Implementation of water-saving irrigation practices in lowland rice results in increased availability of nitrate (NO₃⁻) in the soil and favours germination of upland weeds. Since plant species show a specific preference for either ammonium (NH₄⁺) or NO₃⁻ as nitrogen (N) source, changes in both soil NO₃⁻ concentration and weed flora may affect the competition between rice and weeds. Further, the transpirational demand of the atmosphere might affect growth and competitiveness of lowland (wetland) and upland (dryland) weeds differently due to their adaptation to different ecological environments. Therefore, the study aimed to evaluate the effects of N source on growth, N uptake and competition between rice and common upland and lowland weeds under high and low vapour pressure deficit (VPD). Two rice (Oryza sativa) varieties (NU838 and KD18) differing in growth characteristics and two weed species (Echinochloa crus-galli and Solanum nigrum) differing in their natural habitat were selected and grown hydroponically as monoculture or mixed culture at low or high VPD. N was supplied as 75%/25% or 25%/75% NH₄⁺/NO₃⁻. N uptake rates were measured in the first week, whereas dry matter (DM), N concentration in the plant, total N uptake and the activities of nitrate reductase and glutamine synthetase in the fresh leaves were determined two weeks after the onset of treatments. Independent of N source, both rice varieties and E. crus-galli took up a larger share of NH₄⁺, whereas S. nigrum took up a larger share of NO₃⁻. N uptake of rice and E. crus-galli was hardly affected by N source, whereas high NO₃⁻ led to significantly higher N uptake rates and total N uptake of S. nigrum. NU838 showed a higher competitiveness against weeds than KD18. In competition, high NO₃⁻ decreased the competitiveness of E. crus-galli against NU838 but increased the competitiveness of S. nigrum against NU838. High VPD did not affect DM but increased N uptake of S. nigrum, leading to increased competitiveness of the weed at high transpirational demand. Competitiveness for N uptake appears to be an important trait as the relative N concentration in mixed plant communities was correlated with the activity of N-assimilating enzymes and leaf growth, with a stronger response in rice than in weeds. Our results support the hypothesis that increased availability of NO₃⁻ in aerobic rice soils may be advantageous for the competitiveness of upland weeds, especially at high VPD, whereas it may be disadvantageous for common lowland weeds.

中文翻译：

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高低蒸腾需求下水稻-杂草对氮形态的竞争

在低地水稻中实施节水灌溉措施可增加土壤中硝酸盐 (NO ₃ ⁻ ) 的有效性，并有利于旱地杂草的萌发。_{由于植物物种显示出对铵 (NH 4} ⁺ ) 或 NO ₃ ⁻作为氮 (N) 源的特定偏好，土壤 NO ₃ ^−的变化浓度和杂草菌群可能影响水稻与杂草之间的竞争。此外，由于低地（湿地）和高地（旱地）杂草适应不同的生态环境，大气的蒸腾需求可能对它们的生长和竞争力产生不同的影响。因此，本研究旨在评估氮源在高低水汽压亏缺（VPD）条件下对水稻生长、吸氮量以及水稻与普通旱地和低地杂草之间竞争的影响。两种生长特性不同的水稻 ( Oryza sativa ) 品种（ NU838和 KD18）和两种杂草（稗草和龙葵）) 不同的自然栖息地被选择并在低或高 VPD 下作为单一栽培或混合栽培进行水培。N 以 75%/25% 或 25%/75% NH ₄ ⁺ /NO ₃ ^-的形式提供。在第一周测量 N 吸收率，而干物质 (DM)、植物中的 N 浓度、总 N 吸收以及新鲜叶片中硝酸盐还原酶和谷氨酰胺合成酶的活性在处理开始两周后测定。独立于 N 源，水稻品种和E。 crus - galli占据了较大份额的NH ⁴⁺ _，而S . 黑龙葵在 NO ₃中占有较大份额^-。稻米和E的N吸收。 crus - galli几乎不受N 源的影响，而高NO ₃ ⁻导致显着更高的N 吸收率和S的总N 吸收。 黑龙葵。NU838 比 KD18 表现出更高的抗杂草竞争力。在竞争中，高NO ₃ ⁻降低了E的竞争力。 crus - galli对抗NU838 但增加了S的竞争力。 黑龙葵对抗 NU838。高 VPD 不影响 DM，但增加了S的 N 吸收。 龙葵，导致杂草在高蒸腾需求下的竞争力提高。氮吸收的竞争性似乎是一个重要的性状，因为混合植物群落中的相对氮浓度与氮同化酶的活性和叶片生长相关，水稻的反应比杂草强。我们的结果支持以下假设：好氧水稻土壤中 NO 3 的有效性增加^可能_有利于旱地杂草的竞争力，尤其是在高 VPD 时，而它可能不利于普通低地杂草。