Biological Conservation 34 (1985) 307-332

Techniques for Reducing Soil Fertility for Nature Conservation Purposes: A Review in Relation to

Research at Roper's Heath, Suffolk, England

R. H. M a r r s

Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Cambs PE17 2LS, Great Britain

A B S T R A C T

From 1954-1979 Roper's Heath in Suffolk wasjarmed with heavy inputs of jertiliser. In 1979, it was purchased as a nature reserve, and it is hoped to restore a heathland vegetation. One of the main problems jo t such restoration is that caused by high soil fertility. In order to reduce this fertility, cereal crops were harvested between 1980 and 1982. This paper investigates the effectiveness of this treatment by assessing (I) the amount of nutrients removed, and (2) the effect on soil Jertility using bioassay techniques. In addition, various alternative techniques for helping to reduce soil fertility, e.g. straw/stubble burning, topsoil stripping, specific fertiliser additions to increase crop yield and hence increase the removal of other nutrients with the crop, and grazing, are reviewed and discussed.

I N T R O D U C T I O N

Often the loss of species diversity in many infertile semi-natural habitats, such as grasslands and heathlands, is ascribed to the effects of increased fertility (Green, 1983). A high fertility, usually brought about by 'agri- cultural improvement' leads to an increased dominance of productive species, and a consequent reduction in the densities of species charac- teristic of the original vegetation. If these infertile habitats are to be conserved, the solutions are obvious: (1) prevent further fertiliser ad- dition; and (2) manage the site using an appropriate strategy to remove

307 Biol. Conserv. 0006-3207/85/$03.30 .c Elsevier Applied Science Publishers Ltd, England. 1985. Printed in Great Britain

308 R. H. Marrs

more nutrients from the site than the annual inputs to the site from natural sources. However, on sites which have already suffered from ~agricultural improvement', there is an increasing desire by conservation bodies to restore the 'original' semi-natural vegetation. Historical evidence suggests that, in the past, reversion of improved grasslands and heathlands to their 'former' state was commonplace (Wells et al., 1976; Marrs & Proctor, 1979; Sheail, 1979). However, this reversion cannot be guaranteed, and it would probably take a fairly long time, so management to accelerate this process may be needed. One of the first steps in restoring infertile semi-natural vegetation on sites which have been improved for agriculture should be to try to reduce the soil fertility.

This paper discusses the effectiveness of an attempt to reduce soil fertility at Roper's Heath in Suffolk, England (Grid reference--TL 753725). This site is an area of former heathland once dominated by Calluna vulgaris and Carex arenaria, which separates two heathland National Nature Reserves, Cavenham and Tuddenham Heaths. Roper's Heath was 'improved' for agriculture in 1954, and was farmed until 1979, when it was bought by the Nature Conservancy Council. Between 1954 and 1979 the site was heavily fertilised (with unspecified amounts) and limed. When the site was bought as a nature reserve the management objective was to restore a Calluna vulgaris dominated heathland, and, as a first step towards this objective, an attempt was made to reduce the fertility of the soil by cropping the site with cereals (spring barley Hordeum vulgare in 1980, and cereal rye Secale cereale in 1981 and 1982), with no inputs of artificial fertilisers. After harvesting the cereal rye crop in 1982 the cereal cropping was abandoned in the hope that the site would, with some grazing/cutting regress to a heathland vegetation. By 1984 a short grass heath vegetation covered most of the site; a few seedlings of Calluna vulgaris were found in the spring and summer of 1983, but most of these plants did not survive into 1984. Under this present management it is likely that a short grass heath vegetation will be maintained.

This paper attempts to assess the effectiveness of the use of cereal cropping to reduce the fertility of the soil at Roper's Heath through (1) a comparison of nutrients removed in the grain with inputs, and (2) comparative bioassay experiments on soils from cropped and uncropped areas. In addition, the use of other techniques, which have a potential use in reducing the fertility of soils on fertile sites for conservation purposes, is reviewed and discussed.

Plant species nomenclature follows Clapham et al. (1962).

Reducing soil fertility for nature conservation 309

ASSESSMENT OF SOIL FERTILITY

It is extremely difficult to measure soil fertility; this is because fertility is a function of all soil factors that influence plant growth. To illustrate the difficulties in measuring fertility it is useful to consider the soil as four main nutrient pools (Fig. 1): minerals, soil organic matter, exchange- able/extractable nutrients, and the soil solution. In most soils the bulk of the nutrients are present in the mineral and soil organic matter pools, with only a very small amount in forms available to plants in the short term (exchangeable/extractable pool and the soil solution). The relative sizes of each of these pools can be estimated chemically (Allen et al., 1974), but this will only provide limited information on soil fertility. This is because plant production is related to the rate of supply of nutrients into plant- available forms. Thus this supply depends not only on the amounts of nutrients present in each pool, but also on the relative rates of transfer between these pools (Fig. 1), for example:

1. The net changes in weathering and fixation rates between the mineral pool and plant-available forms.

2. Net changes in soil organic matter turnover--mineralisation and immobilisation.

3. Other processes influencing the nutrient budget of the soil, i.e. leaching, nitrogen fixation, denitrification.

As it would be impractical to measure all of these factors for each

Fig. l .

NUTRIENTS Unavailable Available in Immediately AVAILABLE except in the ~.~ the short term ~ available for TO PLANT long term ~ r plant uptake

I MINERAL POOL ~.,~,.,~. PARENT MATERIALS ~VO AND SECONDARY

MINERALS IEXCHANGEABLE/J S O I L | ~'V',q,v~ ~ EXTRACTABLE J ~ SOLUTION

" I POOL ~ POOL

. . . . . . ~ - ~ J CHA.OED CAT,ONS ~ RAP~--~IO NH,'. K" Co 2" I b U l L J . ~.~.,~¢~9"r/ / ~ J ANIONS NOT NECESS-I EQUILIBRIUM Mg ~÷ HPO 3" & I ORGANIC I ~ f ~ ' ~ I ARILY IN SO.Ur~N ] ~o~-IN SCXUnON

HUMUS AND DEAD REMAINS OF PL~NT~ ~l..

Diagramatic representation of the distribution and main transfers of nutrients in the soil.

310 R. H. Marrs

nutrient element, in this investigation a comparative bioassay technique was used to compare the fertilities of different soils. Here a test plant was grown under carefully controlled glasshouse conditions, and, because all other factors other than soil type were kept constant, the growth and mineral nutrient content of the test plant were assumed to reflect soil fertility. A similar approach has been used to compare the fertility of soils from (1) Calluna vulgaris heathland and adjacent stands of Betulapendula of different ages (Miles, 1981), (2) under Crataegus monogyna and adjacent grassland on the sand dunes at Newborough Warren, Anglesey (Hodgkin, 1984), and (3) to assess the variation in soil fertility throughout Britain (Harrison & Hornung, 1983). The main drawback to the use of comparative bioassay techniques for the assessment of soil fertility is that it is only possible to compare results within any given experiment.

MATERIALS AND METHODS

Bioassay procedure: field and glasshouse methods

Soils collected from the field were sieved through a 4 mm screen to remove large stones and roots; subsamples of this soil (1 kg + 1.0 g) were weighed into pots (either 12.5cm diameter or 12.7 x 12.7cm). Three seedlings of the test species were planted in these pots and, during the first week of the experiment, mortalities were replaced. In a range of experiments three test species were used: Agrostis tenuis (Highland), Lolium perenne ($23) and Triticum aestivum (Rapier). All bioassays were done in the glasshouses at Monks Wood Experimental Station. During the course of the experiment, pots were inspected weekly, and any weed seedlings removed. At harvest the shoots were clipped, and roots extracted by careful washing and sieving. Both roots and shoots were then washed thoroughly, oven dried at 80 °C and weighed.

Chemical analysis

Plants were analysed for nitrogen, phosphorus, potassium, calcium and magnesium, and soils for pH, loss-on-ignition, exchangeable potassium, calcium, magnesium, extractable phosphorus and inorganic nitrogen (nitrate and ammonium forms), using techniques described in Allen et al. (1974).

Reducing soil fertility for nature conservation 311

Statistical analysis

A randomised block experimental design was used in all experiments, with blocks arranged to account for spatial variation in the glasshouse. Analysis of variance was done using either the MINITAB or GENSTAT packages on the NERC Honeywell DPS 300 computer at Bidston; all analyses were done with untransformed and transformed data (log e n and x/n). In this study transformed data (x/n), with detransforms, are presented for all yield and element content data from the bioassay experiments, but untransformed data are presented for soil nutrient analyses.

Study 1: Measurement of the nutrients removed in a cereal rye crop

Approximately one week before harvest in August 1982, five 2m x 2m quadrats were positioned randomly in the cereal rye crop (planted autumn 1981); the vegetation in each quadrat was clipped and removed. This vegetation was sorted into four fractions: (1) grain; (2) chaff, (consisting of glumes, lemma and palea); (3) straw; and (4) other species (mainly arable weeds) and dry weight and nutrient content determined.

Study 2: Effects of a cereal rye crop on soil fertility

Five samples of soil were collected in March 1982 from five randomly chosen points in each of two areas; the first area had been cropped with spring barley in 1980 and cereal rye in 1981, and the second area was an adjacent experimental area which was cropped in 1980 only. The five soil samples from each area were bulked, thoroughly mixed, and a bioassay, to measure the response of Lolium perenne to the soil from each area, set up. In this bioassay five replicate blocks were used, each with two soils (+_cereal rye crop) and five monthly harvests.

Study 3: Soil properties at different depths

It may be possible at sites like Roper's Heath, where the conservation value of the present vegetation is low, to reduce soil fertility by stripping the surface layers of topsoil. However, before this type of operation is contemplated, it is essential to know if the fertility of the deeper layers is indeed lower than the surface soils. To investigate this, soils were collected

312 R. H. Marrs

from five randomly positioned points (April 1982) from four depths (0-5 cm, 5-10 cm, 10-15 cm, and 15-20 cm). A bioassay was set up using Lolium perenne as the test crop; three replicate blocks were used, each with five spatial samples and four soil depths. This experiment was harvested after 16 weeks. An incubation experiment was also done, where the numbers of seedlings emerging in a 12-week period from a sample of each soil were counted; this experiment was also replicated three times.

Study 4: Effects of inorganic fertiliser additions on soil fertility

To use inorganic fertiliser additions to reduce soil fertility may at first seem to be contradictory, but it may be possible to use small additions of one nutrient element to increase crop yield, and thus extract greater amounts of other nutrients than in unfertilised soils (Dyke et al., 1983). A bulk soil collection was made in April 1982, and, after thorough mixing, two bioassays were set up; one with Lolium perenne, the other with Triticum aestivum as test crops. A completely factorial nitrogen (50 kg N ha- 1 as ammonium sulphate), phosphorus (42 kg P ha- 1 as super- phosphate), and potassium (42 kg K ha- ~ as potassium sulphate) fertiliser addition experiment was done. The fertilisers were ground to pass a 1.4 mm screen, bulked in 5 g of acid-washed sand and mixed into the surface soil. In these two experiments the eight fertiliser treatments were replicated three times. At harvest (after 16 weeks), follow-up experiments were done where half the soil was used in a second bioassay. In both follow-up experiments Lolium perenne was used as the test crop. This experiment was set up in mid-August 1982 and lasted 12 weeks.

Study 5: Comparison of soil fertility at Roper's Heath with adjacent unimproved heathland

If nature reserves like Roper's Heath are to be managed to reduce soil fertility, an attempt should be made to assess the magnitude of the reduction in fertility required to bring the soils to similar levels found in unimproved ecosystems. As a preliminary attempt to do this, a bioassay experiment was set up to compare the fertility of Roper's Heath with those of the adjacent unimproved Calluna heath (Cavenham Heath). Soils were collected in April 1984 from three randomly chosen points at both sites. A bioassay experiment was done using three test species of different productivity (growth rate); low productivity Agrostis tenuis, intermediate

Reducing soil fertility for nature conservation 313

Lo l i umperenne , and high p roduc t iv i ty Tr i t i cum aest ivum. Three repl icate blocks were used, each wi th two hea th l and sites, three areas and three species. The exper iment was harves ted af ter 10 weeks' growth.

R E S U L T S

Study 1: Measurements of the nutrients removed in a cereal rye crop (Table 1)

The yield o f grain ob ta ined f rom the unfert i l ised Roper ' s H e a t h was 1 6 6 g m -2 (1.7 t h a - X), with a nu t r ien t con ten t o f 1 8 0 0 m g N m -2 (18kg N ha -1 ) , 482mg P m -2 (5kg P h a - 1), and 1385 m g K m -2 (14kg K h a - 1).

However , it is clear that the nut r ien ts r em o v ed as grain were only a f rac t ion o f the net a m o u n t o f nut r ients r emoved f rom the soil, i.e. the to ta l a m o u n t in the above -g round s tanding crop. Only 50 ~o o f the n i t rogen and phosphorus , and less than 25 ~o o f the potass ium, which was present

TABLE 1 The Standing Crop (g m -2) and Nutrient Content (mg m -2) in a Cereal Rye Crop

Removed from Roper's Heath (Mean values + standard errors (n = 5) of transformed data (~fn) and detransforms are

presented.)

Fraction of Standing N P K the crop crop

Transformed data Straw 15.710 + 0.623 25.317 + 0-926 19.180 + 2.691 54.277 + 2.180 Chaff 6.417 + 0.325 15"069 + 0.762 5-968 + 0.307 16.647 + 0.846 Other species 10.644+ 1.333 28.572 + 3.580 13.327 + 1.672 38.401 + 6.210 Grain 12.891 + 0.673 42.427 + 2.573 21.949 + 1.845 37.219 + 2.320

Detransforms Straw 247 641 368 2 946 Chaff 41 227 36 277 Other species 113 816 178 1 474 Grain 166 1 800 482 1 385

Total 567 3 484 1 064 6 082

°, o removed as grain 29 52 45 23

314 R. H. Marrs

in the standing crop, was removed from the site as grain. If all of the above-ground plant material was removed, the effect of the cropping would be much greater, and 3484mgNm -2 (35kgNha-1) , 1054mg P m -2 (10kgPha -1) and 6082mgKm -2 (61kgNha -1) would be removed.

Study 2: Effects of a cereal rye crop on soil fertility

Both the yield and nutrient content of the test plant (Lolium perenne) were significantly lower on the soil which had previously been cropped with cereal rye compared to the soil where no crop had been removed (Table 2), indicating that rye cropping has reduced the fertility of the soil.

TABLE 2 Yield and Nutrient Content of Lolium perenne Grown on Soils which Have Been

Uncropped or Cropped with Cereal Rye

(Mean values and standard errors of transformed data (x/~) with approximate detransforms in parentheses are presented.)

Yield (g) of Lotium perenne

Harvest (month)

1 2 3 4 5

Treatment mean

Treatment Uncropped 0.282(0-1) 0.974(1) Cropped 0.297(0.1) 0-933(1)

Standard error

2.258(5) 4.295(18) 5.046(25) 2.571(6.6) 2.020(4) 3.339(11) 4.367(19) 2.191(4.8)

0.248 (n = 5) 0.111 (n = 25)

Nutrient content (rag) of Lolium perenne at harvest 5

Element

N P K Ca Mg

Treatment Uncropped 16.48(272) 5.754(33) 18.22(332) Cropped 14.45(209) 4-750(23) 15.65(245)

Standard error (n = 5) 0"93 0.245 0.98

15'88(252) 5.620(32) 13.49(182) 5.101 (26)

0.75 0-294

Reducing soil fertility [or nature conservation 315

TABLE 3 The Effects of Removing a Cereal Rye Crop on the Soil Nutrient Status Before and After a

Bioassay Experiment with Lolium perenne

Loss-on- ignition

°~o)

Element (mg lOO g- ~ )

Exchangeable Extractable Inorganic N P

K Ca Mg NO 3 NH~

Before bioassay a Uncropped 4.6 15.5 390 4-1 6.5 0.34 <0.2 Cropped 3.9 14-0 300 2-9 4.8 2.4 0.36

After bioassay Uncropped 3.96 4.54 372 6.10 2.28 0.06 0.28 Cropped 4.70 3.08 400 6.68 2.98 0.04 0.23 Standard

error (n = 5) 0"09 0'40 7 0'24 0" 10 0.02 0.06

Duplicate analyses.

The chemical analyses of the soil (Table 3) were not as valuable a guide to the fertility of the soils as the growth of plants in the bioassay experiment. For example, at the start of the bioassay the cropped soil had a lower amount of exchangeable potassium, calcium, magnesium, extractable phosphorus and inorganic nitrogen, and a lower loss-on-ignition. After the bioassay the cropped soil had higher levels of all these parameters, except exchangeable potassium, which was higher in the uncropped soil, and inorganic nitrogen where there was no significant difference between soils.

Study 3: Soil properties at different depths

A few of the soil chemical parameters measured showed some significant differences between soil depths (Table 4); for example, exchangeable potassium was lowest in the 10-15 cm depth layer, nitrate-nitrogen was lowest in the surface layer and highest in the 10-15cm layer, and both extractable phosphorus and total nitrogen were lowest in the 15--20 cm depth layer.

In the bioassay experiment no significant differences were found between either the five spatial samples, or the interaction between space and depth; significant differences were only found between the four soil

316 R. H. Marrs

TABLE 4 Significant Differences in Nutrient Status of Soils from Four Depths from Roper's Heath before the Bioassay Experiment, All Other Parameters Measured--Exchangeable Ca,

Mg; Inorganic NH4-N showed No Significant Difference between Depths (Mean values and standard errors are presented.)

Soil BeJbre bioassay depth (cm) Exchangeable P Inorganic N Total N

K (mg lOOg -1) (rag lOOg -1) NO3-N (mg lOOg -~) (%)

0-5 5"93 4.35 1.23 0"10 5-10 2'15 3'35 2'17 0"13

10-15 1'87 3'15 2'75 0'11 15-20 2'58 2.20 2.22 0.10

Standard error (n = 5) 0.28 0.56 0.14 0.01

depths. The yield of Lolium perenne was greater in the 0-5cm and 10-15 cm layers than in the 5-10cm and 15-20cm layers (Table 5). This was also true of the amounts of phosphorus and calcium removed from the soil; no significant differences were found in the amounts of nitrogen, potassium and magnesium removed. The lowest depth layer (15-20 cm)

TABLE 5 The Yield (g) and Nutrient a Content (mg) of Lolium perenne grown on, and Numbers of Seedlings Produced in Germination Trials, from Soils Collected from Four Depths at

Roper's Heath

(Mean values + standard errors of transformed data (x/~) are presented with detransforms in parentheses)

Soil depth Yield Element Seedling ( cm) numbers

P Ca

0-5 2.402(5-8) 3.496(12.2) 7.100(50.4) 2.254 (5) 5-10 2.238(5.0) 3.187(10.2) 6.830(46.6) 2.885 (8)

10-15 2'608(6"8) 3.505(12.3) 7.840(61.5) 3.173(10) 15-20 2.117(4.5) 3.044 (9.3) 6.300(39.7) 1.941 (4)

Standard error (n = 15) 0.125 0.258 0.333 0.151

° No significant differences were found in the nitrogen, potassium and magnesium contents.

Reducing soil Jertility Jor nature conservation 317

was consistently the least fertile soil, as it produced the lowest yield and nutrient content of Lolium perenne. The numbers of buried seed found in the glasshouse tests were highest in the intermediate layers, but were lowest in the soil from 15-20 cm depth.

Study 4: Effects of inorganic fertiliser additions on soil fertility

With Lolium perenne, a significant response to nitrogen and phosphorus was found (Table 6); a greater yield and nutrient content were obtained where nitrogen (except for potassium content) and phosphorus were added. In the follow-up experiment, where Lolium perenne was grown on the soils after the main bioassay experiment, a different result was obtained; here no significant response to nitrogen was found, but significant differences were obtained where phosphorus was added, and in the interaction between nitrogen and phosphorus. The effect of phosphorus addition on its own continued to increase the yield of Lolium perenne, but where nitrogen was added with no phosphorus addition, growth was suppressed.

With Triticum aestivum, the only significant response was to phosphorus addition, which produced a greater yield, potassium and calcium content (Table 7). This significant response was also found on the growth of Lolium perenne in the follow-up experiment.

Apparent fertiliser recoveries (i.e. the amount of nutrient recovered expressed as a percentage of fertiliser addition) of the nitrogen and phosphorus, the two elements which significantly increased the yield of the test plants, were low. A recovery of 20 ~0 for nitrogen and 13 ~o for phosphorus was found with Lolium perenne, and of 6 °/0 for phosphorus with Triticum aestivum. The most interesting result from these experiments is the increased yield and nutrient content of Lolium perenne brought about by nitrogen addition. The lack of response to nitrogen in the follow-up experiment indicates that the nitrogen addition has not left a residual effect in the short term. However, the low apparent recovery of nitrogen (20 ~o) means that 80 °, o of the added fertiliser is unaccounted for, lost through leaching or decomposition, or incorporated in soil organic matter.

The increases in yield brought about by phosphorus additions are unlikely to be of practical value, because (1) residual effects were found in both follow-up treatments, (2) apparent recoveries were low, and (3) phosphorus is insoluble in soils and may persist for many years (Russell, 1974).

TA

BL

E 6

E

ffec

ts o

f F

acto

rial

Add

itio

ns o

f N

itro

gen,

Pho

spho

rus

and

Pot

assi

um o

n G

row

th a

nd N

utri

ent

Con

tent

of

Lo

liu

m p

eren

ne G

row

n in

B

ioas

say

Exp

erim

ents

on

Soi

ls f

rom

Rop

er's

Hea

th

(Onl

y si

gnif

ican

t ef

fect

s ar

e sh

own:

all

oth

er t

reat

men

t co

mbi

nati

ons

show

ed n

o si

gnif

ican

t ef

fect

. M

ean

valu

es a

nd s

tand

ard

erro

rs a

re

pres

ente

d.)

Tre

atm

ents

Y

ield

(g)

N

Eh'

men

t co

nten

t (m

g)

Gro

wth

(g)

in

the

Joll

ow-u

p P

K

C

a M

g

expe

rim

ent

Nit

roge

n N

o N

+

Pho

spho

rus

Po

P+

Sta

ndar

d er

ror

(n =

12)

Nit

roge

n ×

phos

phor

us

NoP

o N

oP

+

N+

Po

N

+P

+

Sta

ndar

d er

ror

(n =

6)

2-72

5 (7

.4)

3-37

4 (1

1.4)

2.75

9 (7

.6)

3.34

1 (1

1.2)

9.32

(8

6.9)

10

.94(

119.

7)

9.46

(8

9.5)

10

.80(

I 16

.6)

3.92

(15.

4)

8-63

(7

4.5)

3.

142

(9-9

) 0.

876(

0.77

) 4'

43(1

9.6)

11

.00(

121.

0)

3.68

6(13

-6)

0-82

0(0.

67)

3.59

(12.

9)

10-6

1 (11

2.6)

8.

94 (

79.9

) 3.

154

(9.9

) 0.

729(

0.53

) 4.

76(2

2.6)

12

.56(

157.

8)

10.6

9(11

4.3)

3-

674(

13.5

) 0"

967(

0.93

)

O. 1

17

0.42

O

. 12

0.57

0.

39

O. 1

41

0-04

3

0.84

4(0-

71)

0.90

8 (0

-82)

0.

615(

0.38

) 1.

026

(1.0

6)

0.06

1

Reducing soil fertility for nature conservation 319

TABLE 7 Effects of Factorial Additions of Nitrogen, Phosphorus and Potassium on Growth and Nutrient Content of Triticum aestivum grown in Bioassay Experiments on Soils from

Roper's Heath (Only significant effects are shown; all other treatment combinations showed no

significant effect. Mean values and standard errors are presented.)

Treatment Yield (g ) Element content (rag) Growth of Lolium perenne (g) in

K Ca the follow-up experiment

Phosphorus Po 2-34 (5.5) 2.79 (7-8) 2.260(5.1) 0-767(0.59) P+ 3.23(10.4) 3.43(11.8) 2.641(7.0) 0.959(0.92)

Standard error (n = 12) 0"25 0.19 0.118 0.041

Study 5: Comparison of soil fertility at Roper's Heath with adjacent unimproved heathland (Table 8)

Significant differences were found between test species and the interaction between test species and site. The differences between test species confirmed the differences in productivity, but differences in species × site interaction showed that:

(1) The species with lowest productivity, Argrostis tenuis, grew better on the Cavenham Heath than on the Roper's Heath soil. The reason for the poor growth on Roper's Heath remains unclear, but may be partly because of slow establishment or poor physical properties of the soil which allowed the soil surface to dry out quickly.

(2) Lolium perenne, the intermediate species, showed no significant difference in response to the two sites.

(3) Growth of the species with highest productivity, Triticum aestivum, was much greater on the Roper's Heath soil than the Cavenham soil, indeed yields were almost doubled on the Roper's Heath soil.

The results for Triticum aestivum show clearly that for productive cereal species, the Roper's Heath soil is much more fertile than unimproved heathland. In terms of this bioassay experiment, the fertility

320 R. H. Marrs

TABLE 8 The Yield (g) of Three Test Plants of Varying Productivity--Agrostis tenu&, Lolium perenne and Triticum aestivum Grown on (1) Improved Heathland Soils from Roper's

Heath and (2) Unimproved Soils from the Adjacent Cavenham Heath

(Mean values + standard errors of transformed data (w/n) are presented with detransforms in parentheses.)

Test species Site Standard mean error

Agrostis Lolium Triticum (n = 27) tenuis perenne aestivum

Roper's Heath Cavenham Heath

Standard error (n = 9)

Species mean

Standard error (n = 18)

0-163(003) 1'095(120) 1418(2.01) 0-756(0'57) 1.090(1'19) 1'013(1.03)

0076 3

0.460 1.092 1.215 (0.21) (1.19) (148)

0.053 9

0.892 (0.80)'~ 0-953 (0.91)) 0.0440

of the Roper's Heath soil would have to be reduced to bring about a 50 reduction in the yield of Triticum aestivum. The results for the other two species show that for comparisons of soil fertility at different sites, either a productive species, or a longer experiment, is required.

DISCUSSION

The main objectives of this work were twofold:

(1) to investigate whether the use of cereal rye crops to reduce soil fertility at Roper's Heath was successful;

(2) to compare alternative methods for reducing soil fertility at Roper's Heath, and to review methods that may be of use on nature reserves elsewhere.

Roper's Heath was used as a test site because cereal crops were already being used to reduce soil fertility, and the site had been farmed for many years with large inputs of fertilisers.

Reducing soil fertility Jbr nature conservation 321

The effects of cropping cereal rye on soil fertility at Roper's Heath

Three cereal crops were taken from Roper's Heath between 1979 and 1981, with no inputs of inorganic or organic fertilisers. The cereal rye crop harvested in 1982 produced a relatively low yield (1.7 t ha - ~) compared to the national mean yield for cereal rye in England between 1974 and 1978 of 3.2 t h a - 1 found using normal agricultural practices (MAFF, 1981). However, for conservation purposes, the yield is not too impor tant - - the main object of the treatment is to produce a net removal of nutrients from the site.

Inputs of major nutrients to Roper's Heath can only occur through natural inputs in rainfall (dry and wet deposition), and in the seed of the rye crop. Estimates for both of these inputs are given in Table 9, and the calculated nutrient budget shows that there was no net removal of nitrogen if grain alone was harvested, although there was a significant net removal of potassium ( x 3) and especially phosphorus ( x 8). However, if all of the above-ground plant nutrients were harvested then there would

TABLE 9 A Comparison of Estimated Nutrient Inputs, and the Amounts Removed from Roper's

Heath in a Cereal Rye Crop

Estimated inputs Element (kg ha- 1)

N P K

Rainfall a 16.9 0.11 3.2 Cereal seed b 2.1 0.6 1.6 Total 19.0 0.71 4.8

Removals ( × excess over inputs in parentheses)

Grain 18.0 5.0 14.0 (0) ( × 8.2) ( x 2.9)

Total above ground 35 10 61 standing crop ( × 1.9) ( × 16.4) ( × 12.7)

a Abstracted data from Saxmundham Experimental Station, Suffolk (Williams 1976), data are generally within ranges quoted by Allen et al. (1968) except for phosphorus, which is almost half their lowest value of 0.2 kg ha- 1. b Calculated using an estimate of 190kg of cereal seed (Nix, 1976), with a nitrogen, phosphorus and potassium content of 1.10 ~, 0.39 °/o and 0.83 ~o respectively.

322 R. H. Marrs

be a net loss of nitrogen (x 2), and the losses of potassium (x 13) and phosphorus ( x 10) would be enhanced.

Although it was difficult to assess the effects of the cereal cropping in the chemical properties of the soil, the results from the bioassay experiment clearly demonstrated that both plant yield and nutrient content were lower in soils which had been cropped with rye. This evidence indicates that the cereal cropping has reduced the fertility of the soil at Roper's Heath, and this was detectable after one year's cropping. Reduction in fertility by cropping vegetation has been demonstrated in longer term experiments, for example in (1) the classical agricultural experiments at Rothamsted (Brenchley, 1969; Garner & Dyke, 1969; Johnston, 1969, 1976; Johnston & Poulton, 1977; Dyke et al., 1983), and (2) in chalk grasslands, where Wells (1980) has shown that removal of grass cuttings for eight years reduced both the soil exchangeable magnesium, but more important, the soil extractable phosphorus.

Alternative methods of assisting the reduction of soil fertility

Straw~stubble burning If all of the above-ground plant nutrients were removed after harvest, but before the nutrients are re-incorporated into the soil, the efficiency of the rye cropping would be enhanced. One inexpensive method of doing this is to use burning, either to burn both straw and stubble together, or as a follow-up technique to burn the stubble after straw removal. The second option would probably be the most effective treatment, as it would be impossible to remove all the stubble and low-growing weeds with mechanical harvesters. Moreover, the burn may be less severe and thus may be more acceptable from a political viewpoint.

Burning generally tends to remove a proportion of all nutrients as smoke, for example, Chapman (1967) showed that after controlled burns on a lowland heath 95 ~o of the nitrogen, 26 ~o of the phosphorus and 21 ~o of the potassium was lost from the ecosystem. However, only nitrogen and phosphorus could not be replaced over a 12-year period by rainfall inputs. After burning, the remaining nutrients are deposited on the soil as ash, and they are either leached or taken up by developing vegetation. Kenworthy (1964) and Allen (1964) have shown that potassium becomes soluble very quickly, but phosphorus is fixed, rendering it less available for plant uptake in the short term.

Thus, the net effect of burning can be: (1) to remove some nitrogen and phosphorus, and this loss cannot be replaced by inputs in the short term;

Reducing soil fertility.[or nature conservation 323

(2) to decrease the availability of phosphorus; and (3) to increase the availability of potassium. In view of the results of the fertiliser addition experiments on soils from Roper's Heath reported here, where nitrogen and phosphorus, but not potassium, yielded a significant increase in growth of test plants, the effect of burning could only help to reduce fertility further.

Topsoil stripping The bioassay experiments testing the fertility of the four different depth layers of soil showed that the 15-20 cm depth layer is least fertile, and has the lowest buried seed content. These results mean that fertility could be reduced quickly by simply removing the surface 20 cm of topsoil. Further studies of changing fertility with depth may show that deeper layers are even more infertile ! Removal of topsoil to expose less fertile layers or even subsoil, although an apparently drastic technique, has several advantages for conservation purposes. These are:

(1) Fertility would be reduced. (2) The topsoil is a saleable product (assuming that a market could be

found!), current prices (1984) range from £3-5m -3. Stripping Roper's Heath (12 ha) to a depth of 20 cm (24 000 m-3) would therefore realise an income between £72000 and £120000, although obviously some of this would be used up in stripping and transport costs.

(3) The topsoil has a buried seed content flora (Table 10) that couldbe used for land reclamation in other areas where a native grass vegetation is required, e.g. roadside verges, country parks.

Stripping topsoil would, of course, be visually unattractive for a short period, but would be no worse than the management practices carried out between 1979 and 1982, when the site was rotavated before sowing cereals. Moreover, topsoil removal by cutting and removing sods for fertilising of agricultural soils (plaggensoils) was a traditional method used for managing heathlands in the Netherlands (Gimingham & de Smidt, 1983), and attempts are now being made to revive this management practice for heathland conservation (Diemont et al., 1982). Extension of this type of management to British heaths may be possible.

This approach has one major drawback in that it would remove a large part of the buried seed pool. However, experiments attempting to restore Calluna heath by additions of heathland litter at Roper's Heath, have

324 R. H. Marrs

TABLE 10 Species which became Established in Field Experiments, presumably from Buried Seed, at Roper's Heath between 1980

and 1982

Monocotyledons Dicotyledons

Agrostis tenuis Aira praecox Alopecurus myosuroides A lopecurus pratensis Apera interrupta Festuca spp. Holcus mollis Lolium perenne Poa pratensis Secale cereale

Arenaria serpyllifolia Capsella bursa-pastoris Campanula rotundifolia Cerastium arvense Cerastium holosteoides Cirsium vulgare Crepis capillaris Echium vulgare Epilobium hirsutum Galium saxatile Hieracium pilosella Hypochoeris radicata Myosotis arvensis Myosotis ramosissima Ornithopus perpusillus Plantago lanceolata Plantago major Ranunculus acris Rumex acetosella Rumex obtusifolius Sagina procumbens Senecio jacobaea Sonchus spp. Stellaria media Trif olium dubium Trifolium repens Urtica dioica Veronica arvensis unidentified = 3

failed (Marrs, unpublished data), mainly because of the rapid establishment of weed species, including Urtica dioica, a species of fertile soils (Pigott & Taylor, 1964). Reducing fertility by topsoil stripping may perhaps allow the establishment of species typical of the unfertile Breck soils, and even if reseeding were required, a lot of seeds can be bought from an income of£120 000. Even if only 10 ~ of this income were profit (£1 200), this would provide £100ha-1 for seed purchase.

Reducing soil fertility Jbr nature conservation 325

The use of inorganic fertilisers Addition of inorganic nitrogen fertiliser has shown to almost completely exhaust the phosphorus store of the soil in the long-term wheat experiments at Broadbalk (Dyke et al., 1983). The increase in crop production brought about by the nitrogen addition, extracts more phosphorus and other elements from the soil than treatments with no fertiliser addition (Table 11). This could be a particularly useful technique if the apparent recovery of the added nitrogen was high, preferably over 100 % or greater (i.e. all the added nitrogen was recovered plus some from the soil organic matter store). In practice, apparent recoveries of between 46 and 71% of applied nitrogen were found at Broadbalk, and thus a

TABLE 11 Apparent Recovery of Added Inorganic Fertiliser Nitrogen and its Effect on Yield and

Nutrient Content of Winter Wheat at Broadbalk between 1970-1975 (Data abstracted from Dyke et al. 1983.)

Apparent recovery a

Mean nitrogen uptake (kg ha- l ) o f grain and straw 1970-75

No nitrogen + Nitrogen (Treatment 3) (Treatment 10)

Fertiliser input

(kg ha- 1 )

Apparent recovery

o ~ (~o)

30 70 96 70 - 30 - - x 100

96

=52~0

Yielc[ ° and nutrient content a (kg ha-1)

Yield P K Ca Mg

No nitrogen (Treatment 3) 2.86 6.2 15 5 2.2

+ nitrogen (Treatment 10) 5.38 9.0 25 10 3.3

Increase by nitrogen fertiliser 2.52 2.8 10 5 1.1

Only data from continuous wheat used b Means of the continuous wheat plots WC(1) and WC(9) were pooled.

326 R. H. Marrs

portion of the added nitrogen remains in the system. However, in the Rothamsted experiments, even where inorganic nitrogen fertilisers have been applied for almost 130 years (Johnston, 1976), the nitrogen content of the soil increased by no more than 0"006 ~ , less than 5 ~ of the total nitrogen which was not accounted for in the crop. Presumably, the unaccounted fraction is lost through a combination of leaching and denitrification.

Thus. there is some scope for using nitrogen fertilisers to reduce phosphate levels (and other elements) for conservation purposes. It is particularly important to reduce phosphate and calcium levels, because most leguminous species will only grow in abundance where levels of these elements are fairly high (Bradshaw & Chadwick, 1980). Even if no nitrogen fertiliser is added, large inputs of nitrogen could occur if legumes invade and fix atmospheric nitrogen. This input could be serious, for example, Sketfington & Bradshaw (1980) showed that Trifolium repens could fix up to 4 9 k g N h a -1 year-1 on the inhospitable china clay wastes, and inputs of at least similar orders of magnitude are likely in other semi-natural ecosystems. If these inputs from biological fixation were to occur even where no fertiliser was added, then a similar amount of fertiliser nitrogen could also be added. Some of this added nitrogen would then be recovered in the crop, at the same time extracting other elements in greater amounts.

At Roper's Heath, where no cereals were planted, there is a profusion of leguminous species, especially where the production of weed species is controlled (Marrs, unpublished data). It would, therefore, be beneficial if the phosphorus and calcium status of the soil were reduced. This site is suitable, therefore, for testing the effects of increased nitrogen fertiliser as a means of accelerating the reduction of soil phosphorus (and other elements) and the consequent effects on vegetation development. Unfortunately, results from the bioassay experiments where nitrogen was added as a fertiliser at low rates, show that:

(1) nitrogen only increased the yield and element content of Lolium perenne; Triticum aestivum was not affected, and

(2) the apparent recovery of nitrogen fertiliser was low.

These results may have been brought about by artefacts under glasshouse conditions, and further field trials are clearly required to confirm these findings and test the effect of adding nitrogen in practice.

Reducing soil fertility for nature conservation 327

The use o f grazing animals It is normally assumed that grazing domestic stock will remove nutrients from a site, and thus reduce fertility. However, this is not necessarily so. In order to assess the benefit of grazing it is necessary to compare removal of nutrients with inputs, and look at the other effects that grazing may have on nutrient cycling.

(1) Nutrient removal. Normally grazing removes some nutrients from the site, either as carcasses or as produce (e.g. wool). However, this removal may be low in comparison with natural nutrient inputs in rainfall. For example, at the montane Agrostis-Festuca grassland IBP site at Llyn Llydaw in Snowdonia, which was grazed by sheep at densities varying between 5.4-15.5 eu (ewe units)ha-1 during daylight hours (Brasher & Perkins, 1978), a very small net removal of phosphorus ( × 1.2) and potassium ( × 3.6) was found (Table 12). No net removal of nitrogen, calcium or magnesium was found. Thus, although some nutrients are removed from the site, grazing may only remove significant quantities of potassium, as all other elements can be compensated by rainfall inputs. The small net removal of phosphorus may not be significant.

(2) Nutrient cycling. Grazing has a major effect on the transfer of nutrients between the plant material and decomposer/soil pools (Table 12). At the Llyn Llydaw site, grazing recycled almost twice the amount of nutrients which were removed from the site by grazing. In addition to this increased nutrient cycling, grazing also prevents or reduces the accumulation of a large standing dead/ litter pool, which locks nutrients out of circulation (BiJlow-Olsen, 1980; Marrs et al., 1980). Thus, grazing may, by increasing the cycling of nutrients, stimulate plant production rather than reduce fertility.

Grazing, therefore, is unlikely to be used to reduce soil fertility in British nature reserves per se, unless its efficiency in removing nutrients is increased. Increased efficiency could be achieved if the grazing animals were only allowed to graze during the day, but taken off the site at night. Under free grazing conditions, sheep may graze very little at night, except perhaps where they are encamped (Hughes & Reid, 1951; Brasher & Perkins, 1978), but within a managed system, daylight grazing could be enforced. This approach is used successfully to manage heathlands in Holland (Gimingham & de Smidt, 1983), and at the L/ineburger Heide in

328 R. H. Marts

TABLE 12 The Effect of Sheep Grazing on the Uptake, Removal and Recycling of Nutrients at the IBP Montane Agrostis-Festuca Grassland at Llyn Llydaw, Snowdonia (abstracted from

Perkins, 1978)

Element (kg ha- 1 year- 1)

N P K Ca Mg

Rainfall inputs 18.4 1.7 3.0 26.0 11.3 Sheep uptake 56.9 5.5 43-1 12.1 9.9

Removal from site Sheep production 3.2 0.8 0.2 1.4 0.05 Loss in urine 7.9 trace 8.5 0.3 1.0 Loss in faeces 5.5 1.2 2.2 2.4 1-5 Total loss 16.6 2.0 10.9 4.1 2.6

( x excess over inputs) (0) ( x 1.2) ( x 3.6) (0) (0) ~o of sheep uptake

lost from system 29.2 36.4 25.3 33.9 26.3 of sheep uptake

recycled within system 70.8 63-6 74.7 66.1 73.7

Germany (Thompson, 1979). Introducing this system to manage British grass and heath nature reserves would be worth considering, but would require some financial investment in terms of fencing, provision of barns for holding the sheep at night, and shepherding. Against this there would, of course, be some financial return, in terms of animal produce (sheep and wool) and the sale of manure. Moreover, a return to using traditional agricultural practices may bring some income from educational and tourist interests.

Soil fertility levels for the conservation of semi-natural vegetation

Although various workers have stressed the importance of the damaging effects of high soil fertility on species diversity in grasslands and other semi-natural vegetation types, and the role of management in reducing this fertility (Green, 1980, 1983; Wells, 1980), little attempt has been made to define the fertility levels that need to be achieved to maintain specific ecosystems. The bioassay comparing the soil fertility at Roper's and Cavenham Heaths was an attempt to assess the reduction in fertility required at Roper's Heath. This experiment showed that a 50 ~o reduction

Reducing soil fertility for nature conservation 329

in the yield of the most productive test species (Triticum aestivum) was required at Roper's Heath. However, the only way that this type of information could be used in practice is if repeated experiments were done over the time course of a management programme designed to reduce soil fertility. Hopefully the differential between the soils would decrease. Nevertheless, it is essential that some form of quantitative assessment of soil fertility should be made in attempts to restore semi-natural vegetation on currently fertile sites, and related if possible to examples of ecosystems that are similar to the desired end result of the management programme.

This problem is, however, analogous to problems encountered in land restoration, where the initial fertility of soils is too low to support vegetation without nutrient inputs and the management objective is to create a self-sufficient ecosystem. Attempts have been made to define the amount of nitrogen which needs to be accumulated for china clay wastes (Bradshaw et al., 1982; Marrs & Bradshaw, 1982; Roberts et al., 1982; Marrs et al., 1983). On these wastes it has been suggested that a soil nitrogen capital of 700 kg N ha-1 is the point at which a non-nitrogen fixing shrub (Salix atrocinerea) ecosystem can sustain itself.

Clearly, if the objective of vegetation management is either to reduce or increase soil fertility to maintain defined ecosystems, then long-term ecological experiments similar to the classical agricultural experiments at Rothamsted (Rothamsted Experimental Station, 1977) are required. Gains and losses of nutrients must be monitored, nutrient budgets drawn up, and fertility is assessed, perhaps using a bioassay procedure under standardised conditions similar to the growth room studies of Grime & Hunt (1975), but where soil parameters rather than the test plant are varied. Further refinements of the bioassay procedure could include: (1) a choice of different species with different productivities; (2) increasing the densities of the test plants (Peace & Grubb, 1982; Hodgkin, 1984); and (3) bioassays with sequential harvests. Hopefully the results from these bioassays could be calibrated against the type of ecosystem the soil supports in the field.

ACKN OWLEDGEMENTS

I am grateful to the Nature Conservancy Council, Eastern Region, for allowing me access to Roper's Heath. Drs G. Radley and R. D. Roberts were involved in the early discussions that led to this research, and D. Malins gave valuable assistance in field, glasshouse and laboratory. The

330 R. H, Marrs

chemical analyses were done by the Chemistry Section at the Institute of Terrestrial Ecology's Merlewood Research Station, and J. D. Roberts in particular is thanked for his help in the analysis. Dr M. D. Hooper and Professor A. D. Bradshaw FRS made many valuable comments on the manuscript.

R E F E R E N C E S

Allen, S. E. (1964). Chemical aspects of heather burning. J. appl. Ecol., 1, 347 -67.

Allen, S. E., Carlisle, A., White, E. J. & Evans, C. C. (1968). The plant nutrient content of rainwater. J. Ecol., 56, 497-504.

Allen, S. E., Grimshaw, H. M., Parkinson, J. A. B. & Quarmby, C. (1974). Chemical analysis of ecological materials. Oxford, Blackwell Scientific Publications.

Bradshaw, A. D. & Chadwick, M. J. (1980). The restoration of land. Oxford, Blackwell Scientific Publications.

Bradshaw, A. D., Marrs, R. H., Roberts, R. D. & Skeffington, R. A. (1982), The creation of nitrogen cycles in derelict land. In The nitrogen cycle, ed. by W. D. P. Stewart and T. Rosswall, 255 9. London, Royal Society.

Brasher, S. & Perkins, D. F. (1978). The grazing intensities and productivity of sheep in the grassland ecosystem. In Production ecology oj'British moors andmontane grasslands, ed. by O. W. Heal & D. F. Perkins, 354 74. Berlin, Springer.

Brenchley, W. E. (1969). The park grass plots oJ' Rothamsted 1856-1949. Harpenden, Rothamsted Experimental Station.

Bfilow-Olsen, A. (1980). Net primary production and net secondary production from grazing an area dominated by Deschampsia flexuosa (L). Trin. by nursing cows. Agro-Ecosystems, 6, 51-66.

Chapman, S. B. (1967). Nutrient budgets for a dry heath ecosystem in the south of England. J. Ecol., 55, 677 89.

Clapham, A. R., Tutin, T. G. & Warburg, E. F. (1962). Flora ojthe British Isles, 2nd edn. Cambridge, University Press.

Diemont, W. H., Blanckenborg, F. G. & Kampf, H. (1982). Blij op de Hei? lnnovaties in het heidebeheer. Intern rapport de Werkgroep Verwerking en Afzet van Heideplaggen. Arnhem, Rijksinstituut voor Natuurbeheer.

Dyke, G. V., George, B. J., Johnston, A. E., Poulton, P. R. & Todd, A. D. (1983). The Broadbalk Wheat Experiment 1968-78: yields and plant nutrients in crops grown continuously and in rotation. Rep. Rothamsted exp. Stn, 1982, Part 2, 5 44.

Garner, H. V. & Dyke, G. V. (1969). The Broadbalk yields. Rep. Rothamsted exp. Stn, 1968, Part 2, 26 49.

Gimingham, C. H. & de Smidt, J. T. (1983). Heaths as natural and semi-natural

Reducing soil fertility for nature conservation 331

vegetation. In Man's impact on vegetation, ed. by M. J. A. Werger and I. Ikusima, 185-99. The Hague, Dr W. Junk.

Green, B. H. (1980). Management of extensive amenity grasslands by mowing. In Amenity grasslands: an ecological perspective, ed. by I. H. Rorison & R. Hunt, 155-161. Chichester, Wiley.

Green, B. H. (1983). The management of herbaceous vegetation for wildlife conservation. In Management of vegetation, ed. by J. M. Way, 99-114. London, British Crop Protection Council.

Grime, J. P. & Hunt, R. (1975). Relative growth rate: its range and adaptive significance in a local flora. J. Ecol., 63, 383-422.

Harrison, A. F. & Hornung, M. (1983). Variation in fertility of UK soils. Annu. Rep. Inst. terr. Ecol., 1982, 98-99.

Hodgkin, S. E. (1984). Scrub encroachment and its effects on soil fertility on Newborough Warren, Anglesey, Wales. Biol. Conserv., 29, 99-119.

Hughes, P. G. & Reid, D. (1951). Studies on the behaviour of cattle and sheep in relation to the utilisation of grass. J. agric. Sci., Camb., 41, 350-66.

Johnston, A. E. (1969). The plant nutrients in crops grown on Broadtbalk. Rep. Rothamsted exp. Stn, 1968, Part 2, 50- 62.

Johnston, A. E. (1976). Additions and removals of nitrogen and phosphorus in long-term experiments at Rothamsted and Woburn and the effect of residues on total soil nitrogen and phosphorus. Minist. Agric. Fish. Fd Tech. Bull., no. 32, 111 -44.

Johnston, A. E. & Poulton, P. R. (1977). Yields on the Exhaustion land and changes in the NPK content of the soils due to cropping and manuring, 1852-1975. Rep. Rothamsted exp. Stn, 1976, Part 2, 53-86.

Kenworthy, J. B. (1964). A stud), oJ the changes in plant and soil nutrients associated with moorburning and grazing. PhD thesis, University of St Andrews.

MAFF (1981). Agricultural statistics, England 1978-1979. London, HMSO. Marts, R. H. & Bradshaw, A. D. (1982). Nitrogen accumulation, cycling and the

reclamation of china clay wastes. J. Environ. Manage., 15, 139-57. Marrs, R. H., Granlund, I. H. & Bradshaw, A. D. (1980). Ecosystem

development on reclaimed china clay wastes. IV. Recycling of above-ground plant nutrients. J. appl. Ecol., 17, 803-13.

Marrs, R. H. & Proctor, J. (1979). Vegetation and soil studies of the enclosed heathlands of the Lizard Peninsula, Cornwall. Vegetatio, 41, 121-8.

Marts, R. H., Roberts, R. D., Skeffington, R. A. & Bradshaw, A. D. (1983). Nitrogen and the development of ecosystems. In Nitrogen as an ecological

Jactor, ed. by J. A~, Lee, S. McNeill & I. H. Rorison, 113-36. Oxford. Blackwell Scientific Publications.

Miles, J. (1981). Effect of birch on moorlands. Cambridge, Institute of Terrestrial Ecology.

Nix, J. (1976). Farm management pocketbook, 7th edn. London, Wye College, University of London.

Peace, W. J. H. & Grubb, P. J. (1982). Interaction of light and mineral nutrient supply in the growth of Impatiens parviflora. New Phytol., 90, 127-50.

332 R. H. Marrs

Perkins, D. F. (1978). The distribution and transfer of energy and nutrients in the Agrostis-Festuca grassland ecosystem. In Production ecology of British moors and montane grasslands, ed. by O. W. Heal and D. F. Perkins, 375-95. Berlin, Springer.

Pigott, C. D. & Taylor, K. (1964). The distribution of some woodland herbs in relation to the supply of nitrogen and phosphorus in the soil. J. Ecol., suppl., 52, 175-85.

Roberts, R. D., Marrs, R. H., Sketfington, R. A. & Bradshaw, A. D. (1982). Importance of plant nutrients in the restoration of china clay and other wastes. Trans. Instn Min. Metall., 91A, 42-50.

Rothamsted Experimental Station (1977). Details of the classical and long term experiments 1968-73. Harpenden, Rothamsted Experimental Station.

Russell, E. W. (1974). Soil conditions and plant growth, 10th edn. London, Longmans.

Sheail, J. (1979). Documentary evidence of the changes in use, management and appreciation of the grass-heaths of Breckland. J. Biogeogr., 6, 277-92.

Skeffington, R. A. & Bradshaw, A. D. (1980). Nitrogen fixation by plants grown in reclaimed china clay waste. J. appl. Ecol., 17, 469-78.

Thompson, J. A. (1979). Impression of the Luneburg Heath and nature reserve. Natur & Nat. Parke., 17, no. 64, 20-21.

Wells, T. C. E. (1980). Management options for lowland grassland. In Amenity grassland." an ecological perspective, ed. by I. H. Rorison and R. Hunt, 175-96. Chichester, Wiley.

Wells, T. C. E., Sheail, J., Ball, D. F. & Ward, L. K. (1976). Ecological studies on the Porton Ranges: relationships between vegetation, soils and land-use history. J. Ecol., 64, 589-626.

Williams, R. J. B. (1976). The chemical composition of rain, land drainage, and borehole water from Rothamsted, Broom's Barn, Saxmundham and Woburn Experimental Stations. Minist. Agric. Fish. Fd Tech. Bull., no. 32, 174--200.