in soil mineral N and plant N uptake, CH4 and CO2 emissions and soil pH, and at multiple depths soil Eh and temperature, dissolved C, Fe and Mn throughout 2015 dry (Boro) season (Jan-Apr). Eh stayed at or above ∼+300 mV except for sudden drops to ∼−200 mV with irrigation events in DSR. Eh quickly dropped to methanogenic conditions, under both AWD and CF; rises to ∼+200 mV were observed during AWD-drainage events but were restricted to upper 5.5 or 12.5 cm depths. Throughout the growing season there was a pronounced increase in reductive dissolution of Fe and Mn (hydro-) oxides, buildup of dissolved C, and CH4 effluxes under AWD and CF but not DSR, likely at least partially driven by the gradual soil warming from ∼20 °C till 28 °C. Predominant aerobic conditions under DSR lead to a nearly doubled C-emissions (CO2 + CH4) compared to AWD and CF, suggesting more soil organic matter (OM) degradation in the former case, while soil mineral N plus plant N build-up rate followed an opposite order. Urea application did not raise soil exchangeable N levels, even prior to significant plant uptake from 28 DAT (days after transplanting), and we forward temporal abiotic NH4+-fixation and N-removal processes as explanations. We conclude that regardless of some distinctions in temporal evolutions of puddle layer Eh, solution C, Fe and Mn, and CH4-emission, soil N-supply was quite comparable under AWD and CF, as was rice yield. In the context of N availability, AWD could be safely adopted for rice growth in the Bangladeshi Boro season. The eventual fertilizer N recovery efficiency was higher for CF (42%) than for AWD (32%), but AWD saved 12% irrigation water. While DSR saved 45% water there was a large yield penalty, likely due to drought stress but also by poor germination caused by cold night temperatures in mid-January, while seedling transplantation in CF and AWD plots was only later on 28 January. Further research should be conducted to investigate the fast and pronounced removal of exchangeable inorganic N after initial N buildup by soil OM mineralization, especially in CF and AWD. At this moment most likely candidate processes appear clay-NH4+ fixation and anaerobic NH4+-oxidation.