| Nitrogen (N), particularly nitrate (NO3-), is a critical pollutant in many northeastern US watersheds that is especially detrimental to coastal marine ecosystems. Agricultural land, which receives fertilizers and/or animal manures, is a principal source of N loading to the environment. The most effective NO3- attenuation mechanism is probably microbial denitrification, i.e., the transformation of NO3- into N gases (e.g., NO, N2O, N2). Currently, our estimates of the magnitudes of denitrification rates at landscape scales are "tentative" at best, usually based on large-scale watershed budgets in which denitrification was estimated by difference. One reason that denitrification is hard to quantify is that a large amount of denitrification occurs in disproportionately small parts of the landscape (i.e., hotspots) and over relatively short periods (i.e., hot moments). Denitrification occurs primarily under anaerobic conditions by heterotrophic microbes and is expected to be vigorous in wet soils high in organic carbon. There is good evidence that these conditions correlate strongly with the likelihood of soil saturating, i.e., hydrological sensitivity, thus, by juxtaposing hydrology and biogeochemistry we can elucidate the distribution of denitrification hotspots across the landscape. We used nitrogen isotopes to quantify and characterize spatial patterns of denitrification in riparian zones and other saturated soils (i.e., the shallow saturated zone) of an agricultural landscape. In situ denitrification rates in shallow groundwater were determined monthly using the 15N-NO3- push-pull method. Annual rates of denitrification showed a strong positive relationship with topographic index, a well-known wetness index and indicator of hydrologic similarity. The resulting relationship was used to distribute denitrification rates across the landscape and estimate denitrification N fluxes from the shallow saturated zone. Denitrification in the shallow saturated zone resulted in a N flux that was nearly half of the total denitrification from the landscape---in about a third of the area---as determined from an unusually well-constrained whole-farm N balance constructed from farm records and field measures. Denitrification N flux rates from saturated riparian soils were among the highest in the landscape, however the contribution of riparian areas to total landscape denitrification was less than 10 percent. |
