Editing IPCC:AR6/SRCCL/Chapter-2 (section)

=== 2.7.3 Soil microbial effects on soil nutrient dynamics and plant responses to elevated CO2 ===

<div id="section-2-7-3-soil-microbial-effects-on-soil-nutrient-dynamics-and-plant-responses-to-elevated-co2-block-1"></div>

Soil microbial processes influencing nutrient and carbon dynamics represent a large source of uncertainty in projecting land–climate interactions. For example, ESMs incorporating nitrogen and phosphorus limitations (but without considering the effects of mycorrhizae and rhizosphere priming) indicate that the simulated future carbon-uptake on land is reduced significantly when both nitrogen and phosphorus are limited as compared to only carbon- stimulation, by 63% (of 197 Pg C) under RCP2.6 and by 67% (of 425 Pg C) under RCP8.5 (Zhang et al. 2013c <sup>[[#fn:r2005|2005]]</sup> ). Mineral nutrient limitation progressively reduces the CO <sub>2</sub> fertilisation effects on plant growth and productivity over time ( ''robust evidence, medium agreement'' ) (Norby et al. 2010 <sup>[[#fn:r2006|2006]]</sup> ; Sardans et al. 2012 <sup>[[#fn:r2007|2007]]</sup> ; Reich and Hobbie 2013 <sup>[[#fn:r2008|2008]]</sup> ; Feng et al. 2015 <sup>[[#fn:r2009|2009]]</sup> ; Terrer et al. 2017 <sup>[[#fn:r2010|2010]]</sup> ). The rates at which nutrient limitation develops differ among studies and sites. A recent meta- analysis shows that experimental CO <sub>2</sub> enrichment generally results in lower nitrogen and phosphorus concentrations in plant tissues (Du et al. 2019 <sup>[[#fn:r2011|2011]]</sup> ), and isotopic analysis also suggest a global trend of decreases in leaf nutrient concentration (Craine et al. 2018 <sup>[[#fn:r2012|2012]]</sup> ; Jonard et al. 2015 <sup>[[#fn:r2013|2013]]</sup> ). However, reduced responses to elevated CO <sub>2</sub> (eCO <sub>2</sub> ) may not be a simple function of nitrogen dilution per se, as they result from complex interactions of ecosystem factors that influence nitrogen acquisition by plants (Liang et al. 2016 <sup>[[#fn:r2014|2014]]</sup> ; Rutting 2017 <sup>[[#fn:r2015|2015]]</sup> ; Du et al. 2019 <sup>[[#fn:r2016|2016]]</sup> ).

Increasing numbers of case studies suggest that soil microbial processes, such as nitrogen mineralisation rates and symbiosis with plants, influence nutrient limitation on eCO <sub>2</sub> effects on plant growth ( ''medium confidence'' ) (Drake et al. 2011 <sup>[[#fn:r2017|2017]]</sup> ; Zak et al. 2011 <sup>[[#fn:r2018|2018]]</sup> ; Hungate et al. 2013 <sup>[[#fn:r2019|2019]]</sup> ; Talhelm et al. 2014 <sup>[[#fn:r2020|2020]]</sup> ; Du et al. 2019 <sup>[[#fn:r2021|2021]]</sup> ). Rhizosphere priming effects (i.e., release of organic matters by roots to stimulate microbial activities) and mycorrhizal associations are proposed to explain why some sites are becoming nitrogen limited after a few years and others are sustaining growth through accelerated nitrogen uptake (limited evidence, medium agreement) (Phillips et al. 2011 <sup>[[#fn:r2022|2022]]</sup> ; Terrer et al. 2017 <sup>[[#fn:r2023|2023]]</sup> ).

Model assessments that including rhizosphere priming effects and ectomycorrhizal symbiosis suggest that soil organic matter (SOM) cycling is accelerated through microbial symbiosis ( ''medium confidence'' ) (Elbert et al. 2012 <sup>[[#fn:r2024|2024]]</sup> ; Sulman et al. 2017 <sup>[[#fn:r2025|2025]]</sup> ; Orwin et al. 2011 <sup>[[#fn:r2026|2026]]</sup> ; Baskaran et al. 2017 <sup>[[#fn:r2027|2027]]</sup> ). Uncertainty exists in differences among ectomycorrhizal fungal species in their ability to decompose SOM (Pellitier and Zak 2018 <sup>[[#fn:r2028|2028]]</sup> ) and the capacity of ecosystems to sustain long-term growth with these positive symbiotic feedbacks is still under debate (Terrer et al. 2017 <sup>[[#fn:r2029|2029]]</sup> ). ESMs include only biological nitrogen cycles, even though a recent study suggests that bedrock weathering can be a significant source of nitrogen to plants (Houlton et al. 2018 <sup>[[#fn:r2030|2030]]</sup> ). In contrast, rock weathering is widely considered to be key for phosphorus availability, and tropical forests with highly weathered soils are considered to be limited by phosphorus availability rather than nitrogen availability (Reed et al. 2015 <sup>[[#fn:r2031|2031]]</sup> ). Yet evidence from phosphorus fertilisation experiments is lacking (Schulte-Uebbing and de Vries 2018 <sup>[[#fn:r2032|2032]]</sup> ) and phosphorus limitation of tropical tree growth may be strongly species-specific (Ellsworth et al. 2017 <sup>[[#fn:r2033|2033]]</sup> ; Turner et al. 2018a <sup>[[#fn:r2034|2034]]</sup> ). Limitation by availability of soil nutrients other than nitrogen and phosphorus has not been studied in the context of land–climate interactions, except potassium as a potentially limiting factor for terrestrial plant productivity in interaction with nitrogen, phosphorus and hydrology (Sardans and Peñuelas 2015 <sup>[[#fn:r2035|2035]]</sup> ; Zhao et al. 2017 <sup>[[#fn:r2036|2036]]</sup> ; Wright et al. 2018 <sup>[[#fn:r2037|2037]]</sup> ).

Anthropogenic alteration of global and regional nitrogen and phosphorus cycles, largely through use of chemical fertilisers and pollution, has major implications for future ecosystem attributes, including carbon storage, in natural and managed ecosystems ( ''high confidence'' ) (Peñuelas et al. 2013 <sup>[[#fn:r2038|2038]]</sup> , 2017 <sup>[[#fn:r2039|2039]]</sup> ; Wang et al. 2017c <sup>[[#fn:r2040|2040]]</sup> ; Schulte- Uebbing and de Vries 2018 <sup>[[#fn:r2041|2041]]</sup> ; Yuan et al. 2018 <sup>[[#fn:r2042|2042]]</sup> ). During 1997–2013, the contribution of nitrogen deposition to the global carbon sink has been estimated at 0.27 ± 0.13 GtC yr <sup>–1</sup> , and the contribution of phosphorus deposition as 0.054 ± 0.10 GtC yr <sup>–1</sup> ; these constitute about 9% and 2% of the total land carbon sink, respectively (Wang et al. 2017c <sup>[[#fn:r2043|2043]]</sup> ). Anthropogenic deposition of nitrogen enhances carbon sequestration by vegetation (Schulte-Uebbing and de Vries 2018 <sup>[[#fn:r2044|2044]]</sup> ), but this effect of nitrogen deposition on carbon sequestration may be offset by increased emission of GHGs such as N <sub>2</sub> O and CH <sub>4</sub> (Liu and Greaver 2009 <sup>[[#fn:r2045|2045]]</sup> ). Furthermore, nitrogen deposition may lead to imbalance of nitrogen vs phosphorus availability (Peñuelas et al. 2013 <sup>[[#fn:r2046|2046]]</sup> ), soil microbial activity and SOM decomposition (Janssens et al. 2010 <sup>[[#fn:r2047|2047]]</sup> ) and reduced ecosystem stability (Chen et al. 2016b <sup>[[#fn:r2048|2048]]</sup> ).

<span id="vertical-distribution-of-soil-organic-carbon"></span>