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Model for decomposition of organic material by microorganisms
MODEL
FOR
DECOMPOSITION
MATERIAL
BY MICROORGANISMS HAWA
Ecology
Center.
Utah
OF ORGANIC
PARNAS*
State Universit).
Logan.
L’tah
83322, U.S.A
Summary-A theoretical lnodel which gives the rate of lnicrobial decomposition ororganic material (plant or animal residues. or soil organic matter) is presented. Explicit equations for the rkite of decomposition. mineralizations and immobilization are given. The main assumption of the model is that the rate of decomposition of any substrate is proportional to the _erowth rate of its decomposers. The main results of the model arc: I. Addition of extra nitrogen to materials poor in nitrogen increase their rate of decomposition; 2. Addition ofextra nitrogen to a substrate whose initial carbon/nitrogen ratio is above a crltical ratio (20~30) causes a decrease in the substrate’s carhon:nitrogen ratio during its decomposition; 3. If the lmtial carhon.!nltrogen ratio is below the critlcal one, no change in the subatratc’s ratio will occur with time; 4. Net mineralization oforganic-nitrogen occurs when the substrate bcmg decomposed hna an initial carbon/nitrogen ratio which is below the critical one; 5. Addition of ammonium to such a substrate (point 4) will increase the rate of organic-nitrogen minerallration hut not necessarily the rate of net mmerali7ation. All the model results are analytical and independent of the values for the various parameters. Neverthe&s the application of the model to real field conditions is discussed while presenting a numerical example.
INTRODLCTION Decomposition processes in the soil have been investigated extensively. hut many questions remain unresolved. For example, it has been shown that the C/N ratio of plant residues low in nitrogen decreases during the process of decomposition if nitrogen is available in CYCCSS to that contained in the residue (Bartholomew. 1965; Brown and Dickey, 1970; Smith and Douglas. 1971). The explanation generally given for this phcnomenon is that inorganic nitrogen is immobilized during the microbial decomposition of the organic carbon of the residue. In this process a net gain in organic nitrogen occurs and at the same time a net loss in organic carbon results from CO1 evolution. The result is a decrease in the organic C/N ratio. This explanation describes the situation in which the “organic matter” includes both the substrate for decomposition and its decomposers. hut, the C/N ratio of the residue its&” is not considered. Harmscn and Kolenbrander (1965) pointed out that the C/N ratio of the substrate itself might decrease during the decay. As the) stated: “This decrease often is in part the consequence of the fact that the fractions with the highest ratio arc the most readily disintegrated parts of the substrate”. During the decomposition of organic matter with a low C.N ratio. inorganic nitrogen accumulates (Bartholomew, 1965). Harmsen and Kolenbrander (1965) reported that addition of fertilizer nitrogen sometimes results in an increased accumulation of inorganic
* Present address: sit). Jerusalem,
Life Science Institute.
Hebrew
Univer-
Israel. Ihl
nitrogen. Broadbent and Nakashima (1971) also found that addition of NH: increases the rate of mineralization of organic nitrogen. and attempted to explain it by osmotic effects. On the other hand, Harmsen and Kolenhrander (1965) mention in their review that a very high concentration of soluble mineral nitrogen in the soil reduces the apparent rate of mincralizatlon of nitrogen. A more difficult question concerns the interrelation between decomposition of organic matter in the soil and that of substrates added to it. It has been reported that loss of carbon from soil organic matter was increased when plant residues were added to the soil (Hallam and Bartholomew. 1953 ; Bingeman, Varner and Martin. 1953; Broadhent. 1947: Broadbent and Norman. 1947). It was also shown that the rate of decomposition both of the added plant residue and the
ISprescntcd. The erect of added nitrogen on decomposition. mineralization and immobilization processes is discussed. The subject of the subsequent report will be a model of the mutual effect between soil organic mattcr and any type of added material. on their respective rates of decomposition. The basic idea of the two models is the same. They arc based on the wcli cst~~biished fact that the organic material supplies the nutrients for the growth of the decomposcrs. Therefore. the rate of decomposition of an organic material is closely linked with the growth of the organisms responsible for its decomposition. Any incrcnsc in the growth rate of the decomposers causes higher rate of decay of that material which supplies the nutrients. By the same rule. any fktor inhihiting the growth of the decomposers, such as suboptimal c~i~~iroiitl~cnt~~jconditions of temperature, moisture. acrrttion, and so on. reduces the rate of decomposition. THE MODEL
Carbon is thus the growth-limiting factor when C: N, < x/J, and nitrogen is when C;N > ~/if,,.as long as the growth rate is still dependent on the concentrations of the substrates (carbon and nitrogen). A flow diagram ofcarbon and nitrogen in the microbial decomposition of R is given in Fig. I. The flo\v diagram and the lnathern~~tic~tl model arc based on the following assumptions. I. Loss of carbon from organic matter (plant. animal or microbial residues. or humic matter) is proportional to the growth rate and the maintenance rcquiroments of the organisms decomposing it. 3. The growth rate ofthe decomposcrs in a given set of environmental conditions (temperature. pH etc.) and for a given type of organic matter increases with increasing carbon and nitrogen coticet~tr~ltions. The exact dependency is shown later. 3. The decomposers use N, and other nitrogen compounds as nitrogen sources according to their rclative availability. 4. Release of nitrogen (N,) from R is proportional to the loss of carbon from the C N compounds. i.c. to the loss of ‘2,. 5. The decomposers can use as energy and carbon source both C, and C2.
Organic matter R is subject to decomposition. R is a mixture of substances. Some of thorn contain nitrogen as a molecular constituent. such as proteins and RNA. and will be termed C N Compounds. The others do not contain nitrogen, such as cellulose and starch. and those will be termed C-Compounds. The amount of carbon per unit soil (weight or area) of the dR dt‘ dN, C N compounds is C, and that of the C-compounds dr df + dr C,. The sum of C, and C2 is defined as C. The amount nitrogen in the C--N compounds is N ,. Although Using assumption 1 several types of C’-N compounds are found in R. JWOfci/l will be considcrcd as the only, decomposing sub(31 stance. The misturc of ~nicroorganlslns which grow on that R. the dccomposers, use R as the source for the where: x is defined above. H is the biomass of the nutrients they need. If all the other nutrients except dccomposers per unit soil. G is the growth rate of the carbon and nitrogen are present in excess, the growth decomposers. rate G of the decomposers is a function of the concentration of the availahlc carbon and nitrogen. Carbon is required both as a source of energy and for building the microbial biomass. Nitrogen is required for the latter purpose onIt. Therefore a certain ratio exists hctwecn the carbon and the nitrogen required for growth. This ratio is defined as:
of
iv here
C
:
= average fraction of carbon
in the decomposer’s
Cdl F
% ,fi
ratio of carbon posed = is thus the total per unit biomass = average fraction
=
CCII
assimilated
to carbon
decom-
carbon used by the organisms increment of nitrogen in the decomposer’s
II
NH;
L
“NH4
1
Fig. I. Flow diagram of carbon and nitrogen as a result of microbial decomposition (for details see text). R = organic matter subject to decomposition. C, + C, = carbon. N, f NH, = nitrogen. G = growth rate. B = decomposcrs biomass, 1~ = mineralization rate. i = immobikation rate. VNH; = input of NH: from an independent source.
16:
Decomposition of organic material by microorganisms Using assumption
4 dN, dt
dC,
N,
(4a)
dt ‘C, ’
-dNl =dC,.,p.
14b) ~ dr 1 where /3 is constant (average) ratio of carbon~nitrogcn in the C-N compounds. From the flow diagram. and since G as stated in assumption 2 is a function of both carbon and nitrogen. the release of N, is also given by:
dr
dN, _ dt
-(
r;,
G B - i) -
111
where ~1is the rate of mineralizatiol~ of N ,. i is the rate of iinmobili~~~tion of NH:. In order to define dC,/df and dC,idt the following should be considered. The role of protein in the growth of the decomposers may be primarily as a nitrogen source. or primarily a source of carbon. However. in both cases C, and N, are coupled in the rate of their release from R. The ratio of the C-compounds to the C-N compounds is derived from the C/N, ratio. There is an increase in the relative concentration of C-N compoutlds as the C,‘N ratio dccreascs. When C:N, = fi no C-compounds are present and only C-N compounds are found. At that extreme condition. the C N compounds are the only source for hoth carbon and nitrogen. However, under conditions where C/N, > /i the question arises whether the rate of protein breakdown is determined by the requirement for nitrogen or for carbon. An approach to express dC,jdt as a function of C/N, is through finding the optimal strategy by which the above questions are answered. According to considcr~~tions of maximal Ion! term growth rate (which will also determine the optlmal strategy) the foltowing function is presently assumed. As long as Ci N, > a#;,, nitrogen is the growth limiting factor. Under this condition, carbon is in excess and there is no advantage in diverting the protein to serve primarily as a carbon source for the following reasons. First, there will be no increase in growth rate at the present (carbon is in excess anyhow). Second, in the future such a diversion wili cause a decrease in growth rate. since nitrogen will be even more limiting (it should be remembered that, fl grams of protein are required per gram carbon). It can be concluded that under such conditions when nitrogen is the growth limiting factor, protein serves primarily as a nitrogen source and the C-compounds as the main carbon source. When C/ N, < r/f;,, carbon is the growth limiting factor. Under this condition. optimal considerati~~n force the diversion of protein to serve primarily as a carbon source together with the C-compounds. simply because otherwise the carbon available for the decomposers will be tower than the existing C. Since when C/N, < q”;,, the tong term growth rate is determined by carbon. decrease in the available carbon wilt lower the growth rate.
In the present work. the optimal distribution between C, and C, as sources for carbon when C:’ N, < x/j,, is not computed. Rather it is assumed that the two contribute according to their relative concentration. When C/N, = r.‘f;, the nitrogen and the carbon are present in the required ratio. thorclore. this is the point at which protein will ~+witchfrom being primarily a liifrogcli source to beco~ne ~rii~~~lr~l~a carbon source. It can be further assumed that the NH: concentration dots not affect the optimal stratcg) as to the shift of protein to serve primarily as a source of carbon. Ammonium. if present. will all&t the concentration 01 nitrogen and therefore the growth rate. Those considerations are expressed in the following equations. When C/N, ~.ff;,,the rate of protein breakdown is according to the requirement for nitrogen. dC,;dr is coupled to the rate of N, which goes to growth. Thus
if C, supplies the rest of tho carbon needed. CIC,
-= dt
-[~.(;.B-p(r,,.t;.H-i)l
17)
When C/N, x/f,,,both C, and C3 serve under this condition as carbon source according to their rclativc concentrations. Thus
(‘7 --C
dCZ - = -3.G.B. dt
The final equation for dC,/dr is therefore given by equation 6 if C:‘N, r$,,, and by equation X if C: N 1 r/f;,. At C/N, = YJ,, equations 6 and X arc equnl. if i = 0. The final equation for dN,/& is given by equation dN, dt
=
-(
G
13 -
i)
if
:’
;
‘I
N,
dN, it-
f,,
=
-KEG%
iE C‘ N,
iIO)
,I
2
I,,
01)
The ilnlnobili7ation (i) describes the rate at which NH; is transferred to microbial biomass. According to assumption 3. it is defined by: i = r;,.G.R.
NH: -~-NH,+ + N,
The change in NH,+ concentration
(13)
is given by
dNH where: 01 = rninera~i~~tioti rate. i = ililtnol~iii~~tioli rate. V,,, = rate of input of NH; into the system from independent sources. The growth rate G is a function of both carbon and nitrogen. It is given by the family of curves in Fig. 2. The mathematical expression for each point on those
HAMA PAKNAS
! h-l
A
G
G
N
1
c
If C/N, = rx(f,no change will occur. If C/N, < r~&, and NH: = 0. dN,,‘dt is given by 11. By incorporating it into equation IS it can be seen that d(C,N,)/dt is equal to zero. Codwiorr I. The C/N, ratio of an organic material being decomposed in the ahscnce of other nitrogen sources wit1 i~cteasr with time if its initial C/N, is higher than x,:/i,. and will not change with time if its C: N, is equal to or lower than xj&. (b) Ej@rt oi’a~i&/ ~irroc/or~ 011 rllc~cirtr~\c wiih lirrlc’ ill ~/IL,CjN, ratio. In this model. though reference is made to ammonium. it should bc remembered that the same holds true for any kind of nitrogen which can serve for decomposcr’s growth. If C;N, 2 $i and NH; > 0. dN,/dr is given by equation (IO) and when incorporated into IS. we get
Fig. 2. Growth rate (G) as a ftlnction of carbon (C) at various
nitrogen
d
(N) concentrations.
C I
y,
--d, curves is given by Michaelis-Menten Kinetics bisubstrate reactions. G ,C‘.N ___“. __. (k,. + c-)(1,,,+ N)
for the
G=
(14)
N C li,
.L
(17) in the paren-
By isolating NH; from the expression theses in equation 17, one can see that d,k -- di ’
where : Glnal
C‘
c;. B
- =g,
= maximal growth rate in given environmental conditions for a given type of R when C and N are high so that ?G,!C:Cand ?G/?N = 0. = N, + NH: =c, +cl = a constant which describes the concentration of c’ at given N which will give G = G,,,/2, for a given R (i.e. the Michaelis constant) = the same as (i(,) for N
> ()
jr
EL’H: <
C.j;,-z.N
1
r
(18)
DiV.ilXVllwulrs
ct
c
:v, dr--
I
= N;
dC
C’
dN,
.-& - (N,)”
-dt
(15)
when
Substituting equation dCldt into 1.5gives:
It shows that if NH; is pwsolt and its concentration is high enough (the exact limit depends on C and N,), the C/N, ratio of R’s which have little protein in them will dccrc~r.sc,with time, while without NH; it would increase with time. Since NH; is not part of equation 15 when C.‘N, ‘. U;f,,, it will have no effect on the x! change ofthc C!N, ratio at that conditions (C:N,
L).
10 for dN,/dt
and equation
3 for
From the starting conditions it follows that the expression in the parentheses is positive and so is I/ N, G B. Therefore d(C/N ,)/dt > 0. Thus, when no NH: is present and the organic material being decomposed has a C/N, ratio higher than z/j;. its C/N ratio will increase with time.
tord1rsio~7 2. Where the initial CiN, ratio in R is greater than s/f,,. the ratio can decrease with time only if other sources of N are present. At this point of the discussion it should be mentioned that part of the confusion in the literature is because the rnc~r,strrc~d C/N ratio includes the microbial biomass in addition to the organic material being decomposed. It is therefore it~port~ltlt to show how the microbial biomass affects the two conclusions. (c) i$Yi 0l’ j/~~~~~~~~~~~ ~~~i(,~~~~;~~/ hic1rrtcrss O/f tl1r c'lzarigc nibqrrl
with
tiu7P
in
tiw
wtrswwl
icrtio
of
ctrrho~~
to
(C$N,). We define: CB = C +.f;.B.
(1%
N, = N, +,f;.B.
(20)
Decomposition
By using equation gets c‘,I d N,
cir
15 together
I h’
of organic material by microorganisms
with i 9 and 20 one
I‘, dN, 1 dC .- - -~ = N, iir N,< dr
(e) E$ct c?f’NH: ou m. The rate of mineralization 111is always zero if C/N, > r,if;,.whether other nitrogen sources are present or not. The effect of NH: on l~itler~~liz~ition has therefore to be checked when C~ N L < x/j;,_3~ combining equations (23). (1 if and (12) and differe&ating it with respect to NH; wc get
ft is very difficult to see a clear picture from equation 21, but in the interesting region when C!N, 2 yjfn. equation 21 becomes: (‘1,
Cl.N"
‘d;”
= :_
G
B
[,,
N
N, N, + NH;
C + ,r;. B N , t ,;, 5
_ _i_ N,,
dB (‘,I d; .f‘ - r. 1,, H
(23)
(C +f;.B)/(N, i- j,.B) approaches f;,$, with increase in B. This means that the increasing B tends to decrease the first expression of equation 22 so that it might become negative. If B is so high that C$N, : j,: ,t;, then the second expression is zero. ‘I’hc conclusion is that the effect of including the microbial biomass on the measured C,INH is in the direction of reducing it with time, even when the C/N, ratio of R itself is increasing with time. Distinction between measured C,,: N, ratio and the S;‘N, of R wili be further discussed in section 3. (d) M~uYYI/~‘~I~~oK Mineralization rate 111is defined here as the rate of converting organic nitrogen to NH:. The mineralization rate can be derived from equations (5) and (12).
Since C/N, : YJ’I’;,. it follows that z/C - (f,,)/(NH, + N, ) > 0. This means that in/iNH, > 0, or that addition of NH; increases the rate of mineralization. The conclusions from sections d and e are: C~)~~~~[~,sj~}~~ 3. Miner~~liz~ltio~l occurs when the C:N 1 ratio for R being decomposed is lower than PY/;,. Cortci~sion 4. The rate ofminerali~ation is increased as C/N, is decreased. COIIC~/~I.S~O~I 5. The higher the amount of NH: in the area, the /t$ro~ will be the rate of mineralization. (f) SKI rtlirlc~~trliltrtio~~.Net mineralization /n,, is defined here by the net change in NH: collc~ntration for a given j~?ifjf~~collcentr~~tlon of NH;, (V,,,L > 0 only at t = 0) caused by mineralization and immohilization. Ill,,
=
111
-
i
(76)
and * f in,,
?NH,
ill1
t-i
(‘NH,
(27)
iNH;
--. N, _+ G’(NH,+
N,)”
.
(‘8)
ii II C’
N,
(‘NH,
I
/,,
is therefore always positive. ~!f;, where 1~ = 0 and c’rni It follows that at C’iN, /‘NH, = 0. fir,, will be zero or negative. When C’;N, ’ q’f,. and nr > 0. The combination gives
From the starting conditions it follows that the expression in the parentheses is positive. which means the mineralization occurs when CiNr < Z/A. The exact rate is given by equation (23).
of equations
(25). (27) and (19)
This derivative is always positive. C’~UIC/US~OI~ 6. Net mineralization is decreased by NH, when C/N, sc,!f;,and increased by NH; when C;N, < y/f’.,1 3.
.vrfflfcTictff
rwfrql/c~
The validity and generality of the results and conctusions presented previously are IZO~dependent on the
HAUUAPAKNAS
I (10
specific values that arc given to the different parameters. The purpose of the numerical example is to dumonstrnte the wav in which the model may be applied to real condi;ions. For the purpose of numerical solutions the environmental conditions temperature, moisture, etc.. were not included. The way in which they could bc included w,ill bc mentioned in the discussion. The numerical v~alucs that have been used for the different parameters arc shown below. . k, and k,, are cl~oscn The numerical values of 6 in SLICKa way to give close agreement with field data, These values arc by no means absolute and reflect only a magnitude of order. Parnas and J. Radford have determined their values in a simulation model on decomposition written for the United States International Biological Program Desert Biome. It has to bc cmphasired that the numerical values of those parameters arc not very important for the present model. though they are of great importance in a simulation model. The numerical values of the other parameters rcllect the most common values appearing in the literature. (‘,ZL<‘CLATE11
___ 2 500z > 400E :: p 3005 ti a zooz E! J too-
Fig. 3. dC;d!. dC,/‘dr and dC2/dt as a function of C/N,. C = IOglm2. B = 10g/mZ. NH: = 0. l = dC,/dt, x = dCj/dt, o = dC/dt.
HESCILTS
dent on C and N, as can be seen in equation 19. The The l’tlnctions dC.!dt, dC,/dt and dC?/dt as a funcdegree of change in C/N, is also dependent on C and tion of CX, ratio arc prcscntcd in Fig. 3. The entity N,; but in addition to this. it will increase as C/N,, 7 ‘/;,. using the values that have been mentioned preG 1,,:,, and NH; increase. viously is equal to 25, It can bc seen in Fig. i that at In Fig. 6 and Table I, the difference between the C‘N , > x ‘1,;both dC,jdr and dC‘,;dt incrcasc as CN, change with time of the C/N, ratio and that of the UIMdecreases. the first one beca~~sc dN,/dt increases and SUI& ratio of organic carbon to organic nitrogen (C,/ the second because dC;dr increases. Starting at C; NH) is shown. In Fig. 6, the ratio between R/B ’ I. N, = r:f,, both C, and C2 scrvc as carbon source Such a ratio is normally much too low for field condiaccording to their rclativje concentrations: therefore tions. A high value of B was chosen on purpose to dC, dt is increasing sharply and dC,/dt decreasing demonstrate the maximal effect of B on the C/N, ratio. sharply as the C’N, ratio approaches p. At this point C‘, = (‘and C. = 0. Addition of NH: will lower dC, 111 To R which had initial C/N, = 50. B was added and C,i/N, dropped to 21-4. It can be seen that when B is dr if C ‘N, > rf,, and will increase it, if CN, < Qj, The rate ofchange of the C‘:N, ratio with time is a11 high the initial value C/N, will inevitably be much smaller than values actually found in plant residues, If increasing function of the initial C;N,. as can be seen in Fig. 4. When the initial C/N, ratio is higher than 25. a high C,.IN, ratio is found. this by itself means that the microbial biomass is not very high compared to R. addition of NH: will cause the C,/N, ratio to decrease But even with such large microbial biomass the change instead of increasing with time as shown in Fig. 5. The in C,/N, did decrease when C/N, increased, but the concentration of NH: which is required to cause a derate ofdccrease was from 21.4 to 21.3 in .10 days. When crease in the C.‘N, ratio with time is evidently depen-
Paramclcr
Meaning
Value
maximal growth rate ofa mixture of decomposers constant equivalent in its meaning to the Michaclis constant. but has area con centration units instead of volumetric concentration units. Means the concentration of carbonwhich gives half the maximal growth rate similar to h, and means the concentration of nitrogen which gives the half maximal growth rate cliicienc> of carbon assimilation fraction of carbon in the microbial hmmass fraction of nltrogcn In the microbial biomass molecular ratio between carbon and nitrogen in protein
0.0
I
20
0.4 0.4 0.5 0.05 4
Units day-
g/m’
g/m’
’
Decomposition 60
of organic
material
I b
ra
As R/B becomes smaller the opposite happens. which means the rate of decrease of C/N, becomes much higher than that of C,/N,. Another point to remember is. that while U/IVsource ofadditional nitrogen (organic or inorganic) will result in decrease in C/N, ratio with time, only ~UOV~LUI~ nitrogen will cause a big chan_ee in C,$jN,,. Without inorganic-nitrogen. the change III C&N,% will be moderate as is shown in the upper part ofTable I when NH, = 0. We can conclude that since the initial carbon/nitrogen ratios found in field conditions are much higher than f;/f;, the microbial biomass has only a very smdll effect. if any. on the change even of the measured carbon,nitrogen ratio. The measured carborq’nitrogcn ratio reflects mainly the C/N, ratio. and the decrease in C! N, is caused mainly by addition of nitrogen. The mineralization rate is shown in Fig. 7 as a function of C/N, ratio. As has been stated in conclusion 4. the rate of mineralkation increases as C/N, decreastx.
DAYS
DAYS
Fig. 4. Change of C.!N, ratio of R being decomposed with time. (a) NH: = 0. l = initial C/N, = 50. x = initial C,: N, = 35. 0 = initial C,N, = 25. (b) NH; = O.Xg!m’. 0 = initial C;N,
= 50. x = initial C;N,
= 30.
5654
50
by microorganisms
NH4
c
N, 48 46
DAYS
Fig. 5. Change of C/N, ratio with time at different NH: concentrations. C = 10g/m2, N, = 0,2g/m’. NH; in g/m2 is: 0 = 0, x = 0.1. q = 0.4, D = 0.X.
NH, was present while C/N, ratio decreased from 50 to 45.X. C,/‘N, decreased only from 21.4 to 190. In Table I the change with time in C/N, and C,/N, ratio’s? when R/B z 10 is shown. The most pronounced effect of the microbial biomass is when no NH: is present. Then C/N, increases with time and C,j/N,j decreases. When NH,’ is present in sutficient amount, both C/N, and C,/N, decreases. The rate in decrease in C,/N,j is slightly higher. when R/B z 10. Table
I. Change
in C/N,
and C,/N,
wth
DAYS Fig. 6. C/N, and C,,‘N, as a function of time. 0 = C/N and NH,’ = 0, 0 = C/N, when NH: = 0.X g;m’. x = C,/N, when NH; = 0, V = C,,‘N, when NH: = 0.X.&m’. C =
IOgjcm’, E = log/m’.
time. as a function
of NH:
concentration
C/N,
N,
NH:
B
(g.‘tn)
km)
(g/m)
0.25
0
0.33
0
I I
0.66 0.25
0 0.7
0.33
0.7
0.66
0.3
I I I
0.25
0.x
I
0.33
0.x
0.25 0.23
wx 0.x
I IO IO
I
B!:R 0.098 0,096 0,093 0098 0.096 0.093 0.09x 0.096 0.98 0.96
and of the B/N ratio C.N,
Initial
30 days later
Initial
30 days later
40.0 30.0 15.0 40.0 3&O
40, I 7 30.05 15.0 39.95 29.92 15.0 39.66 29.73 36.55 27.25
35.0 27.39 14.65 35.0 27.39 14.65 35.0 27.39 20.0 I X.0
34.86 27.25 14.55 34.6 27. I I 14.52 34.75 26.9 17.99
I 50 40.0 30.0 40.0 30.0
16.3
HO-
*
100 go-
E
4o 30 20 IO+ E
IO
20
4 d/fn
1 3’0
1 40
I 50
C/N,
Fig. 7. Mincraliz;ltion
rate (III) as a function of C/N, ratio.
Addition of nitrogen (NH;) stimulates the rate of ~~~i~~er~~ii~~Itio~~ ~1sshown in Fig. X. This is a result of incrcxc in G with increasing NH; concentration. and hc‘ncc incrcasc in dC. dt. When C,‘N < x/J;,, increase in d(‘dr means an increase in dC,;dt. beet not in the rate at which N, is transferred to microbial biomass: therclixc the rata or mincrali/ation incrcascs. One can further xc in Fig. S that i’&iNH: decreasesand NHf incrc;lses and ;I\; C ‘N , decreases. which is according to cyuatinn (271. X’iNHl which multipiics one of the posit~~c cxprcssions in equation (27) decreases both n.ith incrcasc of NH; and with decr~sc of C/N!,. and xincc the other qm2isio11 iii the equation decrcaseh it will mean a decrcasc in iy uith increase of NH:, ‘IS NH: increases. or as C/N, decreases. ;NH; <.
that the C/N ratio was /lighe~. than the initial for the residues that were above and on the soil surlace. Only after 1416 months did it dccrcase. The C:N ratio 01 the buried residue decreased from the beginning. The increase of the CiN ratio in the beginning when the residue (above and on the soil surface) had no 11~1s01 a very small one of extra nitrogen can he cxplaincd h! the model. The residue nitrogen wa the only source ol nitrogen. and carbon was present in excess. Therefore. carbon was accumulated. During the period of the LApcrimcnt, extra nitrogen probably became available (via rain, dust. etc.) and the C/N decreased in the lowcst concentrations of straw (1” I? I kg/ha) when it was above soil and in almost all the concentrations of straw that were on the soil surface. The results with the huried straw suggest also that the CjN ratio mcasurcd is mainly that of the straw its&. IT the on!“. (or the main) reason for decrease in C’~N ratio WBS the iiliiil~~l~ili~~ltion of inorganic N as suggested by most authors. then it should decrease in the course of dccaq of all t?;~xx or R irrcspcctivc of their initial C:N ratio. as long as the concentration of R is such that iGjiR > 0. The conclusion is that the C;N ratio of the mixture of suhstratc and microbial biomass Indeed dccrcnsc bccuusc of iminobili7~~tions of inorganic N. In addition to this. a change in the C‘N ratio of the suhstratc itself occurs if its initial GIN ratio was higher than rr,‘f,,( .-15). This last change is usually masked by the combined change but if the residue can be separxtcd frotn its cicco~nposers when sampling, then it can be obscrkcd. The changu in the C/N ratio of the residue might explain the point of equilibrium which is reached, namely 0,“) to IY;, N, (Bartholomew, 1965). because if the C percentage is taken as 30”,, the <‘,%I mtio which is
100 -
The main results and conclusions of the model agree ver> u;cll with the long known and established cxpcrimental results. It is shown that the lne~~sured organic C’,{: N,, ratio dccrcases during the decay process. The model filrthcr hhows that in addition to the ci~c~~yr ill ~/IV IIKY~,SI~~U/C,j;N, ratio. a change in the CjN, ratio of the ,x~h,srr~~t~occurs. ‘if no nitrogen is present in addition to th:tt contained in the R. the C/N, ratio according to the model will increase with time. Brown and Dickc! (Ic)70) measured loss of wheat straw residue and pcrcentugc of nitrogen in the rcsiduc in three dilrcrcnt conditions. (1)The residue not in contact with soil (2.5 cm ahovc soil surface); (2) residue on the soil surface; (3) rcsiduc huricd 12cm below the soil surface. In addition to those three positions. the snmples were ~4ashed thoroughI\ before the analysis. It is possible that iI1 ~hcx c\porimcnts. the measured C’li ratio was indeed the (‘1, ratio 01 the straw itself. They found
90 Z D .
80-
<
70-
;
60-
cII 50E 40 -
IO00
4000
8000
NH4 s/ha Fig. 8 Mineralization rate (m) as it l’unction of NH; concentration. at three C;N, ratios. 0 = C/N, = 4. 0 = C’: N, = IO. 0 = C’N, = 20.
Decomposition
of organic material
by microorganisms
The application of the model to real field conditions 26. According to the model prcobtained then is is rather simple. The same basic rate equations cxn be sented here, the C.!N ratio will not change anymore used. but environmental conditions should be incorafter reaching, x/f81which is between 2&3(X porated. As has been mentioned brietly in the model. It is well established and is shown hq the model too. is theparameter which is affected b! the cnvironthat if organic material rich in N (C!N, < z;/;,) is c,, mental conditions. G ,, ~can bc corrected for environdecomposed. mineralization will occur. From the way. In an! cyuations developed here. it follows that ~~f~litio~?of mental coefficients in any mathematical case G ,, will bc higher for :I given R if the conditions NH: will increase the rate of rnij~craliz~~tioi~. Broddare appropriate for microbial growth nrld will dccreasc bent and Nakaskima (1971) observed increased minerand might reach zero if the conditions arc inapproali7ation of soil N when NH; salts wcrc added to soil. The authors explained it partially by osmotic effects. priatc. Looking at Fig. 2 in this paper it is clear that This may indeed be a part of the explanation, but if so increase or decrease in G,,,,,\ will raise or dccrcasc the it is vet-! ditficult to understand the ihihire of some curves of G as 21filnction of C and N. However it will other- salts to cause such an increase. not change the dependence of G upon C’ and N. Thus even if the temperature is low and the pH iitr from Lower rates ol’ net immobilization were found in optimal. 2s long as microbi~ii growth conti~iim. all the anaerobic conditions (~arthololne~~. 1965). In anaerobic conditions 1 is higher than in aerobic ones. This is results and the interrelations shown in the model hold. the result of lower F (ratio between C assimilated to ilil~~~or~lf~~!cj~~~~~~,r~f.s-I would like to thank Dr. D. Horn for C’ decomposed). assuming j,, remains unchanged. fruitful discussion udhile bullding the model. Dr. D. W. Go,Therefore, C will he limiting already at higher U& than for critlcnl in acrohic conditions. It means that ifa R of high C’N 1 dall, Dr. J. Skujins and Dr. A. D. Mctaren review of the article. The work reported in the paper \vi~s ratio is decomposed under aerobic conditions. excess supported by the l!.S.,‘IBP Dcvert Bionic progriim under c‘ is available to make possible the imlnobili~Ition of grant No. GBl58X6 from the Nnticxxtl SCIOI~XF~~t~]~~~~ti(~~~. extra N. Howcvcr. in anaerobic conditions. because of the higher x!/,; required this Same C/N, ratio might bc REFERE”;ClCS such that no cxccss of C is present over the N, and ALLNJOYI:. E. and M~IKI~~Y R. M. (1963) Comparative r:ttc\ therefore the rate of ilyililobiii7~~tion of the added N is of dccompositio~l in soil of icood and hark pnrticlcs of decreased. several species of pines. PIYX~. Soii Sri. SK. 4~2. 27, 309 The way of measuring the decomposition rate. and 312. cvcn more important the way of expressing it can BARTHOLOMEW W. V. (1965) Mineralization and immobiliclxwgc the results completely. Smith and Dortglas /ation of nitrogen in the decoinpo~itiol~ of plant ;tnd ani( I c)7 I ) showed that addition of N increased the rate of mal residues. in Soii R’I~JYx+~I. Agronom? 10. (W. V. Bartholomew and F. E. Clark Eds.). pp. 2X5 306. Amcridecomposition. Later during the decomposition. both can Society of Agronomy. Madison. Wisconsm. the treated and the untreated straw decomposed at the Bl\.c;f MA\ C. W.. VARYI R J. E. and MARTlh W. P. (lii5.3) S~IIIX r:lte. and finally the untreated straw decomposed The c&&t of the addition of organic m;ttcrials on the faster, If the average rate of decomposition during a decomposition of an organic soil. Proc. Soil Sci. SW, -1~. long period is taken. the stimulating effect of the added 17, 34 3x. N can be overlooked. This probably explains the failure of Allison and Murphy (1963) to notice stimulation of decomposition of pine material with addition of nitrogen. By examining their curves it is very clear that. except for one case. addition of nitrogen did accclernte dccoiiipositioii in the very beginllili~ of the proccss. Subseyucntly. this acceleration decreased and became negative. By taking averages for 60 or more days they missed the initial increase. As shown in Fig. 2, the growth rate G. and therefore the rate of decomposition. are affected by the concentration of C and N in addition to their dependency HAI.LAM hl. J. and BAl~Tifoi.0Mt.WW. V. (195.3)Inlluencc of rate of plant residue addition in accclcruting the dccomupon the CiN ratio. However. the curves in Fig. 2 position of soil orgimic maitcr. Proc. Soil .%i. SOL. .-i/x 17. show further that ~tlth~ll~h the ~~~?~s~~z~~f~ rate of decom36X-368. position (rate ofloss of material. rate of CO2 evolution HARMSI:F*’G. W. and KOLI NI~RAXIIIR G. V. (1065)Soil inoretc.) is i~~~serl with increasing concentration of subpanic nitrogen, In Soil ~Vitracpu.Agronomy IO (M’. \‘. strate. the ~XWCII~~K~LI(~ of decomposition is decreased Bartholomew. and F. E. C‘lark Edsl. pp. 13 ‘fl. Amcric;in with ilicreasingconcelltration. In lncasuring decompoSociety of Agronomy. Madison. Wisconsin. sition rates in percentage. the stimuhtiry efkct of inKIRKHAM D. and BARTHOLOMEWV. W. (1955) Equations for crcascd concentration is obscured. This is the reason following nutrients translbimntion\ in boil. utilizing for the conclusions sometimes drawn (Brown and tracer d;tta. Proc. Soil Sci. SW. AHI. 18, ii 34. SMITHJ. H. and DOUGLAS C. L. (1971)Whwt strctwdecomDickey, I% 1) that “Perten tagc losses were inversely position in the field. FIX. SOI/ Sri. Sot. .,IIII. 35, 269 272. related to rcsiduc amounts”.
FOR
DECOMPOSITION
MATERIAL
BY MICROORGANISMS HAWA
Ecology
Center.
Utah
OF ORGANIC
PARNAS*
State Universit).
Logan.
L’tah
83322, U.S.A
Summary-A theoretical lnodel which gives the rate of lnicrobial decomposition ororganic material (plant or animal residues. or soil organic matter) is presented. Explicit equations for the rkite of decomposition. mineralizations and immobilization are given. The main assumption of the model is that the rate of decomposition of any substrate is proportional to the _erowth rate of its decomposers. The main results of the model arc: I. Addition of extra nitrogen to materials poor in nitrogen increase their rate of decomposition; 2. Addition ofextra nitrogen to a substrate whose initial carbon/nitrogen ratio is above a crltical ratio (20~30) causes a decrease in the substrate’s carhon:nitrogen ratio during its decomposition; 3. If the lmtial carhon.!nltrogen ratio is below the critlcal one, no change in the subatratc’s ratio will occur with time; 4. Net mineralization oforganic-nitrogen occurs when the substrate bcmg decomposed hna an initial carbon/nitrogen ratio which is below the critical one; 5. Addition of ammonium to such a substrate (point 4) will increase the rate of organic-nitrogen minerallration hut not necessarily the rate of net mmerali7ation. All the model results are analytical and independent of the values for the various parameters. Neverthe&s the application of the model to real field conditions is discussed while presenting a numerical example.
INTRODLCTION Decomposition processes in the soil have been investigated extensively. hut many questions remain unresolved. For example, it has been shown that the C/N ratio of plant residues low in nitrogen decreases during the process of decomposition if nitrogen is available in CYCCSS to that contained in the residue (Bartholomew. 1965; Brown and Dickey, 1970; Smith and Douglas. 1971). The explanation generally given for this phcnomenon is that inorganic nitrogen is immobilized during the microbial decomposition of the organic carbon of the residue. In this process a net gain in organic nitrogen occurs and at the same time a net loss in organic carbon results from CO1 evolution. The result is a decrease in the organic C/N ratio. This explanation describes the situation in which the “organic matter” includes both the substrate for decomposition and its decomposers. hut, the C/N ratio of the residue its&” is not considered. Harmscn and Kolenbrander (1965) pointed out that the C/N ratio of the substrate itself might decrease during the decay. As the) stated: “This decrease often is in part the consequence of the fact that the fractions with the highest ratio arc the most readily disintegrated parts of the substrate”. During the decomposition of organic matter with a low C.N ratio. inorganic nitrogen accumulates (Bartholomew, 1965). Harmsen and Kolenbrander (1965) reported that addition of fertilizer nitrogen sometimes results in an increased accumulation of inorganic
* Present address: sit). Jerusalem,
Life Science Institute.
Hebrew
Univer-
Israel. Ihl
nitrogen. Broadbent and Nakashima (1971) also found that addition of NH: increases the rate of mineralization of organic nitrogen. and attempted to explain it by osmotic effects. On the other hand, Harmsen and Kolenhrander (1965) mention in their review that a very high concentration of soluble mineral nitrogen in the soil reduces the apparent rate of mincralizatlon of nitrogen. A more difficult question concerns the interrelation between decomposition of organic matter in the soil and that of substrates added to it. It has been reported that loss of carbon from soil organic matter was increased when plant residues were added to the soil (Hallam and Bartholomew. 1953 ; Bingeman, Varner and Martin. 1953; Broadhent. 1947: Broadbent and Norman. 1947). It was also shown that the rate of decomposition both of the added plant residue and the
ISprescntcd. The erect of added nitrogen on decomposition. mineralization and immobilization processes is discussed. The subject of the subsequent report will be a model of the mutual effect between soil organic mattcr and any type of added material. on their respective rates of decomposition. The basic idea of the two models is the same. They arc based on the wcli cst~~biished fact that the organic material supplies the nutrients for the growth of the decomposcrs. Therefore. the rate of decomposition of an organic material is closely linked with the growth of the organisms responsible for its decomposition. Any incrcnsc in the growth rate of the decomposers causes higher rate of decay of that material which supplies the nutrients. By the same rule. any fktor inhihiting the growth of the decomposers, such as suboptimal c~i~~iroiitl~cnt~~jconditions of temperature, moisture. acrrttion, and so on. reduces the rate of decomposition. THE MODEL
Carbon is thus the growth-limiting factor when C: N, < x/J, and nitrogen is when C;N > ~/if,,.as long as the growth rate is still dependent on the concentrations of the substrates (carbon and nitrogen). A flow diagram ofcarbon and nitrogen in the microbial decomposition of R is given in Fig. I. The flo\v diagram and the lnathern~~tic~tl model arc based on the following assumptions. I. Loss of carbon from organic matter (plant. animal or microbial residues. or humic matter) is proportional to the growth rate and the maintenance rcquiroments of the organisms decomposing it. 3. The growth rate ofthe decomposcrs in a given set of environmental conditions (temperature. pH etc.) and for a given type of organic matter increases with increasing carbon and nitrogen coticet~tr~ltions. The exact dependency is shown later. 3. The decomposers use N, and other nitrogen compounds as nitrogen sources according to their rclative availability. 4. Release of nitrogen (N,) from R is proportional to the loss of carbon from the C N compounds. i.c. to the loss of ‘2,. 5. The decomposers can use as energy and carbon source both C, and C2.
Organic matter R is subject to decomposition. R is a mixture of substances. Some of thorn contain nitrogen as a molecular constituent. such as proteins and RNA. and will be termed C N Compounds. The others do not contain nitrogen, such as cellulose and starch. and those will be termed C-Compounds. The amount of carbon per unit soil (weight or area) of the dR dt‘ dN, C N compounds is C, and that of the C-compounds dr df + dr C,. The sum of C, and C2 is defined as C. The amount nitrogen in the C--N compounds is N ,. Although Using assumption 1 several types of C’-N compounds are found in R. JWOfci/l will be considcrcd as the only, decomposing sub(31 stance. The misturc of ~nicroorganlslns which grow on that R. the dccomposers, use R as the source for the where: x is defined above. H is the biomass of the nutrients they need. If all the other nutrients except dccomposers per unit soil. G is the growth rate of the carbon and nitrogen are present in excess, the growth decomposers. rate G of the decomposers is a function of the concentration of the availahlc carbon and nitrogen. Carbon is required both as a source of energy and for building the microbial biomass. Nitrogen is required for the latter purpose onIt. Therefore a certain ratio exists hctwecn the carbon and the nitrogen required for growth. This ratio is defined as:
of
iv here
C
:
= average fraction of carbon
in the decomposer’s
Cdl F
% ,fi
ratio of carbon posed = is thus the total per unit biomass = average fraction
=
CCII
assimilated
to carbon
decom-
carbon used by the organisms increment of nitrogen in the decomposer’s
II
NH;
L
“NH4
1
Fig. I. Flow diagram of carbon and nitrogen as a result of microbial decomposition (for details see text). R = organic matter subject to decomposition. C, + C, = carbon. N, f NH, = nitrogen. G = growth rate. B = decomposcrs biomass, 1~ = mineralization rate. i = immobikation rate. VNH; = input of NH: from an independent source.
16:
Decomposition of organic material by microorganisms Using assumption
4 dN, dt
dC,
N,
(4a)
dt ‘C, ’
-dNl =dC,.,p.
14b) ~ dr 1 where /3 is constant (average) ratio of carbon~nitrogcn in the C-N compounds. From the flow diagram. and since G as stated in assumption 2 is a function of both carbon and nitrogen. the release of N, is also given by:
dr
dN, _ dt
-(
r;,
G B - i) -
111
where ~1is the rate of mineralizatiol~ of N ,. i is the rate of iinmobili~~~tion of NH:. In order to define dC,/df and dC,idt the following should be considered. The role of protein in the growth of the decomposers may be primarily as a nitrogen source. or primarily a source of carbon. However. in both cases C, and N, are coupled in the rate of their release from R. The ratio of the C-compounds to the C-N compounds is derived from the C/N, ratio. There is an increase in the relative concentration of C-N compoutlds as the C,‘N ratio dccreascs. When C:N, = fi no C-compounds are present and only C-N compounds are found. At that extreme condition. the C N compounds are the only source for hoth carbon and nitrogen. However, under conditions where C/N, > /i the question arises whether the rate of protein breakdown is determined by the requirement for nitrogen or for carbon. An approach to express dC,jdt as a function of C/N, is through finding the optimal strategy by which the above questions are answered. According to considcr~~tions of maximal Ion! term growth rate (which will also determine the optlmal strategy) the foltowing function is presently assumed. As long as Ci N, > a#;,, nitrogen is the growth limiting factor. Under this condition, carbon is in excess and there is no advantage in diverting the protein to serve primarily as a carbon source for the following reasons. First, there will be no increase in growth rate at the present (carbon is in excess anyhow). Second, in the future such a diversion wili cause a decrease in growth rate. since nitrogen will be even more limiting (it should be remembered that, fl grams of protein are required per gram carbon). It can be concluded that under such conditions when nitrogen is the growth limiting factor, protein serves primarily as a nitrogen source and the C-compounds as the main carbon source. When C/ N, < r/f;,, carbon is the growth limiting factor. Under this condition. optimal considerati~~n force the diversion of protein to serve primarily as a carbon source together with the C-compounds. simply because otherwise the carbon available for the decomposers will be tower than the existing C. Since when C/N, < q”;,, the tong term growth rate is determined by carbon. decrease in the available carbon wilt lower the growth rate.
In the present work. the optimal distribution between C, and C, as sources for carbon when C:’ N, < x/j,, is not computed. Rather it is assumed that the two contribute according to their relative concentration. When C/N, = r.‘f;, the nitrogen and the carbon are present in the required ratio. thorclore. this is the point at which protein will ~+witchfrom being primarily a liifrogcli source to beco~ne ~rii~~~lr~l~a carbon source. It can be further assumed that the NH: concentration dots not affect the optimal stratcg) as to the shift of protein to serve primarily as a source of carbon. Ammonium. if present. will all&t the concentration 01 nitrogen and therefore the growth rate. Those considerations are expressed in the following equations. When C/N, ~.ff;,,the rate of protein breakdown is according to the requirement for nitrogen. dC,;dr is coupled to the rate of N, which goes to growth. Thus
if C, supplies the rest of tho carbon needed. CIC,
-= dt
-[~.(;.B-p(r,,.t;.H-i)l
17)
When C/N, x/f,,,both C, and C3 serve under this condition as carbon source according to their rclativc concentrations. Thus
(‘7 --C
dCZ - = -3.G.B. dt
The final equation for dC,/dr is therefore given by equation 6 if C:‘N, r$,,, and by equation X if C: N 1 r/f;,. At C/N, = YJ,, equations 6 and X arc equnl. if i = 0. The final equation for dN,/& is given by equation dN, dt
=
-(
G
13 -
i)
if
:’
;
‘I
N,
dN, it-
f,,
=
-KEG%
iE C‘ N,
iIO)
,I
2
I,,
01)
The ilnlnobili7ation (i) describes the rate at which NH; is transferred to microbial biomass. According to assumption 3. it is defined by: i = r;,.G.R.
NH: -~-NH,+ + N,
The change in NH,+ concentration
(13)
is given by
dNH where: 01 = rninera~i~~tioti rate. i = ililtnol~iii~~tioli rate. V,,, = rate of input of NH; into the system from independent sources. The growth rate G is a function of both carbon and nitrogen. It is given by the family of curves in Fig. 2. The mathematical expression for each point on those
HAMA PAKNAS
! h-l
A
G
G
N
1
c
If C/N, = rx(f,no change will occur. If C/N, < r~&, and NH: = 0. dN,,‘dt is given by 11. By incorporating it into equation IS it can be seen that d(C,N,)/dt is equal to zero. Codwiorr I. The C/N, ratio of an organic material being decomposed in the ahscnce of other nitrogen sources wit1 i~cteasr with time if its initial C/N, is higher than x,:/i,. and will not change with time if its C: N, is equal to or lower than xj&. (b) Ej@rt oi’a~i&/ ~irroc/or~ 011 rllc~cirtr~\c wiih lirrlc’ ill ~/IL,CjN, ratio. In this model. though reference is made to ammonium. it should bc remembered that the same holds true for any kind of nitrogen which can serve for decomposcr’s growth. If C;N, 2 $i and NH; > 0. dN,/dr is given by equation (IO) and when incorporated into IS. we get
Fig. 2. Growth rate (G) as a ftlnction of carbon (C) at various
nitrogen
d
(N) concentrations.
C I
y,
--d, curves is given by Michaelis-Menten Kinetics bisubstrate reactions. G ,C‘.N ___“. __. (k,. + c-)(1,,,+ N)
for the
G=
(14)
N C li,
.L
(17) in the paren-
By isolating NH; from the expression theses in equation 17, one can see that d,k -- di ’
where : Glnal
C‘
c;. B
- =g,
= maximal growth rate in given environmental conditions for a given type of R when C and N are high so that ?G,!C:Cand ?G/?N = 0. = N, + NH: =c, +cl = a constant which describes the concentration of c’ at given N which will give G = G,,,/2, for a given R (i.e. the Michaelis constant) = the same as (i(,) for N
> ()
jr
EL’H: <
C.j;,-z.N
1
r
(18)
DiV.ilXVllwulrs
ct
c
:v, dr--
I
= N;
dC
C’
dN,
.-& - (N,)”
-dt
(15)
when
Substituting equation dCldt into 1.5gives:
It shows that if NH; is pwsolt and its concentration is high enough (the exact limit depends on C and N,), the C/N, ratio of R’s which have little protein in them will dccrc~r.sc,with time, while without NH; it would increase with time. Since NH; is not part of equation 15 when C.‘N, ‘. U;f,,, it will have no effect on the x! change ofthc C!N, ratio at that conditions (C:N,
L).
10 for dN,/dt
and equation
3 for
From the starting conditions it follows that the expression in the parentheses is positive and so is I/ N, G B. Therefore d(C/N ,)/dt > 0. Thus, when no NH: is present and the organic material being decomposed has a C/N, ratio higher than z/j;. its C/N ratio will increase with time.
tord1rsio~7 2. Where the initial CiN, ratio in R is greater than s/f,,. the ratio can decrease with time only if other sources of N are present. At this point of the discussion it should be mentioned that part of the confusion in the literature is because the rnc~r,strrc~d C/N ratio includes the microbial biomass in addition to the organic material being decomposed. It is therefore it~port~ltlt to show how the microbial biomass affects the two conclusions. (c) i$Yi 0l’ j/~~~~~~~~~~~ ~~~i(,~~~~;~~/ hic1rrtcrss O/f tl1r c'lzarigc nibqrrl
with
tiu7P
in
tiw
wtrswwl
icrtio
of
ctrrho~~
to
(C$N,). We define: CB = C +.f;.B.
(1%
N, = N, +,f;.B.
(20)
Decomposition
By using equation gets c‘,I d N,
cir
15 together
I h’
of organic material by microorganisms
with i 9 and 20 one
I‘, dN, 1 dC .- - -~ = N, iir N,< dr
(e) E$ct c?f’NH: ou m. The rate of mineralization 111is always zero if C/N, > r,if;,.whether other nitrogen sources are present or not. The effect of NH: on l~itler~~liz~ition has therefore to be checked when C~ N L < x/j;,_3~ combining equations (23). (1 if and (12) and differe&ating it with respect to NH; wc get
ft is very difficult to see a clear picture from equation 21, but in the interesting region when C!N, 2 yjfn. equation 21 becomes: (‘1,
Cl.N"
‘d;”
= :_
G
B
[,,
N
N, N, + NH;
C + ,r;. B N , t ,;, 5
_ _i_ N,,
dB (‘,I d; .f‘ - r. 1,, H
(23)
(C +f;.B)/(N, i- j,.B) approaches f;,$, with increase in B. This means that the increasing B tends to decrease the first expression of equation 22 so that it might become negative. If B is so high that C$N, : j,: ,t;, then the second expression is zero. ‘I’hc conclusion is that the effect of including the microbial biomass on the measured C,INH is in the direction of reducing it with time, even when the C/N, ratio of R itself is increasing with time. Distinction between measured C,,: N, ratio and the S;‘N, of R wili be further discussed in section 3. (d) M~uYYI/~‘~I~~oK Mineralization rate 111is defined here as the rate of converting organic nitrogen to NH:. The mineralization rate can be derived from equations (5) and (12).
Since C/N, : YJ’I’;,. it follows that z/C - (f,,)/(NH, + N, ) > 0. This means that in/iNH, > 0, or that addition of NH; increases the rate of mineralization. The conclusions from sections d and e are: C~)~~~~[~,sj~}~~ 3. Miner~~liz~ltio~l occurs when the C:N 1 ratio for R being decomposed is lower than PY/;,. Cortci~sion 4. The rate ofminerali~ation is increased as C/N, is decreased. COIIC~/~I.S~O~I 5. The higher the amount of NH: in the area, the /t$ro~ will be the rate of mineralization. (f) SKI rtlirlc~~trliltrtio~~.Net mineralization /n,, is defined here by the net change in NH: collc~ntration for a given j~?ifjf~~collcentr~~tlon of NH;, (V,,,L > 0 only at t = 0) caused by mineralization and immohilization. Ill,,
=
111
-
i
(76)
and * f in,,
?NH,
ill1
t-i
(‘NH,
(27)
iNH;
--. N, _+ G’(NH,+
N,)”
.
(‘8)
ii II C’
N,
(‘NH,
I
/,,
is therefore always positive. ~!f;, where 1~ = 0 and c’rni It follows that at C’iN, /‘NH, = 0. fir,, will be zero or negative. When C’;N, ’ q’f,. and nr > 0. The combination gives
From the starting conditions it follows that the expression in the parentheses is positive. which means the mineralization occurs when CiNr < Z/A. The exact rate is given by equation (23).
of equations
(25). (27) and (19)
This derivative is always positive. C’~UIC/US~OI~ 6. Net mineralization is decreased by NH, when C/N, sc,!f;,and increased by NH; when C;N, < y/f’.,1 3.
.vrfflfcTictff
rwfrql/c~
The validity and generality of the results and conctusions presented previously are IZO~dependent on the
HAUUAPAKNAS
I (10
specific values that arc given to the different parameters. The purpose of the numerical example is to dumonstrnte the wav in which the model may be applied to real condi;ions. For the purpose of numerical solutions the environmental conditions temperature, moisture, etc.. were not included. The way in which they could bc included w,ill bc mentioned in the discussion. The numerical v~alucs that have been used for the different parameters arc shown below. . k, and k,, are cl~oscn The numerical values of 6 in SLICKa way to give close agreement with field data, These values arc by no means absolute and reflect only a magnitude of order. Parnas and J. Radford have determined their values in a simulation model on decomposition written for the United States International Biological Program Desert Biome. It has to bc cmphasired that the numerical values of those parameters arc not very important for the present model. though they are of great importance in a simulation model. The numerical values of the other parameters rcllect the most common values appearing in the literature. (‘,ZL<‘CLATE11
___ 2 500z > 400E :: p 3005 ti a zooz E! J too-
Fig. 3. dC;d!. dC,/‘dr and dC2/dt as a function of C/N,. C = IOglm2. B = 10g/mZ. NH: = 0. l = dC,/dt, x = dCj/dt, o = dC/dt.
HESCILTS
dent on C and N, as can be seen in equation 19. The The l’tlnctions dC.!dt, dC,/dt and dC?/dt as a funcdegree of change in C/N, is also dependent on C and tion of CX, ratio arc prcscntcd in Fig. 3. The entity N,; but in addition to this. it will increase as C/N,, 7 ‘/;,. using the values that have been mentioned preG 1,,:,, and NH; increase. viously is equal to 25, It can bc seen in Fig. i that at In Fig. 6 and Table I, the difference between the C‘N , > x ‘1,;both dC,jdr and dC‘,;dt incrcasc as CN, change with time of the C/N, ratio and that of the UIMdecreases. the first one beca~~sc dN,/dt increases and SUI& ratio of organic carbon to organic nitrogen (C,/ the second because dC;dr increases. Starting at C; NH) is shown. In Fig. 6, the ratio between R/B ’ I. N, = r:f,, both C, and C2 scrvc as carbon source Such a ratio is normally much too low for field condiaccording to their rclativje concentrations: therefore tions. A high value of B was chosen on purpose to dC, dt is increasing sharply and dC,/dt decreasing demonstrate the maximal effect of B on the C/N, ratio. sharply as the C’N, ratio approaches p. At this point C‘, = (‘and C. = 0. Addition of NH: will lower dC, 111 To R which had initial C/N, = 50. B was added and C,i/N, dropped to 21-4. It can be seen that when B is dr if C ‘N, > rf,, and will increase it, if CN, < Qj, The rate ofchange of the C‘:N, ratio with time is a11 high the initial value C/N, will inevitably be much smaller than values actually found in plant residues, If increasing function of the initial C;N,. as can be seen in Fig. 4. When the initial C/N, ratio is higher than 25. a high C,.IN, ratio is found. this by itself means that the microbial biomass is not very high compared to R. addition of NH: will cause the C,/N, ratio to decrease But even with such large microbial biomass the change instead of increasing with time as shown in Fig. 5. The in C,/N, did decrease when C/N, increased, but the concentration of NH: which is required to cause a derate ofdccrease was from 21.4 to 21.3 in .10 days. When crease in the C.‘N, ratio with time is evidently depen-
Paramclcr
Meaning
Value
maximal growth rate ofa mixture of decomposers constant equivalent in its meaning to the Michaclis constant. but has area con centration units instead of volumetric concentration units. Means the concentration of carbonwhich gives half the maximal growth rate similar to h, and means the concentration of nitrogen which gives the half maximal growth rate cliicienc> of carbon assimilation fraction of carbon in the microbial hmmass fraction of nltrogcn In the microbial biomass molecular ratio between carbon and nitrogen in protein
0.0
I
20
0.4 0.4 0.5 0.05 4
Units day-
g/m’
g/m’
’
Decomposition 60
of organic
material
I b
ra
As R/B becomes smaller the opposite happens. which means the rate of decrease of C/N, becomes much higher than that of C,/N,. Another point to remember is. that while U/IVsource ofadditional nitrogen (organic or inorganic) will result in decrease in C/N, ratio with time, only ~UOV~LUI~ nitrogen will cause a big chan_ee in C,$jN,,. Without inorganic-nitrogen. the change III C&N,% will be moderate as is shown in the upper part ofTable I when NH, = 0. We can conclude that since the initial carbon/nitrogen ratios found in field conditions are much higher than f;/f;, the microbial biomass has only a very smdll effect. if any. on the change even of the measured carbon,nitrogen ratio. The measured carborq’nitrogcn ratio reflects mainly the C/N, ratio. and the decrease in C! N, is caused mainly by addition of nitrogen. The mineralization rate is shown in Fig. 7 as a function of C/N, ratio. As has been stated in conclusion 4. the rate of mineralkation increases as C/N, decreastx.
DAYS
DAYS
Fig. 4. Change of C.!N, ratio of R being decomposed with time. (a) NH: = 0. l = initial C/N, = 50. x = initial C,: N, = 35. 0 = initial C,N, = 25. (b) NH; = O.Xg!m’. 0 = initial C;N,
= 50. x = initial C;N,
= 30.
5654
50
by microorganisms
NH4
c
N, 48 46
DAYS
Fig. 5. Change of C/N, ratio with time at different NH: concentrations. C = 10g/m2, N, = 0,2g/m’. NH; in g/m2 is: 0 = 0, x = 0.1. q = 0.4, D = 0.X.
NH, was present while C/N, ratio decreased from 50 to 45.X. C,/‘N, decreased only from 21.4 to 190. In Table I the change with time in C/N, and C,/N, ratio’s? when R/B z 10 is shown. The most pronounced effect of the microbial biomass is when no NH: is present. Then C/N, increases with time and C,j/N,j decreases. When NH,’ is present in sutficient amount, both C/N, and C,/N, decreases. The rate in decrease in C,/N,j is slightly higher. when R/B z 10. Table
I. Change
in C/N,
and C,/N,
wth
DAYS Fig. 6. C/N, and C,,‘N, as a function of time. 0 = C/N and NH,’ = 0, 0 = C/N, when NH: = 0.X g;m’. x = C,/N, when NH; = 0, V = C,,‘N, when NH: = 0.X.&m’. C =
IOgjcm’, E = log/m’.
time. as a function
of NH:
concentration
C/N,
N,
NH:
B
(g.‘tn)
km)
(g/m)
0.25
0
0.33
0
I I
0.66 0.25
0 0.7
0.33
0.7
0.66
0.3
I I I
0.25
0.x
I
0.33
0.x
0.25 0.23
wx 0.x
I IO IO
I
B!:R 0.098 0,096 0,093 0098 0.096 0.093 0.09x 0.096 0.98 0.96
and of the B/N ratio C.N,
Initial
30 days later
Initial
30 days later
40.0 30.0 15.0 40.0 3&O
40, I 7 30.05 15.0 39.95 29.92 15.0 39.66 29.73 36.55 27.25
35.0 27.39 14.65 35.0 27.39 14.65 35.0 27.39 20.0 I X.0
34.86 27.25 14.55 34.6 27. I I 14.52 34.75 26.9 17.99
I 50 40.0 30.0 40.0 30.0
16.3
HO-
*
100 go-
E
4o 30 20 IO+ E
IO
20
4 d/fn
1 3’0
1 40
I 50
C/N,
Fig. 7. Mincraliz;ltion
rate (III) as a function of C/N, ratio.
Addition of nitrogen (NH;) stimulates the rate of ~~~i~~er~~ii~~Itio~~ ~1sshown in Fig. X. This is a result of incrcxc in G with increasing NH; concentration. and hc‘ncc incrcasc in dC. dt. When C,‘N < x/J;,, increase in d(‘dr means an increase in dC,;dt. beet not in the rate at which N, is transferred to microbial biomass: therclixc the rata or mincrali/ation incrcascs. One can further xc in Fig. S that i’&iNH: decreasesand NHf incrc;lses and ;I\; C ‘N , decreases. which is according to cyuatinn (271. X’iNHl which multipiics one of the posit~~c cxprcssions in equation (27) decreases both n.ith incrcasc of NH; and with decr~sc of C/N!,. and xincc the other qm2isio11 iii the equation decrcaseh it will mean a decrcasc in iy uith increase of NH:, ‘IS NH: increases. or as C/N, decreases. ;NH; <.
that the C/N ratio was /lighe~. than the initial for the residues that were above and on the soil surlace. Only after 1416 months did it dccrcase. The C:N ratio 01 the buried residue decreased from the beginning. The increase of the CiN ratio in the beginning when the residue (above and on the soil surface) had no 11~1s01 a very small one of extra nitrogen can he cxplaincd h! the model. The residue nitrogen wa the only source ol nitrogen. and carbon was present in excess. Therefore. carbon was accumulated. During the period of the LApcrimcnt, extra nitrogen probably became available (via rain, dust. etc.) and the C/N decreased in the lowcst concentrations of straw (1” I? I kg/ha) when it was above soil and in almost all the concentrations of straw that were on the soil surface. The results with the huried straw suggest also that the CjN ratio mcasurcd is mainly that of the straw its&. IT the on!“. (or the main) reason for decrease in C’~N ratio WBS the iiliiil~~l~ili~~ltion of inorganic N as suggested by most authors. then it should decrease in the course of dccaq of all t?;~xx or R irrcspcctivc of their initial C:N ratio. as long as the concentration of R is such that iGjiR > 0. The conclusion is that the C;N ratio of the mixture of suhstratc and microbial biomass Indeed dccrcnsc bccuusc of iminobili7~~tions of inorganic N. In addition to this. a change in the C‘N ratio of the suhstratc itself occurs if its initial GIN ratio was higher than rr,‘f,,( .-15). This last change is usually masked by the combined change but if the residue can be separxtcd frotn its cicco~nposers when sampling, then it can be obscrkcd. The changu in the C/N ratio of the residue might explain the point of equilibrium which is reached, namely 0,“) to IY;, N, (Bartholomew, 1965). because if the C percentage is taken as 30”,, the <‘,%I mtio which is
100 -
The main results and conclusions of the model agree ver> u;cll with the long known and established cxpcrimental results. It is shown that the lne~~sured organic C’,{: N,, ratio dccrcases during the decay process. The model filrthcr hhows that in addition to the ci~c~~yr ill ~/IV IIKY~,SI~~U/C,j;N, ratio. a change in the CjN, ratio of the ,x~h,srr~~t~occurs. ‘if no nitrogen is present in addition to th:tt contained in the R. the C/N, ratio according to the model will increase with time. Brown and Dickc! (Ic)70) measured loss of wheat straw residue and pcrcentugc of nitrogen in the rcsiduc in three dilrcrcnt conditions. (1)The residue not in contact with soil (2.5 cm ahovc soil surface); (2) residue on the soil surface; (3) rcsiduc huricd 12cm below the soil surface. In addition to those three positions. the snmples were ~4ashed thoroughI\ before the analysis. It is possible that iI1 ~hcx c\porimcnts. the measured C’li ratio was indeed the (‘1, ratio 01 the straw itself. They found
90 Z D .
80-
<
70-
;
60-
cII 50E 40 -
IO00
4000
8000
NH4 s/ha Fig. 8 Mineralization rate (m) as it l’unction of NH; concentration. at three C;N, ratios. 0 = C/N, = 4. 0 = C’: N, = IO. 0 = C’N, = 20.
Decomposition
of organic material
by microorganisms
The application of the model to real field conditions 26. According to the model prcobtained then is is rather simple. The same basic rate equations cxn be sented here, the C.!N ratio will not change anymore used. but environmental conditions should be incorafter reaching, x/f81which is between 2&3(X porated. As has been mentioned brietly in the model. It is well established and is shown hq the model too. is theparameter which is affected b! the cnvironthat if organic material rich in N (C!N, < z;/;,) is c,, mental conditions. G ,, ~can bc corrected for environdecomposed. mineralization will occur. From the way. In an! cyuations developed here. it follows that ~~f~litio~?of mental coefficients in any mathematical case G ,, will bc higher for :I given R if the conditions NH: will increase the rate of rnij~craliz~~tioi~. Broddare appropriate for microbial growth nrld will dccreasc bent and Nakaskima (1971) observed increased minerand might reach zero if the conditions arc inapproali7ation of soil N when NH; salts wcrc added to soil. The authors explained it partially by osmotic effects. priatc. Looking at Fig. 2 in this paper it is clear that This may indeed be a part of the explanation, but if so increase or decrease in G,,,,,\ will raise or dccrcasc the it is vet-! ditficult to understand the ihihire of some curves of G as 21filnction of C and N. However it will other- salts to cause such an increase. not change the dependence of G upon C’ and N. Thus even if the temperature is low and the pH iitr from Lower rates ol’ net immobilization were found in optimal. 2s long as microbi~ii growth conti~iim. all the anaerobic conditions (~arthololne~~. 1965). In anaerobic conditions 1 is higher than in aerobic ones. This is results and the interrelations shown in the model hold. the result of lower F (ratio between C assimilated to ilil~~~or~lf~~!cj~~~~~~,r~f.s-I would like to thank Dr. D. Horn for C’ decomposed). assuming j,, remains unchanged. fruitful discussion udhile bullding the model. Dr. D. W. Go,Therefore, C will he limiting already at higher U& than for critlcnl in acrohic conditions. It means that ifa R of high C’N 1 dall, Dr. J. Skujins and Dr. A. D. Mctaren review of the article. The work reported in the paper \vi~s ratio is decomposed under aerobic conditions. excess supported by the l!.S.,‘IBP Dcvert Bionic progriim under c‘ is available to make possible the imlnobili~Ition of grant No. GBl58X6 from the Nnticxxtl SCIOI~XF~~t~]~~~~ti(~~~. extra N. Howcvcr. in anaerobic conditions. because of the higher x!/,; required this Same C/N, ratio might bc REFERE”;ClCS such that no cxccss of C is present over the N, and ALLNJOYI:. E. and M~IKI~~Y R. M. (1963) Comparative r:ttc\ therefore the rate of ilyililobiii7~~tion of the added N is of dccompositio~l in soil of icood and hark pnrticlcs of decreased. several species of pines. PIYX~. Soii Sri. SK. 4~2. 27, 309 The way of measuring the decomposition rate. and 312. cvcn more important the way of expressing it can BARTHOLOMEW W. V. (1965) Mineralization and immobiliclxwgc the results completely. Smith and Dortglas /ation of nitrogen in the decoinpo~itiol~ of plant ;tnd ani( I c)7 I ) showed that addition of N increased the rate of mal residues. in Soii R’I~JYx+~I. Agronom? 10. (W. V. Bartholomew and F. E. Clark Eds.). pp. 2X5 306. Amcridecomposition. Later during the decomposition. both can Society of Agronomy. Madison. Wisconsm. the treated and the untreated straw decomposed at the Bl\.c;f MA\ C. W.. VARYI R J. E. and MARTlh W. P. (lii5.3) S~IIIX r:lte. and finally the untreated straw decomposed The c&&t of the addition of organic m;ttcrials on the faster, If the average rate of decomposition during a decomposition of an organic soil. Proc. Soil Sci. SW, -1~. long period is taken. the stimulating effect of the added 17, 34 3x. N can be overlooked. This probably explains the failure of Allison and Murphy (1963) to notice stimulation of decomposition of pine material with addition of nitrogen. By examining their curves it is very clear that. except for one case. addition of nitrogen did accclernte dccoiiipositioii in the very beginllili~ of the proccss. Subseyucntly. this acceleration decreased and became negative. By taking averages for 60 or more days they missed the initial increase. As shown in Fig. 2, the growth rate G. and therefore the rate of decomposition. are affected by the concentration of C and N in addition to their dependency HAI.LAM hl. J. and BAl~Tifoi.0Mt.WW. V. (195.3)Inlluencc of rate of plant residue addition in accclcruting the dccomupon the CiN ratio. However. the curves in Fig. 2 position of soil orgimic maitcr. Proc. Soil .%i. SOL. .-i/x 17. show further that ~tlth~ll~h the ~~~?~s~~z~~f~ rate of decom36X-368. position (rate ofloss of material. rate of CO2 evolution HARMSI:F*’G. W. and KOLI NI~RAXIIIR G. V. (1065)Soil inoretc.) is i~~~serl with increasing concentration of subpanic nitrogen, In Soil ~Vitracpu.Agronomy IO (M’. \‘. strate. the ~XWCII~~K~LI(~ of decomposition is decreased Bartholomew. and F. E. C‘lark Edsl. pp. 13 ‘fl. Amcric;in with ilicreasingconcelltration. In lncasuring decompoSociety of Agronomy. Madison. Wisconsin. sition rates in percentage. the stimuhtiry efkct of inKIRKHAM D. and BARTHOLOMEWV. W. (1955) Equations for crcascd concentration is obscured. This is the reason following nutrients translbimntion\ in boil. utilizing for the conclusions sometimes drawn (Brown and tracer d;tta. Proc. Soil Sci. SW. AHI. 18, ii 34. SMITHJ. H. and DOUGLAS C. L. (1971)Whwt strctwdecomDickey, I% 1) that “Perten tagc losses were inversely position in the field. FIX. SOI/ Sri. Sot. .,IIII. 35, 269 272. related to rcsiduc amounts”.