Nitrogen Fertilization and Clipping Effects on Green Needlegrass (Stipa viridula Trin.): II. Evapotranspiration, Water‐Use Efficiency, and Nitrogen Recovery1
Soil water and N are the major factors limiting forage production of green needlegrass (Stipa viridula Trin.) on the northern Great Plains. We studied the effects of N fertilization rate and clipping frequency on evapotranspiration (ET), water extraction pattern in soil profiles, water‐use efficiency, and fertilizer N recovery by dryland green needlegrass. Nitrogen was broadcast at 0, 70, and 140 kg of N/ha in November 1968. Green needlegrass was either left unclipped or clipped to a 5‐cm height five times during 1969 at approximately 21‐day intervals.Green needlegrass showed visual signs of water stress when water was deficient in the surface 30 cm of soil. On undipped plots, 80% of the ET was from the 0‐ to 30‐cm depth. Water‐use efficiency of undipped green needlegrass increased during May and June and decreased during July, but that of clipped plants decreased from May through July. Nitrogen fertilization increased and clipping decreased water‐use efficiency.Average N recoveries with 70 and 140‐kg rates were 22% for the first year and 8% for the second year. Recovery was highest with 70 kg of N/ha and frequent clipping. Low recovery of N fertilizer was not due to loss of nitrate‐N by leaching. At the end of the first crop season, 7% of the 70 kg N/ha applied and 17% of the 140 kg of N/ha applied the previous year remained in the soil as nitrate‐N.