16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/12713
Changes in mineral content and CO2 release from organic
greenhouse soils incubated under two different temperatures
and moisture conditions
Pepin, S.1, Dorais, M.2, Gruyer, N.3 & Ménard, C.4
Key words: soil respiration; moisture content; biological activity; soil temperature
Abstract
In organic greenhouse vegetable productions, the turnover rate of organic
amendments may be a limiting factor for optimal crop productivity and quality. Hence,
we determined the mineralization potential of several organic greenhouse soils
maintained at two temperatures (17, 23C) and water potentials (–35, –250 mbars).
Replicate cores of structurally intact soils were collected in plastic cylinders, saturated
with water and adjusted to the appropriate matric potential. Additional soil samples
were sieved, placed in glass jars and incubated under the same treatment conditions.
Soil nutrients, gas concentration (O2, CO2, N2O) and microbial activity (CO2 release)
were measured over a 25-week period during aerobic incubation. Large variations in
nutrient and organic matter content were observed among intact soil samples. CO 2
efflux declined exponentially with time, decreases being most apparent in soils having
high organic matter content. An increase in temperature lead to enhanced soil
respiration rates, mainly during the first weeks of incubation. Overall, mineralization
rates were only slightly affected by moisture level or temperature. Gas diffusion, and
thus soil biological activity, may be momentarily hindered during frequent irrigations.
Yet, our findings indicate that in general matric potentials of –35 and –250 mbars both
result in similar mineralization rates in these soils.
Introduction
The foundation of organic farming is based on soil biological activity, which depends
on soil properties (C/N, % organic matter, pH, O2), cultural practices (fertilization,
amendment, crop rotation, tillage, irrigation) and environmental factors such as
temperature and soil moisture (Dorais, 2008). Although the growing conditions can
easily be controlled in a greenhouse, the turnover of organic amendments for
organically-grown vegetable crops may become a limiting factor for optimal crop
productivity and product quality. Indeed, nutrient requirement of greenhouse tomato
crops is higher than that of field tomato crops, with yield being up to 10 times greater
in greenhouse crops than field crops (Heuvelink & Dorais, 2005). Soil texture and
structure, as well as temperature and moisture may all affect the activity of soil
microorganisms and hence, the mineralization rate of organic matter in the soil
(Angers & Carter 1996; Schjønning et al. 1999; Thomsen et al. 1999). For instance,
1
Horticulture Research Centre, Département des sols et de génie agroalimentaire, Laval
University, Québec, Canada, G1V 0A6, E-mail steeve.pepin@sga.ulaval.ca
2
Agriculture and Agri-Food Canada, Envirotron bldg, Laval University, Québec, Canada, G1V 0A6,
E-mail: doraisma@agr.gc.ca
3
As in 1
4
As in 2
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/12713
pore size distribution and total porosity both impact on soil organic C mineralization by
influencing soil moisture availability for microbes (Yoo et al. 2006). Irrigation
management is thus a key factor to optimize soil biological activity. A better
understanding of mineralization processes under different greenhouse growing
conditions is important for optimizing crop nutrient supply, while minimizing losses to
the environment (i.e. to the groundwater). The objective of the present study was to
determine the mineralization potential of several organic greenhouse soils maintained
at two temperatures and two soil water potentials.
Materials and methods
An incubation experiment was conducted using two temperatures (17°C, 23°C), two
soil matric potentials (–35 mbars, –250 mbars = field capacity) and five incubation
periods (1, 4, 8, 16 and 24 weeks). Three replicate 196-cm3 cores of structurally intact
soil (0–10 cm depth; 5-cm diam. cylinders) per treatment and time period were
collected from five organically managed greenhouse soils in 2006 (n=300; Table 1).
Soil samples were first saturated with distilled water, then adjusted to matric potentials
of –35 and –250 mbars using a tension-plate assembly and a pressure-plate
apparatus. Based on soil water release characteristics, these two matric potentials
resulted in a mean (±SD) water-filled pore space of 80% (±6%) and 68% (±5%),
respectively. Additional samples (n=240) were collected, saturated and brought to the
appropriate matric potential, then sieved through a 6-mm mesh and placed in sealed
glass jars. Cylinders and glass jars were placed into two growth chambers under
constant temperature in completely randomized blocks (n=3). Soil samples were
weighed once a week and distilled water was added when necessary to compensate
for water loss. Samples were rotated weekly both within and between chambers to
minimize chamber effects. Microbial activity (CO2 efflux) was measured in larger soil
samples (~900 cm3) at 4 to 8-week intervals using a portable gas exchange system
(model LI-6400, Li-Cor) and a Soil CO2 Flux Chamber. Three cylinders and glass jars
from each soil and treatment were sampled at the end of each incubation period to
determine water-extractable minerals (K, P, Mg, Ca, Na, etc.; readily available to
plants) and KCl-extractable inorganic nitrogen (NO3, NH4). Soil organic matter content
was determined using the Walkley-Black method (for mineral soil) or the loss by
ignition method (for soil having >20% organic matter). Mean changes in nutrient
content were analyzed using an ANOVA with soil type, temperature, matric potential
and incubation period as fixed factor effects. All statistical analyses were computed
using SAS v.8.2 (SAS Institute, Cary, NC) with a level of significance of P < 0.05.
Results and Discussion
From our glass jar trials, we observed a significant increase in nutrient contents of soil
samples during the 24-week incubation period (Table 1) and a corresponding
decrease in organic matter (data not shown), thus suggesting microbial activity.
Similar trends were obtained with intact soil cores, but a greater variability was
observed due soil heterogeneity within the greenhouse. Microbial activity was also
inferred from soil respiration measurements. CO2 release from incubated samples
declined with time (Figure 1). As expected, CO2 fluxes were greater at 23°C than at
17°C in all types of soil (P<0.01). However, soil matric potential had no significant
effect on CO2 efflux (data not shown). There were no statistical differences in nutrient
and organic matter contents between the different types of containers, hence similar
soil respiration rates were assumed.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/12713
There was a significant effect of incubation temperature on the mineralization of NO 3
in sandy loams (Figure 2; P<0.05), and a consistent trend of increasing nutrient
content with temperature in most soils, except for NH4 and K in OS~20, and P content
in general. Matric potential (i.e. moisture) had no consistent effect on nutrient change
over a 24-wk period (Figure 2). Further, we did not detect any significant effect of
moisture and temperature on Mehlich-3 extractable micro- and macro-nutrients (data
not shown). This was partly due to high soil greenhouse variability in nutrient content
and thus, between intact soil cores.
Table 1: Changes in nutrient content of organically managed greenhouse soils
after a 24-week incubation period.
Textural class
Sandy loam (SL)
Sandy loam (SL)
Loam (L)
Loam (L)
Organic soil (OS)
Cultivation
time
Organic
matter
(years)
(OM, %)
1
2
4
~15
~20
Changes in nutrient content
(mg kg–1 soil day–1)
4.9
6.3
8.5
11.3
33.4
NO3
NH4
K
P
0.77*
3.38*
4.82*
1.77
5.33*
–0.09
0.31*
0.29
<0.01
0.44*
0.08*
0.13*
0.22*
–0.01
0.82
0.0035*
0.0068*
0.0043*
0.0360*
–0.1101*
CO2 Efflux (µmol mĞ2 sĞ1)
NOTE: a soil is considered organic when the content of organic matter > 20%. Cultivation time refers to the number
of consecutive years a crop was organically produced on the same soil. * indicates a significant (P ≤ 0.05) change
in nutrient content between week 1 and week 24.
14
Temperature at 17¡C
Temperature at 23¡C
12
Sandy loam
Loam
Organic soil
10
8
6
4
2
0
0
5
10
15
20
Incubation time (week)
25 0
5
10
15
20
25
Incubation time (week)
Figure 1: Relationship between incubation time and soil respiration for three
types of organically cultivated soil (sandy loam 1-year, loam 15-years, and
organic soil 20-years) exposed to two different soil temperatures
Conclusions
Based on our results and studied soils, we conclude that increasing greenhouse soil
temperature from 17°C to 23°C would significantly enhance soil respiration rates,
particularly during the first few months. However, mineralization rates in intact soil
cores were only slightly increased by higher soil temperature or lower moisture
content. Since gas diffusion and soil biological activity may be momentarily hindered
during frequent irrigations (required by vegetable greenhouse crops), soil moisture
conditions close to field capacity should improve the turnover of soil organic matter.
Yet, similar changes in nutrient contents were observed in soil samples incubated
during 24 weeks at matric potentials of –35 vs. –250 mbars. Enhanced turnover of
organic amendments and release of plant available nutrients may be possible by
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/12713
further improving air-filled porosity (lower matric potential, i.e. drier soil) or by
stimulating the activity of soil microflora and fauna.
1600
1400
NO3
1200
Ğ250 mbars
Ğ35 mbars
NO3
1000
800
*
Change in nutrient content (mg kg Ğ1 soil)
600
400
17¡C
23¡C
*
200
0
180
160
140
120
100
80
60
40
20
0
100
90
NH4
NH4
P
P
80
70
60
9
*
6
3
0
1200
1000
K
K
800
600
100
75
50
25
0
*
SL-1 SL-2 L-4 L~15 OS~20
*
SL-1 SL-2 L-4 L~15 OS~20
Figure 2: Mean changes (±SE) in nutrient contents of soil samples maintained at
two temperature and matric potentials over a 24-wk incubation period
Acknowledgments
We thank le Conseil des recherches en pêche et en agroalimentaire du Québec for its financial
support (project no. 405058), and Serres Jardins-Nature, Serres Naturo and Ferme des Ruisseaux
for providing soil samples. We are grateful to A. Charles, F. Fournier, C. Halde, M. Karimi Youch,
G. Laroche and V. Lavoie for their assistance with soil analysis.
References
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