Nitrogen transformation processes in a forest soil and their influence on tree growth
Tree growth, litterfall and litter decomposition were measured in a stand of 39 year old Sitka spruce (Picea sitchensis, yield class 16), near Aberdeen. A split plot field experiment was used to examine the effect of nitrogen addition [200 kg ha-1 N as (NH4) 2SO4] and carbon addition (1000 kg ha 1 of soluble starch) in the main plots, and trenching in the subplots. Weight loss from litterbags was highest in the carbon treated plots (24%) while that in the nitrogen treated plots was approximately the same as that in the control (22%). Highest levels of nitrogen availability and total nitrogen concentration occurred in the litter of the nitrogen treated plots throughout the course of the experiment. Small amounts of nitrogen were immobilised in the control plots during the first 8 weeks of the experiment. A 3.5% increase in the total amount of nitrogen initially present was observed, this probably being derived from rainfall inputs. This rose to 7.1% in the carbon treated plots during the first 20 weeks, possibly as a result of increased microbial activity increasing the proportion of rainfall nitrogen being immobilised. During the same period, litter in the nitrogen treated plots immobilised 9.7% of total nitrogen initially present as a result of higher nitrogen availabilities in these plots. Rates of CO2 production were measured in the field and found to be highest in the carbon treated plots. Seasonal patterns of litterfall were identified with peaks of production occurring in June and October. Greatest amounts of litter fell in the control plots (3410 kg ha-1 y-1), although the needle litter in those plots also had the lowest nitrogen concentration (ranging from 0.75-1.14% N). Unexpectedly, trenching (the exclusion of live roots from a small plot of soil in the field) slightly decreased the rate of litter decomposition. It also increased the moisture content, availability of nitrogen, and amount of soluble organic nitrogen in the litter. Probably as a result of an adequate nitrogen status at the beginning of the experiment, little increase in tree growth was observed in response to added nitrogen. In an attempt to extend and reinforce the data obtained from the field, a laboratory experiment was designed in which pots of litter were incubated with starch and nitrogen and starch at levels comparable with those in the field. In contrast to the field experiment, nitrogen mineralisation was not increased by the addition of starch, although there was a substantial increase in the amount of KC1-extractable nitrate. The value of using measurements of CO2 evolution to estimate rates of nitrogen mineralisation are discussed.