Changes in soil carbon stocks of New Zealand’s grazed pasture in response to variations in management and environmental factors.

Dairy farming is New Zealand‟s pre-eminent primary industry. It achieves large export earnings but is also responsible for a large proportion of the country‟s greenhouse gas emissions. One of those greenhouse gases is CO₂, and in order to lower New Zealand‟s net greenhouse gas emissions, it is important to identify any management options that can lead to carbon sequestration in pasture soils and thereby minimise net CO₂ emissions to the atmosphere. It is equally important to understand what factors could lead to losses of soil carbon from pasture soils and thereby add further to New Zealand‟s CO₂ emissions. We addressed these questions by using two years of observations from an eddy-covariance system on a dairy farm in the Waikato that provided estimates of the exchanges of water and CO₂ with the atmosphere. We used CenW 4.1, a process-based ecosystem model, to describe these observations in terms of their biophysical drivers and the interactions between them. Agreement between the model and observations was excellent, especially for evapotranspiration and net photosynthesis, for which 91% and 79% of observed daily variations could be explained. The validated model was then used to run different scenarios to assess the effects on soil organic carbon of changes in the application of fertiliser and irrigation water, grazing scheduling, differences in plant-internal resource allocation, and changes in temperature and CO₂ concentration. We found that it was important to consider the combined effect of changes in net primary production, the amount of carbon taken off-site through grazing, the proportion of carbon allocated to pools, especially pools in the soil, that facilitates the stabilisation of carbon in organic matter, and any changes in the rate of organic matter decomposition. Soil organic carbon stocks were positively correlated with rates of fertiliser application and with the rate of water application (rain or irrigation) up to some moderate water application rates. For other changes in key properties, changes in soil organic carbon were often negatively correlated with changes in milk production. That was clearly evident for changes in the grazing regime and in plant root:shoot ratios. Anticipated environmental changes, such as increases in temperature and CO₂ concentration, and both increases and decreases in precipitation from moderate values had either neutral or detrimental effects on soil organic carbon stocks. Milk production was generally more positively affected under most environmental changes

A comprehensive assessment of anthropogenic and natural sources and sinks of Australasia\u27s carbon budget

Regional carbon budget assessments attribute and track changes in carbon sources and sinks and support the development and monitoring the efficacy of climate policies. We present a comprehensive assessment of the natural and anthropogenic carbon (C-CO2) fluxes for Australasia as a whole, as well as for Australia and New Zealand individually, for the period from 2010 to 2019, using two approaches: bottom-up methods that integrate flux estimates from land-surface models, data-driven models, and inventory estimates; and top-down atmospheric inversions based on satellite and in situ measurements. Our bottom-up decadal assessment suggests that Australasia\u27s net carbon balance was close to carbon neutral (−0.4 ± 77.0 TgC yr−1). However, substantial uncertainties remain in this estimate, primarily driven by the large spread between our regional terrestrial biosphere simulations and predictions from global ecosystem models. Within Australasia, Australia was a net source of 38.2 ± 75.8 TgC yr−1, and New Zealand was a net CO2 sink of −38.6 ± 13.4 TgC yr−1. The top-down approach using atmospheric CO2 inversions indicates that fluxes derived from the latest satellite retrievals are consistent within the range of uncertainties with Australia\u27s bottom-up budget. For New Zealand, the best agreement was found with a national scale flux inversion estimate based on in situ measurements, which provide better constrained of fluxes than satellite flux inversions. This study marks an important step toward a more comprehensive understanding of the net CO2 balance in both countries, facilitating the improvement of carbon accounting approaches and strategies to reduce emissions

Ensemble modelling, uncertainty and robust predictions of organic carbon in long-term bare-fallow soils

ACKNOWLEDGEMENTS This study was supported by the project “C and N models inter-comparison and improvement to assess management options for GHG mitigation in agro-systems worldwide” (CN-MIP, 2014- 2017), which received funding by a multi-partner call on agricultural greenhouse gas research of the Joint Programming Initiative ‘FACCE’ through national financing bodies. S. Recous, R. Farina, L. Brilli, G. Bellocchi and L. Bechini received mobility funding by way of the French Italian GALILEO programme (CLIMSOC project). The authors acknowledge particularly the data holders for the Long Term Bare-Fallows, who made their data available and provided additional information on the sites: V. Romanenkov, B.T. Christensen, T. Kätterer, S. Houot, F. van Oort, A. Mc Donald, as well as P. Barré. The input of B. Guenet and C. Chenu contributes to the ANR “Investissements d’avenir” programme with the reference CLAND ANR-16-CONV-0003. The input of P. Smith and C. Chenu contributes to the CIRCASA project, which received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no 774378 and the projects: DEVIL (NE/M021327/1) and Soils‐R‐GRREAT (NE/P019455/1). The input of B. Grant and W. Smith was funded by Science and Technology Branch, Agriculture and Agri-Food Canada, under the scope of project J-001793. The input of A. Taghizadeh-Toosi was funded by Ministry of Environment and Food of Denmark as part of the SINKS2 project. The input of M. Abdalla contributes to the SUPER-G project, which received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no 774124.Peer reviewedPostprin

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those ‘next users’ of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem\u27s carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those ‘next users’ of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem's carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers.</p

The temperature dependence of organic-matter decomposition - still a topic of debate

The temperature dependence of organic matter decomposition is of considerable ecosphysiological importance, especially in the context of possible climate-change feedback effects. It effectively controls whether, or how much, carbon will be released with global warming, and to what extent that release of carbon constitutes a dangerous positive feedback effect that leads to further warming. The present paper is an invited contribution in a series of Citation Classics based on a review paper of the temperature dependence of organic matter decomposition that was published in 1995. It discusses the context and main findings of the 1995 study, the progress has been made since then and what issues still remain unresolved. Despite the continuation of much further experimental work and repeated publication of summary articles, there is still no scientific consensus on the temperature dependence of organic matter decomposition. It is likely that this lack of consensus is largely due to different studies referring to different experimental conditions where confounding factors play a greater or lesser role. Substrate availability is particularly important. If it changes during the course of measurements, it can greatly confound the derived apparent temperature dependence. This confounding effect is illustrated through simulations and examples of experimental work drawn from the literature. The paper speculates that much of the current disagreement between studies might disappear if different studies would ensure that they are all studying the same system attributes, and if confounding factors were always considered and, if possible, eliminated

Climate-change impact potentials as an alternative to global warming potentials

For policy applications, such as for the Kyoto Protocol, the climate-change contributions of different greenhouse gases are usually quantified through their global warming potentials. They are calculated based on the cumulative radiative forcing resulting from a pulse emission of a gas over a specified time period. However, these calculations are not explicitly linked to an assessment of ultimate climate-change impacts. A new metric, the climate-change impact potential (CCIP), is presented here that is based on explicitly defining the climate-change perturbations that lead to three different kinds of climate-change impacts. These kinds of impacts are: (1) those related directly to temperature increases; (2) those related to the rate of warming; and (3) those related to cumulative warming. From those definitions, a quantitative assessment of the importance of pulse emissions of each gas is developed, with each kind of impact assigned equal weight for an overall impact assessment. Total impacts are calculated under the RCP6 concentration pathway as a base case. The relevant climate-change impact potentials are then calculated as the marginal increase of those impacts over 100 years through the emission of an additional unit of each gas in 2010. These calculations are demonstrated for CO _2 , methane and nitrous oxide. Compared with global warming potentials, climate-change impact potentials would increase the importance of pulse emissions of long-lived nitrous oxide and reduce the importance of short-lived methane

Options for including all lands in a future greenhouse gas accounting framework

The current framework through which greenhouse gas emissions and removals in the land use sector are accounted under the Kyoto Protocol has several problems. They include a complex structure, onerous monitoring and reporting requirements, and potential for omission of some important fluxes. One solution that may overcome some of these problems is to include all lands and associated processes within a country's jurisdiction, rather than restrict accounting to specific nominated land categories or activities. Ideally, the accounting approach should cover all significant biospheric sources and sinks, avoid biased or unbalanced accounting, avoid leakage and require no arbitrary adjustments to remedy unintended consequences. Furthermore, accounting should focus on the direct human-induced component of biospheric emissions/removals so that debits/credits can be allocated equitably and provide appropriate incentives to adopt land-use management options with beneficial outcomes for the atmosphere. This paper focuses on biospheric emissions and removals resulting from carbon stock changes. It considers four alternative accounting options that include all land areas: Gross-Net Accounting, Net-Net Accounting, Net Accounting with Negotiated Baselines and the Average Carbon Stocks approach. Each option is described, and assessed with respect to defined criteria for effectiveness. Gross-Net Accounting and Net-Net Accounting do not adequately distinguish the anthropogenic component of carbon-stock changes from indirect and natural effects, so large undeserved credits or debits could be created. Under Net Accounting with Negotiated Baselines, countries' projected emissions and removals during the commitment period would be taken into account in the negotiation of emissions targets. In the commitment period, countries would then gain credits/debits for biospheric removals/emissions. Difficulties with this approach would lie in reaching agreed baselines for emissions and removals for individual countries, and, if desired, in factoring out residual effects of natural variability on emissions/removals. Under the Average Carbon Stocks approach, debits/credits for changes in land use or management practices would be based on the changes in long-term average carbon stocks associated with changes in specific land use and management regimes. This approach thereby directly identifies the anthropogenic component, and assigns debits and credits accordingly. It may prove problematic, however, for countries to accept long-term averages rather than observable realised carbon-stock changes as the basis for accounting. Thus, none of the options is without its drawbacks, but Net Accounting with Negotiated Baselines and the Average Carbon Stocks approach could potentially be used as the basis of developing a future 'all lands' accounting framework