Sub-soil phosphorus placement reduces the competitiveness of barnyard grass [𝘌𝘤𝘩𝘪𝘯𝘰𝘤𝘩𝘭𝘰𝘢 𝘤𝘳𝘶𝘴𝘨𝘢𝘭𝘭𝘪 (L.) Beauv.] on the early growth of rice (𝘖𝘳𝘺𝘻𝘢 𝘴𝘢𝘵𝘪𝘷𝘢 L.) irrespective of the moisture availability

Abstract

Sri Lanka has reached self-sufficiency in rice (𝘖𝘳𝘺𝘻𝘢 𝘴𝘢𝘵𝘪𝘷𝘢 L.). However, productivity of rice is low due to constraints such as high weed competition, low phosphorus (P) utilization efficiency, and water scarcity. As the abundance of grass weeds, water and P availability are closely inter-linked, two pot experiments were conducted under glasshouse conditions to investigate the impact of P placement, and different soil moisture regimes on the competitive effect of barnyard grass [BG; 𝘌𝘤𝘩𝘪𝘯𝘰𝘤𝘩𝘭𝘰𝘢 𝘤𝘳𝘶𝘴𝘨𝘢𝘭𝘭𝘪 (L.) Beauv.] on early growth of rice. In the first experiment, the impact of two P placement depths [0-5 cm (top soil) and 15-20 cm (sub-soil) from the soil surface of a pot] were tested on the early growth of two species in combination (i.e. rice-rice, rice-BG, BG-BG) and two destructive samplings were done at 14 days (appearance of the first tiller of both species; 5-leaf stage of rice and 3-leaf stage of BG), and 28 days (maximum or active tillering of both species) after germination. In the second experiment, two soil moisture levels were imposed as continuous flooding (3-cm water level above the soil surface throughout the experiment) and alternate wetting and drying (continuous flooding until tillering and then watered at 12 day intervals) apart from the P placement and plant combination. The soil used was a Reddish Brown Latasol (RBL) with a low total P (98 μg/g) and available P (1.5 μg/g) concentrations. Shoot and root dry weights, tissue P concentrations, root length and diameter, plant height and leaf counts per plant were measured. In the first experiment, sub soil P placement reduced the shoot and root dry weights (by 70-90 %), P concentrations (by 30-60 %) and the amount of P taken up (by 90 %) in BG than those in the top soil P placement. Root length and root surface area of BG were significantly higher when P was placed in the top soil layer (985 cm/plant and 71 cm²/plant, respectively) than those under sub-soil P placement (561 cm/plant and 34 cm²/plant, respectively). However, such a response was not observed in rice. Even though sub soil P placement enhanced the penetration of rice roots into sub-soil layers that of BG was reduced by 50 %. In both experiments, plant height and the number of leaves per plant of BG were reduced (p<0.05) when P was placed in the sub-soil layer irrespective of the moisture treatment and plant combination. The results indicate that subsoil P placement reduced the growth and development of BG while the growth of rice was unaffected providing a competitive advantage for rice over BG. Soil moisture treatments had no significant effect (p>0.05) on the growth and development of both the species during the early growth stages.