LIVE AND DEAD ROOTS IN FOREST SOIL HORIZONS:CONTRASTING EFFECTS ON NITROGEN DYNAMICS

Roots are widely acknowledged to be major contributors to ecosystem nutrient cycles. However, live roots may have very different effects than dead roots, the quality of the soil surrounding the roots may itself affect the influence of the roots, and the nature of the soil profile may modify all of these interactions. We have used experimental, field‐based mesocosms to dissect the differing effects of live and dead roots in organic and mineral horizons of spodosolic forest soils. Mesocosms were constructed in a simple ecosystem dominated by pitch pine (Pinus rigida Mill.) and blueberry (Vaccinium corymbosum L.) in the New Jersey Pinelands. They contained 5 cm deep surface horizons of either organic matter or an inert substitute (vermiculite), placed above 10 cm of mineral material from the E horizon, with defined amounts of live or dead roots added. Sampling was conducted over a 2‐yr period. In mineral soil, live roots stimulated ammonification rates but had little effect on extractable N, and the stimulation occurred only if an organic surface horizon was present. In contrast, live roots in organic material reduced both ammonification rates and extractable N. Dead roots increased extractable N in the mineral material, but not the organic material. The presence or absence of an organic horizon had a dominating effect on all aspects of nitrogen dynamics in the mineral soil; not only extractable inorganic N, but also transformation rates, were higher in soils under organic matter. The results showed that the effects of roots on soil nutrients depended on (1) the ratio of live to dead root biomass, (2) the quality of the soil material in which the roots occurred, and (3) the structure of the soil profile. These findings necessitate a revision of the concept of “rhizosphere effect” in forest soils.