Growing season variability in carbon dioxide exchange of irrigated and rainfed soybean in the southern United States

Authors: Wagle, Pradeep; Gowda, Prasanna; Anapalli, Saseendran; Northup, Brian; Reddy, Krishna

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2017
Publication Date: 3/27/2017
Citation: Wagle, P., Gowda, P., Anapalli, S.S., Northup, B.K., Reddy, K.N. 2017. Growing season variability in carbon dioxide exchange of irrigated and rainfed soybean in the southern United States. Science of the Total Environment. 593-594:263-273.

Interpretive Summary: Soybean is one of the most widely grown crops in the world. It has been a very valuable crop in the southern United States (U.S.) as well. It is crucial to quantify carbon dynamics of major agroecosystems in all climatic regions to improve our understanding of how those globally important agroecosystems respond to a wide range of climatic conditions. This study compares carbon fluxes between irrigated (Stonville, MS) and rainfed (El Reno, OK) soybean in the southern U.S. during the 2016 growing season. Grain yield was about 1.6 t ha-1 for rainfed soybean and 4.9 t ha-1 for irrigated soybean. Daily (7-day average) net ecosystem CO2 exchange (NEE), gross primary production (GPP) and ecosystem respiration (ER) reached seasonal peak values of -4.55, 13.54, and 9.95 g C m-2 d-1 in rainfed soybean and -7.48, 18.13, and 14.93 g C m-2 d-1 in irrigated soybean, respectively. Grain yield and the magnitudes and seasonal sums of carbon fluxes for irrigated soybean in this study were comparable to those for soybean in the U.S. Corn Belt, but they were lower in rainfed soybean. Rainfed soybean was a net carbon sink for only two months, but irrigated soybean appeared to be a net carbon sink for about three months.

Technical Abstract: Measurement of carbon dynamics of soybean (Glycine max L.) ecosystems outside Corn Belt of the United States (U.S.) is lacking. This study reports carbon dioxide (CO2) fluxes from a rainfed soybean field in El Reno, Oklahoma and an irrigated soybean field in Stoneville, Mississippi during the 2016 growing season. The major objective was to compare the seasonal variability of net ecosystem CO2 exchange (NEE) and its components (gross primary production, GPP and ecosystem respiration, ER) and relevant controlling environmental factors between irrigated and rainfed soybean fields in the southern U.S. Grain yield was about 1.6 t ha-1 for rainfed soybean and 4.9 t ha-1 for irrigated soybean. The magnitudes of diurnal NEE (~2-weeks average) reached seasonal peak values of -23.18 and -34.78 µmol m-2 s-1 in rainfed and irrigated soybean, respectively, approximately two months after planting during peak growth. Similar threshold values of air temperature (Ta, slightly over 30 ºC) and vapor pressure deficit (VPD, ~2.5 kPa) for NEE were observed at both sites. Daily (7-day average) NEE, GPP, and ER reached seasonal peak values of -4.55, 13.54, and 9.95 g C m-2 d-1 in rainfed soybean and -7.48, 18.13, and 14.93 g C m-2 d-1 in irrigated soybean, respectively. The growing season (DOY 132-243) NEE, GPP, and ER totals were -54, 783, and 729 g C m-2, respectively, in rainfed soybean. Similarly, the growing season (DOY 163-256, flux measurements initiated on DOY 163) NEE, GPP, and ER totals were -291, 1239, and 948 g C m-2, respectively, in irrigated soybean. Rainfed soybean was a net carbon sink for only two months, while irrigated soybean appeared to be a net carbon sink for about three months. Grain yield and the magnitudes and seasonal sums of CO2 fluxes for irrigated soybean in this study were comparable to those for soybean in the U.S. Corn Belt, but they were lower for rainfed soybean.