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Title: Grazing and Nitrous Oxide

item Del Grosso, Stephen - Steve

Submitted to: Nature
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2010
Publication Date: 4/8/2010
Citation: Del Grosso, S.J. 2010. Grazing and Nitrous Oxide. Nature. 464:843-844.

Interpretive Summary: Nitrous oxide (N2O) is an important greenhouse gas that also affects atmospheric chemistry and stratospheric ozone levels. Agriculture is the primary anthropogenic source, with both cropped and grazed soils being major contributors. High grazing intensity is thought to increase N2O emissions because grazing increases the rate of nitrogen cycling and faster cycling generally leads to higher nitrogen losses from the soil-plant-animal system. However, on page xxx of this issue, Wolf et al. present evidence that grazing decreases emissions from semi-arid grasslands that experience soil freeze/thaw cycles during the spring season. When these types of grasslands are not grazed, the majority of annual emissions often occur during spring when sub-surface soil layers are still frozen and melting snow saturates surface soil layers, creating anaerobic conditions which facilitate denitrification, one of the primary biochemical pathways of N2O emissions. Grazing, however, reduces above ground standing biomass and plant height, so less snow is captured and soil water content during spring thaw is diminished. Less surface biomass and snow cover also reduce insulation and lead to more extreme diurnal fluctuations of soil temperatures. Lower water inputs from melting snow and colder low temperatures inhibit the soil microbial activity that is responsible for N2O emissions. Consequently, current methodologies that do not account for how grazing intensity impacts soil environmental conditions and microbial activity probably over-estimate emissions from some grasslands where snow melt and freeze/thaw cycles are important processes.

Technical Abstract: Nitrous oxide emission rates are typically measured using ‘bottom up’ methods which involve placing airtight chambers over the soil surface and measuring the change in gas concentration over short time periods (e.g., 15-60 min.). Emission rates vary substantially both spatially and temporally and many studies are limited because of low sampling frequency (less than one sample per week) or by sampling only during the growing season (i.e., when plants are active). Wolf et al.1 measured emission rates year round every 3 hours using automated chambers at 2 sites and weekly using manual chambers at 8 sites in Mongolian grasslands under different stocking rates ranging from ungrazed to heavily grazed. During the growing season, emissions increased with grazing intensity at most of the sites, consistent with previous studies and with models commonly used to estimate emissions. But in these grasslands, the majority of annual emissions from the ungrazed sites occurred during the non-growing season. In contrast, only a small portion of annual emissions (less than 10% on average) occurred during the non-growing season at the heavily grazed sites. On an annual basis, emissions were over two times greater from the ungrazed compared to the heavily grazed sites. This shows that previous studies that only considered growing season emissions could lead to faulty conclusions regarding the impact of grazing on total N2O emissions.