“Nitrogen and Phosphorus Addition Affect Soil Respiration in Northern Hardwood Forests,” published in the August 2024 Ecosystems, describes findings from research the authors conducted over a 10-year experiment to assess the effects of N and P additions on soil respiration in 13 northern hardwood forests including the Hubbard Brook Experimental Forest.
The study found that N addition consistently suppressed soil respiration by up to 14%, primarily in warmer months. According to the authors, “the N suppression of soil respiration was detected primarily in the summer season when soil temperature is highest and fluxes are greatest.”
The suppressive effect of nitrogen on soil respiration is likely due to a reduction in microbial activity. When nitrogen is abundant, the soil’s microbes produce fewer enzymes needed to break down organic materials, such as lignin, a complex organic compound found in the cell walls of plants, particularly in wood and bark, that provides structural support and rigidity. This reduced enzyme activity slows the decomposition of organic matter, leading to less carbon dioxide being released from the soil. As a result, overall soil respiration decreases.
Interestingly, the suppressive effect of N was lessened when P was also added. In one period, adding P reduced N’s suppression from 24% to 1%, suggesting that P availability can offset the effects of added N, possibly by altering nutrient dynamics in the soil.
Unexpectedly, the authors observed a 118% increase in summer soil respiration over the 10-year study.
This increase could not be explained by soil temperature, nutrient treatments, or instrumentation changes. One hypothesis proposed by the authors is that increasing atmospheric CO2 levels might have stimulated photosynthesis, leading to increased belowground carbon allocation. However, the study also found that the increase in fine-root biomass from 2011 to 2020 was minimal, making it unlikely to fully explain the doubling of soil respiration. Solving that mystery is for another day.
