The uptake of mineral nutrients from the soil by plants is greatly aided by mutualistic associations with mycorrhizal fungi, which grow into and extend out of the plant roots. Of these symbioses the arbuscular mycorrhizal fungi is the oldest, most anatomically intimate and ecologically widespread (Govindarajulu et al. 2005). As well as benefiting plants by aiding phosphorus uptake from the soil, AMF can take up and transfer significant amounts of nitrogen to their host plants (Huang et al. 2020; He et al. 2003). We determined that the colonization level of AMF in GJDN1 was significantly higher than that of ZZ35 in the field, which was consistent with the results of Huang (2020). In addition, our research indicates that nitrogen use efficiency of GJDN1 were higher than in ZZ35 (Table 7). The same result was obtained in field experiments at Pingxiang. This finding strongly supports the positive effect AMF have on nitrogen uptake and utilization of rice. Numerous studies have shown that the extracellular mycorrhizal mycelia of AMF can take the place of root hairs in absorbing ammonia, nitrate, and some simple amino acids from the soil, transferring these nitrogen sources to the host plant and effectively improving the nitrogen utilization rate (Leigh et al. 2009; Tanaka and Yano 2005). Meanwhile, a considerable amount of nitrogen remained in the mycorrhizal mycelia, forming a ‘nitrogen bank’ to provide nitrogen to host plants in exchange for carbon (Messenguy et al. 1980). However, how the "nitrogen pool" of mycelium exchanges carbon sources, whether there is a fixed threshold is not clear yet, and further research is needed.
Promotion by natural variation of OsCERK1 to nitrogen uptake and utilization in rice is negatively correlated with soil nitrogen status
The effect of AMF on plant N uptake is related to the availability of nutrients in the environment. Compared with high nutrient conditions, plant growth responds more strongly to AMF under low nutrient conditions (Corrêa et al. 2015; Geneva et al. 2010; Johnson 2010; Ma et al. 2021; Bakhshandeh et al. 2017; Lu et al. 2020). Compared with ZZ35, the enhanced NAE, NRE, and NPFP of GJDN1 became less pronounced as N supply increased from 50–90%. These results indicated that AMF contribute significantly to nitrogen utilization in rice under low nitrogen conditions, which in turn suggests that the promoting effect of AMF on plant nutrient uptake is affected by soil nutrient status. Under nutrient deficient conditions, plant growth is more limited by nutrient absorption. To obtain more nutrients, plants can rely more heavily on their symbiosis with AMF (Bonneau et al. 2013). However, with sufficient soil nutrients, plants can absorb enough simply through their roots, reducing their dependence on AMF (Schalamuk et al. 2011). At high enough nitrogen applications, the total nitrogen uptake of rice plants increased although the efficiency of nitrogen uptake and utilization decreased (Zhu et al. 2017). However, some experimental results are inconsistent with the above conclusions. Compared with 50–90% nitrogen applications, AMF had a stronger contribution to nitrogen utilization and increased its nitrogen utilization rate at 100% nitrogen application level, which may be affected by external environmental conditions such as water and light and needs further study. In the field experiment (Pingxiang), because the field environment was difficult to be controlled artificially, the promotion effect of AMF on nitrogen absorption and utilization of rice was not significant negatively regulated by soil nitrogen nutrition.
Promotion by natural variation of OsCERK1 to nitrogen uptake and utilization in rice is negatively regulated by total nitrogen accumulation
The NRE of GJDN1 was higher than that of ZZ35 before the heading stage. However, compared with ZZ35, the NRE of GJDN1 first increased and then decreased as N application levels increased, reaching the highest value of 27.60% at 75% N application. These results indicate that AMF symbiosis promotes the absorption and utilization of nitrogen in rice before heading, and the effect of AMF on nitrogen accumulation was better in low N conditions than in high N conditions. From heading stage to mature stage, the NUPE of GJDN1 was 32.52–147.96% higher than that of ZZ35 under low N conditions (0–50%), but it was 8.53–20.16% lower than that of ZZ35 under high N conditions (75–100%). These results showed that the promoting effect of AMF symbiosis on nitrogen uptake and utilization in rice was affected by total nitrogen accumulation. When rice reached a certain level of nitrogen accumulation (about 3.8 g N plant− 1 in this study), the promoting effect of AMF on nitrogen absorption weakened or was lost completely.
Nitrogen fertilizer management has different effects on NUE under different soil fertility levels, and the appropriate nitrogen ratio can improve NUE in rice (Peng et al. 2021; Li et al. 2017b). Nitrogen accumulation and NUE of rice can be improved by properly increasing the ratio of basal fertilizer (basal fertilizer: tillering fertilizer: ear fertilizer = 4:3:3). Li (2017) found that decreasing the ratio of basal/tillering N to total N, NUE, NAE, NPE, and NPFP all increased at first but then decreased, determining the ideal ratio of basal/tillering N to panicle N was 7:3. A single application of nitrogen to the root zone saves fertilizer and is an efficient fertilization method, significantly increasing the storage time of nutrients in the soil, reducing the risk of nutrient loss, and improving NUE in rice (Liu et al. 2017a; Liu et al. 2017b). AMF could possibly be regarded as a regulatory device for nitrogen uptake in rice. When the amount of nitrogen accumulation is low, AMF can accelerate nitrogen uptake and promote vegetative growth in rice, and when the vegetative growth reaches a certain level, it can cooperate with the rice to reduce nitrogen uptake. Therefore, AMF can help appropriately reduce the amount of total nitrogen applied to rice. In addition to guaranteeing yield, AMF can improve NUE and reduce the risk of nitrogen loss.