4.1. The Influence of Interannual Factors on Soybean Growth and Yield
The influence of soybean flooding on plant height, the number of solid pods, 100-grain weight, yield, and dry matter mass has clear interannual differences, which can exclude the direct factors linked to flooding, such as flooding duration and growth stage. This study was conducted in the warm-temperate semi-humid monsoon climate zone of the Huaibei Plain, and the annual flooding test strictly controlled changes in the direct flooding factors. For different years, under uniform soil and crop variety conditions, the interannual changes in meteorological factors such as temperature and light should be the main indirect factors influencing the interannual variation in soybean growth and yield. For this reason, the number of high-temperature days (≥35 °C), accumulated temperature, and cumulative hours of sunlight were counted for the entire period of crop growth. Among them, the annual total number of high-temperature days for the whole growth period was 2014 < 2011 < 2015 < 2017 = 2016 = 2012 < 2018. Although 2018 had the greatest number of high-temperature days, and its average maximum temperature was 36 °C, the average maximum temperatures of 2012 and 2016 rose to 36.6 °C and 36.3 °C, respectively. This is consistent with the smallest soybean plant height, number of solid pods, yield, and dry matter mass in 2016; the lower number of solid pods, yield, and dry matter mass in 2012; and the largest soybean plant height, number of solid pods, yield, and dry matter mass in 2011, when the number of high-temperature days and the average temperatures were lower.
The cumulative temperatures across the whole growth period were 2011 < 2015 < 2014 < 2016 < 2017 < 2012 < 2018, and the cumulative sunlight hours across the whole growth period were 2014 < 2011 < 2015 < 2012 < 2017 < 2018 < 2016. The interannual order of accumulated temperature and hours of sunlight was opposite to the interannual variation in soybean plant height, number of solid pods, 100-grain weight, yield, and dry matter weight, where the cumulative hours of sunlight and 100-grain weight were significantly negatively correlated. Therefore, high temperature may be an indirect factor that exacerbates soybean flood stress.
Studies on other crops have also shown that high temperatures significantly affect crop growth [
11]. When cotton was under the stresses of high temperature, this caused a significant reduction in seed cotton yield in a trial where cotton was flooded in a concrete-bottomed measuring cylinder in the middle and lower reaches of the Yangtze River [
12]. Furthermore, a barrel experiment used to set different levels of flooding and high temperatures at the cotton seedling stage found that these two factors aggravated declines in cotton root activity [
14]. Previous studies have predominantly focused on crops such as cotton or rice, and few studies have focused on the influence of high temperatures and other meteorological environmental factors on soybean growth and yield. This study combined years of experiments analyzing high-temperature data and found that the flooding effect of soybeans is influenced not only by direct factors, such as flooding duration and growth stage, but also by indirect factors such as meteorological conditions changing with interannual flooding. In particular, the superposition of high-temperature factors could change soybean flooding response patterns.
4.2. Influence of Flooding Duration on Soybean Growth and Yield
The impact of flooding duration on the number of soybean pods, 100-grain weight, yield, and dry matter mass was significant, but was not so on plant height. Considering the interaction between the year and growth stage, the variation in soybean indicators with flooding duration presented three trends. The first trend occurred at the branch stage in 2011 and the flowering-podding stage in 2017 and 2018, where all soybean indicators excluding plant height decreased with increased days of flooding, and the decrease was more pronounced for longer days of flooding. Judging from the control treatments, which were not flooded throughout the entire growth period, all indicators were relatively large in these three years, indicating that soybean growth was less restricted by other indirect factors, such as drought or high temperature. Solely under the condition of flooding stress, the number of solid soybean pods and the 100-grain weight decreased as the duration of flooding increased, resulting in a decrease in yield [
7,
15]. Considering that all indicators of soybeans in the control treatment decreased in the remaining years, the two changing trends in those years were likely the result of the combined effects of indirect factors, such as drought or high temperatures [
16].
The second variation trend occurred at the seedling stage in 2012, the branching and flowering-podding stages in 2014, and the flowering-podding stage in 2015–2016. Excluding plant height, all soybean indicators increased when flooded for 3 or 6 days, and only decreased significantly when flooding was prolonged. The reason may be that short-term flooding alleviated the impact of indirect factors such as drought or high temperatures on soybeans, which was similar to the compensatory effect of cotton flooding on early drought stress [
13]. For instance, in 2016, when the average high temperature and quantity of sunlight were relatively large, all soybean indicators in the non-flooded control treatment were relatively small, indicating that soybeans were likely to be in a state of drought or high-temperature stress. At this time, the flooding had mitigated the impact of this stress, and the crop itself had some tolerance to flooding. Therefore, short-term flooding improves the number of solid pods, 100-grain weight, yield, and dry matter mass of soybeans. However, when flooding is prolonged, soybean organ growth is restricted, which leads to a significant decrease in dry matter accumulation and yield. At this time, high temperatures or drought stress will further aggravate the flooding stress.
The third trend occurred at the branch stage in 2012, when all soybean indicators excluding plant height decreased at the third day of flooding, increased at the sixth day of flooding, and decreased again at the ninth day of flooding. Because the number of solid pods and yield of the control in 2012 were significantly lower than those of other years, it can be speculated that soybeans may have been under the stress of indirect factors, such as drought or high temperature. The yield-reducing effect of soybean flooding on day three may be stronger than the yield-increasing effect of drought or high-temperature mitigation, resulting in a decrease in soybean yield. When flooding continued for 6 days, the yield-reducing effect of flooding may have been weaker than the yield-increasing effect of drought or high-temperature mitigation, resulting in a recovery in soybean yield. After flooding for more than 6 days, the number of solid pods, 100-grain weight, yield, and dry matter mass of soybeans were significantly reduced, due to prolonged flooding.
In summary, when flooded for ≤3 days, the number of solid pods, yield, and dry matter of soybeans were similar to those that were not flooded. When flooded for ≤6 days, the 100-grain weight of soybeans was almost unaffected by flooding. Therefore, the critical flooding duration for the soybean pod number, yield, and dry matter mass is 3 days, and the critical flooding duration for 100-grain weight is 6 days. Flooding within the critical duration may alleviate the drought or high-temperature stress of soybeans, and the impact of soybean flooding beyond the critical duration is likely to be amplified by indirect factors such as high temperature or drought.
4.3. Effects of Flooded Growth Stage on Soybean Growth and Yield
The changes in the flooded growth stage had significant effects on soybean plant height, the number of solid pods, yield, and dry matter mass. In contrast, while 100-grain weight was not sensitive to the influence of the flooded growth stage, it decreased significantly when the soybean plant was flooded for more than 6 days at the flowering-podding stage. Flooding at the seedling stage had the greatest impact on soybean plant height and dry matter mass; flooding at the flowering-podding stage had the greatest impact on the number of solid pods, 100-grain weight, and yield; and flooding at the branch stage had the lowest impact on various soybean indicators. Flooding at the seedling stage, when soybeans are in the stage of vegetative growth, predominantly affects soybean plant height, which then leads to a decrease in soybean dry matter [
17]. Once the flooding stress is relieved, the roots and leaves continue to grow in the seedling stage and have enough time to recover, so flooding at the seedling stage has little long-term effect on flowering, pod production, and yield [
18]. Flooding stress during the branch stage may promote the vegetative growth of soybean straw, so it has little effect on the physiological indicators and yield of soybeans. In contrast, the flowering-podding stage is the most nutrient-demanding stage in soybean growth, and waterlogging through prolonged flooding will cause the soybean root system to gradually decline. With prolonged flooding, the soybean root system function decreases, and the flower pods fall off, eventually leading to a significant decrease in soybean yield [
19,
20]. Therefore, the flood-sensitive growth stage for soybean yield, the number of solid pods, and 100-grain weight is the flowering-podding stage.