It would be great to get access to this report and assess their conclusion that “Biochar addition could possibly increase soil carbon stocks but it is not yet an economical option for large-scale application in New Zealand.”
Management practices to reduce losses or increase soil carbon stocks in temperate grazed grasslands: New Zealand as a case study
DavidWhiteheadaLouis A.SchipperbJackProngercGabriel Y.K.MoinetaPaul L.MudgecRobertoCalvelo PereiradMiko U.F.KirschbaumeSam R.McNallyfMike H.BearefMartaCamps-Arbestaind
- aManaaki Whenua – Landcare Research, PO Box 69040, Lincoln 7640, New Zealand
- bEnvironmental Research Institute, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
- cManaaki Whenua – Landcare Research, Private Bag 3127, Hamilton 3240, New Zealand
- dSoil & Earth Sciences Department, School of Agriculture & Environment, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand
- eManaaki Whenua – Landcare Research, Private Bag 11052, Palmerston North 4442, New Zealand
- fNew Zealand Institute for Plant and Food Research, Canterbury Agriculture and Science Centre, Private Bag 4704, Christchurch Mail Centre, Christchurch 8140, New Zealand
Received 12 December 2017, Revised 18 June 2018, Accepted 22 June 2018, Available online 8 July 2018.
- •We review farm management options to increase grassland soil carbon stocks.
- •Carbon saturation deficit defines the potential to increase soil carbon stocks.
- •Increasing carbon stock is dependent on carbon inputs and stabilisation processes.
- •Models highlight trade-offs between increasing soil carbon and milk production.
- •We recommend assessment criteria and priorities for further research.
Even small increases in the large pool of soil organic carbon could result in large reductions in atmospheric CO2 concentrations sufficient to limit global warming below the threshold of 2 °C required for climate stability. Globally, grasslands occupy 70% of the world’s agricultural area, so interventions to farm management practices to reduce losses or increase soil carbon stocks in grassland are highly relevant. Here, we review the literature with particular emphasis on New Zealand and report on the effects of management practices on changes in soil carbon stocks for temperate grazed grasslands. We include findings from models that explore the trade-offs between multiple desirable outcomes, such as increasing soil carbon stocks and milk production.
Farm management practices can affect soil carbon stocks through changes in net primary production, the proportions of biomass removed, the degree of stabilisation of carbon in the soil and changes to the rate of soil carbon decomposition. The carbon saturation deficit defines the potential for a soil to stabilise additional carbon. Earlier reviews have concluded that, while labile carbon is the dominant substrate for soil carbon decomposition, a fraction of soil carbon stocks is stabilised and protected from decomposition by the formation of organo-mineral complexes. Recent evidence shows that the rate of organic carbon decomposition is determined primarily by the extent of soil organic carbon protection and, therefore, the availability of substrates to microbial activity.
New Zealand grassland systems have moderate to high soil carbon stocks in the surface layers (i.e., upper 0.15 m) where most roots are located, so the carbon saturation deficit is relatively low and the scope to increase soil carbon stocks by carbon inputs from primary production may be limited. International studies have shown that the addition of fertilisers, feed imports, and applications of manure and effluent can increase soil carbon stocks, especially for degraded soils, but the responses in New Zealand soils are uncertain because of the limited number of studies. However, recent evidence shows that irrigation can reduce soil carbon stocks in New Zealand, but neither the processes nor the long-term trends are known. Studies of sward renewal have shown that short-term losses of carbon losses resulting from the disturbance can be mitigated using rapid replacement of the new sward, minimum tillage and avoidance of times when the soil water content is high. Swards comprising multiple species have also shown that soil carbon stocks may be increased after periods of several years. Model simulations have shown that the goal of increasing both soil carbon and milk production could be achieved best by increasing carbon inputs from supplementary animal feed. However, losses of carbon at feed export sites need to be minimised to achieve overall net gains in soil carbon. Grazing intensity can have a big influence on soil carbon stocks but the magnitude and direction of the effects are not consistent between studies.
Biochar addition could possibly increase soil carbon stocks but it is not yet an economical option for large-scale application in New Zealand. There is some evidence that the introduction of earthworms and dung beetles could potentially increase soil carbon stabilisation, but the greenhouse gas benefits are confounded by possible increases in nitrous oxide emissions. The new practice of full inversion tillage during grassland renewal has the potential to increase soil carbon stocks under suitable conditions but full life-cycle analysis including the effects of the disruptive operations has yet to be completed.
We conclude with a list of criteria that determine the success and suitability of management options to increase soil carbon stocks and identify priority research questions that need to be addressed using experimental and modelling approaches to optimise management options to increase soil carbon stocks.