Effect of pasture improvement managements on physical properties and water content dynamics of a volcanic ash soil in southern Chile

Soil & Tillage Research 178 (2018) 55–64

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Effect of pasture improvement managements on physical properties and water content dynamics of a volcanic ash soil in southern Chile

T

Iván Ordóñeza,b,c, Ignacio F. Lópezb, Peter D. Kempb, Constanza A. Descalzic,d, Rainer Horne, ⁎ Felipe Zúñigac,d, Dorota Decc,f, José Dörnerc,f, a

Magister en Ciencias Mención Producción Animal, Escuela de Graduados, Facultad de Ciencias Agrarias, Universidad Austral de Chile, Valdivia, Chile Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand c Centro de Investigación en Suelos Volcánicos, Universidad Austral de Chile, Valdivia, Chile d Doctorado en Ciencias Agrarias, Escuela de Graduados, Facultad de Ciencias Agrarias, Universidad Austral de Chile, Valdivia, Chile e Institute for Plant Nutrition and Soil Science, Christian Albrechts University zu Kiel, Hermann Rodewaldstr. 2, 24118, Kiel, Germany f Instituto de Ingeniería Agraria y Suelos, Facultad de Ciencias Agrarias, Universidad Austral de Chile, Valdivia, Chile b

A R T I C L E I N F O

A B S T R A C T

Keywords: Andisol Pasture management Soil physical quality Degraded grasslands

The pastures in Chile are sustained on volcanic ash soils covering an area of 1,340,000 ha. Since 44% of these pastures are degraded, different strategies to improve these prairies have been implemented. This study examined the impact of the different pasture improvement managements (PIMs) on soil physical properties, water content dynamics and pasture productivity of a volcanic ash soil under sheep grazing. The experiment was established on a Duric Hapludand and considered four types of PIMs (fertilised-naturalised pasture, cultivated pasture, direct-drilled pasture, diverse-direct drilled pasture) including the initial situation (non-fertilised naturalised pasture). The effect of PIMs and grazing events on bulk density (BD), plant available water (PAW), air permeability (ka), saturated hydraulic conductivity (Ks) and pre-compression stress (Pc) in the topsoil (0–10 cm) was determined taking undisturbed soil samples, whereas the penetration resistance (PR), field air permeability (kl field) and herbage mass production were measured in the field. The volumetric water content, matrix potential and soil temperature were continuously registered at different depths. The fertilisation of degraded naturalised pastures, without soil structure disturbance, improved the pasture yield (140%), reaching values comparable to those improved with conventional systems. In the short term, the volume of macropores does not change significantly as a function PIMs. However, tilled soils presented less connected pores compared to the non-cultivated PIMs. The conservation of soil structure plays an important role in water accessibility by plants, so that fertilised-naturalised pastures were able to absorb water to depths of up to 60 cm. Compared to the improved pastures, the degraded non-fertilised pasture presented the lowest above ground herbage biomass as well as negative effects on soil physical properties (e.g. Pc increased by 57% and lower physical resilience) after grazing events.

1. Introduction

ha−1) and with a low nutritive value (Siebald et al., 2000). The pasture improvement managements (PIMs) mostly used directly on the increase of the low dry matter production of these degraded pastures in southern Chile are: i) the improvement of the chemical soil condition and grazing management; ii) sowing high production species through zero-tillage techniques plus an improvement of soil chemical conditions, and iii) new pasture establishment by traditional soil tillage, sowing high production species plus an improvement of soil chemical conditions (Cuevas, 1980; Balocchi and López, 1994; Chauveau et al., 2015; Zúñiga et al., 2015).

Sheep, beef and dairy cattle farming in southern Chile are based on grazing systems located mainly in the zone of volcanic ash soils (Balocchi, 2002). These pastures in Chile cover an area of 1,340,000 ha. However, 44% (594,000 ha) of these pastures are degraded or present low levels of productivity (INE, 2007), consequently different strategies to improve these prairies have been implemented (Zúñiga et al., 2015). The latter is a relevant topic in southern Chile, since naturalised pastures produce low annual accumulated herbage (less than 5000 kg DM

⁎ Corresponding author at: Instituto de Ingeniería Agraria y Suelos, Facultad de Ciencias Agrarias, Centro de Investigación en Suelos Volcánicos, Universidad Austral de Chile, Valdivia, Chile. E-mail address: [email protected] (J. Dörner).

https://doi.org/10.1016/j.still.2017.11.013 Received 26 May 2017; Received in revised form 6 November 2017; Accepted 20 November 2017 0167-1987/ © 2017 Elsevier B.V. All rights reserved.

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It is well known that different tillage intensities generate different changes in soil physical quality (Strudley et al., 2008; Dörner et al., 2013a, 2013b; Ivelic-Sáez et al., 2015; Zúñiga et al., 2015). The more intensive the intervention of the soil structure, the more drastic the changes in pore functions will be, in comparison with the original status of the soil (Schwen et al., 2011a). Traditional tillage implies a destruction of soil aggregates, and then, soil structure formation processes will occur depending on wetting/drying cycles, cementation of the particles of the soil, and the stabilisation of the structure by the biological factors made by soil macro, meso and microfauna (Hartge, 2000; Bronick and Lal, 2005). This kind of tillage negatively affects the continuity of the functional pores of the soil (Dörner and Dec, 2007; Dörner et al., 2013b; Zúñiga et al., 2015) and generates a change in the relation between macro, meso and micropores by increasing total porosity but destroying the pore continuity at the same time (Reszkowska et al., 2011a; Zúñiga et al., 2015). On the other hand, less intensive tillage systems, like zero-tillage, maintain the soil structure, the architecture and the pore size distribution (Zúñiga et al., 2015) allowing preferential fluxes of water as well as a minor resistance to root growth and exploration of deeper soil depths (Xua and Mermoud, 2003; Uteau et al., 2013). There is wide scientific knowledge related to the impact of traditional, zero and reduced tillage on mineral soils (Reynolds et al., 1995; Angulo-Jaramillo et al., 2000; Neves et al., 2003; Xua and Mermoud, 2003; Zibilske and Bradford, 2007; Strudley et al., 2008; Abid and Lal, 2009; Schwen et al., 2011a, 2011b), however, this does not hold true for soils derived from volcanic ashes (Dörner et al., 2012; Ivelic-Sáez et al., 2015; Zúñiga et al., 2015), which normally have very low bulk densities (< 0.9 Mg m−3) and excellent physical properties (WRB, 2006). Dörner et al. (2013a) pointed out that the impact of tillage and of different botanical composition at these andic soils result in very different soil physical properties which also affect their relationships to soil water content dynamics. Considering that the physical properties of volcanic ash soils are specific and that animals graze pastures throughout the whole year in southern Chile (Dec et al., 2012), it is necessary to understand how the soil responds after the implementation of different pasture improvement managements (PIMs) under grazing conditions. The latter is relevant in order to: i) evaluate the interaction between the different managements (e.g. different kind of tillage and pastures) on soil-plant-water relationships, and, ii) determine how these different managements systems may affect the physical properties of the soil in the medium- and long-term. Therefore, the aim of this research was to analyse the impact of the implementation of different pasture improvement managements (PIMs) on soil physical properties, water content dynamics and pasture productivity of a volcanic ash soil under grazing.

Fig. 1. Temperatures and rainfalls between December of 2012 and January of 2016.

5.6, organic matter levels of 13.1% as well as 8.6 mg/kg of Polsen and aluminium saturation of 8.6%. Detailed information about the physical and chemical properties of the soil series under different land uses and soil management intensities can be found in Dörner et al. (2013a, 2013b, 2015); Zúñiga et al., (2015). The experiment considered five types of pastures, including the initial situation, defined as ‘non-fertilised naturalised pasture (NFNP)’. This pasture was spontaneously growing in the utilised site and was not sown neither fertilised nor limed. The other four pastures corresponded to pasture improvement managements (PIMs, Table 1) that farmers can use in southern Chile: 1) fertilised naturalised pasture (FNP) without tillage treatment: the initial naturalised pasture was improved through fertiliser addition and liming to upgrade soil pH conditions. Fertiliser and lime were applied over the current pasture (improvement of the initial situation, NFNP) without tillage; 2) cultivated pasture (CP): the initial pasture was eliminated through two consecutive applications of glyphosate (3 weeks apart), after which the soil was ploughed, harrowed and a L. perenne and T. repens pasture was sown; 3) direct drill pasture (DP), zero-tillage treatment: the initial naturalised pasture received two consecutive glyphosate applications, as it was performed for CP, afterwards a L. perenne and T. repens pasture was direct drilled; 4) diverse direct drill pasture (DDP), zero-tillage treatment: the initial naturalised pasture was eliminated by herbicide application, as it was done for CP, subsequently a B. valdivianus, L. perenne, D. glomerata, H. lanatus and T. repens pasture was direct drilled. All the pasture treatments, except by NFNP, were annually limed and fertilised as follows: 180 kg nitrogen ha−1 year−1 (Nitromag 21%N); 52.3 kg phosphorus ha−1 year−1 (triple superphosphate, 20% P), 99.6 kg potassium ha−1 year−1 (potassium chloride, 60% K+) and 800 kg calcium ha−1 year−1 (lime). The establishment of the PIMs were conducted in April 2013, when the soil water content was close to field capacity (soil water content ≤ 40%). The botanical composition was performed according to Grant (1981) and measured before the PIMs were implemented (March 2013) as well as during spring of 2014 as indicated in Table 1 (more information about this parameter can be found in Descalzi, 2017). The pastures were sown on April 17th 2013 and then were grazed by 25 sheep per plot (equivalent to 625 sheep ha−1). The grazing criteria were sheep introduced at 2100–2300 kg DM ha−1 and removed at 1000–1200 kg DM ha−1 (Parga et al., 2007; Flores et al., 2017).

2. Material and methods 2.1. Geographic and treatment descriptions The experimental field was located at the Universidad Austral de Chile’s Estación Experimental Agropecuaria Austral (EEAA) (39°46′ S, 73°13′ W, 12 m a.s.l.) in Valdivia, Chile. The average annual temperature is 12 °C and yearly precipitation is between 1901 and 2005 mm, with a 2440 mm mean (González-Reyes and Muñoz, 2013), but with a major concentration of the rainfall in winter (Huber, 1970), as well as, a well-defined reduction in the rainfall in recent years (González-Reyes and Muñoz, 2013). Rainfall and average daily air temperature during the experiment (March 2013 till December 2015) were collected at the INIA station, located 40 km north from the study site (Fig. 1). The soil is derived from volcanic ashes, classified as a Duric Hapludand (Valdivia Series according to CIREN, 2003) with a profile that can reach 3 m depth. The topography is complex, with dominant slopes of 3 to 8%, and sectors that are slightly curved from 2 to 5%. At the study site the slope was less than 2%. The soil presented pHwater of

2.2. Soil physical parameters, water content dynamics and dry matter production: soil sampling and field measurements In order to determine the effect of the different pasture improvement managements (PIMs) on soil hydraulic properties, undisturbed soil samples were collected in the topsoil (2–8 cm depth) in September 2013 by using metallic cylinders (230 cm−3, with h = 5.60 cm and Ø = 7.15 cm). Additionally, samples were also collected before and 56

57

DP

DDP

Diverse direct-drilled pasture

FNP

Fertilised naturalised pasture

Direct-drilled pasture

NFNP

Acronym

Non-fertilised naturalised pasture

Before the PIMs

Treatments

Zero-tillage

Zero-tillage

Without tillage

Without tillage

Tillage

Lolium perenne L. cv. Rohan; Trifolium repens L. cv. Weka; Bromus valdivianus Phil.; Dactylis glomerata L. cv. Safin and Holcus lanatus

Lolium perenne L. cv. Rohan and Trifolium repens L. cv. Weka.

None

None

Sown species

T. repens: 4 kg ha−1 B. valdivianus: 16 kg ha−1 H. lanatus: 1 kg ha−1 D. glomerata: 3 kg ha−1

L. perenne: 25 kg ha−1 T. repens: 4 kg ha−1 L. perenne: 25 kg ha−1

L. perenne: 25 kg ha−1 T. repens: 4 kg ha−1

None

None

Sowing rate

Table 1 Pasture improvement managements (PIM’s) of a degraded pasture and botanical composition in an Andisol in southern Chile.

L. perenne (12.1%); H. lanatus (26.2%); Other grasses (50.9%) and broad leaf (10.3%)

Initial botanical composition

L. perenne (28.1%); H. lanatus (7.9%); T. repens (6.8%); B. valdivinaus (26.1%); D. glomerata (8.9%); other grasses (13.2%) and broad leaf (9.1%)

L. perenne (68%); H. lanatus (1.5%); T. repens (7.2%); B. valdivinaus (0.8%); other grasses (20.4%) and broad leaf (2.0%)

L. perenne (1,28%); H. lanatus (8.2%); T. repens (6.2%); B. valdivianus (0.05%); other grasses (49.1%) and broad leaf (35.2%) L. perenne (24.2%); H. lanatus (13.5%); T. repens (15.5%); other grasses (32.3%) and broad leaf (14.5%)

Botanical composition (spring 2014)

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after the first grazing, to evaluate the effect of tillage and grazing, respectively. The saturated hydraulic conductivity (Ks, n = 21 per treatment) and water retention curves (WRC, n = 12 per treatment) were determined. To measure the pre-compression stress (Pc; n = 9 per treatment), soil samples were taken in October 2014, with a cylinder of 120 cm3 (h = 3.00 cm, Ø = 7.15 cm). To determine the grazing effect, the sampling was undertaken before and after grazing. To evaluate the pore system under the different PIMs and grazing events, the following field analyses were performed: i) field air conductivity (kl field; Air Permeability PL-300, UGT GmbH, Germany), ii) penetration resistance (PR; Hand Penetrometer, Eijkelkamp Agrisearch Equipment, Giesbeek, The Netherlands) and iii) soil water content (SWC; HH2 Moisture Meter readout unit with WET-2 sensor, Delta-T devices, England). These measurements were conducted before and after every grazing between August 2013 – December 2014, with the exception of kl field, which was performed in all the treatments, before and after the first grazing. For both, PR and SWC there were 30 repetitions per treatment, which were registered, whereas 9 repetitions per treatment were carried out for the kl field. To evaluate the water content dynamics, data loggers and sensors (Decagon devices, USA) were installed in the field. For the temporal changes in the matric potential and volumetric water content MPS-2 (two repetitions per depth) and 5TM (two repetitions per depth) were installed at 10, 20 and 60 cm soil depth. All sensors were connected to a data logger (EM50, Decagon Devices, USA) in which the data were collected at 30 min intervals. The 5TM sensors were calibrated using undisturbed soils samples collected from the same field and soil depths. The calibration was conducted according to Dörner et al. (2010). The determination of accumulated herbage mass production was performed with the Rising Plate Meter (n = 300 per treatment) before and after every grazing until March 2015. The plate was calibrated according to the available pasture herbage mass following the methodology of Earle and Mc Gowann (1979). Each measurement was performed before and after every grazing.

used to evaluate the impact of PIMs on soil mechanical and hydraulic properties, as also conducted by Dörner et al. (2013b) and Zúñiga et al. (2015) for the same soil series. While capacity parameters define a general status of the soil (e.g. bulk density, pre-compression stress), intensity parameters quantify the functionality (e.g. air conductivity) of the pore system (Horn and Kutilek, 2009). To evaluate the soil capacity parameters, the bulk density (BD), pre-compression stress (Pc), air capacity (AC) and plant available water (PAW) were calculated as in Dörner et al. (2015). On the other hand, for the evaluation of soil intensity parameters, air permeability (ka), saturated hydraulic conductivity (Ks) and pore continuity indexes (C2) were used. The air permeability was registered during the determination of the WRC at a matric potential of −6 kPa according to Dörner and Horn (2006). The C2 pore continuity index was calculated from the relationship between air permeability and volume of air-filled porosity at a matric potential of −6 kPa (Groenevelt et al., 1984). 2.4. Statistical analyses The experiment was a complete randomised block design (5 treatments x 3 blocks). The normality of the data was evaluated using the Shapiro-Wilk test (p ≤ 0.05) and the homogeneity of variance by Levene test (p ≤ 0.05). If the data were not normally distributed, data were transformed with the natural logarithm or non-parametric statistics were used (box diagrams, mean, median, percentiles, and standard deviation). The Kruskal-Wallis’s test (p ≤ 0.05) was used to compare the means of the treatments (for not normal distribution). The data with normal distribution were analysed by analysis of variance (ANOVA), and the difference between treatments was tested with the Fisher’s least significance difference test (LSD) (p ≤ 0.05). 3. Results 3.1. Effects of the pasture improvement managements (PIMs) and grazing on soil penetration resistance, soil capacity and intensity parameters

2.3. Laboratory analysis Penetration resistance (PR) increased with decreasing SWC (y = −36.89× + 2963; R2 = 0.87; P < 0.001) and no statistical differences between treatments were observed (P > 0.05; Fig. 2). PR values ranged between 1.2–1.3 MPa (at an average SWC of 41.4%) and 2.9–3.0 MPa (by volumetric water contents ≤ 10%). When the soil reached field capacity (SWC 40% in Fig. 2), PR ranged between 1.343 and 1.520 MPa. Treatments without tillage (NFNP and FNP) and direct-drilled (or zero-tillage DP and DDP) treatments exhibited a similar behaviour with no statistical differences for all parameters (Fig. 3). Regarding capacity parameters, the traditional tillage treatment (CP) presented the lower

To determine the water retention curve (WRC), the samples (n = 12 for each treatment, before and after grazing) were saturated from beneath for 48 h and then equilibrated at matric potential values of −1, −2, −3, −6 (in sand tables), −15, −33, −50 kPa (in pressure chambers). The volume of fine pores was defined from the data provided by Zúñiga et al. (2015) for the same soil. To determine saturated hydraulic conductivity (Ks), the undisturbed soil samples (n = 21 for each treatment, before and after grazing) were saturated from beneath for 48 h. Thereafter, the samples were introduced into the water permeameter (Eijkelkamp, model 09.02.01.25, The Netherlands) and saturated again for 24 h. The Ks measurements were conducted 1, 3, 6, 12, 24 and 48 h after beginning the constant water flow through the soil samples. The saturated hydraulic conductivity was calculated using Darcy’s law (Klute and Dirksen, 1986). Finally, the samples were dried at 105 °C for 24 h (Hartge and Horn, 2009). To determine the consolidation curve, the samples (n = 9, before and after the grazing) were saturated from beneath for 48 h, and thereafter equilibrated at a matric potential value of −6 kPa (at sand tables). Using an odometer with free drainage (T-controls T303) the samples were mechanically stressed with a static loading of 1; 6.25; 12.5; 25; 50; 100 and 200 kPa for 6 min as conducted by Zúñiga et al. (2015) in the same soil. The unloading of the mechanical stress was realised in the following order 200; 50; 6.25 and 1 kPa. During the stress application, the vertical deformation of the soil was measured (0.001 mm of precision). The pre-compression stress (Pc) was calculated from the stress strain curve using the mathematical model proposed by Baumgartl and Köck (2004), which is based on the graphical method of Casagrande (1936). The capacity and intensity concept (Horn and Kutilek, 2009) was

Fig. 2. Penetration resistance (kPa) as a function of soil water content (Vol.%) for the different pasture improvements strategies (PIMs).

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Fig. 3. Tillage effect over both, capacity and intensity soil parameters. Bulk density (BD; n = 12), air capacity (AC; n = 12), air permeability (ka; n = 12) and field air conductivity (kl field; n = 9), before the first grazing, for the different pasture improvements strategies (PIMs). Bars indicate ± 1 error standard. Uppercase letters indicate statistical differences between treatments.

BD (p ≤ 0.01) and AC (p ≤ 0.001) compared with the zero-tillage treatments (Fig. 3A and B). According to results of intensity parameters, the air permeability of the traditional tillage treatment (CP) measured in the lab (ka; p ≤ 0.001) as well as in the field (kl field; p ≤ 0.001) presented the lowest values compared to the other PIMs. Regarding the grazing effect (Fig. 4), NFNP showed an increase of Pc (p ≤ 0.01) and a decrease in C2 index (p ≤ 0.001) as a consequence of animal trampling. Under NFNP and FNP treatments was there a decrease of PAW (p ≤ 0.05) after the grazing event. No other treatment exhibited statistical differences in the studied parameters due to grazing (p ≥ 0.05). Regarding CP, there was an increase of PAW (p ≤ 0.05), and decrease in Ks (p ≤ 0.001) and C2 index (p ≤ 0.001).

3.2. Water content dynamics and pasture productivity after the implementation of the pasture improvement managements (PIMs) Rainfalls between April-August 2013 (autumn and winter) were 1160 mm ( ± 138.2 mm differences between years), whereas September-December 2013 (spring) reached 350 mm ( ± 15.6 mm differences between years). Rainfalls between January-March 2014 (summer) were 251 mm, while in the same period during 2015, only 19 mm were collected (Fig. 1). The pasture growth distribution was quite similar between treatments with some seasonal differences also observed for the water content dynamics (Figs. 5 and 6). The cultivated pasture (CP) presented a greater growth in winter compared to the other treatments, reaching Fig. 4. Grazing Effect over both, capacity and intensity soil parameters. Plant available water (PAW; n = 12), Bearing capacity (Pc; n = 4), saturated hydraulic conductivity (Ks; n = 10) and C2 index (n = 12). Black columns indicate data before grazing and grey columns indicate data after grazing for the different pasture improvement strategies (PIMs). Bars indicate ± 1 error standard. Uppercase letters indicate statistical differences between treatments and lowercase letters indicate statistical differences because of the grazing.

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Fig. 5. Effect of the pasture improvement strategies (PIMs) over herbage dry weight (HDW) and soil water content (SWC) at different depths. NFNP (A); FNP (B); CP (C); DP (D) and DDP (E).

the maximum growth in this season (2015) of 2450 kg DM ha−1 (approximately 30 kg DM ha−1 day ± 2.3 kg). In spring, all pastures presented similar growth; however, in late spring (November − December) the higher growth was for FNP with herbage production of 4400 (approximately 105 kg DM ha−1 day−1) and 3300 kg DM ha−1 (approximately 64.5 kg DM ha−1 day−1) in spring of 2014 and 2015, respectively. Greater differences were detected between summer periods of 2014 and 2015 (Fig. 5). There was a significant reduction in the production of herbage mass growth per day between the summer of 2014 and 2015, with the exception of FNP, which had a production of 23.2 kg DM ha−1 ( ± 3.2 kg) in summer 2014 and 20.1 kg DM ha−1 day ( ± 2.7 kg) in summer 2015. On the other hand, the treatment that was most affected, between all the treatments under PIMs, was CP with a herbage mass growth per day of 12.4 kg DM ha−1 day ( ± 4.4 kg) summer 2014 and 2.5 kg DM ha−1 day ( ± 0.2) in summer of 2015. The accumulated herbage at summer 2015 for FNP was 2306 kg DM ha−1, CP was 714 kg DM ha−1, DP was 1088 kg DM ha−1, DDP was 1119 kg DM ha−1 (Fig. 6). The soil volumetric water contents were near saturation (< 6 hPa) between April − October for 10 cm depth, April–November for 20 cm and between May − November for 60 cm depth (data not shown). In general, the CP treatment at 10 cm depth, presented the lowest average

of SWC (indicating higher water uptake) reaching 16.3% ( ± 1.18% difference between summers). At 20 cm depth, a change in this trend was observed: FNP was the treatment with the lowest average of SWC between the summers of 2014–2016 with an average of 22.7% ( ± 1.26% difference between summers) and at 60 cm depth with a 24.3% ( ± 1.48% difference between summers). On the other hand, the average of SWC at 10 and 60 cm depths, during all the seasons were higher for the DDP treatment. If taken into account, the average of SWC of the driest summer (2015) at 10 cm was 14.7% and at 60 cm depth was 26.1%. Conversely, the maximum average of SWC at 20 cm was for CP treatment with 26.2% indicating the lowest water uptake. Due to the effect of the dry season in 2015, the depth dependent changes in SWC, matric potential and soil temperature for the day before the first rain in autumn (40.2 mm on 01/04/2015) are presented in Fig. 7. The matric potential values at 10, 20 and 60 cm soil depth were over the permanent wilting point, with the exception of: CP at 20 cm depth (-12,500 hPa); NFNP and CP at 60 cm (-13,600 and −13,800 hPa, respectively). The CP presented the lowest matric potential at 10 cm (-40,500 hPa) compared to the other treatments. While the matric potential increased towards the 20 cm depth for DP and CP, the opposite was observed for the other treatments. At 20 cm depth, the pastures with the lowest matric potential were the more diverse ones, Fig. 6. Seasonal changes and herbage dry weight (HDW, accumulated value for each season and pasture) by the different PIMs as affected by soil water content (SWC, mean values for each season) and the accumulated rainfall per season. Seasons are abbreviated as follows: S = summer; A = autumn; W = winter; Sp = spring.

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effect of tillage as well as of grazing events on the mechanical stability measured in the field (PR) were assessed (Fig. 2). It is well known that PR increases with decreasing SWC (Franzluebbers and Stuedemann, 2008; Dec et al., 2011), which is in agreement with our results. Furthermore, when regarding the physical means of PR values, it can be assessed that at field capacity (40% of SWC) none of the pastures reached PR values higher than 2000 kPa, which is considered as a critical value to indicate soil compaction problems (e.g. Horn and Fleige, 2009). We used capacity parameters to define the effect of PIMs on soil porosity and stability, whereas the intensity parameters have been used to reveal the impact of PIMs on soil pore organisation and functionality. Regarding soil capacity parameters, soil tillage normally induced a decrease of BD (Schwen et al., 2011a, 2011b), an increase of AC (Dörner et al., 2012) and a decrease in PAW (Abid and Lal, 2009); however, in the present study a decrease of BD accompanied with a decrease of AC and an increase of PAW under traditional tillage treatment (CP) was observed. These contradictory results, can be ascribed to rain events which occurred after tillage and which allowed the rearrangement of soil particles, changing the pore size distribution after soil tillage (Seguel and Horn, 2005; Dörner et al., 2012). Capacity parameters are less sensitive to changes in soil structure compared to intensity since they provide the status of the continuity and architecture of soil porosity (Horn and Kutilek, 2009; Uteau et al., 2013). Their sensitivity to soil structural changes has been assessed by several authors (Strudley et al., 2008; Dörner et al., 2013b; Ivelic-Sáez et al., 2015) and also proven in the present study (Figs. 3C,D, 4C,D) by the negative effects of soil tillage (CP treatment) on ka and kl field, Ks and C2 index due to the destruction of continuity of soil porosity (Reynolds et al., 1995; Schwen et al., 2011a). These results are also consistent with outcomes of several authors that underline the sensitivity of pore space at the macropore range (Gebhardt et al., 2009; Dörner et al., 2011; Reszkowska et al., 2011a, 2011b). In these terms, the zero-tillage treatments (DP and DDP) as well as those treatments without mechanical disturbance (NFNP and FNP) did not affect the C2 index (as well as Ks, ka and kl field) which means that soil pores remain stable and able to conduct fluids, highlighting the conservation of soil structure as well as the continuity and architecture of soil porosity. Finally, unclear effects of grazing on soil PAW, Pc, Ks and C2 index were observed (Fig. 4). NFNP and FNP treatments and soils under zerotillage (DP and DDP) have a similar behaviour whereas the tilled soil differs from them. Soil physical attributes of CP treatment were not altered by grazing, however, CP showed a diminishment of the parameters of Ks and C2 index, showing the interruption of the soil porosity (Schwen et al., 2011a; Dörner et al., 2012; Zúñiga et al., 2015), and an increase of PAW indicating the change of pore distribution compared to the original situation or less intensive soil intervention (Dörner et al., 2012; Zúñiga et al., 2015). On the other hand, negative effects of grazing on PAW, Pc and C2 index of the soil under NFNP were observed. This meant that the treatment with the lowest accumulated herbage mass (see Fig. 6 with NFNP reaching 4 ton DM ha−1 year−1) and therefore probably with the lowest below ground production (root mas production), was affected by grazing, which indicates that animal trampling on the soil with least resilience (understood as the ability of the soil to recuperate its pore functions after removing the stress according to Lal, 1994) of the degraded pasture induced a higher impact on the mechanical strength compared to the other pastures. In these terms, it has been assessed that the fertilised pastures presented a higher resilience capacity compared to non-fertilised or degraded pastures (as NFNP) in the same soil, similar treatments in a long-term trial (Zúñiga et al., 2015; Ivelic-Sáez et al., 2015; Dec et al., 2012). The resilience capacity as an indicator of soil elasticity is related to the type of pasture since forage production is positively correlated with the nutrient recycling (Whitehead, 2000; Franzluebbers et al., 2004). The higher above ground biomass production (e.g. FNP) is positively related to the below ground biomass production (Yan et al., 2013). Therefore a

Fig. 7. Effect of the dry season over soil water tension (A); soil water content (B) and soil temperature (C) at different depth before the raining season started (31/03/2015).

NFNP with −36,400 hPa and FNP with −31,500 hPa. At 60 cm depth, the FNP reached the lowest matric potential of −19,800 hPa. CP and NFNP show the highest tension at 60 cm, with −13,800 and −13,500 hPa, respectively. Corresponding to the matric potential, the depth dependent change in SWC showed that FNP presented the lowest values at 10, 20 and 60 cm with 21.0%, 18.6% and 12.2%, respectively. Finally, the pasture with the lowest temperatures (31/03/2015 before the first autumn rain) at all depths was FNP with 17.6 °C, 17.6 °C and 18.2 °C at 10, 20 and 60 cm, respectively. On the other hand, for the same period the highest temperatures were registered in NFNP with 17.8 °C, 18.2 °C and 18.6 °C at 10, 20 and 60 cm depth, respectively.

4. Discussion 4.1. Effect of pasture improvement managements (PIMs) and grazing over penetration resistance, the capacity and intensity parameters According to the results of soil penetration resistance, almost no 61

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have the higher values of SWC being constant in all depths evaluated during almost all seasons (Figs. 5 and 6), but with a similar accumulated herbage mass production per year (Fig. 6). This may indicate a lower absorption of water by these pastures since the soil fertility was not a limiting growth condition. Clearer differences were observed regarding the plant growth rate during summer, where FNP, DP and DDP were the improved pasture with the highest production during this period (2306; 1087 and 1119 kg DM ha−1 respectively for summer 2015; Fig. 6). On the other hand, regarding SWC, DP and DDP were the pastures with higher values, indicating less water absorption between 0 and 60 cm soil depth (20.4% and 22.2% respectively; Fig. 6) compared to FNP. Finally, the differences of water uptake (given by the SWC) and the similar accumulated herbage mass per year between all the treatments indicate that there are some mechanisms which influence the SWC, e.g.: the hydraulic lifting generated by species with deeper root systems. This mechanism must be taken into account in order to better understand the distribution and absorption of water through the soil profile (Richards and Caldwell, 1987; Caldwell et al., 1998; Skinner et al., 2004; Skinner, 2008). The hydraulic lifting by the deeper root species along with the effect of the type of tillage influencing the continuity of soil porosity and soil water content dynamics, may allow the movement of water from the bottom to the top of the soil and, this process may give some insights of the differences between the treatments, especially during the drought seasons. All the PIM’s had the same accumulate herbage mass (Fig. 6) for both 2013 (between 6700 and 8100 kg DM ha−1) and 2014 (between 9000 and 9600 kg DM ha−1). According to these, it seems for this specific soil type (Andisol), that the connectivity of pores it is not the priority for biomass production, but it may have an effect over the persistence and survival of the species at stress seasons, such as summer (soil water redistribution). According to these results, such as the high efficiency in capturing water in summer showed by FNP (2% more in summer 2015 than CP between 0 and 60 cm soil depth; Fig. 6), the high accumulated herbage mass and its high production during summer (Fig. 6) could support the hypothesis that a high diversity pasture may deliver a higher stability and persistence in time of both survival of the species and dry matter production (Garwood and Sinclair, 1979; Tillman and Downing, 1994; Volaire and Thomas, 1995; Volaire, 1998). However, there are still several questions, which have to be clarified in order to better understand these mechanisms in this complex soil-water-plant continuum, such as hydraulic lifting in the soil profile (Richards and Caldwell, 1987; Caldwell et al., 1998; Skinner et al., 2004; Skinner, 2008), the water use efficiency of different pasture species (Evans, 1978; Skinner, 2008) and the role of root growth rate and roots turnover effects on soil structure (Matthew et al., 1986; Reid et al., 2015).

higher root surface, generated by higher above ground biomass production, is one of the important factors to improve the soil resilience (Zúñiga et al., 2015). 4.2. Pasture improvement managements (PIMs) and their consequences on soil water content dynamics Soil structural changes, related to soil water content dynamics as a result of rainfall, evaporation, infiltration and water uptake by the roots (Simunek et al., 2003), are more pronounced on superficial soil horizons (Hartge and Horn, 2009). Direct-drilled treatments (DP and DDP) and those without tillage (NFNP and FNP) did not present a negative alteration of the architecture and continuity of soil porosity, allowing the existence of a continuous pore system within the soil profile (Uteau et al., 2013). The latter increased the water infiltration to the deepest soil layers, improving the water distribution, which facilitated root growth (Xua and Mermoud, 2003). On the other hand, the traditional tillage treatment (CP) exhibited a change in the soil porosity distribution, increasing the retention of water in the superficial soil layer (Zúñiga et al., 2015). The latter is related to the disruption of pore continuity through the soil profile, e.g. due to the well-known formation of platy soil structure, which can also induce a direct dependent behaviour of mechanical and hydraulic properties (Dörner and Horn, 2006,2009). According to these findings and from the soil water content dynamics presented in this research, it is possible to indicate that the fertilised soil under the diverse pasture managements with a minor intervention in the soil structure (FNP), had major access to the pool of water stored in the soil as assessed in the water stress season (Figs. 5–7). The latter is related to both soil physical and root development, which again is in agreement with the differences in field soil water content and matric potential registered at the end of an extreme drought season (Fig. 7). While the soil physical condition is related to the presence of a continuous pore system across the soil profile (Uteau et al., 2013), the root development across these pores plays a key role in the access to water and nutrients. The higher water uptake, derived by lower SWC (or also higher matric potential) and higher daily productivity during the dry seasons in FNP, is in concordance with those presented by Neal et al. (2012). They found differences of SWC at different depths with different perennial species, indicating that L. perenne extracted less water from deeper soil horizons as reflected by higher SWC’s, in comparison with Bromus wildenowii, Festuca arundinacea and Phalaris aquatica. In the present study the SWC was related to the pasture species root system architecture that is shallow (e.g Lolium perenne) or deep roots systems (e.g Bromus valdivianus, Dactylis glomerata). Evans (1978); Crush et al. (2005) found that some cultivars of L. perenne have a higher concentration of roots in the first 10 cm of the soil compared with other species (e.g. D. glomerata). Similarly, Crush et al. (2009) indicated that variations of 59.0%–77.5% of L. perenne root concentrations in the first 10 cm were related to its origin (wild types; cultivars and breeding lines). This is consistent with our results, since the SWC registered in CP and DP (pastures highly dominated by L. perenne according to Table 1) had the lowest SWC at 10 cm depth during summer 2015, reflecting a higher concentration of roots. Descalzi (2011) shows that B. valdivianus had higher tiller stability compared with L. perenne, under water restriction, indicating a higher tolerance to water stress, due to the presence of a deeper root system (Stewart, 1996). Likewise, the increase of root morphological traits (or a higher diversity of species) may magnify the water uptake of the pasture and even improve its resilience (Barkaoui et al., 2016). In the present study, a high percentage of highly productive species, but also a high percentage of less productive grasses and broad leaf species were observed in FNP (Table 1), being highly probable that the diversity of pasture species in FNP and the soil structure conservation are responsible for the higher quantity of living roots between 20 and 60 cm, which may explain the lower levels of SWC (Fig. 7). These results differ in the case of DP and DDP, since they

5. Conclusions The implementation of pasture improvement managements without alteration of structure dependent properties (soil fertilisation without tillage): i) allows the conservation of the continuity of the pore system in the soil profile and throughout this, ii) improves the water accessibility by plants, and therefore, iii) can produce the same herbage mass production as those improved pastures by conventional systems (cultivar establishment, soil fertilisation and tillage). According to this, the fertilisation of the degraded naturalised pastures without soil tillage is a viable way to improve the herbage mass production and soil physical functions, keeping a higher summer production since the root system is able to explore the soil and absorb water from deeper soil horizons. The soil response to grazing events differs between pastures; only the degraded non-fertilised pasture presented negative effects on soil structure-dependent properties after grazing. The non-fertilised pasture (NFNP), which reached the lowest above biomass herbage production, was more exposed to animal trampling and presented a lower resilience compared to the other pastures. 62

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Acknowledgements

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