Mayank Krishna et al. - PNAS October 1, 2019 116 (40) 19786-19788

The article by Perakis and Pett-Ridge in PNAS (1) stimulated an interesting discussion (2) on the role of nitrogen-fixing alder in enhancing rock weathering and supply of rock-derived nutrients. The authors ascribe the accelerated rate of weathering to the generation of acidic conditions in the soil due to excess nitrogen (1).

Our work on Alnus nepalensis (Himalayan alder) may offer an alternative explanation to the role of alder on rock weathering. We focused on the changes in bacterial diversity and community structure in soil with the development of alder stands. These stands were designated as juvenile, young, and mature depending upon the stages of alder growth. We assessed the patterns, degree of chemical weathering, and the enrichment and depletion of major elements in the soil with alder growth. Our results suggest that the rate and pattern of weathering have remained similar even after many decades of alder development (Fig. 1). Moreover, enrichment and depletion of major elements in the soil did not vary prominently (Fig. 1B). The chemical index of alteration that surrogates for the degree of chemical weathering has also not varied conspicuously. We do not question the findings of the paper, but we argue that if the increased acidity leads to rock weathering and leaching of nutrients so much that alder “eats away the rock” (3), we would have seen notable changes in the weathering parameters with alder growth. Soil pH remained in the acidic range throughout the alder growth, ranging from 5.8 to 6.4. This motivates us to suggest an alternative explanation to what has been offered in the paper. We argue that there is a high demand for mineral elements (e.g., magnesium, phosphorus, iron, etc.) in the juvenile stage for their indispensable role in nitrogen fixation by Frankia inhabiting the root nodules (4, 5). This may be due to carboxylate secretion from cluster roots (2) or via rhizodeposition that promoted rock weathering in the juvenile stage. However, with alder development, nitrogen enrichment and the addition of other nutrients in the soil due to ecological processes such as nitrogen fixation and litter decomposition is expected. Consequently, alder would have elicited changes in the soil bacterial community. Consistent with the previous results (6), our findings revealed the shift in the bacterial community from oligotrophs to copiotrophs (Fig. 2) with Proteobacteria phyla dominating at later stages of alder growth. Most of the bacteria from the phylum Proteobacteria play a role in nitrogen fixation (5, 6). Microbes at later stages of alder development may also assist alder in nitrogen fixation (7). This may have led to the reduction in demands for the mineral elements at later stages. Consequently, chemical weathering and the nutrient mobility remain similar even with alder development.

See more: https://www.pnas.org/content/116/40/19786

Figure: Stacked-bar chart depicting variations in relative abundance of soil bacteria.