Nutrient cycling in relation to decomposition and organic-matter quality in taiga ecosystems

Abstract

A variety of evergreen and deciduous forests in the taiga of interior Alaska were studied over a 5-year period to examine how the chemical quality of forest-floor organic matter affected its rate of decomposition and mineral cycling within and outside the tree vegetation. Litterbag and respiration studies were used to monitor decomposition. Natural forest-floor substrates and others altered by addition of N, P, and K fertilizer and glucose as a carbon source were studied in the laboratory and field for rates of weight loss and O2 consumption. Forest floors differing in C/N ratios, including those deficient in N, were used to measure substrate quality influences on seedling growth, nutrient content, and tannin content. Microbial (bacteria and fungi) biomass was measured across a range of forest types along with pH, base saturation total pool sizes of N and P, and annual mineralization of organic matter per square metre. Under identical moisture and temperature conditions average respiration rates in evergreen forest-floor L, F, and H substrates were 1.8, 2.8, and 2.0 times less than in the corresponding deciduous forest horizons, respectively. Birch L and F horizons had respiration rates 11.5 times higher than the corresponding black spruce layers. Weight losses in birch L, F, and H horizons were 6, 3, and 2 times higher, respectively, than in the corresponding black spruce substrates. Substrates had a quality-dependent decay rate which did not change when they were relocated within or between sites indicating that measured field climatic differences were not as influential on decay rates as substrate quality components. Fungal biomass was significantly correlated with the quantity of organic matter in all sites (n = 15, r = 0.62) but correlations were better for deciduous (n = 9, r = 0.89), and evergreen (n = 6, r = 0.82) forests separately. Strong correlations exist also between grams of organic matter decayed per square metre per year and fungal biomass (n = 13, r = 0.86), and fungal biomass and grams of N and P mineralized per square metre per year (n = 14, r = 0.95) and (n = 11, r = 0.94, respectively). Seedlings on mineral-deficient substrates produced more tannins than the controls, and seedlings on substrates with widening C/N ratios had successively less tissue with lower N content, and proportionally more roots. Nitrogen content of litter fall in increasingly nitrogen-poor forest floors was correspondingly lower. Nitrogen content of litter fall on N rich forest floors and N fertilized forest floors was proportionately higher. Nitrogen withdrawal in leaves at senescence was inversely correlated with grams N mineralized per square metre per year in forest floors. Fertilization did not influence microbial processes in the field, though lab studies indicated a negative influence of NH4, P, and K on microbial respiration. Glucose added in the laboratory and field markedly increased forest-floor microbial respiration. In vitro glucose-induced increases in respiration were not influenced by addition of ammonium nitrate and were significantly depressed by addition of P and K. In the field, fertilization had no effect on either glucose-induced respiration or microbial biomass.