Pasture degradation, secondary forest regrowth, and mature forest productivity: Do nutrients matter? Eric A. Davidson, The Woods Hole Research Center Adam Hirsch, National Oceanographic and Atmospheric Administration Cláudio J. R. de Carvalho, Embrapa Amazônia Oriental Ricardo de O. Figueiredo, Embrapa Amazônia Oriental Daniel Markewitz, University of Georgia

Evidence that nutrients are important in mature Amazonian forests C:N:P ratios in litterfall indicate a very conservative P cycle (Markewitz et al Ecol. Appl. still in press) : Paragominas, BR, litterfall C:N:P = 1500:45:1 Hubbard Brook, USA, litterfall = 620:14:1 Recent basinwide surveys indicate faster tree growth in the more fertile soils near the Andes (Malhi et al Global Change Biology) If nutrients can limit primary productivity in mature Amazonian forests, why wouldn’t they also be important in pastures and secondary forests?

Evidence that nutrients have little effect on pasture productivity and rates of secondary forest regrowth pastures in western Amazonia are seldom fertilized soil testing seldom shows a correlation between soil nutrient analyses and rates of secondary forest regrowth. Absence of seed banks and poor physical soil properties have been proposed as more important factors limiting rates of forest regrowth (Buschbacher, R., C. Uhl, and E.A.S. Serrão J. Ecology 76: successful exotic grasses and native forest species are well adapted to nutrient deficient soils through mycorrhizal associations

Evidence that nutrients have a significant effect on pasture productivity and rates of secondary forest regrowth pastures in eastern Amazonia are often fertilized with P fertilization experiments have shown growth responses of secondary forest species to fertilization with N and P (Davidson et al. Ecol. Appl. still in press; Gehring et al Biogeochemistry 45: ). significant nutrient stocks are lost when fire is used for pasture formation and management (Kauffman et al Oecologia 104: ; Kauffman et al Oecologia 113:

Davidson et al. Ecol. Appl. In press.

Evidence that nutrients cycling processes are altered when mature forests are replaced with pastures and secondary forests soil emissions of N gases (N 2 O and NO) and rates of net N mineralization decrease in old pastures (Verchot et al GBC; Melillo et al., JGR). indices of plant available-P decline in old pasture soils (Garcia-Montiel et al SSSAJ; Townsend et al JGR). hydrologic export of N declines in pastures; export of P increases in some cases (Markewitz et al. Ecol. Appl., still in press; Neill et al. Ecol Appl.2001).

Perenial stream Ephemeral stream Mature forest Managed pasture Secondary forest ~ 1 km Degraded pasture

Markewitz et al., Ecol. Appl., in press

Key questions for nutrient cycling after forest clearing: Do logging and burning remove enough nutrients to slow rates of pasture and secondary forest growth? How fast are N and P released from soil organic matter? How do plant demands for N and P compare to rates of N and P mineralization in pastures and secondary forests?

D) N and P uptake; the most limiting is taken up 100%; the other according to N:P ratios E) The nutrient in excess goes to extra hydrologic loss C) Calculate partitioning of uptake to wood & foliage B) hydrologic N loss A) gaseous N loss foliage wood litter Soil 0-10 cm fast, slow, passive * Soil cm fast, slow, passive * Mineral N and P Inputs from atmosphere and BNF * from Trumbore et al. (1995) partitioning of soil carbon fire ash smoke & gases beef export

Net N mineralization  30 kg N/ha/yr

Conclusions Significant stocks of N and P are lost from the ecosystem with forest clearing and burning More N is lost than P, and net N mineralization declines more than net P mineralization (net min N:P = 20). However, pasture grasses demand less N (grass N:P = 16), so the pasture remains P limited Secondary forest vegetation requires more N (N:P = 35), so the depletion of N stocks during the pasture phase results in N limitation during regrowth of the secondary forest following pasture abandonment Multiple burns accelerate the nutrient degradation process Mineralization rates of nutrients bound in the soil are key to understanding productivity of pastures and secondary forests