Global rice agriculture will be increasingly challenged by water scarcity, while at the same time changes in demand (e.g. changes in diets or increasing demand for biofuels) will feed back on agricultural practices. These factors are changing traditional cropping patterns from double-rice cropping to the introduction of upland crops in the dry season. For a comprehensive assessment of greenhouse gas (GHG) balances, we measured methane (CH4 )/nitrous oxide (N2 O) emissions and agronomic parameters over 2.5 years in double-rice cropping (R-R) and paddy rice rotations diversified with either maize (R-M) or aerobic rice (R-A) in upland cultivation. Introduction of upland crops in the dry season reduced irrigation water use and CH4 emissions by 66-81% and 95-99%, respectively. Moreover, for practices including upland crops, CH4 emissions in the subsequent wet season with paddy rice were reduced by 54-60%. Although annual N2 O emissions increased two- to threefold in the diversified systems, the strong reduction in CH4 led to a significantly lower (P < 0.05) annual GWP (CH4  + N2 O) as compared to the traditional double-rice cropping system. Measurements of soil organic carbon (SOC) contents before and 3 years after the introduction of upland crop rotations indicated a SOC loss for the R-M system, while for the other systems SOC stocks were unaffected. This trend for R-M systems needs to be followed as it has significant consequences not only for the GWP balance but also with regard to soil fertility. Economic assessment showed a similar gross profit span for R-M and R-R, while gross profits for R-A were reduced as a consequence of lower productivity. Nevertheless, regarding a future increase in water scarcity, it can be expected that mixed lowland-upland systems will expand in SE Asia as water requirements were cut by more than half in both rotation systems with upland crops.

The nutritional responses to drying and rewetting cycles of partial root-zone irrigation still remains elusive. The effect of alternate partial root-zone irrigation (PRI) on water use efficiency and nitrogen (N) accumulation compared with deficit irrigation (DI) and full irrigation (FI) were investigated in maize (Zea mays L.) grown under three N-fertilization rates (1.5, 3.0, and 6.0 g N pot-1) and moderately and severely water-stressed levels (60 and 40% of soil water holding capacity). The plants were grown in split-root pots and exposed to FI, DI and PRI treatments from the fourth leaf to silking stage. Analysis across the N-fertilization treatments showed that both PRI and DI significantly decreased plant water use as well as plant height, girth, leaf area and shoot biomass, leading to similar WUE compared with the FI control. Carbon isotope composition (δ 13 C) was highest in PRI plants indicating a fine-tuned long-term stomatal control over gas exchange. Across the N-fertilization rates, FI plants accumulated significantly greater amount of N than deficit irrigation treatments. PRI and DI plants had similar plant δ 15 N, indicating the similar soil N mineralization. Plant dry biomass, which was linearly associated with plant N uptake, was similar for PRI and DI plants. Both resulted in the equivalent amount of N accumulation in the shoots of PRI and DI plants. It was noted that increased soil moisture level, e.g., from 40% to 60%, showed the tendency of increasing N uptake for PRI plants relative to DI plants. Therefore, in order to facilitate N uptake, the soil water availability in the wet soil compartment of PRI treatment should remain at high water levels.

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Worldwide, rice production contributes about 10% of total greenhouse gas (GHG) emissions from the agricultural sector, mainly due to CH4 emissions from continuously flooded fields. Alternate Wetting and Drying (AWD) is a promising crop technology for mitigating CH4 emissions and reducing the irrigation water currently being applied in many of the world's top rice‐producing countries. However, decreased emissions of CH4 may be partially counterbalanced by increased N2O emissions. In this case study for the Philippines, the national mitigation potential of AWD is explored using the process‐based biogeochemical model LandscapeDNDC. Simulated mean annual CH4 emissions under conventional rice production for the time period 2000–2011 are estimated as 1,180 ± 163 Gg CH4 yr−1. During the cropping season, this is about +16% higher than a former estimate using emission factors. Scenario simulations of nationwide introduction of AWD in irrigated landscapes suggest a considerable decrease in CH4 emissions by −23%, while N2O emissions are only increased by +8%. Irrespective of field management, at national scale, the radiative forcing of irrigated rice production is always dominated by CH4 (>95%). The reduction potential of GHG emissions depends on, for example, number of crops per year, residue management, amount of applied irrigation water, and sand content. Seasonal weather conditions also play an important role since the mitigation potential of AWD is almost double as high in dry as compared to wet seasons. Furthermore, this study demonstrates the importance of temporal continuity, considering off‐season emissions and the long‐term development of GHG emissions across multiple years.