Dynamics of c in a pasture grass (panicum maximum var. Trichoglume)—soil system

Soil Bid. Biochem. Vol. 24, No. 4, pp. 381-387, Printed in Great Britain. All rights reserved

1992 Copyright

DYNAMICS OF C IN A PASTURE GRASS MAXIMUM VAR. TRICHOGLUME)-SOIL

0

0038-0717/92 $5.00 + 0.00 1992 Pergamon Press plc

(PANICUM SYSTEM

H. V. A. BUSHBY,* I. VALLIS and R. J. K. MYERS CSIRO Division of Tropical Crops and Pastures, 306 Carmody Road, St Lucia, Brisbane 4067, Australia (Accepted 14 October 1991) Summary-A lack of available nitrogen is the primary mineral constraint to the maintenace of productivity by grass-based pastures in northern Australia. It is hypothesised that under N stress, plants maximize soil exploration by the allocation of a large proportion of their resources below-ground which imposes constraints on yield from tops. A carbon balance of mature plants of the C, grass green panic (Panicurn maximum) was conducted to investigate this hypothesis. Plants were grown for 88 days in a chamber containing atmospheric concentrations of CO, labelled with ‘YY.The effects of nitrogenous fertilizer, removal of shoots and shading on the partitioning of dry matter and 14Cbetween plant parts and to the soil, and respirational losses of 14Cfrom roots and rhizosphere microorganisms were measured. Additions of fertilizer restored the productivity of rundown plants by increasing the dry matter (DM) and r4C content of shoots and crowns. Root DM was not affected by fertilizer but the 14Ccontent was increased. Thus, N fertilizer increased the proportion of DM allocated to shoots relative to roots whether or not shoots were removed. Although the absolute amounts of 14Clost as CO, from root-rhizosphere respiration increased as a result of applications of fertilizer N, the proportional losses were not affected. Over all treatments, loss of 14Cby root-rhizosphere respiration was highly correlated with total plant DM (r* = 0.78). Removal of shoots did not affect shoot DM production but it did decrease the size of the crowns for both N treatments. The amount of 14Cin the shoots of rundown (but not of fertilized) plants was increased by cutting, while for crowns, roots and the microbial biomass, the amount of 14Cwas reduced in both rundown and fertilized plants. Shading rundown plants decreased shoot DM and 14C in the shoots, crowns, roots, biomass and soil organic matter, plus that lost through respiration compared to rundown plants in full sunlight. It is concluded that rundown in green panic pastures is primarily due to the allocation of a large proportion of dry matter to the roots in response to low availability of soil N. Increased yields associated with the alleviation of N deficiency in uncut plants resulted primarily from the direction of recent photosynthate to shoots and to a lesser extent, from mobilization of ‘*C from roots.

INTRODUCTION

Sown grass pastures in northern Australia decline in productivity with age. Thus, of the 3.3 million ha of sown grass in Queensland, 70% exhibit a marked decline in production 3-5 yr after establishment, a decline that is particularly noticeable on fertile clay soils (Catchpoole, 1981; Myers et al., 1987; Robbins et af., 1987). Although organic N contents in the clay soils are often relatively high (0.254.30%, Russell, 1981) and cereal crops show no decline in productivity after 20 yr continuous cropping (J. S. Russell, pers. commun.) observations that fertilizer N will restore productivity of sown grass pastures (Henzell, 1977) have led to the conclusion that the decline is due to inadequate rates of mineralization of soil organic N. Root-derived C compounds of high C:N ratio can exert major effects on the N cycle since they increase the requirement of soil microbes for N in direct competition with the plant. It follows therefore, that *Author for correspondence.

the movement of C and N both within green panic plants (Panicum maximum) and between plant and soil could be an important factor in the rundown of these pastures. The work reported in this paper was part of a project to define a C and N balance for green panic. In particular, our aim was to describe the effect of cutting and the application of N fertilizer on the distribution of photosynthate from the shoots to the roots, to the soil organic matter and to the microbial biomass, and to measure the loss of C as CO, via respiration of roots and rhizosphere microorganisms. The hypothesis was that under conditions of N deficiency, root growth of green panic plants is stimulated in relation to shoot growth, and this acts as a large sink for photosynthate which imposes significant limits upon the growth of shoots. A further aim was to investigate the effects of shading on the allocation of C to various components of the plant-soil system. This was an attempt to explain the findings of Wilson et al. (1986) who observed that a shaded rundown green panic pasture produced greater yields and higher tissue N contents than adjacent plants in full sunlight.

381

382

H. V. MATERIALS

A. BUSHBYet

AND METHODS

Intact cores (0.24 m dia, 1 m deep) containing one green panic plant were taken (Marchant et al., 1987) from a 20 yr old, rundown pasture on a prairie soil (Dr 4.13, Northcote, 1979, D. J. Ross, pers. commun.) at the CSIRO Narayen Research Station near Mundubbera, SE. Queensland (25=42’S, 150”53’E). The cores were transported to the CSIRO Division of Soils, Adelaide, where they were placed in a cabinet in which the atmospheric concentration of 14C02 was kept constant (Martin et al., 1992). Day-night air temperatures inside the cabinet were maintained between 15-35°C. Plants were grown in an atmosphere containing 0.03-0.10% CO1 with a specific activity of 0.53 MBq gg’ C. There were five treatments arranged in a randomized block with 4 replications: (1) uncut, nil N; (2) cut, nil N; (3) uncut, plus N fertilizer; (4) cut, plus N fertilizer and (5) cut, nil N and shaded with “Sarlon” shade cloth (43% light transmission). Light transmission was measured with a quantum flux meter. Under the perspex cover of the cabinet (unshaded), transmission was 93% of full sunlight, and under the shade cloth, there was a 60% reduction from full sunlight. At the commencement of the experiment, the shoots of plants in the cut treatments were harvested to within 8 cm of the crown. Thereafter, shoots in the cut and shaded treatments were harvested at 40 and again at 69 days. Shoots in the uncut treatments were harvested once only, at the end of the experiment. Urea, equivalent to 100 kg ha-’ of N was added to each N fertilized treatment at the start of the experiment and 34 and 57 days later giving a total of 300 kg haa’ of N (1357mg of N per core). 15N (51.72mg N as (NH,),SO, at 72.3% enrichment, equivalent to 11.4 kg ha-’ of N) was mixed with i4N fertilizer and added as a single dose to each core at the start of the experiment. This enabled comparisons of the distribution of “N within the plant-soil system with results obtained from a field experiment, to determine whether cores in the artificial environment simulated field conditions. Gas tight seals and correction for leaks Gas tight seals to isolate the soil atmosphere from the shoots of the plants were difficult to obtain. Dow Corning 3 110 RTV Silicone Rubber was used with catalyst M (stannous octoate). This catalyst was somewhat toxic at recommended rates (0.5%) and it was necessary to reduce the concentration to 0.07%. This was the minimum which would vulcanize the elastomer and still give a seal strong enough to withstand the air pressures (7 kPa) required to flush CO, that accumulated in the air space above the roots to collection vessels. Even at this low catalyst concentration leaks occurred in some cores when the outer leaves died and shrank away from stem and elastomer. Inevitably some of the CO2 respired by roots and microbial biomass was then lost. Martin et al.

al

(1992) encountered similar problems but found that the amount of CO2 respired by roots and biomass could be accurately estimated from an equation involving the ratio of the specific activity of respired CO, to the specific activity of root tissue. Thus AC CO, respired = (l _ A) where A = Specific activity of respired CO, Specific activity of roots C = CO, respired cores (mgC)

from unplanted

This equation was used in the present experiment to calculate the total CO, respired by below-ground components (i.e. roots and microbial biomass). However, as the plants employed in this experiment had large root systems prior to commencement of labelling, meaningless results were obtained if the specific activities of the roots were used in the equation. Instead, the specific activity of the 14C02 in the enclosed atmosphere in which the plants were growing was used on the premise that respiration largely involved recently assimilated C rather than endogenous sources from earlier growth. This is an agreement with the published data (Noggle and Fritz, 1976). The amounts of 14C labelled C in shoots, crowns, roots and biomass were obtained from the following equation: 14C label (mg) per component

_

specific activity of component x mgC in component specific activity of enclosed

atmosphere

The ratio of specific activities was used because of the large amount of unlabelled C in each component of these mature plants prior to beginning the experiment. Harvest After 88 days the shoots and crowns were harvested. Roots from the 0 to 7.5, 7.5 to 15, 15 to 30 cm depths of each core were separated by washing from 1 kg subsamples of soil that had been passed through a 10 mm sieve. Additional 500 g subsamples of soil were air freighted to Brisbane where the total C and N, and the 14C and “N contents of the soil and the microbial biomass were determined. All plant and soil materials for total C, N, 14C and “N analyses were dried at 60°C. The 14Ccontent of the soil organic matter (SOM) pool for each soil depth was determined by subtracting the 14C content of roots plus biomass from the total 14C content of soil, which included roots plus biomass plus SOM. A correction of the root mass data was required because of the adhesion of fine clay particles to roots. The correction involved multiplying the measured mass of the roots plus clay particles by a factor y/40, where y = the measured percentage C content and 40 = the C content (%) of uncontaminated roots.

Carbon cycling in green panic Chemical and microbial analyses

The total C content and “C activity in plant, soil and respired CO2 from roots plus microorganisms were measured by the techniques described by Amato (1983). Microbial C was measured by the fumigation-incubation method with CO2 released from non-fumigated soil during l&20 days after the start of incubation as a control (Jenkinson and Powlson, 1976). The K, factor was 0.45 (Jenkinson, 1989). Microbial N was also measured by the fumigation-incubation method (Brookes et al., 1985) using a kN factor of 0.57 (Jenkinson, 1989). 14Cin plant material was determined with a Packard Tricarb combustion unit and Packard Tricarb 4000 series scintillation counter. 14Cin CO2 collected in NaOH for microbial biomass determinations was measured with the scintillation counter on 0.5 ml aliquots of diluted NaOH. Total N in soil was determined by Kjeldahl digestion after pretreatment with sodium thiosulphate to include nitrate (Dalal et al., 1984), and total N in plant material by digestion using the salicylic modification (Ashton, 1936). The “N enrichment was determined after distillation and titration of the digests (Buresh et al., 1982). The dried distillates were reacted with lithium hypobromite to produce N, gas in a preparation system (Ross and Martin, 1970) fitted to a Micromass 602E mass spectrometer. RESULTS

Distribution of “N in core andjeld

experiments

The distribution of “N within various components of the plant-soil system for the cores was compared with results from a field experiment conducted at the site from which the cores were collected. The distributions of “N within the plant and in the SOM were similar (Table 1) and indicated that in general, conclusions derived from this study using soil cores could be applied to field situations. The smaller proportions of added “N retained in the microbial biomass and mineral fractions in the cores than in the field microplots could be caused by differences in water regimes. Water in the soil cores was not limiting whereas in the field experiment significant periods of water stress occurred. This difference would favour turnover of N through the microbial biomass in the Table 1. Partitioning of 15N within various plant-soil components of the cores 88 days after application of “N compared with the distribution of “N within these components in the field 91 days after addition of ” N “N distribution (% total added + SE) component

cores

Field*

Shoots Roots SOM Microbial biomass Mineral

29.1 _+1.2 8.7 + 0.5 20.3 + 1.3 5.8 + 0.4 1.4 + 0.2

28.2 k 9.4 + 20.7 f 15.0 f 8.9 +

Total recovered

65.3 f 2.2

82.2 f 5.0

1.0 0.8 4.9 1.9 1.5

‘I. Vallis, V. R. Catchpoole, M. M. Ludlow and W. B. McGill, pers. commun.

383

cores. The lower recovery of mineral lSN and total 15N in the cores could have been caused by a combination of denitrification and leaching to below the sampling depth. Plant growth

The application of N fertilizer to plants increased the dry matter of shoots and crowns of both cut and uncut plants, but it had no effect on the yield of roots [Table 2(A)]. In contrast, cutting did not affect the yield of shoots but it decreased the dry matter of crowns in both N treatments and roots in the nil N treatment [Table 2(A)]. The only effect of shading was to greatly reduce the yield of shoots. No treatment affected the size of the soil microbial biomass. Mean monthly photosynthetically active radiation (PAR) levels in the cabinet for March, April and May were 36.6,26.9 and 14.7 E mm2 day-’ respectively. At 40% transmittance, mean monthly PAR for the shaded treatment was 14.5, 10.6 and 5.9 E m-’ day-’ for the 3 months. Nitrogen fertilizer increased the shoot:root (S:R) ratio in both cutting treatments by increasing the yield of shoots, while cutting increased the S:R ratio, although to a lesser extent in the nil N treatment, by reducing the mass of roots [Table 2(A)]. The low S:R ratio for shaded plants reflected the reduction in yield of shoots. Partitioning of ‘T Effects of N fertilizer. Fertilizer N increased the amount of 14Cin the shoots, crowns and roots of both cut and uncut plants, and in the soil microbial biomass associated with uncut plants [Table 2(B)]. It also increased the amount of 14C lost as CO2 by root +microbial respiration. The 14C content of SOM was unaffected by additions of N fertilizer. When the distribution of 14C to each component was expressed as a percentage of net photosynthate (14Ccontent of the plant + soil + root respiration), N increased the proportion retained in the shoots of the uncut, but not of the cut plants [Table 2(C)]. The interaction between these factors was therefore significant. Other effects of N were to reduce the proportion of 14C in the microbial biomass in both cutting treatments and in SOM in the uncut treatment. Fertilizer N did not alter the proportion of 14C in the crowns or roots or the proportion lost by respiration in either the uncut or cut treatments. N fertilizer had no significant effect on the ratio of 14Cin the shoots and roots [Table 2(B)], in contrast with the marked effect on the S:R ratio for dry mass [Table 2(A)]. Effects of cutting. Cutting caused a small increase in the amount of 14C in the shoots of unfertilized grass, but reduced the 14C content of crowns, roots and microbial biomass in both plus N and nil N treatments [Table 2(B)]. There was no effect of cutting on the 14Ccontent of SOM. Cutting reduced the loss of 14Cvia root and rhizosphere respiration from the unfertilized, but not from the fertilized treatment.

384

H. V. A. BUSHBY et

al.

Table 2. Dry masses of plant parts and of the microbial biomass, and the distribution of “‘C between plant, soil and root-rhizosphere respiration components for green panic grown for 88 days in a ‘*CO, atmosphere. Data in parts B and C were analysed after logarithmic and angular transformation respectively, and the means derived by back-transformation (A) Dry weight of components Biomass C Treatment Plus N

Uncut cut

Nit N

UWUt

cut cut ishade F-test for nitrogen x cut

Shoot: root ratio?

Shoots (n)

Crowns (9)

Roots (a)

87.0af 84.4a 34.3b 36.5b 18.2~

27.0a 18Sb 17.5b 10.2c 9.lc

62.6a 55.3ab 64.6a 47.2b 42.Ib

8.6a 8.3a 8.3a ?.Sa 8.6a

0.81~ 1.06b 0.67C

NS

NS

NS

NS

NS

1.85 I .wa

(B) Assimilated 14C(mg) Treatment Plus N Nil N

Uncut cut uncut cut cut + shade

F-test for nitrogen x cut

Shoots

Crowns

Roots

Biomass

SOM

ResDiration

Shoot: root+

21827a 22751a 6026~ 8110b 3882d

385% 1629b 1361b 407c 145d

6223a 2612b 2208b 895c 305d

519a 28Obc 358b 212c 62d

1312a 1820a I466a 1059a 277b

7396a 6138ab 4721b 2143~ 706d

4.2~ 9.8b 3.4c 7.6b 14.0a

NS

NS

NS

NS

NS

*

NS

(C) Percentage distribution of assimilated ‘*C Treatment Plus N Nil N

uncut Cut UftCUt cut Cut + shade

F-test for nitrogen x cut

Shoots

Crowns

Roots

Biomass

SOM

Resuiration

Below around6

53.0a 64.0b 35.7c 59.9b 72.ld

9Sa 4.7b 8.8a 3.8bc 2.6c

15.1 7.Sbc 14.9a R.6b

1.3bc 0.8~ 2.4a 1.7ab I.lbc

3.4b 5.2b 9.8a 8.2ab 5.2bc

18.2ab 17.4a-c 28.4a 17.3a-c 13.lbc

37Sab 31.2c SS.6a 36.2a-c ?S.lC

t*

NS

NS

NS

NS

NS

NS

5.6C

*,**lnteraction significant at P 10.05, P > 0.01. NS = not significant. tShoot:root ratios include crowns. fValues within columns having the same letter are not significantly different (P z 0.05). @Below-ground dist~bution not always equal to sum of components because of statistical adjustment for missing values.

As a percentage of net photosynthesis, cutting increased the proportion in the shoots but decreased the proportion in the crowns and roots in both plus and nil N treatments Fable 2(C)]. There was no effect of cutting on the percentage of net “C in the microbial biomass or in the SOM, or in the percentage of 14C lost as respired CO,. Cutting caused a marked increase in the S:R ratio for 14Ccontent in both fertilizer treatments in contrast with the small increase in the S:R ratio for dry matter [Table 2(A)]. Eficts of shading. Shading decreased the amount of 14Cin the shoots, crowns, roots, microbial biomass and SOM, and respiration losses compared to the equivalent unshaded treatment [Table 2(B)]. Estimates of the latter component were quite variable because they were derived by subtracting the amounts in the root and biomass components from those in the whole soil. Shading increased the proportion of 14Cretained in the shoots and reduced the proportion retained in roots [Table 2(C)]. Thus the S:R ratio for 14Cwas increased [Table 2(B)] in contrast to the effect of shading on the S:R ratio for dry matter [Table

2(A)]. Shading did not alter the total percentage 14C allocated below-ground nor the distribution between the below-ground components [Table 2(C)]. Losses of “Cfrom

roots

Carbon from roots may be incorporated into microbial biomass or SOM or respired as CO, by roots and microorganisms. Incorporation of 14Cinto mirobial biomass was proportional (r2 = 0.67) to the specific 14Cactivity (SA) of roots (Fig. 1). 14Clost via “C content respiration increased with root (v” = 0.60), but it was best correlated with the total dry matter of shoots (r’ = 0.78) and was only poorly correlated with the total dry matter of the roots (r2 = 0.21).

Piants used in this experiment were from a 20 yr old pasture and as such contained substantial amounts of 12C prior to transfer to the 14C atmosphere and the imposition of treatments. The total plant 12C content and distribution of 12C between shoots, crowns and roots at the end of the experiment

385

Carbon cycling in green panic

l

.

was relocated to the shoots. Thus, the total ‘*C per cylinder remained constant (Fig. 2). In contrast, when rundown plants were cut or cut and shaded, the ‘*C from the roots was lost. This 12C formed a large proportion of the total “C lost from plants subjected to these treatments.

DISCUSSION y = 116.2 i 3.12x , = 0.02

The distribution of C both within green panic and between the plant and the soil depended on the .’ nitrogen status of the plant and whether the shoots i I I I I I I I were removed or shaded. We believe that these results 140 100 120 20 40 60 80 0 provide an explanation for the decline in productivity Root specific activity (BqlmgC) of grass based pastures on heavy clay soils in Queensland. In the field, the yield of green panic pastures is Fig. 1. Relationship between the specific activities of roots often firstly limited by water availability and then by of green panic grass and the “C content of microbial biomass for each plant after 88 days growth in a “‘CO2 N only when water is in adequate supply (I. Vallis, W. atmosphere. McGill, pers. commun.). With this qualification, however, as pastures age, soil reserves of mineral N decline from an initial flush associated with the is ikstrated in Fig. 2. Assuming that the total amount preparation of land for sowing to lower amounts that of “C and its dist~bution within the uncut, unfertilare characteristic of the soil that is no longer cultiized plants represented the situation at the beginning vated. Plants respond by allocating an increasing of the experiment (ignoring an unknown amount lost proportion of their dry matter to roots as they through respiration during the experiment), then the explore larger volumes of soil. This commitment of effects of each treatment can be seen. Firstly, all resources to below-ground is at the expense of top treatments reduced the amount of 12C in roots by growth, hence the above-ground productivity of pasremarkably similar amounts. The final destination of tures declines. A further stress would be introduced “C removed from roots depended upon the when rundown plants were heavily grazed. Defoliatreatment. When rundown plants were fertilized or tion caused a reallocation of dry matter and 14Cfrom fertilized and cut, most of the 12Cremoved from roots roots to top growth (see S:R ratios), and under heavy grazing most of this would be lost from the plant. Contrary to the results of Wilson er al. (1986), 50 shading green panic in this experiment did not inI +N-C crease yields of shoots relative to an unshaded treatm +N+C ment. In S.E. Queensland, mean daily PAR in o -N-C m -N+C summer is 60 E rn^“ day-’ and Wilson et al. (1986) 40 m -N+C+SH obtained enhanced growth by reducing this to about 22 E m-* day-’ by shading. Shaded plants in the 3 lj cores received about 14 E m-* day-’ which was below 2 30 optimum (J. Wilson, pers. commun.), i.e. sunlight 9 0 rather than nitrogen limited growth. In spite of lower yields however, the proportion of 14C in shoots of shaded plants was much higher than in plants in full sunlight, and the S:R ratios for 14Cof shaded plants were higher than those of plants in other treatments. These results are compatible with Thornleys theory 10 for the control of S:R ratios based on the supply, transport and utilization of C and N (Thornley, 1972; Wilson, 1988). Under low light intensity, the model predicts that a greater proportion of assimilates will 0 Crowns Total Shoots Roots be partitioned to shoots rather than to roots. ConFig. 2. The distribution of unlabelled carbon (‘zC>in green versely, when N is deficient, resources would be panic as influenced by each treatment. Treatment codes are: preferentially directed below-ground. In our exper+N -C = Plus N fertilizer, not cut; +N+C = Plus N iment, the S:R ratios for dry matter were always fertilizer, cut; -N-C = no N fertilizer, not cut; lower for N deficient than for N fertilized plants. - N + C = no N fertilizer, cut; - N + C + SH = no fertilizer, Thornley’s model also predicts that S:R ratios will cut and shaded. Vertical bars show the LSD between treatments. increase after plants are defoliated (Wilson, 1988) due /

r

386

H. V. A.

BUSHBYet al.

to reduced root growth associated with a preferential retention of carbohydrates in shoots. In general, our results were in agreement with Thornley’s model. The effects of treatments on the distribution of 12C within green panic were surprising. Firstly, taking unfertilized, uncut green panic as the reference, upon which all treatments were imposed, then cutting, application of fertilizer N and shading all reduced the root 12C content by about 25%. In the absence of fertilizer, the 12C removed from roots was lost (i.e. the total 12C content per cylinder decreased) when shoots were cut or shaded. This was presumably due to the death of roots in response to the removal of plant shoots. If the shoots were removed from plants that were not rundown (i.e. had received N fertilizer) then the “C removed from roots was not lost but relocated to the shoots. A possible explanation for this is that there was very little death of roots in rapidly growing -green *panic even though the shoots were removed. and the decline in root 12 C was due to the relocation of labile pools. Bokhari (1977) reported significant remobilization of storage carbohydrates following the removal of herbage. Additions of fertilizer without cutting also resulted in the relocation of labile, non-structural root 12C. Root-rhizosphere respiration by green panic was primarily a function of the size of the shoots (r2 = 0.78). a conclusion that was also reached for hative grasslands by Warembourg and Paul (1977) and for wheat by Martin and Kemp (1986). A similar relationship between total root C content and respiration was not obtained because of the large amounts of old, inactive or even dead root material which would not have contributed significantly to respiration losses. These roots were not separated from active roots at harvest. In conclusion, the response of green panic plants to N deficiency appears to be to allocate a large proportion of its resources to below-ground components, much of which can be lost to the soil or via respiration. Amelioration of the deficiency by applications of fertilizer N, or presumably also by chisel ploughing (Catchpoole, 1984), allows the plant to conserve resources and to divert them to the shoots: increased growth results. thank Dr D. E. Smiles, Chief of the Division of Soils, for the use of the growth chamber and laboratory facilities in Adelaide. Also, we thank Dr J. N. Ladd (Officer-in-Charge of the laboratory of Adelaide), Dr J. K. Martin and Mr M. Amato for their advice and assistance with 14C analyses and we acknowledge the competent technical assistance of Mr R. F. Adey who maintained the growth chamber during the experiment. In the Division of Tropical Crops and Pastures, we thank Mr H. U. Liebich, Miss M. L. Lim and Mr R. J. Daley for the C analyses and Mr C. W. McEwen for the “N determinations. We also thank Professor W. B. McGill, Department of Soil Science, University of Alberta, Edmonton, Canada and Dr M. M. Ludlow (CSIRO) for helpful discussions during the preparation of this manuscript.

Acknowledgements-We

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Noggle G. R. and Fritz G. J. (1976) Plant respiration. In Introductory Plant Physiology. Prentice-Hall, New Jersey. Northcote K. H. (1979) A Factual Key for the Recognition of Australian Soils, 4th Edn. Rellim, Adelaide. Robbins G. B., Bushel J. J. and Butler K. L. (1987) Decline in plant and animal production from ageing pastures of green panic (Panicum maximum var. trichoglume). Journal of Agricultural Science 108, 407417. Ross P. J. and Martin A. E. (1970) A rapid procedure for preparing gas samples for nitrogen-15 determination. Analyst

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Warembourg F. R. and Paul E. A. (1977) Seasonal transfers of assimilated “‘C in grassland: plant production and turnover, soil and plant respiration. Soil Biology & Biochemistry 9, 295-30

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Wilson J. B. (1988) A review of evidence on the control of shootxoot ratio, in relation to models. Annals of Botany 61,433449. Wilson J. R., Catchpoole V. R. and Weier K. L. (1986) St~u~ation of growth and nitrogen uptake by shading rundown green panic pasture on brigalow clay soil. Tropical Grasslands 20, 134-143.