Elevated CO₂ Impacts on Grazed Pasture: Long-Term Lessons from the New Zealand FACE

Elevated CO₂ Impacts on Grazed Pasture: Long-Term Lessons from the New Zealand FACE

A grazed pasture Free Air CO₂ Enrichment (FACE) experiment has been operating for almost 15 years in the Rangitikei Region of New Zealand, following earlier work studying CO₂, temperature and moisture effects on pasture in growth chambers. The longer-term results are reviewed with a focus on primary productivity, pasture compositional dynamics, nutrient cycling feedbacks and soil carbon supply. In terms of the direct effects of CO₂ elevated to 475 ppm, increases in primary production were reflected in belowground allocation and legume stimulation, with the latter diminishing in the longer term and annual aboveground net herbage accumulation showing a low and variable response over time. The grazing animal has strongly influenced nitrogen (N) transformations, in terms of carbon (C) and N decoupling and return to soil, a feedback that appears to mitigate the recognized effects of increased C inputs on nitrogen limitation. Nevertheless, soil mineral N has decreased and labile fractions of soil carbon have increased – effects only detectable over a decade. Key lessons unique to this study include the animal and soil feedbacks that have driven longer-term effects differing from earlier growth chamber and the results of FACE in the first few years. Remaining questions for further investigation include mechanisms influencing legume responses and the effects of elevated CO₂ on N processes (biological fixation, microbial processing, mineral N losses). The facility is currently undergoing major refurbishment in order to explore the effects of elevated CO₂ and interactions with temperature and moisture availability.