Simulated long-term effects of different soil management regimes
on the water balance in the Loess Plateau, China
Shulan Zhang
a,c,*
, Elisabeth Simelton
b, Lars Lo¨vdahl
c, Harald Grip
c, Deliang Chen
b,d aCollege of Resources and Environmental Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, ChinabDepartment of Physical Geography, Earth Sciences Centre, Gothenburg University, Box 460, SE-405 30 Gothenburg, Sweden cDepartment of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea, Sweden
dNational Climate Centre, Beijing, China
Received 11 January 2006; received in revised form 18 August 2006; accepted 19 August 2006
Abstract
A soil management regime that improves water use efficiency (WUE) is urgently required to increase the sustainability of the winter wheat-summer fallow system in the Loess Plateau, China. However, the long-term partitioning of the water balance must be understood in order to evaluate the viability of possible soil management regimes. Therefore, an ecosystem model (CoupModel) was used to explore the effects on components of the water balance of five types of soil management regimes: conventional practice, wheat straw mulching, incorporation of high organic matter contents, compaction, and use of a harvested fallow crop. Three variants of the fallow crop approach were also considered, in which the crop was harvested 15, 30 and 45 days before sowing the wheat (designated Fallow-15d, Fallow-30d and Fallow-45d, respectively). Simulations were used to identify the relative magnitude of soil evaporation, wheat transpiration and deep percolation and to elucidate the temporal variability in these components for a selected location using climate records spanning 45 years. However, the soil management regime significantly influenced the magnitude of every component of the water balance (in terms of minimum, maximum and mean values) over the long periods considered. Consequently, wheat yield and WUE differed significantly among the simulated treatments. Mulching led to significantly lower soil evaporation, higher transpiration, and more frequent and extensive deep percolation than other regimes, thereby improving fallow efficiency (percentage of rainfall stored in the soil during the fallow period at the end of the fallow period), wheat yields and WUE. In contrast, soil compaction gave the opposite results, leading to the most unfavourable partitioning of the water balance reflected in the lowest wheat yield and WUE values of all the regimes. In 90% of the years no deep percolation occurred in the soil compaction simulations. Use of a fallow crop with optimal harvest timing (Fallow-30d) improved partitioning of the water balance (decreased soil evaporation) and did not significantly reduce wheat yield compared with conventional practice. High organic matter contents in the soil also had a positive influence on the water balance and improved wheat yield and WUE relative to conventional practice. Therefore, mulching appears to be the best management practice for the winter wheat-summer fallow system in the Loess Plateau, according to the simulations. Increasing soil organic matter may be the best option if mulching cannot be implemented. The ideal time for harvesting a fallow crop for use as green manure or fodder appears to be ca. 30 days before sowing the winter wheat.
#2006 Elsevier B.V. All rights reserved.
Keywords:Long-term effects; Mulching; Soil compaction; Fallow crop; Deep percolation
1. Introduction
Water shortages pose the greatest threat to dryland farming in semiarid areas. In China, 14.7 million ha of arable land can be categorized as semiarid, mainly distributed in the Chinese Loess Plateau, the world’s largest Loess Plateau (Li and Xiao,
1992). The main crop on a large part of the Loess Plateau is winter wheat, conventionally cultivated with a single crop being produced per year, followed by about three months summer fallow. The fallow period falls in the rainy season, in which water can be stored in the soil and used by the following wheat crop. During the past 20 years both fertilizer applications and wheat yields have increased, resulting in increased soil-water depletion (Huang et al., 2003). Consequently, soil water is not being fully replenished during the fallow, and at sites where a dry subsoil layer has formed the crop yield varies strongly with rainfall during the growing season (Li, 2001). The dry soil layer, where water content is close to the wilting point,
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* Corresponding author at: College of Resources and Environmental Sciences, Northwest A&F University, 712100 Yangling, Shaanxi, China. Tel.: +86 29 87088120.
E-mail address:zhangshulan@nwsuaf.edu.cn(S. Zhang).
0378-4290/$ – see front matter#2006 Elsevier B.V. All rights reserved.
may be replenished with plant available water during the fallow period if sufficient rain falls to penetrate the soil profile. However, the conventional practice of keeping the soil bare during the fallow period results in very low precipitation storage efficiency or fallow efficiency (ratio of stored water to rainfall during fallow) (Li and Xiao, 1992; Latta and O’Leary, 2003). Furthermore, the infiltration depth is shallower under high fertilization than low fertilization conditions (Huang et al.,
2002). Therefore, the conventional practice with high
fertilization does not appear to be a sustainable management option in the long-term.
One way to increase water storage is to retain crop residues on the soil surface. Mulching can be an effective measure for reducing soil evaporation and increasing water storage (Unger,
1978; Steiner, 1989; Baumhardt and Jones, 2002). Furthermore,
short-term studies have shown that mulching is beneficial for water storage and crop yields in the Loess Plateau (Wang et al.,
2001; Huang et al., 2005; Zhang et al., 2006a).Zhang et al.
(2006a)also found that deep percolation occurred earlier and
more extensively under mulching than under conventional practice, which should favour the recovery of the soil water content of the dry sub-layer. However, mulching has not always been shown to increase crop yields, and its effectiveness depends on crop, soil and climate (Wicks et al., 1994; Gajri
et al., 1994). In addition, mulching has not been widely
practiced in China for two practical reasons: (1) the presence of mulch reduces the quality of wheat sowing by standard machines and (2) mulch material is also needed for animal food stuff and/or fuel.
Another way to improve water storage is to change the hydraulic properties of the soil in a way that increases rain infiltration and decreases soil evaporation. Increasing the soil organic matter content has been found to meet these objectives
(Unger and Stewart, 1974). Unfortunately, the most rapid and
convenient way to increase soil organic matter – applying animal manure – cannot be implemented in this region in practice, because such resources are very limited. Hence, growing a crop during the fallow period, and ploughing it in as green manure at an appropriate time before the next crop, might be a more viable strategy to increase soil organic matter contents and hence enhance fallow efficiency. However, various short-term studies have shown somewhat inconsistent results. After testing different crop rotation systems, Li et al. (2000)
reported that growing a fallow crop for forage does not greatly influence the quantity of water stored in the soil for use by subsequent winter wheat crops in the middle-west Loess
Plateau. Vigil and Nielsen (1998) found that wheat yields
following the application of green legume manure were lower than those obtained with traditional summer fallow in a 2-year study. In a 3-year study, use of a fallow crop as green manure reduced the yield of the following winter wheat in a relatively dry year compared with conventional practice, but did not reduce the yield in a wet year (Zhang et al., 2006a). Therefore, interpreting these direct measurements and assessing the value of the examined management strategies is complicated by year-to-year variations in both the total amount and the seasonal distribution of rainfall (Asseng et al., 2001).
In the Loess Plateau region there is an increasingly urgent need not only to improve water use by the crops but also to elucidate side-effects caused by modern mechanised operations in the field. Soil compaction has been recognized as one of the most serious factors promoting soil degradation in the world
(Oldeman et al., 1991). A laboratory study has shown that soil
compaction greatly affects the hydraulic properties of silt loam soil from the Loess Plateau (Zhang et al., 2006b); saturated hydraulic conductivity in compacted soil amounted to less than 15% of that in non-compacted soil. However, there is little information on the effects of compaction on crop yields.
Due to the inter-annual climate variability, only long-term analysis is likely to give clear indications of the risks associated with management effects on crop yield and valid measures of the water balance components involved (Keating et al., 2002). Simulations are, therefore, powerful tools for extrapolating short-term experimental results and for analyzing how different measures are likely to influence production and the water balance across the range of climatic conditions within a given location. In the study presented here, simulations were applied to identify the relative magnitude of the effects of different soil management strategies on key water balance components (such as transpiration, soil evaporation, and deep percolation). For this purpose we explored the temporal variability in these variables for the winter wheat zone in the Loess Plateau using 45 years of meteorological data. We also identified the impact of the duration of fallow crop growth (forage or green manure) on subsequent wheat yields. The overall purpose was to identify a sustainable management strategy for agricultural production in the Loess Plateau and similar regions.
2. Materials and methods
2.1. Model description
The CoupModel is a physically based one-dimensional model to simulate fluxes of water, energy, carbon and nitrogen in the soil–plant–atmosphere system, which incorporates both
the former SOIL (Jansson and Halldin, 1979) and SOIL-N
(Eckersten et al., 1998) models. A detailed description of the
model was given byJansson and Karlberg (2004).
The model predicts water and heat flows between soil layers, processes at the surface and the base of the profile including runoff, infiltration, and deep percolation. In addition, evapo-transpiration, surface energy balance, heat storage, soil frost and snow dynamics are calculated. The water and heat flow are described byRichard’s (1931)solution of Darcy’s law coupled with Fourier’s law. Calculation of soil evaporation was based on solving the soil surface energy balance while plant transpiration and evaporation of intercepted water are based on the Penman–
Monteith equation (Monteith, 1965). Deep percolation will
growth reduced by linear response functions for unsuitable air temperature, nitrogen availability and water stress. No reduction from pests or nutrients deficiency (except nitrogen) was included. In this simulation nitrogen is assumed to be sufficient, thus plant growth was not reduced because of nitrogen supply. The daily total assimilation is translocated to a temporary storage pool, and then is partitioned to the roots, leaves and stem. Grain development started according to air temperature sum, and assimilates to grain were translocated from leaf, stem and root, respectively.
2.2. Model calibration and long-term application
The model was first calibrated against a 3-year winter wheat field experiment in the Loess Plateau byZhang et al. (2006a)
including three different soil management regimes, which were conventionally managed winter wheat-summer fallow, mulch-ing (a conventionally managed, but unploughed treatment in
which air-dried, unchopped wheat straw (0.8 kg m2
) was evenly distributed over the soil surface and kept at relatively constant levels), and winter wheat-summer fallow crop.
The long-term simulation was run for the same soil type as the above field experiment during a 45-year period using daily weather data at Luochuan (35.82N, 109.50E, 1159 m a.s.l.) as driving variables. The long-term simulation was run for five types of soil management regimes totalling seven treatments
(seeTable 1). The crop and soil parameters applied were the
same for all treatments as the calibrated values found from the field experiment, except those hydraulic properties, which were
derived from laboratory studies for soil compaction (Zhang
et al., 2006b) and high organic matter treatments (Zhang et al.,
2006c). In order to investigate the impact from climate
variability the soil properties of each treatment were kept constant during the simulation period (from 20 September 1955 to 19 September 2000) and no nutrient limitation was considered.
The simulation was started on 20 September 1955. In the
simulations, the date of sowing winter wheat (Triticum
aestivum L.) was fixed at 20 September for all years and treatments, while the harvest date was calculated by the model according to the temperature sum. The fallow crop, black bean (Aphis fabae), was sown on 16 June every year. The annual
water balance was calculated from previous fallow period until the end of the following wheat season. The first year (1955) the fallow period was not included in the simulation. Therefore, the annual water balance was calculated from summer in 1956 to summer in 2000, totally for 44 years.
2.3. Analysis methods
2.3.1. Water balance
The field water balance can be written as
P¼EþTþDþRþDSþEi (1)
wherePis the precipitation,Ethe soil evaporation,Tthe crop
transpiration, R the surface runoff, D the deep percolation
below the root zone,DS the change in soil water storage and
Eiis evaporation from intercepted rainfall. In this study surface
runoff was zero because the topography was flat, and Ei was
neglected because it was quite constant and constituted a very small proportion of the water balance compared with the other terms (Zhang et al., 2006a). DS can be either positive or negative. Therefore, the water balance was calculated as
PDS¼EþTþD (2)
2.3.2. Water use efficiency
Water use efficiency (WUEgor WUEb) was defined as
WUE¼
Y
ET (3)
where WUEgor WUEbrepresents the water use efficiency for
the grain or biomass yield (kg m3
),Y the grain or biomass yield of the wheat, respectively, and ET is the evapotranspira-tion during the wheat season.
2.3.3. Water stress level
The wheat water stress level was expressed as
WSL¼1
Ta
Tp
where WSL is the water stress level,Taactual transpiration and Tpis the potential transpiration. The higher the value of WSL
the more severe the water stresses.
Table 1
Description of the different treatments
Treatment Fallow period (harvest–19 September) Growing season (20 September–June) Soil properties
Conventional management Ploughed bare soil Winter wheat Zhang et al. (2006d)
Mulch Wheat straw no summer ploughing Winter wheat + wheat straw As conventional management
Fallow-15d Black bean growing 16 June–5 September Winter wheat As conventional management
Fallow-30d Black bean growing 16 June–20 August Winter wheat As conventional management
Fallow-45d Black bean growing 16 June–5 August Winter wheat As conventional management
HOM (soil with high organic matter content)
Ploughed bare soil Winter wheat Zhang et al. (2006c); soil-OM
3%-units (0–10 cm depth) and 1.5%-unit (10–20 cm depth)
Compacted Bare soil Winter wheat Zhang et al. (2006b); assumed to
2.4. Meteorological data
In the simulations, measured daily climatic data spanning 45 years (20 September 1955 to 19 September 2000) was used. The measured variables were precipitation, air temperature, relative air humidity, wind speed and sunshine hours, which was used to estimate global radiation. Values were missing for about 10% of the total number of records for the first variable, and less than 0.1% for the other four. The data were provided by the National Climate Centre, Beijing. The missing values were replaced by zeroes for precipitation and interpolated for the other variables in the simulations.
3. Results
3.1. Climatic conditions
In the present study the annual precipitation was calculated as rainfall within the previous fallow period (from wheat harvest to wheat sowing) plus precipitation during the following wheat season. Over the 44 years, the annual precipitation varied from 351 to 819 mm, with an average of 568 mm (Fig. 1a). Fallow rainfalls accounted for 39–85% (average 61%) of the annual precipitation. Ten-year average annual precipitation was 604 mm in the 1960s, 544 mm in the 1970s, 607 mm in the 1980s and 538 mm in the 1990s. For about 10%, 25% and 20% of the years the annual precipitation was <400, 400–500, and >700, respectively. Fallow rainfalls were between 300 and
400 mm for nearly 50% of the years (Fig. 1b). Thus, annual precipitation showed great variability; some years could be very dry or very wet. The annual average temperature was 9.48C, while the daily maximum and minimum temperatures were 28.2 and16.48C, respectively (data not shown).
3.2. Simulated components of the water balance
The different soil management regimes resulted in different magnitudes of soil evaporation, transpiration and deep percolation (Table 2). Annual mean soil evaporation comprised
Fig. 1. Observed (a) and accumulated probability (b) of annual (fallow + wheat season) and fallow precipitation from 1957 to 2000.
58–82% of the water balance, regardless of the management regime, with wheat transpiration the next largest term at 17– 32%. Fallow crop transpiration and deep percolation made up the remainder of the balance, amounting to 2–12% and 1–9%,
respectively. The proportions of the water balance accounted for by soil evaporation were highest in the soil compaction treatment (ranging from 68% to 90%) and lowest in the mulching treatment (43–76%) during the simulation period. Growing a fallow crop decreased the contribution of annual mean soil evaporation by 2–8% compared with conventional practice. The high organic matter treatment resulted in 2% lower soil evaporation than that in the conventional treatment. The contribution of wheat transpiration to the water balance was highest under mulch and lowest in the compaction treatment. The longer the period with a preceding fallow crop, the lower the transpiration in the subsequent wheat season compared with conventional practice, but the maximum transpiration was the same (35%) and large differences were found in minimum values (Table 2). The high organic matter treatment gave similar mean transpiration values to the conventional treatment, but the maximum value was higher than that in the conventional treatment. The contributions of deep percolation to the water balance differed significantly among treatments; being highest under mulch (9%; ca. two- to nine-fold higher than in the other treatments), and lowest in the compaction treatment. Fallow crop treatments decreased deep percolation to some extent compared with conventional practice, except Fallow-45d, for which the proportions (including maximum values) were the same.
3.3. Temporal variability of water balance components
The temporal variability in soil evaporation and wheat transpiration in each treatment was relatively minor compared to the temporal variability in deep percolation (Table 2 and
Fig. 2). For all treatments the coefficients of variation were
around 10% for soil evaporation, less than 30% for wheat transpiration, but higher than 100% for deep percolation. The accumulated probability showed that for 50% of the years deep percolation amounted to less than about 30 mm under mulching, but less than 5 mm for all other treatments
(Fig. 3). Growing a fallow crop affected the amount of deep
percolation to some extent, but did not substantially affect the probability of its occurrence compared with conventional practice. The amount of deep percolation significantly
Fig. 2. Simulated temporal variation of soil evaporation (E), transpiration (Tw for wheat andTffor fallow crop, respectively) and deep percolation (D) under various soil management regimes from 1957 to 2000. From the top the treatments are Fallow-45d (a), Fallow-30d (b), Fallow-15d (c), compaction (d), mulch (e), high organic matter (HOM, f) and conventional (g).
Fig. 3. Accumulated probability of deep percolation occurring under the conventional, high organic matter (HOM), compaction, mulch, Fallow-15d, Fallow-30d and Fallow-45d treatments.
Table 3
Long-term simulated fallow efficiencya under different soil management regimes (%)
Treatments Mean Maximum Minimum S.D.
Conventional 28 bc 46 3 11
Mulch 38a 57 10 11
HOMb 30 b 48 1 11
Compaction 19 d 46 15 11
Fallow-15d 19 d 46 29 16
Fallow-30d 23 cd 46 12 13
Fallow-45d 27 bc 47 5 12
S.D. refers to standard deviation. Different letters in the same column indicate differences that are significant at theP<0.05 level (LSD).
a Fallow efficiency expresses ratio of stored water to rainfall during fallow period.
correlated with yearly precipitation, but was most strongly correlated with fallow rainfall (R2= 0.46 for conventional practice, 0.28 and 0.63 for compaction and mulch, respec-tively). To generate deep percolation 405 mm rain was needed under mulch conditions, compared to 504 and 589 mm for the conventional and compaction treatments, respectively.
3.4. Fallow efficiency
The mean ratio of stored water to rainfall during the fallow period (fallow efficiency) under various treatments ranged from 19% to 38%; the highest value was found in the mulch treatment and the lowest values in the compaction and Fallow-15d treatments (Table 3). In comparison with the conventional treatment, growing a fallow crop decreased the fallow efficiency by percentages ranging from 1% to 9%, depending on the growing duration. High organic matter resulted in 2% higher fallow efficiency than conventional practice. However, all treatments except the mulching and high organic matter treatments had similar maximum fallow efficiencies, but very different negative minimum fallow efficiencies (ranging from
29% to3%).
Fallow efficiency significantly correlated with fallow rainfall and rainfall distribution during the fallow period
(Table 4). The lowest correlation coefficient between fallow
efficiency and rainfall throughout the fallow period was for the compaction treatment (0.57), followed by mulching (0.66) and the Fallow-15d treatment (0.70); the other treatments had similar correlation coefficients, all>0.7. However, rainfall in the last month of the fallow period was also highly correlated with fallow efficiency, with correlation coefficients of 0.71 for the compaction treatment, and 0.5–0.6 for all of the other treatments.
3.5. Water stress probability
The simulations indicated that the wheat was subjected to
some degree of water stress (WSL0.05) during all years
(Fig. 4). During 50% of the years the water stress level was less
than or equal to 0.19 for the mulch treatment, and 0.29, 0.30, 0.33, 0.34, 0.36 and 0.45 for the high organic matter, conventional, Fallow-45d, Fallow-30d, Fallow-15d and com-paction treatments, respectively. The treatments could be separated into three groups according to the water stress levels associated with them: slight stress (the mulching); severe stress (compaction); and moderate stress (the other treatments). In the moderate stress group Fallow-15d generated the most severe water stress.
3.6. Wheat yield and water use efficiency
The simulated average annual wheat biomass yield ranged from 0.59 to 0.98 kg m2
, regardless of treatment effects, and biomass water use efficiency ranged from 2.00 to 3.09 kg m3
(Table 5). Mulching increased both the grain and biomass
yields, and consequently resulted in higher water use efficiency than conventional practice. Conversely, the compaction treatment greatly decreased the wheat yield and resulted in the lowest WUE values. The treatment in which the fallow crop was grown for the shortest time (Fallow-45d) gave the same wheat yield as the conventional treatment, while wheat yields were reduced under the other two fallow crop treatments, and the WUE values followed the same trends. The high organic matter treatment slightly increased wheat yield and WUE.
The accumulated probability of the treatments to increase wheat biomass yields relative to the conventional practice is
Table 4
Linear correlation coefficients for the relationship between fallow efficiency and: rainfall throughout the fallow period, rainfall in the last month of the fallow period and the rest of the time (n= 45)
Treatment Whole fallow The last month Other time
Conventional 0.73 0.59 0.40
Note: all coefficients are significant at theP<0.01 level except the value of 0.14.
Fig. 4. Accumulated probability of water stress occurring under the conven-tional, mulch, high organic matter (HOM), compaction, 15d, Fallow-30d and Fallow-45d treatments.
Table 5
Long-term simulated wheat yield (kg m2) and water use efficiency (WUE) (kg m3) under various treatments
Treatment Grain S.D. Biomass S.D. WUEg S.D. WUEb S.D. Conventional 0.39 b 0.09 0.84 b 0.19 1.26 b 0.25 2.68 b 0.39 Mulch 0.44 a 0.09 0.98 a 0.19 1.41 a 0.24 3.09 a 0.36 HOM 0.40 b 0.09 0.86 b 0.19 1.27 b 0.26 2.73 b 0.40 Compaction 0.28 d 0.09 0.59 d 0.18 0.97 d 0.20 2.00 e 0.33 Fallow-15d 0.35 c 0.12 0.74 c 0.26 1.09 c 0.31 2.29 d 0.60 Fallow-30d 0.37 bc 0.11 0.79 bc 0.24 1.17 bc 0.28 2.46 cd 0.49 Fallow-45d 0.39 b 0.10 0.83 b 0.21 1.24 b 0.25 2.62 bc 0.40
shown inFig. 5. Mulching increased the yields in 95% of the years, and in 50% of the year yields increased by about 20%. The high organic matter treatment increased the yields in more than 85% of the years, although by less than 5% in 50% of them. The effects of the fallow crop treatments differed according to harvest time. In 50% of the years yields were decreased by about 10% in the Fallow-15d treatment, less than 3% in the Fallow-30d treatment, and zero in the Fallow-45d treatment. The probability that yield would be reduced by growing a fallow crop was highest when the fallow crop period was longest (i.e. higher for the Fallow-15d treatment than for the Fallow-30d and Fallow 45-d treatments). However, soil compaction was the only treatment for which yields were reduced in all years, relative to the conventional practice, and the reductions were substantial—amounting to about 30% in half of the years.
4. Discussion
The soil management regime strongly influenced the magnitude of the water balance components. Although this study considered seven types of management, two had extreme effects, namely mulch and compaction. Mulching decreased soil evaporation, increased transpiration and deep percolation, leading to increased wheat yields and WUE. In contrast, compaction caused significantly higher soil evaporation, which led to lower yields and WUE compared with the other treatments.
Soil evaporation depended on the rates and frequency of rainfall, atmospheric demand, soil moisture and during periods with the degree of crop cover (LAI). Generally, soil evaporation proceeds through two stages (Ritchie, 1972); the amounts of water evaporated in the first and second stages being determined by atmospheric evaporative demand and soil hydraulic properties, respectively. Mulch significantly reduces
soil evaporation in the first stage (Steiner, 1989; Ji and Unger, 2001). In the present study, fallow rainfall accounted for more than 60% of the yearly precipitation, hence the soil evaporation was often in the first stage. During the wheat season soil evaporation was often in the second stage because the surface layer was very dry due to water uptake by the crop and the lower rates of precipitation during that season. Therefore, the ratios of soil evaporation to potential evaporation were similar between mulching and conventional practice (data not shown). On an annual basis mulching reduced soil evaporation by 12%, on average, compared with the conventional practice. Conse-quently, the fallow efficiency was significantly improved; more water was available for wheat transpiration and (thus) both wheat yield and WUE increased. The potential of mulching to increase soil organic matter, which has lower unsaturated
conductivity than the other soil components (Zhang et al.,
2006c), should further decrease soil evaporation, as shown by
the data for the high organic matter treatment (Table 2). In contrast, soil compaction adversely affects the pore size distribution and increases unsaturated hydraulic conductivity, thereby enhancing soil evaporation in both the first and second stages (Tamari, 1994). In our simulations this resulted in higher maximum and minimum evaporation relative to the conven-tional treatment, causing the wheat to be subjected to more water stress, and thus reduced yields. This conflicts somewhat with the results of a short-term field study of heavy textured soils byRadford et al. (2000), who found that soil compaction reduced wheat emergence, but not the yield. In the present long-term simulation, alleviation of soil compaction by shrinking and swelling cycles between rainfall events was not considered. Furthermore, under field conditions field operations can loosen the topsoil and water stress may be less severe than what the simulation suggested. Therefore, results of the present simulation might somewhat overestimate the side-effects of soil compaction. On the other hand our simulations did not account for eventual reduced root development due to soil compaction.
The simulations indicate that the value of using fallow crops depends on the duration of fallow crop growth. The outcome of the Fallow-45d treatment was similar to that of conventional practice for all of the water balance, fallow efficiency, wheat yield and WUE terms. Although the Fallow-30d treatment resulted in significantly lower soil evaporation and WUEb, the
fallow crop transpiration was higher than in Fallow-45d. The Fallow-15d treatment significantly decreased the fallow efficiency, more during a dry year (minimum value) and less
during a wet year (maximum value) (Table 3). This adverse
effect was not fully compensated by a reduction in soil evaporation during the fallow crop period and, consequently, less water was available for wheat transpiration (Table 2). Consequently, wheat yield and WUE were lower than for conventional practice. The negative effects of long fallow crop periods are in agreement with the findings ofVigil and Nielsen
(1998).
Deep percolation is a crucial component of the water balance in the Loess Plateau region. Short-term investigations have found a dry subsurface-layer in the soil profile of agricultural
land (Huang et al., 2002) and in a long-term (15-year) experiment it was found that less soil water replenishment occurred in plots subjected to high current levels of fertilization than in unfertilized control plots during fallow periods (Huang
et al., 2003). These results imply that conventional practice
reduces the frequency and extent of deep percolation. Our long-term simulations indicate that during the 1960s, 1970s and 1980s, the mean annual deep percolation amounted to 26, 33 and 25 mm, respectively, under conventional practice. How-ever, in the last 10 years (1991–2000) annual mean deep percolation amounted to only about 1 mm. Mulching nearly doubled the probability of deep percolation occurring compared to conventional practice in our simulations
(Fig. 3), and approximately tripled the quantities of water
involved (Table 2). This is because mulch reduces soil
evaporation by changing the surface energy balance (Horton
et al., 1996), favouring rainfall infiltration (Baumhardt and
Lascano, 1996), transporting more water to deeper soil layers
and possibly recharging groundwater. Moreover, deep percola-tion has the potential to maintain higher crop yields by buffering against a following drought year. For example, high rainfall in 1989 generated deep percolation which maintained the yield in 1990 when the precipitation was low. Hence, one advantage of mulching is that it could help conserve water in irrigated areas. Furthermore, mulching reduced the amount of precipitation required to generate deep percolation compared to the conventional and compacted treatments (especially the latter). Therefore, soils compaction, by the use of heavy machinery for example, can be detrimental to a sustainable hydrological cycle.
The huge temporal variability in deep percolation in dryland cropping systems (Fig. 2) indicated that use of long-term data is important for estimating water balance components, either in simulations or by empirical measure-ments. Even with mulching, which resulted in the most frequent deep percolation, there was still no deep percolation in 30% of the years. In such cases short-term experiments can easily give biased estimates of long-term water balance performance because of the likelihood that the period considered will be unrepresentatively wet or dry. Furthermore, in this study we assumed a condition with no nutrient (nitrogen) limitation. It should be similar to the conditions in
the study by Huang et al. (2003) where high levels of
fertilization were used and a dry soil layer was formed. The interaction between nutrient and water is an important issue, but out of the scope of this paper.
5. Conclusions
In the long-term, mulching improved the partitioning of the water balance between different components compared with conventional practice and increased both the winter wheat yield and WUE. Thus, mulching in the winter wheat-summer fallow system could be a sustainable management strategy in the Loess Plateau, China. In contrast, soil compaction generated the most unproductive water balance. Considering all of the factors involved, the duration of the bare fallow should not be
less than 30 days (Fallow-30d or Fallow-45d) to maximise the benefits of producing green manure or fodder without interfering significantly with the wheat yield.
Acknowledgements
We thank John Blackwell for linguistic improvements. This study was part of a joint project between the Swedish University of Agricultural Sciences and Northwest A&F University of Agriculture and Forestry in China, funded by the Swedish International Cooperation Development Agency (INEC-KTS/453/01) and project from Sida/SAREC (SWE-2002-038).
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