An article titled “Quantifying water and CO2 fluxes and water use efficiencies across irrigated C3 and C4 crops in a humid climate” by Anapalli et al. (Science of the Total Environment 663, Jan. 2019) paints an interesting picture of how choice of crop either in monoculture or in a rotation can influence water use efficiency of irrigated crops. (See below addendum for C3 and C4 crop differences.)
Abbreviations used in the article are as follows:
• EWUE–crop-ecosystem water use efficiency, or the amount of CO2 removed from the soil-crop-air system per unit of water used in ET.
• NEE–net ecosystem exchange, measured in lb. CO2 per acre; represents the balance from the amount of CO2 fixed in photosynthesis minus the CO2 released in plant respiration and as a byproduct of organic matter decomposition in the soil; expressed as a negative value if the net flux of CO2 is coming down toward the crop or sink.
• WUE–net carbon gain from photosynthesis to water lost through transpiration at the leaf level. In agricultural science, WUE is used to denote the ratio of the amount of harvested yield, either grain or biomass, to the amount of water used in producing the crop yield.
• EC–eddy covariance–provides quantification of EWUE at the crop-ecosystem level by measuring the CO2 and water fluxes between the atmosphere and land surface. This is achieved by measuring the covariance of the vertical wind speed for eddy transport and the concentrations of CO2 and water vapor in the eddies.
The objectives of the study were to quantify 1) NEE and ET from C4 (corn) and C3 (soybean and cotton) cropping systems in the lower Miss. Delta and 2) EWUE across these three cropping systems for irrigation water management applications.
• The research reported in this article was conducted at Stoneville, Miss., which has a sub-tropical humid climate with mild winters and warm summers. Dominant soil series in all crop fields is Tunica clay.
• Corn was planted in 38-in.-wide rows on Mar. 21, 2017, was fully emerged on Mar. 28, and reached physiological maturity (PM) on July 17, or 111 days after emergence (DAE).
• Soybean was planted in 30-in.-wide rows on Apr. 21, 2017, was fully emerged on Apr. 28, and reached PM on Sept. 10, or 135 DAE.
• Cotton was planted in 30-in.-wide rows on Apr. 22, 2017, was fully emerged on May 1, and reached PM on Sept. 10, or 132 DAE.
• All study sites were irrigated by the furrow method to maintain soil water content in the upper 12 in. of soil at >65% plant available water.
• Eddy covariance methodology was used to measure CO2 and water fluxes between the atmosphere and land surface in all fields (click here for a link to details about the setup and use of a network of towers throughout the Delta to quantify carbon and water fluxes in the Lower Miss. River Basin).
• In general, there were only minor differences in the air temperatures above the canopies of the three crops.
• Compared to soybean and cotton, soil temperatures under corn remained cooler throughout the growing season. Soil temperatures under cotton were the hottest and soil temperatures under soybean were intermediate to those under corn and cotton. This was likely related to early-season corn leaf area index (LAI) being the greatest and cotton LAI being the lowest. The LAI of crop plants in a cropping system has the most influence on NEE.
• All three crops were a net sink for CO2: corn, soybean, and cotton fixed -31,331, -23,563, and -8,856 kg CO2 per ha in exchange for 483, 552, and 367 mm of ET, respectively (negative values for fixed CO2 show that CO2 fixed in the plant is removed from the atmosphere).
• Corn grain yield averaged 203.7 bu/acre, soybean seed yield averaged 71.1 bu/acre, and cotton lint yield averaged 1124 lb/acre.
• Daily biomass accumulation was highest in corn, followed by soybean, and least in cotton.
• Highest NEE was in corn, followed by soybean followed by cotton. Seasonal NEE estimated for cotton was 72% less than that for corn and 62% less than that for soybean. Thus, corn, the C4 crop, fixed more CO2 for a given amount of resources than did the C3 crops soybean and cotton.
• Maximum ET from corn, soybean, and cotton was 0.252, 0.26, and 0.224 in./day, respectively. Whole season ET for corn, soybean, and cotton was 19.0, 21.7, and 14.5 in., respectively. The lower cotton ET was due to its lower LAI compared to soybean and corn.
• Half-hour estimates of ET from all crops were most correlated with solar radiation (R2 = 0.77) vs. air temperature (R2 = 0.34) and vapor pressure deficit (R2 = 0.53) in this study that was conducted in the sub-humid climate of the Delta.
• Overall, daily EWUE in all three crops followed the LAI growth patterns. Higher rates of EWUE were achieved in all three crops during the peak LAI stages of those crops.
• The EWUE in corn (53 kg CO2/ha per mm of ET) was greater than that of soybean (43 kg CO2/ha per mm of ET) which was greater than that of cotton (24 kg CO2/ha per mm of ET).
• The corn crop’s grain production WUE (expressed as the ratio of the grain weight to the amount of ET) was 26 kg/ha per mm of ET. The WUE of soybean seed production was 9 kg/ha per mm of ET, and that of cotton lint yield was 3 kg/ha per mm of water.
• The authors conclude that the results from this study can be used to make decisions about the proper crop mix to use in the Miss. Delta to achieve increased WUE while also sequestering more CO2 in cropping systems. These results also indicate that C4 corn is a more efficient user of water that are C3 soybean and cotton crops.
These results should no be misconstrued to mean that C3 crops should be tossed out in favor of C4 crops, but they do suggest the logical question of “Why not work at changing major C3 crop plants to C4 plants?” so that WUE of all crops can be increased. That does seem logical on the surface. However, according to a statement in the article titled “Integrating C4 photosynthesis into C3 crops to increase yield potential” by Covshoff and Hibberd (Curr. Opinion in Biotechnology 2012, 23:209-214), “Owing to complex changes associated with C4 photosynthesis, it is no understatement to define this conversion as one of the Grand Challenges for Biology in the 21st Century.” They “outline the challenges of installing a C4 system and assess how new approaches and knowledge may help achieve this goal.” And of course, this task will require sophisticated and complex genetic engineering techniques and methodology. But this endeavor just might be worth the effort when the economic and societal benefits are taken into account; i.e., a reduction in wasteful photorespiration by C3 plants that might be converted to C4 plants could result in a significant increase in useable yield from these plants. (Caveat: Since the above article was published 7 years ago, it is likely that numerous strides have been made in reaching this goal.)
At least producers in the Midsouth who irrigate using water from the declining MRVAA should take note of how selection of a rotation partner with soybeans can affect the amount of product that is produced with a unit water, and the efficiency of that production.
Composed by Larry G. Heatherly, Feb. 2019, firstname.lastname@example.org
C3 AND C4 PLANTS–WHAT ARE THE DIFFERENCES?
C3 and C4 are two of the three different processes (see video) that plants use to fix carbon (C) during photosynthesis (PS). This carbon fixing is the way plants remove C from atmospheric carbon dioxide (CO2) to create organic molecules such as carbohydrates.
A C3 plant uses a pathway that produces a 3-carbon molecule called 3-phosphoglyceric acid. About 85% of the earth’s plants use this C3 pathway to fix C via the Calvin cycle. During this one-step process, the enzyme RuBisCo (ribulose-1,5-biphosphate carboxylase/oxygenase) causes an oxidation reaction in which some of the energy used in PS is lost through photorespiration, and this results in about a 25% reduction in the amount of C that is fixed by the plant. This lost C is released back to the atmosphere as CO2. Soybeans, cotton, wheat, and rice are common C3 plants.
In C4 plants, the light-dependent reactions and the Calvin Cycle are physically separated. The light-dependent reactions occur in the mesophyll cells (spongy tissue in the middle of the leaf), and the Calvin Cycle occurs in bundle-sheath cells (cells around the leaf veins) (this cellular structure is known as Kranz anatomy). A C4 plant produces the intermediate 4-carbon molecules malic or aspartic acid during the carbon-fixing process. This intermediate step in the pathway before the Calvin Cycle reduces the amount of C that is lost to the atmosphere in the overall process. The CO2 that is taken in by a C4 plant is moved to the bundle sheath cells (also contain chloroplasts) by the malic or aspartic acid molecules (now called malate and aspartate molecules). The oxygen content in the bundle sheath cells is very low, so the RuBisCo enzymes are less likely to catalyze oxidation reactions and waste C molecules. The malate and aspartate molecules release the CO2 in the chloroplasts of the bundle sheath cells and the Calvin Cycle begins. C4 plants include corn, sugar cane, and sorghum.
The photorespiration that occurs in C3 plants when the Calvin Cycle enzyme RuBisCo acts on oxygen rather than CO2 is wasteful; C3 plants have no feature to reduce or eliminate this photorespiration. C4 plants minimize photorespiration by separating initial CO2 fixation and the Calvin Cycle in the different cell types described above. Photorespiration uses up fixed C, wastes energy, and tends to happen when plants close their stomata to reduce water loss. High temperatures make it worse.