Carmen A. Nezat

Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems

The depletion of calcium in forest ecosystems of the northeastern USA is thought to be a consequence of acidic deposition and to be at present restricting the recovery of forest and aquatic systems now that acidic deposition itself is declining. This depletion of calcium has been inferred from studies showing that sources of calcium in forest ecosystems—namely, atmospheric deposition and mineral weathering of silicate rocks such as plagioclase, a calcium-sodium silicate—do not match calcium outputs observed in forest streams. It is therefore thought that calcium is being lost from exchangeable and organically bound calcium in forest soils. Here we investigate the sources of calcium in the Hubbard Brook experimental forest, through analysis of calcium and strontium abundances and strontium isotope ratios within various soil, vegetation and hydrological pools. We show that the dissolution of apatite (calcium phosphate) represents a source of calcium that is comparable in size to known inputs from atmospheric sources and silicate weathering. Moreover, apatite-derived calcium was utilized largely by ectomycorrhizal tree species, suggesting that mycorrhizae may weather apatite and absorb the released ions directly, without the ions entering the exchangeable soil pool. Therefore, it seems that apatite weathering can compensate for some of the calcium lost from base-poor ecosystems, and should be considered when estimating soil acidification impacts and calcium cycling.

Nitrogen budget of the Mobile–Alabama River System watershed

We have determined the nitrogen mass balance for the Mobile–Alabama River System (MARS) for two years of different hydrologic regimes (i.e. low flow vs. high flow). The maximum riverine export of N from the watershed is only 7%, suggesting relatively high retention and/or losses of N by denitrification within the watershed. Previous investigations of other watersheds within the USA demonstrate export percentages of c. 20–25%. Our calculations indicate that during a high flow year such as 1990, c. 13% of the new N introduced to the watershed annually is lost within the riverine system either through diatom uptake or denitrification. Another 4% is lost to the groundwater while 25–38% is sequestered by the terrestrial biomass (i.e. crop production and forest growth). Thus, as much as 51% of the N input to the landscape in the MARS is unaccounted for. We believe the location of this ‘missing’ N is probably within the soil, or the N has been lost through denitrification within the terrestrial ecosystem. The relatively low N yield from the MARS suggests that the watershed is not as saturated with respect to N as are many other U.S. drainages.

Mycorrhizal weathering in base-poor forests: Ecology (communication arising)

because Ca/Sr ratios can vary greatly in different parts of a plant, Ca/Sr ratios in foliage are poor indicators of the source of calcium for trees. Differences in Ca/Sr ratios in tree foliage should be interpreted with caution until more is known about the cycling of these elements in forests. Depletion of the exchangeable calcium pool in soil is of widespread concern and may have serious implications for future forest health and productivity. Blum et al. propose that apatite-derived calcium is used largely by ectomycorrhizal tree species, and that mycorrhizae may weather apatite and absorb the released ions directly, without ions entering the exchangeable pool. The mineralogy of the shallow soils and bedrock at Plastic Lake (referred to here as PC1) in south-central Ontario is dominated by quartz, plagioclase, K-feldspar, hornblende, vermiculite, imogolite and goethite; there is no evidence that apatite is present at PC1 (ref. 9). Furthermore, base-cation massbalance budgets that consider only silicate weathering are consistent with the measured changes in stream water and soil chemistry, which together indicate that substantial calcium losses have occurred from the exchangeable pool over the past two decades. Despite the absence of calcium-rich minerals at PC1, molar Ca/Sr ratios in tree foliage at PC1 vary widely and range from about 330 to 3,200 (Table 1), which is similar to the range (about 500 to 2,200) reported at Hubbard Brook. As shown previously, we found that Ca/Sr ratios vary widely across plant parts within a single tree species (Table 1). The annual cycling of calcium in the above-ground biomass (litter fall plus foliar leaching) of temperate forests in eastern North America is typically an order of magnitude greater than the rate of calcium weathering expected in base-poor forests. Because the ‘mining’ of calcium cannot supply a large proportion of a forest’s annual calcium demand, the direct uptake of calcium from minerals will not protect trees from the negative effects of low calcium or high aluminium availability that may occur in acidified soils. The ecological significance of direct calcium uptake through symbiotic mycorrhizal association lies in whether this process increases the annual calcium weathering rate, which will determine the size of the exchangeable calcium pool under given acid-deposition and harvesting conditions. Shaun A. Watmough, Peter J. Dillon Environmental and Resource Studies Department, Trent University, Peterborough, Ontario K9J 7B8, Canada e-mail: swatmough@trentu.ca

Ecology - Mycorrhizal weathering in base-poor forests - Reply

because Ca/Sr ratios can vary greatly in different parts of a plant, Ca/Sr ratios in foliage are poor indicators of the source of calcium for trees. Differences in Ca/Sr ratios in tree foliage should be interpreted with caution until more is known about the cycling of these elements in forests. Depletion of the exchangeable calcium pool in soil is of widespread concern and may have serious implications for future forest health and productivity. Blum et al. propose that apatite-derived calcium is used largely by ectomycorrhizal tree species, and that mycorrhizae may weather apatite and absorb the released ions directly, without ions entering the exchangeable pool. The mineralogy of the shallow soils and bedrock at Plastic Lake (referred to here as PC1) in south-central Ontario is dominated by quartz, plagioclase, K-feldspar, hornblende, vermiculite, imogolite and goethite; there is no evidence that apatite is present at PC1 (ref. 9). Furthermore, base-cation massbalance budgets that consider only silicate weathering are consistent with the measured changes in stream water and soil chemistry, which together indicate that substantial calcium losses have occurred from the exchangeable pool over the past two decades. Despite the absence of calcium-rich minerals at PC1, molar Ca/Sr ratios in tree foliage at PC1 vary widely and range from about 330 to 3,200 (Table 1), which is similar to the range (about 500 to 2,200) reported at Hubbard Brook. As shown previously, we found that Ca/Sr ratios vary widely across plant parts within a single tree species (Table 1). The annual cycling of calcium in the above-ground biomass (litter fall plus foliar leaching) of temperate forests in eastern North America is typically an order of magnitude greater than the rate of calcium weathering expected in base-poor forests. Because the ‘mining’ of calcium cannot supply a large proportion of a forest’s annual calcium demand, the direct uptake of calcium from minerals will not protect trees from the negative effects of low calcium or high aluminium availability that may occur in acidified soils. The ecological significance of direct calcium uptake through symbiotic mycorrhizal association lies in whether this process increases the annual calcium weathering rate, which will determine the size of the exchangeable calcium pool under given acid-deposition and harvesting conditions. Shaun A. Watmough, Peter J. Dillon Environmental and Resource Studies Department, Trent University, Peterborough, Ontario K9J 7B8, Canada e-mail: swatmough@trentu.ca