The effect of rock content on nutrients in a Sierra Nevada forest soil

The effects of rock content on nutrient concentrations and contents (kg ha− 1) in a skeletal soil derived from andesite were investigated. Rock content to a 60 cm depth ranged from 8 to 69% among 20 quantitative soil pits within a 10 ha area. On average, rocks contained 9% of total soil C, 19% of total soil N, 3% of total ecosystem C, and 12% of total ecosystem N. Percent rock content also appeared to cause increases in organic matter concentrations: rock content was significantly, positively correlated with total C and total N concentrations in the fine earth (> 2 mm) fraction. Soil C was organic; no carbonates were present. Percent rock content was also significantly, positively correlated with fine earth C:N ratio, suggesting that organic matter in rockier soils was in an earlier stage of decomposition at any given depth. Other consistent correlations with rock content included Bray P (positive) and exchangeable Mg2 + (negative). The decrease in fine earth mass with increasing percent rock content offsets the increase in C concentration such that there was no correlation between percent rock content and either fine earth or total soil (fine earth plus rock) C content (kg ha− 1). Percent rock content was significantly negatively correlated with total N, Bray P, inorganic N, exchangeable K+, Ca2 +, and Mg2 + contents, however. The results of this study show that high rock content can affect soil C and nutrient pools not only because of the contributions of the rocks themselves, but also because high rock content appears to cause higher concentrations in the fine earth fraction.

► Soil C, N and extractable P concentrations were inversely related to rock content.
► Soil C contents (kg ha–1) did not increase with rock content.
► Rocks contained 8% of total C and 16% of total N in soils.