Elevated CO₂effects on decomposition processes in a grazed grassland

Abstract

The effects of elevated atmospheric CO 2 (475 lL L À1 ) on in situ decomposition of plant litter and animal faecal material were studied over 2 years in a free air CO 2 enrichment (FACE) facility. The pasture was grazed by sheep and contained a mixture of C 3 and C 4 grasses, legumes and forbs. There was no effect of elevated CO 2 on decomposition within plant species but marked differences between species with faster decomposition in dicots; a group that increased in abundance at elevated CO 2 . Decomposition of mixed herbage root material occurred at a similar rate to that of leaf litter suggesting that any CO 2 -induced increase in carbon allocation to roots would not reduce rates of decomposition. Sheep faeces resulting from a 'high-CO 2 diet' decomposed significantly slower during summer but not during winter. The overall outcome of these experiments were explored using scenarios that took account of changes in botanical composition, allocation to roots and the presence of herbivores. In the absence of herbivores, elevated CO 2 led to a 15% increase in the rate of mass loss and an 18% increase in the rate of nitrogen (N) release. In the presence of herbivores, these effects were partially removed (11% increase in rate of mass loss and 9% decrease in N release rate) because of the recycling occurring through the animals in the form of faeces.

The rate of progression and stability of progressive nitrogen limitation at elevated atmospheric CO2 in a grazed grassland over 11 years of Free Air CO2 enrichment

“…Nutrients returned to the soil through excretion, especially urine, have much greater potential for losses compared to returns via litter where C and N are combined with the potential for loss greater at elevated CO 2 in some circumstances (Allard et al 2003). Grazing also influences botanical composition through direct selection of species for consumption and through indirect processes such as physical damage by treading and changes in botanical composition may modify nutrient cycling (see Allard et al 2004 for an example from this experiment).…”

Climate Impacts on Agriculture: Implications for Forage and Rangeland Production

“…Nutrient cycling is sensitive to changes in plant species composition, which in turn is sensitive to global climate change. Shift s in the abundances or composition of species that diff er in C/N could aff ect element cycling (Weatherly et al, 2003;Allard et al, 2004;King et al, 2004;Dijkstra et al, 2006;Gill et al, 2006;Shaeff er et al, 2007). Increasing [CO 2 ] may reduce decomposition by altering the leaf litter N concentration (Gill et al, 2006); however, Norby et al (2001) found that litter quality may not be the best predictor of tissue decomposition.…”

Selective grazing modifies previously anticipated responses of plant community composition to elevatedCO2in a temperate grassland

“…Globally, managed grasslands are an important mixed‐species community covering 25% of the land area (Asner et al ., ) and providing services that include food and fibre supply, erosion control and a potential sink for carbon (C). Experiments with grasslands almost invariably report botanical changes under eCO 2 (Owensby et al ., ; Allard et al ., ; Morgan et al ., , ; Polley et al ., ) with consequences that include changes in the responsiveness of the system to CO 2 (Polley et al ., ), decomposition rates (Allard et al ., ), forage chemical composition (Allard et al ., ; Polley et al ., ) and soil C sequestration (Pendall et al ., ). Temperate grasslands support 35% of global ruminant livestock production (Radcliffe & Baars, ) and are very often mixed‐species pastures that frequently include legumes.…”