|Conant, Richard - CSU NREL, FT. COLLINS, CO|
|Paustian, Keith - CSU NREL, FT. COLLINS, CO|
|Del Grosso, Stephen|
|Parton, William - CSU NREL, FT. COLLINS, CO|
Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2004
Publication Date: March 1, 2005
Citation: Conant, R., Paustian, K., Del Grosso, S.J., Parton, W. 2005. Nitrogen pools and fluxes in grassland soils sequestering carbon. Nutrient Cycling in Agroecosystems. 71:239-248. Interpretive Summary: Carbon storage in soils has been promoted as a means to offset human related emissions of carbon dioxide (CO2) to the atmosphere. However, strategies intended to sequester carbon also impact nitrous oxide (N2O) emissions. This is important because N2O is a potent greenhouse gas and also impacts stratospheric ozone levels. We combined field data and modeling results from different grassland study sites and calculated carbon storage and N2O emission rates. Although changes in grassland management can sequester significant amounts of carbon, these changes can also increase N2O emissions and partially nullify the benefits of carbon storage. We conclude that when evaluating different land management strategies, changes in N2O emissions, as well as carbon levels, should be considered.
Technical Abstract: Carbon sequestration in agricultural, forest, and grassland soils has been promoted as a means by which substantial amounts of CO2 may be removed from the atmosphere, but few studies have evaluated the associated impacts on changes in soil N or net global warming potential (GWP). The purpose of this research was to (1) review the literature to examine how changes in grassland management that affect soil C also impact soil N, (2) assess the impact of different types of grassland management on changes in soil N and rates of change, and (3) evaluate changes in N2O fluxes from differently managed grassland ecosystems to assess net impacts on GWP. Soil C and N stocks either both increased or both decreased for most studies. Soil C and N sequestration were tightly linked resulting in little change in C:N ratios with changes in management. Within grazing treatments N2O made a minor contribution to GWP (0.1 to 4 percent), but increases in N2O fluxes offset significant portions of C sequestration gains due to fertilization (10-125%) and conversion (average=27%). Results from this work demonstrate that even when improved management practices result in considerable rates of C and N sequestration, changes in N2O fluxes can offset substantial portion of gains by C sequestration. Even for cases in which C sequestration rates are not entirely offset by increases in N2O fluxes, small increases in N2O fluxes can substantially reduce C sequestration benefits. Conversely, reduction of N2O fluxes in grassland soils brought about by changes in management represents an opportunity to reduce the contribution of grasslands to net greenhouse gas forcing.