Interactions between Hydrochemistry N Dynamics and Plants at Betsele A Model System

At Betsele, 64°N in northern Sweden, there is a remarkable N-sup-ply gradient from a groundwater rcchargc area to a discharge area (Giesler et al., 1998). The gradient encompasses a wide range of the variation in soil pH, %N in soils, ctc., and in plant community composition found in Fennoscandian boreal forests (Figs. 2 and 4). Similar but less pronounced gradients arc found along any major slope, but this site was chosen to enable studies of large variability on the same type of coarsc-tcxturcd till soil within a distance of 90 m only. It should be pointed out that the gradient docs not encompass a complete slope from the water divide to the discharge area; in fact, some of the discharge water may have flowed up to 700-800 m through the soils and bcdrock upslope of the discharge area. Surface discharge usually occurs for about a week in connection with snow melt during the spring, but may also occur under unusually wet conditions later in summer and autumn. Discharge events arc short and dynamic; the groundwater level may rise and fall several decimeters in a day. In the area in general, the process of podzolization dominates and is associated with an acid mor layer and acidification of the upper mineral soil. In the discharge area, this proccss is, however, sometimes interrupted by "titration" events, bringing in base-rich water (and no doubt some N).

Along the 90 m transect, forest productivity increases by a factor of 3 in the direction of the discharge area. The forest stand is about 125 years old throughout, but changes from a dominance of Pinus sylvestris L. to a dominance of Picea abies (Karst.) L. The tallest P. abies in the discharge area are 36 m tall, compared with

22 m tall P. sylvestris in the recharge area. Also, there is a remarkable shift in the understory from mainly 1- to 2-dm-tall dwarf shrubs of Vaccinium spp. through a zone of dwarf shrubs and low herbs to a luxurious stand of tall herbs, e.g., 1- to 2-m-tall Aconi-tum septentrionale L., in the discharge area.

Soil solution pH in the mor-layer increases from about 3.5 in the recharge area to about 6.5 in the discharge area (Fig. 4). In the upper mineral soil, AI1+ dominates the exchange complex in the recharge area, but is gradually replaced by Ca2+ toward the discharge area. Moreover, in mor-layer of the recharge area, levels of inorganic N are low ( < 30 |xmol L~'), but as one approaches the discharge area, levels of NH4 + rise (> 100 |xmol L_l), and finally NO," becomes the dominant inorganic N species in the discharge area, where levels of inorganic N are highest (180 fimol L_l). Levels of total P rise gradually at first, but then increase more rapidly toward the discharge area. In contrast, levels of PO, in the soil solution fall remarkably toward nil in the discharge area.

Thus, analyses of soil chemistry confirm the suspected increase in N supply in the direction of the discharge area. Foliar analysis and plant growth bioassays also confirm this increase in N supply. The low availability of P04 in the discharge area is confirmed by plant growth bioassays as well as 32P root uptake bioassays on

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FIGURE 4 Variations in soil chemistry (Giesler et al., 1998) along the 90-m long gradient at Betsele (which starts at 0 m in a recharge area and ends at 90 m in a discharge area) along which forest productivity increases threefold, (a), pH in the mor-layer (open symbols, the upper Ol horizon; closed symbols, the lower 02 horizon); (b), exchangeable cations in the mineral E horizon (CECe = effective cation exchange capacity). The same trend in Al and Ca saturation is found in the mor-layer and the mineral Bs horizon, but in the mor-layer Al is organically com-plexed, while it occurs as Al!+ in the mineral E and Bs horizons; (c), %N in the mor-layer (symbols as in a); (d), concentration of NH4" in soil solution in the mor-layer (symbols as in a); (e), concentration of NO in soil solution in the mor-layer (symbols as in a); (f), amount of total P in the mor-layer; (g), concentration of P04 in the soil solution in the mor-layer (symbols as in a).

and plant growth bioassays also confirm this increase in N supply. The low availability of P04 in the discharge area is confirmed by plant growth bioassays as well as 32P root uptake bioassays on roots collected in the field. Apparently, the low availability of P in the discharge area is caused by high levels of organically com-plexed Fe and Fe-oxyhydroxides with a high capacity to bind P, e.g., into Fe phosphates.

The study at Betsele confirms that the supply of N is strongly correlated with soil pH, and especially the supply of Ca (Giesler et al., 1998; Fig. 4). This should partly reflect the fact that both N and Ca are components of the groundwater, but it is noteworthy that NO," is a dominant inorganic N species in the discharge area, which indicates that also in situ processes are important, as NO only occurs at trace levels in the soil solution elsewhere. Also, preliminary data suggest that calculated rates of N mineralization increase with the increase in pH (P. Flogberg, A. Nordgren and M, Hogberg, unpublished). It is also of interest that P, rather than N, is limiting in the discharge area; hence this is a naturally N-saturated system. As regards discussions about ratios between base cations (BC) and Al as predictors of effects of acid rain on forest growth, it is clear here, that in an area with very low levels of deposition of N and S, there is a correlation between the Ca/Al ratio and productivity (Fig. 4). Some authors have argued that the correlations between such ratios and forest growth are evidence of the impact of acid rain on forest growth, and that ratios of BC/A1 < 1, or of Ca/Al < 1, in the mineral soil, represent situations when soil acidity is detrimental to root function and forest growth (Sverdrup et al., 1992; Cronan and Grigal, 1995). Any effects of such ratios on forest growth in Fennoscandia are confounded, if not totally obscured, by effects of the variability in N supply, which is correlated with pH (which, in turn, correlates positively with BC/A1 or Ca/Al, cf. Fig. 4).

Interestingly, the relations between soil pH and N supply found along the short transect at Betsele are very much the same as those proposed by Read (1986; 1991) as typical for long latitudinal and altitudinal gradients from polar/Alpine conditions through temperate coniferous forests and nemoral deciduous forests to dry and warm temperate "steppe" conditions. Read proposed that under cold and wet conditions, decomposition of organic matter would be slow and incomplete and result in the formation of acid, peaty soils or thick mor-layers with slow mineralization of organic N. Dominant plants are typically ericaceous dwarf shrubs with er-coid mycorrhiza and a high potential capacity to use organic N sources, e.g., amino acids. Under slightly warmer (and drier) conditions, in temperate forests, soil organic matter is more readily decomposed, and along with organic N sources, plants can use more NH4+ produced by mineralization. Dominant plants are ec-tomycorrhizal trees with a high potential capacity to use both simple organic N sources and NH4 h. Under warmer, drier conditions, in temperate grasslands ("steppe"), mineralization is rapid, soil pH is high, and NO,~ becomes an important N source. Dominant plants have arbuscular mycorrhiza. At Betsele, the sequence in terms of changes in soil pH, C/N ratio, concentration and species of inorganic N, and type of mycorrhiza, is, indeed, very much the same (Giesler et al., 1998). It has not yet been demonstrated in the field that plants along the transect differ very much in their use of the different potential N sources. Preliminary analyses have, however, shown that amino acids are the dominant N species in the soil solution of the mor-layer in the recharge, but not in the discharge area (A. Nordin, P. Hogberg, and T. Nasholm, unpublished). This is most interesting, since plants with all three major types of mycorrhiza, ericoid, ecto-, and arbuscular, are capable of taking up at least glycine, as was recently demonstrated in the field (Nasholm et al., 1998). Also, nonmycorrhizal sedge has been demonstrated to use simple organic N sources (Chapin et al., 1993). However, despite many years of research on N cycling and plant N uptake, we do not know more exactly the fractional contribution of different potential N sources to plant uptake in the field and how this varies latitudinally and across landscapes.

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