Fallow management increases soil water and nitrogen storage

Agricultural Water Management 186 (2017) 12–20

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Fallow management increases soil water and nitrogen storage Ketema Tilahun Zeleke a,b,∗ a

School of Agricultural and Wine Sciences, Charles Sturt University, Wagga Wagga, Australia Graham Centre for Agricultural Innovation (an alliance between NSW Department of Primary Industries and Charles Sturt University), Wagga Wagga, Australia b

a r t i c l e

i n f o

Article history: Received 25 September 2016 Received in revised form 4 February 2017 Accepted 10 February 2017 Keywords: APSIM Australia Fallow Plant available water Weed

a b s t r a c t In regions where rainfall during the cropping season is low and variable, such as most parts of Australia, stored soil moisture determines the yield and sowing time of the following crop. A long-season fallow experiment was conducted in south-eastern Australia, and a biophysical simulation model, APSIM, was evaluated and applied. Stubble cover did not significantly affect fallow soil water storage; once the soil profile was filled during the winter fallow, the presence or absence of stubble cover during the summer fallow made little difference. However, weed growth during the summer period significantly affected the soil water storage. By the time of winter crop sowing, the plant available water (PAW) was depleted by 11% (18 mm) in weed free – stubble covered treatment, 14% (23 mm) in weed free – stubble free treatments, 34% (52 mm) in the weedy – stubble covered treatment, and 42% (64 mm) in weedy – stubble free treatment. The weedy (39 kg ha−1 ) and weed free (98 kg ha−1 ) treatments differed significantly in the amount of soil mineral nitrogen at the end of the fallow period. APSIM was able to simulate the change in soil water storage under the weedy treatment accurately (R2 = 0.93, NRMSE = 4%). Long term simulation showed that there was an 88% probability of accumulating 140 mm PAW by the time of sowing, compared with only 13% probability when weeds were present. If the summer fallow period was not properly managed, the water stored during the winter season could be lost to weeds. While soil water and nitrogen storage may vary with soil type, rainfall amount, rainfall distribution, and weed pressure, fallow weeds must be controlled to ensure accumulation of fallow soil water and nitrogen for a subsequent crop. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In the summer-dominant rainfall regions of Australia, a short fallow over summer between successive winter grain crops is commonly used to conserve rain water. However, rainfall in southeastern Australia is mainly winter-dominant with less frequent and higher intensity rainfall during summer (Sadras, 2003; Gentilli, 1971). In the latter environment, a long fallow (>6 months), which spans over winter and summer seasons, is likely to be more effective. However, studies on the effects of long fallow on water storage in winter-dominant rainfall environments are generally rare. The difference in stored water between a summer (short) fallow and a long fallow depends on the summer rainfall and the soil type (Oliver and Sands, 2013). If there is high summer rainfall, the soil profile can be filled even in a short/summer fallow, so the stored

∗ Corresponding author at: School of Agricultural and Wine Sciences, Charles Sturt University, Wagga Wagga, Australia. E-mail address: [email protected] http://dx.doi.org/10.1016/j.agwat.2017.02.011 0378-3774/© 2017 Elsevier B.V. All rights reserved.

soil water can be similar under short or long fallow. Long fallow can be more effective in soils with high water holding capacity which can also store water from the winter season. Thus, the soil profile can be filled from rainfall during the summer-autumn-winter period. Depending on the spring-summer management condition, however, this stored soil water can remain stored for the winter crop or be lost to soil evaporation and weed transpiration. Therefore, strategies to better capture and store fallow rain include (i) retention of crop residues on the soil surface to improve water infiltration and reduce evaporation; and (ii) control of summer fallow weeds to reduce transpiration (Kirkegaard and Hunt, 2011). Improvement in the capture and storage of water derived from stubble management depends on soil type and rainfall pattern (Incerti et al., 1993; Gregory et al., 2000). Stubble mulch is more effective in reducing evaporation in frequent low-intensity rainfall and clay soils than in less frequent high-intensity rainfall and sandy soils (Gregory et al., 2000). Residues slow the flow of water on the soil surface, allowing more time for infiltration (Freebairn and Boughton, 1981), as well as slowing soil evaporation following rainfall events. However, if conditions remain dry for an extended

K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

period, total evaporation can be unaffected by residues (Verburg et al., 2012). As a result, increases in fallow efficiency due to reduced evaporation are minor, and they occur only when large amounts of residue are present and rainfall patterns are favourable (Kirkegaard et al., 2007b; Browne and Jones, 2008). That means stubble mulch might not be effective in the less frequent, less intense south eastern Australian summer rainfall on well drained Red Kandosol soil. O’Leary and Connor (1997) compared water accumulation under long fallows with residue retention and weeds controlled during the summer fallow, and found that, although the retention of residues during the long fallow did not increase soil water accumulation on sandy loam soils, there were significant increases measured in heavy clay soils. Hunt et al. (2013) found no effect of stubble on soil water accumulation on a clay soil in the Mallee region of Victoria. However, they found that controlling summer weeds increased soil water accumulation (mean 45 mm) and mineral N (mean 45 kg ha−1 ) before sowing on sand and clay soils. Sadras et al. (2012) reported that stubble treatments did not affect soil water content during a fallow period. Summer weed control was found to be more important than stubble management in conserving both water and N with complete control increasing the level of mineral nitrogen by 69 and 45 kg N ha−1 in the two years of study (McMaster et al., 2015). The above studies leave uncertainty regarding the relative effects of residue management and weed control on soil water and N accumulation, as most of the experiments focussed on either residues or tillage. In addition, studies on the effect of weed on summer fallow soil water storage are mostly based on soil profiles already depleted by the winter crop. However, if the soil profile is full, the chance of germinating and growing abundant summer weeds increase. Although summer weeds can grow on episodic summer rain, in a long fallow the soil water can be full from the winter/spring rainfall and weeds can grow easily (Hunt et al., 2009). This study evaluates how stubble and weed affect an already full soil profile from a winter fallow. It assesses the effect of stubble and weed on soil water and nitrogen stored during the winter period of a long fallow in an equiseasonal rainfall environment of Wagga Wagga (NSW), a transition zone between summer and winter dominant rainfall. The objective is: (1) to experimentally verify the relative effects of stubble management and weed control on both soil water and nitrogen accumulation; and (2) to validate and apply a biophysical model to analyse the effect of summer fallow weed on soil water storage. In order to overcome season-specific factors over-riding the conclusions from the experimental data, the biophysical simulation model is used to extend the findings across multiple seasons.

2. Materials and methods

13

vested at the end of November 2014, the stubble was removed and left fallow. On 30 April 2015, the plots were sprayed with herbicide to kill the weeds and four treatments, stubble/stubble free and irrigation/nonirrigated, were applied as factorial design with six replications. Stubble was applied at a rate of 5 t ha−1 . The irrigation treatments were to see the effect of rainfall on fallow storage efficiency. For the irrigated treatments, three irrigations were applied, two in May and one in October. Soil water content was measured, at two-three week intervals at six depths (15, 30, 45, 60, 90 and 120 cm) using neutron moisture meter calibrated at the site. Soil water tension was measured with gypsum blocks fitted with data loggers recording soil water tension every 2 h at four depths 30, 50, 75 and 105 cm. Daily recording of weather data during the experimental period was obtained from the bureau of meteorology weather station located near the site. There was a high amount of rainfall (102 mm) between 7 October 2015 and 20 November 2015 which brought the water content of all the treatments almost to the same level and triggered a large amount of weed growth. On 20 November 2015, neutron probe was read and the treatments commenced. While weed was left to grow on half of the plots, on the remaining half it was completely controlled using herbicide and manual weeding. Thus, as of 20 November 2015, the experimental design was a factorial comprising two levels of stubble (nil, 5 t ha−1 ) and two levels of weeding (nil, weeded), with six replications. The most abundant weed in the experimental plots was witchgrass (Panicum capillare) which has a fibrous and shallow root system. Fleabane (Conyza spp.) was the second most abundant weed. Heliotrope (Heliotropium europaeum) was the third abundant weed. Established heliotrope has a well developed tap root and will grow under conditions dry enough to wilt-out most other plants (Hunt et al., 2009). The other weed species were wireweed (Polygonum aviculare) and cathead (Tribulus terrestris) both of which have long tap root systems. Summer-fallow weed density was estimated by identifying and counting all weeds within three 1 m by 2 m quadrates in each plot. To determine the dry biomass of the weeds, biomass cuts of 2 m2 were taken and dried in a dehydrator (70 ◦ C) for 48 h. Weed ground cover was mea® sured using GreenSeeker (NTech Industries Inc., Ukiah, CA, USA), a handheld tool that determines Normalized Difference Vegetative Index (NDVI). The Greenseeker canopy reflectance sensor was used to measure the proportion of green-coloured material in the field against the background of exposed soil as described by Holzapfel et al. (2009). To determine the amount of mineral nitrogen in the soil, on 31 March 2016, 12 soil samples (4 treatments and 3 replications) were taken from 0 to 60 cm depth (composite). The soil was oven dried (70 ◦ C) for 24 h, clods broken, thoroughly mixed and passed through 2 mm sieve. The effects of the treatments on all data were analysed using analysis of variance (ANOVA) in R (R Core Development Team, 2015).

2.1. Field experiment 2.2. Modeling – validation and application This study involves two phases: field experiment and computer simulation. The field experiment was conducted at Wagga Wagga, NSW in Australia during the 2015-16 seasons. The soil of the experimental site is a sandy clay loam Red Kandosol with good drainage characteristics (Isbell, 2002). The soil hydrologic characteristics of the Red Kandosol soil are as described in Zeleke et al. (2014b). The mean bulk density, drained lower limit (DLL) (also called wilting point) and drained upper limit (DUL) (also called field capacity) of this soil (0–120 cm) are 1.46 g cm−3 , 0.185 cm3 cm−3 and 0.277 cm3 cm−3 . The experimental plots were fitted with a drip irrigation system, neutron probe access tubes for soil water content measurement, and gypsum blocks for soil water tension measurement. The experimental plots were sown to wheat during the 2014 winter cropping season (May–November). After wheat was har-

The biophysical model APSIM (Agricultural Production Systems sIMulator) (Holzworth et al., 2014) was used to simulate soil water balance. The performance of APSIM in simulating the soil water dynamics under bare and cropped conditions has already been tested in the area (Zeleke et al., 2014a). Here only the validation of APSIM in simulating soil water balance in weed and weed plus stubble conditions is presented. The soil water balance in weedy, weed free, stubble, stubble free conditions was simulated during the 2015-16 spring-summer-autumn period (20 November 2015–31 March 2016). Statistical performance parameters (RMSE, NRMSE, R2 ) were used to validate the model. The SILO patched point climate dataset of the Wagga Wagga Agricultural station was used (Jeffrey et al., 2001). In the stubble plots, 4 t ha−1 wheat stubble

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K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

was used. This is on the assumption that of the 5 t ha−1 applied in April 2015, 1 t ha−1 would be lost in different ways (eg. degradation). The starting moisture content in the simulation was set to be the one measured on 20 November 2015. The weed module in APSIM simulated the growth of weed in response to climatic parameters temperature, rainfall and radiation. The soil water extraction parameters in APSIM (CLL: crop lower limit; kl: maximum extraction rate; xf: exploration factor) were adjusted for weed based on the observed soil water change during the season. The CLL, for example, may differ for different crops due to differences in root density, root depth, crop demand and duration of crop growth. The following parameters were used at each of the six depths for CLL (0.110, 0.130, 0.150, 0.170, 0.190, 0.220), kl (0.06, 0.06, 0.04, 0.03, 0.02, 0.01) and xf (1, 1, 1, 0.7, 0.2, 0.1). The agreement between measured and simulated grain yield was evaluated using the root mean square error (RMSE), coefficient of determination (R2 ), and normalised root mean square error (NRMSE) as

  n 1 2 RMSE =  (Mi − Si ) n

(1)

i=1 n 

R2 = 1 −

(Si − Mi )

i=1

n  

M−M

2

2

(2)

i=1

NRMSE =

RMSE

∗ 100

(3)

Mi Where Mi and Si represent measured/observed and simulated values of the respective variable, n is the number of values of a give variable, M is the mean of the measured/observed values. RMSE has the same unit as that of the variable being simulated. The closer the value of RMSE is to zero and the closer the value of R2 is to unity, the better the model simulation. The validated model was used to simulate the soil water content and predict the effect of weed on plant available water (PAW). Similar to the experimental treatments, APSIM was used to predict the PAW on 31 March of each year for 116 years (1901–2016). The simulation was initialised and reset on 1 April of each year to remove the effect of previous year. For the weed scenario, summer weed was allowed to grow from the beginning of November and for the weed-free scenario weed was assumed to be completely controlled/no weed growth. From the simulated soil water content of 31 March, PAW was determined by subtracting the DUL of the Red Kandosol soil over the 1.2 m depth. Then the PAW was arranged in a descending order in order to determine the exceedance probability of different PAW levels. 3. Results and discussion 3.1. Effect of stubble on winter fallow soil moisture Fig. 1 shows the dynamics of soil water content during the winter fallow period (30 April–20 November 2015). At the start of the monitoring period (30 April 2015), the soil profile water content of all four treatments was similar. By the next measurement (21 May 2015), the soil water content of the irrigated plots had increased substantially due to the 52 mm irrigation in early May, except at the deepest monitoring depth (120 cm). The soil water content of the unirrigated plots did not change. The rainfall in June brought the irrigated plots close to full capacity, and also increased the water

content of the unirrigated plots, especially at shallow depths. By the middle of August, the soil profile of the unirrigated plots also filled to a level which was not different from that of the irrigated plots. The soil profiles of the irrigated plots, which were already filled by mid June, must have drained water below the rootzone depth during the winter months. The presence of stubble did not alter this soil profile water content during the winter months (Fig. 1a–d). Since evaporation was not an important factor during the winter period, stubble did not have an effect on soil water storage. Although two irrigations, 52 mm in May and 26 mm in October, were applied to the irrigated plots, there was no difference in the soil water contents of the treatments at the end of the winter/autumn period (20 November 2015). The mean soil water content at the start of the winter crop sowing period (30 April 2015) was 0.219 cm3 cm−3 . The mean soil water content of 0.272 cm3 cm−3 on 20 November 2015 indicated an increase of 24% relative to that of the starting soil water content on 30 April 2015. This increase indicated that autumn-winter was the main period of soil water recharge. The fact that there was no significant difference between the treatments shows that stubble did not affect winter period soil water storage. Autumn rain also did not affect winter period soil water storage, as both irrigated and unirrigated plots had similar soil moisture content by mid September. High autumn rainfall would not change the eventual amount of soil water stored at the end of the winter period, as the soil profile would be recharged during the winter period. Rather, if the soil profile were filled as a result of autumn rainfall, it would result in deep drainage during the winter period. A 42 years simulation study in southern NSW found that long fallowing is associated with more drainage (5–79 mm) within the fallow period than a short fallow system (Fischer et al., 1990). After mid September, the soil water content did not increase as the soil had already reached field capacity. As the air temperature started to increase in spring, the soil water storage started to decrease, mainly in the surface layers. The decrease was much greater in bare plots than in the stubble covered plots. This can be seen from the significant difference between stubble covered and bare fallow plots on 20 November (Fig. 1). Fig. 2 shows that by 20 November (i.e., the last data point), the soil water content of the two bare plots was the lowest, while that of the two stubble covered plots was the highest. This was despite the soil water content of all the treatments being almost equal on 7 October. This is consistent with the earlier report that stubble affected soil water conservation only if the near-surface soil was wet, or if there was frequent rainfall (Zeleke et al., 2014a). 3.2. Effect of stubble and weed on summer fallow soil moisture From Fig. 3a and c it can be seen that the soil profile water content of the weed free plots changed only slightly irrespective of the stubble cover. However, the soil water content of the weedy plots (Fig. 3b and d) decreased substantially. A similar pattern was observed in Fig. 4 which showed the variation in plant available water (PAW). The decrease was especially pronounced at 30, 45, and 60 cm depths, where the majority of the roots were expected to be concentrated (Kirkegaard et al., 2007a). At 30 cm depth in the weedy plot, the soil water content decreased from 0.293 cm3 cm−3 on 20 November 2015 to 0.175 cm3 cm−3 on 31 March 2016, which was a 40% decrease. Moreover, the variation in soil water content at 90 cm depth showed the effect of weeds even at this depth. The increase in soil water content for the measurements made on 08 December 2016 was due to high amount of rainfall in the preceding days. The weed free plots had significantly higher soil water content at 15 cm and 30 cm depths for the 20 November 2015 and 04 December 2015 measurements. At deeper depths there was

K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

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Fig. 1. Soil water content at different depths and times during the winter fallow period (30 April 2015–20 November 2015) under irrigated/unirrigated and stubble covered/stubble free treatments.

Fig. 2. (a) Soil water content in the root zone (0–120 cm) and the cumulative water (rainfall and irrigation) (b) change in soil water content during the winter fallow period (30 April – 20 November 2015) relative to the value at the start of the monitoring period. The arrows indicate timing of irrigation.

no significant difference yet. This could be due to the hydraulic conductivity of the soil being too low to permit sufficient water movement to the plant roots. Furthermore, the stubble covered weedy plots had slightly higher soil moisture content than only-

weed plots especially during January, February and March. This could be due to the suppression of weed growth by the stubble. The decrease in moisture content from the 20 November 2015 to the 31 March 2016 was 5% for the weed free – stubble covered,

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K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

Fig. 3. (a) Soil water content in the root zone (0–120 cm) at different times during the summer fallow period (20 November 2015–31 March 2016) under with/without stubble cover and weed treatments.

Fig. 4. Plant available water in the 120 cm soil depth at different measurement times during the fallow period (20 November 2015–31 March 2016) under with/without stubble cover and weed treatments. Standard error bars and cumulative rainfall during the summer fallow period are also shown.

7% for weed free – stubble free, 20% for the weedy – stubble free and 16% for the weedy – stubble covered treatments. This showed the effect of weed on fallow period soil moisture. In the equiseasonal rainfall environment of central west NSW, Haskins and McMaster (2012) also demonstrated that the control of summer fallow weeds with herbicides and retained residues in a continuous crop sequence increased both soil water (0–53 mm) and N accumulation (32–57 kg ha−1 ). There was no significant change in soil water content in weed free plots, whether bare or stubble covered, while the soil water content in weedy plots was decreasing, with a sharp decrease in their soil water content during December and January. In late February and March, however, the soil water content started recovering due to reduced soil evaporation and drying weeds. In a study by Fernandez et al. (2008) which measured fallow soil water storage in 10 field trials, weed control was found to be more important than residue level. They did, however, also observe a strong interaction between residue level and weed control, with weeds being suppressed under high levels of residue (10 t ha−1 ). In this study, the weed density was 74 and 49 per square meter under stubble free and stubble covered treatments, respectively. This effect has been used as a tactic in ecological approaches to weed management (Anderson, 2005).

K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

Weed ground cover was measured three times: 9 December 2015, 11 February 2016 and 24 March 2016. The mean value for weedy – stubble covered treatments was 5.3%, 11.3% and 3.6%, respectively, at these three measurement times. For the weedy – stubble free treatment, these were 5.6%, 12.7% and 4.6%, respectively. However, statistical analysis showed that there was no significant difference between the treatments. The mean weed biomass was 1.78 t ha−1 . Fromm and Grieger (2002) found that summer fallow weed control with retained residue increased soil water stored at sowing by 6–21 mm depending on weed density and rainfall pattern. Weeds growing in summer fallows reduce the long-term water-use efficiency of paddocks by transpiring water and sequestering nitrogen that would otherwise be used for crop production. At the start of the summer fallow period (20 November 2015), there was insignificant difference between the water content of the soil profiles of the four treatments. Due to a high amount of spring rainfall, the soil profile was at field capacity. Although there was a slight change in soil moisture content at the shallowest monitoring depth (15 cm), overall, the soil profile for weed free treatments (both stubble free and stubble covered) did not change over the monitoring period. However, the soil moisture content of the weedy plots (both stubble free and stubble covered) decreased significantly, especially in the top 60 cm depth. By 18 December 2015, the soil moisture content of the weedy plots has decreased further by 15% from the values on 20 November 2015. Because of the high amount of rainfall between 25 January and 08 February, the soil moisture content of the weedy plots at 30 cm and 60 cm depths increased. Although the soil moisture content at these depths rebounded because of the rainfall, it quickly decreased again indicating the high water use by the weeds. However, by March, the decline in soil moisture content slowed, as evaporative demand decreased with weed drying and declining air temperature. Table 1 shows the statistical analysis of the effect of weed, stubble and their interaction on the soil water content. On 4 December 2015 the soil water content of the stubble covered – weed free treatments was significantly higher (P < 0.05) than the two treatments: weedy – stubble covered and weedy – stubble free plots. From 18 December 2015–31 March 2016, both weed free treatments had significantly higher soil water content than the weedy treatments. There was also significant difference between stubble covered – weedy and stubble free – weedy treatments. However, there was no difference between stubble covered – weed free and stubble free – weed free treatments. There was significant difference between the mineral nitrogen content of weed free (98 kg ha−1 ) and weedy treatments (39 kg ha−1 ). However, there was no significant effect of stubble (Table 2). In addition to what is shown in the above table, there was significant difference between the PAW of weedy – stubble covered and weedy – stubble free treatments on 25 January, 08 February, 26 February and 15 March, but no difference between the weed free – stubble covered and weed free – stubble free treatments. Consequently, stubble had effect only in the weedy treatments. PAW differed significantly between weedy – stubble covered (131 mm) and weedy – stubble free (108 mm) treatments on 08 February, and also on 26 February and 15 March. This might have been due to suppression of the weeds by the stubble which reduces the vigour

17

and number of weeds. By the end of the summer season (31 March 2016), the volumetric soil water contents (cm3 cm−3 ) were 0.265 (weed free – stubble covered), 0.255 (weed free – stubble free), 0.223 (weedy – stubble covered), and 0.214 (weedy – stubble free treatments). The difference was significant between the weedy (0.219) and weed free (0.260) treatments, but not between the stubble treatments. This corresponded to a decrease in soil water content of 5% for the weed free treatments and 24% for the weedy treatments. When weeds were present, the soil water profile was depleted. If a farmer were to choose to sow on a few mm of rainfall, and no subsequent rain fell, crop failure could result, as soil water was already depleted. These results show that long fallow had benefit on high PAW soils (e.g., clay loam) to a greater extent than on low PAW soils (e.g., shallow sand). This study showed that once the soil profile was full from the previous winter period rainfall, the presence or absence of stubble would not make difference. However, if the weeds were not controlled, this could have a substantial effect on the soil water and nitrogen, and the whole stored soil water profile could be depleted. The effect of weed was important at shallow depths (the top 30 cm). The transfer of soil moisture from one winter over the hot summer to the next autumn is the major advantage of long fallow. However, the growth of summer weed can deplete soil moisture even from deeper soil profile. If summer weeds establish themselves in a long fallow where there is already stored soil moisture, the rate of removal of moisture will be high. 3.3. Soil water tension as monitored by gypsum blocks Figs. 6 and 7 show the soil water tension (SWT) measured using gypsum blocks at four depths. In autumn, the unirrigated plots had high soil water tension. However, it decreased during the winter period. The irrigated treatments had low soil water tension in autumn and winter periods. During the winter months, the soil water tension was the lowest (about 50 kPa) and did not change in any irrigated treatment. From Fig. 6 it can be seen that the soil water tension of the weed free treatment remained low during the summer months as well. In contrast, Fig. 5 shows the interesting dynamics of soil water tension in the weedy plots. For the weedy plots, soil water tension at the 30 cm depth started increasing by 23 November 2015, and 3 weeks later (14 December 2015), SWT reached 500 kPa (the highest that could be recorded by the data logger). However, SWT bounced back again to 66 kPa by 26 December 2015 due to the consecutive rainfall events in the preceding week. SWT dropped again to the 500 kPa level by the end of the first week of January 2016. In the period 30 January 2016–04 February 2016 SWT decreased again to about 90 kPa. By 13 February 2016, SWT again increased to the 500 kPa level. These fluctuations are in response to the rainfall during these periods. Fig. 6 shows that, although the soil in the top layer was rewetted by rainfall, it quickly dried again due to the active weed root system and weed water use. As the season progressed and the roots deepened, the lower soil depths also started increasing in SWT. The deepest depth started responding only later in the season, while the shallow blocks responded to the rainfall events quite readily. The soil water tension at the 50 cm depth also started increasing by mid-December

Table 1 Statistical analysis of the effect of fallow period treatments on plant available water. Factor

Weed Stubble Weed × Stubble

Dates of measurement 20Nov

04Dec

18Dec

05Jan

25Jan

08Feb

26Feb

15Mar

31Mar

* ns ns

*** ns ns

*** ns ns

*** ns ns

*** * ns

*** ** ns

*** * ns

*** * ns

*** * ns

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K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

Table 2 Mineral nitrogen content of the soil at the end of the fallow period under different weed and stubble cover treatments. Treatments

Mineral nitrogen (kg ha−2 )

Weed free – Stubble free Weed free – Stubble covered Weedy – Stubble free Weedy – Stubble covered

125a 71ab 44b 34b

(16 December 2015), three weeks after the 30 cm depth started increasing. SWT increased rapidly in the second half of January 2016 and reached the 500 kPa mark by the end of January 2016. Unlike the shallow depth (30 cm), SWT did not decrease again, but stayed at the 500 kPa level for the remainder of the monitoring period. By the end of the first week of January 2016, the tension of the gypsum block at the 75 cm depth also started increasing. However, SWT reached the highest 500 kPa level only after mid March. SWT decreased sharply in late January 2016 and early February 2016 but decreased only slowly in late February 2016and March 2016. The fact that the gypsum block at the 75 cm depth was also increasing indicated that there were active plant roots present, although not as many as in the top 60 cm. The soil water suction at the 105 cm depth did not show change (30 kPa) for the most part of the monitoring period. But SWT started decreasing in late February 2016 and by the end of the monitoring period (31 March 2016), SWT had reached 200 kPa while all the shallow depth blocks were at or above the 500 kPa level. For the 75 cm depth during 04–07 December 2015, the soil water tension increased sharply during the day and remained stable, and in some cases, even decreased during the night. Although not clear from these Figures, a closer look at these fluctuations in the gypsum block readings showed that it was related to day time temperature; showing fast decline during hot days. The NDVI reading on 9 December 2015 showed that the weeds were drying, but still taking up soil moisture from deeper layers. 3.4. Effect of weeds and stubble on summer fallow soil nitrogen At the end of the fallow period (31 March 2016), the total nitrogen (ammonium and nitrate) in the top 60 cm of the soil profile was found to be 39 kg ha−1 and 98 kg ha−1 in the weedy and weed free plots, respectively, which is a significant difference

(59 kg ha−1 , at P < 0.05). However, there was no significant difference between the stubble covered and stubble free treatments, and the interaction was also not significant, which was comparable with other studies. Osten et al. (2006) conducted experiments in the summer-dominant (Emerald, Queensland), equi-seasonal (Wagga Wagga, New South Wales), and winter-dominant (Merredin, Western Australia) rainfall environments and observed negative effect of weed biomass on soil N accumulation. In a study conducted on two soil types in the Mallee region of Victoria, Hunt et al. (2013) found that weeds reduced mineral N by 44 kg ha−1 . Although, any weed level consumes some amount of the nitrogen mineralised during the fallow period, the effect depends on the weed biomass. In this study the mean weed biomass on the weedy plots was 1.8 t ha−1 while the weed biomass on the weedy − stubble covered plots was 1.5 t ha−1 . The conservation of water and nitrogen during the summer fallow period was recognised to be interrelated, as water was required to capture the benefits of the extra nitrogen and vice versa (Sadras et al., 2012). Summer fallow weeds reduce mineral N by drying the soil and reducing N mineralisation (Angus et al., 1998). As a result, the additional nitrogen benefit can be due to increased mineralisation in a wetter soil as well as reduced nitrogen loss through uptake by summer weeds. 3.5. Validation and application of APSIM in simulating fallow period soil water content APSIM was used to simulate soil moisture dynamics in weedy (with and without stubble) and weed free (with and without stubble) treatments during the 2015–16 fallow period. The simulated soil water content was compared against the measured values in the 1.2 m soil depth. The RMSE values of the stored soil water content were: stubble covered – weed free (12.6 mm), stubble free – weed free (17.6 mm), stubble free – weedy (8.9 mm), and stubble covered – weedy treatments (15.2 mm). The corresponding NRMSE of the treatments were: stubble covered – weed free (3.9%), stubble free – weed free (5.6%), stubble free – weedy (3.3%), and stubble covered – weedy treatments (5.3%). APSIM has already been validated for stubble covered and stubble free summer fallow conditions at this site (Zeleke et al., 2014a,b). Therefore, regression analysis of the soil water contents of only the two weedy treatments, stubble covered and stubble free, were presented in Fig. 7. These results show that APSIM has accurately simulated the soil water content

Fig. 5. Soil water tension at different depths in the weedy – stubble free plot at different times during the fallow period (April 2015–November 2016). Daily rainfall events and amounts during this period is also shown.

K.T. Zeleke / Agricultural Water Management 186 (2017) 12–20

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Fig. 6. Soil water tension at different depths in the weed free plots (a) dry plot during the winter fallow period (b) irrigated plot during the fallow period.

Fig. 7. Regression of measured and simulated soil water content during the fallow period in the stubble covered and stubble free weedy plots. Filled markers (䊉) are for stubble covered weedy plots while stubble free weedy plots are shown by unfilled markers ().

during the fallow period under weedy conditions with R2 of 0.93. This shows that APSIM can be used to analyse the effect of weed and weed control on fallow period soil water storage. As discussed earlier, there were different types of weeds on the experimental plot. Some of the weeds such as witchgrass are shallow-rooted and some such as heliotrope are deep-rooted. However, in APSIM database only summer grass, which is shallow-rooted, was available as an option for simulation. That might be why the model overestimated some of the measured soil water content values. A simulation experiment was carried out to see year to year variability of stored soil moisture as a result of summer weeds and stubble in a long fallow. The validated model APSIM was used to simulate 116 years (1901–2016) soil water dynamics under weedy and weed free environments. This is presented in Fig. 8 as exceedance probability. It can be seen that weed had a significant effect on plant available water in any given year. If there were no weeds during the summer fallow, there would be a high probability of having the soil profile filled during the winter period, ready for winter crop sowing. There was an 88% probability that a PAW of 140 mm or more would be available, if the land were weed-free. However, if weeds were present, there would be only a 13% probability of exceedance. In a long fallow, controlling summer weed is

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Fig. 8. Exceedance probability of the plant available water on 31 March in a Red Kandosol soil (based on simulation analysis of 1901–2016 weather data).

essential for a full plant available water profile in the next season. If the summer weed is not controlled, most of the soil water stored during the winter can be taken up by the weeds. This simulation was done for a single weed density. For alternative weed pressures, the results would vary, but the principles remain. 4. Conclusion Controlling summer weeds is the single most effective way to maximise fallow efficiency. This is because weeds extract moisture at depth and also take up nutrients. Summer weed control conserves both stored moisture and nitrogen. In comparison to controlling summer weeds, retaining stubble had little impact on conserving summer rainfall. Because the weed free treatments did not show significant change in soil water content and soil water tension, (whether stubble free and stubbled covered), weed control is the prime determinant of soil water conservation under long fallow. Acknowledgements Graeme Poile for the laboratory analysis of the soil nitrogen, Deirdre Lemerle and Goeffrey Burrows for weed identification, and Len Wade for reviewing the presentation of this manuscript are acknowledged. References Anderson, R.L., 2005. A multi-tactic approach to manage weed population dynamics in crop rotations. Agron. J. 97, 1579–1583. Angus, J.F., van Herwaarden, A.F., Fischer, R.A., Howe, G.N., Heenan, D.P., 1998. The source of mineral nitrogen for cereals in south-eastern Australia. Aust. J. Agric. Res. 49, 511–522, http://dx.doi.org/10.1071/A97125. Browne, C., Jones, B., 2008. How important is straw for yield of no-till crops on heavy soils in the low-rainfall southern Mallee? In ‘Global issues, paddock action. In: Adelaide Aust, S. (Ed.), Proceedings of 14th Australian Agronomy Conference’. 21–25 September 2008. MUnkovich. Australian Society of Agronomy/The Regional Institute Ltd, Gosford, NSW. Fernandez, R., Quiroga, A., Noellemeyer, E., Funaro, D., Montoya, J., Hitzmann, B., Peinemann, N., 2008. A study of the effect of interaction between site-specific conditions, residue cover and weed control on water storage during fallow. Agric. Water Manage. 95, 1028–1040. Fischer, R.A., Armstrong, J.S., Stapper, M., 1990. Simulation of soil water storage and sowing day probabilities with fallow and no-fallow in Southern New South Wales. I. Model and long term mean effects. Agric. Syst. 33, 215–240. Freebairn, D.M., Boughton, W.C., 1981. Surface runoff experiments on the eastern Darling Downs. Aust. J. Soil Res. 19, 133–146, http://dx.doi.org/10.1071/ SR9810133.

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