D Y N A M I C S O K S O U , M I C R O B I A L I I I O M A S S A N D A C T I V I T Y IN
CONVENTIONAL AND OROANIC I ARMINO SYSTEMS
N. G U N A P A I A A N D K M S C O W
Dcpt of I a n d . Air and W a t e r R e s o u r c e s . University of California, Davis 9 5 6 1 6
Dynamics of Soil Microbial Biomass and Activity in
Conventional and
Organic Farming Systems
Ninnala (iunapala and Kale M. Scow*
)cpt. DI I -and. Air and Water Resources, University ol California, Davis 9 5 6 1 6
phone: 916-752-4632
fax:916-752 1552
email: kmscowCn'ucdavis.edii
ille: Microbial dynamics in different farming systems
SummaryD y n a m i c s of microbial populations d u r i n g t w o growing s e a s o n s were
c o m p a r e d in soils under t o m a t o e s m a n a g e d by conventional ( 2 and 4 yr rotations), low
input, or organic practices. Fumigation cxtractable c a r b o n ( L L C ) and nitrogen ( F E N ) ,
potentially mincralizablc nitrogen, arginine ammoniricalion and substrate induced
respiration w e r e significantly higher in organic and low input than conventional systems on
most s a m p l e d a t e s . Microbial parameters w e r e significantly negatively correlated with
levels of soil mineral nitrogen in the conventional 4 yr s y s t e m , whereas they were
positively correlated with mineral nitrogen in the o r g a n i c s y s t e m T h e carbon to nitrogen
ratios of material released after fumigation extraction were significantly higher in the
c o n v e n t i o n a l than o r g a n i c soils. In all farming s y s t e m s , soil moisture w a s positively
correlated w i t h F E C or F E N , but negatively correlated with the ( 7 N ratio of the microbial
b i o m a s s a n d substrate induced respiration. Soil temperature w a s negatively correlated with
FKC and F F N . but positively correlated with the C/N ratio of microbial biomass.
INTRODUCTION
N u l n e n t cycling and energy flow in terrestrial e c o s y s t e m s are tied to the turnover of
o r g a n i c mailer in soil. Although small in m a s s , the microbial b i o m a s s is a m o n g the most
labile p o o l s of o r g a n i c mailer and thus s e r v e s a s an important reservoir of plant nutrients,
s u c h as N a n d P ( M a r u m o t o el <4. 1982: J e n k i n s o n and l_a<M 1981). T h e microbial
b i o m a s s is a sensitive indicator of c h a n g e s resulting from a g r o n o m i c practices and other
perturbations of the soil e c o s y s t e m ( S m i t h and P a u l . 1990: D o r a n . 1987). Its size and
activity is directly related t o the a m o u n t a n d quality of c a r b o n and other nutrients available
from plant r e s i d u e s , o r g a n i c a m e n d m e n t s and root e x u d a t e s (Frascr el tit. 1988: Adams
and Latighlin. 1 9 8 1 ; Martyniuk et at., 1978: P o w l s o n el al.. 1987)
Other factors
influencing microbial p o p u l a t i o n s are soil moisture and temperature (Campbell and
B i e d e r h c c k . 1976), physical d i s t u r b a n c e of the soil ( D o r a n . 1987). and interactions with
soil fauna ( B e a r e el al.. 1992).
A n u n d e r s t a n d i n g of microbial p r o c e s s e s is important lor the management of
farming s y s t e m s , particularly those that rely on o r g a n i c inputs of nutrients (Smith and Paul.
1990).
M a n y studies concerned with Ihe i m p a c t s of farm management systems on
microbial population d y n a m i c s c o m p a r e different tillage practices (IXiran. 1987: Angers el
<//.. I 9 9 2 ; C a r t e r . I 9 8 6 ) . S t u d i e s c o m p a r i n g c h a n g e s in microbial populations resulting
from different a m o u n t s and types of o r g a n i c inputs, but subjected lo ihe same tillage
practices, arc fewer. A general conclusion of such studies is the development of a higher
microbial b i o m a s s in soils receiving c o v e r c r o p s a n d m a n u r e s than in Ihe same soils
receiving only mineral fertilizers ( K i r c h n e r el al.. I99.E Bolton et al.. 1985: Fraser el al..
I9K8. Nannipicri eial.
1990: A n d e r s o n and D o m s c h . I 9 8 9 : P o w l s o n el al.. I 9 8 7 ; Doran.
1987).
I h e Sustainable Agriculture F a r m i n g S y s t e m s ( S A F S ) Project al University ol
California. D a v i s is a loiigterm. muttidisciplinary study that c o m p a r e s the a g r o n o m y , soil
fertility, soil b i o l o g y , pesls and w e e d s , and e c o n o m i c s of four farm management systems
(Temple el
I 9 9 4 : S c o w el at.. I 9 9 4 ) . A major goal of the project is l o improve
m a n a g e m e n t of soil fertility a n d structure through a greater understanding of soil microbial
p o p u l a t i o n s and their activities T h e objectives of this study were lo: i) compare ihe
seasonal d y n a m i c s of soil microbial populations and their activity in four farming systems:
ii) identify relationships b e t w e e n microbial and soil m l r o g e n parameters: and iii) determine
relationships b e t w e e n soil microbial parameters and environmental variables. During the
g r o w i n g s e a s o n s in 1992 and I99.V m e a s u r e m e n t s w e r e m a d e in tomato plots of ihe carbon
and nitrogen associated with Ihe microbial b i o m a s s . ihe potential activity ol the microbial
populations, as well as of
populations.
environmental
variables that
might
influence
microbial
MATERIALS AND MKT HODS
The study was conducted as part of the Sustainable Agriculture harming Systems
i S A F S ) Project, initialed ill 1989 (Temple viol., 1994). It is located on 8.1 ha of land at
Ihe Department of Agronomy Held Facility. University of California. Davis. I h e soils arc
recent alluvial soils classified as Reiff and Yolo loams. T h e climate is Mediterranean with
average summer temperature of 32 °C and winter 8 °C. The majority of rainfall occurs
between December and March with a yearly total of 6 3 5 m m .
Tlie fanning systems under study are three 4 year rotational systems which include
organic, low input, and conventional4 y e a r ( c o n v 4 yr), and a 2 year conventional rotation
system (Temple et al., 1994). The summer crop rotations for the 4 year systems aatomatoes, safflower, corn and beans. During winter, cover crops are grown in the low
input and organic systems, whereas the soil is fallow in the conventional systems with the
exception of the bean year which is precceded by winter wheal. T h e conventional 2 yr
rotation consists of tomato and wheat/bean (Temple el al., 1994).
Production practices during 1993 are summarized in Table I. The organic system
derives its fertility from a winter cover crop, Lana woolypod vetch (1'iVm
(Li\\\ai/\i).
manure, seaweed and fish powder.
N o pesticides are used and the plots are managed
according lo California Certified Organic Farmers requirements (California Certified
Organic Farmers. 1994). T h e low-input system relies on cover crops as a source of N but
is supplemented, as needed, with inorganic fertilizer and pesticides. The conventional
systems use inorganic fertilizers and pesticides.
The plots are arranged in a randomized complete block, split plot design with four
replicates per system. Each farming system is divided into a number of plots (67 m x 18.1
m) equal lo the number of crops in rotation within that system, and all crops in the rotation
are grown each year.
Soil samples were collected 3 lo 4 days following irrigation in order lo eliminate
some of the potential variability due to extremes in moisture content. Thirty randomly
selected 2.5 cm diameter soil cores per plot from the 0 to 15 cm depth were collected.
Samples from 15-30 cm were also collected on one dale in 1992. Sampling was confined
lo the area between 10 c m from the row and 5 c m from the bed edge. T h e cores were
pooled and well-mixed into a single composite sample per plot and transported to the
laboratory in a cooler on ice. l a t e r , samples were subdivided for the different assays. In
Ihe laboratory, replicated 10 to 15 g subsamples were used for gravimetric soil moisture
determination (105 °C for 24 h). Field moist soil was sieved through a 4 m m sieve and
immediately stored at 4 °C until assayed.
For 4 0 to 4 8 hours prior to the activity
measurements, sieved soil was incubated at 2 4 ° C in plastic bags loosely tied for sufficient
aeration and lo prevent moisture loss. Soil temperature at Ihe 0 to 15 cm depth of each
sampled plot was recorded in ihe field. All measurements are presented as Julian days (JD)
where, for example. J D I - January I and J D 3 2 = February 1.
Analyses of KCI cxtractable ammonium N and nitrate N were conducted by the
Division of Agriculture and Environmental Resources Analytical Laboratory using a
Carlson autoanalyzer(Carlson, 1978). Fumigation extractabte carbon (FEC) and nitrogen
(FEN) released after fumigation and extraction of soil were measured using a method
adapted from Vance eial. (1987) and Sparling and West (1988). The fumigation lime w a s
24 hours and a soil exlrat lant ratio of 1:2.4 was used. All extracts were stored at 20 °C
until analysis. Before analysis, samples were thawed and incubated at room temperature lor
3 0 minutes. Exlraclable carbon following fumigation was determined by a I M i n n . m Model
I X ' 8 0 I O C analyzer in 1992 and by a Shimadzu Model 5 0 5 0 T O C analyzer in 1993. l o r
the analyses with the Shimadzu T O C analyzer, samples and standards were diluted lo 3 and
6 fold prior to injection to prevent precipitation of K ,S(), from the extraction medium onto
the catalyst pellets. The catalyst was removed and replaced every 7 2 samples and new
standards were run every 24 samples to ensure there was no degeneration of ihe catalyst
Extraetablenitrogen following fumigation extraction was determined as ninhydnu tractive
N. which includes N H , N. amines, amino acids. |>eptidcs and proteins ( A m a t o a n d liidd,
1988; Carter; 1991), The values reported are differences in cxtractable carbon and nitrogen
between the fumigated and uiilumigated samples and are not converted to "total" microbial
biomass C and N because of the uncertainly associated with using existing correction
factors (Horwath and Paul. 1994).
Substrate induced respiration (SIR) was measured according to Smith et at. (1985)
in which both the control and glucose amended soils are provided w itli nutrients
Five »
of soil were amended with nutrient broth (Beeton Dickinson Microbiology Systems.
Coekeysville. MD)containing beef extract and gelatin digest and then either amended or not
amended ("uninduced") with 200 /<g glucose g soil
This glucose concentration was
determined in preliminary tests lo be the level at w Inch the short term respiration rale was at
its maximum. The soil was incubated for 2 h at room tenqieralure and the head space CO,
concentration was determined using a C t ) , Analy/er with an infrared detector (Applied
Electrochemistry. Inc.. Sunnyvale. C A ) .
Arginineammonificalion(AA) was measured according lo Alt 1 and Kleiner {1987)
Following preincubation at 3 0 ° C f o r I h, 5 g soil was amended or not amended with an
arginine solution lo give a final concentration of 196 /«» N g ' , Alter stopping the reaction
al 4 5 minutes by immersion in liquid nitrogen, the soil was thawed, extracted with 2 M
KCI. and NH^ N in the extract was analyzed by colorinietry.
Potentially mineralizablenitrogen ( P M N ) was measured in 1992 by the autoclave
method adapted from Stanford and Smilh (1976) and Doran (1987). and in both 1992 and
1993 by the anaerobic incubation method adapted from Kceney (1982)
Analysis of variance. Fisher's Protected I .cast Significant Difference (PESD) lest,
and the / test of significance were performed where appropriate (Ceng and Hills. 1989,
Littleand Hills, 1978). Linear regression and correlation analyses were carried out using a
Macintosh StatView 4.0 application program (Abacus Concepts. Inc.. Berkeley. CA).
1
RESULTS
Comparison of 4 farming systems in 1992 and 1993.
All four farming systems were sampled on al least four dates during the growing
season in 1992 and 1993. The year 1992 was the last year of the first 4 year rotation
sequence and 1993 was the first year of ihe second rotation, fumigation exlraclable carbon
( F E C ) was significantly higher in the organic and low input systems than in conventional
systems on most dales (Fig. IA and IB). These differences were present even al the
beginning of the growing season, before the incorporation of organic residues, and
incorporation was followed by little change or a decline in FEC in the organic and low
input systems. Differences a m o n g systems were observed in soil samples from deeper in
the profile as well. In September, 1992, FEC was significantly higher in the 0 to 15 cm
than the 15 lo 30 cm layer in all systems (Table 2) At this depth. I EC was significantly
higher in low input than other systems, whereas the organic did not differ from Ihe
conventional systems.
In 1993, PEN also was higher in organic and low input than
conventional (Fig. 2A). Potentially mincrali/.abic nitrogen ( P M N ) . on the other hand, was
significantly different before incorporation of cover crop only in the low input system (Fig.
2B). After that, conventional 4 yr was significantly lower than all other systems in mid
June, and later in the season there were no differences among systems. On the last
sampling date there had been 3 weeks since the conventional tomatoes were harvested and
its residues incorporated, whereas it was still 10 days prior lo harvest in ihe organic and
low input systems.
Estimates were made ol Ihe total carbon (C) and nitrogen (N) contained in inputs
(
ling external inputs and above-ground residues from the preceding crop) lo Ihe four
fa
ng systems in 1992 and 1993 (Table 3). In 1993. Ihe inputs lo the organic system
had a considerably higher C/N ratio than in previous years because a large fraction of the
cover crop consisled of volunteer oals, which contain lower N levels than vetch, and
because the manure contained larger than usual quantities of slraw. In contrast, in 1992 the
C7N ratios for inputs to the conv 4 yr and organic systems were 8.2 and 19.9. respectively.
As would be expected under the immobilization conditions associated with the high C/N
ratios in 1993. soil nitrate and ammonium concentrations in the organic system were low
over the entire season (Fig. 3A and 3B). In ihe conv 4 yr system, however, mineral N
levels increased dramatically following application of sidedress fertilizer and remained
relatively high for three w e e k s .
Intensive sampling of six microbial parameters describing biomass or potential
activity was conducted in the organic and conv 4 yr systems on a total of 13 dales in 1993.
On most dates, all six parameters were significantly higher in ihe organic than conv 4 yr
system (Figs. 4 b). Fluctuations in F E C were minor in the conv 4 yr (± 7 % of a mean of
7 6 //g/g) in comparison to the organic plots (± 18% of a mean of I 10). In contrast,
fluctuations in FEN were high in both conv 4 yr (± 2 6 % of a mean of 5 /«g/g) and organic
(± 19% of a mean of 27 /ig/g) plots (Fig. 4A and 4 B ) .
l e v e l s of SIR and AA were significantly higher over most of Ihe growing season in
the organic than conv 4 yr plots (Fig 5). Arginine ammonification (Fig 5C) substantially
increased in the organic plots following input of organic residues, whereas SIR did not
show any obvious response (Fig. 5A). SIR showed a substantial drop in the conv 4 yr and
a small drop in organic plots immediately following the time of sidedressing (between JD
110 and 155) in the conv 4 yr plots. Uninduced respiration, was significantly higher in
organic than conv 4 yr plots early to mid-season and. by the end of the season, declined lo
about 19% of its original level in both systems (Fig. 5B).
Potentially mineralizable
nitrogen showed the same differences between organic and conv 4 yr plots (Fig. 6) as seen
for other parameters and its behavior was most similar to, of all the parameters measured,
that of F F N .
In the conv 4 yr system, PMN was not detectable on the two dales
immediately following sidedress application. T h e decline may have been due to an inability
of the method lo detect mineralizable N above the high background level resulting from
side dressing or toxicity; however, this period also corresponded with a time when Ihe
SIR showed a temporary decline in the conv 4 yr plots.
T h e parameter SIR has been proposed as an estimate of microbial biomass
(Anderson and Domsch, 1978 ) based on the assumption that short-term metabolic rates
reflect the size of the microbial population at the time of measurement. Therefore, it could
be concluded that differences in SIR, and perhaps A A . may simply reflect differences in the
magnitude of microbial biomass. T o test whether there were differences between the two
farming systems with respect to the intrinsic activity of the microbial community, AA and
SIR were converted to a per unit biomass basis by dividing by FEC determined on the
same date. T h e ratio SIR/FEC was significantly higher in the organic than conventional
system during more than 6 0 % of the observations (Fig. 7 ) , and continuously during the
time between J D 128 and 172. For 2 0 days following cover crop incorporation, AA/FEC
was significantly higher in the organic than conv 4 yr system (Fig. 7) and then (here was
little difference between systems. In striking contrast to the other measures, the ratios of
uninduced respiration/EEC were virtually identical for the t w o systems except on one dale
shortly after cover crop incorporation (Fig. 7 ) .
Most bacteria have lower C/N ratios than do fungi (Holland and Coleman. I9K7)
and therefore the C/N ratio of the microbial biomass may reflect the domination of the
microbial community by fungi or bacteria. T h e C/N ratios of the microbial biomass were
calculated for each dale in 1993. Because biomass N is measured by ninhydrin reactive
nitrogen, rather than total N , and because neither C nor N have been corrected lor total
biomass, the ratios serve for purposes of comparison and not as absolute values. Ihe ( 7 N
ratios were significantly higher in conventional than organic soils on i
lates (fig 8)
Microbial C/N ratios in the organic plots were remarkably stable over in.... of the grow in.'
season, whereas in the conv 4 yr system the ratios showed greater fluctuations and
increased over the last third of the growing season
Relationship between microbial and soil fertility parameters
Determining relationships among microbial parameters and soil nitrogen levels is
complicated because of the short term lliictiialions in soil fertility and microbiological
properties. Therefore, looking at relationships among data collected on the same dale ma)
not reflect important trends occurring prior lo the sample dale Nevertheless, correlations
among all microbial and soil fertility parameters showed that when data from ihe organic
and conventional 4 yr systems < 13 dales) ( f a b l e 4) or from all farming systems (4 dales)
(data not shown) were combined, there were consistently significant ( p - 0 . 0 5 ) negative
relationships among microbial parameters (IPX'. I I N SIR. AA and PMN) and soil nitrate
or ammonium. Correlation analysis of data lor the conv 4 vr system alone showed the
same trends as for all farming systems combined
However, analysis of the organic
system alone revealed no significant (p=0.05) ncgatu e relationships between microbial and
soil fertility parameters; in fact, these analyses showed significant ( p - 0 . 0 5 ) positive
relationships between FEC and soil nitrate or ammonium and between AA and ammonium
(data not shown). In alt systems, there was a strong positive correlation between mosl of
Ihe microbial parameters and PMN
Relationship between microbial parameters and cm ironmciual factors
Frequent irrigation (shown as bars on f i g 9) maintained soil moisture in organic
and conv 4 yr plots al levels between 12 and 307, m the top 15 cm of soil (fig. 9)
Irrigation was more frequent and soil moisture was usually higher in (he organic than conv
4 yr system. Values in the coin 4 yr system dropped considerably alter irrigation was
terminated after JD 200 (mid July). Seasonal variations in soil moisture levels were not as
great in organic as in conv 4 yr soils. As these samples were collected 3 lo 4 days
following irrigation and intervals between irrigations were often 8 to 10 days, soils hecamc
even drier than the measurements indicated and es|iecially so in the lop layer of soil, for
example. Amezkclael ui < 1996) found thai moisture levels in the same soils during much
of the 1994 growing season ranged from 4 - 9 % , with a mean of 5.4%. in the top 5 cm of
soil.
Because the gravimetric moisture content lor the SAPS soils at 0.33 bar is
approximately 2 4 % (Scow, unpublished data), it is likely that moisture levels were
frequently suboptimal for microbial activity
Correlation analyses for climatic and microbial data (organic and conv 4 yr systems
combined) indicated significant positive relationships between soil moisture and either 11 <
or FEN, but negative relationships between moisture and the ( 7 N ratio of the microbial
biomass. SIR, and SIR/FEC ( f a b l e 5).
Soil temperature had significant negative
relationships with levels of PEC and FEN. but a positive relationship with (he (7N ratio oi
microbial biomass.
DISCUSSION
Differences among farming systems
Estimates of microbial biomass and activity used in this study indicated that these
parameters are almost always significantly higher in soils of the organic and low input than
conventional farming system Though management of the farming systems differs in man)
ways, we believe Ihe mosl important factor differentiating the microbial populations in ihe
different farming systems is Ihe amount of carbon entering Ihe systems. The fact that more
pesticides arc used in the conventional than organic systems is presumed to be uumi|M>rlaui
for several reasons, First, pesticide use is minimal in the conventional systems because
they arc managed according lo integrated |>esl management (II'M) practices Also, although
different measurements. The organic system showed a higher AA/TFC immediately alter
incorporation whereas SIR/FKC was higher in the organic system in the middle of the
growing season.
The large difference between patterns of nninduced respiralion/FKC ami SIR/FI ("
' w a s unexpected. The ratio of nninduced respiralion/FHC was virtually identical in ilkorganic and conv 4 yr systems. Uninduced respiration reflects mineralization ol soil
carbon, as well as of the complex carbon contained in the nutrient broth amendment
(pancreatic digest of gelatin and beef extract) in the method we employed, whereas SIR
reflects the net niineralizalionof added glucose The short term mclalvolism of soil carbon
and nutrient broth was a direct function of the size of the biomass and thus m o u r study,
was a better estimate of biomass than glucose-induced respiration. Wardle and Parkinson
(1940b) discuss the likelihood that not all of the soil biomass can respond rapidly to added
glucose and thus SIR is not always proportional to microbial biomass estimates by
fumigation methods.
Differences in microbial community composition.
Though the emphasis of our study was not on microbial coinmunil) structure, there
were indications of differences in the communities of the organic and conventional
systems.
Fungal dominated communities are favored over bacterial dominated
communities in farming systems in which plant residues are not tilled, at least in part due to
spatial stratification of the organic inputs (Beare el at.. 1992, Holland and Coleman. 1987)
These studies have compared systems receiving the same amounts of organic matter btu
differing in whether or not the residues are incorporated. In contrast, our study considers
systems differing in their organic inputs but not substantially in their tillage practices. The
lower C/N ratios in microbial biomass in organic than conv 4 yr soils supported that
bacteria may be more important and fungi not as dominant in organic compared to
conventional soil. Other supporting evidence for the importance of bacterial populations in
organic soils was the fact that in 1993 bacterial feeding nematodes were more prevalent in
organic than conventional systems, with little difference between systems in fungal feeders
(Ferris el at., 1996). Also, Scow el at. (1994) found higher levels of fungal feeding
nematodes in conventional than organic and low input soils of the SAFS project in 1992.
T w o factors distinguishing the two farming systems that may have, in turn,
contributed to differences in their soil communities were the types of organic inputs and the
soil moisture levels. The conv 4 yr system receives no carbon inputs after harvest of beans
in the previous October; therefore the major sources of C for soil organisms in the
subsequent cropping season are soil humus and the exudates and sloughing of tomato
roots. T h e organic system, on the other hand, receives large single inputs of manure and
cover crops. Bremer and van Kesscl (1992) found that microbial biomass growing on
green manure crops had significantly lower C7N ratios than biomass growing on straw
residues and Necly el at. (1991) found a direct correspondence between the C7N ratio of
organic inputs and the C/N ratio of microbial biomass. With respect to soil moisture, fungi
arc known to be favored at low water potentials in soil (Paul and Clark, 1989) and thus the
consistently lower moisture content in the conventional soils may also have selected for
fungal-dominated populations. The significant negative relationship between soil moisture
and microbial C/N ratios supported this hypothesis. (Xhcr researchers have also used
changes in C/N ratios of microbial biomass to infer changes in the community (Garcia and
Rice. 1994; Whcallcyif u/., 1990; Collins el at., 1992)
Seasonal variations.
Seasonal fluctuations in FHC arc minor at longterm siles s u c h as Kothamsled
(Jenkinson, 1990; Jcnkinson and Rayncr, 1977; Pslra el til., 1990), where carbon inputs in
the form of animal manures have been occurring for over a century and presumably a
steady stale has been achieved with respect to microbial biomass. However, seasonal
fluctuations in FHC and/or FKN have been observed in numerous other systems, such as
pesticides are used in the low input but not organic plots, the levels of microbial biomass
are similar in the two systems Finally a large body of literature supports the premise ifiat
most pesticides applied ai recommended application rales, with the exception of fumigants,
do not significantly impact microbial biomass and activity (Fraser el til., 1988; Martviiiuk
and Wagner. 1978; Hicks ct ai.. 1989). Because carbon is so often a limiting factor lor
soil microbial populations, the higher cartkMi associated w ith (he organic and low input than
conventional systems presumably overrides small, if any. differences in microbial
populations due to toxicity from pesticides
Since the establishment of the SAFS project in 1989, C a n d N inputs have
quantitatively and qualitatively differed among the farming systems (Scow el at., 19941.
In 199 V total C inputs to the organic system were 2 J limes the levels of C lo the low iiipui
system, yel differences in microbial biomass estimates were not substantial. This lack of
dillereuce may be due to the recalcitrance o f the carbon in the poultry manure; however,
this hypothesis has not been tested. I c v e l s of C to (he organic system were, on the other
hand. 6.2 limes greater (han inputs to the conv 4 yr system and (he differences in microbial
biomass estimates were always significant and as great as 2 fold. The impact of ihe organic
inputs was seen beyond I he "rowing season, as evidenced by the presence of differences in
FHC and FFN between organic and conv 4 yr systems even before cover crop
incorporation and even following tomato harvest. Activity measurements, on the oilier
hand, were usually not significant!) different among systems before and afler the growing
season.
Another factor contributing l o differences among the farming systems is unique lo
Mediterranean and other seasonally warm climates. I he mild winters of California make ii
possible lo maintain plant cover over « i n t e r w iih only a I l o 2 month period of bare fallow,
as is done in Ihe organic and low input fanning systems
In Ihe conventional systems,
however, winter management in Ihe Sacramento Valley involves keeping ihe soil fallow
Irom September until March l o conserve moisture and lo permit field operations l o he
carried oul lor (he next year's crop The warm, wet winters of California can support
relatively high microbial activity compared to lev els in climates with colder winters, and
litis activity would l>e even more enhanced in rhi/osphere, e.g. of the cover crop, than
fallow soil. Microbial populations tend lo be lower in systems wilh fallow periods than
with continuous cropping (Collins el til.. 1992). The presence of a high microbial biomass
in early spring in the organic and low input plots may be beneficial if the biomass could
rapidly mineralize organic residues into plant available nitrogen. On the other hand, a
possible consequence of a high microbial biomass fostering high rales of mineralization and
nitrification could be nitrate leaching during periods of heavy rainfall in spring. The
possible benefits and disadvantages of maintaining large and active microbial populations
over winter needs further investigation.
There was indirect evidence lhai side dressing of (he conventional system may have
had temporary negative impacts, possibly due lo osmotic shock (Paul and Clark. 1989), on
microbial populations. Between Julian days 125 and 140 (early to mid May), declines in
the conventional system occurred during a period when high concentrations of mineral N
were present following side-dressing Substrate induced respiration, uninduced respiration
and SIR/FFX" were Ihe parameters that showed ihe greatest declines during this period.
I .ess substantial declines in the same parameters within Ihe same time period in ihe organic
plots also occurred, suggesting that part of (lie decline may have been due lo climatic
conditionsex|>eriencedby both systems (see below); however, declines were clearly more
severe in (he conventional system.
Measures of microbial biomass or activity, alone, do nol relied all iinpiri.nn
differences between systems localise a large p o r t i o n of (he microbial community may lie
iuaclivc((iray and Williams, 1971) Thus, Ihe ratio of an activity measurement--e.g.. S I R .
A A , or uninduced respiration to FHC gives an indication of ihe specific aclivily ol ihe
microbial biomass. Generally, when lliere were differences, microbial populations were
more active in the organic than conv 4 yr system, although not al ihe same time for the
tall grass prairie (Garcia ami Rice. I W I and agricultural systems ll.vncli ami Panting.
1980: Campbell and Biedcrbeck. 1976; Wheat Icy <7 <i/.. 1990: MeGill el til.. 1986). In
both years of our study, mosl microbial parameters showed a shaqi decline billowing II)
155 (early June) which corresponded lo a period during which the temperature increased
substantially. Similar declines in FEC during this same time period have been observed in
previous years in all 4 farming systems (Scow el til.. 1994). In the organic system, early
season fluctuations were mosl likely due to incorporation of organic material.
Even though Ihe SAKS plots were irrigated, soil moisture sometimes drop|>cd lo
levels low enough, particularly in ihe conventional system, to limit microbial populations
Numerous studies have reported decreases in microbial biomass due to drying of the soil
(Van Gcstel <•/<//.. 1992; I a d d eial.. 1986: Wardle and Parkinson. 1990a)! Soil moisture
and microbial biomass levels were significantly positively correlated in our study. High
temperatures were also associated with lower microbial biomass. and. as w iih low moisture
conditions, were associated with high microbial C/N ratios. Because soil moisture is
closely related lo temperature, it is difficult lo isolate the contribution of each of these
variables in governing population levels. Also. Ihe very high temperatures in Ihe lale
growing season may have led lo additional short-term moisture limitations between
irrigation events thai we were unable to d e l e d with our sampling frequency
On the
regional scale, temperature and moisture are positively correlated with dccom|>osiiiou rales
and negatively correlated with biomass levels (Insam el <J.. 1989). however, these
relationships are rarely based on data collected in arid climates and do not consider
irrigation as a variable.
Relationship between microbial and soil fertility parameters
An important and challenging objective of the SAFS project is lo assess whether Ihe
striking differences in microbial parameters correspond to differences in soil fertility among
the different farming systems. In previous years we have shown thai seasonal patterns in
soil nitrate levels differ among fanning systems (Scow el al.. 1994). I he pattern changed
from previously higher (e.g. in 198*). 1990) to lower nitrate levels in organic than
conventional tomatoes.
In 1993. the cover cropland manure inputs into the organic system had considerably
highcrC/N ratios than in previous years and mineral N levels in organic soils showed few
fluctuations and remained well below levels in the conventional soil. Nevertheless, tomato
yields were equivalent in the organic and conventional systems. Due to poor weather
conditions. Ihe mean yield of conventional tomaloes in Yolo County was low in 1993
relative lo other years ( S A F S . unpublished data); thus il was not a high yielding year for
tomatoes in general. However, we believe the high minerali/alion activity of the large
microbial biomass in the organic soil lead lo release of sufficient N for thai year. This N
was then taken up by the tomaloes so rapidly thai il never accumulated in Ihe soil
The
observed increases in microbial activity and increased nematode activity (Ferris el al..
1996) after incorporation of inputs in the organic system support this hypothesis.
A side benefit of organic inputs with high C/N ratios is their potential to reduce
nitrate leaching. Il is likely that ( 7 N ratios as high as those observed in the organic system
in 1991 would, in years when tomato yields are not so unusually low. lead lo microbial
immobilization and nutrient deficiency t o crops, T h u s , though such high ( 7 N ratios might
be beneficial in their ability to facilitate the capture of nitrate, these ratios would not be
rccommcndcd from the perspective of soil fertility It is possible, however, thai ( 7 N ratios
intermediate hclwcen the high levels observed in 1993 and the relatively low ratios usually
recommended (e.g.. ratios of around 15) may provide fertility that is sufficient, while being
less likely lo negatively impact Ihe environment
Studies are in progress al Ihe SAI S plots
lo determine Ihe effect of ( 7 N ratio of organic fertilizers on tomato yields under organic
management
Substantial increases in mineral nitrogen concentrations have lieen shown lo
correspond lo declines in soil microbial biomass (Haynes. I9K6; Bondc el al.. 1988) The
inverse relationship in the conventional 4 yr system between microbial biomass and
mineral nitrogen supported this observation.
In contrast, the relationship bclw.
microbial biomass and mineral N was positive in the organic system. Presumably in
organic system then- w a s a continuous release of plant available N from ihe micn>l
biomass. e.g. via wet dry cycles (Marunioto el til.. 1977. 1982) and/or predai
(Clarholm. 1985: Ferris el al. 1996). as indicated by the f a d that there was enough N
sustain adequate tomato yields in (his system The absence of an obvious net decliix
F F C in the organic system during the pcruxl of maximum crop demand (in lale May
June) may imply (hat Ihe high carbon inputs to this system could maintain micro!
|M>pulalions at levels thai matched their losses d u e lo death
There is considerable interest in conserving or increasing organic matter level
agricultural soils (Pausliau etti.. 1995). A commonly held, but largely untested, belie
that il is not possible to increase organic matter contents in irrigated agricultural soil
California due to their very high rates of carbon mineralization. After one four \
rotation cycle, total organic matlerconleul had increased in the organic system by 8 I .V
comparison lo Ihe conventional systems (Scow el til.. 1994) and. since ihen. organic ma
levels have been consistently and significantly higher in low input and organic U
conventional systems ( S A P S , unpublished results) More significant was Ihe increasi
amount of C associated with microbial biomass in the organic and low input relative to
conventional system, as has been observed by others (Powlson el al.. 1987). Microi
biomass is one of the most labile of the |HH>ls comprising organic matter (Paul and CI
1989) and. if soil fertility is Ihe main interest, an increase in F F C is likely lo In
represent changes in ihe nutrient-supplying capacity of organic matter than is an increas
total organic mailer.
ACKNOWLEDGMENTS
This work would nol have been possible without the assistance of Hui f i . Inn
Onlla. Jennifer Douglas. Oscar Somasco. Don Stewart. Diana Priedman. and Mu
Volat. Funding was provided primarily by the USDA Cooperative Stale Research Sen
under Agreement No. 9 1 - C O O P I -6590 jointly funded by the U.S. EPA for thecondm
the Agriculture in Concert with the Environment (ACE) Program and Ihe Califoi
Department of Food and Agriculture Fertilizer Research and Fducalion Progi
Additional support was provided by USDA Western Regional Sustainable Agriculi
Research and Education (SARE) Program: Ihe University of California Sustain
Agriculture and Research Program ( S A R E P ) and the U.S. EPA (R8I9658) Center
Ecological Health Research al UC Davis. Although Ihe information in this document
been funded in part by (he United Stales Environmental Protection Agency, it may
necessarily reflect the views of (he Agency and no official endorsement should be inferi
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figure I
Fumigation exlraclable carbon in the four cropping systems during
1992 (A) and 1 9 9 3 ( B ) . Vertical bars _ standard error ( n = 4 ) .
figure 2. fumigation exlraclable nitrogen (A) and jioteiilially mincrali/ablc
uilrogen (B) in I he lour cropping systems in 1993. Vertical bars =
standard error (n 4).
figure 3. Changes in soil NIP) N (Al and soil N l f i N (ID in com 4 yr and
organic systems iu 1993. Vertical bars
figure 4.
standard error t n - 4 )
Changes in fumigation exlraclable carbon (A) and
fumigation
exlraclable nitrogen (B) in conv 4 yr and organic systems in 1993.
Vertical bars = standard error (u
4)
figure 5. Changes in substrate induced respiration (A), uninduced respiration
(H) and arginine ammouification(C) iu conv 4 yr and organic systems
in 1993. Vertical bars = standard error (n =4)
Figure 6. Changes in PMN in conv 4 yr and organic systems in 1993 Vertical
bars = standard error ( n - 4 ) .
Figure 7.
Changes iu substrate induced respiration / I P C (A), uninduced
respiration/ FEC (B) and arginine ammoiulicalion/ FEC <C) in conv
4 yr and organic systems in 1993. Vertical bars - standard error
(n=4).
Figure K
Changes in fumigation exlraclable ( 7 N ratios in conv 4 yr and
organic systems in 1993 Vertical bars = standard error ( n - 4 ) .
figure 9. Changes in soil moisture and the amount ol irrigation waler applied
over Ihe season in conv 4 yr and organic systems in 1993.
—
FEC
O
—
Q...
Organic
1992
Low input
1993
Conv 4 yr
m
Conv 2 yr
vetch incorporation in
E
g
organic and low input
/
80
tomato harvest
120
160
240
200
J U L I A N
D A Y S
1
2 0
1 6 0
J U L I A N
»0
—
O
—
-•- A
*
OO -
Organic
2 0 0
2 4 0
D A Y S
1 9 9 3
Low input
Conv4yi
B
Conv2yr
45
PMN
—
O
—
O
Organic
—
-
-
Low input
Conv 4 yr
Conv 2 yi
'10 35
GO tomato harvest in
conventional
40
-
80
v e t c h incorporation in
organic a n d low input
120
I
160
200
Julian
E
«
^
I
-
25
harvest in organic
and low input
15
-
240
days
8 0
1 2 0
1 6 0
J U L I A N
- « 5
|
2 0 0
D A Y S
19<
I
I
k
Ammonium
90
1993
N
Organic
I
70 H
a.
50
Conv 4 yr
30
I
10
V
-i
•10
80
80
1 20
200
160
Julian
'20
240
160
Julian
200
240
days
days
14
B
1993
FEN
1?
10 H
S I
8 H
6
V
,
80
120
Conventional
160
Julian
200
days
240
120
160
Julian days
200
240
1993
SIR/FEC
-o—
—• -
Organic
C o n v 4 yr
Fumigation extractable C / N
199
25 H
20 i
15
10
120
1G0
Julian days
200
80
120
160
Julian
Julian DAYS
200
days
240
Table 1. Tomato production practices in 1993
Operation"
Organic
Low input
Transplants
Transplants
Conv 2 yr
''1D69
JD69
lDl"09
Planting date
Method of planting
Cropping System
Conv 4 yr
Seeding
Seeding
Fertilizer
Preplant and/or
Starter
Vetch/oats cover crop
176 kg N h a '
Vetch/oats cover crop
121 kg N h a '
1
1
Mineral fertilizer
(6-20-20) 112 kg ha-
Mineral fertilizer
(6-20-20) 112 kg ha-
Turkey manure (1 % N)
67 kg N h a '
1
Fish powder
(12-0.25-1) 4.5 kg h a "
1
Seaweed
1
(3-0.25-0.15) 4.7 L h a '
Sidedxess
Mineral fertilizer
Mineral fertilizer
Fish powder
( 1 2 - 0 . 2 5 - 1 ) 4 . 5 kg h a "
(8-24-6) 94 kg ha"
1
1
(34-0-0) 134 kg h a '
Mineral fertilizer
1
(34-0-0) 134 kg h a '
Seaweed
(3-0.25-0.15) 9.4 L h a "
1
Herbicides
* JD=Julian day
Irrigation
O
*"
water
O
O
O
N
o
o
—
CO
(liters)
m
a
in
O
O
o
to
o
o
% ajnjsioai nog
CM
O
Gramoxone
Gramoxone
Devrinol
Devrinol
1
Organic
Table 2. Fumigation exlraclable carbon al two sampling depths in September 1992
Cropping syslem
^
o
£y£pu»
'
1992
Carbon
Fumigation exlraclable C (pg/g soil)
a t O - 15 cm
al 15 30 cm
47 42
a
93'.S b
105-95 b
28 78 I)
23 85 he
35 2H a
•Each value is a mean of three replicates. Means followed by the same letter within each
column are not statistically different according to Fisher's PLSD lest (p=0 05)
Nitrogen
Cropping syslem
Low input
(kg/ha)
Crop residues
Cover crop
Manure
Total
2520
1509
ISH?
... 1
1760
I S'K,
Crop residues
Cover crop
Manure
Fertilizer
Total
92
1(11
101
2
296
42
101
0
3-1
177
I )o2
. ...
1410
3424
3276
HI 36
I 19K
2120
0
3524
I3IK
0
0
MIS
Crop residues
Cover crop
Manure
Fertilizer
Total
43
121
67
1
2S2
W
171,
II
10
225
Y)
0
0
141
180
C:N of inputs
35 1
IK»
24
0
0
|4|
|f,5
C:N of inputs
1 9 9 3 "
Carbon
Crop residues
Cover crop
Manure
Tolal
Nitrogen
19.9
Conv 4 yr
l_5_<>
H2
7.3
* The cover crop in Ihe organic syslem had a large |K>pulalion ol volunteer oats which
led lo a higher than normal C/N ratio. Also, iu 1993 the manuie has a higher propurtiuii
of straw lhau in previous years. Thus, the organic system li.nl a substantially higher C
and lower N than the low input syslem.
Table 4. Linear correlation coefficients of 1993 soil microbial and nitrogen data from
conv 4 yr and organic systems
Variables
Number of
observations
Correlation
coefficient (1 )*
FEC.
PMN
94
0 550
FEC.
soil N H 4 - N
104
0216
FEC,
soil N O 3 - N
104
0244
FEN,
PMN
94
0 522
100
-0269
, sodNH4-N
94
-0.279
PMN , sod NO $ N
94
0236
AA , s o i l N 0 3 N
76
-0.318
SIR . s o i l N P P I N
88
0.371
S01INO3N
88
0.446
104
0876
FEN , soil N O 3 - N
PMN
SIR
.
S01INH4-N
, sod N O ) N
* significant at the 0.05 probability level
Table 5. Linear correlation coefficients ol combined data
from 1992 and 1993 indicating relationships among soil
moisture, temperature, microbial and niirogen par .nuclei s
32
32
24
Correlation
coellicient
(.)'
0 121
0.479
0 SH')
I) I'Jd
I) sou
0 41)1
117
89
89
117
0.418
0 354
0 296
0.192
Number of
observations
Variables
Soil
Soil
Soil
Soil
Soil
Soil
moisture
moisture
moisture
moisture
moisture
moisture
,
,
,
,
,
,
Soil
Soil
Soil
Soil
temperature
temperature
temperature
temperature
124
%
FEC
FEN
C/N
SIR
SIR/FEC
AA/FEC
%
, FEC
, FEN
, C/N
, soil N H N
4
* significant al the 0.05 probability level