Effect of fine root contact on plant-induced weathering of basalt

The objective of the present study was to provide experimental evidence on the active role of plant roots in rock weathering and the importance of the proximity of roots to rock in the weathering process. The analysis was based on the release of different elements from basalt rock particles by three crop species: rice, soybean and maize. Quantitative results were obtained by chemical analyses. We designed two types of hydroponic crop pots, in which fine roots were allowed (or not allowed) to make contact with rock particles by using coarse (or fine) net bags. Experiments were carried out in a controlled glasshouse during a 42 d period. The release of elements in the presence of the plants was calculated by subtracting the decrease in the amounts of elements in the media from the amounts absorbed by the plants. We observed the positive effect of plants on the release of elements from the rock particles and the highest amounts were released in the soybean pots. The amounts of Si, Ca, Mg, Mn and Al released increased by a factor of 2-5, 2-7, 16-112, 3-19 and 6-60, respectively. The amount of Fe released by soybean plants from the rock particles was 4-6 times higher than that by other plants. Between the coarse and fine net pots, the amount of released elements differed significantly only for soybean (Si, Mg and Mn at p < 0.01 level and Fe at p < 0.05 level), which displayed the most vigorous growth. Our results imply that weathering may be caused partially by the absorption of nutrient elements directly through the interface of fine roots and rock particles, and is most likely associated with alterations of the local rhizosphere conditions surrounding the roots.