Photosynthetic limits on carbon sequestration in croplands

This is the final version. Available on open access from Elsevier via the DOI in this recordHow much C can be stored in agricultural soils worldwide to mitigate rising carbon dioxide (CO2) concentrations, and at what cost? This question, because of its critical relevance to climate policy, has been a focus of soil science for decades. The amount of additional soil organic C (SOC) that could be stored has been estimated in various ways, most of which have taken the soil as the starting point: projecting how much of the SOC previously lost can be restored, for example, or calculating the cumulative effect of multiple soil management strategies. Here, we take a different approach, recognizing that photosynthesis, the source of C input to soil, represents the most fundamental constraint to C sequestration. We follow a simple “Fermi approach” to derive a rough but robust estimate by reducing our problem to a series of approximate relations that can be parameterized using data from the literature. We distinguish two forms of soil C: ‘ephemeral C’, denoting recently-applied plant-derived C that is quickly decayed to CO2, and ‘lingering C,’ which remains in the soil long enough to serve as a lasting repository for C derived from atmospheric CO2. First, we estimate global net C inputs into lingering SOC in croplands from net primary production, biomass removal by humans and short-term decomposition. Next, we estimate net additional C storage in cropland soils globally from the estimated C inputs, accounting also for decomposition of lingering SOC already present. Our results suggest a maximum C input rate into the lingering SOC pool of 0.44 Pg C yr−1, and a maximum net sequestration rate of 0.14 Pg C yr−1 – significantly less than most previous estimates, even allowing for acknowledged uncertainties. More importantly, we argue for a re-orientation in emphasis from soil processes towards a wider ecosystem perspective, starting with photosynthesis.Biotechnology and Biological Sciences Research Council (BBSRC

Towards optimal use of phosphorus fertiliser

This is the final version. Available on open access from Nature Research via the DOI in this recordData availability: The dataset analysed during the current study is available from the corresponding author on reasonable request.Because phosphorus (P) is one of the most limiting nutrients in agricultural systems, P fertilisation is essential to feed the world. However, declining P reserves demand far more effective use of this crucial resource. Here, we use meta-analysis to synthesize yield responses to P fertilisation in grasslands, the most common type of agricultural land, to identify under which conditions P fertilisation is most effective. Yield responses to P fertilisation were 40–100% higher in (a) tropical vs temperate regions; (b) grass/legume mixtures vs grass monocultures; and (c) soil pH of 5–6 vs other pHs. The agronomic efficiency of P fertilisation decreased for greater P application rates. Moreover, soils with low P availability reacted disproportionately strong to fertilisation. Hence, low fertiliser application rates to P-deficient soils result in stronger absolute yield benefits than high rates applied to soils with a higher P status. Overall, our results suggest that optimising P fertiliser use is key to sustainable intensification of agricultural systems.De Heus b.v

Synthesizing the evidence of nitrous oxide mitigation practices in agroecosystems

This is the final version. Available from IOP Publishing via the DOI in this record. Data availability statement: The data that support the findings of this study are openly available at the following URL/DOI: https:// doi.org/10.17605/osf.io/2fjhw.Code availability statement: The code to reproduce the findings of this study is openly available at the following URL/DOI: https://doi.org/10.17605/osf.io/2fjhw. The synthesized data after curation is provided (Supplementary data).Nitrous oxide (N2O) emissions from agricultural soils are the main source of atmospheric N2O, a potent greenhouse gas and key ozone-depleting substance. Several agricultural practices with potential to mitigate N2O emissions have been tested worldwide. However, to guide policymaking for reducing N2O emissions from agricultural soils, it is necessary to better understand the overall performance and variability of mitigation practices and identify those requiring further investigation. We performed a systematic review and a second-order meta-analysis to assess the abatement efficiency of N2O mitigation practices from agricultural soils. We used 27 meta-analyses including 41 effect sizes based on 1119 primary studies. Technology-driven solutions (e.g. enhanced-efficiency fertilizers, drip irrigation, and biochar) and optimization of fertilizer rate have considerable mitigation potential. Agroecological mitigation practices (e.g. organic fertilizer and reduced tillage), while potentially contributing to soil quality and carbon storage, may enhance N2O emissions and only lead to reductions under certain pedoclimatic and farming conditions. Other mitigation practices (e.g. lime amendment or crop residue removal) led to marginal N2O decreases. Despite the variable mitigation potential, evidencing the context-dependency of N2O reductions and tradeoffs, several mitigation practices may maintain or increase crop production, representing relevant alternatives for policymaking to reduce greenhouse gas emissions and safeguard food security.Danish Council for Independent ResearchDanish Council for Independent ResearchEU H2020 Marie Skłodowska-Curie Action

Increasing trends of soil greenhouse gas fluxes in Japanese forests from 1980 to 2009

Forest soils are a source/sink of greenhouse gases, and have significant impacts on the budget of these terrestrial greenhouse gases. Here, we show climate-driven changes in soil GHG fluxes (CO2 emission, CH4 uptake, and N2O emission) in Japanese forests from 1980 to 2009, which were estimated using a regional soil GHG model that is data-oriented. Our study reveals that the soil GHG fluxes in Japanese forests have been increasing over the past 30 years at the rate of 0.31 Tg C yr−2 for CO2 (0.23 % yr−1, relative to the average from 1980 to 2009), 0.40 Gg C yr−2 for CH4 (0.44 % yr−1), and 0.0052 Gg N yr−2 for N2O (0.27 % yr−1). Our estimates also show large interannual variations in soil GHG fluxes. The increasing trends and large interannual variations in soil GHG fluxes seem to substantially affect Japan's Kyoto accounting and future GHG mitigation strategies

The Effects of Carbon Dioxide Removal on the Carbon Cycle

Increasing atmospheric CO2 is having detrimental effects on the Earth system. Societies have recognized that anthropogenic CO2 release must be rapidly reduced to avoid potentially catastrophic impacts. Achieving this via emissions reductions alone will be very difficult. Carbon dioxide removal (CDR) has been suggested to complement and compensate for insufficient emissions reductions, through increasing natural carbon sinks, engineering new carbon sinks, or combining natural uptake with engineered storage. Here, we review the carbon cycle responses to different CDR approaches and highlight the often-overlooked interaction and feedbacks between carbon reservoirs that ultimately determines CDR efficacy. We also identify future research that will be needed if CDR is to play a role in climate change mitigation, these include coordinated studies to better understand (i) the underlying mechanisms of each method, (ii) how they could be explicitly simulated, (iii) how reversible changes in the climate and carbon cycle are, and (iv) how to evaluate and monitor CDR