Tillage Effect on Seasonal Accumulation of Labeled Fertilizer Nitrogen in Sorghum

Abstract

Limited information is available regarding tillage effects on seasonal N accumulation in sorghum [Sorghum bicolor (L.) Moench], especially where labeled N was used. Tillage effects on fertilizer N accumulation in 1985 and 1986 were evaluated in a field study on a Weswood silt loam (Fine‐silty, mixed, thermic Fluventic Ustochrept) near College Station, TX. Nitrogen‐15 depleted NH4NO3 (150 kg N ha−1) was surface broadcast or subsurface banded under no‐tillage or conventional tillage management to monitor fertilizer N accumulation in sorghum at boot, anthesis, soft dough, and harvest. Both years, most tillage effects on fertilizer N accumulation were manifested early in the season. The tillage‐by‐placement interaction was not significant for N uptake parameters, indicating a similar N accumulation response to placement for both tillage systems. Most fertilizer N uptake occurred by anthesis (84% averaged across years). Fertilizer N accumulation in no‐tillage sorghum was generally higher throughout the 1985 season, resulting in significantly greater fertilizer N recovery by harvest. This effect occurred even though yields and total N accumulations for both tillage systems were similar. No tillage difference in whole plant fertilizer N accumulation at harvest was observed in 1986. No‐tillage delayed early development of sorghum approximately 2 wk in 1986. Fertilizer N accumulation in leaves was less with conventional tillage than no‐tillage early in the 1986 season. Leaf fertilizer N did not differ between tillage treatments after anthesis when vegetative demand declined. Conventional tillage culms contained more fertilizer N thoughout the season. Tillage effects on panicle fertilizer N accumulation coincided with periods of high N requirement, such as at soft dough, when no‐tillage was higher. The plant growth delay for no‐tillage sorghum in 1986 may have subjected the crop to additional moisture or temperature stress resulting in a tillage by fertilizer placement interaction, whereby banded treatments produced more grain than broadcast in no‐tillage plots. Yields for banded no‐tillage sorghum did not differ from those for either placement treatment in conventional tillage plots.