Reply to Gupta and Igamberdiev: Mechanisms and feedbacks in N fixation and NO production

Abstract

Igamberdiev (1) present a cogent argument for the importance of examining mechanisms when studying biogeochemically significant phenomena, and we appreciate their kind words regarding our efforts (2). We wish to emphasize several points that are relevant to their position. First, we argue that the key feature of kudzu that allows it to have such a dramatic impact on nitrogen cycling and nitric oxide emission is its capacity to form symbiotic relationships with bacteria capable of converting N 2 to NH 3 (nitrogen fixation). In contrast to Gupta and Igamberdiev's penultimate sentence, it is not kudzu's invasive tendencies per se that lead it to be involved with increases in nitrogen fluxes, it is the combination of these tendencies with its capacity for symbiotic nitrogen fixation that leads to the increases. That is, we predict that an invasive plant that does not dramatically alter rates of nitrogen inputs will not alter nitric oxide fluxes.Second, our data fit well within the conceptual model developed originally by Firestone and Davidson (3,4), which is often referred to as the "hole-in-the-pipe model." In this model, the rate of NO flux from soils depends on both the "flow through the pipe"-the rates of nitrification and denitrification-and the size of the "holes in the pipe," which are determined by environmental conditions such as soil moisture and temperature. The more N "flowing through the pipe," the higher NO fluxes will be for a given set of environmental conditions. As an N-fixer, kudzu increases the amount of N flowing through the "pipe" and thus the amount of NO lost from the "holes."Finally, there is a mechanistic aspect to the question of soil NO that has not received sufficient attention in the literature and that may be relevant to our results. Increases in soil air space NO concentrations are correlated with increases in nitrogen uptake by roots (5). This increase in N uptake may spur further growth, biomass accumulation, and subsequent litterfall and decomposition. These changes may be part of a positive feedback in which increased NO production leads to increased N uptake and cycling between plants and soils, with increased NO fluxes an outcome. There is also the possibility that the NOinduced stimulation of N uptake may decrease rates of N fixation by the plant, reducing the magnitude or even changing the direction of the feedback. This uncertainty highlights the need for mechanistic research as a complement to phenomenological efforts.