Short And Long-Term Effects Of Nitrogen Fertilization On Carbon And Nitrogen Cycles In Northeastern Forest Soils

Abstract

Humans now dominate the nitrogen cycle by producing more biologically available nitrogen (N) than all natural processes combined. Over the past 150 years, northeastern forests have seen a four to ten-fold increase in N deposition as a result of anthropogenic activities such as agriculture and industry. Most of this added N is retained in surface soils. N deposition has the potential to alter atmospheric carbon dioxide (CO2) by influencing rates of decomposition of the vast reservoir of soil organic matter. N deposition may also change the soil nitrogen cycle, influencing the ability of soils to retain N. The objective of this work was to measure the response of decomposition and N retention to added N in northeastern forest soils. I used a cross-site lab experiment with soils from five long-term N addition experiments to measure the effects of long-term N fertilization on CO2 release, microbial biomass, heavy soil C (ie: mineral-associated), light soil C (ie: not mineralassociated), dissolved organic carbon (DOC), and the activities of four C-degrading enzymes. In response to a short-term pulse of N applied to soils in the lab, I focused on changes in CO2 and C degrading enzymes. To study the effects of N addition on N retention, I added a 15N tracer to incubating soils, and measured the partitioning of the tracer into several soil N pools. I found evidence to support the hypothesis that N fertilization can stimulate decomposition of easily degraded labile C such as sugars and starches; I measured transient increases in CO2 respiration and a cellulose-degrading enzyme with a shortterm pulse of N at one site. N inhibited decomposition of harder to degrade, ligninbased C compounds, decreasing CO2 respired during the first week of a year long incubation at four sites, along with microbial biomass and the lignin-degrading enzyme phenol oxidase. Soils with more organic matter appear to retain more N; light fraction 15N retention was positively correlated with soil %C. N addition reduced 15N retention in mineral soils.