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Rice–weed competition in response to nitrogen form under high and low transpirational demand
Journal of Agronomy and Crop Science  (IF3.473),  Pub Date : 2021-10-19, DOI: 10.1111/jac.12562
Duy Hoang Vu, Sabine Stuerz, Folkard Asch

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 Snigrum 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 Snigrum. NU838 showed a higher competitiveness against weeds than KD18. In competition, high NO3 decreased the competitiveness of Ecrus-galli against NU838 but increased the competitiveness of Snigrum against NU838. High VPD did not affect DM but increased N uptake of Snigrum, 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.