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Managing Agricultural Fields to Maximize Their Ability to Sequester Carbon in a CO2-Enriched World of the Future
Volume 8, Number 42: 19 October 2005

There are a number of naturally occurring phenomena - negative feedbacks, if you will - that tend to mute the rate of rise of the anthropogenically-driven increase in the air's CO2 content, while there are other phenomena that perform the same function that owe their existence to human ingenuity.  In a recent report of a study that deals with a phenomenon of the latter type, Prior et al. (2005) describe a multifaceted field management system from the world of agriculture that was developed to help conserve resources and increase crop yields, but which simultaneously serves the newer purpose of stimulating carbon sequestration on agricultural fields.

This conservation as opposed to conventional management system employs little to no tillage and uses special crop rotations.  In the southern United States, where Prior et al. have been testing the two approaches for the past five years, the conventional cropping system consists of a rotation cycle where grain sorghum and soybean are rotated each year with spring tillage after winter fallow that produces only a light growth of weeds.  In the conservation cropping system, grain sorghum and soybean are also rotated, but in the place of weeds are three cover crops: crimson clover, sunn hemp and wheat, which are similarly rotated but without tillage.

To see how the two management systems presently compare in terms of crop production and soil carbon sequestration, as well as how they likely will compare in the high-CO2 world that is expected to prevail a half century or so from now, the five scientists from the U.S. Department of Agriculture's Agricultural Research Service employed them for a period of four years (two complete cropping cycles) in 7-meter-wide x 76-meter-long x 2-m-deep bins filled with a silt loam soil, upon which they constructed a number of clear-plastic-wall open-top chambers that they maintain at atmospheric CO2 concentrations that have averaged either 375 ppm (ambient) or 683 ppm (enriched) over the first four years of the projected ten-year study.

In the first cropping cycle, elevated CO2 increased clover residue production in the conservation system by 23%, but it had no detectable impact on weed growth in the conventional system.  In the case of the following sorghum crop, management, i.e., conservation vs. conventional) had no effect on residue production, but elevated CO2 increased sorghum residue production by 14%.  The production of residue by the sunn hemp that was planted next in the conservation system was increased by 32% by the elevated CO2, while there was no weed production in the conventional system due to a prior herbicide application.  The wheat that followed the sunn hemp also experienced a CO2-induced yield enhancement of 32%, while more residue was produced in the conservation system, as opposed to the conventional system that only grew weeds at that time.  Last of all, elevated CO2 increased soybean grain and residue production by 40% and 49%, respectively, while conservation management added an extra 6% to grain production and 4% to residue production.

In the second cropping cycle, elevated CO2 increased clover residue production by 22% in the conservation system, but it once again had no effect on weed production in the conventional system.  It then increased sorghum residue and grain production by 24% and 22%, respectively, while conservation management led to additional increases of 13% in these two growth parameters.  The elevated CO2 next increased the production of sunn hemp residue by 61%, while there again was no weed growth in the conventional management system due to another herbicide application.  Wheat responses were as observed in the first cropping cycle.  Last of all, elevated CO2 increased soybean yield and residue production by 52% and 49%, respectively, while conservation management actually resulted in a 9% decrease in soybean residue in this cropping cycle.

In terms of the cumulative residue produced over the two cropping cycles, there was little interaction between management practices and atmospheric CO2 concentration, with conservation practices increasing this parameter by about 90% in both CO2 treatments, with elevated CO2 increasing it by approximately 30% in both management treatments, and with conservation practices and elevated CO2 together increasing it by 150%.  In terms of the carbon retained and incorporated into the first 5 cm of the soil at the end of the two cropping cycles, however, there were significant interactions.  Elevated CO2 increased this important soil property by about 10% in the conventional system, but by 45% in the conservation system, while the application of conservation practices increased 0- to 5-cm soil carbon storage by close to 45% in ambient-CO2 air but by nearly 90% in elevated-CO2 air.  Together, the two treatments increased surface soil carbon storage by close to 110%.

Clearly, increasing atmospheric CO2 concentrations and best-management conservation practices work hand-in-hand to boost crop yields and residue production while increasing soil carbon storage, with each factor bringing out the best in the other.  In addition, as noted by Prior et al., "in an elevated CO2 environment there will be larger amounts of crop residue and consequently more ground cover," so that "accumulation of additional surface litter may improve water infiltration (and storage) and help ameliorate water quality problems by reducing runoff and soil erosion."

All of these observations bode well for the biosphere, including humanity, and all have their origin in human ingenuity.

Sherwood, Keith and Craig Idso

Reference
Prior, S.A., Runion, G.B., Rogers, H.H., Torbert, H.A. and Reeves, D.W.  2005.  Elevated atmospheric CO2 effects on biomass production and soil carbon in conventional and conservation cropping systems.  Global Change Biology 11: 657-665.