Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity and turnover for the perennial bunchgrass Bouteloua gracilis in the shortgrass steppe of eastern Colorado, USA. We hypothesized that root longevity would be partially controlled by root diameter, following previously observed patterns in woody plants. In addition, we hypothesized that root turnover would be greatest in surface soil horizons and decrease with depth due to variation in soil moisture availability and temperature. Root longevity was correlated with root diameter. Median life span of roots > 0.4 mm was approximately 320 days, while roots < 0.2 mm had a median life span of 180 days. There was approximately a 6% decrease in the likelihood of mortality with a 0.10-mm increase in root diameter, controlling for the effect of depth in the soil profile. Root length production and mortality were highest in the upper 20 cm of the soil profile and decreased with depth. However, because root length density also decreased with depth, there were no significant differences in turnover rate of root length among sampling intervals. Turnover was approximately 0.86 yr(-1) based on root length production, while turnover was 0.35 yr(-1) using root length mortality as a measurement of flux. The imbalance between turnover estimates may be a consequence of the time the minirhizotrons were in place prior to imaging or may result from our lack of over-winter measures of mortality. Our work suggests that Bouteloua gracilis roots have complex life history strategies, similar to woody species. Some portion of the root system is highly ephemeral, while slightly larger roots persist much longer. These differences have implications for belowground carbon and nitrogen cycles in the shortgrass steppe.