Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2016
Publication Date: 4/15/2016
Citation: Skinner, R.H., Goslee, S.C. 2016. Defoliation effects on pasture photosynthesis and respiration. Crop Science. 56:2045-2053. doi:10.2135/cropsci2015.12.0733.
Interpretive Summary: Pastures have the ability to store carbon in the soil and help mitigate global climate change, but the amount of carbon that can be stored depends on many factors including how the pastures are managed. This study was undertaken to determine how the type of defoliation, i.e. cutting vs. grazing affects the uptake and loss of carbon from pasture systems. Pastures that were cut experienced a larger reduction in carbon uptake immediately after defoliation (70% vs. 46%) and the reduction lasted longer (five weeks vs. three weeks) than when pastures were grazed. The loss of carbon after defoliation due to plant and soil respiration did not differ from the rate of loss before defoliation, regardless of the defoliation method. However, on average, the rate of carbon loss was greater when pastures were cut compared to when they were grazed. This study suggests that grazed pastures should be able to store more soil carbon than pastures that are cut and thus, play a greater role in mitigating climate change.
Technical Abstract: Ecosystem C gain or loss from managed grasslands can depend on the type and intensity of management practices that are employed. However, limited information is available at the field scale on how the type of defoliation, specifically grazing vs. cutting, affects gross primary productivity (GPP) and ecosystem respiration (RE) immediately after defoliation and during the regrowth process. This study takes advantage of daily field-scale micrometeorological measurements of GPP and RE made over a nine-year period on two pastures that were grazed or cut approximately five times per year to examine the relative effects of grazing and cutting on GPP and RE. The null hypotheses were that no difference existed between defoliation regimes in the initial reduction in GPP and RE following defoliation or in the time required for GPP and RE to recover to pre-defoliation levels. Overall, RE was greater when pastures were cut compared to when they were grazed. Regardless of defoliation method, RE did not change immediately after defoliation, or during the six weeks following. The initial reduction in GPP2000 (GPP at full sunlight) was greater when pastures were cut (70%) than grazed (46%) and recovery to pre-defoliation levels took five weeks when pastures were cut compared to three weeks when grazed. This study suggests, all other management practices being the same that grazed pastures should be stronger annual C sinks than pastures that are cut. However, cut and grazed pastures are rarely managed in the same way so that the realized balance between GPP and RE will depend on both physiological responses to defoliation and management intensity.