Climate variability, soil fertility, and pedogenic thresholds

Background/Question/Methods

Pedogenic thresholds occur where soil properties change abruptly along a continuous gradient in environmental forcing. Several thresholds have been identified along rainfall gradients on basaltic soils in Hawaii. Here, we asked if rainfall variation (as well as average rainfall) could play an important role in the development and dynamics of pedogenic thresholds. We also asked if pedogenic thresholds can be observed on lithologies other than basalt, and if so how the wide spectrum of weathering rates of the individual minerals in most non-basalt substrates could affect the position and nature of pedogenic thresholds. To evaluate rainfall variation, we developed a simple model that included cation weathering and leaching and several scales of rainfall variation. To evaluate parent materials other than basalt, we sampled soils developed in arkosic loess deposited on moraines dating to the last glacial maximum along a rainfall gradient from 400-4250 mm/year in New Zealand, and compared their soil properties with those observed along similar rainfall gradients on Hawaiian basalt. 

Results/Conclusions

Patterns in base saturation and associated soil properties are similar on basalt- and loess-derived soils; base saturation is >50% in dry sites, and goes through a sharp transition (a threshold) to near 0% in wetter sites. However, the pattern for overall loss of elements is very different, in that cations in primary minerals are almost entirely depleted in high-rainfall basalt-derived soils, but 20-50% of the cations from parent material are retained in the wetter sites on New Zealand loess. We suggest that while the basalt threshold represents an irreversible change, the loess threshold is kinetically controlled. The broader suite of minerals in loess contains recalcitrant minerals that weather more slowly than cations are removed by leaching. Our model supports the plausibility of this interpretation. The model also suggests that rainfall variation plays two important roles in the dynamics of pedogenic thresholds. First, the position of thresholds shifts to lower rainfall where rainfall variability is high, reflecting the influence of wet years. Second, a shift to drier conditions poses special problems for basalt-derived soils; weathering cannot rejuvenate the fertility of cation-depleted basaltic soils.