Agriculture, Ecosystems and Environment, 9 (1983) 83--96 83 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

SLASH A N D B U R N AGRICULTURE AT HIGHER ELEVATIONS IN NORTH-EASTERN INDIA. II. SOIL FERTILITY CHANGES

B.K. MISHRA and P.S. RAMAKRISHNAN

Department of Botany, School of Life Sciences, North-Eastern Hill University, Shillong 793 014 (India)

(Accepted 10 November 1982)

ABSTRACT

Mishra, B.K. and Ramakrishnan, P.S., 1983. Slash and burn agriculture at higher elevations in north-eastern India. II. Soil fertility changes. Agric. Ecosystems Environ., 9 : 83--96.

The effect of 'slash and burn ' agriculture (jhum) on soil fertility at higher elevations of Meghalaya, north-eastern India was investigated, comparing and contrasting 15, 10 and 5- year jhum cycles. The pH of the surface soil increased after the burn and gradually decreased during cropping and subsequent fallow development. Nutrients such as carbon and nitrogen are volatilized by the burn and declined, at least in the initial phase of cropping. However, nitrogen recovery started during the later phase of cropping. Available phosphorus followed a more or less similar pattern as nitrogen. On the other hand, cations increased markedly after the burn and were depleted during cropping. In general, the nutrient level under a 5- year jbum cycle was significantly lower than under 10 and 15-year cycles. The recovery pattern during fallow development had an initial phase of depletion, up to about 5 years, followed by recovery. This suggests that a jhum cycle of 5 years, now prevalent in the region, is definitely too short. The generally lower nutr ient status under a terrace system after cropping, even when compared to a 5-year jhum cycle, suggests that a terrace system could not be sustained without a heavy input of fertilizers.

INTRODUCTION

In an earlier paper (Mishra and Ramakrishnan, 1983), the sediment, water and nutrient loss under different jhum cycles were considered and compared with fallow developed after cropping. The present study assesses the nutrient status of the soil during jhum under three cycles of 15, 10 and 5 years and the recovery pattern during fallow development up to a period of 15 years. The jhum system was compared with a terrace agro-ecosystem during a 2-year period of cropping. The terrace system had 'also been cropped for the two preceding years. This study is significant from the point of view of: ~(1) an understanding of the role of soil nutrients in decreased yield under shorter jhum cycles; and (2) assessing the length of the optimum jhum cycle which would ensure satisfactory recovery of soil fertility.

0167-8809/83/$03.00 © 1983 Elsevier Science Publishers B.V.

84

METHODS OF STUDY

Four replicate plots under jhum and terrace cultivation were selected at Shillong taking care to ensure similar aspects (south-facing slopes) and topo- graphic conditions (Mishra and Ramakrishnan, 1983). For the studies per- taining to jhum, deforested plots under 15, 10 and 5-year cycles were identified within the same area on the basis of local records. In each plot, soil was sampled at ten random points from a depth of 0--40 cm, at intervals of 0--7, 7--14, 14--28 and 28--40 cm of the profile. Samples were taken from both ridges and furrows for calculation of quantities of different nutrients on a hectare basis. However, the data for concentrations presented here pertain only to the ridges as the soil in the furrows was less affected by the burn. Soil sampling was carried out on five different occasions in the plots under 15 and 10-year jhum cycles: (1) after slashing of the vegetation and I day before burning; (2) immediately (1 day) after burning; (3) 30 days after burning during the early monsoon; (4) 100 days after burning during mid-monsoon; and (5) 365 days after burning at the end of cropping when the land was left fallow. All samplings were also carried out under a 5-year jhum cycle except that an additional sampling was taken at the end of 730 days (after 2 years of cropping). In the plots under terrace cultivation, soil was sampled once before the onset of the monsoon before seeds were sown (1 month after manuring) fol lowed by 3 0 , 1 0 0 , 3 6 5 and 730 days from the time of the first sampling.

Four replicate plots under 1, 5, 10 and 15-year jhum fallows were also identified within this area on the basis of local records, with comparable aspects and topographic conditions. These fallows were developed in plots under a 15-year jhum cycle. Sampling was carried out only once (in March) and is based on ten random collections through a depth of 0--40 cm of the soil profile as discussed earlier.

Soft carbon was estimated by the Walkey---Black method (Allen, 1974). The soil pH was determined with a pH meter using a soil--water suspension of 1:5 ratio. Other soil analysis procedures were the same as those given in the first paper in this series (Mishra and Ramakrishnan, 1983). The soil bulk density determinations were made using a core sampler and the values were used for subsequent conversion of analytical data to field weight per unit area.

RESULTS AND DISCUSSION

Slash and burn agriculture involving low or high intensity burning, depending on the quanti ty of the slash under different jhum cycles, brings about a num- ber of changes in the physico--chemical properties of the soil. This is known to alter the availability of some of the nutrients important for plant growth (Ahlgren and Ahlgren, 1960; Nye and Greenland, 1960); it also affects the amount of water passing through the soil sub-system, dissolved substances and the particulate matter lost from the top soil as discussed earlier (Mishra and Ramakrishnan, 1983).

o

no

n~

n~

~ 0 0 ~ 0 I ~ ~

I ~ ~

I ~ ~

~ ~ 1

~ ~ 1

~ ~ 1

~ N ~ I

~ 1

~ 1

8.5

86

pH during cropping

A l t h o u g h the p H decl ined wi th soil dep th , the e f f ec t o f j h u m was m o r e ob- vious in the sur face layers on ly (Table I). I m m e d i a t e l y a f t e r the burn , the pH increased (P < 0 .01) u n d e r all j h u m cycles and this increase was m o r e obv ious in the sur face layers o f the soil prof i le , and u n d e r a 15-year cycle c o m p a r e d to a 10 or 5-year cycle . This is a l ready well d o c u m e n t e d (Ahlgren and Ahlgren, 1960 ; R a m a k r i s h n a n and T o k y , 1981) and cou ld be exp la ined as due to an increase in ca lc ium and o t h e r ca t ions l ibe ra ted a f t e r burning. Dur ing cult iva- t ion , the p H decl ined m a r k e d l y (P < 0 .05) s ta r t ing f r o m 30 days a f t e r the burn u p to 365 days , and 730 days a f t e r c ropp ing unde r a 5-year j h u m cycle , t he pH was even lower . T h e surface soil pH u n d e r t e r race cu l t iva t ion increased (P ~ 0 .05) a f t e r 100 days o f c r o p p i n g and dec l ined s u b s e q u e n t l y to t he original level, t he s ignif icance o f which is n o t clear.

TABLE II

Changes in the concentration of carbon, nitrogen (%) and available phosphorus (ppm) on the ridges during cropping

Nutrient Sam- Jhum cycle (years) piing time 15 (days)

Soil depth (cm)

0--7 7--14 14--28 28--40

10

Soil depth (cm)

0--7 7--14 14--28 28--40

Carbon

Nitrogen

Phosphorus

--1 1.9 2.1 1.6 0.2 1 1.6 1.5 1.5 0.2

30 1.6 1.2 1.4 0.2 100 1.6 1.2 1.2 0.2 365 1.9 1.4 0.7 0.1 730 . . . .

1.8 2.7 1.6 0.2 1.7 1.6 1.5 0.2 1.6 1.2 1.4 0.2 1.6 1.1 1.1 0.2 1.8 1.3 0.8 0.1

--1 0.26 0.22 0.12 0.11 0.26 0.22 0.12 0.11 1 0.25 0.19 0.12 0.11 0.25 0.19 0.19 0.10

30 0.22 0.16 0.08 0.10 0.21 0.17 0.10 0.10 100 0.16 0.14 0.08 0.08 0.17 0.15 0.09 0.10 365 0.23 0.24 0.08 0.08 0.23 0.22 0.08 0.10 730 . . . . . . . .

--1 3.5 2.3 1.2 0.6 1 3.6 2.1 1.2 0.6

30 6.9 1.8 1.1 0.5 100 5.2 1.4 1.0 0.5 365 4.3 1.3 0.9 0.4 730 . . . .

3.4 2.2 1.2 0.6 3.6 2.1 1.2 0.6 8.1 1.8 1.0 0.5 6.2 1.6 1.0 0.5 4.3 1.4 1.2 0.5

87

Carbon during cropping

The concentration and total quantity of soil carbon was significantly higher (P < 0.05) under longer jhum cycles of 10 and 15 years compared to that under a 5-year cycle, and in all cases it was significantly low (P < 0.01) at the 28--40-cm depth compared to the profile above. After the burn, carbon con- centration, particularly in the 7--14~m depth, and the total quantity in the surface layers, declined significantly (P < 0.05) with time under all jhum cycles, more markedly so under longer cycles of 10 and 15 years (Table II, Fig. 1). The marked decrease in carbon content under 10 and 15-year jhum cycles may be explained as due to a comparatively higher intensity burn than occurs under a 5-year jhum cycle where the fuel load is obviously low. Ac- cording to Meiklejohn (1955) and Ahlgren and Ahlgren (1965), burning damages soil microbial populations, but they soon recover to pre-burn levels

5

Soil depth (cm)

Terrace

Soil depth (cm)

0--7 7--14 14--28 28--40 0--7 7--14 14--28 28--40

1.6 1.8 1.5 0.2 . . . . 1.6 1.6 1.5 0.2 1.6 1.8 1.5 0.I 1.6 1.1 1.3 0.2 1.6 1.7 1.3 0.1 1.6 1.1 1.0 0.2 1.3 1.3 1.2 0.1 1.7 1.0 0.4 0.1 1.3 1.3 1.2 0.1 1.8 1.1 0.4 0.1 1.2 1.2 1.1 0.1

0.21 0.18 0.10 0.09 . . . . 0.20 0.18 0.11 0.09 0.21 0.18 0.10 0.09 0.17 0.19 0.10 0.08 0.21 0.17 0.08 0.08 0.16 0.16 0.08 0.07 0.20 0.16 0.08 0.08 0.20 0.19 0.07 0.06 0.19 0.15 0.07 0.07 0.21 0.20 0.07 0.04 0.09 0.14 0.06 0.07

3.3 2.1 1.1 0.5 . . . . 3.5 2.1 1.1 0.5 3.3 2.2 1.1 0.6 3.5 1.7 0.9 0.5 4.8 2.6 1.0 0.5 4.9 1.4 0.7 0.3 4.0 1.9 0.9 0.5 4.4 1.0 0.6 0.2 3.2 1.4 0.8 0.5 4.1 0.9 0.7 0.2 3.1 1.3 0.8 0.5

88

300

E v

- ' 20C tO

E

0 0 J F M A M J J A $ O N D

M o n t h s

IOG

2~

16 u

D

2

E O

Fig. 1. Ombrothermic diagram for the study area. Monthly average maximum (4) and minimum (zx) temperature; monthly total rainfall (o).

or even higher. Such an increase in microbial activity, together with improved soil temperature conditions could possibly result in an increased breakdown and utilization of the organic matter in the burn t plots. This may be accen- tuated by greater losses of organic matter due to increased surface run-off (Mishra and Ramakrishnan, 1983). Under the terrace system, the initial carbon level was significantly lower (P<0.05) than even the pre-burn level under a 5-year jhum cycle and it also showed a decline during cropping.

Nitrogen during cropping

Nitrogen concentration was generally higher in the surface layers of the soil and declined with depth. Nitrogen concentration in the soil under a 5-year cycle was significantly lower (P<0.05) than under 10 and 15-year cy- cles. The concentration of this nutrient declined sharply after the burn in the surface layers, particularly the 7--14-cm depth layer {Table II), and a similar decline in the total quanti ty was also noted {Fig. 2). Such a decline after the burn has been observed by others {Wells, 1971; White et al., 1973) and could be at tr ibuted to the conversion of organic nitrogen to volatile forms during

89

6016

4 5 1 2

~3008

1504

C D

T--OOLO °~0~0~ ---- OOLO0 --001-'10 ~0~ V -O°u~ f o~O~)O~ o~ OCO~ ~ ¢m c-- - - ¢m ¢ . .

Sampling period (days)

Fig. 2. Changes in total quantity of carbon (within a soil profile of 40-cm depth) during the various stages of jhum and terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7---14 cm; stippled column, 14--28 cm; open column, 28--40-cm depth of soil.

the pyrolysis (Allen, 1964; Knight, 1966; Debell and Ralston, 1970).: The degree of volatilization is dependent on the intensity of the burn and, there: fore, under a 5-year jhum cycle the post-burn decline is minimal compared to longer cycles with greater fuel load. Nitrogen concentration in the soil improved at the end of 1 year under all cycles, but such an improvement in total quantity was observable only under 10 and 15-year cycles. An increased nitrification after the burn has been reported by some workers (Ahlgren and Ahlgren, 1965; Jorgensen and Wells, 1971), and they have related this to changed pH and micro-environmental conditions while others have suggested release of allelopathic suppression by the removal of the vegetal cover (Smith et al., 1968; Rice, 1974}. Under terrace cultivation the loss of nitrogen con- tinued up to the end of 2 years of the observed cropping period.

Available phosphorus during cropping

Available phosphorus concentration in the soil declined with depth. The concentration was generally lower under a 5-year cycle than under 10 and 15- year cycles (Table II). Available phosphorus under the three jhum cycles showed a significant decline (P < 0.05) after the burn. Others have either reported no significant effect (Allen, 1964; Viro, 1974) or even an increase (Nye and Greenland, 1960; Stark, 1971) in the available phosphorus level after a burn. Though there are no obvious mechanisms for volatilization of phosphorus, Lloyd (1971) has reported a massive loss of this element due to burning which agrees with the present results. Recovery occurred after 30 days under 10 and 15-year cycles and after 100 days for the 5-year cycle with

90

5 o~ v

z

C D

-- --00U'~ T - - O 0 cO -- --oOU~ 0 --OOu~ 0

Sampling period (days)

Fig. 3. Changes in total quanti ty of nitrogen (within a soil profile of 40-cm depth) during the various stages of jhum and terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7--14 cm; stippled column, 14--28 cm; open column, 28--40-cm depth of soil.

TABLE III

Changes in the concentration of cations (mg/100 g soil) on the ridges during cropping

Cation Sam- Jhum cycle (years) piing time 15 10 (days)

Potassium --1

Soil depth (cm)

0--7 7--14 14--28 28--40

Soil depth (cm)

0--7 7--14 14--28 28--40

13 9 6 5 11 7 5 5

Calcium

Magnesium

1 61 39 9 7 30 29 22 10 6

100 21 17 18 9 365 15 10 14 13 730 . . . . .

--1 10 11 7 3 1 32 27 19 5

30 23 16 11 4 100 19 11 8 3 365 15 9 5 2 730 . . . . .

--1 8 7 7 5 1 23 16 9 7

30 14 10 7 5 100 9 9 6 5 365 7 7 5 5 730 . . . .

56 34 7 7 32 2O 9 7 22 16 13 11 13 14 11 16

12 10 6 4 28 20 15 6 19 18 13 6 16 14 11 4 13 12 9 3

10 6 4 2 21 19 9 3 13 12 8 3 11 10 7 4

7 6 5 5

aND ffi Not detectable.

91

subsequent decline during cropping (Fig. 3). In the terrace system also, after an initial increase the available phosphorus declined during the cropping period. This decline during cropping may partly be related to the uptake by the crop and to phosphorus fixation into unavailable forms reported for tropical soils rich in sesquioxides of iron and aluminium in their clay fraction (Gebhardt and Coleman, 1974, Tinker, 1977; Parfitt and Lee, 1979).

Exchangable cations during cropping

The concentrat ion (Table III) and total quanti ty (Figs. 4, 5 and 6) of all cations in the surface soil layers under the three jhum cycles improved marked- ly (P < 0.01) after the burn due to their release through ash (Nye and Green- land, 1960; Zinke et al., 1978), and subsequently declined sharply. This decline is partly related to absorption by the developing crop, but more due to losses through sediment and water as discussed earlier (Mishra and Ramakrishnan, 1983). The larger quanti ty of cations released in plots under 15 and 10-year jhum cycles is due to the greater quant i ty of slash burnt compared to that un- der a 5-year cycle. This, together with a more marked depletion of cations af- ter cropping for 1 year under a 5-year cycle, would again make a short cycle unfavourable. Under terrace cultivation, the cation levels are low to start with,

5

Soil depth (cm)

0--7 7--14 14--28 28--40

Terrace

Soil depth (cm)

0--7 7--14 14--28 28--40

12 7 6 6 51 32 7 6 27 17 6 5 16 10 5 5 11 7 3 7

8 5 3 9

9 8 8 3 21 17 13 6 16 15 13 6

8 6 5 6 5 4 5 5 3 2 5 4

9 6 5 3 20 16 8 3 15 13 7 3 11 9 6 3

8 6 4 2 6 4 3 2

13 10 7 5 11 7 5 7 7 6 9 9 5 3 5 5 3 5 5 5

13 11 6 5 12 9 5 5

8 7 5 4 7 6 4 2 5 4 3 2

10 7 5 3 7 6 4 3 4 4 4 3 2 3 3 2

ND a 3 4 2

92

and here too there was a significant decline (P < 0.01) in the level at the end of the cropping period compared to the initial level.

mqt IliUm,

q'--O0 tO .7- OOLO ---- OOtO0 --OOLO0 o~0~) coOLO I c~ O~oc,~ c~O~O o~

Sampling period (days)

Fig. 4. Changes in total quantity of available phosphorus (within a soil profile of 40-cm depth) during the various stages of jhum and terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7--14 cm; stippled column, 14--28 cm; open column, 28--40-cm depth of soil.

63

56

v

y .

42

1L

0 "7 -- OOu~ T--OOtn .-~- -- OOU~ O ~OOu~ O

~O~D O~ O(.D c~O cDc~ o'~O toc~

5ampLincJ period (days)

Fig. 5. Changes in total quantity of potassium (within a soil profile of 40-cm depth) during the various stages of jhum mad terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7--14 cm; stippled column, 14--28 cm; open column, 28--40-cm depth of soil.

93

04

52

3g

E o~ 26

A

13

0 "T" - - O 0 t~'~

C D

~ 0 U ~ O T - o o ~ ~ o ~ "T -o,,., ~ ~o ,.,-,~° ,- ~,.o .-,

Sampling period (days)

Fig. 6. Changes in total quant i ty of calcium (within a soil profile of 40-cm depth) during the various stages of jhum and terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7--14 cm; stippled column, 14--28 cm; open column, 28- -40~m depth of soil.

Recovery patterns during fallow development

Soil pH declined gradually with the age of the fallow and reached a level inherent to the soil type (Table IV).

Starting from a low level in a 0-year fallow (just after cropping), and after an initial phase of further depletion in a 1-year fallow, the recovery of carbon, nitrogen and available phosphorus occurred in older fallows of 5 and 10 years, with a slight decline in a 15-year fallow. This suggests that a longer fallow period favours greater improvement in humus and some of these nutrients in order to sustain slash and burn agriculture, an observation also noted by others (Nye and Greenland, 1960). After a sharp initial depletion of cations up to 5 years, the recovery occurred in 10 and 15-year fallows (Fig. 7).

TABLE IV

Changes in pH during fallow development

Jhum fallows Soil depth (cm) of different age (years) 0--7 7--14 14--28 28--40

0 5.8 5.7 5.2 5.2 1 5.4 5.1 5.2 5.0 5 5.5 5.2 5.1 5.1

10 5.3 5.1 5.1 5.0 15 5.1 5.0 5.0 4.9

9 4

E

£

, ~ 8 , ~

B C D

~ 0 ~ I ~ 0 ~

Sampling period (days)

~ o O l . O 0

Fig. 7. Changes in total quantity of magnesium (within a soil profile of 40-cm depth) during the various stages of jhum and terrace cultivation. A, 15-year jhum cycle; B, 10-year jhum cycle; C, 5-year jhum cycle; D, terrace. Dark column, 0--7 cm; hatched column, 7--14 cm; stippled column, 14--28 cm; open column, 28--40-cm depth of soil.

e

v U

693

462

231 Z

0

0.G75

~~E 0.450

o_ 0.225

O.C

~ 33 22

11

I I I I 0

/ 36 24

, , , ' 0

3O

N zo

~-7 7"14,4-'28 28"4o

P /

o /

S I I I I

J / I I I I

, . o / . / j

- I I

7,4 ;4 8 2A0 Soil depth

Fig. 8. Changes in cumulative quantity of various nutrients (within a soil profile of 40-cm depth) during the various stages of fallow development, o . . . . --o, 0-year; o o, 1-year; o - - - - - % 5-year; • • , 10-year; * . . . . . , 15-year jhum fallow.

95

CONCLUSIONS

One of the chief conclusions arising out of the present study relates to the poorer nutrient status of the plots under a 5-year jhum cycle which together with the weed problem under shorter cycles (Ramakrishnan and Mishra, 1982) would account for the poorer crop yield under this cycle compared to that under longer cycles of 10 and 15 years (Mishra and Ramakrishnan, 1981). It is also seen that, in general, nutrient depletion occurs up to about 5 years of fallow regrowth and the recovery process is not well under way until after about 10 years of fallow development. Arising out of the work on nutrient loss patterns (Mishra and Ramakrishnan, 1983), it is evident that the losses through water and sediment are more severe under a shorter cycle of 5 years. Because of the greater frequency of jhum under a shorter 5-year cycle than when it is 10 years or more, the losses naturally occur more frequently causing faster depletion of fertility. For all these reasons, shorter cycles are not viable economically and cause serious environmental problems (Ramakrishnan, 1980; Ramakrishnan et al., 1981). The conclusion arising out of the earlier paper (Mishra and Ramakrishnan, 1983) on terrace cultivation being a non- viable alternative is further strengthened by the results of the present study.

ACKNOWLEDGEMENT

This research was partly supported by the Department of Science and Technology, Government of India, under its 'Man and Biosphere' programme.

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