Biogeochemistry of a temperate forest nitrogen gradient

Abstract

Wide natural gradients of soil nitrogen (N) can be used to examine fundamental relationships between plant-soil-microbial N cycling and hydrologic N loss, and to test N-saturation theory as a general framework for understanding ecosystem N dynamics. We characterized plant production, N uptake and return in litterfall, soil gross and net N mineralization rates, and hydrologic N losses of nine Douglas-fir (Pseudotsuga menziesii) forests across a wide soil N gradient in the Oregon Coast Range (U.S.A.). Surface mineral soil N (0-10 cm) ranged nearly three-fold from 0.29% to 0.78% N, and in contrast to predictions of N-saturation theory, was linearly related to 10-fold variation in net N mineralization, from 8 to 82 kg N.ha(-1) x yr(-1). Net N mineralization was unrelated to soil C:N, soil texture, precipitation, and temperature differences among sites. Net nitrification was negatively related to soil pH, and accounted for <20% of net N mineralization at low-N sites, increasing to 85-100% of net N mineralization at intermediate- and high-N sites. The ratio of net: gross N mineralization and nitrification increased along the gradient, indicating progressive saturation of microbial N demands at high soil N. Aboveground N uptake by plants increased asymptotically with net N mineralization to a peak of approximately 35 kg N.ha(-1) x yr(-1). Aboveground net primary production per unit net N mineralization varied inversely with soil N, suggesting progressive saturation of plant N demands at high soil N. Hydrologic N losses were dominated by dissolved organic N at low-N sites, with increased nitrate loss causing a shift to dominance by nitrate at high-N sites, particularly where net nitrification exceeded plant N demands. With the exception of N mineralization patterns, our results broadly support the application of the N-saturation model developed from studies of anthropogenic N deposition to understand N cycling and saturation of plant and microbial sinks along natural soil N gradients. This convergence of behavior in unpolluted and polluted forest N cycles suggests that where future reductions in deposition to polluted sites do occur, symptoms of N saturation are most likely to persist where soil N content remains elevated.

Species-specific trajectories of nitrogen isotopes in Indiana hardwood forests, USA

“…Increased nitrification could also be a consequence of forest development, as has been demonstrated on a 100-year chronosequence of upland hardwood forests close to our study sites (Idol et al, 2003). Finally, net nitrification rate is positively correlated with net mineralization rate, and both processes are sensitive to a variety of ecosystem parameters that may have varied over time in these stands, including soil C:N, soil pH, moisture, temperature, and NH + 4 uptake (Perakis and Sinkhorn 2011;Matson and Vitousek, 1981).…”

Nitrogen deposition and soil nitrogen dynamics in subtropical evergreen broad-leaved stands along an age-sequence

“…Therefore, the C/N ratio and DON concentration in the surface mineral soil could be used as a predictor of net N mineralization rate in this subtropical forest. Our finding was in agreement with the result reported by Chen and Mulder (2007) and Perakis and Sinkhorn (2011). This study demonstrates that the concentrations of N forms and the net N transformation rates varied during the forest development with higher N availability in the older stands than in the young stands.…”

“…In addition, the N mineralization rate, an important process regulating N dynamics, has been suggested as an index for potential risk of N leaching in forest ecosystems (Perakis and Sinkhorn, 2011). At our site, the annual N mineralization potential in the surface 10 cm of the mineral soil was significant correlation with the total inorganic N concentration of soil solution at the 30 cm depth (Figure 3).…”

“…Researches along N deposition gradients in the USA and Europe revealed a negative correlation between the C/N ratio in the organic horizon and N mineralization, nitrification, and leaching rates (Dise et al, 1998). The soil mineral N flux and net N mineralization rate has been proposed as an indicator for NO 3 -leaching from forest soils (Perakis and Sinkhorn, 2011).…”

Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

“…For example, it has been argued that hypotheses developed for N-polluted sites have limitations when applied to ecosystems that are naturally N-rich (Perakis and Sinkhorn 2011); that is, systems where net N mineralization should decline when N availability is above soil and plant demand, but it does not. In other ecosystems, elevated N inputs do not always result in increased rates of N mineralization and nitrification or higher foliar N content (Lovett and Goodale 2011), and in xeric systems, significant N losses occur in catchments where the vegetation remains N-limited (Vourlitis and others 2009).…”