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Title: Effects of thinning, residue mastication, and prescribed fire on soil and nutrient budgets in a Sierra Nevada mixed conifer forest

item JOHNSON, DALE - University Of Nevada
item WALKER, ROGER - University Of Nevada
item GLASS, DALLAS - University Of Nevada
item STEIN, CHAD - University Of Nevada
item MURPHY, JAMES - University Of Nevada
item Blank, Robert - Bob
item MILLER, WATKINS - University Of Nevada

Submitted to: Forest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2013
Publication Date: 2/28/2014
Citation: Johnson, D.W., Walker, R.F., Glass, D.W., Stein, C.M., Murphy, J.B., Blank, R.R., Miller, W.W. 2014. Effects of thinning, residue mastication, and prescribed fire on soil and nutrient budgets in a Sierra Nevada mixed conifer forest. Forest Science. 60:170-179.

Interpretive Summary: To lessen the chance of catastrophic fires, forests in the Sierra Nevada are being thinned. There are limited data on the effect of various thinning practices on ecosystem stocks of carbon and nitrogen, and long-term effects on soil nutrient availability. We evaluated several thinning strategies and their effect of ecosystem carbon, nitrogen, phosphosus, potassium, sulfate, and calcium. The most ecologically significant effect was the loss of 13% of total ecosystem nitrogen from and combination of harvesting and prescribed burning. Prescribed burning also resulted in elevated availability of soil mineral nitrogen, calcium and sulfate two years after the burn.

Technical Abstract: The effects of thinning followed by residue mastication (THIN), prescribed fire (BURN), and thinning plus residue mastication plus burning (T+B) on nutrient budgets and resin-based (plant root simulator [PRS] probe) measurements of soil nutrient availability in a mixed-conifer forest were measured. Because of site differences, removals of carbon (C) and nutrients by harvesting were greater in the T+B than in the THIN (Mg) treatment. Harvesting caused greater exports of C, phosphorus, potassium (K), calcium (Ca), and magnesium than burning, harvesting and burning caused approximately equal exports of nitrogen (N), and burning caused greater exports of sulfur than harvesting. Burning caused greater volatilization losses of C and N from the T+B than from the BURN treatment because the addition of chips from mastication in the T+B treatment caused greater combustion of the native forest floor. The most significant effect of the chips on soil nutrients was the addition of K to the forest floor pools, which was also reflected in resin-based measurements of soil nutrients. Burning caused increases in soil NO3-–N, mineral N, Ca2+, and SO42- as measured by PRS probes, which persisted for 2 years after the burn. The more intense burning in the T+B treatment also caused elevated orthophosphate levels 2 years after the burn. Burning effects on K+ and Mg2+ were not significant. Overall, the most ecologically significant effect of treatment was the export of N from the combination of harvesting and burning in the T+B treatment, which totaled 932 kg ha-1 and 13% of the total ecosystem N capital.