Knute J. Nadelhoffer

Nitrogen saturation in northern forest ecosystems

This article describes ways in which excess nitrogen from fossil fuel combustion may stress the biosphere. Nitrogen emissions can have a direct effect on air quality through both the oxidizing potential of nitrogen oxides and the role these compounds play in the formation of ozone. The complexity of these effects on water quality and on forest nutrition is discussed.

Nitrogen Saturation in Temperate Forest Ecosystems

N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

Roots exert a strong influence on the temperature sensitivity of soil respiration

The temperature sensitivity of soil respiration will largely determine the effects of a warmer world on net carbon flux from soils to the atmosphere. CO2 flux from soils to the atmosphere is estimated to be 50–70 petagrams of carbon per year and makes up 20–38% of annual inputs of carbon (in the form of CO2) to the atmosphere from terrestrial and marine sources,. Here we show that, for a mixed temperate forest, respiration by roots plus oxidation of rhizosphere carbon, which together produce a large portion of total effluxed soil CO2, is more temperature-sensitive than the respiration of bulk soil. We determine that the Q10 value (the coefficient for the exponential relationship between soil respiration and temperature, multiplied by ten) is 4.6 for autotrophic root respiration plus rhizosphere decomposition, 2.5 for respiration by soil lacking roots and 3.5 for respiration by bulk soil. If plants in a higher-CO2 atmosphere increase their allocation of photosynthate to roots these findings suggest that soil respiration should be more sensitive to elevated temperatures, thus limiting carbon sequestration by soils.

Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter

Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope, analyses of C and N.Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of ‘lignocellulose’ (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials (‘lignin’) dominate decay in the latter part of the continuum.Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopic composition were observed during litter decay. Larger changes were observed with depth in the soil profile.An understanding of factors that control ‘lignin’ degradation is key to predicting the patterns of mass loss and N dynamics late in decay. The hypothesis that labile C is needed for ‘lignin’ degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows ‘lignin’ decay must be evaluated in soil systems.

BELOWGROUND CARBON ALLOCATION IN FOREST ECOSYSTEMS: GLOBAL TRENDS'

Carbon allocation to roots in forest ecosystems is estimated from published data on soil respiration and litterfall. On a global scale, rates of in situ soil respiration and aboveground litter production are highly and positively correlated, suggesting that above- and belowground production are controlled by the same factors. This relationship also allows us to predict rates of total soil respiration and total carbon allocation to roots in forest ecosystems from litterfall measurements. Over a gradient of litterfall carbon ranging from 70 to 500 g m-2 yr-1, total belowground carbon allocation increases from 260 to 1100 g m-2 yr-1. The ratio of belowground carbon allocation to litterfall decreases from 3.8 to 2.5 as litterfall carbon increases from 70 to 200 g.m-2 yr-1, but changes little (2.5 to 2.2) as litterfall carbon increases from 200 to 500 g.m-2 yr-1. Use of this relationship permits the construction of simple carbon budgets that can be used to place upper limits on estimates of fine root production in forest ecosystems. Determining live-root respiration rates in forest ecosystems will further constrain the range of possible root production rates.

Resource-based niches provide a basis for plant species diversity and dominance in arctic tundra

Ecologists have long been intrigued by the ways co-occurring species divide limiting resources. Such resource partitioning, or niche differentiation, may promote species diversity by reducing competition. Although resource partitioning is an important determinant of species diversity and composition in animal communities, its importance in structuring plant communities has been difficult to resolve. This is due mainly to difficulties in studying how plants compete for belowground resources. Here we provide evidence from a 15N-tracer field experiment showing that plant species in a nitrogen-limited, arctic tundra community were differentiated in timing, depth and chemical form of nitrogen uptake, and that species dominance was strongly correlated with uptake of the most available soil nitrogen forms. That is, the most productive species used the most abundant nitrogen forms, and less productive species used less abundant forms. To our knowledge, this is the first documentation that the composition of a plant community is related to partitioning of differentially available forms of a single limiting resource.

EFFECTS OF TEMPERATURE AND SUBSTRATE QUALITY ON ELEMENT MINERALIZATION IN SIX ARCTIC SOILS

We compared the effects of temperature on rates of microbial respiration, N mineralization, nitrification, and P mineralization in soils from six arctic ecosystems located along a toposequence on Alaskas North Slope. Soils from these ecosystems were incubated aerobically in the laboratory for 13 wk and at temperatures representative of field values during a typical growing season. Rates of C and N mineralization were insen- sitive to temperature between 30 and 90C but increased by factors of 2 or more between 90 and 15?. For both C and N, differences in mineralization rates among soils were greater than differences due to incubation temperature within single soils. This suggests that the quality of soil organic matter varies widely among these ecosystems and is more important than soil temperature differences in controlling rates of these processes in the field. Nitri- fication occurred in all soils, even at 30, but there were large differences among soils in nitrification potentials. Overall differences in P mineralization between soils were small. Rates of P mineralization, however, decreased with increasing temperature in soils from some sites and increased with temperature in others. Carbon respired during the 1 3-wk incubations ranged between 1.5 and 8% of total soil organic C across soil types and incubation temperatures. In contrast to the relatively high C mineralization rates in these soils, net N and P mineralization rates were very low and were likely due to high microbial demands for these nutrients. High microbial demand for mineral nutrients can severely limit plant N and P availability in arctic soils.

Fine Roots, Net Primary Production, and Soil Nitrogen Availability: A New Hypothesis

The relationships between above- and belowground net primary production and soil nitrogen availability were studied at nine temperate forest sites. Annual allocations of nitrogen (N) and net primary production to leaf litter, perennial tissues (wood + bark), and aboveground biomass all increased significantly (P < .01) in relation to apparent N uptake by vegetation (NJ) as calculated using field measures of net N mineralization (Nj) and other major N fluxes to and from available N pools. Mean annual N content and biomass of fine roots (:<3.0 mm diameter) were both negatively correlated with NU (r= -0.71, P < .05; r -0.63, P < .10, respectively). However, only -50% of Nm at each site could be accounted for by allocation to aboveground litter and perennial tissues. Assuming that mineralized N not accounted for by allocation to these components was taken up by vegetation and allocated to fine roots, annual N allocation to fine roots (Nfr) was a constant fraction of N uptake. Therefore, Nfr increased in absolute terms with both Nm and apparent N uptake. Fine-root N turnover rates (or Nfl/fine-root N content) also increased as Nm and NU increased. Provided that fine-root biomass and N turnover rates were similar within individual sites, allocation of production to belowground biomass also increased relative to increases in soil N availability.Furthermore, the proportion of total net primary production allocated to belowground biomass did not decrease with increased N avail- ability.

Global Change and the Carbon Balance of Arctic EcosystemsCarbon/nutrient interactions should act as major constraints on changes in global terrestrial carbon cycling

n the cold, arctic climate, global warming due to greenhouse gas accumulation in the atmosphere might be expected to increase both primary production and heterotrophic (mainly soil) respiration. Which of these processes will increase more or more rapidly? The answer to this question is critical in understanding the effects of warming on the net carbon balance of arctic ecosystems and of the earth itself. If primary production increases faster than heterotrophic respiration, carbon will be removed from the atmosphere and will accumulate on land. If the reverse happens, carbon will be lost to the atmosphere. Eventually, a new equilibrium may be reached, but this equilibration could take decades or even centuries. Meanwhile, dramatic losses or gains of carbon may occur over the entire arctic region (5.7 x 106 km2; Oechel 1989), with potentially important feedbacks on the global atmospheric concentration of

Biogeochemical Diversity Along a Riverside Toposequence in Arctic Alaska

Nitrogen and phosphorus pool sizes, distribution, and cycling rates were described and compared for six different ecosystem types occurring along a single topose- quence in northern Alaska. The toposequence was located on a series of old floodplains of the Sagavanirktok River, in the northern foothills of the Brooks Range. From tussock tundra in the uplands, the toposequence passed through a relatively dry hilltop heath zone, a hillslope shrub/lupine/Cassiope zone, a footslope Equisetum zone, a wet sedge tundra, and a riparian shrub zone. A late-melting snowbank covered the hillslope site in early June of each year, and the sites consistently varied in soil temperature, soil moisture, thaw depth, and the seasonal pattern of soil thaw. The standing stocks of N, P, and C in soils of these six ecosystem types varied dra- matically but not monotonically along the toposequence, as did the turnover rates of these elements. Several measures were used in comparisons of N and P availability, including soil solution concentrations, in situ accumulation on ion-exchange resins, and levels of KCl-extractable N and P. Annual rates of net N mineralization were assayed using a buried bag method, and ecosystem respiration was measured by trapping CO2 in soda lime (NaOH + Ca (OH)2). Soil P pools were characterized by sequential extraction methods into four major pools, including loosely bound P, Al- and Fe-bound P, primary mineral P, and organic P. Both N and P availability were low in all six ecosystems when compared with temperate forests or wetlands. Among ecosystems, however, there was considerable variation in the relative availability of N vs. P, and in the apparent relative importance of nitrate as a nitrogen source.