Soil responses to management, increased precipitation, and added nitrogen in ponderosa pine forests

• Published in: Ecological applications 2007-07, Vol.17 (5), p.1352-1365
• Main Authors: Hungate, Bruce A., Hart, Stephen C., Selmants, Paul C., Boyle, Sarah I., Gehring, Catherine A.
• Format: Article
• Language: English
• Subjects: afforestation; Arizona; atmospheric deposition; Chemical Precipitation; Climate; climatic change; Coniferous forests; Ecosystem; fertilizer application; Fires; Forest ecology; Forest litter; forest restoration; Forest soils; forest thinning; microbial biomass; Mineral soils; mineralization; N isotope tracer; nitrification; Nitrites - metabolism; Nitrogen; Nitrogen - metabolism; nitrogen fertilization; nitrogen fertilizers; nitrogen mineralization; nutrient availability; Pinus ponderosa; Pinus ponderosa - growth & development; Pinus ponderosa - metabolism; precipitation; prescribed burning; Soil - analysis; Soil ecology; soil fertility; Soil microorganisms; soil pollution; Soil water; Soil water content; stable isotopes; stand management; Time Factors; Water - chemistry; δ15N
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/06-1187.1

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 δ¹⁵N 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.