Microclimatic control of microbial C, N, and P pools in Spodosol Oa horizons

We examined effects of soil moisture and temperature on CHCl3-labile C, N, and P (fumigation-extraction method) in Oa horizons of two Spodosols. Temperature, moisture, and CHCl3-labile pools were observed from May to November 1994 in an unmanipulated (reference) plot in a spruce forest and, except for P, in experimental plots in both spruce and northern hardwood forests, where temperature was regulated with buried heating cables and clear plastic tarps were used to exclude rain. Spruce reference plot microbial C, N, and P were 5.9, 0.49, and 0.092 g.kg(-1), respectively, and comprised 1.2, 4.8, and 21% of the elemental totals for the Oa horizon. Pools were positively correlated with moisture (r = 0.38-0.49; p = 0.0001), and N/C in CHCl3-labile pools was positively correlated with temperature (r = 0.34; p = 0.0007). Neither temperature nor moisture explained a decline in P/C over time. In control plots for the hardwood soil warming study, microbial C and N were 6.4 and 0.74 g.kg(-1), comprising 1.5 and 3.4% of total Oa horizon pools, respectively. Excluding rain reduced variability in CHCl3-labile C in Oa horizons of both forests (p < 0.05). Warming decreased CHCl3-labile C and N in hardwood Oa horizons (p = 0.01) by 0.05 and 0.007 g.kg(-1).degrees C-1, respectively. Spruce forest results suggested a net transfer of P out of microbial pools during the growing season.