Soil Responses to Management, Increased Precipitation, and Added Nitrogen in Ponderosa Pine Forests

Abstract

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.

Functional groups show distinct differences in nitrogen cycling during early stand development: implications for forest management

“…Means with the same letter within each year are not significantly different (Fisher's LSD,α=0.05) presented) indicated N UP and root biomass increased concurrently. Lower N UP in I relative to F and IF shows site nutrient supply capacity (i.e., N MIN ) was not enhanced by irrigation and suggests native N supply capacity, not moisture availability (Groffman et al 2009;Hungate et al 2007), limited N UP at this site.…”

Long-term antagonistic effect of increased precipitation and nitrogen addition on soil respiration in a semiarid steppe

“…Changing precipitation regimes (Zhang et al., ) and atmospheric N deposition (Erisman et al., ; Gruber & Galloway, ) can both affect plant growth and C uptake via altering soil water and N availability especially in ecosystems where they are limited (LeBauer & Treseder, ; Niu et al., ), with consequent impacts on autotrophic and heterotrophic soil respiration. Numerous previous studies have reported soil respiration responses to precipitation change (Flanagan et al., ; Liu et al., ; Suseela et al., ; Wan, Norby, Ledford, & Weltzin, ; Yan et al., ) or N addition (Chen, Li, Lan, Hu, & Bai, ; Graham et al., ; Janssens et al., ; Ramirez, Craine, & Fierer, ), but few have investigated the combined effects of these two factors (Eisenhauer, Cesarz, Koller, Worm, & Reich, ; Hungate, Hart, Selmants, Boyle, & Gehring, ; Niu et al., ). Meta‐analyses have revealed that the responses of plant biomass to N addition increase with mean annual precipitation (Harpole, Potts, & Suding, ; Xia & Wan, ).…”

Forest restoration treatments have subtle long‐term effects on soil C and N cycling in mixed conifer forests

“…In semiarid mixed conifer forests like those prevalent throughout the intermountain west, moisture frequently limits primary productivity (e.g., Hungate et al. ), but productivity in these low‐N adapted forests frequently responds positively to N additions (Shafii et al. , Brockley et al.…”