Effects of annual grass senescence on 15NH4+ and 15N-glycine uptake by blue oak (Quercus douglasii) seedlings and soil microorganisms in California oak woodland

Annual grasses are stronger competitors for available soil N than blue oak seedlings and soil microorganisms. However, little is known about the dynamics of N competition during annual grass senescence. We conducted a field experiment in a California oak woodland to study effects of annual grass senescence on N uptake by grasses, blue oak seedlings and soil microorganisms. Labeled N was applied at the beginning of April, May and of June in the form of 15NH4+ or 15N-glycine. Plants and soils were harvested after 5 days (15NH4+ and 15N-glycine treatments) and after 26 days (15NH4+ treatment only). We evaluated effects of N form, season and labeling period on N competition among oak seedlings, annual grasses and soil microorganisms. N forms did not affect competition among grasses, oak seedlings and soil microorganisms, but more 15N was incorporated into the soil organic N pool in the 15N-glycine treatments than in the 15NH4+ treatments. There were no seasonal (May vs June) effects on 15N recovery in blue oak seedlings and soil microorganisms. Plant samples from April harvest were lost. In June, when grasses were senescing, more 15N was found in the soil inorganic pool than in May. Extremely dry soils in June may have limited inorganic N availability to oak seedlings and soil microorganisms. After 26-day labeling period, 15N recovery in blue oak seedlings and the soil organic N pool significantly increased, while 15N recovery in both the soil microbial and inorganic N pools decreased compared to the 5-day labeling period. Although blue oak seedling biomass changed little from early May to late June, N concentrations in oak roots increased 53%. In contrast, annual grass biomass peaked in May, and then decreased rapidly. Our results suggest that blue oak seedlings and annual grasses have different temporal competitive abilities. Blue oak seedlings appear to have a long-term strategy for N competition. Blue oaks take up N slowly but steadily, increasing N uptake from 5 to 26 days. This extended time period has a greater positive effect on N uptake than does reduced grass uptake caused by senescence.