The role of microbes in retaining soil nitrogen with depth at Hubbard Brook
Understanding how nitrogen (N) is retained in ecosystems is important given N that often limits primary productivity in temperate forests. Ecological theory suggests that ecosystem nitrogen retention should be positively correlated with biomass accumulation of the ecosystem, meaning young forests with high rates of biomass accumulation should have high net N retention rates, whereas mature forests should retain little N. However, stream export at the Hubbard Brook Experimental Forest (New Hampshire) has decreased with time, despite steady N inputs and unchanging live biomass, coarse woody debris and forest floor pools, contradicting the theory. Given this N budget, the mineral soil is a likely candidate for the missing N sink. Microbes can play an important role in soil N retention, as microbes can (1) produce decomposition enzymes that release N from organic matter, (2) transform organic N into more plant available forms (NH4+, NO3-) through mineralization and nitrification, and (3) immobilize N into their biomass, thereby limiting NO3- leaching losses. This study aimed to determine the microbial mechanisms controlling N retention in mineral soil across succession at Hubbard Brook. To address this question, we measured decomposition enzyme activity, gross N transformation rates, and N stocks for deep soil cores (Oe/Oa and 0-50 cm mineral) collected across a chronosequence (a 40 yr, 90yr, and old-growth stand) at Hubbard Brook.
We expected to find the highest N-degrading enzyme activity at the youngest site, as evidence of N-mining by vegetation with a high N demand, and lowest activity in the old-growth site when plant N demand is low. However, N-degrading enzyme activity (g dry soil-1) in the mineral soil was significantly lower in the mid-aged stand as compared to the young or old-growth stands. N-degrading activity also decreased significantly with depth between the organic and mineral horizons across sites. Significant differences were also found in extractable NH4+ and NO3-between organic and mineral horizons but no differences in extractable NO3- were found across sites. The highest concentrations of NO3- were found at the old-growth site and lowest at the young site. Conversely, no difference was found in extractable NH4+ across sites or with depth through the mineral soil. These results suggest that depth, not stand age, may be the more important factor in understanding microbial control on N cycling and that microbial mechanism are likely not largely contributing to the N retention pattern observed at Hubbard Brook.