Periodic cropping of pasture for summer-grazed turnips leads to substantial carbon loss

Abstract

Understanding how management practices impact carbon (C) cycling in agroecosystems is critical from the perspective of greenhouse gas emissions and to ensure ongoing soil quality, production and profitability of the land. Here, we quantified the net ecosystem C balance (NECB) of two grazed summer turnip crops grown as part of the pasture renewal process – a common management practice of New Zealand dairy grasslands. The NECB was calculated from measurements of net ecosystem production obtained using eddy covariance and measurements or estimates of all other major flows of C into and out of two adjacent paddocks. The NECB of the two turnip crop periods (∼7 months in length) indicated C losses of −539 and −596 g C m⁻² period⁻¹. Carbon was also lost from the adjacent pasture paddock during both crop periods, thus reducing the net effect (i.e., turnips minus pasture) of the periodic cropping of grazed turnips to −441 and −413 g C m⁻² period⁻¹. Returns of C during grazing via grazing wastage and excreta deposition offset some of the C lost from ecosystem respiration during the long periods with no or limited photosynthesis partly reducing total C loss. Soil C stocks were also measured via direct soil sampling to 0.6 m before and after the turnip crops. However, high spatial variability of direct soil sampling prevented corroboration of the C loss calculated by the NECB. The NECB approach was able to detect much smaller changes in carbon than practical soil sampling approaches allowed. We concluded that the production of grazed turnips for supplemental feed as part of the pasture renewal process resulted in significant C loss that must be recaptured under a return to pasture before further cropping to avoid a downward staircase of C stocks in these agroecosystems.