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 wate r use efficiency of an alfalfa (Medicago sativa L.) crop were followed by m easuring the above-canopy fluxes with the Bowen ratio/energy balance/CO2 gr adient technique and correcting for CO2 released by the soil and roots via respiration. Measurements were taken late in the season, just before the la st cutting, after cutting, during regrowth, and during the initial senescen ce 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 assi milation 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 betw een 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 s ign of light saturation. Immediately after cutting, ET was reduced markedly ; A(c) was reduced even more drastically but remained positive. Energy bala nce 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 f luxes are presented, spanning pre-cutting, immediately after cutting, regro wth and the start of senescence. Before cutting, water use efficiency of th e 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), de clined to 7.5 g CO2 m(-2) per day after cutting, but increased afterwards c ontinuously, reaching 46.3 g m(-2) per day 3 weeks later, before declining under less favorable growing conditions as the end of autumn approached. Un der favorable conditions, alfalfa water use efficiency appears to be as hig h as most other crops when the cost in assimilates for symbiotic nitrogen f ixation and mycorrhizal fungi, larger proportion of root biomass, and highe r protein content are taken into account. Its reputation as a water spender is not deserved. (C) 2000 Published by Elsevier Science B.V.