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Hill slope effects on the vertical fluxes of photosynthetically fixed [sup.14]C in a grazed pasture.

Abstract

New Zealand hill-country farms consist of an amalgamation of land units in different slope and aspect categories, each with unique production potentials. Information on the influence these slope categories have on carbon (C) partitioning is imperative for more accurate and complete understanding of C inputs and fluxes through a grazed hill pasture ecosystem. The effects of 3 slope categories [representing 1-12 [degrees], 13-25 [degrees], and [is greater than or equal to] 26 [degrees] microtopographical units corresponding to low (L), medium (M), and steep (S) slopes] on the vertical translocation of photosynthetically fixed C was studied by using a [sup.14]C-[CO.sub.2] pulse-labelling chamber technique. Pasture and soil samples were taken after 4-h, 7-day, and 35-day chase periods, to examine the fluxes of [sup.14]C in the pasture plant-root-soil system. Total C and [sup.14]C were determined in the pasture shoot, root, and soil components. Microbial biomass C and [sup.14]C contents in each soil were also determined using the chloroform fumigation-extraction technique.

Pasture composition and growth varied with slope category. High fertility grasses (90%) were dominant on the L slope while low fertility grasses ([is greater than or equal to] 60%) were dominant on the M and S slopes. Shoot growth over 35 days amounted to 4470, 2045, and 1308 kg/ha at the L, M, and S slopes, respectively. The standing root biomass did not differ significantly among the slopes. Allocation of the [sup.14]C-labelled assimilate below-ground was rapid, with 23-35% detected in the roots within 4 h of pulse-labelling. The above- and below-ground partitioning of [sup.14]C varied with the length of the chase period, and was strongly influenced by slope. Pasture plants allocated more C below-ground in the M and S slope categories. During the study period, 173 kg C/ha was assimilated daily at the L slope site, with 73 kg being respired, 50 kg remaining above-ground in the shoot, and 43 kg being partitioned into the root. In comparison, at the S slope, of the 56 kg/ha C assimilated daily, 22 kg was respired, 14 kg remained in the shoot, and 18 kg was partitioned into the root, and the daily input to the soil varied between 2 and 7 kg C/ha. By using annual growth measurements from adjacent areas, the amounts of C translocated annually to roots and soil at each slope category were also estimated from the [sup.14]C distribution of spring growth. At the L slope site, 9340 kg C/ha was respired, 6375 kg remained above-ground in the shoot, and 5510 kg was translocated to roots and 930 kg to soil. At the S slope site, 5710 kg C/ha was respired, 3490 kg remained in the shoot, and 4490 kg was translocated to the roots and 555 kg to soil.

Additional keywords: [sup.14]C pulse-labelling, microbial biomass, carbon inputs, carbon fluxes, carbon budgets.

Introduction

Soils are the major sink for carbon (C) in terrestrial ecosystems and account for two-thirds of the total carbon pool (Schimel et al. 1995). Soil organic C is continuously decomposed and mineralised. Maintenance of soil organic C levels is dependent on continuous inputs of organic materials. Organic C inputs into the soil are strongly influenced by land use (Walker et al. 1959; Jackman 1964; Giddens et al. 1997; Saggar et al. 1997; Lambert et al. 1998). Within a pasture system, Saggar et al. (1997), using a [sup.14]C-[CO.sub.2] pulse-labelling chamber technique (Saggar and Searle 1995), showed that above- and below-ground partitioning of photosynthetically fixed C was strongly influenced by phosphorus (P) fertility, with a greater proportion of assimilated C also partitioned below-ground under low P status. This suggested that, at low fertility, pasture plants allocated more resources below-ground for the acquisition of nutrients. A number of studies support this suggestion. The study by Saggar et al. (1997) provided the first data on C fluxes for constructing C budgets for a grazed hill-country ecosystem. However, these data were limited to pasture plants growing on only one slope class. Hill-country consists of an amalgam of land slope and aspect categories, each with different production (Lambert et al. 1983) and nutrient levels (Saggar et al. 1990a, 1990b), despite a common annual fertiliser input. The uneven return of nutrients through animal excreta, grazing intensity, and differences in soil depth and development with slope are some of the dominant factors causing these differences. Pasture production and nutrient accumulation levels are higher on gentle slopes and decrease with increasing slope. Changes in pasture production in slope categories will impact not only on the amount of C assimilated, but also on the amount of assimilated C partitioned to roots, the amount of root-derived soil C, and the amount respired. Thus, information on the influence that slope categories have on C-partitioning is an essential addition to the current working model describing C inputs and fluxes in the grazed hill pastures in New Zealand.

This paper reports on the effect of slope category (1 [degrees] to [is greater than or equal to] 26 [degrees]) on the assimilation and partitioning of photoassimilated C in the plant-soil-root components of a sheep-grazed pasture. We used the [sup.14]C pulse-labelling technique which has the advantage of ease and rapidity of use for studying fluxes through different pools in the plant, but gives little information on the degradation of older parts (Whipps 1990). We investigated a number of factors which possibly influence the [sup.14]C distribution after labelling, such as the time of day and the duration of the allocation period (Saggar and Searle 1995), and suggested that an allocation period of 35 days is appropriate for pasture pulse-labelling.

Materials and methods Sites

The [sup.14]C pulse-labelling experiments were conducted in Spring 1995 at 'Ballantrae' AgResearch Hill Country Research Station, 20 km north-east of Palmerston North, in the foothills of the Southern Ruahine Range. The average annual rainfall at the site is 1200 mm. Three hill pasture sites representing 1-12 [degrees], 13-25 [degrees], and [is greater than or equal to] 26 [degrees] microtopographical units corresponding to low (L), medium (M), and steep (S) slope categories were used for this study. The microtopographical units ranged from 1 to 10 [m.sup.2]. The sites were in a high fertility (HF) farmlet that had continuously received 375 kg/ha.year of single superphosphate (SSP) since 1980, with previous applications of 500 kg SSP/ha.year from 1975 to 1980. Aspect classes centred approximately around east, south-west, and north-west cover climatic effects. We used 3 slope classes on the east aspect, which has intermediate climatic effects and allowed effective partitioning of the source of variation in pasture production (Lambert et al. 1983) and species composition (Table 1). The average annual pasture production at the L (1-12 [degrees]), M (13-25 [degrees]), and S ([is greater than or equal to] 26 [degrees]) sites was 15400, 12070, and 8890 kg DM/ha, respectively (M. G. Lambert, pers. comm).

Table 1. Chemical and biological properties of soil on each slope category

Values represent mean of 3 replicates [+ or -] s.e. within a slope category
Soil property Slope category
 Low Medium

p[H.sub.w] 5.46 [+ or -] 0.12 5.36 [+ or -] 0.10
Total C (%) 7.10 [+ or -] 0.21 5.47 [+ or -] 0.32
Total N (%) 0.56 [+ or -] 0.07 0.39 [+ or -] 0.05
Total P (%) 0.13 [+ or -] 0.03 0.09 [+ or -] 0.02
Inorganic N (mg/kg soil) 63 [+ or -] 7 38 [+ or -] 9
Olsen P (mg/kg soil) 80 [+ or -] 8 41 [+ or -] 7
Microbial C (mg/kg soil) 1234 [+ or -] 11 897 [+ or -] 17
Microbial N (mg/kg soil) 291 [+ or -] 5 260 [+ or -] 7

Soil property
 Steep

[pH.sub.w] 5.40 [+ or -] 0.08
Total C (%) 4.99 [+ or -] 0.24
Total N (%) 0.32 [+ or -] 0.07
Total P (%) 0.07 [+ or -] 0.02
Inorganic N (mg/kg soil) 29 [+ or -] 9
Olsen P (mg/kg soil) 27 [+ or -] 5
Microbial C (mg/kg soil) 737 [+ or -] 6
Microbial N (mg/kg soil) 217 [+ or -] 8


Full details of techniques for [sup.14] [CO.sub2]-C pulse-labelling, shoot, root, and soil sampling and analyses are presented elsewhere (Saggar and Searle 1995; Saggar et al. 1997) and summarised below.

Labelling

Three replicates were established on each slope category. Pasture was cut to 2 cm height, 1 week before [sup.14]C pulse-labelling. A representative area (25 cm diam.) was pulse-labelled using a fishbowl-PVC flanged chamber.

Sampling

Four soil cores (36 mm diam.) were taken to a depth of 10 cm, from each of the 3 replicates at each site on Days 0 (4 h after labelling), 7, and 35. The above-ground parts of the labelled pasture plants were clipped from all 4 cores. Roots were then separated from 2 soil cores by gentle shaking and wet-sieving. Wet-sieved subsamples from the 2 remaining cores were used for total C, [sup.14]C, and microbial biomass C and N analyses. Oven-dried (65 [degrees] C) samples of the above-ground pasture, root biomass, and subsamples of the sieved soil were analysed for total C, N, and P, cation exchange capacity, and pH.

Analyses

Soil chemical properties

Soil pH (1:2.5 water) and cation exchange capacity (CEC) were determined according to Blakemore et al. (1987). Total N was determined by a semi micro-Kjelda, hl digestion method, followed by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] measurement by autoanalyser, and total P was measured on the same Kjeldahl digest (Twine and Williams 1971).

Total C

Total C in soils, pasture shoots, and roots was determined following oxidation and digestion by using a modified digestion-tube apparatus incorporating a [CO.sub.2] trap. A known aliquot of trapping solution was used for liquid scintillation counting and for estimation of total [sup.14]C.

Microbial C

Microbial C was determined by a fumigation-extraction method (Vance et al. 1987). The additional oxidisable C and [sup.14]C counts obtained from the fumigated soils were taken to represent the microbial-C flush and converted to microbial-biomass C by using the relationship (Sparling and West 1988):

Microbial C = C flush/0.35

Microbial N

Microbial N was determined from [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentrations of the persulfate-digested 0.5 M [K.sub.2] [SO.sub.4] extracts from fumigated and non-fumigated soil samples (Ross 1992) and microbial N, estimated by using the relationship (Jenkinson 1988):

Microbial N = N flush/0.45

[sup.14]C-[CO.sub.2] respiration loss

A total [sup.14]C budget for labelled pools was calculated from the total [sup.14]C recovered. The respiratory losses of [sup.14]C [CO.sub.2] after 7 and 35 days were calculated as the difference between total recoveries of assimilated [sup.14]C after these periods and the recoveries after 4 h.

Annual C fluxes

Estimates of the annual amounts of C assimilated, translocated to roots and added to soil, were made according to Saggar et al. (1997) using the annual dry matter production measurements from adjacent areas at each slope category. It was assumed that, at steady state, the distribution of net fixed [sup.14]C in the pasture-root-soil system represents the average partitioning of assimilate:

(1) Estimated assimilated C = [A.sub.shoot] [C.sub.shoot]/[sup.14]C [C.sub.shoot]

where `estimated assimilated C' is the annual flux (kg C/ha), [A.sub.shoot] is the annual shoot growth (kg C/ha), [C.sub.shoot] is the shoot C concentration (%), and [sup.14]C shoot is the percentage of net assimilated [sup.14]C in shoots at Day 35. The estimated assimilated C was then divided among plant-soil components on the basis of the percentage of the [sup.14]C distribution at Day 35.

The moisture content of the soil was determined by oven-drying at 105 [degrees] C to a constant weight. All results are expressed on an oven-dry (105 [degrees]C) weight basis, unless otherwise stated.

Statistical analyses

The significance of differences between sites was assessed by analysis of variance and Fisher's l.s.d. test. The [sup.14]C data for each replicate were expressed as percentages of net assimilated [sup.14]C in the plant-soil system. After transformation to arcsine square roots, they were subjected to analysis of variance (Little and Hills 1977) to determine the statistical significance of the effect of slope and time after labelling.

Results

Site characteristics

Soils from the 3 slopes differed in mineral N, inorganic P, and total C, N, and P contents (Table 1), reflecting the different amounts of nutrient returns to these sites in animal excreta. Animal excreta represent a critical pathway in the nutrient cycle of these grazed pasture ecosystems. Microbial biomass C and N also varied between these slope categories, with microbial C and N declining with increasing slope. Soil moisture contents decreased with increasing slope.

Pasture growth

During the 35-day study, pasture shoot growth amounted to 4470, 2045, and 1308 kg DM/ha in the L, M, and S slopes, respectively (Table 2). High fertility grasses (90%) dominated at the L site, while low fertility grasses ([is greater than or equal to] 60%) dominated at the M and S sites. There was an appreciably higher proportion of legumes on the S slope (Table 2) reflecting the lower N status of the S site. Standing root biomass in the 0-10 cm soil depth remained almost stable over the period of the field experiment in each of the slope categories, and therefore data are presented as averages for the 3 samplings (Table 2). Root biomass did not differ with slope, averaging 11330, 13310, and 12210 kg dry matter/ha for the L, M, and S slopes, respectively.

Table 2. Pasture growth on each slope category during the 35-day [sup.14]C-labelling study, and pasture composition and standing root biomass at the end of the 35-day growth

Values represent mean of 3 replicates [+ or -] s.e.
 Low

Pasture shoot biomass (kg DM/ha) 4470 [+ or -] 266
Tiller density (tillers/[m.sup.2]) 39 827 [+ or -] 4031
Pasture composition (%)
 High fertility grasses 90
 Low fertility grasses 9
 Legume <1
 Others <1
Pasture growth rate (kg DM/ha-day) 120 [+ or -] 8
Standing root biomass (kg/ha)(A) 11 330 [+ or -] 1214

 Slope category
 Medium

Pasture shoot biomass (kg DM/ha) 2045 [+ or -] 50
Tiller density (tillers/[m.sup.2]) 26 523 [+ or -] 1647
Pasture composition (%)
 High fertility grasses 29
 Low fertility grasses 60
 Legume 7
 Others 3
Pasture growth rate (kg DM/ha-day) 55 [+ or -] 2
Standing root biomass (kg/ha)(A) 13 310 [+ or -] 1340

 Steep

Pasture shoot biomass (kg DM/ha) 1308 [+ or -] 116
Tiller density (tillers/[m.sup.2]) 20 629 [+ or -] 1987
Pasture composition (%)
 High fertility grasses 8
 Low fertility grasses 61
 Legume 21
 Others 10
Pasture growth rate (kg DM/ha-day) 35 [+ or -] 3
Standing root biomass (kg/ha)(A) 12 210 [+ or -] 763


(A) Values represent mean ([+ or -] s.e.) of 3 replicates averaged for the 4-h, 7-day, and 35-day samples.

Mean pasture shoot and root C contents ranged between 39-41% and 34-37%, respectively (Table 3), and varied slightly with slope and harvest; however, there was no consistent influence of either slope or sampling period (data not shown). Nitrogen and P contents of shoots were consistently higher than those of roots, and decreased in shoots and roots with increasing slope (Table 3). The C:N ratios of the shoots at L, M, and S slopes ranged from 16 to 29, compared with 39 to 55 for the roots. The C:P ratios of the shoots at L, M, and S slopes ranged from 91 to 131, compared with 240 to 293 for the roots.

Table 3. Pasture shoot and root carbon (C), nitrogen (N), and phosphorus (P) contents, and nutrient ratios from each slope category

Values represent mean ([+ or -] s.e.) of 3 replicate 7-day, and 35-day samples
Nutrient Slope category
content Low Medium

Shoot C (%) 41-4 [+ or -] 0.8 39.2 [+ or -] 2.1
Root C (%) 36.8 [+ or -] 2.2 35.1 [+ or -] 0.4
Shoot N (%) 2.59 [+ or -] 0.40 1.53 [+ or -] 0.05
Root N (%) 0.96 [+ or -] 0.02 0.70 [+ or -] 0.03
Shoot P (%) 0.45 [+ or -] 0.02 0.41 [+ or -] 0.03
Root P (%) 0.15 [+ or -] 0.01 0.15 [+ or -] 0.01
Shoot C: N 16 26
Shoot C: P 92 96
Shoot N: P 5.7 3.7
Root C: N 39 50
Root C: P 240 234
Root N:P 6.2 4.7

Nutrient
content Steep

Shoot C (%) 39.3 [+ or -] 1.5
Root C (%) 34.2 [+ or -] 0.7
Shoot N (%) 1.34 [+ or -] 0.03
Root N (%) 0.62 [+ or -] 0.01
Shoot P (%) 0.30 [+ or -] 0.05
Root P (%) 0.12 [+ or -] 0.01
Shoot C: N 29
Shoot C: P 131
Shoot N: P 4.5
Root C: N 55
Root C: P 293
Root N:P 5.3


[sup.14]C-distribution

The distribution of [sup.14]C, expressed as a proportion of net assimilated [sup.14]C -[CO.sub.2] (sum of [sup.14]C recovered in shoot, root, and soil within 4 h after labelling), varied with the length of the chase period and slope (Table 4).

Table 4. Distribution of [sup.14]C (%) in the shoot, root, soil, and microbial biomass of pastures on each slope category at 4 h, 7 days, and 35 days after pulse-labelling

For each property, values followed by the same letter are not significantly different at P = 0.05; values for respired [sup.14]C-[CO.sub.2] were determined by difference and are not compared statistically
System Slope category
component Low Medium Steep

 4h
Shoot 61c 66b 72a
Root 35a 30b 23c
Soil 4.1a 4.1a 4.6a
Microbial biomass 0.7c 1.3b 1.7a
Respired (n.d.) (n.d.) (n.d.)

 7 days
Shoot 39f 45e 48d
Root 34a 31b 25c
Soil 3.9a 4.5a 4.6a
Microbial biomass 0.5c 1.4b 2.2a
Respired(A) 24 20 23

 35 days
Shoot 29g 27h 25i
Root 25c 30b 32b
Soil 4.2a 4.0a 3.9a
Microbial biomass 0.7c 1.3b 2.2a
Respired(A) 42 40 40


(n.d.), not determined.

(A) Calculated as a difference between amounts of [sup.14]C-[CO.sub.2] at 4 h, and not received in standing herbage, root, or soil.

Shoot

Initially (4 h after labelling), the majority of the [sup.14]C (61%, 66%, and 72% in the L, M, and S slopes, respectively) was in the shoot biomass. The percentage of [sup.14]C recovered in the shoots declined with time on all 3 slope categories, with the amount remaining highest on the L slope and lowest on the S slope at Day 35.

Root

Within 4 h of pulse application, 23-35% of the [sup.14]C was detected in roots below-ground. Photoassimilate translocation continued between 4 h and 7 days on the M and S slopes, reflecting the slow biological activity, as indicated by slower growth rates for these 2 slope categories. By Day 35, 32% of gross assimilated [sup.14]C was present in the root biomass on the S slope pasture site, compared with 30% and 25% on the M and L slopes. Thus, about 7% more [sup.14]C was partitioned below-ground in the S slope pasture compared with the L slope pasture.

Soil microbial biomass

Although the microbial biomass C pool did not vary significantly during the study period, significant differences were observed in the specific activity of the microbial biomass with slope. Microbial biomass [sup.14]C content was highest (1.7-2.2% of the total [sup.14]C assimilated) on the S slope soil and lowest (0.5-0-7% of the total [sup.14]C assimilated) on the L slope soil, reflecting the size of, and flux through, the respective pools (Table 4).

Respiration

At Day 35, 40-42% of the [sup.14]C label was not recovered (Table 4) and is assumed to have been lost by shoot, root, and soil respiration. A small proportion of this unaccounted [sup.14]C may have been transferred to greater depths, but our below-ground measurements were confined to only 10 cm. As in our previous study (Saggar et al. 1997), it was not possible to distinguish between shoot, root, and soil respiration.

Carbon budget

After 35 days, the L, M, and S slope pasture had assimilated 173, 81, and 56 kg C/ha.day, respectively. This implies a decrease in carbon as the slope increases. Photosynthetic C remaining in the above-ground biomass was 50, 22, and 14 kg C/ha. day on the L, M, and S slopes, respectively (Fig. 1). By using the distribution of [sup.14]C after 35 days, the amount of C translocated to the roots was calculated as 43, 24, and 18 kg C/ha.day for the L, M, and S sites, respectively. Estimates of the annual C budget from the annual dry matter production on the 3 slope classes were 5510, 5210, and 4490 kg C/ha translocated to root, and 930, 700, and 555 kg to soil in the L, M, and S slope categories, respectively (Fig. 2).

[Figures 1-2 ILLUSTRATION OMITTED]

Discussion

In accordance with previous studies (e.g. Rattray et al. 1995; Saggar and Searle 1995; Saggar et al. 1997), there was an initial rapid allocation of the [sup.14]C-labelled assimilate below-ground (6-9% per h) in these legume-based pastures. The translocation of [sup.14]C -labelled assimilate continued at a reduced rate, and by Day 35, less than 30% of the [sup.14]C was found in the shoot. The respiratory [sup.14]C-[CO.sub.2] losses, calculated as the difference between the total amounts of [sup.14]C recovered in the soil-plant system at 7 and 35 days, suggest that about 20% of [sup.14]C was lost during each period. This respiratory loss was equivalent to 3% per day of the gross assimilated [sup.14]C between 4 h and 7 days and declined to [is less than] 1% between Days 7 and 35. This pattern of translocation and respiration of labelled C is typical of pulse-labelling studies (Swinnen et al. 1994; Rattray et al. 1995; Saggar et al. 1997). The recently assimilated C is rapidly translocated to the roots. The C remaining in the shoot tissue is presumably in relatively stable forms as storage and structural compounds. Prosser and Farrar (1981) have indicated that an initial pulse of [sup.14]C appears in the soil from soluble organic root exudates followed by a more constant release from labelled storage and structural materials. The final balanced allocation of [sup.14]C in the plant is reached when the metabolic components of the shoot and root are depleted of [sup.14]C and the plant-soil system is at steady-state. In our previous studies (Saggar and Searle 1995; Saggar et al. 1997), based on the dynamics of both the decrease of [sup.14]C in shoots and [sup.14]C respiration rate, we used an allocation period of 35 days.

Under low-slope conditions of high fertility and non-limiting moisture, a higher proportion of [sup.14]C remained in the above-ground component of the sward, whereas under steep-slope conditions of low fertility and limited water, a higher proportion of [sup.14]C was recovered in the below-ground component of the sward at Day 35. This phenomenon is consistent with the hypothesis that, under limited fertility and moisture conditions, plants respond to a relative shortage of any essential resource by increasing allocation to the structures and functions responsible for the acquisition of that limiting resource or by decreasing the loss of that limiting resource (Chapin 1991). As the capacity of plants to obtain nutrients is dependent on C substrate for root production (Clement et al. 1978), an increase in photoassimilate allocation to roots is expected under conditions of decreased nutrient and moisture availability (Bloom et al. 1985; Hamblin et al. 1990; Mehrag and Killham 1990; Palta and Gregory 1997; Saggar et al. 1997). In our study, the root biomass did not differ significantly with slope and there was little change in the standing root biomass during the study period. However, there was a consistent increase in the proportion of [sup.14]C assimilate translocated below-ground on the S slope pasture (Table 4). The chemical analysis of the shoot and root materials showed decreased N and P concentration with increased slope (Table 3). Our results demonstrate that the response of pasture plants on steep slopes is to allocate a greater proportion of the assimilated C to roots for the acquisition of moisture and nutrients. The study also shows that increased slope alters sward composition and reduces pasture growth and C assimilation. At Day 35, the total C assimilated by the S slope pasture was only one-third that of the L slope pasture. In addition to the lower C assimilation, the proportion of [sup.14]C retained in the shoot decreased and that translocated below-ground increased with an increase in slope. The C:N and C:P ratios in the pasture shoots and roots (Table 3) are a further reflection of the nutrient availability. These results suggest that, at S slopes, limiting nutrient and moisture conditions lead to low concentrations of limiting nutrients in plant biomass and to accumulation of carbohydrates. The pasture plants responded by increasing the proportional allocation to roots.

The proportion of [sup.14]C recovered in the microbial biomass on the S slope was 3 times greater than on the L slope, despite 68% more rhizodeposition on the L slope. This suggests that, although pasture plants were releasing similar proportions of C compounds into the rhizosphere on all 3 slopes (3.9-4.2%), these compounds were utilised to a greater extent by the microbial population on the steep slope. Enhanced microbial utilisation of rhizodeposition may partly be due to the higher quality of compounds released (Paterson et al. 1996) compared with most of the organic residues in the bulk soil, or the greater dependence of the microorganisms on steep slopes on root exudates (because of low amounts) as a substrate for growth.

The amount of assimilated C decreased with an increase in pasture slope and was 173, 81, and 56 kg/ha [multiplied by] day on the L, M, and S slope pasture sites, respectively. The C remaining in the shoot was 50, 22, and 14 kg C/ha-day at the L, M, and S slope pasture sites, respectively, and the amount translocated to the roots was 43, 24, and 18 kg C/ha [multiplied by] day for the L, M, and S sites, respectively (Fig. 1). The difference in total daily C assimilated in the shoot was 3.6-fold between the L and S slope sites, whereas the daily C allocated to roots differed by only 2.4-fold; this indicates that a higher proportion of assimilated C was translocated below-ground at the S slope site. The estimates of C transferred to the pasture roots are similar to previous data for grazed pastures (Saggar et al. 1997), but higher than those reported (3-8 kg C/ha [multiplied by] day) for field-grown wheat (Whipps 1990). As perennial pasture plants maintain roots throughout the year, they are likely to invest more of their productivity in root material than annuals. Native grasslands (Agropyron koeleria) in Canada have been reported to transfer 34 [multiplied by] 5-54 [multiplied by] 1% of assimilated C to roots (Warembourg and Paul 1977) compared with 6 [multiplied by] 5-35 [multiplied by] 6% in newly sown pastures. Established pastures under a temperate climate may, therefore, transfer more C to roots compared with wheat or barley. Secondly, our experiments were undertaken during a very active and vigorous pasture growth period in spring. During this 35-day period, herbage growth amounted to 15-30% of the annual production. Therefore, these estimates are likely to be higher than the annual daily average values of C fluxes.

Estimates of the annual amounts of C assimilated, and translocated to roots and soil, were made by relating the distribution of [sup.14]C-labelled photosynthate at steady-state (35 days) to these annual measurements of dry matter production. Accumulated over the year, the quantity of C translocated to the roots was 4490-5510 kg C/ha, and to soil 555-930 kg (Fig. 2). Recent work examining long-term changes in soil carbon status of these soils by Lambert et al. (1998) shows that these below-ground inputs are insufficient to sustain current soil C levels. They showed an annual net loss of 200 kg C/ha on these sites. Our annual estimates of C inputs of 555-930 kg C/ha to these soils would suggest an annual loss of 755-1130 kg C/ha at these sites.

Conclusions

This study provided data on the fluxes and quantities of C partitioned in the pasture-root-soil system of a New Zealand hill pasture. Our results indicate that, where slope influences plant growth, there are likely to be concurrent effects on rhizosphere translocation and deposition. The utilisation of 14C-labelled rhizodeposits by the rhizosphere microbial biomass increased with slope. This study has also demonstrated the reduction in pasture growth with increasing slope due to decreased moisture and nutrient availability, and sward composition resulted in greater partitioning and translocation of photoassimilated C to roots, to compensate for the limited resource available. Data from the present study and our previous study (Saggar et al. 1997) will be used to model C turnover in grazed pastures and for determining C budgets for pastoral systems, which cover 50% of the New Zealand land surface. However, these estimates are based on the patterns of C distribution during very active pasture growth in spring. Work by Barker et al. (1988) and Saggar et al. (1997) shows that annual root growth rates vary 3- to 4-fold, being the highest in spring and the lowest in winter, in keeping with the above-ground biomass accumulation. The extent to which these differences in growth rate influence the partitioning of photoassimilated C to the root and to soil has not been measured. Studies addressing this issue and the effect of season on respiration rates are the focus of our current research.

Acknowledgments

Our work was funded by the Foundation of Research, Science and Technology (contract number C09811).

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Manuscript received 30 November 1998, accepted 25 March 1999

S. Saggar(AC), A. D. Mackay(B), and C. B. Hedley(A)

(A) Landcare Research, Private Bag 11052, Palmerston North, New Zealand.

(B) AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand.

(C) Corresponding author, email: Saggars@Landcare.cri.nz
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Author:Saggar, S.; Mackays, A.D.; Hedley, C.B.
Publication:Australian Journal of Soil Research
Geographic Code:8NEWZ
Date:Jul 1, 1999
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