Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/11/2013
Publication Date: 4/27/2013
Citation: Skinner, R.H. 2013. Nitrogen fertilization effects on pasture photosynthesis, respiration, and ecosystem carbon content. Agriculture, Ecosystems and Environment. 172:35-41. Interpretive Summary: Well-managed pasture systems have the ability to help alleviate some of the adverse effects of climate change by increasing soil carbon sequestration, thus removing carbon dioxide from the atmosphere. However, it is not clear if increasing pasture productivity by increasing nitrogen fertilization can lead to more soil carbon sequestration than would occur in unfertilized or low fertility pastures. This nine-year study found that increasing nitrogen fertilization increased photosynthesis (carbon inputs) and forage yield, but also increased respiration (carbon loss) from grazed pastures under a temperate central Pennsylvania climate, resulting in no change in the net amount of carbon entering or leaving the system. Because of the increased forage yield with increased fertilization, more carbon was also removed from the system through hay harvests or by grazing animals which increased the total carbon loss. In this case, increasing nitrogen fertilization was not an effective strategy for increasing soil carbon sequestration.
Technical Abstract: Some studies have shown that increasing nitrogen (N) fertility can increase soil carbon (C) sequestration, whereas others suggest that N fertilization has no effect on sequestration. Increasing N fertilization typically increases annual photosynthetic C uptake (gross primary productivity or GPP) and forage yield but also increases ecosystem respiration (Re), such that net ecosystem exchange (NEE) and soil C sequestration can increase, remain unchanged, or decrease depending on the relative impact of the fertilizer application on these two competing processes. A nine-year study monitoring carbon dioxide flux at two pasture sites examined in detail the relationship between photosynthesis and respiration under a range of environmental conditions. As expected, forage yield was highly correlated with GPP (P = 0.0006). However, yield was also positively correlated with Re (P = 0.0004), and no significant relationship existed between NEE and GPP or Re. Regressing Re against GPP yielded an equation with y-intercept near zero and slope of -0.96 (R**2 = 0.87, P is less than 0.0001) (GPP is negative and Re is positive in accordance with the meteorological sign convention). Although the results suggest that GPP should exceed Re as productivity increased, the difference was so small that NEE was predicted to be only slightly negative even under the most productive conditions. Including other C inputs and outputs, such as forage removal or manure deposition caused increased N fertilization to increase the net loss of C from the ecosystem because of the increased forage yield and subsequent removal.