Alteration of pasture root carbon turnover in response to superphosphate and irrigation at Winchmore New Zealand

Abstract

Grassland net primary productivity, carbon (C) allocation to roots, root production and subsequent turnover time in grazed pastures have significant implications for modelling landuse effects on global carbon dynamics. The objective of this work, using the decline of assimilated ¹³C, was to quantify C allocation to roots, root production and turnover time in response to long-term fertiliser and irrigation treatments. The treatments on the pastures on a long-term research site at Winchmore, Canterbury, New Zealand were 0 or 375 kg ha⁻¹ y⁻¹ superphosphate under irrigation and, on a nearby irrigation experiment, unirrigated or irrigated (with fertilisation of 250 kg ha⁻¹ y⁻¹ superphosphate) when the soil water content fell to 20% w/w (50% available moisture). The pasture treatments were pulse-labelled using ¹³CO₂ within portable gas-tight enclosures. Separate micro-plots were ¹³CO₂ pulse-labelled in late spring, summer and autumn. Below ground net primary production (206 g C m⁻² y⁻¹) was similar in unfertilised, unirrigated, fertilised and irrigated pastures, despite marked differences in above ground production. Unfertilised/irrigated and unirrigated/fertilised treatments had greater root biomass, root C allocation and longer root turnover time (1.9 y and 2.0 y, respectively) than fertilised and irrigated treatments (1.3 y). These root turnover times appeared to be consistent with the improved substrate quality attributed to the species found in the irrigated and fertilised treatments.

Keywords:

• below ground net primary production
• carbon pulse-labelling
• ¹³C recovery
• irrigation
• root allocation
• root turnover
• fertiliser

Acknowledgements

Experimental work and data analysis were conducted while the authors were employed by AgResearch. We thank Peter Carey and Yin Chow Woo for helping with field work and sample preparation, Anjana Rajendram of the University of Waikato Stable Isotope Unit for carbon analyses, Roger Cresswell for soil C analysis, David Whitehead of Landcare Research for the loan of a gas analyser, the staff of Winchmore, David Baird for statistical advice, the New Zealand Foundation for Research Science and Technology for funding this research. Finally, thanks go to both the Agricultural and Marketing Research and Development Trust and the New Zealand Fertiliser Manufacturer's Research Association for their support, which enabled this work to be completed after the sad and untimely death of Dr Dean Stewart.