Slash and burn agriculture at higher elevations in North-Eastern India. I. Sediment, water and nutrient losses

Agriculture, Ecosystems and Environment, 9 (1983) 69--82 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

69

SLASH AND B U R N A G R I C U L T U R E A T H I G H E R E L E V A T I O N S IN N O R T H - E A S T E R N I N D I A . I. S E D I M E N T , W A T E R A N D N U T R I E N T LOSSES

B.K. MISHRA and P.S. RAMAKRISHNAN

Department o f 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. I. Sediment, water and nutrient losses. Agric. Ecosystems Environ., 9:69--82 The hydrology and the pattern of sediment and nutrient loss through water that may occur under 'slash and burn' agriculture (jhum) at the time of cropping, as well as during the subsequent fallow development, was studied at higher elevations of Meghalaya, northeastern India and compared with terrace cultivation. A comparison of an agro-ecosystem under a 10-year jhum cycle with that under a 5-year cycle suggests that the loss of sediment, water and nutrients such as nitrogen and phosphorus is greater under the latter, though cationic losses show a reverse trend. All losses were markedly reduced during fallow development during secondary succession. Terrace cultivation resulted in a general reduction of water and nutrient loss. However, these losses increased during the second year of terrace cultivation. While jhum cannot be sustained with the shorter cycle introduced in recent times, terracing does not seem to offer an alternative. INTRODUCTION 'Slash a n d b u r n ' a g r i c u l t u r e (locally k n o w n as ' j h u m ' ) , w h i c h involves slashing t h e n a t u r a l v e g e t a t i o n , b u r n i n g t h e dried slash a n d cultivating t h e land f o r a few years b e f o r e it is a b a n d o n e d f o r n a t u r a l r e g e n e r a t i o n , is t h e p r e d o m i n a n t f o r m o f agriculture in t h e n o r t h - e a s t e r n hill region o f India. T h e j h u m cycle, w h i c h is t h e t i m e interval b e t w e e n t h e o n s e t o f forest fallow d e v e l o p m e n t d u e t o natural r e g e n e r a t i o n a n d t h e r e c u l t i v a t i o n o f t h e same site, has in r e c e n t times been r e d u c e d f r o m a m o r e f a v o u r a b l e 2 0 - - 3 0 years t o a p e r i o d as s h o r t as 5 years. F u r t h e r , in t h e r e c e n t past, terracing o f land has been i n t r o d u c e d i n t o this region as an alternative t o j h u m . J h u m is practised a l o n g t h e steep slopes 6 f t h e hills (average slope angle, 3 0 - - 4 0 ° ) and t h e r e f o r e large losses o f t o p soil, w a t e r a n d n u t r i e n t s o c c u r d u r i n g t h e c r o p p i n g p e r i o d d u e t o r u n - o f f and infiltration. H o w e v e r , a steadystate e q u i l i b r i u m m a y be r e a c h e d a f t e r a c r o p c o v e r is established, t h e t i m e

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© 1983 Elsevier Science Publishers B.V.

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taken for this being dependent upon the crop species (Zachar, 1982). Similarly, during fallow development, losses are progressively reduced as has also been shown elsewhere during forest regeneration after clear-cutting (Bormann and Likens, 1970; Marks and Bormann, 1972). The loss of nutrients, chiefly through infiltration of water, is also likely to occur under terrace cultivation. The present work, which was carried o u t at Shillong (25.34°N and 91.56°E), is an attempt to study the hydrology and the pattern of sediment and nutrient loss through water, that may occur under jhum at the time of cropping as well as during the subsequent fallow development and to compare it with the pattern under terrace cultivation. STUDY A R E A AND DESCRIPTION OF THE SYSTEM

Shillong, located in the Khasi Hills of Meghalaya, is at an altitude of 1500 m. The climate is monsoonal with most of the annual rainfall of 1800--1900 mm occurring during the m o n s o o n period from May to September. The average maximum and minimum temperatures during this season were 24 and 16°C, respectively. The percentage humidity remained very high during this period (84%). Winter, extending from November to February, is cold with occasional showers, with an average m a x i m u m temperature of 17°C and a minimum of 8°C. Frosts are very c o m m o n during December and January. The brief summer which is warm and dry extends from March to April with a maximum temperature of 23°C and minimum of 12°C (Fig. 1). 'Slash and burn' agriculture (jhum) is extensively practised by the local Khasi tribe at higher elevations of Meghalaya and is also prevalent in the entire north-eastern hill areas of India. While at lower elevations in the subtropical forests, the entire forested cover is slashed which is the typical and more c o m m o n form of j h u m (Toky and Ramakrishnan, 1981), at higher elevations the lower branches of trees alone (chiefly Pinus kesiya), which are already sparsely spaced due to previous land-use practice, are slashed. Further, unlike at lower elevations where slash is burnt in situ and seeds of the crop mixture are planted directly on the soil--ash complex by dibbling, in the present case the slash is placed in parallel rows running down the slope, covered over with a layer o f soil placed on the dried slash so as to make ridges and furrows. Thus the slash is burnt under more controlled conditions and the seeds of the crop mixture are planted on the ridges. While fertilizer is n o t used in the typical form of jhum at lower elevations, in the present case b o t h organic (pig dung and compost) and inorganic (NPK, 1:1:1) fertilizers are used depending upon the type of jhum cycle. A g o - e c o s y s t e m s under a 10-year jhum cycle generally receive 600 kg ha -~ year -1 (oven
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A mixture of eight crops consisting of r o o t and tuber crops such as Solanum tuberosum, Colocasia antiquorum and Ipomea batatus; cereals such as Zea mays; legumes such as Phaseolus vulgaris; and vegetables such as Brassica oleracea, Cucurbita maxima and Cucumis sativus are grown together in the same plot. Emphasis on the mixture of crops m a y vary according to the type of jhum cycle imposed on a given site. Thus, while all the crops mentioned may be grown together under a 10-year cycle, only S. tuberosum, Z. mays and B. oleracea are grown under a 5-year cycle. While cultivation is only for 1 year in the case of a 10-year cycle, under a 5-year cycle cropping m a y continue for 2--3 years before the land is fallowed for natural regeneration of vegetation. Solanum tuberosum, however, is the major crop in both cases. After the burn and a few weeks before the onset of the monsoon, the tuber crops are planted on the ridges while the rest o f the crops are planted after the first few showers. Along each ridge three distinct rows are planted by dibbling the seeds of S. tuberosum; Z. mays seeds are planted at intervals. While C. maxima and C. sativus are planted at random, b u t widely scattered on the ridges, planting of C. antiquorum is confined mostly to the top and b o t t o m of the ridges. Likewise, the planting of P. vulgaris is confined around the pine trees which m a y provide support. During the period July--August

72 t h e first crop of tubers are harvested and then a second crop of winter p o t a t o is raised together with B. oleracea on the ridges. Harvesting of the cereals and legumes occurs during the period September--October. The harvesting of winter p o t a t o and B. oleracea is carried o u t in November after which the land is left uncultivated for the period December--March. If cropping is n o t carried o u t in subsequent years, the plot is left aside for natural regeneration of forest. The jhum cycles studied during the present investigations are of 10 and 5 years. It m a y be mentioned here that at present a 5-year cycle is more common. The soils are podsolic and since the erosion outstrips weathering, the soil cover is scant. T h e soil is sandy, loose and highly porous with a pH of 4--6. A number of w e e d y colonizers such as Eupatorium adenophorum, Erigeron linifolius, Hypochaeris radicata, Imperata cylindrica and Oxalis latifolia, are dominant in a 1-year jhum fallow. In a 5-year fallow, together with these weeds, Pteridium aquilinum and Dichranopteris linearis are also dominant. In 10 and 15-year fallows the weedy species are replaced by rapidly regenerating Pinus kesiya trees, besides other broad-leaved tree saplings of Schima wallichii and Quercus griffithii and shrubs such as Rubus micropetalus and Rubus ellipticus (Ramakrishnan and Mishra, 1982). In recent times, terrace cultivation was introduced to this area by governmental agencies. Terraced lands are cultivated for a m a x i m u m period of 6--8 years after which they are abandoned. In the present case, the terraces had been under cropping during the preceding 2 years only. Either a mixture of crops as in jhum or a monoculture of one crop species, namely S. tuberosum or B. oleracea may be raised on bench terraces. The terraces have a width of 3--4 m. This cultivation involves heavy inputs of organic (3000 kg ha -1 year-~ and inorganic fertilizer (1:1:1 ratio of NPK; .74 kg ha-1 year-l) in April before the onset o f the monsoon. The crop mixture in the present study under terrace consisted of S. tuberosum, Z. mays and B. oleracea. METHODS

OF S T U D Y

F o u r replicate plots under jhum under a 5 and 10-year cycle and under terrace cultivation were identified at Shillong, taking care to ensure similar aspects and topographic conditions (the average slope angle was 40 °). One, 5 and 10-year-old fallows (four replicates} which had developed after cropping under a 10-year jhum cycle, were also identified taking care to ensure similar aspects and topographic Conditions. Each plot had an area of a b o u t 2 ha. For studies pertaining to sediment and water loss due to erosion and runoff, the loss from a confined area of 2 X 20 m along the slope was collected in large collectors and sampled periodically for chemical analysis. For the study o f percolation loss of water, 'Russian' t y p e lysimeters were employed (Buckman and Brady, 1960}. Soil was cut vertically in each plot to expose

73 the profile. A small tunnel was excavated at a depth of 40 cm (the depth to which most roots penetrate) and the lysimeter (30 X 30 X 15 cm) was placed inside it. By pressing from below, the rim of the lysimeter was firmly inserted in the undisturbed soil above. The percolated water was tapped out from the lysimeter from time to time for analysis. The results are based on the mean of four observations in each plot with four replicates. After analysing the water/soil samples for NO3-N and PO4 -P, soon after collection, the water samples were preserved in polythene jars for subsequent analysis. Soil samples were air
The monthly pattern of sediment and water loss (Fig. 2) showed two distinct trends. While the sediment loss showed two distinct peaks, one in May and another in September, the water loss through run-off and percolation had one major peak only, in June. The major loss of sediment and water during the early part of the monsoon from jhum agro-ecosystems may be attributed to the lack of crop cover. The subsequent decrease in these losses may partly be due to the development of an effective crop cover and partly due to the decreased intensity of rainfall. Zachar (1982) has discussed in detail the ability of the crop cover and the arrangement pattern of crops in reducing soil loss. The second peak of sediment loss in September may be closely related to soil disturbances during the harvest of the tuber crops. In the agro-ecosystems under a 5-year jhum cycle the loss of sediment was more than that under a 10-year cycle (Table I), which may be related to the poor physical characteristics of the soil due to more frequent cultivation and thinner crop cover. The run-off water loss significantly increased ( P < 0.05) in the agro-ecosystem under a 5-year jhum cycle compared to a 10-year cycle. The shortening of the jhum cycle, however, does not seem to have any effect on the loss of percolation water. During the second year of jhum (5-year cycle) the loss of water due to run-off increased while percolation loss decreased which could be related to the sealing of the soil pores by the finer soil particles suspended in the run-off water (Lowdermilk, 1930; Auten, 1934; Weaver and Harmon, 1935). Sediment and run-off water losses were drastically decreased (P < 0.01) with the development of the jhum fallows (Table I). However, the percola-

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a v a l u e s w i t h i n p a r e n t h e s e s i n d i c a t e losses d u r i n g t h e s e c o n d y e a r o f o b s e r v a t i o n .

75 tion water increased (P < 0.05} in the jhum fallows compared to the afroecosystems. Under a forested ecosystem, much of the rainfall reaching the forest floor is first intercepted by the canopy cover and branches of the forest species and then by the litter layer on the soil surface. This actually reduces the hitting force of the rain drops and thereby the erosion is minimised (Lowdermilk, 1930; Rowe, 1955; Langbein and Schumm, 1958). With the development of the fallow, the loss of sediment and run-off water are thus drastically reduced. The increase in percolation loss o f water in a 1year fallow compared to that under the agro~ecosystem may be due to the better physical structure of the soil and also due to a decline in the run-off losses. Percolation loss reached its peak in a 5-year fallow to decline again in an older 10-year fallow. This decline in the latter case may be related to increased evapo--transpiration/absorption b y the vegetal cover. Sediment loss during cropping under terrace cultivation (Table I) was significantly lower than that under jhum (P < 0.01) and this reduction was to the extent of 38 and 25% in the first and second year of cropping, respectively, compared to the first year cropping under a 5-year jhum cycle. During the second year of terrace cultivation, the sediment loss increased significantly (P < 0.01) compared to the loss during the first year of cropping. The runoff-water under the terrace system was significantly lower (P < 0.05) compared to j h u m afro-ecosystems; this increased during the second year of cropping with a corresponding decline in percolation losses during this year, which suggests clogging up of the pores by finer soil particles.

Nu trien t losses Data for nutrient concentration during the early and late m o n s o o n period are presented in Table II. However, m o n t h l y sampling carried o u t througho u t the m o n s o o n period (but n o t presented here) showed a gradual decline in concentration with the passage of time. The sediment always showed a higher concentration o f nutrients compared to water. In general, the concentration of various nutrients in the sediment, and water lost from the afroecosystem under a 10-year jhum cycle was higher compared to a 5--year cycle. This m a y be due to the low nutrient status o f the soil, the smaller quantity o f slash to burn and also qualitative differences in the nutrient content of the slash in the agro~ecosystem under a 5-year cycle compared to that under a 10-year cycle. Tables III and IV present the total quantities of nutrients lost through sediment and water. In general, the nutrient losses are heavy under j h u m agro-ecosystems compared to the fallows where the losses are markedly reduced and very often there are no detectable losses. The only exception is the significantly increased (P < 0.05) loss o f NO3-N in a 5-year fallow through percolation water. This increased leachability of the nitrate ions in a 5-year fallow dominated b y Imperata cylindrica and Eupatorium adenophorum is contrary to earlier findings that suggest that acidifying grass vegeta-

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80 tion dominated b y I. cylindrica may repress nitrification in the soil (Nye and Greenland, 1960). The total quantity of nutrients lost from the agro-ecosystem was generally higher under a 5-year j h u m cycle compared to a 10-year cycle, which m a y be related to the poor physical quality of the soil in the former case resulting in less crop cover (Mishra and Ramakrishnan, 1981). This is more evident for the loss through sediment and run-off water than for the loss through percolation. During the second year of cropping under a 5oyear cycle the losses increased further which further strengthens the argument given above. The reduction in nutrient loss through percolation in the second year of cropping may be related to a smaller volume of percolation water as discussed earlier. In a single pulse, fire releases the nutrients, particularly the cations, tied up in the slash so much so that their availability may exceed the retention capacity o f the jhum agro-ecosystems. Hence, many of the cations are lost through sediment and water before they could be incorporated into the plant biomass. The large a m o u n t of cations released after the burn increases the soil pH which in turn m a y p r o m o t e nitrification (Granhall and Hendricksson, 1969). This, together with an increased concentration of bicarbonate ions, results in an increase of anions which are balanced by the free cations so that t h e r a t e of nutrient loss through water may increase (Bormann et al., 1968; Lewis Jr., 1974). The greater loss of potassium through percolation water compared to divalent cations under the jhum system confirms the findings of others (Allen, 1964; Lloyd, 1971), who have shown that potassium was much more readily dissolved than calcium and magnesium ions following a burn. Heavy loss of nutrients by erosion was chiefly due to a higher concentration of nutrients in the eroded sediment. In the latter months of the monsoon, the nutrient loss was halted to some extent by an effective vegetation cover of crops and associated weeds. The nitrogen and phosphorus losses from terrace agro-ecosystems are often less than under a 5-year j h u m agro~ecosystem, though the percolation losses of these nutrients tended to be consistently higher. The cationic losses, however, were invariably low from terrace agro-ecosystems compared to jhum under a 5-year cycle. Inspite of this, there is evidence of a significant decrease in crop yield in the subsequent years of terrace cultivation which m a y be related to a reduction in soil fertility and an increased weed problem (Mishra and Ramakrishnan, 1981). For this reason the farmer often discards the terraced plots after a b o u t 6 years of cultivation as the land becomes uneconomical to maintain. If the total loss of nutrients such as N and K is calculated as a percentage of the total input of these through inorganic fertilizer, this would work out to 56% for N and 28% for K. This loss would further increase (64% for N and 33% for K) in the second year of cultivation due to a decline in the physical characteristics of the soil. Thus, terracing does n o t seem to be a viable alternative to jhum.

81 CONCLUSIONS E c o l o g i c a l l y , t h e increase in t h e f r e q u e n c y o f t h e j h u m c y c l e u p s e t t h e h y d r o l o g y a n d n u t r i e n t cycling resulting in h e a v y losses o f soil a n d n u t r i e n t s as s h o w n b y this s t u d y . F r o m a n e c o n o m i c p o i n t o f view, t h e c r o p y i e l d is m a r k e d l y r e d u c e d u n d e r s h o r t e r cycles (Mishra a n d R a m a k r i s h n a n , 1 9 8 1 ) , p a r t l y d u e t o r e d u c e d soil f e r t i l i t y (Mishra a n d R a m a k r i s h n a n , 1 9 8 3 ) . T h u s , a r e d u c e d j h u m c y c l e o f a b o u t 5 years, as p r e v a l e n t at p r e s e n t , d o e s n o t s e e m t o be s u s t a i n a b l e o n ecological a n d e c o n o m i c g r o u n d s , as m u c h as terracing. I t is f o r these r e a s o n s t h a t a l o n g t e r m s t r a t e g y f o r a s h i f t in land-use p r a c t i c e t o p l a n t a t i o n a n d h o r t i c u l t u r a l c r o p s ( R a m a k r i s h n a n et al., 1 9 8 1 a , b ) has b e e n suggested. ACKNOWLEDGEMENT This r e s e a r c h was p a r t l y f i n a n c e d b y t h e D e p a r t m e n t o f Science a n d T e c h n o l o g y , G o v e r n m e n t o f I n d i a , u n d e r its ' M a n a n d B i o s p h e r e ' p r o g r a m m e .

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