Letter to the Editor pubs.acs.org/est
Letter to the Editor: Answer to the Viewpoint “Sequestering Soil Organic Carbon: A Nitrogen Dilemma” Rattan Lal§ Claire Chenu∥ Philippe Ciais⊥
I
n their viewpoint, van Groeningen et al.1 argue that a “nitrogen dilemma” could reduce the feasibility of the global increase in soil organic carbon (SOC) stocks targeted by the 4 per 1000 Initiative2 and suggest the need for a ‘spatially diversified strategy’ for climate change mitigation from agricultural soils. More specifically, they point that the ‘increase in plant N uptake required to meet the 4 per 1000 goal is unrealistic’ given policies targeting reductions in N surpluses from intensive agriculture. The 4 per 1000 target provides a guideline for locally increasing SOC stocks in the top soil by 0.4% per year, implying an annual SOC sequestration rate proportional to the initial SOC stock. With this aspirational target, degraded soils with low SOC stocks need to sequester less carbon and, hence, immobilize less N and P than soils rich in organic matter. Therefore, the 4 per 1000 target is spatially differentiated in terms of nitrogen and phosphorus requirements. Global reactive N (Nr) availability in agricultural ecosystems is large (ca. 300 Tg Nr per year7), but unevenly distributed. In most intensive agricultural systems, since Nr and P are in excess, additional SOC sequestration, for example, through cover crops,3 could reduce environmental pollution thereby matching environmental policies targeting reductions in Nr and P surpluses. In contrast, in extensive cropping systems with low initial SOC stocks, the modest Nr and P supplies required for SOC sequestration at an annual rate of 0.4% could be provided by biological N fixation (BNF) from legumes, with possible cobenefits for climate change adaptation.4 BNF could be restricted by low P bioavailability in some soils, but symbiotic N2 fixation plants possess an advantage in P acquisition especially in warm climates,for example, through root phosphatase5 and legume trees and shrubs can mobilize phosphorus from deep soil layers. Additionally, recycling organic fertilizers derived from livestock or urban wastes6 could help counter N and P deficiencies. In the long term, since global N:P stoichiometry is increasing under human influence,7 P limitation could become more critical for global SOC sequestration than N limitation. In most cases, providing Nr and P through inorganic fertilizers applications only would be too expansive (ca. 170 USD per Mg of C carbon sequestered8) and would lead to additional Nr release to the environment. On a global scale, soil erosion by water induces annual nutrients losses (23−42 Mt (megaton) N and 15−26 Mt P) in agricultural land, which are of the same order of magnitude than annual fertilizer application rates.9 Soils nutrients lost by erosion need to be replaced through fertilization at an economic cost, which is too high in poor regions such as sub-Saharan Africa. Limiting erosion and land degradation, through the 4 per 1000 strategy, could preserve a source of nutrients both for plants and for the buildup of SOM and thereby reduce the needs for additional fertilizer inputs.
†
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) van Groenigen, J. W.; van Kessel, C.; Hungate, B. A.; Oenema, O.; Powlson, D. S.; van Groenigen, K. J. Sequestering Soil Organic Carbon: A Nitrogen Dilemma. Environ. Sci. Technol. 2017, 51, 4738− 4739. (2) 4 per 1000. The 4 per 1000 Initiative. Soils for food security and climate. 2015. http://4p1000.org/ (accessed on April 26, 2017). (3) Poeplau, C.; Don, A. Carbon sequestration in agricultural soils via cultivation of cover crops − A meta-analysis. Agric., Ecosyst. Environ. 2015, 200, 33−41. (4) Lipper, L.; Thornton, P.; Campbell, B. M.; Baedeker, T.; Braimoh, A.; Bwalya, M.; Caron, P.; Cattaneo, A.; Garrity, D.; Henry, K.; et al. Climate-smart agriculture for food security. Nat. Clim. Change 2014, 4 (12), 1068−1072. (5) Houlton, B. Z.; Wang, Y.-P.; Vitousek, P. M.; Field, C. B. A unifying framework for dinitrogen fixation in the terrestrial biosphere. Nature 2008, 454 (7202), 327−330. (6) Chabbi, A.; Lehmann, J.; Ciais, P.; Loescher, H. W.; Cotrufo, M. F.; Don, A.; SanClements, M.; Schipper, L.; Six, J.; Smith, P.; et al. Aligning agriculture and climate policy. Nat. Clim. Change 2017, 7, 307−309. (7) Peñuelas, J.; Poulter, B.; Sardans, J.; Ciais, P.; Velde, M.; van der Bopp, L.; Boucher, O.; Godderis, Y.; Hinsinger, P.; Llusia, J.; et al. Human-induced nitrogen−phosphorus imbalances alter natural and managed ecosystems across the globe. Nat. Commun. 2013, 4, 2934. (8) Richardson, A. E.; Kirkby, C. A.; Banerjee, S.; Kirkegaard, J. A. The inorganic nutrient cost of building soil carbon. Carbon Manage. 2014, 5, 265−268. (9) FAO and ITPS. Status of the World’s Soil Resources (SWSR) − Main Report; Food and Agriculture Organization of the United Nations and Intergovernmental Technical Panel on Soils: Rome, Italy, 2015.
Jean-François Soussana*,† Suzanne Lutfalla† Pete Smith‡
© XXXX American Chemical Society
INRA, Paris, France University of Aberdeen, Aberdeen, AB24 3FX, U.K. § The Ohio State University, Columbus, Ohio United States ∥ UMR ECOSYS, INRA, AgroParisTech, Université Paris-Saclay, F-78850 Thiverval-Grignon, France ⊥ LSCE, IPSL, Gif sur Yvette, France ‡
Received: July 31, 2017 Accepted: September 12, 2017
A
DOI: 10.1021/acs.est.7b03932 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
