healthmedicinet

Annualized crop yield

Annualized crop grain yield varied with treatments and years, with a significant (P ? 0.05) treatment × year interaction. Grain yield was greater in NTCW, STCW, FSTCW,
and FSTW–B/P than STW-F in 1986, 1987, 1992, 1994, 1997, 1998, 2004, 2005, 2009, 2012,
and 2013 (Figure 1). In 1990 and 1996, grain yield was greater in NTCW than FSTCW, FSTW-B/P, and STW-F.
In 2000 and 2011, grain yield was greater in NTCW and FSTW–B/P than STW-F and STCW.
In 2008, grain yield was greater in STCW than STW-F. Averaged across years, grain
yield was 23 to 30% lower in STW-F than NTCW, STCW, FSTCW, and FSTW–B/P (Figure 1). Biomass (stems + leaves) yield also followed trends similar to grain yield.

Figure 1. Effect of tillage and cropping sequence combination on annualized crop yields from
1984 to 2013. FSTCW denotes fall and spring till continuous spring wheat, FSTW–B/P fall and spring till spring wheat–barley (1984–1999) followed by spring wheat–pea
(2000–2013), NTCW no-till continuous spring wheat, STCW spring till continuous spring wheat and STW-F spring till spring wheat-fallow. Annualized crop yield in FSTW–B/P includes average
yield of spring wheat and barley from 1984 to 1999 and spring wheat and pea from 2000
to 2013. Bars at the top denote least significant difference among treatments at P = 0.05. Number in parenthesis along with different letters in the treatment legend denotes significant mean annualized crop yield from 1984
to 2013.

Absence of crops during fallow resulted in lower annualized crop grain yield in STW-F
than the other treatments during the years when the growing season precipitation was
near or similar to the 105-year average (Figure 2). Similar results of lower annualized crop grain yield in crop-fallow than continuous
cropping in dryland cropping systems during the years with near normal precipitation
in the northern Great Plains have been reported by several researchers (Halvorson
et al 2000]; Campbell et al 2004]; Tarkalson et al. 2006]; Sainju 2014]). Crop yields were not different among treatments during years with below-normal
precipitation, such as in 1984, 1985, 1988, 1995, 2006, and 2007 or above-average
precipitation, such as in 1991, 1993, 2003, and 2010. This suggests that increased
soil water conservation by fallow during years with below-normal precipitation increased
crop yield in STW-F, thereby resulting in similar annual crop yields among all treatments
in these years. During years with above-average precipitation, anaerobic condition
due to increased soil water content reduced crop yields, resulting in non-significant
differences in yields among all treatments.

Figure 2. Total precipitation during the growing season (April–August) and throughout the year
(January–December) from 1984 to 2013 at the experimental site.

Orthogonal contrasts indicated non-significant differences among NTCW, STCW, and FSTCW,
suggesting that tillage had no effect on crop yield. Several researchers (Halvorson
et al 2000]; Sainju et al. 2009]; Lenssen et al 2014]) also found that tillage had minimal effect on dryland crop grain yields. Similarly,
non-significant difference in yields between FSTCW and FSTW–B/P indicates that crop
rotation also had no effect on annualized grain yield compared with monocropping.
Greater average yields in NTCW, STCW, FSTCW, and FSTW–B/P than STW-F suggest that
continuous cropping can increase annualized crop yield compared with crop-fallow under
dryland cropping systems in the northern Great Plains. Differences in grain yields,
nutrient removal in grain, and the amount of crop residue returned to the soil resulted
in variations in soil chemical properties and nutrient concentrations, as described
below.

Soil phosphorus and potassium

Soil Olsen-P concentration varied among treatments and soil depths, with a significant
treatment × depth interaction (Table 2). At 0–7.5 cm, Olsen-P was greater in NTCW, STCW, FSTCW, and FSTW–B/P than STW-F
(Table 3). At other depths, treatment had no effect on Olsen-P and averaged 4.6, 2.4, 2.0,
2.1, and 2.8 mg P kg
^?1
at 7.5–15, 15–30, 30–60, 60–90, and 90–120 cm, respectively. Olsen-P was greater at
0–7.5 cm but lower at 15–30 cm under continuous wheat than wheat-fallow. Similarly,
Olsen P at 7.5–15 cm was lower under continuous wheat than wheat–barley/pea. Olsen-P
concentration decreased from 0–7.5 to 7.5–15 cm and remained constant thereafter at
other depths in all treatments. After 30 year, P balance at 0–120 cm was greater in
STW-F than STCW and FSTW–B/P (Table 4). Phosphorus balance was lower under continuous wheat than wheat-fallow, but greater
than wheat–barley/pea.

Table 2. Analysis of variance for the effects of tillage and cropping sequence combination
treatment and soil depth on soil chemical properties

Table 3. Effect of tillage and cropping sequence combination on soil Olsen-P, K, and SO
₄
–S concentrations at the 0–120 cm depth in 2013

Table 4. Effects of tillage and cropping sequence combination on P and K balance at 0–120 cm
depth after 30 year (1983–2013)

Reduced amount of P fertilization to crops, P uptake, and/or crop residue returned
to the soil probably resulted in lower Olsen-P concentration at 0–7.5 cm in STW-F
than the other treatments. Phosphorus fertilizer was applied to spring wheat once
in 2 years in STW-F compared to other treatments where fertilization was done annually
to spring wheat, barley, and pea. Non-significant differences in P concentration among
treatments and depths below 7.5 cm were probably a result of immobile nature of P.
It has been well known that P moves slowly relative to N and K in the soil profile
(Kuo 1996]; Tarkalson et al. 2006]). Overall, tillage to a depth of 10 cm had no effect on Olsen-P concentration even
after 30 year. Increased crop residue returned to the soil likely increased Olsen-P
at 0–7.5 cm, but increased P uptake from subsoil layers probably reduced Olsen-P at
7.5–15 and 15–30 cm under continuous wheat than wheat-fallow and wheat–barley/pea.

As with Olsen-P, the trend in K concentration among treatments and depths was similar
(Tables 2, 3). Potassium concentration at 0–7.5 and 7.5–15 cm was lower in STW-F than the other
treatments, except for the concentration at 0–7.5 cm in FSTCW. Absence of crops and
lack of K fertilization during fallow reduced K concentration in STW-F. At 15–30,
30–60, 60–90, and 90–120 cm, K concentration was not affected by treatments and averaged
153, 100, 96, and 103 mg K kg
^?1
, respectively. Similar to Olsen-P concentration, tillage had no effect on K concentration.
This was similar to that reported by Lal et al. (1994]) and Sainju et al. (2011]), but in contrast to that documented by Tarkalson et al. (2006]) who found greater K concentration at 0–5 and 5–10 cm in conventional tillage than
no-tillage due to increased residue incorporation into the soil. Increased amount
of crop residue returned to the soil and/or K fertilization increased K concentration
at 0–60 cm under continuous wheat than wheat-fallow. Similar to Olsen-P, K concentration
decreased from 0–7.5 to 15–30 cm and then remained constant with depth in all treatments.
Application of K fertilizer increased K concentration in surface soil layer, a case
similar to that observed for Olsen-P. Significant differences in K concentration among
treatments at 7.5–15 cm compared with non-significant differences for P at this layer
suggests that K is more mobile than P.

Differences in the rate of P and K fertilizers applied to crops, removal of P and
K in crop grains, and soil total P and K contents at 0–120 cm at the end of the experiment
resulted in variations in P and K balances among treatments (Table 4). Although soil total P and K contents at 0–120 cm at the end of the experiment were
greater in NTCW than STCW, FSTW-B/P, and STW-F, lower P and K fertilization rates
to crops and removal in grains resulted in higher P and K balances in STW-F than STCW
and FSTW-B/P. Increased P and K fertilization rates compared to grain P and K uptake
may have increased soil residual P and K levels, which likely increased P and K losses
and therefore negative balances under continuous wheat compared to wheat-fallow. Reduced
mineralization of soil organic matter and crop residue likely increased K balance
in no-till than conventional till. Both P and K balances were, however, negative in
all treatments, suggesting that P and K were lost from the surface soil probably due
to surface runoff and leaching after 30 year, a case similar to that reported by various
researchers (Kirkby et al. 2011]; Wang et al. 2014]).

Olsen-P and K concentrations at 0-7.5 cm (25.0–40.0 mg P kg
^?1
and 272–348 mg K kg
^?1
, respectively) were greater than the critical levels of 12.0 mg P kg
^?1
and 120 mg K kg
^?1
, respectively, for optimum dryland crop production in the northern Great Plains (Agvise
Laboratories 2010]). As shown above, crop grains were able to remove only 12–22% of applied P and K
through fertilizers and annual application of P and K fertilizers can increase P and
K losses from the agroecosystem. As a result, P and K fertilization rates can either
be reduced or suspended for several years until their concentrations in the soil falls
near the critical levels. This will help in reducing the cost of fertilization and
improving soil and environmental quality without altering crop yields.

Soil pH and buffer pH

Soil pH and buffer pH varied among treatments and depths, with a significant treatment × depth
interaction (Table 2). Soil pH at 0–7.5 cm was greater in FSTW–B/P and STW-F than STCW and FSTCW (Table 5). At 7.5–15 cm, pH was greater in STW-F than the other treatments, except NTCW. At
15–30, 30–60, 60–90, and 90–120 cm, pH was not different among treatments and averaged
7.65, 8.26, 8.58, and 8.69, respectively. Soil pH was lower under continuous wheat
than wheat-fallow at 0–7.5 and 7.5–15 cm and lower than wheat–barley/pea at 0–7.5 cm.
Soil pH increased with depth, regardless of treatments.

Table 5. Effect of tillage and cropping sequence combination on soil pH, buffer pH, and electrical
conductivity (EC) at the 0–120 cm depth in 2013

The trend for soil buffer pH among treatments was similar to pH (Table 5). At 0–7.5 cm, buffer pH was greater in FSTW–B/P and STW-F than the other treatments.
At 7.5–15 cm, buffer pH was greater in STW-F than STCW and FSTCW. At other depths,
buffer pH was not different among treatments and averaged 7.44, 7.59, 7.69, and 7.72
at 15–30, 30–60, 60–90, and 90–120 cm, respectively. Buffer pH was lower under continuous
wheat than wheat-fallow at 0–7.5 and 7.5–15 cm and lower than wheat–barley/pea at
0–7.5 cm. Buffer pH was 1.07–1.41 units greater at 0–7.5 cm and 0.21–0.85 units greater
at 7.5–15.0 cm than pH. At other depths, buffer pH was either similar to or less than
pH.

The greater soil pH and buffer pH at 0–7.5 and 7.5–15 cm in FSTW–B/P and STW-F were
probably a result of reduced amount of N fertilizer applied. Nitrogen fertilizer was
either applied at 5 kg N ha
^?1
to pea compared with 34–70 kg N ha
^?1
applied to spring wheat and barley in FSTW–B/P in each year or was not applied during
the fallow phase in STW-F. In contrast, N fertilizer was applied to spring wheat at
34–70 kg N ha
^?1
every year in NTCW, STCW, and FSTCW. Continuous application of NH
₄
-based N fertilizers to crops can reduce soil pH, resulting in the development of
infertile soils and decreased crop yields (Liebig et al. 2002]; Tumuslime et al. 2011]; Schroder et al. 2011]). Several researchers (Lal et al. 1994]; Liebig et al. 2002]) have found that soil pH was higher in crop rotations containing legumes and nonlegumes
than continuous nonlegumes, a case similar to that obtained for higher pH and buffer
pH in FSTW–B/P than FSTCW in our experiment (Table 4). Tillage had no effect on soil pH and buffer pH. This was similar to that observed
by Lal et al. (1994]), but different from that found by Tarkalson et al. (2006]) who reported that soil pH varied with tillage at various depths due to variations
in depth of incorporation of N fertilizer into the soil. Greater differences in buffer
pH and pH at 0-7.5 cm among treatments showed that the acidity in the surface soil
layer can be reduced by liming, especially in NTCW, STCW, and FSTCW. Because soil
pH was 6.0 below 7.5 cm, lime can be applied at variables rates depending on soil
pH among treatments in the surface layer without the need for incorporating it into
the soil to neutralize acidity and increase the availability of most nutrients, thereby
improving crop yields.

Soil calcium and magnesium

Soil Ca and Mg concentrations varied among depths, with a significant treatment × depth
interaction (Table 2). Soil Ca concentration at 0–7.5 cm was greater in FSTW–B/P and STW-F than NTCW,
STCW, and FSTCW (Table 6). At 7.5–15, 15–30, 30–60, 60–90, and 90–120 cm, Ca concentration was not different
among treatments and averaged 1.88, 3.65, 4.68, 4.58, and 4.19 g Ca kg
^?1
, respectively. At 0-7.5 cm, Ca concentration was lower under continuous wheat than
wheat-fallow and wheat–barley/pea. Calcium concentration increased with depth from
0–7.5 to 30–60 cm and then remained constant thereafter in all treatments, except
for FSTCW.

Table 6. Effect of tillage and cropping sequence combination on soil Ca, Mg, and Na concentrations
at the 0–120 cm depth in 2013

The trend for soil Mg concentration was similar to Ca concentration. At 7.5–15 cm,
Mg concentration was greater in STW-F than NTCW, STCW, and FSTCW (Table 6). At 0–7.5, 15–30, 30–60, 60–90, and 90–120 cm, Mg concentration was not different
among treatments and averaged 0.22, 0.50, 0.73, 1.21, and 1.44 g Mg kg
^?1
, respectively. Magnesium concentration was lower under continuous wheat than wheat-fallow
at 0–7.5 and 7.5–15 cm. Unlike Ca concentration, Mg concentration increased with depth
in all treatments.

Greater Ca and Mg concentrations at 0–7.5 and 7.5–15 cm in STW-F and FSTW–B/P than
the other treatments were similar to higher soil pH and buffer pH in these treatments
(Table 5). It is likely that the absence of N fertilization to crops during fallow or reduced
N fertilization to pea increased soil pH and therefore Ca and Mg concentrations in
STW-F and FSTW–B/P compared with the other treatments. In contrast, increased soil
acidity resulting from N fertilization to spring wheat every year probably increased
dissolutions of Ca and Mg which were either taken up by the crop or moved down the
soil profile, resulting in lower Ca and Mg concentrations at the surface layer and
increased with depth in NTCW, STCW, and FSTCW. Increased Mg concentration with depth
as opposed to similar levels of Ca concentration below 30 cm indicates that the proportion
of Mg-containing minerals increased with depth while the amount of Ca-containing minerals
remained the same. As with pH and buffer pH, tillage had no effect on these nutrients,
a case in contrast to those reported for various levels of Ca and Mg in no-tillage
and conventional tillage systems at various depths (Tarkalson et al. 2006]). This could be a result of differences in the amount of crop residue returned to
the soil among tillage systems. Mean annualized crop residue returned to the soil
was not different among NTCW, STCW, and FSTCW in this study, but was greater in no-till
than conventional till in the experiment described by Tarkalson et al. (2006]). Increased amount of crop residue returned to the soil can increase soil Ca and
Mg concentrations (Lal et al. 1994]; Liebig et al. 2002]).

Soil sodium, sulfate-sulfur, and zinc

Soil Na and SO
₄
–S concentrations varied among treatments and depths and Zn concentration among depths
(Table 2). The treatment × depth interaction was significant for Na, SO
₄
–S, and Zn concentrations. At 0–7.5 cm, Na concentration was greater in FSTW–B/P than
STCW and STW-F (Table 6). At 60–90 and 90–120 cm, Na concentration was greater in FSTCW and FSTW–B/P than
NTCW, STCW, and STW-F. At 7.5–15, 15–30, and 30–60 cm, Na concentration was not different
among treatments and averaged 16.1, 19.4, and 25.9 mg Na kg
^?1
, respectively. At 60–90 and 90–120 cm, Na concentration was lower in no-till than
conventional till. Similar to Mg concentration, Na concentration increased with depth.

Increased tillage intensity and/or amount of crop residue returned to the soil probably
increased Na concentration at 0–7.5, 60–90, and 90–120 cm in FSTCW and FSTW–B/P compared
with other treatments. It is likely that increased mineralization of crop residue
and soil organic matter due to enhanced tillage increased mobility of Na, some of
which moved down the soil profile, thereby increasing Na concentration in FSTCW and
FSTW-B/P, especially at deeper layers. This was similar to that reported by Sainju
et al. (2011]) who found greater soil Na concentration in conventional tillage than no-tillage,
where tillage was conducted to a depth of 20 cm compared to 10 cm in our study. The
increased Na concentration with depth was proportional to increased soil pH and Ca
and Mg concentrations, suggesting that continuous N fertilization to crops increased
dissolution of Na that was either taken by the crop or moved down from the surface
to the subsurface layers.

Soil SO
₄
–S concentration at 90–120 cm was greater in FSTCW and FSTW–B/P than NTCW, STCW, and
STW-F (Table 3). At 0–7.5, 7.5–50, 15–30, 30-–60, and 60–90 cm, SO
₄
–S concentration was not different among treatments and averaged 8.0, 5.9, 5.7, 6.7,
10.4 mg SO
₄
–S kg
^?1
, respectively. At 90–120 cm, SO
₄
–S concentration was lower in no-till than conventional till. Similarly to Na concentration,
enhanced tillage and/or increased amount of crop residue returned to the soil likely
increased SO
₄
–S concentration at 90–120 cm in FSTCW and FSTW-B/P. Although not significant, SO
₄
–S concentration was lower under continuous wheat than wheat-fallow and wheat–barley/pea.
This was similar to that observed by Sainju et al. (2011]) who reported greater SO
₄
–S concentration at subsurface layers in wheat-fallow than continuous wheat. Increased
SO
₄
–S concentration with depth in FSTCW and FSTW–B/P as opposed to similar levels at
all depths in the other treatments suggest that enhanced tillage intensity increased
mineralization of crop residue and soil organic matter that accelerated the mobility
of SO
₄
–S, some of which moved down the soil profile and accumulated in the deeper layers.

In contrast to Na and SO
₄
–S concentrations, Zn concentration at 0–7.5 cm was greater in NTCW than FSTW–B/P
and STW-F (Table 7). At 7.5–15, 15–30, 30–60, 60–90, and 90–120 cm, Zn concentration was not different
among treatments and averaged 0.45, 0.20, 0.20, 0.21, and 0.29 mg Zn kg
^?1
, respectively. Zinc concentration was greater under continuous wheat than wheat-fallow
at 0–7.5 cm and greater than wheat–barley/pea at 90–120 cm. Reduced soil disturbance
and greater amount of crop residue returned to the soil appeared to increase Zn concentration
at 0–7.5 cm in NTCW. Increased N fertilization appeared to increase Zn concentration
under continuous wheat than wheat-fallow and wheat–barley/pea. As with P and K concentrations,
Zn concentration decreased from 0–7.5 to 7.5–15 cm and remained constant with depth
thereafter.

Table 7. Effect of tillage and cropping sequence combination on soil Zn concentration and cation
exchange capacity at the 0–120 cm depth in 2013

Soil cation exchange capacity and electrical conductivity

Soil CEC varied among treatments and depths and EC among depths, with a significant
treatment × depth interaction for both CEC and EC (Table 2). Soil CEC at 0–7.5 cm was lower in STW-F than NTCW, STCW, FSTCW, and FSTW–B/P (Table 7). At 7.5–15, 15–30, 30–60, 60–90, and 90–120 cm, CEC was not different among treatments
and averaged 13.6, 22.9, 31.1, 33.5, and 33.5 cmol
_c
kg
^?1
, respectively. The CEC was greater under continuous wheat than wheat-fallow at 0–7.5
cm. The CEC increased from 7.5–15 to 30–60 cm and remained constant with depth for
all treatments.

Reduced amount of crop residue returned to the soil due to the absence of crops during
fallow likely decreased CEC at the surface layer in STW-F compared with the other
treatments. Sainju et al. (2011]) also found lower CEC in wheat-fallow than continuous wheat after 9 years in dryland
cropping systems in western Montana. As with most other soil parameters, tillage had
no effect on CEC. This was in contrast to those reported by several researchers (Lal
et al. 1994]; Tarkalson et al. 2006]) who found that CEC was greater in no-tillage than conventional tillage at the surface
soil. Differences among tillage depths and the amount of crop residue returned to
the soil likely resulted in variation in CEC among tillage systems in various locations.
Increased CEC below 15 cm was due to increased Ca, Mg, and Na concentrations (Table 6).

In contrast to CEC, EC at 90–120 cm was greater in FSTCW and FSTW–B/P than NTCW, STCW,
and STW-F (Table 5). At 0–7.5, 7.5–15, 15–30, 30–60, and 60–90 cm, EC was not different among treatments
and averaged 0.19, 0.17, 0.24, 0.29, and 0.34 dS m
^?1
, respectively. The EC was lower in no-till than conventional till at 90–120 cm. Increased
tillage intensity and/or amount of crop residue returned to the soil likely increased
EC at 90–120 cm in FSTCW and FSTW-B/P. Greater EC with these treatments at this depth
were also associated with higher Na and SO
₄
–S concentrations (Tables 3, 6), suggesting that increased accumulation of these nutrients increased soil salinity
at deeper soil layer. The EC 0.25 dS m
^?1
at 0–7.5 and 7.5–15 cm indicates that soils are not saline at the surface layers and
are optimal for crop growth and microbial activity (Liebig et al., 2002]). Our results were similar to that reported by Sainju et al. (2011]) who found greater EC with conventional tillage than no-tillage at 30–60 cm, but
in contrast to higher EC in wheat-fallow than continuous wheat due to increased Ca,
Mg, and Na concentrations. Similar to CEC, EC also increased with depth below 15 cm,
suggesting increased salinity.

Implication of management practices

Mean annualized crop yield was not different among NTCW, STCW, FSTCW, and FSTW-B/P,
and the yield was greater in these treatments than STW-F (Figure 1). Most soil nutrients and chemical properties at the surface soil in FSTW–B/P were
either greater than or similar to the other treatments. As a result, FSTW–B/P can
be used as a superior management practice to reduce N fertilization rate and maintain
long-term soil fertility and crop yield. Tillage had no effect on dryland wheat and
pea yields (Lenssen et al. 2007], 2014]) and also on annualized crop yield and soil properties as observed in this experiment.
Furthermore, crop rotation had little effect on annualized crop yield and soil properties.
As a result, no-tillage with legume-nonlegume crop rotation may be used to enhance
the long-term sustainability of dryland soil fertility and crop yields with reduced
chemical fertilizer and tillage-related inputs. Because of reduced crop yields, lower
nutrient concentrations, and degraded chemical properties, conventional tillage with
crop-fallow system should be avoided in dryland cropping systems.