6. TEST OF SPECIFIC HYPOTHESES

This section will examine each of the specific hypotheses set up in Chapter 4.

6.1. HYPOTHESIS 1

“HIGH LEVELS OF N, P AND K RESULT IN A GREATER ABUNDANCE OF THISTLES, DOCKS AND RAGWORTS”.

Table 6.1. shows the mean accumulative abundance of problem weeds per 0.25 m2 (Cirsium arvense, Cirsium vulgare, Rumex obtusifolius and Senecio jacobaea) for each field and the nutrient status of the soil.

Table 6.1: Abundance of problem weeds and nutrient concentration of the soil (mg kg-1).

Abundance* NH4 NO3 NO2 Total N PO4 K
Woodham Lodge 1.94 2.40 9.74 0.76 4.29 1.50 38.00
Front Field 1.77 3.60 8.17 1.33 4.81 2.90 33.00
Back Field 1.54 0.90 1.71 0.29 1.88 2.00 42.00
South Field 1.49 1.40 0.60 0.10 1.26 7.80 51.00
Saunders 1.20 1.95 1.81 0.24 1.99 2.30 20.00
Springfield 0.94 2.50 7.15 0.41 2.68 1.85 31.75
Cowbridge 0.75 3.10 2.71 0.32 3.12 4.53 54.33
Chaplefield 0.40 2.20 1.11 0.19 2.02 2.00 18.00
North Field 0.33 0.50 0.35 0.05 0.35 3.70 33.00

* Mean cumulative abundance of Cirsium arvense, Cirsium vulgare, Rumex obtusifolius and Senecio jacobaea per 0.25 m2.

 

Correlation data:

Abundance and Total N

0.606

Abundance and PO4

-0.025

Abundance and K

0.276

The two fields with the highest abundance of problem weeds are Woodham Lodge and Front Field, which also have the two highest levels of nitrates. North Field has the lowest abundance, and also the lowest nitrate, nitrite and ammonium levels. The results do suggest that higher levels of nitrate cause a greater abundance of problem weeds. South Field has low nitrate levels, but high phosphates and potassium levels make more nutrients available to plant growth. The high nitrate concentration at Woodham Lodge has allowed Cirsium arvense to thrive where other arable weeds and grasses do not have the competitive ability to do so as a result of the rigorous mowing regime.

A correlation for abundance and total N gave a value of 0.606. However, this is not statistically significant.

Figure 6.1. Scatter plot of abundance against total N

6.2. HYPOTHESIS 2

“GREATER SPECIES RICHNESS IS EXPERIENCED ON FIELD MARGINS”.

Figure 6.2: Comparison of field margins and field cores

For the four fields studied, the field margins are more diverse in each case. The core areas have fewer species, however they are not dominated by annuals, suggesting that the fields are maturing. Table 6.2 gives a full species list for the margins and cores.

 

Table 6.2

6.3. HYPOTHESIS 3

“SOWING A GRASS COVER PRODUCES A LOWER SPECIES DIVERSITY AND SPECIES RICHNESS AFTER FIVE YEARS THAN NATURAL REGENERATION”.

Table 6.3: Species richness and species diversity for sown and unsown fields

Species Richness Species Diversity
SOWN Springfield

21

3.52

FIELDS Cowbridge

19

3.48

Saunders

18

3.00

Chaplefield

13

2.59

Front Field

18

2.11

Back Field

23

3.00

MEAN

18.7

2.45

UNSOWN North Field

15

2.39

FIELDS South Field

32

3.69

Woodham Lodge

25

3.74

MEAN

20.7

3.27

T-test for species richness                                 0.18

T-test for species diversity                    0.28

The T-test shows that these relationships are not statistically significant.

The mean values for both species richness and species diversity are higher on the naturally regenerated fields. There is not a great difference in the diversity index and species richness for the sown and unsown fields. On North Field the dominance by Festuca spp. has resulted in a much lower diversity, as commented by Burch (1980), and has in effect acted like a sown Lolium perenne sward in preventing the colonisation by other species. As the sown swards have developed, more perennial species have the opportunity to colonise the land, especially in the more marginal areas.

6.4. HYPOTHESIS 4

“SOWING A GRASS COVER RESTRICTS THE SPREAD OF WEEDS”.

Table 6.4: Comparison of abundance of weeds on sown and unsown fields


Abundance *
SOWN Springfield

0.94

FIELDS Cowbridge

0.75

Saunders

1.20

Chaplefield

0.40

Front Field

1.77

Back Field

1.54

MEAN

1.10

UNSOWN North Field

0.33

FIELDS South Field

1.49

Woodham Lodge

1.94

MEAN

1.25

T-test               0.39

* Mean cumulative abundance of Cirsium arvense, Cirsium vulgare, Rumex obtusifolius and Senecio jacobaea per 0.25 m2.

The mean abundance of problem weeds is higher under the naturally regenerated land. However, there is not a great difference between the two. North Field has a lower abundance of weeds than any of the other fields. This is as a result of Festuca spp. dominating the field and preventing the colonisation of other species. Chaplefield has a very low abundance, and also has a very high cover of Lolium perenne (some quadrat cells with 100% cover). Perennial rye grass is very efficient in preventing other species colonising. Although North Field has a low abundance of weeds, ragwort has reached problem levels. Pesticide drift from neighbouring fields may have contributed to the relatively low abundance, as well as the high Festuca cover. Woodham Lodge has a very high frequency of Cirsium arvense. The only grass on Woodham Lodge to have reached any significant levels is Alopercurus myosuroides, a problem weed itself, and is not effective in preventing the spread of Cirsium arvense.

The T-test shows that this relationship is not statistically significant.

6.5. HYPOTHESIS 5

“GREATER SPECIES DIVERSITY AND SPECIES RICHNESS IS FOUND IN AREAS OF LOWER NUTRIENT SUPPLY”

Table 6.5: Species richness, species diversity and nutrient supply (mg kg-1).

Species richness Species diversity NH4 NO3 NO2 Total N PO4 K
Woodham Lodge 25 3.74 2.40 9.74 0.76 4.29 1.50 38.00
Front Field 18 2.11 3.60 8.17 1.33 4.81 2.90 33.00
Back Field 23 3.00 0.90 1.71 0.29 1.88 2.00 42.00
South Field 32 3.69 1.40 0.60 0.10 1.26 7.80 51.00
Saunders 18 3.00 1.95 1.81 0.24 1.99 2.30 20.00
Springfield 21 3.52 2.50 7.15 0.41 2.68 1.85 31.75
Cowbridge 19 3.48 3.10 2.71 0.32 3.12 4.53 54.33
Chaplefield 13 2.59 2.20 1.11 0.19 2.02 2.00 18.00
North Field 15 2.39 0.50 0.35 0.05 0.35 3.70 33.00

South Field has the greatest species richness and also low nitrate, nitrite and ammonium content. However, the phosphate and potassium levels are high, which may reflect the previous crop of potatoes or the fact that the diverse cover has improved the nutrient supply. Woodham Lodge also has a relatively high species diversity and species richness, but has a high nitrate content. North Field and Chaplefield which have relatively low species richness and species diversity also have low nitrate, nitrite and ammonium contents. The results do not show any clear relationship between species diversity and species richness with nutrient supply.

Correlation data:

Species richness and Total N

0.03317

Species diversity and Total N

0.04782

Species richness and PO4

0.53779

Species diversity and PO4

0.24512

Species richness and K

0.65011

Species diversity and K

0.51964

The correlation data shows that species richness and species diversity are not related to the mineral nitrogen. However, species richness correlates well with both phosphate and potassium content.

Figure 6.3: Scatter plot of potassium against species richness

There is a positive correlation between potassium and species richness. The hypothesis will have to be rejected as there is no statistical evidence that lower nutrient levels cause greater species richness, in fact the opposite appears to be true for phosphates and potassium.

6.6. HYPOTHESIS 6

“SOWING CLOVER RESULTS IN HIGHER NITROGEN LEVELS IN THE SOIL”.

Table 6.6: Mean % cover and frequency of clover with nutrient content (mg kg-1)

% Cover Frequency NH4 NO3 NO2 Total N
SOWN Cowbridge

2.2

4.5

3.10

2.71

0.32

3.12

WITH Springfield

20.0

54.0

2.50

7.15

0.41

2.68

CLOVER Chaplefield

13.3

32.8

2.20

1.11

0.19

2.02

Saunders

9.3

22.2

1.95

1.81

0.24

1.99

MEAN

11.2

28.4

2.60

3.66

0.31

2.45

NOT North Field

0.50

0.34

0.05

0.35

SOWN South Field

1.40

0.60

0.10

1.26

WITH Front Field

0.6

2.2

3.60

8.17

1.33

4.81

CLOVER Back Field

0.90

1.71

0.29

1.81

Woodham Lodge

1.0

16.1

2.40

9.74

0.76

4.29

MEAN

0.32

3.7

1.79

3.73

0.46

2.50

T-test for Total N

0.48

T-test for NH4

0.16

T-test for NO3

0.36

T-test for NO2

0.21

The T-test shows that none of the relationships are actually statistically significant.

For the fields sown with clover, nitrates and nitrites are lower, and ammonium is higher. Springfield has the highest cover of clover on Birkett Hall, and also the highest nitrate concentration for Birkett Hall. However, other than this, the evidence is not conclusive that clover increases the mineral nitrogen concentration of the fields. This may change however once the field has been ploughed and mineralisation occurs. During mineralisation the ammonium will be converted to nitrate. Also, organic matter will be mineralised.

6.7. HYPOTHESIS 7

“HEAVILY GRAZED FIELDS HAVE A HIGHER NITROGEN CONTENT THAN LIGHTLY GRAZED FIELDS”.

To test this hypothesis the fields of Birkett Hall will be examined alone.

Table 6.7: Comparison of heavily and lightly grazed fields


NH4
NO3 NO2 Total N
HEAVILY Bell Meadow 5.20 11.42 2.38 7.34
GRAZED Springfield ‘grazed’ 0.90 8.71 0.29 2.75
Saunders 1.95 1.81 0.24 2.00
Front Meadow 4.00 13.19 1.81 6.64
MEAN 3.01 8.78 1.18 4.68
LIGHTLY Springfield 3.03 6.62 0.44 3.97
GRAZED Cowbridge 3.10 2.71 0.32 3.12
Chaplefield 2.20 1.11 0.19 2.02
MEAN 2.78 3.48 0.32 3.04
T-test NH4

0.41

T-test NO3

0.07

T-test NO2

0.10

T-test Total N

0.16

Nitrates are more twice as high on the heavily grazed fields. Saunders has been heavily grazed, but has very low NO3, NOand NH4 levels. Bell Meadow and Front Meadow generally have very high nutrient contents. Heavily grazed fields increase the cycle of nitrogen through the system by returning nitrogen back to the soil in faeces and urine.

The T-test shows that for both nitrate and nitrite the relationship is statistically significant at the 90% confidence level.

6.8. HYPOTHESIS 8

“THE ANNUAL CUTTING OF SET-ASIDE PROMOTES THE SPREAD OF PROBLEM WEEDS”.

Table 6.8: Cutting regime and abundance of problem weeds at farm level

Farm Cutting Regime Abundance*
Woodham Lodge 2-3 times per year 1.94
White House Once a year 1.66
Reeds Farm Once a year 0.91
Birkett Hall Grazed 0.83

* Mean cumulative abundance of Cirsium arvense, Cirsium vulgare, Rumex obtusifolius and Senecio jacobaea per 0.25 m2.

Figure 6.4: Mean abundance of problem weeds per 0.25 m2 for each farm

Cutting could be responsible for the spread of problem weeds. Birkett hall has the lowest mean abundance of weeds and is not cut at all, but grazed. Woodham Lodge has the highest mean abundance and is cut the most frequent. At Woodham Lodge, Cirsium arvense is responsible for the majority of the cover.

 

6.9. SUMMARY

Although the mean data suggests that a majority of the hypotheses are correct, statistical testing does not give significant results. The only significant relations were for the heavily and lightly grazed fields, where nitrates and nitrites were higher. The frequent cutting of a field produces a higher abundance of weeds. Field margins have a greater species richness.

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