VARIATION IN SUGAR MAPLE ROOT RESPIRATION WITH ROOT DIAMETER AND SOILDEPTH

Root respiration may account for as much as 60% of total soil respirat ion. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of terrestrial carb on budgets. Root systems are often sampled by diameter and depth class es to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the dist al end of the root system have much greater tissue N concentrations th an larger roots, and this led to the hypothesis that the smallest root s have significantly higher rates of respiration ration than larger ro ots. This study was designed to determine if root respiration is relat ed to root diameter or the location of roots in the soil profile. We e xamined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (A cer saccharum Marsh.) forests in Michigan. Root respiration declined a s root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0-10 cm depth) respired at rates up t o 40% greater than deeper roots, and respiration rates for roots < 0.5 mm in diameter were 2.4 to 3.4 times higher than those for roots in l arger diameter classes. Root N concentration explained 70% of the obse rved variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depth s appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in z ones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These result s suggest that broadly defined root classes, e.g., fine roots are equi valent to all roots < 2.0 mm in diameter, do not accurately reflect th e functional categories typically associated with fine roots. Tissue N concentration or N content (mass x concentration N) may be a better i ndicator of root function than root diameter.