灌溉对沙拐枣幼苗生长及氮素利用的影响

采用盆栽试验,比较了5个灌溉梯度下(4.6、6.1、7.7、9.2、13.0 kg·株-1·次-1)沙拐枣幼苗氮素累积分配、利用和回收特征及其生长特性差异。结果表明:随灌溉量增加,沙拐枣幼苗整株氮素累积量和干物质量均显著增加,但在最高灌溉量下沙拐枣幼苗出现严重病害。生长旺季干物质和氮素主要分配在同化枝中,平均分别占总株的39.5%和66.1%,随灌溉量增加分配比例显著增加;季末茎和老枝是干物质和氮素的主要累积器官,平均分别占总株的54.7%和47.8%,分配比例也随灌溉量增加而显著增加。干旱条件下沙拐枣幼苗具有较高的根冠比,增加灌溉量后显著下降。生长旺季沙拐枣幼苗具有较高氮素回收效率,平均为64.4%,灌溉后明显增加;季末平均为58.1%,灌溉后有下降趋势。在两个生长季平均氮素利用效率分别为120.5和235.8g/g,增加灌溉量虽可提高植物氮素利用效率,但在最高灌溉量下氮素利用效率出现降低。由此可见,沙拐枣幼苗物质分配特征具有明显的季节性和可塑性,灌溉量过高和过低都不利于沙拐枣幼苗生长及氮素回收和利用效率的提高,因此中等灌溉量(7.7—9.2 kg·株-1·次-1)更有利于其生长及自身特性发挥。

Effects of irrigation on plant growth and nitrogen use characteristics of Calligonum caput-medusae Schrenk seedlings

Water and nitrogen (N) are two primary factors controlling plant growth in desert ecosystems. Most studies have focused on water stress resulting from the low rainfall and high evaporation rates in arid areas. However, irrigation has become the main strategy for vegetation recovery in the southern rim of Tarim Basin. Many studies have shown that irrigation is most effective when nutrients are not limited, and fertilization is most effective when plants are not water-stressed. In addition, N not only affects drought tolerance through changing dry matter partitioning, but also plays an important role in ecosystem functioning and vegetation succession. Therefore, the combined effects of water and N on vegetation recovery and reconstruction in this area should be evaluated. We conducted a pot experiment to study characteristics of N allocation, use, and resorption, and growth of Calligonum caput-medusae Schrenk seedlings under different irrigation treatments (4.6, 6.1, 7.7, 9.2, 13.0 kg/plant per irrigation event). The results showed that the amounts of both N and dry matter per whole plant significantly increased with increasing amounts of irrigation. However, C. caput-medusae Schrenk seedlings were infected with powdery mildew at the high irrigation level (13.0 kg/plant). During the early growth stage, irrigation promoted dry matter accumulation in and N allocation to, assimilating branches. On average, these assimilating branches accounted for 39.5% of whole-plant dry matter accumulation and 66.1% of whole-plant N-allocation. During the late growth stage, stems and older branches became the main organs for dry matter and N accumulation, on average accounting for 54.7% and 47.8% of whole-plant dry matter accumulation and N accumulation, respectively. The dry matter accumulation in and N allocation to stems and older branches was positively affected by irrigation at the end of the growing season. The plants allocated more dry matter and N into assimilating branches in their early growth stage to obtain more photosynthates, and to stems and older branches at the late growth stage to accumulate more energy for plant growth the following year. The root/shoot ratio of C. caput-medusae Schrenk seedlings was markedly higher under dry conditions than under irrigated conditions. The mean value of N resorption efficiency (NRE) at the early and late growth stages was 64.4% and 58.1%, respectively. The NRE was positively affected by irrigation at the early growth stage, but negatively affected by irrigation at the late growth stage (at the end of the growing season). There was a clear seasonal variation in N use efficiency (NUE), with a low mean value (120.5 g/g) at the early growth stage and a high mean value (235.8 g/g) at the late growth stage. Irrigation significantly enhanced the NUE of assimilating branches, stems, and older branches, and roots at the late growth stage. Although the whole-plant NUE increased significantly under higher irrigation levels, excessive irrigation did not increase the NUE. These findings suggested that both N and biomass could be distributed to the most appropriate organ of C. caput-medusae Schrenk at different growth stages to adapt to the arid and barren natural environment. Since plant growth and N use could be limited by over-irrigation and water stress, medium irrigation levels (7.7-9.2 kg/plant) were appropriate for establishing C. caput-medusae Schrenk seedlings in this area.