Plan de Aseguramiento PDA 2014 Guaviare

F igure 4 . 1 N i trif ication rate i n the Toko�aru s i l t l o am mea sured at 2 2 °C f o r 1 9 hours af ter t h e addi t i on of a��oniu� substrate

upwards curv i l inear mode l ( F igure 4 . 1 , dotted l ine ; R2 = 0 . 9 8 , p< 0 . 1 % ) wh i c h i s wha t wou l d be e xpec ted f o r a growing populat ion . _ It was theref ore concluded that the incubat ion time selected would have to be a comprom ise which sati s f i ed thre e basi c requirements ; ( a ) that ni trate product i on should be approximately l inear with time ; ( b ) the incubation t ime would have to be long enough to a l low for product ion o f a measurable increase in No3 - concentration ; and ( c ) not too long that s igni f icant popu l a t i on g r owth o c c urred , or t he expe r i ment be c a m e ph ys i c a l l y i mpo s s ible t o do . A l inear model was f itted to the data between 1 and 8 hours ( Darrah e t al . , 1 9 8 6b ) and s ince this gave a good f i t ( Figure 4 . 1 s o l i d l in e ; R2 = 0 . 8 2 , p < 0 . 1 % ) these time s s eemed sui table for SNA measurements on the Tokomaru s i l t l oam .

As indicated above , the t ime taken to complete an SNA analysis was an important cons ideration . I t was apparent that the maximum number · of tubes that could be deal t with at a time wi thout incurring a t ime error whi lst s ampl ing the suspens i ons , was between 1 5 and 2 0 . ( In fact w i th pract ice , 1 5 tubes could be sampled in 2 minutes . i . e . the t ime o f sampl ing given as 1 or 8 hours is accurate to ± 3 % a f ter 1 hour and ± 0 . 4 % after 8 hours . ) By s t art ing the Buchner suct ion at 6 . 3 0 am ( and a l l owing for a period of 5 hours for the samples to equil ibrate fol lowing the addition o f a c i d or a l ka l i - s ee Chapter 7 and Darrah e t a l . ( 1 9 8 6b ) ) , and separat ing the tubes into f our groups o f 1 5 with a stagger o f hal f an hour between the f irst and second and the third and f ourth groups , and 4 5 minutes between the second and third , i t was possible to complete 6 0 SNA analyses by.midnight i f an incubat ion of 8 hours was used . Accordingly , i n a l l S NA anal yses reported here , the incubat ing media w ere sampled a f ter 1 and 8 hours , and the SNA value calcu l ated as the diff erence between N03 -N produced at these t imes per g soil per hour .

iii . Selection o f ammonium substrate concentrati on for SNA

Macduff and Whi te ( 1 985 ) measured nitrif ication rates over a range of so i l mo i s ture con t ents and i ncub a t i o n t emperatures , and f ound that irrespect ive of temperature, nitrification was l imited by the supply of NH 4 -N . Gilmour ( 1 9 8 4 ) predicted that nitri f ication rates fol lowing zero­ order k i ne t i c s w i l l increase as the i n i t i a l c.<:mcentrat ion of NH4 · increases according t o the equation :

NR = k )I NXt

( 4 .

3 )

where NR is the absolute nitri f i cation rate , k is the rate constant , and NXt i s the init i a l NH4 • concentrat ion at the start of the t ime period t . Mo l ina ( 1 9 8 5 ) a s s e r t ed tha t n i t r i f i c a t i on proceeded from pulses of ammonium oxidation generated by microbial c lusters . The size of the pulse mus t be subj ect to a negat ive feedback system , however , because if large app l i c a t ions o f a mmon i um are supp l i e d , the e f f ec t of l o w osmotic po t en t i a l w i l l i n h i b i t n i t r i f i ca t i on ( Darrah et a l . , 1 9 8 7 a ) . Thus equation

( 4 .

3 ) must be treated with c ircumspection because i t suggests that NR will continue to increase l inearly with increas ing initial NH 4 concentrat ion even a t very high concentrations . Furthermore , C l ay e t a l .

( 1 9 8 5 ) noted that i f clusters of NH 4 · oxidizers were to generat e a pulse of N02 - large enough , the ir microni che may be acidi f ied to toxic levels and nitr i f icat i on would be reduc ed ( Chapter 7 ) . Neverthe less equat ion ( 4 . 3 ) does indicate that nitri f ication rates can be l imited by inadequate NH 4 - N subs t r a t e concentra t i ons . Thus , the concentr at i o n of NH 4 - N subs trat e supp l ied i n SNA incuba tions must b e such that i t i s non­ l imiting in the sense that it is in excess , but not so much so as to generate conditions which are toxic to the nitr i f iers .

The form in which the ammonium was to be supplied for SNA measurements was also an important cons iderat ion , the obvious choice being between NH4Cl and ( NH 4 )

;

so4 since these are readily ava i l able forms o f ammonium . Darrah et a l . ( 1 9 8 5 a ) mon i tored t he response of soil n i tr i f i e rs to additions of both NH4Cl and ( NH4 ) 2 S04 . Addi t ions of more than 7 . 3 � moles N g_ , soil as NH4 C l were found to inhibit ni trif icat ion , but . a s imi l ar e f fect was not f ound w i th ( NH 4 ) 2S04 sugges ting that the c hl oride ion

work ( Darrah et a l . , 1 98 7 a ) demonstrated that the inhibitory ef f ec t of

the c l - i on was dis propor t iona t e to its con tri but ion to the . o smotic po t en t i a l of the s o i 1 s o l u t i on . i . e . C l - i s t o x i c t o nitri f i ers . Accordingly , ( NH 4 ) 2 S04 was chosen as the subs trate to be used for SNA me a su r em e nt s , bu t a n e x p e r i men t w a s r e qu i re d t o e s t a b l i s h what concentration should be use d , s ince Darrah et a l . ( 1 9 85a ; 1 9 87a ) had shown that this salt was a l so inhibitory to nitr i f i er activity in high concentrat i ons .

Methods and Materials

A bul k s o i l sample was col lected as for the l ineari t y experiment ( Section i i . above ) and l eached overnight with 0 . 0 0 5 M KC! . A f ter the remova l of exc e s s m o i s t ur e , 6 0 i ncuba t i on s were s e t up f o l l ow i ng t he method des cri bed abo ve , exc ept tha t the tubes were sp l i t into 6 groups and ( NH 4 ) 2 S 0 4 substrate added a s 1 0 cm3 of 0 . 0 0 1 , 0 . 0 0 5 , 0 . 0 0 8 , 0 . 0 1 0 , 0 . 0 1 5 or 0 . 0 2 0 M ( 1 0 repl ica tes each ) . The incuba tions were carried ou t as described , with sampling of the suspens ion after 1 and 8 hours .

Result s and D is cuss i on

The mean SNA values f o r each concentrat ion o f ( NH 4 ) 2 S 0 4 added to the incubat i ons are shown i n Table 4 . 1 . An analysis of variance showed that there was no s igni f icant di f ference in SNA value ( p < 0 . 1 % ) over the range of NH4 -N concentrat ions tes ted . i . e . there was ei ther no inhibition of nitr i f ier activity in any treatment , or the amount o f inhibition was the same for e ach t reat ment . D i r ec t c ompa rison between thes e resul ts and those o f Darrah et a l . ( 1 9 8 7 a ) i s made diff icult by the fact that ( a ) in this experiment there was no means of measuring the osmotic potent ial ( an osmometer was not ava i l able ) ; ( b ) the ionic s trengths o f the incubat ion med i a were quite d i f ferent ( 0 . 0 1 M CaCl 2 compared to 0 . 0 0 5 M KC! ) ; and most important ly ( c ) the ionic s trength of the soil solut ion in the f ield i n the s o i l s tudi ed by Darrah et a l . ( 1 987a ) was probably d i f ferent f rom that i n the Tokomaru s i l t l o a m , wi th the consequence that the soil

Concentrat i on o f ( NH4 ) 2 S04 0 . 00 1 M 0 . 00 5 M 0 . 0 0 8 M 0 . 0 1 0 M 0 . 0 1 5 M 0 . 02 0 M Mean SNA ( IJ mol N03 -N g- 1 h_ , ) 0 . 0 1 6 0 . 0 1 5 0 . 0 1 5 0 . 0 1 5 0 . 0 1 3 0 . 0 1 5 Standard e:r.ror 0 . 0 0 1 0 0 . 0 0 0 8 O . OO J 1 0 . 00 1 5 0 . 00 1 0 0 . 00 1 7

d i f ferent between the two soi ls , certainly in terms o f i ts magni tude . Nevertheless , i f one assumes that the dif ferences between the incubating media were insign i f icant in terms of their ef f ect on the overal l osmot i c potent ial in the two exper iments , s imple comparisons can b e made .

Osmotic pressures were calculated for each substrate concentrati on us ing the equat ion ( Hi l le l , 1 9 80 ) :

r = MRT ( 4 .

4 )

where r i s the osmotic pres sure ( Pa ; 1 bar = 1 0 5 Pa ) , M i s the total mol ar concentration of solutes ( mo l m- 3 ) , T is the temperature ( degrees Kelvin ) and R is the gas constant ( 8 . 3 1 4 J K_ , mo l - ' ) . Us ing equat ion ( 4 . 4 ) , i t was apparent that the osmotic potent ial o f the substrates used wa s high ( osmotic potent ial = - osmotic pressure ) in relat ion to those of Darrah et a l . ( 1 9 8 7a ) , and therefore one was l ed to conclude that there was no inhibit ion of nitrif icat ion in any of these incubations . In view o f thi s , and mindful that no account had so far been taken of the po s s i b le seasonal f l uc tuat ion in S NA va lue ( se e Chapter 7 ) and the consequent poss ibilty of much higher values at other times of the year than t hose measured here , 0 . 0 1 M ( NH4 ) 2 S04 was chosen as the substrate to be u sed in a l l f uture SNA ana l yse s . S ince the SNA va lue for 0 . 0 1 M ( NH 4 ) 2 S04 was 0 . 0 1 5 ± 0 . 0 0 1 5 � mol N g- ' h_ , indications were that this concentration o f NH4- was wel l in excess of the nitrif ier requirements , but not too high to cause toxicity problems .

iv . o f ammonium

I n s e v e r a l o f the e x pe r i me n t s d e s c r ibed in the fol lowing chapters , exchangeable ammonium was anal ysed . Thi s was a pa r ticularly impo rtant ana l ys i s in the spa t i a l vari abi l ity exper imen t ( Chapter 6 ) s ince one p o s s ib i l ty w a s t ha t v a r i a b i l i ty in n i tr i f i e r act i v i t y f o l lowed variab i li ty in Ex-NH 4 - ·

6 g s o i l ( oven-dry equival ent , s ieved < 2 mm ) was weighed into 5 0 c m3 plas t i c tubes ( the same as those used for SNA measurement s ) . To these , 3 0 cm3 2 M KCl was added and the tubes were shaken i n an end-over-end shaker for 2 hours . The suspens i ons were then f i ltered through Whatman No . 3 2 f i l ter paper , and the f i l trate frozen and stored . This was l ater analysed for N H 4 -N on a Techn i c on au toanalys er f o l l owing the s tandard method ( Technicon Users Manual , 1 9 7 6 ) .

B . FIELD SAMPLING

i . S ite Deta i l s

Unless otherwise indicated , a l l the experimental work reported here was carried out us ing soi l sampled from the Massey Univers i ty No . 4 Dairy Farm , spec i f i ca l l y from two adj acent f ields in the Soil Sci ence Research Area - No . 6 , which had no previous history of fert i l izer trials or other experimental wo rk , was u sed for t he bul k of the s tudy , and No . 2 , a f ormer l ime tria l , whose hi story i s detai led in Chapter 7 , was used for the pH related work . Both f ie lds had been under a ryegrass -white clover p a s t ur e f o r severa l y e a r s and f o r the durat ion of thi s s tudy were peri odically grazed by sheep ( 3 - 4 days each graz ing ) at approximately 1 4 0 s tock units ha- ' ; one stock unit is equivalent to a 5 5 kg ewe ( weight at mating ) which consumes suf f i c ient dry matter to produce one weaned l amb per year ( Cornforth & Sinclair , 1 9 8 4 ) .

The s o i l at this s i te , the Tokomaru s i l t loam ( Cowie , 1 9 7 4 ) i s c lass i f i ed a s a Yel low Grey Earth ( Taylor & Pohlen , 1 9 68 ) or Typic f ragiaqual f ( So i l Survey Staf f , 1 9 7 4 ) . I t i s a poor ly drained s o i l of low nutrient sta tus and was not recommended by Cowie ( 1 9 7 4 ) for cropping or hort icultural use . He recommended regular dress ings o f phosphate , l ime and potash to a c h i eve opt i mum pasture growth . According l y , land on this soil is typical ly used for town dairy supply ( serving Palmerston North ) and the f a t t e n ing of s heep , a l th ough produ c t i on can be l imi ted by the wet c ond i t ions in winter and spr ing , and by the drying out o f the soil in s umme r .

Mean annual rainfal l a t the s i te , which i s approximately 7 5 m above sea l eve l , is 9 9 5 mm and mean monthly temperatures range from 8 oc i n. Jul y to

1 7 oc in February ( New Zealand Meteorological Service ;· Figure 4 . 2 ) .

i i . Soil

S o i l samp l es for a l l e xperiment s except the pH work ( Chapter 7 ) were taken in sect ions us ing a core auger 3 cm deep with a diameter of 5 cm . For a l l SNA measurements ( see below ) , the top 3 cm l ayer was discarded to m inimise any inhibitory ef fects that grass root s may have on the rate of nitrif ication ( Mo l ina & Rovira , 1 9 6 4 ; Neal , 1 9 69 ; Moore & Waid , 1 9 7 1 ) , and the 3 - 9 cm l ayer retained f or analysis . The rat i onale behind us ing this depth range f or experimental work is more ful ly explained in Chapter 5 . On a l l sampling occasions , samples were s i eved ( < 2 mm ) as soon as pos s ible a f ter sampl ing and stored in sealed plastic bags at 3 oc ( see

Section C bel ow ) .

Prior to soil sampl ing , there was always a per i od o f three weeks during which there was no gra z i ng . Th i s was done i n an a t tempt to min imise graz ing e f f ects such as ho tspo ts o f high nitrate concentrat i on caused by the urine and excreta o f grazing animals ( Ryden e t a l . , 1 98 4 ; Bal l & Ryden, 1 9 8 4 ; Whi te , 1 9 8 4 ) .

i i i . Correlation between moisture contents of sieved and rinsieved soil

As out l ined in sec t ion A ( above ) , the amount o f s oi l re quired for a s ingle SNA analysis was approximatel y 5 g ( oven-dry equivalent ) of s i eved s oi l . Each s o i l sampl e was analysed in dupl icate for nitri f er activi t y , a n d i n a dd i t i o n , t w o f urther s ub - s a m p l e s o f 6 g e a c h ( oven - dr y equivalent ) were n eeded for ana l y s i s o f exchangeab le ammonium , and a f urther 1 0 g needed f or measurement ( in dupl i cate ) o f the s o i l moisture content so that results could be calculated on a per g dry soil bas i s . Overa l l , approximately 3 5 g s ieved soil was needed f or analysis . S ince the purpos e of this wo rk was to study f ield n itrif ier activity , e ach

200

In document Implementación de la política pública Planes Departamentales para el manejo empresarial de los servicios de agua y saneamiento en los departamentos del Magdalena y del Guaviare 2008 2015 (página 115-120)