Effects of treatments and depth on soil chemical properties

CHAPTER 4 EFFECTS OF LONG-TERM BURNING FREQUENCY ON SOIL

4.3 Results

4.3.1 Effects of treatments and depth on soil chemical properties

Soil depth had highly significant effect on total C (P <0.01), while N, P, K, Ca, Na, soil pH and the C:N ratio were not significantly affected by either burning frequency or soil depth at (P >0.05). Although all statistical tests were performed at ∞ = 0.05, the P-values (Table 4.1) indicate that most of the soil properties were very close to significantly affected by burning treatments at ∞= 0.05. Soil depth also had significant effect (P ≤0.01) on the soil variables except for Na and the C:N ratio. The interaction between burning frequency and soil depth did not have significant effect on any of the soil properties (P >0.05).

Table 4. 1 P-values for a 2-way ANOVA for soil chemical properties (n = 48; ∞= 0.10 ).

Source d.f. pH Ca K P Na N C C:N

Treatment (A) 5 0.718 0.485 0.718 0.485 0.072 0.919 0.338 0.934 Soil depth (B) 3 0.056 0.088 0.056 0.088 0.215 0.0530 0.000 0.168 A x B 15 0.994 0.659 0.994 0.659 0.745 0.832 0.441 0.566

The trends in soil properties as affected by burning frequency can be divided into two categories: those that increased with fire frequency, and those that decreased with increase in fire frequency. Frequent burning resulted in an increase in soil pH, K, Ca, and Na, while it decreased C, N, and P concentrations. The results of burning treatments on the soil properties are presented in Table 4.2, while those of the soil depth are presented in Table 4.3.Carbon concentrations generally decreased with increase in soil depth, with the highest recorded in the uppermost layer (0-2 cm), and the lowest (1.23) in the 6-8 cm depth. These two values also differed significantly. A similar pattern was observed with nitrogen, though the treatment means were not

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significantly different (Figure 4.1). Soil pH and K generally showed an increasing trend with increase in soil depth, while Ca, P and N concentrations showed a declining trend. The mean C:N ratio was highest on the 0-2 cm depth and lowest on the 2-4 cm depth (Table 4.3).

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Table 4. 2 Means (± SE) for the main effect of burning frequency on soil properties in a long-term fire experiment (P < 0.05; n =60).

Treatment pH C (%) N (%) C:N ratio Ca (mg/kg) K (mg/kg) P (mg/kg) Na (mg/kg)

Annual 6.42±0.02 1.45±0.13 0.12± 0.0 16.8±1.3 538±137 64.1±0.3 0.54±0.14 5.4±1.4 Biennial 6.39±0.01 1.75±0.17 0.13±0.01 16.0±0.8 516±116 63.9±0.1 0.52±0.12 5.2±1.2 Triennial 6.41±0.01 1.6±0.02 0.10±0.02 16.7±1.5 311±89 64.1±0.3 0.31±0.09 3.1±0.12 Quadrennial 6.40±0.01 1.56±0.02 0.13±0.01 14.0±0.8 336±64 64.1±0.3 0.34±0.06 3.4±0.9

Sexennial 6.36±0.04 1.51±0.05 0.14±0.01 14.9±0.8 294±106 63.7±0.1 0.29±0.11 2.9±0.11 No burn 6.29±0.11 1.62±0.04 0.14±0.04 14.9±0.6 410±10 62.9±0.9 0.41±0.01 4.1±0.1

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Figure 4. 1 Means (±SE) for the effect of soil depth on C and N concentration in a long-term fire experiment. Means with the same letters are not significantly different (P >0.05).

c

bc

b b

a a a

a 0

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

0-2cm 2-4cm 4-6cm 6-8cm

Nutrient concentration (%)

Soil Depth

C N

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Table 4. 3 Means (±SE) for the effect of soil depth on soil properties in a long-term fire experiment.

*Values with different letters in the same column are significantly different (P ≤0.05)

Depth (cm) pH C:N Ratio Ca (mg/kg) K (mg/kg) P (mg/kg) Na (mg/kg) 0-2 6.29(0.11)^a 18.0(2.5)^c 609(208)^c 62.9(1.7)^a 0.61(0.21)^c 6.09(2.1)^c 2-4 6.34(0.10)^a 12.5(3.0)^a 358(14)^b 63.4(0.1)^a 0.36(0.04)^b 3.58(0.4)^b 4-6 6.41(0.01)^b 14.4(1.1)^b 333(68)^ab 64.1(0.3)^ab 0.33(0.07)^ab 3.33(0.7)^ab 6-8 6.48(0.04)^b 17.1(1.6)^bc 302(99)^a 64.8(0.4)^ab 0.30(0.04)^a 3.02(1.0)^a

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4.3 2 Interactive effects of fire frequency and soil on chemical properties

Post-hoc (LSD) analyses also revealed some observable trends in the soil properties.

The triennial and quadrennial burns resulted in the highest mean pH (6.57), while the no burn treatment resulted in the lowest mean pH of 6.06 in the uppermost layer (Table 4.3). Mean separation revealed significant differences between the pH values for the no burn uppermost layer and the annual 4-6 and 6-8 cm layers. The pH for the no burn uppermost layer was also significantly lower than those of the 6-8 cm layers in the biennial 6-8 cm; triennial, quadrennial and sexennial burns (Table 4.4) Except for potassium, the concentrations of all the other nutrients decreased with increase in soil depth across all the burning treatments, with the highest mean values mostly recorded in the no burn treatment (Table 4.4). Calcium and phosphorus concentrations were highest on the 2-4 cm layer in the annual burn treatment. Potassium concentrations portrayed an opposite pattern by increasing with increase in soil depth.

The highest value of 65.6 mg/kg K was recorded at the 6-8 cm depth in the triennial and quadrennial burns, while the lowest was at the topmost layer in the no burn treatment (Table 4.4). The no burn treatment in the topmost layer resulted in the highest C concentration, which differed significantly from all the other treatment means (except the uppermost biennial and triennial burns). The topmost layer in the no burn treatment had the highest C:N ratio (23.2), which was significantly higher than the 2- 4 cm layer in the quadrennial burn, as well as the 4- 6 cm layer in the no burn and annual burns respectively. All the treatment means for total N were not significantly different at 5 % (Table 4.4).

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Table 4. 4 LSD test results for soil mineral concentrations at different levels of depth in a long-term fire experiment . Means with the same letter in a column for each depth are not significantly different (P < 0.05; n =60).

Treatment pH Total C (%) Total N (%) C:N ratio Ca (mg/kg) K (mg/kg) P (mg/kg) Na (mg/kg) 0-2 cm

No burn 6.09 ^a 2.48^d 0.12^a 23.2^a 690^b 60.9^a 0.69^b 6.90^c

Annual 6.31 ^a 1.52^b 0.08^a 22.4^a 1280^c 63.1^a 1.28^b 2.75^a

Biennial 6.40 ^a 2.04^c 0.11^a 19.4^a 660^b 63.7^a 0.66^b 6.60^c

Triennial 6.31 ^a 2.08^c 0.13 ^a 16.7^a 405^a 63.1^a 0.41^a 4.05^bc

Quadrennial 6.35 ^a 1.92^bc 0.15 ^a 13.1^a 310^a 63.5^a 0.31^a 3.10^a

Sexennial 6.34 ^a 1.84^bc 0.14 ^a 13.2^a 320^a 63.4^a 0.31^a 3.10^a

2-4 cm

No burn 6.28 ^a 1.69^b 0.16 ^a 12.1^a 375^a 62.8^a 0.38^a 3.75^b

Annual 6.43 ^b 1.52^b 0.14 ^a 13.3^a 280^a 64.3^a 0.28^a 2.80^a

Biennial 6.40 ^a 1.86^bc 0.21 ^a 11.8^a 425^b 63.7^a 0.43 ^a 4.25^bc

Triennial 6.32 ^a 1.71^bc 0.10 ^a 17.7^a 240^a 63.2^a 0.24 ^a 2.40^a

Quadrennial 6.30 ^a 1.77^bc 0.20 ^a 9.8^b 510^b 63.0^a 0.51 ^a 5.10^bc

Sexennial 6.32 ^a 1.66^b 0.20 ^a 10.5^a 320^a 63.2^a 0.32 ^a 3.20^ab

4-6 cm

No burn 6.39 ^a 1.34^a 0.21 ^a 9.3^b 310^a 63.9^a 0.21 ^a 3.10^b

Annual 6.46 ^b 1.44^a 0.23 ^a 9.4^b 350^a 64.6^b 0.23 ^a 3.50^b

Biennial 6.40 ^a 1.62^b 0.14 ^a 11.8^a 520^b 63.7^a 0.14 ^a 5.20^bc

Triennial 6.47 ^b 1.48^b 0.09 ^a 17.1^a 305^a 64.7^b 0.09 ^a 3.05^bc

Quadrennial 6.42 ^a 1.33^a 0.09 ^a 19.6^a 225^a 64.2^a 0.08 ^a 2.25^a

Sexennial 6.34 ^a 1.34^\a 0.08 ^a 19.1^a 290^a 63.4^a 0.08 ^a 2.90^a

6-8 cm

No burn 6.43 ^a 0.99^a 0.08 ^a 14.9^a 265^a 64.3^a 0.08 ^a 2.65^ab

Annual 6.45 ^b 1.34^a 0.06 ^a 21.9^a 240^a 64.5^b 0.06 ^a 2.40^a

Biennial 6.44^b 1.47^b 0.07 ^a 21.0^a 460^b 64.4^b 0.07 ^a 5.60^c

Triennial 6.56 ^b 1.54^b 0.08 ^a 15.6^a 295^a 65.6^b 0.08 ^a 2.95^b

Quadrennial 6.56 ^b 1.24^a 0.09 ^a 13.6^a 300^a 65.6^b 0.09 ^a 3.00^b

Sexennial 6.47 ^b 1.20^a 0.08 ^a 16.0^a 255^a 64.7^b 0.08 ^a 2.55^a

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Soil pH was not significantly correlated with total C, Ca, P, Na and the C:N ratio, but exhibited significant negative correlation with nitrogen and highly significant positive correlation with potassium concentrations (Table 4.5). There was no significant correlation between soil pH and fire frequency (r = 0.34; P >0.05).

Table 4. 5 Correlation coefficients (r) for the relationship between soil pH and nutrient concentrations. (* Significant P ≤0.05; ** Highly significant P ≤0.01).

Soil nutrient Correlation coefficient (r)

Total C -0.25

Total N -0.67*

C:N ratio 0.46

Ca 0.12

K 0.99**

P 0.12

Na 0.12