Climate change refers to the statistically significant changes in the mean and dispersion values of meteorological factors. Characterizing potential evapotranspiration (ET₀) and its climatic causes will contribute to the estimation of the atmospheric water cycle under climate change. In this study, based on daily meteorological data from 26 meteorological stations in Heilongjiang Province from 1960 to 2019, ET₀ was calculated by the Penman–Monteith formula, linear regression method and the Mann–Kendall trend test were used to reveal the seasonal and inter-annual changing trend of ET₀. The sensitivity-contribution rate method was used to clarify the climatic factors affecting ET₀. The results showed that: (1) From 1960 to 2019, the maximum temperature (T_max), minimum temperature (T_min) and average temperature (T_mean) showed an increasing trend, with climate tendency rate of 0.22 °C per decade (10a), 0.49 °C/(10a), 0.36 °C/(10a), respectively. The relative humidity (RH), wind speed (U) and net radiation (Rn) showed a decreasing trend, with a climate tendency rate of −0.42%/(10a), −0.18 m/s/(10a), −0.08 MJ/m²/(10a), respectively. (2) ET₀ showed a decreasing trend on seasonal and inter-annual scales. Inter-annually, the average climate tendency rate of ET₀ was −8.69 mm/(10a). seasonally, the lowest climate tendency rate was −6.33 mm/(10a) in spring. (3) ET₀ was negatively sensitive to T_min, and RH, while positively sensitive to T_max, T_meanU and Rn, its sensitivity coefficient of U was the highest, which was 1.22. (4) The contribution rate of U to ET₀ was the highest on an inter-annual scale as well as in spring and autumn, which were −8.96%, −9.79% and −13.14%, respectively, and the highest contribution rate to ET₀ were Rn and T_min in summer and winter, whose contribution rates were −4.37% and −11.46%, respectively. This study provides an understanding on the response of evapotranspiration to climatic change and further provides support on the optimal allocation of regional water resource and agricultural water management under climate change.

Predicting the impact of future climate change on food security has important implications for sustainable food production. The 26 meteorological stations’ future climate data in the study area are assembled from four global climate models under two representative concentration pathways (RCP4.5 and RCP8.5). The future maize yield, actual crop evapotranspiration (ET_a), and water use efficiency (WUE) were predicted by calibrated AquaCrop model under two deficit irrigation (the regulated deficit irrigation (RDI) at jointing stage(W1), filling stage(W2)), and full irrigation (W3) during the three periods (2021–2040, 2041–2060, and 2061–2080). The result showed that the maize yields under W1, W2, and W3 of RCP4.5 were 2.8%, 2.9%, and 2.5% lower than those in RCP8.5, respectively. In RCP8.5, the yield of W3 was 1.9% and 1.4% higher than W1 and W2, respectively. Under the RCP4.5, the ET_a of W1, W2, and W3 was 481.32 mm, 484.94 mm, and 489.12 mm, respectively. Moreover, the ET_a of W1 was significantly lower than W2 under the RCP4.5 and RCP8.5 (p > 0.05). In conclusion, regulated deficit irrigation at the maize jointing stage is recommended in the study area when considering WUE.

Understanding the methods leading to rice yield increase is vital for sustainable agricultural development. Improving the harvest index (HI) is an important way to increase rice yield. To explore the effects of different water and nitrogen management modes on the rice HI in the black soil region of Northeast China, a field experiment was conducted in 2019 (Y1) and 2020 (Y2). Two irrigation methods, conventional flooding irrigation (FI) and controlled irrigation (CI), were established in the experiment, and four nitrogen application levels (0 kg/ha, 85 kg/ha, 110 kg/ha, and 135 kg/ha) were set during the entire growth period, named N0, N1, N2, and N3. The dry matter weight and the rice yield at the maturity stage were determined, and the HI was then calculated. The results showed that different irrigation modes and nitrogen application levels had significant effects on the rice HI. Under different irrigation modes with the same nitrogen application level during the two years, the comparison regular of HI was consistent. In Y1 and Y2, the HI of FN0 was 3.36% and 5.02% higher than that of CN0 (p < 0.05), and the HI of CN1 was 0.31% and 2.43% higher than that of FN1 (p > 0.05). The HI under CI was significantly higher than that under FI under N2 and N3 (p < 0.05), the HI of CN2 was 4.21% and 4.97% higher than that of FN2, and the HI of FN3 was 13.12% and 20.34% higher than that of CN3. In addition, during the two-year experiment, the HI first increased and then decreased with an increase in the nitrogen application rate under FI and CI. Under the FI treatments, the HI of N1 was the highest, and that of N2 was the highest under the CI treatments. A variance analysis showed that the irrigation pattern and nitrogen application level had significant interactions on the rice HI (p < 0.01), and the appropriate water and N management mode could increase rice the HI by 26.89%. The experimental results showed that the HI of the 110 kg/ha nitrogen application rate under CI was the highest, reaching 0.574 and 0.572, respectively, in two years. This study provides a data reference and theoretical support for realizing water savings, nitrogen reduction, and sustainable agricultural development in the black soil region of Northeast China.

Appropriate irrigation schedules could minimize the existing imbalance between agricultural water supply and crop water requirements (ET_c), which is severely impacted by climate change. In this study, different hydrological years (a wet year, normal year, dry year, and an extremely dry year) in Heilongjiang Province were calculated by hydrological frequency methods. Then, the single crop coefficient method was used to calculate the maize ET_c, based on the daily meteorological data of 26 meteorological stations in Heilongjiang Province from 1960 to 2020. Afterward, the CROPWAT model was used to calculate the effective precipitation (P_e) and irrigation water requirement (Ir), and formulate the irrigation schedules of maize in Heilongjiang Province under different hydrological years. The results showed that ET_c and Ir decreased first and then increased from west to east. The P_e and crop water surplus deficit index increased first and then decreased from west to east in Heilongjiang Province. Meanwhile, the average values of the Ir in were 171.14 mm, 232.79 mm, 279.08 mm, and 334.47 mm in the wet year, normal year, dry year, and extremely dry year, respectively. Heilongjiang Province was divided into four irrigation zones according to the Ir of different hydrological years. Last, the irrigation quotas for the wet year, normal year, dry year, and extremely dry year were 0~180 mm, 20~240 mm, 60~300 mm, and 80~430 mm, respectively. This study provides reliable support for maize irrigation practices in Heilongjiang Province, China.

Straw incorporation has a variety of impacts on greenhouse gas (GHG) emissions. However, few studies have focused on the effects of multi-year straw incorporation. In this study, a field experiment was established to study the effects of straw incorporation and water-saving irrigation on GHG emissions in the cold region of Northeast China. The following four treatments were included: (i) controlled irrigation (CI) with 1-year straw incorporation (C1), (ii) controlled irrigation with 5-year straw incorporation (C5), (iii) flooded irrigation (FI) with 1-year straw incorporation (F1), and (iv) flooded irrigation with 5-year straw incorporation (F5). The fluxes of N₂O, CO₂, and CH₄ were measured by the static chamber–gas chromatography method, and their global warming potential (GWP) and greenhouse gas intensity (GHGI) in units of CO₂-equivalent at the 100-year scale were calculated. The results showed that the 5-year straw incorporation reduced N₂O emissions but increased CH₄ emissions. Compared with C1 and F1, C5 and F5 reduced N₂O emissions by 73.1% and 44.9%, respectively, while increasing the CH₄ emissions by 101.7 and 195.8%, respectively. Under different irrigation regimes, CI reduced CH₄ emissions by 50.4–79.7% while increasing CO₂ emissions by 8.2–44.9% compared with FI. The contribution of N₂O and CO₂ emissions were relatively high at the mature and milk stages, respectively, with a range of 16–54% and 41–52% for the treatments. In contrast, CH₄ emissions were mainly manifested at the tillering stage, with a contribution of 36–58% for the treatments. Affected by higher CH₄ emissions in FI, the GWP of CI was 1.4–47.6% lower than FI. In addition, the yield of CI was 10.0–11.5% higher than FI, which resulted in a GHGI of 11.5–52.4% lower than FI, with C5 being the lowest. The irrigation regime of CI combined with 5-year straw incorporation was an effective agronomic measure to increase yield and reduce GHG emissions from paddy fields in the cold region of Northeast China.

For agricultural production, improving the rice harvest index (HI) through agricultural management practices is a major means to enhance water and N utilization efficiency and yield. Both irrigation regimes and nitrogen (N) rates are important aspects of agricultural management practices. However, it is unclear how the rice HI is affected by water and N. This study aimed to clarify the mechanism underlying the response of the rice HI to water and N, and to explore the most suitable water-saving and N reduction management practices to ensure yield. A two-year (2021~2022) field experiment was conducted on Mollisols in Northeast China. In this experiment, nine treatments were performed, involving three irrigation regimes (flooded irrigation, controlled irrigation, and “thin-shallow-wet-dry” irrigation) and three N rates (110, 99, and 88 kg/ha). The rice agronomic traits and transfer of photoassimilates under different water and N management practices were observed and studied; rice HI, WUE, and the NUE of rice was calculated and analyzed. The highest HI was achieved under controlled irrigation with a 99 kg/ha N rate, at values of 0.622 and 0.621 in 2021 and 2022, respectively. Controlled irrigation (CI) with an appropriate reduction in the N rate increased the proportion of productive tillers, the transfer rate of dry matter and non-structural carbohydrates (NSCs), the sugar–spikelet ratio, the grain–leaf ratio, and the leaf area index (LAI) during the heading–flowering stage. A subsequent analysis indicated that the main reason for the increase in the HI was the increase in the sugar–spikelet ratio during the heading–flowering stage. A high HI increased the rice yield by increasing the thousand-grain weight. The present study suggested that water-saving irrigation regimes and appropriate N reduction not only led to water and fertilizer resource savings but also improved agronomic characteristics during rice growth and enhanced transport capacity. Thus, these practices improved the rice HI and have enormous potential for increasing yield. Therefore, regulating the rice HI through water and N management methods should be considered an important strategy for improving rice yield.