Routine Measurement of Dissolved Inorganic 15N in Precipitation and Streamwater

The difficulty of measuring 15N in dilute solutions has limited the potential of ecosystem labeling experiments and model comparisons. By concentrating the N from large (up to 20 L) water samples on ion-exchange resin columns, we obtained enough N for accurate and reproducible 15N measurement using the Teflon tape diffusion method. Analysis of standards demonstrated >95% recovery of inorganic N from samples, and 15N values comparable to those obtained using standard distillation methods. The value of the blank at our laboratory was 0.16 moles N. Analytical precision was within 2 15N when samples of streamwater from the Bear Brook Watersheds (1800 eq · ha-1 · yr-1 N addition at 192 15N) were kept frozen until analysis. The analytical process is lengthy, but once set up can easily be performed for large numbers of samples.     

Controls on N Retention and Exports in a Forested Watershed

We conducted a 15N-tracer study in a fertilized, forested catchment at the Bear Brook Watersheds in Maine (BBWM), USA, in order to characterize N cycling processes, identify sinks for ammonium-N additions, and determine the contribution of the experimental ammonium additions to nitrate exports from the treated catchment. Distributions of 15N in plant tissues, soils, precipitation and streamwater collected before adding tracers showed that nitrate-N (the dominant form of inorganic N deposition at the site) inputs under ambient conditions were depleted in 15N relative to plants and that soil was enriched in 15N relative to plants. The 15N content of streamwater nitrate was within the range of 15N contents in natural plant tissues, suggesting that nitrate deposited from the atmosphere is reduced and assimilated into soil and plant N pools before being leached as nitrate from the catchment. Variations in 15N natural abundances also suggested that most N uptake by trees is from the forest floor and that nitrification occurs in soils at this catchment under ambient conditions. Changes in 15N contents of plant tissues, soils and streamwater after adding a 15N tracer to the ammonium sulfate fertilizer applied to the treated catchment showed that soils were the dominant sink for the labeled ammonium. Surface soils (Oca horizon plus any underlying mineral soil to 5cm depth) assimilated 19 to 31 percent of the 42 kg ha-1 of 15N-labelled ammonium-N during the tracer study. Aboveground biomass assimilated 8 to 17 percent of the labeled ammonium-N additions. Of the three forest types on the catchment, the soil:biomass assimilation ratio of labeled-N was highest in the spruce forest, intermediate in the beech-dominated hardwood forest and lowest in the mixed hardwood-spruce forest. Although ammonium sulfate additions led to increases in streamwater nitrate, only 2 of the 13 kg ha-1 of nitrate-N exported from the catchment during the 2 years of tracer additions was derived from the 42 kg ha-1 of labeled ammonium-N additions.     

Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (< 1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C:N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N x ha(-1) x yr(-1) above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.     

Mineral Soil and Solution Responses to Experimental N and S Enrichment at the Bear Brook Watershed in Maine (BBWM)

Buried mineral soil-bags and natural solutions were studied as indicators of forest ecosystem response to elevated N and S inputs at the Bear Brook Watershed in Maine (BBWM). The BBWM is the site of a paired watershed manipulation experiment in a northern New England forested ecosystem. The study includes two small (10 ha each) catchments dominated by northern hardwood forests with red spruce in the upper elevations. Treatments consist of (NH4)2SO4 applied to the West Bear watershed six times per year, increasing N and S deposition 3× and 2× above ambient values, respectively. Buried mineral soil-bag changes over time reflected both the native soil environment and the treatments. Most of the treatment effects on mineral soils were evident as higher inorganic S found in the treated watershed soils. Adsorbed SO4 in the buried mineral soil-bags increased by approximately 40% under softwood stands and 50% under hardwood stands over the study period. Hardwood soil solutions responded with significant increases in NO3 and SO4 concentrations that resulted in accelerated cation leaching, primarily Ca and Al. Few differences that could be attributed to treatments were evident in soil solutions under softwoods. No treatment effects were evident in throughfall and stemflow chemistry.