Sixty years of seasonal irrigation affects carbon storage in soils beneath pasture grazed by sheep

Agriculture, Ecosystems and Environment 148 (2012) 29–36

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Sixty years of seasonal irrigation affects carbon storage in soils beneath pasture grazed by sheep F.M. Kelliher a,b,∗ , L.M. Condron b , F.J. Cook c , A. Black b a b c

AgResearch, Lincoln Research Centre, Private Bag 4749, Christchurch 8140, New Zealand Lincoln University, Department of Soil and Physical Sciences, P.O. Box 84, Lincoln 7647, New Zealand CSIRO Land and Water, 41 Boggo Road, Dutton Park, QLD 4102, Australia

a r t i c l e

i n f o

Article history: Received 24 May 2011 Received in revised form 25 October 2011 Accepted 25 October 2011 Available online 15 December 2011 Keywords: Irrigation Soil carbon Grassland Vertical distribution Respiration

a b s t r a c t For sixty years at Winchmore, South Island, New Zealand (43◦ 48 S, 171◦ 48 E, 160 masl), stoney soils under continuous pasture grazing by sheep have received rainfall (nil irrigation) or rainfall and irrigation as required during summer. This consistently managed, replicated field trial presents a unique opportunity to examine long-term treatment effects on pastoral soil. Samples were recently excavated at intervals to a depth of 1 m and the total carbon (C) storage measured. In the irrigated plots, soil C storage (9.1 ± 0.3 kg C m−2 , mean ± standard error, n = 3) was significantly less (p < 0.05) than in plots receiving rainfall alone (13.4 ± 0.8 kg C m−2 ). We estimated irrigation induced a 36% increase of C inputs to the soil on an annual basis, mostly as litter fall. Using a respiration model based on soil temperature and water content inputs, irrigation was also estimated to have induced a 97% increase in rate of annual C loss to the atmosphere. On this basis, the estimated irrigation effects had reduced C storage by 61% (97–36%), reasonably accounting for the 47% treatment effect determined by soil sampling. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Grassland covers about one-quarter of Earth’s land area, storing about the same proportion of the carbon (C) in soils (Jobbagy and Jackson, 2000). Consequently, changes in net C exchange between grassland and the atmosphere may significantly affect the global C balance (Soussana et al., 2010). Net C exchange in grassland varies seasonally, reflecting solar irradiance, rainfall and plant production. During the spring and summer months, solar irradiance will be maximal and net C uptake and evaporation rates as well, provided sufficient rainfall and soil water storage capacity. Summer drought can limit the realisation of this potential. For pastoral agriculture, where farmed animals are fed by grazing grassland, irrigation during summer can prevent soil water deficit, eliminating this constraint on herbage production and feed availability. The resilience of this intensified food and fibre production system also depends on soil services such as water storage and fertility that will be provided by the stored organic matter including C. The C in soils has come from plants. While C abundance in soils has been associated with climate, global meta-analysis determined an additional effect of plant functional type on the vertical

∗ Corresponding author at: AgResearch, Lincoln Research Centre, Private Bag 4749, Christrchurch 8140, New Zealand. Tel.: +64 3 321 8785; fax: +64 3 321 8811. E-mail address: [email protected] (F.M. Kelliher). 0167-8809/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2011.10.022

distribution of C in soils and temperate-climate grassland was a mean type (Jobbagy and Jackson, 2000). Soil C storage will be the net result of inputs and outputs including respiration generating C emissions to the atmosphere. For soils, C output can be postulated to be proportional to C input as done for plants (Lloyd and Farquhar, 2008). By this argument, while irrigation of seasonally dry soils should increase plant production, a change in soil C storage will continue to depend on a net balance of the C input and output rates. Changes in soil C storage are likely to be relatively small, so detecting them will be aided by representative sampling, careful analysis and the passage of time. As an example, by re-sampling soils after 17–30 years beneath grassland at thirty sites that had been grazed by dairy cattle across the North Island of New Zealand, the mean change in C storage to a depth of 1 m was a significant decrease of 6 ± 3% (p < 0.05, ±standard error of the mean; Schipper et al., 2007). These soils had not been irrigated because rainfall was generally sufficient at the sampled sites (averaging 1306 mm year−1 ). In the Otago region of New Zealand’s South Island, where mean annual rainfall was 500–550 mm, sampling soils beneath pasture grazed by sheep at forty sites, half watered by rainfall and the others by rainfall as well as irrigation when required in summer over periods up to 49 years, indicated irrigation had not affected C content nor bulk density of the soil to a depth of 0.15 m (Houlbrooke et al., 2008). Near Deniliquin in southeastern Australia where the mean annual rainfall was 405 mm, 5 years of irrigation

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onto ungrazed pasture did not significantly affect C content of the soil to a depth of 0.075 m (bulk density was measured in 2 years and assumed to be constant) according to Rixon (1966). Under irrigation, C accumulation had been associated with the formation of an organic matter mat on the surface that can be attributed to a lack of faunal activity incorporating plant litter into the soil (Kleinig, 1966). This may account for an incongruous synopsis of Rixon’s data reported by Conant et al. (2001). In northern China, different levels of particulate organic matter and irrigation, weekly to achieve a 50% increase of rainfall from 264 to 400 mm over 100 days during summer, were applied to soil beneath ungrazed grassland (Xiao et al., 2007). Both treatments significantly affected plant production and soil microbial biomass and activity, but not C storage in the soil to a depth of 0.2 m. At a mid-western American golf course where the mean annual rainfall was 860 mm, irrigation effects on C input to the soil, C output and the change in C storage over four years after grass establishment have recently been quantified by Qian et al. (2010). The irrigation treatment had been applied twice a week to meet 70% of the calculated evaporation and the plot’s grass was also mown twice a week to a height of 51 mm, while the rain-fed plots were mown as required to maintain the same height of grass. Irrigation induced a 13% increase (p < 0.05) in the soil C storage to a depth of 0.2 m and 240 and 300% increases in the estimated C input and output rates, respectively (Qian et al., 2010). While generally consistent, this literature was sparse and further research warranted determining if seasonal irrigation to intensify grassland food and fibre production systems significantly affects soil C storage and resilience through the sustained provision of soil services. There have been few long-term field trials with consistent, recorded land use and management history, though data from the UK and USA has been used to calibrate soil C turnover models (Jenkinson and Rayner, 1977; Parton et al., 1987) and recently, determine the long-term effect of liming on soil C sequestration under permanent grassland in the UK (Fornara et al., 2011). For sixty years at the Winchmore trial located near Ashburton, South Island, New Zealand (43◦ 48 S, 171◦ 48 E, 160 masl), soils under continuous pasture and grazed by sheep have been watered by natural rainfall (nil irrigation) or rainfall plus flooded irrigation (by a border-dyke system) as required during summer (on average, seven times per year) (Condron et al., 2006). Recently, soils subjected to these two treatments have been sampled and soil mass (kg m−2 ) and C density (kg C m−3 ) measured at intervals to a depth of 1 m. These data will be analysed to determine if sixty years of seasonal irrigation has affected soil C storage. This paper reports the results of our data analyses and interpretations. 2. Materials and methods 2.1. Theory Many soil properties change with depth and such relations are called vertical distributions. Analysis requires a continuous function, but corroborative measurements are usually made on samples excavated at discrete depth intervals. Analysis has been developed to eliminate a bias error in nonlinear relations between mean values and depths from sampling intervals (Cook and Kelliher, 2006). Here, the analysis has been applied, beginning with an exponential function for the vertical distribution of C stored in soils (S, kg C m−3 ) S(z) = S0 e−z/zS

(1)

where z is depth (m) and there are two parameters, S0 is S at z = 0 and zS is a characteristic length scale. Integrating Eq. (1) yields C storage in soils from the surface to the deepest depth sampled. The deepest depth sampled was 1 m (thus, the numerator of the

exponential term in Eq. (2) below) and the integral, S1 m , kg C m−2 , may be written S  1 m = S0 zS (1 − e−1/zS )

(2)

 = S z (Cook et al., Integrating Eq. (1) from z = 0 to z = ∞ yields S∞ 0 S  , the completeness of soil C 2007). Calculating a ratio of S1 m and S∞

storage by sampling to depth 1 m may be quantified by proximity of the ratio to unity. The length scale may be illustrated by comparing zS = 0.1, 0.2 and 0.3 m, whereby the proportion of C stored over 1 m depth of soil located in the uppermost 0.075 m would be 53, 31 and 23%, respectively. Thus, the value of parameter zS conveys a physical meaning, increasing it corresponds with proportionally lesser C stored near the surface and vice versa. 2.2. Irrigation trial During the late 1940s at Winchmore in New Zealand (9 km west of Ashburton, South Island), a series of long-term field trials was established to determine the seasonal irrigation and fertiliser requirements of pasture grazed by farmed sheep (Condron and Goh, 1989; Condron et al., 2006). Since then, these replicated field trials have been consistently managed, presenting a unique opportunity to examine long-term treatment effects on the vertical distribution of organic matter in a pastoral soil. The soil is classified as shallow, free-draining Lismore silt loam (Orthic Brown in New Zealand and Udic Ustochrept according to the USDA), and, like most in New Zealand, is non-calcareous and does not contain any free carbonate (Soil Bureau, 1968). The trial began in June 1947 when the soil was cultivated to a depth of 0.2 m following degradation of brown top (Agrostis sp.) pasture (Rickard, 1972), then the soil was ‘disced’ and harrowed. Thus, soil to the cultivated depth had been disturbed and mixed, so immediately afterwards, it was homogeneous (Saville et al., 1997). Cultivation would have decreased C storage near the surface and increased it deeper to a depth of 0.2 m (Jackman, 1964). During February 1948, twenty-four plots (each 9 m wide by 100 m long) were created by a grader with mound-shaped, border dykes along the long axes (Rickard, 1972). A cereal crop was then planted, superphosphate fertiliser and lime applied at rates of 125 and 5000 kg ha−1 , respectively, and during September 1948, the crop was ploughed into the soil. The site was fallowed until ryegrass (Lolium perenne) and clover (Trifolium repens, though T. subterraeum) in the irrigated plots (White et al., 2008), pasture seeds were sown during March 1949. Thereafter, each winter, all plots have been subjected to superphosphate fertiliser applied at 250 kg ha−1 , but no nitrogenous fertiliser has been applied. In 1965, lime was applied for a final time at 1900 kg ha−1 . This study was based on soil samples collected during April 2009 in six plots. For three of these plots, beginning September 1949, an irrigation treatment was applied, while the others were not irrigated (nil irrigation treatment). Irrigation was applied at weekly intervals, 100 mm depth of water was flood irrigated with infiltration into the soil for approximately 50 min. For the summers of 1949/50, 1950/51, 1951/52 and 1952/53, there were 25, 22, 32 and 30 irrigation events, respectively (Rickard, 1972). Although not previously reported, for October–February during these summers, the corresponding rainfall was 357, 520, 354 and 642 mm. Ironically, over 60 summers from 1949/50 to 2008/09, 1950/51 and 1952/53 had by far the most rainfall during October–February and the longterm mean was 304 ± 11 mm (±standard error of the mean, n = 60). From the summer of 1953/54 onwards, the basis for irrigation was changed to reflect the soil water deficit. Each year until 1997, during September through April, six soil samples were taken to a depth of 0.1 m about three times per week (Rickard and McBride, 1987). When the gravimetric water content (kg water kg−1 soil) was less than 20%, about seven times per year, the plots had 100 mm depth of water flood irrigated onto them. Hereafter, this will be called a

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20% irrigation treatment. From 1997 onwards, irrigation scheduling has been based on the same soil water content and sampling strategy, but the measurements have been done using the time domain reflectometry method (Srinivisan and McDowell, 2009). Sheep have rotationally grazed the 20% irrigation treatment plots each year during September through March. There were ∼40 ha−1 , one-year-old, females, weighing ∼40 kg up to ∼50 kg when removed for mating seven months later (Ray Moss, personal communication). Sheep have also grazed the nil irrigation plots at a stocking density varied according to the available herbage. To emulate sheep grazing, but exclude the sheep, open-top, wire netting cages (1 m by 3 m) were placed over areas of pasture and the herbage regularly harvested by cutting to a height of 25 mm (Rickard and McBride, 1986b). After harvest, the remaining or residual pasture herbage dry matter (DM) was 1000 kg DM ha−1 according to White et al. (2008). The areas where cages were placed were changed regularly and harvesting occurred at approximately monthly intervals during grazing and at the end of the non-grazing period. 2.3. Soil sampling The S data came from soil samples. The soil is stoney, and midway between the borders in each plot, a mechanical digger was used to excavate a pit, approximately 1 m wide, 2 m long and 1.5 m deep. Along a pit face, using a steel frame (0.4 m by 0.4 m by 0.25 m) to delineate the extracted volume, soil and stone samples were taken from six depth intervals (0–0.075, 0.075–0.15, 0.15–0.25, 0.25–0.50, 0.50–0.75 and 0.75–1.00 m). In total, to obtain the soil samples, approximately 1500 kg of stones and soil was excavated from the six pits. Each extracted sample was transported to a laboratory, weighed, separated into soil and stones using sieves and weighed again. From each sample, approximately 3 kg of (fresh) soil was set aside for analysis, and the remainder returned to the pit which was refilled. From each fresh soil sub-sample, a portion was weighed, dried at 105 ◦ C for 24 h and weighed again. Applying the water content to the sample’s fresh soil and stone weights, and accounting for sample volume, each sample’s stone content and bulk density were determined. In the laboratory, sample processing also included the removal of roots, followed by C content measurement using a combustion method. For each replicate, the six S and z-interval data pairs were subjected to rectilinear vertical integration to determine measured values of S1 m . Eq. (1) was then applied to the six S and z-interval data pairs of each replicate in order to estimate parameters S0 and zS by minimising a root mean square error between the measured and fitted values of S1 m . This procedure generated unique parameter pairs. Following logarithmic transformation of the measured and fitted S(z) data, a coefficient of determination (r2 ) was determined as an indicative statistic. 2.4. Rainfall and evaporation The balance of rainfall and evaporation will determine the soil water deficit or surplus and drainage. Rainfall has been recorded daily at Winchmore. For 1 January 1950 through 31 December 2009, daily rainfall was extracted from a national climate database (www.clifo.co.nz) maintained by New Zealand’s National Institute of Water and Atmosphere. These data were compiled as monthly sums for analysis. For well-watered pasture, potential evaporation (E) was estimated using the Priestley and Taylor model that may be written E=˛



s (s + )



An

(3)

31

where ˛ is a dimensionless parameter, s is the slope of a curve relating saturated vapor pressure and temperature,  the latent heat of vaporisation,  the psychrometric constant and An the net all-wave available energy flux density. Parameter ˛ was 1.26 following Scotter et al. (1979). Parameters s and  were related to the monthly mean air temperature, measured at Winchmore beginning 1 January 1966 through 31 December 2009. For the period 1 January 1950 through 31 December 1965, we substituted measurements that had been made in Ashburton. For 1 January 1966 through 31 December 1970, monthly mean air temperature at Ashburton averaged 8% more than those at Winchmore according to linear regression analysis that accounted for 99% of the variation (data not shown). Air temperature estimates using the substituted data were adjusted accordingly. The cooler the air the lower will be s and . For example, at 5 ◦ C, {s/[(s + )]} will be 56% of its value at 25 ◦ C. The mean monthly An was estimated from shortwave irradiance, Is , measured at Winchmore according to the relation An (MJ m−2 d−1 ) = 0.62Is (MJ m−2 d−1 ) – 1.47 reported by Scotter et al. (1979). For 1 January 1960 through 31 December 1972, missing data meant we had to substitute measurements that had been made at the Christchurch airport, located about 100 km northeast of Winchmore. For 1 January 1973 through 31 December 1977, monthly means of Is at Christchurch airport averaged 3% less than those at Winchmore according to linear regression analysis that accounted for 94% of the variation (data not shown). The estimates of Is by the substituted data were adjusted accordingly. For 1 January 1950 through 31 December 1959, Is had not been measured at Winchmore or Christchurch airport, so we substituted sunshine duration measurements that had been made at Waimate, located 158 km South of Winchmore. This included a relation between monthly means of sunshine duration at Waimate (hW ) and Is at Christchurch airport, based on data for 1 January 1960 through 31 December 1965 (Is = 4.93hW − 8.49, r2 = 0.58). Sunshine duration at Waimate and Christchurch airport (hC ) were also compared over the same period (hC = 1.15hW , r2 = 0.79).

3. Results Annual rainfall and evaporation of well-watered pasture averaged 741 ± 18 and 783 ± 10 mm, respectively (Table 1). The mean monthly rainfall was remarkably consistent throughout the year, though variable from year to year. There was no statistically significant trend in rainfall over the 60-year-long record. Mean monthly potential E ranged from 18 ± 2 mm in winter (June) up to 122 ± 3 mm in summer (January). There was relatively little variation in monthly potential E and no trend over 60 years of calculation, in agreement with the analysis of daily Is and air temperature data from 1966 to 2004 by Srinivisan and McDowell (2009). Combining mean monthly values of rainfall and potential E, a soil water deficit would be expected for seven months (September through March) and surplus or drainage for five months. The 20% irrigation treatment has included seven 100 mm water application events per year, on average (Srinivisan and McDowell, 2009), so about twice the annual water input to the soil as the nil irrigation treatment. Our procedure of fitting the soil C storage vertical distribution function to the data minimised differences between measured and fitted S1 m , and achieved reasonable agreement across most depth layers (Figs. 1 and 2). The parameters were informative about the vertical distribution of soil C storage. For example, for plots 16 and 6 that were subjected to the nil and 20% irrigation treatments, respectively, the corresponding fitted values of S0 and zS were 50.9 and 45.8 kg C m−3 and 0.24 and 0.20 m (Table 2). Thus,  = 12.2 and 8.7 kg C m−2 , respectively while corresponding valS∞ ues of S1 m were 12.0 and 9.2 kg C m−2 , so on average for these plots, it was estimated 96% of C storage had been obtained by

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Table 1 Monthly values of rainfall recorded at Winchmore during the years 1950–2009 (mean ± standard error of the mean, n = 60) and evaporation estimated for well-watered pasture by the Priestley and Taylor model using solar irradiance and air temperature measurements as described in the text. Also shown are difference calculations of the monthly water surplus (drainage) or deficit including root mean square errors. Month July August September October November December January February March April May June Year

Rainfall (mm) 66.7 69.2 51.3 60.0 60.4 66.2 59.1 58.0 66.5 64.0 64.3 55.5

± ± ± ± ± ± ± ± ± ± ± ±

Potential evaporation (mm)

5.1 7.3 4.5 4.1 5.2 4.5 4.2 4.0 5.6 6.5 5.5 5.3

19.9 33.4 56.3 83.0 103.8 115.9 121.6 92.5 70.5 43.2 25.1 17.6

741.1 ± 18.4

± ± ± ± ± ± ± ± ± ± ± ±

Drainage (mm)

Deficit (mm)

46.8 ± 5.2 35.8 ± 4.2

1.6 1.6 2.0 2.1 2.7 2.7 2.7 2.8 1.3 1.8 1.7 1.8

5.0 23.0 43.5 49.8 62.5 34.5 3.9

± ± ± ± ± ± ±

0.5 1.7 3.9 3.6 4.7 2.6 0.3

20.8 ± 2.3 39.2 ± 4.3 38.0 ± 5.3

782.8 ± 9.5

Table 2 Estimates of the soil carbon (C) storage vertical distribution characteristic length scale (zS ), C concentration at the surface (S0 ) and C storage to a depth of 1 m (S1 m ) for the nil and 20% irrigation treatment plots. Treatment

zS (m)

Nil irrigation Plot 11 Plot 16 Plot 23

0.32 0.24 0.23

Mean Standard error of the mean

0.26 0.03

S1 m (kg C m−2 )

Treatment

zS (m)

S0 (kg C m−3 )

S1 m (kg C m−2 )

48.0 50.9 61.7

14.6 11.8 13.9

20% irrigation Plot 6 Plot 9 Plot 18

0.20 0.26 0.18

45.8 38.0 49.8

8.8 9.7 8.9

53.5 4.2

13.4 0.8

0.21 0.02

44.5 3.5

9.1 0.3

S0 (kg C m−3 )

sampling to a depth of 1 m. On average, parameter zS in the nil irrigation treatment was not significantly greater than in the 20% irrigation treatment (0.26 ± 0.03 versus 0.21 ± 0.02 m, p > 0.05) nor was parameter S0 (53.5 ± 4.2 versus 44.5 ± 3.5 kg C m−3 ). However, inserting the parameter values into Eq. (2), the estimated mean soil C storage to a depth of 1 m in the nil irrigation treatment was significantly greater than in the 20% irrigation treatment (13.4 ± 0.8 versus 9.1 ± 0.3 kg C m−2 , p < 0.05). Similarly, rectilinear, vertical integration of the soil C storage measurements to a depth of 1 m yielded plot means of 13.6 ± 0.8 kg C m−2 under nil irrigation versus 9.3 ± 0.3 kg C m−2 under the 20% irrigation treatment.

Fig. 1. Relation between soil carbon storage (S) and depth (z) for plot 16 that was subjected to the nil irrigation treatment. Solid lines are measurements, shown in the primary graph according to the rectilinear integration method with a short, horizontal line denoting the midpoint of each sampling interval. The dashed lines were determined by fitting Eq. (1) to the data as described in the text. A secondary graph portrays the relation between normalised data for S and z.

4. Discussion Soil C storage has been determined in plots by sampling across depth intervals, measuring C and bulk densities, and vertically integrating “stepped” profiles by a rectilinear method. Analysis can be developed by fitting a function to such data as described by Cook and Kelliher (2006). In this study, we used an exponential decay function and minimised the difference between measured and fitted values of the vertically integrated C storage to a depth of 1 m. At a plot level, the difference was 1–2% and the function’s two parameters could be interpreted according to their physical meaning. Alternatively, we also fitted equal-area spline functions to these data following Ponce-Hernandez et al. (1986). Integrating

Fig. 2. Relation between soil carbon storage (S) and depth (z) for plot 6 that was subjected to the 20% irrigation treatment. Solid lines are measurements, shown in the primary graph according to the rectilinear integration method with a short, horizontal line denoting the midpoint of each sampling interval. The dashed lines were determined by fitting Eq. (1) to the data as described in the text. A secondary graph portrays the relation between normalised data for S and z.

F.M. Kelliher et al. / Agriculture, Ecosystems and Environment 148 (2012) 29–36

Fig. 3. Relation between volumetric soil water content and depth based on the data reported by Stoker (1982). The open symbols, connected by a dashed line, were minimum, measured values. The solid symbols, connected by a solid line, were maximum values, allowing for drainage and called the field capacity. The difference between minimum and maximum values is the available water storage capacity for plants.

these alternatively determined vertical distributions to a depth of 1 m, we obtained nil and 20% irrigation plot means that were only 3–5% less than values determined by the measurements. The soil C storage has been determined over the same volume to a depth of 1 m under nil and 20% irrigation and there was a statistically significant treatment effect. Alternatively, C storage can be determined on an equivalent soil mass basis (Ellert et al., 2002). In the nil irrigation plots, mean soil mass per unit area to a depth of 1 m was significantly greater than in the 20% irrigation plots (736 ± 33 and 584 ± 102 kg m−2 , respectively). However, mean soil mass per unit area to a depth of 0.75 m in the nil irrigation plots (585 ± 33 kg m−2 ) was virtually identical to mean soil mass per unit area to a depth of 1 m in the 20% irrigation plots. On this basis, by rectilinear, vertical integration of our samples, we compared mean measured soil C storage to a depth of 0.75 m in the nil irrigation plots with that to a depth of 1 m in the 20% irrigation plots, and there was a significant difference (12.6 ± 0.9 for nil irrigation versus 9.3 ± 0.3 kg C m−2 , p < 0.05). Thus, analysing these data on an equivalent soil mass basis did not affect the treatment effect in terms of order (nil irrigation > irrigation) or statistical significance. The requirement for irrigation may be estimated by the difference between minimum and maximum soil water storage, the latter allowing for drainage and called the field capacity. This quantity may be considered water available to the pasture plants. Analysis of two sets of soil samples taken to a depth of 0.9 m indicated the available water was 99.4 mm, 62% of which was stored to a depth of 0.3 m (Fig. 3). This calculation supported the quantity of water that had been applied each irrigation. For five months, October through February, potential E was 517 mm in total, significantly exceeding the rainfall. During this period of potential soil water deficit, on average, there were seven 100-mm flood irrigation events. Thus, the seasonal irrigation was equivalent to 74% of the potential E. To further explore the available water, the vertical distribution was analysed by applying Eq. (1) to the data, again minimising the root mean square error between measured and fitted values of S1 m . For the available water, the fitted values of S0 and zS were 274 kg m−3 and 0.41 m, respectively, and r2 was 0.98. Thus, the value of zS for the available water was significantly greater than the corresponding term for C storage (0.41 m versus 0.25 m). Moreover, below a depth of 0.3 m, the maximum water content was 0.08–0.19 m3 m−3 , which was significantly less than for the soil above. Soil texture became increasingly coarser with depth below

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0.3 m. For the nil and 20% irrigation treatments, the mean volumetric stone contents were statistically indistinguishable (0.30 ± 0.04 and 0.25 ± 0.04 m3 m−3 , respectively) and maxima (0.48 m3 m−3 ) occurred across the depth interval of 0.15–0.25 m (data not shown). Texture and stone content can also affect the soil’s capacity for C storage, indicated by the C storage vertical distributions portrayed in Figs. 1 and 2. The vertical distribution of roots has not been measured at Winchmore, but for the roots of temperate grassland, zS was 0.17 m according to Cook et al. (2007). Accordingly, the proportion of roots located in the uppermost 0.1 and 0.3 m of soil would be 42 and 64%, respectively. As indicated above, little of the soil’s available water was located below a depth of 0.3 m. These vertical distributions accounted for a strong correspondence between the temporal courses of gravimetric water content in the uppermost 0.1 and 0.3 m of soil, the latter considered the root zone (Rickard and McBride, 1987). For the uppermost 0.1 m of soil under nil irrigation, mean gravimetric water content ranged from 0.13 kg kg−1 during January and February up to 0.32 kg kg−1 during July and August (Rickard and McBride, 1987) (Table 3). Corresponding gravimetric water contents for the 20% irrigation treatment were 0.28 and 0.36 kg kg−1 . The C emissions from soil to the atmosphere, soil respiration (Rs ) including roots and microbes, can be strongly responsive to temperature, provided water and C substrate availabilities are sufficient (Lloyd and Taylor, 1994). However, unlike soil temperature and water content, C availability is not a state variable that can be readily measured. The C in soils comes from plants, thus the C supply rate can be affected by the weather and nutrient supply. At Winchmore, monthly mean soil temperature (Ts ) at a depth of 0.1 m ranged from 4 to 20 ◦ C, though 1 ◦ C cooler during summer for the 20% irrigation treatment (Rickard and McBride, 1986a). Under well-watered conditions, the response of Rs to Ts has been estimated by an Arrhenius type function, written following Lloyd and Taylor (1994) as Rs = R10 exp

  E0

1 1 − 283.15 − T0 Ts − T0



(4)

where R10 was Rs when Ts = 283.15 K (10 ◦ C) and parameters E0 and T0 were 308.56 and 227.13, respectively. This relation has been verified at a grassland site near Twizel, located about 192 km southwest of Winchmore (Hunt et al., 2004). Accordingly, as Ts increases from 5 to 20 ◦ C, there would be a four-fold increase in Rs . At Twizel, for a given Ts , as soil water content increased from the driest value to the field capacity, there was a four-fold increase in Rs (Hunt et al., 2004). By varying soil water content at intervals from the driest value to the field capacity, the relation between water content and Rs was linear according to Orchard and Cook (1983). To estimate annual Rs for the nil and 20% irrigation treatments at Winchmore, we combined the relation between Rs and Ts (Eq. (2) with R10 set to a normalised value of 1.0), a four-fold, normalised linear relation between Rs and soil water content and the monthly mean data of Ts and water content (Table 3). These calculations suggested 20% irrigation had induced a 97% increase in the annual rate of soil C loss to the atmosphere compared to nil irrigation. Earlier, for a hypothetically wet year in eastern Oregon, USA, annual Rs was estimated to be 150% greater than estimated for a normal year when annual precipitation was 552 mm and water deficit prevailed throughout the summer (Kelliher et al., 2004). Mean annual rainfall at Winchmore was 34% greater than normal annual precipitation at the eastern Oregon site, so the proportional effects of soil water deficit on annual Rs at the two sites were considered broadly similar. While Rs determines C emissions to the atmosphere, the soil C storage rate also depends on C input rates. Roots are one form of C input to soils. Root production rates for nil and 20% irrigation were estimated following Metherell (2003). This combined

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Table 3 Mean gravimetric water content for the uppermost 0.1 m of soil (1959–1986, 27 years) and temperature at a depth of 0.1 m (1958–1985, 27 years) in the nil and 20% irrigation plots at Winchmore after Rickard and McBride (1987) and Rickard and McBride (1986a,b), respectively. Month

Treatment Nil irrigation

20% irrigation −1

Water content (kg kg

◦

)

Temperature ( C)

Water content (kg kg−1 )

Temperature (◦ C)

June July August September October November December January February March April May

0.30 0.32 0.32 0.28 0.23 0.18 0.16 0.13 0.13 0.17 0.20 0.26

4.5 3.9 5.8 8.9 12.3 15.6 18.2 19.6 19.0 16.3 12.3 8.0

0.36 0.37 0.36 0.32 0.30 0.29 0.28 0.27 0.27 0.29 0.30 0.33

4.6 3.9 5.8 8.8 12.1 15.1 17.2 18.4 18.0 15.8 12.3 8.0

Year

0.22

12.0

0.31

11.7

Winchmore plot measurements of root mass (to a depth of 0.2 m) and time for a pulse-labelled C isotope (13 C) in roots to dissipate by respiration, death and decomposition that had been reported by Stewart and Metherell (1999). There suggested no significant treatment effect of irrigation on the root production rate that averaged 0.18 kg C m−2 y−1 (Table 4). Pastoral agriculture aims to maximise herbage production and the proportion ingested by farmed, grazing animals. Uneaten herbage dies and falls onto soil, another form of C input to soils. As intended, the 20% irrigation treatment increased pasture herbage production compared to nil irrigation. The treatment effect was statistically significant (11.2 ± 0.3 versus 6.4 ± 0.3 t DM ha−1 y−1 where DM means dry matter or biomass) according to 25 years (1960–1985) of pasture harvesting at Winchmore that had been described earlier (Rickard and McBride, 1986b). The pasture herbage production measurements estimated sheep DM intake. For a “hard-grazed” sward, leaf death rate or litter fall rate should be equal to the feed intake rate according to Parsons et al. (1983). For ryegrass pasture, they argued leaf turnover will be rapid, so uneaten herbage soon dies. On this basis, assuming C content of the herbage was 42%, we estimated mean values of litter fall were 0.27 and 0.47 kg C m−2 y−1 under nil and 20% irrigation, respectively (Table 4). Urine and dung excretion by grazing sheep returns undigested herbage to soils, another form of C input. Pasture herbage DM production, DM digestibility and C content of the DM (42%) were combined to estimate urine and dung C excretion rates for the nil and 20% irrigation treatments. For sheep fed grass in a calorimeter chamber, the urine C excretion was 5% of the DM intake (Blaxter and Graham, 1955). For pasture grazed by sheep in the South Island of New Zealand that was not subjected to soil water deficit, the mean digestibility was 79% (Clark and Ulyatt, 2002). Thus, for the 20% irrigation treatment, recalling pasture herbage production was

herbage consumed by the sheep and equal to 1.12 kg DM m−2 y−1 , an estimated urine and dung C excretion rate was 0.12 kg C m−2 y−1 ([0.05 × 1.12] + [1 − 0.79] × 1.12 × 0.42). Soil water deficit reduces the mean digestibility of pasture to 72% according to Clark and Ulyatt (2002), so for the nil irrigation treatment with herbage consumed by the sheep of 0.64 kg DM m−2 y−1 and the estimated urine and dung C excretion rate was 0.09 kg C m−2 y−1 ([0.05 × 0.64] + [1 − 0.72] × 0.64 × 0.42). Estimation has suggested 20% irrigation induced a 36% increase in C inputs to the soil compared to nil irrigation (root production + litter fall + urine and dung excretion, Table 4), mostly attributable to increased litter fall. We have taken this percentage and that determined earlier from the annual Rs calculations to indicate treatment effect on C inputs to the soil and losses to the atmosphere, respectively. Combining, we have estimated that soil C storage of the nil irrigation treatment would have been 61% greater than for 20% irrigation (97–36%). The mean, measured soil C storage determined to a depth of 1 m in the nil irrigation treatment was indeed greater than a corresponding mean of the 20% irrigation treatment, but by 47% (re-iterating, these means were 13.4 ± 0.8 and 9.1 ± 0.3 kg C m−2 ). Estimating C inputs to the soil and losses to the atmosphere has not completely accounted for the 20% irrigation treatment effect on soil C storage. There have been uncertainties in the measurements and calculations, and possibly, two other potentially influential variables. Firstly, while not measured in this study, it has recently been reported that downwards, dissolved soil C transport contributed to the vertical distribution of soil C storage (Baisden and Parfitt, 2007) and C loss (Parfitt et al., 2009; Ghani et al., 2010; Kindler et al., 2011) from grassland. Based on the latter three published studies that reported measurements, a mean, annual value of dissolved C loss from soils beneath pasture would be 0.04 kg C m−2 . Coincidentally, this was virtually identical to a mean, annual value

Table 4 Annual carbon (C) inputs to soils (kg C m−2 y−1 ) under the nil and 20% irrigation treatments estimated according to the sources, including measurements made at Winchmore, as described in the text. Treatment

Source −2

Nil irrigation (kg C m

−1

y

)

−2

20% irrigation (kg C m

Root production

0.19

0.16

Litter fall Urine + dung excretion

0.27 0.09

0.47 0.12

Root + litter + urine + dung

0.55

0.75

−1

y

) Stewart and Metherell (1999) and Metherell (2003) Parsons et al. (1983) Blaxter and Graham (1955) and Clark and Ulyatt (2002)

F.M. Kelliher et al. / Agriculture, Ecosystems and Environment 148 (2012) 29–36

of our earlier estimates for sheep urine C excretion onto the soils at Winchmore. If applied to the 20% irrigation treatment over 60 years, this estimated loss would account for nearly 60% of the treatment effect reducing soil C storage by 4.3 kg C m−2 , on average (Table 2), assuming no such loss from soils under nil irrigation. A second potentially influential variable at Winchmore has been subjected to measurement there during earlier studies. Under 20% irrigation, monthly sampling to a depth of 0.25 m indicated no systematic variability in earthworm density throughout the year. However, during October–May, there were significantly more earthworms under 20% irrigation than nil irrigation (mean densities were 517 and 194 m−2 , respectively, Fraser and Piercy, 1996). The dominant earthworm was Aporrectodea caliginosa. Under 20% irrigation, assuming earthworms had been solely responsible for incorporating C inputs to the soil (0.75 kg C m−2 y−1 , Table 4) and there was a 50% respiration loss prior to ingestion (0.75–0.38 = 0.37 kg C m−2 y−1 ), an estimated C ingestion rate was 0.7 × 10−3 kg C worm−1 y−1 (0.37/517). For context, earlier, fieldcollected (Tasmania, Australia) Aporrectodea longa worms were incubated in moist soil (0.3 kg kg−1 ) without plants and maintained at 15 ◦ C (3 ◦ C warmer than the mean soil temperature at Winchmore, Table 4) in a laboratory by Svendsen and Baker (2002). When they applied sheep dung to the soil surface, it was incorporated at a rate suggesting ingestion of 1.4 × 10−3 kg C worm−1 y−1 . While twice the ingestion rate estimated for Winchmore, the Australian study did not seem to have accounted for dung respiration loss prior to its ingestion by the worms. Earthworm activity can significantly enhance microbial respiration and organic matter decomposition rates in soils (Stout and Goh, 1980; Speratti and Whalen, 2008; Ernst et al., 2009). Thus, earthworms could have contributed to the 20% irrigation treatment effect on soil C storage by increasing Rs . There had been no earthworms in the soil at Twizel where the relations between Rs and Ts and soil water deficit had been determined and applied to the soils at Winchmore for this study. Moreover, soil water deficit negatively affected earthworm density that has corresponded with significantly reduced sheep dung decomposition rate during summer in New Zealand pastures compared to winter when conditions were wetter (Allard et al., 2004). Winchmore has been the site of New Zealand’s longest running pastoral agriculture trial and there appears to have been no other published, long-term studies examining irrigation effects on grassland soil C storage. An alternative approach might have been a natural experiment whereby soil C storage was compared for a pair of sites with a similar thermal regime and soils similar in texture, mineralogy, and structure, but differing in precipitation to a degree that reflects the irrigation requirement at Winchmore. To consider this approach, we begin with the long-term mean, annual rainfall at Winchmore of 741 mm. As stated earlier, for October through February, the mean rainfall was less than E by 213 mm. This water deficit and the annual rainfall yield an annual total water requirement of 954 mm. An effect of irrigation would then be estimated by comparing soil C storage at grassland sites differing only by their annual precipitation of 741 mm versus 954 mm. Statistically, samples of such sites would be preferred for the comparison. New Zealand data were not available to us. However, soil surveys were done during the 1980s in grasslands across the Great Plains region of the USA (Burke et al., 1989) and the 1980s and 2000s in grasslands across Northern China (Yang et al., 2010). Analysis of the American data for 500 soils, based on data reported by Burke et al. (1989) in their Fig. 2b, suggested a saturating (concave) relation between annual precipitation (independent variable, ranging from 250 to 950 mm) and C storage to a depth of 0.2 m. By this relation, increasing annual precipitation from 741 to 954 mm corresponded with a 5% increase in soil C storage. The data from China was reported by Yang et al. (2010) and portrayed in their Fig. 3b, representing around 300 sites and an annual precipitation

35

range of 200–700 mm. There was a logarithmic relation with soil C storage that was convex in shape and extrapolation by increasing annual precipitation from 741 to 954 mm suggested a 50% increase in soil C storage. Thus, the alternative approach yielded wide-ranging results that differed greatly from those obtained by the Winchmore trial. It can be difficult to understand treatment effects under field conditions and subsuming the complex interactions into available variables has not always led to an informative meta-analysis. 5. Conclusions For soils beneath pasture grazed by sheep and subjected to seasonal water deficit, sixty years of irrigation reduced carbon storage significantly, compared to un-irrigated control plots watered solely by rainfall. The irrigated soil’s carbon storage had changed by nearly 50% to 1 m depth with nearly half the integrated change in the upper 0.25 m. Irrigation during summer significantly increased the estimated rates of carbon input to soils by litter fall, root production and excreta returns from the grazing animals. However, there was estimated to have been a much stronger effect by increasing soil water content when temperatures were highest during summer, increasing the rates of respiration and carbon loss to the atmosphere. These conclusions emphasise the enduring value of long-term field trials to challenge and improve our understanding of carbon storage in soils. Acknowledgements Funding to FMK was provided by the New Zealand Agricultural Greenhouse Gas Research Centre. Additional funding was provided by the Agricultural and Marketing Research and Development Trust (AGMARDT), the New Zealand Ministry of Agriculture and Forestry, and Environment Canterbury. FMK thanks Ray Moss, Tony Parsons and Louis Schipper for valuable discussions. Scott Rains kindly shared his compilation of daily weather measurements made at Winchmore during the years 1966–2006. Chikako van Koten analysed the rainfall and evaporation time series data. References Allard, V., Newton, P.C.D., Lieffering, M., Soussana, J.-F., Grieu, P., Matthews, C., 2004. Elevated CO2 effects on decomposition processes in grazed grassland. Glob. Change Biol. 10, 1553–1564. Baisden, W.T., Parfitt, R.L., 2007. Bomb 14 C enrichment indicates decadal C pool in deep soil? Biogeochemistry 85, 59–68. Blaxter, K.L., Graham, N.M.C.C., 1955. Plane of nutrition and starch equivalents. J. Agric. Sci. 46, 292–306. Clark, H., Ulyatt, M., 2002. A recalculation of enteric methane emissions from New Zealand ruminants 1990–2000 with updated emissions predictions for 2010. Report to Ministry of Agric. and Forest, Wellington, pp. 1–38. Conant, R.T., Paustian, K., Elliot, E.T., 2001. Grassland management and conversion into grassland: effects on soil carbon. Ecol. Appl. 11, 343–355. Condron, L.M., Goh, K.M., 1989. Effects of long-term phosphatic fertiliser applications on amounts and forms of phosphorus in soils under irrigated pasture in New Zealand. J. Soil Sci. 40, 383–395. Condron, L.M., Sinaj, S., McDowell, R.W., Dudler-Guela, J., Scott, J.T., Metherell, A.K., 2006. Influence of longterm irrigation on the distribution and availability of soil phosphorus under permanent pasture. Aust. J. Soil Res. 44, 127–133. Cook, F.J., Knight, J.H., Kelliher, F.M., 2007. Oxygen transport in soil and the vertical distribution of roots. Aust. J. Soil Res. 45, 101–110. Cook, F.J., Kelliher, F.M., 2006. Determining vertical root and microbial biomass distributions from soil samples. Soil Sci. Soc. Am. J. 70, 728–735. Ellert, B.H., Janzen, H.H., Entz, T., 2002. Assessment of a method to measure temporal change in soil carbon storage. Soil Sci. Soc. Am. J. 66, 1687–1695. Ernst, G., Henseler, I., Felten, D., Emmerling, C., 2009. Decomposition and mineralisation of energy crops governed by earthworms. Soil Biol. Biochem. 41, 1548–1554. Fornara, D.A., Steinbeiss, S., McNamara, N.P., Gleixner, G., Oakley, S., Poulton, P.R., Macdonald, A.J., Bardgett, R.D., 2011. Increases in soil organic carbon sequestration can reduce the global warming potential of long-term liming to permanent grassland. Glob. Change Biol. 17, 1925–. Fraser, P.M., Piercy, J.E., 1996. Effects of summer irrigation on the seasonal activity, population size, composition and biomass of Lumbricid earthworms in a

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