Academic journal article The American Midland Naturalist

Temporal and Spatial Patterns in Root Nitrogen Concentration and Root Decomposition in Relation to Prescribed Fire

Academic journal article The American Midland Naturalist

Temporal and Spatial Patterns in Root Nitrogen Concentration and Root Decomposition in Relation to Prescribed Fire

Article excerpt

ABSTRACT.-This study examined the temporal and landscape-scale patterns of root nitrogen concentration [N], root decomposition and N release from decomposing roots in oak-hickory forest ecosystems in southern Ohio. Sampling was conducted in three watershed-scale treatment units with different prescribed burning regimes, and each of these treatment units was divided into xeric, intermediate and mesic landscape positions. Root [N] decreased through the growing season in live roots but increased in dead roots. Root [N] was significantly lower in the xeric landscape positions, but only during the driest parts of the growing season. There was no consistent effect of prescribed burning on live or dead root [N]. Where differences among landscape position were detected, live root [N] was greatest in the most mesic landscape positions whereas dead root [N] was greatest in relatively xeric landscape positions. Overall, an average of 70% of original mass was lost from root litterbags and 80% of total N was released over one year of decomposition, with no significant differences among burning treatments in the instantaneous decay rate (k) or rate of N release. There were no differences among landscape positions in root decay rate or N release, even though prior studies have demonstrated strong landscape-position effects on leaf litter decomposition in similar sites. Overall, muss loss and N release from root litter is rapid in these ecosystems and relatively unaffected by either landscape position or fire frequency.

INTRODUCTION

Root dynamics are an integral part of overall forest ecosystem dynamics. In temperate forest ecosystems, biomass is approximately equally distributed above- and belowground (McClaugherty et al., 1984; Jackson et al., 1997). Similarly, approximately equal amounts of belowground detritus (root litter) are produced as aboveground detritus (loaf litter) (Hendrick and Pregitzer, 1992, 1993). However, root litter typically contains higher nutrient concentrations (particularly N) because resorption of nutrients before abscission is less in roots than leaves (Nambiar, 1987; Gordon and Jackson, 2000). As a result, root litter is also more rapidly decomposed and mineralized (Ostertag and Hobbie, 1999) and belowground turnover and overall throughputs of nutrients and C can exceed aboveground fluxes.

Root decomposition typically follows a two-stage pattern (McClaugherty et al., 1982,1984). First is an initial stage of rapid mass loss from labile C substrates, such as nonstructural carbohydrates, that has been attributed to simple leaching processes (McClaugherty et al., 1982, 1984). This is followed by a second stage of slower mass loss from recalcitrant C substrates such as suberin. Microbial activity is more important for the decomposition of remaining recalcitrant substrates after the initial loss of more labile substrates through leaching.

Broad-scale patterns of rates of decomposition among ecosystems are most commonly attributed to variations in macroclimate and litter quality (Meetenmeyer, 1978). Macroclimate effects on litter decomposition can typically be resolved best across broad geographic ranges with relatively large fluctuations in temperature and moisture. At smaller spatial scales, litter quality exerts greater control over litter decomposition (Boerner, 1984). For root decomposition, nitrogen concentration [N] is the best predictor of overall litter quality and subsequent decomposition (Hendricks et al., 2000). This creates potentially strong feedbacks between root [N] and overall ecosystem N budgets and N availability. Hendricks et al. (2000) have shown a significant positive relationship between NO^sub 3^^sup -^ availability and root [N], an important component of substrate quality (McClaugherty et al., 1984). This creates a positive feedback in which increased turnover of roots with high [N] increases N returns to the soil, further increasing N availability (Hendricks et al. …

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