Geoderma 152 (2009) 283–289
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Geoderma
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Microbial biomass and N cycling under native prairie, conservation reserve and
no-tillage in Palouse soils
T.J. Purakayastha a,⁎, J.L. Smith b,1, D.R. Huggins b,2
a
b
Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, India
USDA-ARS, Paciﬁc-West Area Land Management and Water Conservation Research Unit, Pullman, WA 99164-6421, USA
a r t i c l e
i n f o
Article history:
Received 10 June 2008
Received in revised form 25 May 2009
Accepted 22 June 2009
Available online 18 July 2009
Keywords:
Conservation reserve program
Conservation tillage
Land use change
Soil microbial biomass
Nitrogen mineralization
Particulate organic matter
a b s t r a c t
Tillage management practices that improve soil quality are needed to maintain agricultural productivity. Soil
quality can be improved through increases in microbial biomass carbon (MBC) and the nitrogen (N) contents
in different pools of soil organic matter (SOM). The objective of our experiment was to study the dynamics of
MBC and N cycling in soils collected from the Palouse area of eastern Washington State. Soil managements
were: no tillage for 4 (NT4) and 28 (NT28) years, conservation reserve program (CRP), native prairie (NP),
and conventional tillage (CT). Microbial biomass carbon was 50% less in the CT soil than the NP soil and there
was 74% less total soil N (TSN) in the surface CT soil compared to the NP soil. Conversion of CT to CRP, NT4,
and NT28 increased MBC in the 0–5 cm depth by 40, 6 and 78%, respectively. Compared to CT, the TSN
content was 20, 57, and 94% higher in CRP, NT4 and NT28, respectively. The net N mineralized (Nmin) over
180 days in the surface 5 cm was highest (69.5 µg N/cm3) in NT28 soil and lowest (14.2 µg/cm3) in CRP.
Interestingly, the CT soil mineralized as much N as the NT systems but had less TSN than NT. Compared to NP,
CT had a 70% reduction in particulate organic matter nitrogen (POM-N) to a depth of 20 cm. The conversion
of CT soil to CRP, NT28, and NT4 enhanced the POM-N content by 65, 216, and 101% at 0–5 cm. The proportion
of POM-N to TSN was lowest in CT soil whereas the NT and CRP soils were considerably higher. The lower
Nmin/MBC ratios (qN) of the conservation systems imply a higher immobilization capacity and tighter soil N
cycling. Mineralized N per unit of respiration (Nmin/CO2-C) was highest in CT and NT28 management
systems suggesting different factors were responsible in these systems for exhibiting almost similar values.
The Nmin ratios along with MBC and POM-C relationships could be useful soil quality indicators in judging
tillage-induced changes in soil management systems.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Carbon (C) is an indispensable necessity for soil fertility; it is
strongly correlated with nitrogen (N) and fuels the microbial engine
that drives the nitrogen cycle (Korschens, 1998). There is considerable
interest in the concept of soil quality as it relates to the use of
agricultural lands in sustainable production including N management
(Doran and Parkin, 1994; Salinas-Garcia et al., 1997). For example
microbial biomass is a sensitive indicator of N fertilizer management
affects on soil quality (Rice et al., 1986; McCarty and Meisinger, 1997;
Staben, et al., 1997; McCarty et al., 1998). In addition to microbial
biomass carbon (MBC), N mineralization (Nmin) and ratios such as
Nmin/MBC (qN) and Nmin/CO2–C evolved respond readily to
⁎ Corresponding author. Tel.: +91 11 25841494; fax: +91 11 25841529.
E-mail addresses: [email protected], [email protected] (T.J. Purakayastha).
1
Tel.: +1 509 335 7648; fax: +1 509 335 3842.
2
Tel.: +1 509 335 3379; fax: +1 509 335 3842.
0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.geoderma.2009.06.013
changes in soil management and can provide an effective early
warning of deterioration of soil quality (Smith, 1994, 2002).
This loss is most rapid during the ﬁrst few years of cultivation, and
eventually an apparent equilibrium is established, provided constant
management practices are employed. It has been well documented
that tillage has reduced soil organic matter (SOM) stocks. Between 35
and 50% of the SOM and N were lost during the ﬁrst 50 years of tillage
in the Great Plains of the United States (Bauer and Black, 1981). Over a
14-year period in Canada, cultivated, prairie soils lost 26% of native
SOM and 33% of native N. In the next 20 years, losses decreased by half
(Doughty et al., 1954; Campbell et al., 1975). In other Canadian soils,
over a 60- to 80-year period, C and N losses ranged from 50 to 60% and
40 to 60%, respectively (Campbell et al., 1976; Voroney et al., 1981).
Furthermore, in Queensland Australia, annual tillage caused a loss of
36% of the soil C and N over a 20–70 year period (Dalal and Mayer,
1986). The long-term effect of tillage is to decrease the SOM level,
which, in turn, disrupts nutrient cycling and fertility and degrades soil
quality.
Reduced tillage increases the standing stock of macroaggregateprotected soil organic C compared with conventional tillage (CT)
284
T.J. Purakayastha et al. / Geoderma 152 (2009) 283–289
mostly at the soil surface (Campbell et al., 1995; Franzluebbers and
Arshad, 1997). In a 23 year ﬁeld study, Dolan et al. (2006) found that
no-tillage managed soil had N30% more soil organic C (SOC) and soil
organic N (SON) than the moldboard plow and chisel plows soils.
McCarty et al. (1998) showed that a soil proﬁle typical of plow tillage
management can transform to a soil proﬁle characteristic of no tillage
(NT) within a 3-year period of transition. In this time period,
stratiﬁcation of organic matter in the proﬁle progressed signiﬁcantly
toward that which occurs after 20 yr of no tillage. A similar pattern
was reported by several workers (Carter and Rennie, 1982; Burle et al.,
1997; Salinas-Garcia et al., 1997; Kandler et al., 1999; Bayer et al.,
2000), who also identiﬁed increased accumulation of C and N with
greater periods of no-till management.
Microbial biomass C (MBC) is also stratiﬁed under no-tillage
compared to being homogeneous in the 0- to 15-cm layer under plow
tillage (Alvarez, 1998). With the transition to no-tillage, MBC, microbial
biomass N, and active N pools were shown to increase more rapidly in
the upper soil proﬁle than did the total pools of SOC and SON (McCarty
et al., 1998; Woods, 1989). Doran (1987) reported that microbial
biomass and potentially mineralizable nitrogen in the 0–7.5 cm surface
layer of no till soils were 34% higher than those of ploughed soils,
although the opposite was true at 7.5- to 15-cm depth. Wright et al.
(2005) found MBC to be greatest under no-till management but only in
the surface 2.5 cm with little tillage effect to 20 cm.
The importance of reduced tillage in N cycling and soil quality may be
dependent upon the length of time the soil has been subject to change in
management. In this study, we assessed the magnitude of change of a
Palouse silt loam soil from pre-cultivation native prairie conditions with
respect to total soil C (TSC), total soil N (TSN), particulate organic matter
C and N (POM-C, POM-N), MBC, N mineralization (Nmin), and microbial
N quotient (qN) from soil management systems consisting of: conventional tillage N100 years (CT), no-tillage for 4 (NT4) and 28 (NT28)
years; 11 years of perennial grass (CRP); and native prairie (NP). We
hypothesize that SON stocks based on length of time in perennial versus
annual vegetation and no-tillage versus tillage would be NP N NT28 N
CRP N NT4 N CT. Deviations from this sequence would indicate the
importance of other factors inﬂuencing MBC and its activity.
2. Materials and methods
2.1. Soil sampling
Five ﬁelds with differing management history, but the same soil
classiﬁcation and landscape position, were identiﬁed to represent
diverse land uses in the Palouse region of eastern Washington State,
near the city of Pullman. The climate type of this zone is
Mediterranean with an annual precipitation averaging 544 mm and
mean annual air temperature of 8.3 °C. Historically, conversion of NP
to agricultural uses in the Palouse region occurred in the late 1800s,
was dependent on inversion tillage using a moldboard plow and
resulted in annual soil erosion rates exceeding 25 Mt ha– 1 (USDA,
1978). The sites were located in summit positions (0 to 3% slope) on
Palouse silt loam soils (ﬁne-silty, mixed, superactive, mesic pachic
ultic Haploxerolls, Palouse series), where the inﬂuence of soil
deposition from historic erosion processes would be minimized. All
soils had a similar soil texture, with particle size distribution of 21%
sand, 59% silt and 20% clay. Management, vegetation and site
descriptions for each location are given in Table 1.
Soil samples consisted of forty soil cores (4 cm diameter) from
each location in depth increments of 0- to 5-, 5- to 10- and 10- to 20-cm
were collected using a 4 cm diameter hammer driven soil core sampler.
Immediately after collection, soil samples were brought to the
laboratory in a cooler and stored at 4 °C. Four intact soil samples
from each location for a C mineralization study and 27 cores (3 intact
cores with 9 sampling days) were used for MBC and N mineralization
study (Section 2.3). An additional 5 cores were taken for the
determination of water content, ﬁeld capacity and bulk density. Soil
moisture content was determined after drying at 105 °C for 24 h. Soil
samples from rest of the 4 cores per treatment were air dried, ground,
and sieved through a 2-mm sieve for analyses of total soil C (TSC) total
soil N (TSN), particulate organic matter C (POM-C) and particulate
organic matter N (POM-N).
Soil bulk density was calculated from weights of ﬁeld-moist soil,
the oven-dried sub-sample and the volume of the core sampler
following the method of Veihmeyer and Hendrickson (1948).
2.2. Soil microbial biomass, C and N mineralization
Soil MBC and N mineralization were measured by destructive
sampling a set of soil cores after 0, 5, 10, 20, 40, 60, 100, 140, 180 days
of initial incubation. Soil cores collected from the 0- to 5-, 5- to 10-,
and 10- to 20-cm depths at each site were moistened to ﬁeld capacity
(25% water by weight, − 0.033 MPa) and placed in a 500-mL canning
jar and a vial of water to maintain high humidity, and incubated for
26 wk at room temperature (20 °C). On each sampling date 3 soil
cores from each site were passed through 2-mm sieve. Soil microbial
biomass was measured by the substrate induced respiration (SIR)
method (Anderson and Domsch, 1978; Smith et al., 1985) in which
0.5 ml of a 12 g glucose/L solution was added to moist soil equivalent
to 10 g of oven dry soil and incubated for 3 h at 22 °C. Vials were
covered for 2 h, ﬂushed with moist air, septa capped and CO2 in the
head space measured by a gas chromatograph (Schimadzu GC 17-A)
after 1 h incubation. The MBC was estimated by using the equation of
Anderson and Domsch (1978);
x = 40:04y + 0:37
ð1Þ
Where
x
y
mg microbial biomass carbon 100 g soil−1
ml CO2 100 g soil−1 h−1
Soil from the harvested cores was also used to determine NH4–N
and NO3–N over the 180 day incubation. The NH4–N and NO3–N in a
Table 1
Location and management history of seven sites used for the study.
Site description
Locationa
Management
Native Prairie (NP)
Never cultivated, Idaho fescue, blue bunch, wheat grass, June grass grows naturally
Conservation Reserve Program (CRP)
28 year no tillage (NT28)
Kramer Palouse Natural Area
(KPNA) near Colton
Grower Field near Albion
Grower Field, near Palouse
4 year no tillage (NT4)
Palouse Conservation Field Station, Pullman
Conventional tillage (CT)
Grower Field, adjacent to KPNA near Colton
a
All the management practices are situated in Washington State, USA.
Smooth brome grass grows for 11 years, no fertilization or cultivation
Winter wheat-spring barley/spring grain legume (pea or lentil), seeding was accomplished
directly into the preceding years crop residue by means of a coulter-type planter
Winter wheat-spring barley-spring wheat, seeding was accomplished directly into the
preceding years crop residue by means of a coulter-type planter
Cultivated for the past 100 year, currently under winter wheat-spring pea rotation, mould
board plowed to a depth of 20 to 25 cm and disked to a depth of 8 to 10 cm
T.J. Purakayastha et al. / Geoderma 152 (2009) 283–289
5 g soil sub-sample was extracted with 2 M potassium chloride
(KCl) at a soil solution ratio of 1:5. KCl extracts were refrigerated at
4 °C and subsequently analyzed for NH4–N and NO3–N (Net N
mineralization) colorimetrically on a continuous rapid ﬂow analyzer
(Alpkem RFA-301).
Soil C mineralization was studied from four intact cores (preserved
at 4 °C) collected from the 0- to 5-, 5- to 10-, and 10- to 20-cm depths
at each site. Soil cores were moistened to ﬁeld capacity (25% water by
weight, −0.033 MPa) and placed in a 500-mL canning jar with a vial
containing 5 mL of 1 mol L ha– 1 NaOH to trap evolved CO2 and a vial of
water to maintain high humidity, and incubated for 26 wk at room
temperature (20 °C). During the initial 2 wk, the NaOH trap was
replaced at half-week intervals and subsequently at weekly intervals
through 26 wk. Control jars lacking a soil core were incubated in the
same manner. The jars, including controls, were vented to the
atmosphere during replacement of the NaOH trap. The quantity of
CO2–C evolved was determined by titration with 1 mol L ha– 1 HCl
using phenolphthalein as an indicator (Anderson, 1982). Amounts of
respired CO2 were calculated as milligrams CO2 –C per cubic
centimeter and cumulative CO2–C evolution was calculated over
26 wk.
2.3. Total soil total C (TSC), total soil N (TSN), particulate organic matter
C (POM-C) and particulate organic matter N (POM-N)
Total C, N, POM-C and -N in soil were determined at the start of the
experiment. For total soil C and N concentrations a soil sub-sample
was air dried, ground and determined by dry combustion using a Leco
CNS 2000 carbon analyzer (Tabatabai and Bremner, 1970). As the soils
used in the present study did not have inorganic C, TSC was
considered as soil organic C (SOC). For the determination of POM-C
and -N fractions, soil sub-samples were air dried, mixed with 30 ml of
0.5% sodium hexametaphosphate, shaken for 24 h, and poured onto a
53 µm sieve. The solids were rinsed through the sieve with minimal
amounts of water and the portions retained on and passed through
the sieve were collected separately in pre-weighed plastic boats. Boats
were dried in a forced-air oven at 60 °C for 72 h and the ﬁnal weights
of the boats were recorded. Material in the boats was transferred to a
glass bottle containing stainless steel rods and placed on a rotary
grinder for 24 h. Total C and N content of the POM and mineral
fractions were determined by dry combustion.
2.4. Statistical analysis
The experiment was conducted using completely randomized
design with Duncan's multiple range test (DMRT) for separation of
means. One way analysis of variance (ANOVA) was performed on all
variables using the DOS based MSTATC version C program developed
by S. P. Eisensmith. Mean comparisons under various soil and land
management practices for individual parameter at speciﬁc soil depth
was done using DMRT at the 0.05 probability level of signiﬁcance.
Management history had signiﬁcant effects on soil bulk density
suggesting that the data be expressed on an equivalent mass basis;
however, due to the mass-altering impacts of signiﬁcant soil erosion
over time, data were expressed on a soil volume basis (Ellert and
Bettany, 1995).
3. Results and discussion
3.1. Microbial biomass carbon (MBC)
Microbial biomass C has been advanced as a sensitive biological
indicator because it is strongly inﬂuenced by management practices
and system perturbations (Smith and Elliot, 1990). In the surface
(0–5 cm) soil depth, MBC for NP and NT28 sites were greater than
the other management sites (Table 2); however, only NP maintained
285
Table 2
Soil microbial biomass and net N mineralized under different tillage management
practices.
Tillage
management
Microbial biomass C
Net N mineralized
mg cm− 3
µg cm− 3
Initial
Maximum
180 day
180 day
0−5 cm
NP
CRP
NT28
NT4
CT
2.56a
1.78b
2.26a
1.34c
1.27c
2.97b
2.41
2.85
2.26
2.51
2.49a
1.51b
1.17bc
0.85cd
0.69d
33.0c
14.2d
69.5a
19.4d
45.4b
5–10 cm
NP
CRP
NT28
NT4
CT
1.90a
0.79c
0.72c
0.88c
1.26b
2.66
0.91
0.89
1.05
1.36
2.29a
0.81b
0.58b
0.46b
0.79b
18.0b
18.4b
40.9a
39.5a
40.2a
10–20 cm
NP
CRP
NT28
NT4
CT
1.82a
0.83b
0.78b
0.74b
0.76b
2.18
0.84
0.78
0.74
1.11
2.04a
0.53b
0.38b
0.21b
0.53b
10.2c
11.8bc
20.2a
21.8a
13.6b
a
Signiﬁcant management differences for each parameter in a particular soil depth
are indicated by different letters (P = 0.05) using Duncan's multiple range test.
b
Maximum values may have occurred on different days, statistics are not
appropriate.
larger amounts of MBC in subsurface soils. The CT soil, being cultivated
over 100 years, had 50% less MBC than NP in the surface 5 cm.
Compared to the CT soil, the adoption of no tillage and conservation
reserve programme increased the MBC in the surface 5 cm by 78, 40, 6%
in NT28, CRP and NT4, respectively. The greater MBC in the NP soil can
be explained by the larger accumulation of labile organic C over time
due to non disturbance of the soil. In contrast, the MBC in the CT soil
decreased because of low labile organic C which continuously declined
due to cultivation over 100 years (Purakayastha et al., 2008). Similar
increases in MBC and N have been observed in tropical cropping
systems (Franchini et al., 2007) and long-term cropping sequences
(Wright et al., 2005) where tillage management had the highest
impact.
As was expected, there was a signiﬁcant decline of MBC with soil
depth (Table 2). The MBC in CT did not vary widely at different depths
which may be due to a more uniform distribution of organic C in the
plow layer resulting from physical disturbance in the CT system
(Carter and Rennie, 1982). In contrast, there was signiﬁcant stratiﬁcation of MBC from the 0–5 cm to 5–20 cm depth in CRP and NT soils,
with the lower depths being similar. The CT soil had signiﬁcantly more
MBC than NT4, NT28 and CRP in the 5–10 cm depth but similar
amounts in the 10–20 cm depth which may have been due to soil
mixing during repeated tillage under cropping. Previous reports in NT
systems have observed the stratiﬁcation of residue near the soil
surface and the increased surface distribution of organic matter and
MBC (Holland and Coleman 1987; Dalal et al., 1991, Madejon et al.,
2007). In the same climate and soil, Granatstein et al. (1987) also
reported that microbial biomass, total C and total N were highest
under no-till surface soils (0–5 cm) of Palouse, WA with minimal
differences for tillage or depth below 5 cm. However, in a hot humid
sub-tropical soil of Texas, Wright et al. (2005) found an increase of
MBC and mineralizable N in the surface soil with corn and cotton
cropping sequences for twenty years under no-till and minimum
tillage systems but little change in MBC concentration in the 2.5 to
20 cm depth.
The dynamics of MBC over 180 day period revealed that the MBC
increased from the initial amount and peaked somewhere between 5
to 20 days depending upon management and depth (Table 2). The
286
T.J. Purakayastha et al. / Geoderma 152 (2009) 283–289
Table 3
Total soil N (TSN), particulate organic matter N (POM-N) and net soil N mineralization
(Nmin) with different tillage management practices.
Tillage
management
TSN
NP
CRP
NT28
NT4
CT
6.48aa
2.50c
3.57b
3.51b
2.63c
POM-N
Net Nmin
mg ha− 1
kg ha− 1
1.30a
0.39c
0.60b
0.52bc
0.31c
49.3c
25.5d
69.5a
57.9b
55.2bc
a
Signiﬁcant management differences for each parameter are indicated by different
letters (P = 0.05) using Duncan's multiple range test.
MBC then declined over the 180 day incubation period to be generally
lower than the initial MBC. This indicates a rapid growth of the
microbial biomass initially by utilizing the most easily mineralizable
soluble carbon substrates. Thereafter, the microbial growth was
reduced due to absence of a labile pool of carbon and presence of
only intermediate or more resistant C pools (Staben et al., 1997).
3.2. Net nitrogen mineralization (Nmin)
The amount of net N mineralized over 180 day period was
generally stratiﬁed with depth (Table 2). In all the soil depths, NT28
showed the highest amount of net N mineralization. In the 0–5 cm soil
depth, CRP and NT4 showed the lowest N mineralization. CRP and NP
soils showed the lowest amount of N mineralization in the 5–10 and
10–20 cm soil depths. On an aerial basis (0–20 cm soil depth) nitrogen
mineralization was greatest (69.5 kg ha− 1) in the NT28 soil and
lowest (25.5 kg ha− 1) in the CRP soil (Table 3). The range in the NT
and CT soils (55 to 70 kg ha− 1) is a typical amount of N mineralized
from the Palouse soil on a yearly basis. The enhanced N mineralization
in NT28 is partially due to its higher POM-N contents (Table 5) which
are associated with enhanced N supply (Koutika et al., 2001). This
result is similar to the ﬁndings of Scharenbroch et al. (2005) who also
reported an enhanced microbial biomass N (71%), potential C
mineralization (20%) and potential N mineralization (83%) in
undisturbed older urban Palouse soil (mean landscape age of
64 years). In another study from Canada reported an increased
proportion of mineralizable N in the more labile N pools under NT
than under CT (Shariﬁ et al., 2008). The eleven year period without
fertilization in the CRP system was not sufﬁcient for the build up of
SON and the mineralizable N pool. However, Burke et al. (1995) has
shown that cultivated ﬁeld which was abandoned 50 years ago to
short grass steppe in north-east Colorado, USA has recovered much of
the active soil organic matter (microbial biomass, potentially mineralizable N) but recovery of the total soil organic matter pool will take a
much longer time period. Robles and Burke (1997) reported that
grasses grown in CRP soils (80% legume:20% grass mixture) in
Wyoming, USA, had the higher rates of potential net N mineralization
than the same grasses grown in the more nutrient-depleted CRP ﬁelds
(20% legume:80% grass mixture). In the present study CT and NT soil
mineralized twice as much as the NP and CRP soils probably due to the
effects of surface disturbance in their management history and the
addition of fertilizer N every year (Smith and Elliot, 1990).
Interestingly, all of the soils exhibited linear N mineralization over
the 180 day incubation period (data not shown) which indicates high
soil N mineralization potential (Smith et al., 1980). In addition to the
lack of trends across managements, the net soil N mineralization was
not correlated to either soil TSN or POM-N. Even though conservation
management tended to increase SON, Liang et al. (2004) found that
depending upon the soil this did not translate directly into an increase
in mineralizable N. However, in a Brazilian oxisol there was a
consistent increase in biological activity and N mineralization with
no-till management (Green et al., 2007).
Net N mineralization is a net balance between SOM mineralization
and microbial immobilization which is inﬂuenced by the magnitude of
the labile C and N pools. The varied response of the N mineralized in
each soil during the 180 day incubation is due to the complex
interaction of microbial biomass and enzymatic activity with different
labile C and N pools in each soil management system (Smith, 1994;
Staben et al., 1997). This supports the concept that the complexity of N
cycling and the interaction of C and N pools make it difﬁcult to predict
the available soil N for plant growth (Smith and Paul, 1990).
3.3. Total soil N (TSN) and particulate organic matter N (POM-N)
In general, the TSN contents in the 0–5 cm depth were higher than
the 5–10 cm or 10–20 cm depths (Table 4). As compared to lower
depths, a greater accumulation of TSN was observed in surface depth
due to conversion of CT to NT and CRP. This indicated a stratiﬁcation of
crop residues and organic matter in the surface depth under NT or CRP
systems. TSN contents in NP were greater than other management
systems at all soil depths, while the lowest values were observed in CT
and CRP soils. The long-term no tillage (NT28) had hardly any
inﬂuence on TSN contents as compared to short-term no tillage (NT4).
TSN contents in CT soil, compared to NP soil, have decreased by 74, 51
and 55% in the 0–5, 5–10 and 10–20 cm depths, respectively. On the
other hand in the 0–5 cm soil depth, there was an increase in TSN over
CT by 20, 94 and 57% in CRP, NT28 and NT4, respectively. As compared
to lower depths a greater accumulation of TSN was observed in surface
depth due to conversion of CT land to NT and CRP. The stratiﬁcation of
N under NT system has also been reported by several others
(Franzluebbers et al., 1995; Alvarez et al., 1998; Mrabet et al., 2001).
Fuentes et al. (2004) reported a 64% and 58% reduction in TSN in 0–5
and 5–10 cm soil depths, respectively due to conventional tillage of
native prairie soils of eastern Washington. Solomon et al. (2000)
showed a 56% reduction in C and 51% reduction in N contents in A
horizon of chromic luvisol in Tanzania as a result of clearing and
subsequent cultivation of native tropical woodland. Similar decreases
in total soil C and N contents in 0–17.5 cm soil depth in Brazil have also
been reported (Bayer et al., 2000). In the deeper depths the TSN
concentrations in NT increased by 25% (5–10 cm) and 19% (10–20 cm)
in comparison to the CT soil. Similar increases with depth have
been observed in arid wheat based systems where TSN increased by
38–68% (Dou and Hons, 2006) and in corn-soybean rotations where
Table 4
Total soil N (TSN), particulate organic matter N (POM-N) and the ratio as affected by management practices.
Tillage
management
NP
CRP
NT28
NT4
CT
a
TSN (mg cm− 3)
POM-N (mg cm− 3)
Soil depth, cm
Soil depth, cm
POM-N/TSN (%)
Soil depth, cm
0–5
5–10
10–20
0–5
5–10
10–20
0–5
5–10
10–20
4.93aa
1.55c
2.50b
2.03b
1.29c
3.09a
1.27c
1.75b
2.02b
1.51c
2.74a
1.09c
1.45b
1.48b
1.23c
1.08a
0.33cd
0.62b
0.40c
0.20d
0.68a
0.16c
0.24bc
0.33b
0.15c
0.48a
0.15b
0.16b
0.15b
0.14b
21.9ab
21.3ab
24.8a
19.7bc
15.5c
22.0a
12.6bc
13.7bc
16.3b
9.9c
17.5a
13.8ab
11.0b
10.1b
11.4b
Signiﬁcant management differences for each parameter in a particular soil depth are indicated by different letters (P = 0.05) using Duncan's multiple range test.
T.J. Purakayastha et al. / Geoderma 152 (2009) 283–289
no-tillage increased both SOC and TSN by more than 30% (Dolan et al.,
2006). In Wyoming, USA Robles and Burke (1997) reported that the
net impacts of increased plant inputs and cessation of tillage in CRP
land increased coarse POM-C and -N by factors of two to four relative
to wheat-fallow ﬁelds, but had negligible effects on total SOM.
Stocks of TSN (0–20 cm) with these agricultural management
histories follows the order: NP N NT28 = NT4 N CT = CRP which differs
from our original hypothesis (Table 3). Deviations from the hypothesized sequence could possibly be due to differences among the sites in
initial levels of TSN prior to management changes and relatively poor
biomass production in CRP due to lack of fertilization. Insights of the
inﬂuence of these factors on TSN can be gained through examining
TSN depth-stratiﬁcation and measures of labile TSN constituents. The
equivalent amount of TSN in NT28 and NT4 is probably being due to
higher levels of TSN contents in the former than the latter at the
initiation of experiment. The reduction of TSN content in CT is
primarily due to over 100 years of cultivation of NP.
The POM-N stock in 0–20 cm depth was greatest in NP soil, and
lowest in CT and CRP soils (Table 3). Though the no tillage practices for
long-term (NT28) did not inﬂuence the POM-N stock over short-term
no tillage (NT4), conversion of CT to long-term NT (NT28) signiﬁcantly increased POM-N stock.
The greatest concentrations of POM-N were observed in the NP soil
at all soil depths (Table 4). The NP, NT28, NT4 soils were signiﬁcantly
higher in POM-N than in the CT and CRP soils at 0–5 cm but similar by
the 10–20 cm depth. Compared to the NP soil, the POM-N in the CT soil
decreased by 81, 78 and 71% in the 0–5, 5–10 and 10–20 cm depths,
respectively. The adoption of NT increased POM-N in the 0–5 cm
depth by 210, and 100% in NT28 and NT4, respectively. At the 10–20 cm
depth, CRP, NT28, NT4 and CT treatments did not differ signiﬁcantly for
POM-N concentration. This suggests that at the lower depths recovery
of POM-N is very slow. The CRP sites, however, did not achieve the
subsurface storage of POM-N found in NT even after four years. This
result may have been reversed if burning had been eliminated from
bluegrass seed production and the CRP site fertilized in order to
promote biomass additions to soil (Huggins et al., 1997). The POM
fractions or light fraction organic matter tends to be greater under NT
systems (Malhi et al., 2006) and is more sensitive to management
practices (Dou and Hons, 2006). Considering all management treatments POM-N was linearly correlated to SON (R2 = 0.96, P b 0.01) at all
depths.
The ratio of POM-N/TSN was highest in NT28 (along with CRP and
NT28) and lowest in CT for the 0–5 cm soil depth (Table 4). But at
depth NP and CRP showed higher ratio than the others. In the NP soil
18–22% of the total N was POM-N whereas in the CT soil it accounted
for 10 to 15%. The low proportion of POM-N/TSN in CT soil was due to
rapid oxidation of POM-N, due to conventional tillage (Paustian et al.,
1997). Similarly, Fabrizzi et al. (2003) reported a decreased ratio of
POM-N/TSN (14:1) in conventional tillage than in no tillage (20:1)
with and without N fertilization at the Balcarce Experimental Station,
Argentina. On the contrary, the adoption of NT and CRP dramatically
Table 5
Total soil C:N and POM C:N ratios as affected by management practices.
Tillage
management
NP
CRP
NT28
NT4
CT
TSC/TSN
POM-C/POM-N
Soil depth, cm
Soil depth, cm
0–5
5–10
10–20
0–5
5–10
10–20
10.6ba
15.6a
15.2a
14.3a
11.1b
10.2b
13.4a
14.2a
14.0a
9.3b
10.0c
14.9a
14.9a
14.2ab
11.4bc
16.0ab
17.2a
15.3b
13.4c
11.1c
14.7b
15.2b
10.9c
11.4c
16.9a
14.5ab
15.6a
12.8bc
11.7c
12.6bc
a
Signiﬁcant management differences for each parameter in a particular soil depth
are indicated by different letters (P = 0.05) using Duncan's multiple range test.
287
Table 6
N mineralized per unit of soil microbial biomass C and N mineralized per unit of C
mineralized as affected by tillage management practices.
Tillage
management
NP
CRP
NT28
NT4
CT
Nmin/MBC × 10− 2a
Nmin/CO2−C × 10– 3a
Soil depth, cm
Soil depth, cm
0−5
5–10
10–20
0–5
5–10
10–20
1.3bb
0.9b
5.9a
2.3b
6.6a
0.8c
2.3c
7.1ab
8.6a
5.1b
0.5d
2.2cd
5.3b
10.4a
2.6c
6.3b
3.0c
12.8a
5.5b
12.2a
5.4b
8.7b
14.6a
15.5a
12.7a
5.6c
7.8c
18.2a
14.9ab
12.7b
mg N mineralized 180 d kg− 1soil/mg CO2–C or MBC kg− 1soil.
Signiﬁcant management differences for each parameter in a particular soil depth
are indicated by different letters (P = 0.05) using Duncan's multiple range test.
a
b
increased the amount of POM-N as a percentage of TSN ranging from
15 to 25% in the surface soil. The greater POM-N/TSN ratio under NT
systems reﬂects the inﬂuence of tillage on SOM quality, with a highest
proportion of total N present in the active fractions. The build up of a
labile N pool with a size of N53 µm can positively impact soil physical
structure and aggregation (Fansler et al., 2005) and increase N
immobilization and retention (Purakayasttha et al., 2008).
3.4. C and N relationships
The short-term no tillage (NT4), long-term no tillage (NT28) and CRP
soils showed mostly higher C:N ratio (13–16) of SOM in all the soil
depths (Table 5). Contrarily the NP and CT soils had signiﬁcantly lower C:
N ratios ranging from 9 to 11. The C:N ratios of SOM fractions have often
been used as indices of quality; however, interpretation of trends can be
difﬁcult. Agricultural use of soils reduces total soil C:N ratios, which
typically range from 14–8, as labile SOM is lost (Duxbury et al., 1989;
Kaffka and Koepf, 1989). However, Miller et al. (2004) reported that soil
C:N ratio decreased during cultivation only with decreasing temperature. Aggradations of SOM in whole soils is reﬂected by increasing C:N
ratios. However, the mechanisms behind the lower C:N ratios for CT and
NP soils are different. The CT soil has reduced C concentrations from
oxidation of labile SOM due to tillage (Purakayastha et al., 2008). But
fertilizer N inputs to CT soil that results in an apparent low soil C:N ratio,
whereas, the NP soil has built up C reserves and has increased the soil N 2
fold thus reducing the soil C:N ratio. The relatively greater TSN in the NP
soil is presumably due to more conserved N cycling and high input from
native leguminous grasses.
When the ratios of POM-C, N are calculated (Table 5) the NT and CT
systems showed similar values to the whole soil C:N ratios, however
they decreased with 5–10 cm soil depth for the NT soils and increased
with the CT soil. This is a classic example of turning over of residues in
a CT system verses a NT system. Higher POM-C:N ratio in NT28 system
than in CT system is mainly due to protection of POM inside the soil
aggregates which inhibited its decomposition by microbes. Similarly
Scharenbroch et al. (2005) reported higher particulate organic matter
C:N ratios from less disturbed older urban soils (mean landscape age
of 64 years) than more disturbed newer urban soils (mean landscape
age of 9 years) in the same Palouse region. No N inputs for the NP and
CRP along with uniform stratiﬁcation of plant biomass make the POMC/POM-N ratio more uniform with depth. Nevertheless, N limitation
in CRP soil likely restricted the decay in the highly rooted smooth
brome grass (Malhi et al., 2003a,b). On the contrary, the higher POM
C:N ratio in NP soil reﬂects the large labile C and N pools in this system
(Purakayastha et al., 2008). Thus higher POM-C:N ratio is attributed
due to aggradations of SOM.
The Nmin/MBC ratio (qN) can be an indicator of substrate quality
and efﬁciency of N cycling (Smith, 1994). This ratio was higher in CT
and NT28 than in the NP, NT4 or CRP systems at 0–5 cm depth
(Table 6). Although both the CT and NT28 soils exhibited similar ratios,
there might be different factors responsible for the similarity. As
288
T.J. Purakayastha et al. / Geoderma 152 (2009) 283–289
NT28 has more POM-N than CT, providing greater substrate for N
mineralization and N loss that would increase the ratio. In the CT soil
the POM-N was almost 1/3 of that in NT28 at 0–5 cm, the N
mineralization was even less than 1/2. Thus it clearly indicates that
other labile pools of N except POM-N in CT soil possibly contributed
mostly to N mineralization. These N pools in CT soil might originate
from fertilizer N that is applied to the crops every year. The lower
metabolic N quotient in other management systems has been
associated with a higher immobilization capacity and lower mineralization rates resulting in the suggested tighter N cycling (Smith,
1994). The consequence of a tighter N cycle is the potential for less
inorganic N loss through microbial trace gas production, leaching, and
run-off.
In contrast to a pool ratio the Nmin/CO2–C ratio from microbial
respiration is an activity based indicator describing the net N
produced per unit of microbial activity or energy use. The lowest
ratios were observed in the NP and CRP soils. This indicates more
preservation of N in these management systems. On the contrary, the
ratio was highest in the CT and NT28 soils at 0–5 and 5–10 cm soil
depths, with the NT4 soil being equivalent at the lower depths. Higher
values indicate less immobilization and more inorganic N in the
system. In NT28 system, microbes might be proportionately converting CO2–C more to MBC than respiring it as CO2 that increased the
ratio. In contrast, C substrates were reported to be low in the CT soil,
hence the lower CO2–C that was respired (Purakayastha et al., 2008).
The ratios are an order of magnitude lower than the qN values since
energy expended overtime, even in a maintenance situation, is many
times the pool size of MBC. Since, generally, C and N mineralization
can be tied to the C/N ratio of the soil (Smith, 1994), comparison of
Nmin/CO2–C ratio may provide information on the ability of a soil to
immobilize and conserve inorganic N.
4. Conclusions
There was an overall increase in MBC, TSN, and POM-N in NT and
CRP soil compared to CT soil. The highest microbial biomass was in NP
and the lowest amount in CT, clearly indicated a rapid deterioration of
soil quality due to over 100 years of cultivation. Considering strategies
for increasing TSN of depleted soil, our hypothesis was that practices
that included NT and CRP would result in an increased soil N pool as
compared to CT in the following order: CT b NT4 b CRP b NT28 b NP.
Our data, however, deviated signiﬁcantly from this sequence.
Unexpectedly CRP failed to increase TSN contents as compared to
CT. In addition, the adoption of NT seemed to improve the soil
biological N cycling and N retention. However, MBC and POM-N, along
with the ratios of Nmin/MBC and Nmin/CO2–C may be sensitive
indicators of management-induced changes in TSC and TSN. The
potential for better utilization of MBC and TSN reserves in NT and CRP
soils depends on our understanding of their distribution in soil and
associated factors of management and time which inﬂuence their
depletion and accretion. Understanding the microbial response to
tillage related changes and N cycling under different NT and CRP
systems is important to the development of precise nutrient management systems to maintain soil quality.
Acknowledgements
The authors are grateful to the Department of Science and
Technology, Ministry of Science and Technology, Government of
India for providing necessary ﬁnancial support in the form of a
BOYSCAST fellowship to the ﬁrst author to carry out this work. We also
acknowledge the technical assistance of Debbie Bikfasy and David
Uberuaga, USDA-ARS, Paciﬁc West Area Land Management and Water
Conservation Research Unit, Pullman, WA.
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