Anthropogenically-enhanced nutrient availability is often cited among the most important drivers of altered ecosystem function and loss of services worldwide. Although the above-ground consequences of nutrient enrichment on plant growth patterns are numerous and well-documented, below-ground impacts are less clear but nonetheless critical from a global change perspective. In coastal wetlands, for example, plant-soil-nutrient dynamics directly affect the capacity to sequester carbon as soil organic matter, keep pace with sea level rise, and resist storm-induced erosion. Here, we investigate the effects of excess nutrient loading on below-ground plant growth in an oligohaline marsh fertilized for seven years with a factorial combination of nitrogen (N) and phosphorus (P). We used two common assessment procedures, the ingrowth and standing crop methods, to simultaneously quantify distinct aspects of below-ground plant growth: 1) below-ground biomass accumulation into unexploited open resource space, and 2) in situ, or maintenance, below-ground biomass of plants in equilibrium with their environment, respectively. Our objective was to determine if plant growth responses to nutrient enrichment differed depending on process and/or biomass component measured. We show that excess N concurrently increased live root biomass accumulation in ingrowth cores and reduced in situ live root standing crop. Similar, albeit non-significant, response trajectories were apparent for other below-ground biomass pools using both methods, excepting dead biomass and total standing crop. A review of previously published research supports our results and suggests that nutrient enrichment consistently has contrasting effects on below-ground plant growth depending on whether biomass accumulation or standing crop is measured, and that living biomass components are most responsive to enhanced nutrient availability. Synthesis. We conclude that eutrophic conditions can be both beneficial and detrimental to ecosystem function by either stimulating below-ground biomass accumulation in unexploited soil or reducing the below-ground standing crop required to sustain the nutritional needs of established plants in mature communities. Thus, nutrient enrichment may, in the short-term, contribute to soil organic matter (i.e., carbon) accumulation by increasing below-ground growth as plants exploit new resource space. Over the long-term, however, nutrient enrichment has the potential to negatively impact soil organic matter content as plants equilibrate to excess nutrient availability by down-regulating below-ground standing crop.