Nitrogen Fertilization Increases Pasture Canopy Photosynthesis and Ecosystem Respiration.

Proper management of agricultural lands has the potential to increase soil C sequestration and help reduce the rate that CO₂ concentration is increasing in the atmosphere. Grazing lands, in particular, are thought to have a high C sequestration potential. Eddy covariance systems have continuously monitored CO₂ flux from two low-input temperate pastures since January 2003. These pastures received only limited fertilizer inputs and as a consequence had relatively low productivity. Averaged over the first five years of C flux measurements (2003-2007) net ecosystem exchange (NEE) on one pasture was 69 g CO₂ m^-2 yr^-1 (positive values indicate a net loss of CO₂ to the atmosphere, whereas negative values indicate net ecosystem uptake). Beginning in 2008, we applied 200-250 kg N ha^-1 yr^-1 to one of the two pastures to determine if improved fertility could increase gross primary productivity (GPP) but also to determine if concurrent increases in ecosystem respiration (Re) would offset the gains in GPP. Gross primary productivity averaged over three years (2008-2010) increased 20% under the higher fertility regime compared to the first five years, but Re also increased by 16%. The net result was to change NEE from a net source to a net sink of -118 g CO₂ m^-2 yr^-1. It appears that increased N fertilization of previous low-input pastures can improve soil C sequestration despite the concurrent increase in Re.