Impact of irrigation on soil carbon and nitrogen stocks

Dr Paul Mudge1, Dr Jack Pronger1, Alesha Roulston3, Dr Scott Fraser1, Dr Andre Eger2, Veronica Penny2, Danny Thornburrow1, Jamie Millar4, Prof Louis Schipper3

1Manaaki Whenua-Landcare Research, Hamilton, New Zealand, 2Manaaki Whenua-Landcare Research, Lincoln, New Zealand, 3University of Waikato, Hamilton, New Zealand, 4Pioneer brand products NZ , Hamilton, New Zealand

Irrigation in New Zealand has increased rapidly in recent decades, yet there is surprisingly little known about the impact of irrigation on soil carbon (C) and nitrogen (N) stocks. Processes affecting both C and N inputs and outputs are altered by irrigation and therefore it is not easy to predict the net effect on soil C and N stocks and direct measurements are needed. A recent study revealed that on average irrigated pastoral soils from 30 sites across New Zealand had significantly less C and N than adjacent unirrigated pastures, with differences of 7 t C ha–1 and 0.6 t N ha–1 in the uppermost 0.3 m. Causes for these differences in C and N stocks are not well understood, but could have important implications for national carbon budgets and soil quality. Subsequently, we have sampled an additional 70 paired sites to determine whether the impact of irrigation on soil C and N differs by region, soil type and irrigation duration. Initial results from a directly aligned MSc student project focusing on Pumice Soils in the Reporoa Basin are consistent with the previous study, with less C in the irrigated compared to the unirrigated soils. Soil samples from paired sites in Hawkes Bay, Wairarapa, Canterbury and Otago are currently being analysed for total C and N and results from the full set of 100 paired sites (sampled to ≥0.3 m depth) will be presented. For a subset of paired sites we are also investigating what pools of C and N differ, rates of C and N cycling, the sensitivity of respiration to temperature and microbial community composition. Our ultimate aim is to identify where and how irrigation can be used to maintain, or increase soil organic matter and the multiple associated benefits.


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