Carbon and nitrogen mineralization in soil microcosms from a northern hardwood forest show no response to phosphorous additions

Background/Question/Methods

Anthropogenic nitrate deposition is altering soil nitrogen enrichment and acidification, affecting biogeochemical processes in northern hardwood forests.  High nitrogen availability can influence organic matter (SOM) cycling by depressing degradation of recalcitrant soil organic matter. In some ecosystems, available P limits decomposer activity and this may be linked to SOM quality.  We need to better understand responses by microbial decomposer organisms to these important nutrients, because of their potential influence on ecosystem carbon sequestration.

We manipulated N&P availability in laboratory microcosms that were amended with several different C sources of varying complexity. Organic horizon material was collected in mid-July from 3 beech-dominated mature plots in the Hubbard Brook Experimental Forest (Thornton, NH) and incubated with factorial N, P, dextrose, cellulose, xylan and sawdust additions. Carbon mineralization was measured intensively, at several-hour intervals, for several days and then at several-day intervals over 23 days, and N mineralization was quantified by extracting inorganic N at the beginning and end of incubations.

Results/Conclusions

Nitrogen fertilization depressed respiration and increased nitrogen mineralization, and we did not observe any response to phosphorus fertilization.  All carbon additions generated significantly different substrate induced respiration effects in a whole-model comparison (MANOVA d.f. (4, 55), P < 0.0001). The clearest differences between C addition were visible in the first 70 hours and a time*C*NP interaction was significant (Wilk’s lambda, d.f.(36, 113), P = 0.018). While carbon substrates of differing recalcitrance stimulated growth at contrasting rates , they did not influence the response of C and N mineralization to nutrient additions. Nitrogen mineralization was not affected by P and increased in response to N and N+P additions (ANOVA d.f. (3, 56) P < 0.0001 and Tukey-Kramer, P < 0.0001), across carbon addition types. However, glucose showed depressed N mineralization in comparison to the control and other carbon additions.

Our results show a lack of interaction between N and P and suggest that high N availability actually increases mineralization of additional N, for a variety of C sources.  The mechanism underlying this response may be of interest for understanding N saturation processes.