Halo‐hydromorphism alters nitrogen fertilization responses of tall wheatgrass pastures: Capture and use of resources, tiller dynamics and forage production

Abstract

Halo-hydromorphismlimits productivity in approximately 100 million hectaresworldwide. Tall wheatgrass (Thinopyrum ponticum) is a species widely used in theseenvironments for its seeding potential, being the addition of nitrogen a consideredtechnological tool to increase forage quality and production. The objective of thestudy was to determine the impact of nitrogen fertilization on the capture and useof resources (radiation, water and nitrogen) in a cool season perennial sward growingin contrasting halo-hydromorphicconditions. Cultivated pastures from threeindependent sites were used. Sites were described according to the degree of halo-hydromorphismusing soil salinity and water table attributes (salinity and depth) as environmentalindicators: low HHM site [electrical conductivity (EC1:2.5) 0.97 dS/m; watertable salinity 2.03 dS/m; depth 85 cm], intermediate HHM site (EC1:2.5 3.86 dS/m; watertable salinity 7.40 dS/m; depth 134 cm) and high HHM site (EC1:2.5 4.49 dS/m; watertable salinity 7.85 dS/m; depth 31 cm). At each site, a late spring regrowth (~750°Cd)was studied by applying two treatments (n = 5): without (N0) and nitrogen fertilization(N150; 150 kg/ha of nitrogen in the form of urea). The response of tall wheatgrass tonitrogen fertilization in halo-hydromorphicconditions depends on soil salinity andwater table attributes. N150 treatments production was twice as high as in N0 in lowHHM and intermediate HHM environments (from 1750 to 3500 kgDM/ha and from1080 to 1985 kgDM/ha, respectively). Meanwhile, in high HHM conditions, forageproduction was only 40% higher when nitrogen was added (from 625 to 870 kgDM/ha). In low HHM the higher N150 production was related to tiller density and size,whereas in intermediate HHM and high HHM was linked only to tiller size. In N150treatments, the nitrogen nutrition index was negatively affected with the increase inHHM conditions (0.77, 0.62 and 0.55 for low HHM, intermediate HHM and high HHM,respectively). Instead, nitrogen nutrition index of N0 was similar in all the environments(~0.42). In N150, forage production capacity analysed in terms of radiationand water use efficiency (RUE and WUE, respectively) was similar in low HHM and intermediateHHM environments (RUE ~0.81 gDM/Mj and WUE ~13 kgDM/mm). Thesefindings emphasize the importance of conducting analyses based on resource use and capture to understand productive responses to the increase in growth-limitingfactors.Furthermore, they contribute to the identification of environments suitable fornitrogen fertilization.