10-11-2012, 05:05 PM
Soil and residue carbon mineralization as affected by soil aggregate size
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Abstract
The nature of the contact between fresh organic matter and soil depends mainly on the characteristics of
the plant residues and on the physical properties of the soil. In a cultivated cropping system, changes in
soil organic C cannot be entirely attributed to changes in organic matter input. Breakdown of aggregates
caused by cultivation not only affects soil organic matter but also influences the rate of mineralization of
added organic matter. Many models simulating organic matter decomposition in the field are calibrated
with laboratory data from experiments where crop residues are ground and mixed homogeneously with
soil aggregates. In the present study, soil aggregate size was used as a means of varying the contact
between crop residue and the soil. The results demonstrated that cumulative soil carbon mineralization
from different aggregates had a significant (r = 0.60, p = 0.05) and positive relationship with their
oxidizable soil carbon content. Residue carbon mineralization in different aggregate size classes was
inversely related to aggregate oxidizable soil carbon content (r = 0.95, p = 0.01), cumulative soil carbon
mineralization (r = 0.89, p = 0.01) and resistant soil carbon pool (r = 0.80, p = 0.01). Residue carbon
mineralization in different aggregate size classes was also inversely (r = 0.61, p = 0.05) related to the
active carbon content (KMnO4 oxidizable carbon) of the aggregates.
Introduction
In a cultivated cropping system, changes in soil organic C
cannot be entirely attributed to changes in organic matter input
(Balesdent et al., 1998). Cultivation practices can stimulate
biodegradation of the initially physically protected C in soil,
and hence it could be responsible for the decrease of soil organic C
(Tisdall and Oades, 1982; Balesdent et al., 1998). Soil aggregation
can provide physical protection of organic matter against rapid
decomposition (Pulleman and Marinissen, 2004), and the aggregate
formation seems to be closely linked with soil organic matter
storage in soils (Golchin et al., 1995). Many studies (Cambardella
and Elliott, 1993; Six et al., 1998; Ashagrie et al., 2007) have
demonstrated that breakdown of aggregates caused by cultivation
was responsible for the loss of soil organic matter. The soilresidue
contact has 2 main physical components: the residue’s
potential contact area and the actual contact area.
Materials and methods
Collection of soil samples
Soil samples were collected from the Research Farm (conventionally
tilled) of Indian Institute of Soil Science, Bhopal (Latitude
238200N; Longitude 778300E). Samples were collected using a core
augar from 0 to 15 cm soil depth from the field of soybean–wheat
crop rotation. The bulk sample was mixed thoroughly, homogenized
and stored for further analysis. The study area is in the subtropical
humid region that receives an average annual precipitation
of 1080 mm and mean annual temperature of 25 8C. The soil of
the study area (Vertisol) was clayey in texture with neutral to
slightly alkaline in soil reaction. These soils belong to the fine
montmorillonitic hyperthermic family of Typic Haplusterts.
The samples were air-dried for 2 weeks at room temperature
(25 8C), ground and sieved through different mesh size. Samples of
soil were fractionated using a standard dry sieving technique to
determine different aggregate size fractions. Five 20-cm soil sieves
(4 mm, 2 mm, 1 mm, 0.5 mm, 0. 25 mm) were used to create 5 soil
fractions for analysis and incubation. Air-dried soil was gently
poured onto a sieve with a screen size matching the largest sized
aggregates in a size class (passed through 4 mm and collected on
the 2 mm sieve). The sieve was held in one hand above a piece of
brown paper and tapped with the other hand. The number of taps
was same for all the aggregate size classes. Accordingly, aggregate
sizes of 2–4 mm, 1–2 mm, 1–0.5 mm, 0.5–0.25 mm and <0.25 mm
were collected and subsequently used for chemical characterization
and incubation. The descriptions of each aggregate size class
are provided in Table 1.
Carbon mineralization experiment
Potential C mineralization was studied in a laboratory
incubation experiment for 183 days. The soil moisture was
adjusted at 80% of FC (field capacity) to simulate average field
moisture conditions and incubation was carried out at room
temperature (25 8C). Each soil aggregate size class (50 g) was
amended with and without 1% of wheat straw (weight basis), and
each size class was replicated 3 times. The carbon and Kjeldahl N
content of ground material were 423 and content of 5.2 g kg1,
respectively
Soil carbon pools
Soil carbon content was divided into 3 pools (active, slow and
resistant pool). Resistant organic C (Cr) in soil samples was
determined using the method suggested by Rovira and Vallejo
(2002). In brief, one gram of oven-dry sieved (<0.25 mm) soil sample
was hydrolyzed with 25 ml of 6 M HCl at 110 8C for 18 h with
occasional shaking. After cooling, the unhydrolyzed residue was
recovered by centrifuging. The process of centrifuging (at 20 8C) and
decantation was repeated several times with de-ionized water until
samples were free from chloride. Residues were then transferred to
pre-weighed vials, dried at 60 8C to constant weight, and total C was
measured by dry combustion technique using Shimadzu TOC
analyzer (SSM5000A). The remaining pools, Ca (labile/active) and
Cs (slow) were estimated by using the double decomposition model.
SOC (soil organic carbon) pools are divided into Ca and Cs pools
according to their turnover time with the assumption that negligible
amount of CO2 was evolved from the resistant pool (Cr) during the
short incubation period (Paul et al., 1999).
Model fitting and statistical analysis
The double decomposition equation was fitted with the nonlinear
regression (SPSS window version) that was used in the
Marquardt algorithm and an iterative process to find the parameter
values that could minimize the residual sum of squares. The resultant
pool sizes and their mineralization rate constants are generally
sensitive to the initially assigned parameter values, and the iterative
steps size. It was found that the automatically estimated initial
parameters resulted in acceptable parameter values. The only
caution was taken that decay constant of carbon pools should not be
negative, and the sum of the slow and active pool should not exceed
the acid hydrolyzable (6 M HCl) soil carbon pool. The model which
gave the lowest value of RMS (root mean square) and high F-value
was chosen as the best fit. The mean rates of CO2-C evolutions at
different day intervals were used to compare the treatment effects on
soil organic C mineralization.
Conclusions
The study clearly demonstrated that residue carbon mineralization
is significantly affected by the size of soil aggregate. Residue
carbon mineralization was inversely related to soil oxidizable
carbon content and soil carbon mineralization. Higher the soil
carbon mineralization lesser will be the residue carbon mineralization.
In other words, greater will be the chances of residue
carbon stabilization.