Soil responses to management, increased precipitation, and added nitrogen in ponderosa pine forests.
Forest management, climatic change, and atmospheric N deposition can affect soil biogeochemistry, but their combined effects are not well understood. We examined the effects of water and N amendments and forest thinning and burning on soil N pools and fluxes in ponderosa pine forests near Flagstaff, Arizona (USA). Using a 15N-depleted fertilizer, we also documented the distribution of added N into soil N pools. Because thinning and burning can increase soil water content and N availability, we hypothesized that these changes would alleviate water and N limitation of soil processes, causing smaller responses to added N and water in the restored stand. We found little support for this hypothesis. Responses of fine root biomass, potential net N mineralization, and the soil microbial N to water and N amendments were mostly unaffected by stand management. Most of the soil processes we examined were limited by N and water, and the increased N and soil water availability caused by forest restoration was insufficient to alleviate these limitations. For example, N addition caused a larger increase in potential net nitrification in the restored stand, and at a given level of soil N availability, N addition had a larger effect on soil microbial N in the restored stand. Possibly, forest restoration increased the availability of some other limiting resource, amplifying responses to added N and water. Tracer N recoveries in roots and in the forest floor were lower in the restored stand. Natural abundance delta15N of labile soil N pools were higher in the restored stand, consistent with a more open N cycle. We conclude that thinning and burning open up the N cycle, at least in the short-term, and that these changes are amplified by enhanced precipitation and N additions. Our results suggest that thinning and burning in ponderosa pine forests will not increase their resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change. Restoration plans should consider the potential impact on long-term forest productivity of greater N losses from a more open N cycle, especially during the period immediately after thinning and burning.
Hungate
Bruce A
BA
Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011, USA. bruce.hungate@nau.edu
Hart
Stephen C
SC
Selmants
Paul C
PC
Boyle
Sarah I
SI
Gehring
Catherine A
CA
D001130:Arizona; D011232:Chemical Precipitation; D002980:Climate; D017753:Ecosystem; D005390:Fires; D009573:Nitrites / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D041604:Pinus ponderosa / Q000254:growth & development / Q000378:metabolism*; D012987:Soil / Q000032:analysis*; D013997:Time Factors; D014867:Water / Q000737:chemistry
D009573:Nitrites; D012987:Soil; D014867:Water; D009584:Nitrogen