The consequences of acid rain on fine-root carbon allocation in a hardwood forest

Background/Question/Methods

Acid rain, by acidifying soil, can lead to a decrease in bioavailable phosphorus (P). In order to acquire P and other things, plants invest carbon (C) into both fine-root production and mycorrhizal relationships. However, it is still unclear how below-ground C allocation will respond to changes in bioavailable P. Does an abundance of P reduce the plant’s dependence on fine-root production and mycorrhizal association? We hypothesized that under high P conditions plants will invest less C into fine-root production and mycorrhizal associations than when bioavailable P is low. This was tested on separate field manipulations of elevated P, and elevated pH, in acidic temperate deciduous forest soils in southeastern Ohio (n=9). Surface soil was cored, pH was measured, and fine-roots were analyzed. Two acid-phosphatase enzymes responsible for P-acquisition were measured fluorometrically.

Results/Conclusions

Consistent with our hypothesis, fine-root biomass was 26% lower in elevated P when compared with the control (P=0.03). Likewise, microbial phosphatase activity was 48% lower in elevated P treatments (P<0.01). This suggests that availability of P results in the plant’s demand being met with less mycorrhizal support, and with lowered investment of C into fine-root production. While fine-roots were sensitive to P, there was no significant change in fine-root biomass with elevated pH. This suggests P is a greater determinant of plant C allocation than soil pH. Field studies with elevated P in natural systems are limited; past work with similar-aged soils have shown elevated P to increase fine-root turnover. It is well understood that nitrogen (N) availability also affects C allocation; increased N results in both a decrease in fine-root biomass, and an increase in fine-root turnover. Similarly, this work implies that levels of bioavailable P can influence C allocation to fine-roots. Also, our results suggest that while acid rain deposition does decrease soil pH, it more importantly limits bioavailable P. In conclusion, P-limitation caused by acid rain deposition may include a tradeoff between plant nutrient acquisition and growth.