Canopy CO2 assimilation, energy balance, and water use efficiency of an alfalfa crop before and after cutting

Canopy photosynthesis, energy balance and evapotranspiration (ET), and water use efficiency of an alfalfa (Medicago sativa L.) crop were followed by measuring the above-canopy fluxes with the Bowen ratio/energy balance/CO2 gradient technique and correcting for CO2 released by the soil and roots via respiration. Measurements were taken late in the season, just before the last cutting, after cutting, during regrowth, and during the initial senescence phase near the end of autumn. Before cutting, canopy photosynthesis rate was similar to those for other high yielding crop species and ET was very close to reference ET rate, with most of the energy for evaporation coming from net radiation and a small part from sensible heat. Net canopy CO2 assimilation rate (A(c)) reached 1.7 mg (39 mu mol) CO2 m(-2) s(-1) around noon on sunny days and up to 2.4 mg (55 mu mol) CO2 m(-2) s(-1) in periods between clouds on a windy and relatively cool day. A(c) increased with increase in photosynthetic active radiation up to 1700 mu mol m(-2) s(-1) with no sign of light saturation. Immediately after cutting, ET was reduced markedly; A(c) was reduced even more drastically but remained positive. Energy balance changed so that only a minor fraction of the net radiation was used for ET, and the major part was dissipated as sensible heat and as storage heat flux into the soil. Evapotranspiration dropped to about 1 mm per day, but started to increase after cutting over a period of about 2 weeks. A(c) also increased and reached 84% of pre-cutting value about 3 weeks after cutting. Seven diurnal curves of A(c), ET, and parameters associated with energy fluxes are presented, spanning pre-cutting, immediately after cutting, regrowth and the start of senescence. Before cutting, water use efficiency of the alfalfa normalized for the evaporative demand was between 49.1 and 52.8 g CO2 m(-2) per day (approximately 10.5-11.3 g dry matter m(-2) per day), declined to 7.5 g CO2 m(-2) per day after cutting, but increased afterwards continuously, reaching 46.3 g m(-2) per day 3 weeks later, before declining under less favorable growing conditions as the end of autumn approached. Under favorable conditions, alfalfa water use efficiency appears to be as high as most other crops when the cost in assimilates for symbiotic nitrogen fixation and mycorrhizal fungi, larger proportion of root biomass, and higher protein content are taken into account. Its reputation as a water spender is not deserved. (C) 2000 Published by Elsevier Science B.V.