suite of five experimental glasshouses is described In each house, the CO~ ..... m each of the five hnes to shut down the supply of CO2 to a particular house.
Agricultural and Forest Meteorology, 40 (1987) 27~292
279
Elsevier Scmnce P u b h s h e r s B V , Amsterdam - - Printed m The Netherlands
COMPUTER CONTROL OF CARBON DIOXIDE CONCENTRATION IN E X P E R I M E N T A L G L A S S H O U S E S A N D ITS USE TO E S T I M A T E N E T CANOPY P H O T O S Y N T H E S I S
R B MATTHEWS*, B MARSHALL**, R A SAFFELL*** and D HARRIS
ODA M~crochmatology Group, School of Agriculture, Sutton Bon~ngton, Loughborough, Le~cs (Gt Britain) (Received September 10, 1986, revismn accepted J a n u a r y 25, 1987)
ABSTRACT Matthews, R B , Marshall, B , Saffell, R A and Harris, D , 1987 Computer control of carbon diomde concentration in experimental glasshouses and its use to estimate net canopy photosynthesis Agmc For Meteorol, 40 279-292 A computer controlled system for m o m t o r i n g and controlling the concentration of CO 2 m a suite of five experimental glasshouses is described In each house, the CO~ concentration, measured using an infra-red gas analyser, was maintained close to the ambmnt c o n c e n t r a t m n by rejecting pure CO 2 t h r o u g h a metering system controlled by the computer The required injection rate for the interval between concentration measurements was calculated with an algomthm t h a t used v e n t i l a t m n rate and prewous and current CO 2 concentrations as inputs V e n t d a t l o n rate was calculated using a relation based on ventilator positron and windspeed, determined prevmusly from measurements using m t r o u s oxide as a tracer gas Soil resp~ratmn was measured by the gain m weight of soda lime inside a container reverted over the soil Net canopy photosynthesis was estimated as the residual after the other CO 2 fluxes had been accounted for The system worked rehably and was used to obtain light-response curves of net canopy photosynthems for a number of tropical crops INTRODUCTION The concept of enclosing a leaf or part of a leaf m a gas-tight cuvette and m e a s u r i n g t h e d e c r e a s e m CO2 c o n c e n t r a t i o n has been used on occasions to estimate net photosynthesis (e g , M a r s h a l l and Blscoe, 1977) Subsequent development led to the use of larger chambers that could enclose entire plants (Glauert, 1983) to provide reformation on the way in which individual leaf photosynthesis ~s I n t e g r a t e d t o g i v e w h o l e p l a n t r e s p o n s e s Lake (1966) has suggested that it should be possible to use an entire glasshouse, enclosing a crop stand growing in natural soil, to integrate individual plant performance to estimate photosynthems by a canopy Hand and Bowman (1969) descmbe a v e r m o n o f s u c h a s y s t e m , m w h i c h a s o l e n o i d w a s a c t i v a t e d t o a l l o w p u r e CO2 * Present address Oxford Forestry Institute, South Parks R d , Oxford, Gt Britain ** Present address Scottish Crops Research Institute, Invergowrle, Dundee, Gt Britain *** Present address Campbell Scmntffic L t d , College Road, Sutton Bonmgton, Loughborough, Lems, Gt Britain 0168-1923/87/$03 50
© 1987 Elsevier Science P u b h s h e r s B V
280 to flow to the glasshouse for varying lengths of time, depending on the dev]a t m n of the measured from the desired CO2 concentratmn Flow rate to the house was controlled manually In an experimental programme studying tropical crops growing m controlled e n w r o n m e n t glasshouses m the U K (Montelth et a l , 1983), the substantml depletmn of CO2 caused by high rates of crop photosynthesis made it necessary to develop a system to supplement the amount of COs m the atmosphere of each glasshouse However, it was found that by quantifying all other fluxes of CO2 into and out of the glasshouse, it was also possible to use the system to estimate rates of canopy photosynthesis Th~s allowed measurement of photosynthetic responses over a very much shorter time interval (I h) than conventmnal growth analysis techmques (1 week). Previous papers m this serms have described the ratmnale and design of the controlled environment system (Montelth et a l , 1983), the dlstmbuted mmroprocessor control (Saffell and Matthews, 1986), and the control of temperature (Saffell and M a r s h a l l 1983) and humldlty (Matthews and Saffell, 1986) PROCEDURES COe balance The mare CO2 fluxes m the system proposed by Lake (1966) are transfer by ventflatmn, CO2 rejection, soil resptratmn, changes m internal concentration, and photosynthesis Of the first four, transfer by ventilation is the most difficult to measure, and is hkely to cause the greatest errors in estimates of the CO~ balance However, these errors can be mmlmlsed if the difference between internal and external CO2 concentrations is small Thus, the control system was designed to maintain the COs concentration reside the house as near as possible to that outside Assuming t h a t the mr reside the glasshouse is Ideally stirred to give a uniform dlstributmn of CO2, the equatmn for the CO2 balance of a glasshouse can be wmtten as I+
8-P-
VN(C~- C e ) -
v dv~C = 0 dt
where I = rate of CO2 rejection into the house (gCO2h ~), S = rate of CO2 evolution from the so11 (g CO2 h- 1), p = rate of CO2 uptake by photosynthesis (g CO2 h-l), V = volume of glasshouse (120m3), N = ventdatlon rate (changes h 1), C, = internal COs concentration (gCO2m-3), Ce = external COs concentratlon (gCO2m-3) All these parameters were measured, except P, whmh was calculated as the residual Although not included m eq 1, another slgmficant source of CO2 may be respiration by humans. However, as this quantity ~s very difficult to est~mate, care was taken to calculate COs balances only for pemods when people were excluded from the glasshouses In practme, because regular detailed
281
p h y s m l o g m a l m e a s u r e m e n t s w e r e b e i n g made, the g l a s s h o u s e s w e r e free f r o m h u m a n i n t e r f e r e n c e o n l y at w e e k e n d s T h e f o l l o w i n g s e c t m n s descmbe t h e m e t h o d in w h i c h the o t h e r fluxes w e r e m e a s u r e d or e s t i m a t e d
Sotl resptratton Soll r e s p l r a t m n (S) w a s m e a s u r e d u s i n g t h e m e t h o d described by M o n t e l t h et al (1965), m w h i c h a p l a s t m c o n t a i n e r c o n t a i n i n g a w e i g h e d q u a n t i t y of soda lime w a s i n v e r t e d o v e r t h e soft S w a s e s t i m a t e d f r o m t h e g a i n m w e i g h t of the soda h m e o v e r a specffied t i m e i n t e r v a l , u s u a l l y 3 d a y s A l t h o u g h S depends on t e m p e r a t u r e , it c h a n g e s o n l y slowly due to t h e t h e r m a l i n e r t i a of t h e soil We a s s u m e d it to r e m a i n c o n s t a n t o v e r the i n t e r v a l s b e t w e e n m e a s u r e m e n t
Venttlatton rate E s t i m a t e s of t h e v e n t i l a t i o n r a t e ( N ) w e r e m a d e by r e j e c t i n g into the glassh o u s e a set a m o u n t of t h e t r a c e r gas, m t r o u s oxide (N20), w h m h is n o t a b s o r b e d or p r o d u c e d by p l a n t s or soil, a n d m o m t o r l n g ~ts d e c a y m c o n c e n t r a t m n ( G o e d h a r t et a l , 1984) An ADC M k II S e r m s 225 N 2 0 infra-red gas a n a l y s e r (IRGA) was used for this p u r p o s e N is g i v e n by N
-
-1
ln¢,l-~o
tl - t----O q~
~bo
(2)
w h e r e ~bm = I R G A r e a d i n g at s t a r t of m e a s u r e m e n t period, ~btl = I R G A r e a d i n g at end of m e a s u r e m e n t period, ~bo = I R G A r e a d i n g of N20-free air, tO = t i m e at s t a r t of m e a s u r e m e n t period, tl = t i m e a t end of m e a s u r e m e n t p e m o d C o m p l e t e d e t a i l s of this m e t h o d a r e g i v e n by M a r s h a l l (1987), w h o f o u n d t h a t for t h e s e g l a s s h o u s e s , N w a s m a i n l y d e p e n d e n t on v e n t i l a t o r p o s i t i o n a n d w m d s p e e d O t h e r possible factors, s u c h as wind d i r e c t i o n a n d b u o y a n c y due to t e m p e r a t u r e d i f f e r e n c e s b e t w e e n reside a n d o u t s i d e (Bot, 1983), h a d r e l a t i v e l y m i n o r influences To o b v i a t e the n e e d for d i r e c t m e a s u r e m e n t s of v e n t l l a t m n , t h e r e l a t i o n b e t w e e n w m d s p e e d a n d N w a s d e t e r m i n e d for a r a n g e of v e n t i l a t o r positions, f r o m w h i c h t h e a l g o r i t h m N
=
233 + 014p + 013u + 012pu
w a s derived, w h e r e p is the v e n t i l a t o r p o s i t i o n ( m seconds of o p e n i n g f r o m the c o m p l e t e l y closed positron), a n d u is w m d s p e e d (m s ~) T h e c o m p u t e r used this a l g o r i t h m to e s t i m a t e N
Measurement of ambtent C02 concentration Air w a s d r a w n f r o m t h e o u t s i d e a t m o s p h e r e at t h e s a m e g l a s s h o u s e v e n t i l a t o r s , a n d p a s s e d t h r o u g h a n ADC M k II series operating m absolute mode Readings were taken every minute, of the a m b i e n t CO2 c o n c e n t r a t i o n (Ce) s t o r e d F o r t h e first 2 m m
h e i g h t as the 225 CO2-IRGA and the value m e v e r y hour,
282 the external mr flow was switched off for automatic c a h b r a t m n of the IRGA Air that had been passed over soda lime to remove all CO2 was diverted through the IRGA, and after 1 mm to allow the analyser to stablhse, a reading was taken to give a zero c a h b r a t m n The flow was then switched to allow mr containing a known CO2 concentratmn, usually 400 p p m , to pass through the IRGA Again, after 1 mm, a reading was taken, the IRGA cahbratmn updated m the program, and the air flow switched back to normal measurement of the external CO2 c o n c e n t r a t m n for the remainder of the hour
Measurement of glasshouse C02 concentratmn A representative sample of air for analysm was extracted from the atmosphere within each house through an retake mamfold mounted below the central cross-strut and extending east-west for the full width of the glasshouse The mamfold was a copper tube with an reside diameter of 8 mm, drilled with 1-mm diameter inlets at 23-cm intervals along its length The air was then pumped at a rate of 101 m m - 1 through 8-mm diameter copper tubes to the gas sample board The eqmvalent velomty was 3 3 m s- ~, resulting m a typical time delay of 30 s for a sample to travel the 100 m from the furthest glasshouse to the control cabin To prevent condensation, the tubes were heated to 50-60°C by similar tubes carrying hot water pumped from the control cabin The sample, rejection and heating tubes were bound together, wrapped m insulating material, enclosed m a waterproof PVC d r a m pipe, and channelled through a concrete condmt running underground between glasshouses and control cabin As there was only one dli~erentml COs-IRGA, a system of switching the incoming sample mr flow sequentmlly from house to house was devmed (Fig 1), and was housed m the same control cabin as the computer and data logger As an example of operation, glasshouse 1 was selected by activating the appropriate digital output line, which operated the solenmd valve SV1 (Danfoss EVP302) w a a relay, shutting off the exit port El, and passing the gas sample from glasshouse 1 into the sample mamfold SM. Most of the sample (9 t ram- 1 ) was exhausted t h r o u g h the port EX, whale the remaining gas flow (11 mln -1 ) was passed to the sample cell of the dlfferentml CO s-IRGA (ADC Mk II, serms 225). This measured the difference between the COs concentratmn of the mr of the glasshouse, C1, and t h a t of mr drawn from above the control cabin at the same helght as the glasshouse ventalators, Ce. Th~ty seconds after the house was seldcted, the voltage output of the I R G A was read and stored m the computer The sample and reference hnes were reversed by the changeover valve CH, and after a further 30s, a second reachng was taken Thus, the change m output voltage between the mltml and reversed readings was eqmvalent to double the CO2 concentratmn difference Thin techmque of reversal removed the necessity of making a zero check and avmded problems assocmted with zero drift. It was fmmd t h a t 30 s between readings was qmte adequate for the mr flow through t h e IRGA to statnhse after each change,before the next reading was taken The calibratmn of the 1RGA was checked weekly using
283
EX
