Nutrient and soil organic matter dynamics under shifting cultivation in semi-arid northeastern Brazil

Agriculture, Ecosystems and Environment, 38 ( 1992 ) ! 39-15 l

139

Elsevier Science Publishers B.V., Amsterdam

Nutrient and soil organic matter dynamics under shifting cultivation in semi-arid northeastern Brazil H. Tiessen a, I.H. Salcedo b and E.V.S.B. Sampaio b aDepartment of Soil Science, Universityof Saskatchewan, Saskatoon, S7N 0 WO, Canada bDepartamento Energia Nuclear, UniversidadeFederalde Pernambuco, Recife, PE, Brazil (Accepted 19 June 1991 )

ABSTRACT Tiessen, H., Salcedo, I.H. ~ ,J Sampaio, E.V.S.B., 1992. Nutrient and soil organic matter dynamics under shifting cultivation in semi-arid northeastern Brazil. Agric. Ecosystems Environ., 38:139151. in subsistence farming systems using shifting cultivation, crop production depends on the natural fertility of the soil and depletion of fertility during cultivation is the cause for land abandonment. During the subsequent bush fallow, fertility levels are improved and the land may he available for further cultivation cycles, in the present study, the effects of cultivation and the regrowth of bush fallow on soil organic matter and phosphorus fertility were evaluated, using adjacent plots with different histories of cultivation and lengths of bush fallow in semi-arid northeastern Brazil. Six years of cultivation with minimal fertilisation resulted in reductions of C, N and organic P by 30%, or about 10 t ha -~ of C. Available N and P were greatly reduced under these conditions, in a fertilised and limed field, this trend was to some extent avoided, but such inputs are usually not an option for local farmers. Eight to 10 years of bush fallow were sufficient to re-establish fertility levels similar to the original uncultivated site. Phosphorus fractionation indicated that the decline in P fertility was not a result of net export of P in the crop, but arises from the mineralisation of organic P and subsequent lransformation of the surplus inorganic P to unavailable forms.

INTRODUCTION

The Sert~o area of north-eastern Brazil covers approximately 106 km 2. It has a semi-arid climate, and the natural vegetation is deciduous thorn forest or thorn bush savannah known as 'caatinga' (Hayashi and Numata, 1976). With the exception of limited areas under irrigation, normal cultivation practices involve slash/burn shifting cultivation with cropping cycles of 3-5 years. The main crops are cotton, maize, beans and cassava. Little fertJ,liser is used in the area because economic resources are limited and the low and irregular rainfall limits the potential return on fertiliser investment. Yields nolmai!y show a marked decline after the first year of cultivation when the fertiliser © 1992 Elsevier Science Publishers B.V. All rights reserved 016%8809/92/$05.00

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effect of the ashes and decomposing organic materials declines. This pattern is similar to the one described for the humid tropics by Nye and Greenland (1960) and Sanchez ( 1976 ). The cropping cycle is normally followed by ten or more years of bush fallow during which soil fertility is restored while the native vegetation is partially re-established. Little quantitative information exists on the processes of the fertility decline during the cropping cycle or the recovery of soil fertility during bush fallow in semi-arid regions. Nutrient transformations associated with traditional shifting cultivation form the basis of agricultural productivity in large areas of the semi-arid tropics. Rational use of alternative cropping systems and supplementary fertilisation should be based on an understanding of the nutrient transformations of the traditional system. We therefore studied some of the changes in soil organic matter and fertility during a cycle of cultivation and subsequent bush fallow. Changes in fertility occur so slowly during the complete cycle of shifting cultivation that it is difficult to monitor them with field experimentation. An alternative approach is to compare similar fields representing different stages during the shifting cultivation cycle. Land abandonment on private farms in the area is normally followed by variable periods of grazing during the reestablishment of native vegetation. This and the lack of adequate records on small farms prompted us to choose a fenced site belonging to the Agricultural Research Service of Pernambuco, even though all cultivated fields had received small basic dressings of NPK fertiliser. It was postulated that the turnover of organic matter from the original vegetation and the reintroduction of litter and extensive root biomass after abandonment of the cultivated sites were a critical factor in the nutrient cycling under shifting cultivation. Therefore, an attempt was made to analytically separate minor fertiliser effects from the effects of cycling of organic matter on levels and forms of N and P. The choice of existing plots with different histories of cultivation and length of bush fallow implied that no 'treatment' replication and no randomisation of plots was possible. This imposes limits on the statistical interpretation of data, but is ,~ situation frequently encountered in comparisons of cultivated and uncultivated sites (Tiessen et al., 1982: Schimel et al., 1985 ). Differences seen between sites could therefore be due to trends or variability across the experimental area or to the effects of site histories we tried to investigate. Natural site variabilit) usually is large under caatinga (Tiessen and Santos, 1989) and this variability may mask a large proportion of the effects of cultivation and reestablishment of native vegetation on soil organic matter contents and fertility. Therefore, a site was selected on a plateau of an exceptionally homogeneous cretaceous marine sediment that was evenly and deeply weathered to an oxisol (paleustox). Geo-statistical methods of analysis (Trangmar et al., 1985) which have been applied successfully to the evaluation of soil fertility parameters on non-replicated sites (O'Halloran et al.,

SOIL FERTILITY AND SHIFTING CULTIVATION IN BRAZIL

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1985) require transects with large numbers of samples, which can not normally be obtained on the small fields common in the tropics. Site variability along transects was evaluated using a simple analysis for random number series (Tiessen, 1988) in an attempt to separate effects of plot history from site variability. M ATERIALS AND M E T H O D S

Soils were collected from the IPA Experimental Station on the Chapada do Araripe (40°20 ' west, 7 o35' south, in Pernambuco, NE Brazil). The region has a climate with a highly variable annual rainfall of 235-1146 mm with an average of 830 mm year-i over the last 50 years (BSWH, K6ppen). More than 80% of the rain falls from December to April. Average annual temperature is around 24°C with variations of less than 5 ° from the coldest to the warmest months. Native vegetation is composed of low shrubs (3-5 m ), often thorny, usually deciduous, in a dense formation with a few emergent trees reaching 10 m height and a herbaceous layer which occupies the small open spaces and dries out entirely during the dry season. Normal cultivation practices on the Chapada involve shifting cultivation with 4-5 years of cropping, frequently to cassava, followed by 15 or more years of bush fallow. The sampling site consisted of 6 adjacent plots with the following land use history: ( 1 ) A plot abandoned in the previous year after a rotation of cassava, cotton, fallow, cassava (BF 1 ). (2) A plot under 4 years of bush regrowth following a rotation of cassava and beans (BF4). ( 3 ) A plot under 8 years of regrowth after cassava (BF8). (4) A plot under 10 years of regrowth following cassava (BF 10). (5) A field cropped continuously with sorghum and millet for an unusually long period of 12 years with regular fertiliser additions and liming (CRP). (6) An area adjacent to the cultivated and 8 year regrowth sites just outside the experimental station under climax native vegetation that had not been cultivated in the memory of any of the locals (CAr). All plots were at least 50 m X 60 m (0.3 ha) and were sarapled along regular transects traversing 2 or 3 different plots (Fig. 1). in all, 4 transects were sampled along 2 parallel lines and at right angles to each other. In this way all plots with the exception of BF8 and CAT were crossed by 2 transects at right angles to each other. Transect lengths varied between 21 and 36 points, and the total sample number was 122. Sampling interval was 5 m, except for the native CAT site which could not be traversed with regular transects without undue slashing and destruction of the vegetation. Samples were analyzed for organic C by automatic combustion, total N and P by H202/H2SO4 digestion and autoanalysis (Thomas et al., 1967), effec-

142

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cRP Bf 1

Trans ,r-,

i,ea,s'~','~l-,e

~-

Tr,,n~ Bf 4

,., B.f l O. .

.

.

.

.

.

.

.

.

.

.

B f 8. - * .

50 m

Fig. I. Plan of the experimental site showing plot and transect arrangements.

tive cation exchange capacity (CEC) by unbuffered NH4CI extraction and cation summation upon plasma arc determination of Ca, Mg, K, AI, Fe and Mn and pH was measured in a 1:10 soil:water ratio. Phosphorus was fractionated by a sequential extraction procedure distinguishing resin extractable inorganic P (Pi), bicarbonate extractable Pi and organic P (Po), hydroxide extractable Pi and Po, acid extractable Pi and a final residue that remained unextractable and was digested in a mix of H202/H_,SO4 (Tiessen et al., 1983, 1984). Site variability across the relatively small fields was assessed using transects and a random number series analysis (Tiessen, 1988) which tests for the presence of data clusters or trends along any one transect. This analysis was used to independently evaluate data across the site. Plot comparisons were subsequently made using pooled within-plot data regardless of transect orientation. Deviations between mean and median values for some variables within plots indicated skewed data distributions. In such situations parametric tests for analysis of variance are not recommended (Sokal and Rohlf, 1981 ). A distribution-free multiple comparison based on Kruskal-Wailis rank sums for unequal sample sizes (Hollander and Wolfe, 1973) with an experiment error of cv= 5% which represents a conservative approach to multiple comparisons. RESULTS AND DISCUSSION

Spatial data patterns Significant trends in exchangeable AI existed across the site (Fig. 2 ). Aluminium saturation percentage increased from about 5% to 60% along the first 12 points oftransect 3. This trend was not seen in the parallel transect 4 which showed a randomly distributed variability of AI saturation between 20% and 60% This range is similar to the 20%-50% range observed under native veg-

143

SOIL FERTILITY AND SHIFTING CULTIVATION IN BRAZIL 80

80 BF 1

70

p

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Fig. 2. Exchangeable aluminum levels along the 4 transects.

etation (not shown). Occasional lower A! saturation percentages in the regrowth sites may arise from an accumulation of ashes during the slash/burn cycle. The regular trends in AI saturation percentage observed in transect 3 and also transect 2 are within the range of natural variability but their nonrandom distribution remains unexplained. All trends observed in AI saturation percentage and total exchangeable A! contents are mirrored by opposite trends in Ca and Mg but not K (not shown ). Similarly, pH reflects these trends, ranging from 4.6-5 9 along the transects of decreasing AI. Correlation coefficients of AI with exchangeable bases and pH were - 0.86 and - 0.82, respectivt:ly. Because of these strong trends, these parameters were excluded from the median comparisons used to evaluate the effects of differences in cultivation history. Only the very low A! and Fe levels and elevated pH, Ca and Mg values of the cultivated CRP plot which received dolomitic lime could clearly be attributed to a treatment effect (Fig. 2, Table 1 ). In all areas except CAT, levels of exchangeable Fe are high, almost matching those of AI (Table 1 ). In CAT, Fe values are only a quarter of the AI values. This suggests that iron may be held in native organic matter and be

H, TIESSEN ET AL.

144 TABLE 1 Median values o f p H and exchangeable cations ( m m o l k g - ~) in different plots Plot

nI

pH

Ca

Mg

K

AI

Fe

CRP BF ! BF4 BF8 BFIO CAT

34 21 24 8 27 8

6.07 5.27 5.50 5.22 5.26 5,20

22.5 a 8. i b 2.9 b 5.2 b 6.7 b 3.2 b

05.4 a 2.2 b 1.4 b 2.3 b 2.7 b 2.4 b

1.7 a 0.9 b 0.9 b 1.0 ~b i.4 a 1. I~b

0.3 b 2.4 • 4.7 a 4.0 a 3.4 ~ 6.3 a

0.8 c 2.5 bc 4.8 a 4.6 ~b 4.3 a 1.7 b~

N u m b e r o f samples per plot. M e d i a n values within each c o l u m n with the same superscript do not differ significantly at P < 0 . 0 5 .

19

1.8"

1,6

CRP

1.6"

Bf4

Of 1

1.4

1.4"

1,2

1.2"

[]

Bf 10

[]

o

[]

[]

1.0

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0,8

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0 0

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1.0"

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1 2o sample number transect 1

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08"

0.6

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06

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eample number transect 2

1.8

1.6"

B 1,6"

B; 1

CRP

1.6

1.4 ¢d

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Bf 10

Bf 8

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1.2

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08

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Fig. 3. C a r b o n levels a l o n g t h e 4 transects.

released into exchangeable forms upon cultivation. No trends were observed for the distribution of C (Fig. 3), N and P across the site, and the following discussion based on median comparisons is not affected by site variability.

SOIL FERTILITY AND SHIFTING CULTIVATION IN BRAZIL

145

Effects of land use and bush fallow The CRP and BFI areas show the lowest organic C contents because of recent cultivation. With increasing fallow period C contents increase until, at BFI0, levels are not significantly different from CAT (Fig. 4). Twelve years of continuous cropping of CRP have not lowered carbon contents beyond the levels reached after about 6 years of cropping in BFI. This may be the effect of continuous fertilisation and liming of CRT at levels much higher than any of the other plots. Janzen (1987) similarly reported that high levels of P availabilfly from fertilisation resulted in increased amounts of mineralisable C in a Canadian soil. The effect of fertilisation is particularly noticable in the N contents of CRP that have been maintained at levels which are not significantly lower than those of CAT (Fig. 4). For the other plots, depletio~ and recovery of N levels followed those of carbon, and C:N ratios varied between 13 and 15 with no significant differences among plots. Soil mineral N contents (NO3 and NH4) were highest under natural vegetation, lowest after cultivation (BFI), and recovered with increasing fallow period, following a pattern similar to that of the organic matter contents (Fig. 5 ). Labile organic matter in the soil is the source for mineral nitrogen in the absence of recent fertilisation and this is reflected in these trends. The relatively low, but significant, correlation (R = 0.34) between mineral N and total N is explained by the more transient nature of the mineral N which depends on organic N reserves but also on soil moisture status. Relatively greater amounts of NH4 than NO3 are found under native vegetation and re~='owth and increases in mineral N in these plots are entirely due to increased NH4 levels. Fertiliser additions in the CRP plot were reflected predorv mantly in the NO~ fraction. II

1.2]

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1.o-1 ]

qz

1b=

c

o.8 = c

*'=

I

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h,,

c

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~

carbon nitrogen

0.4 0.2 0.0

BI 1

Bf 4

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Fig. 4. Carbon and nitrogen levels in native, cropped and differentbush fallow plots. Same letters over columnsindicatethe absenceof significantdifferencesfor each separate element.

H. TIESSEN ET AL.

146 30

2O

[]

..,4

NH4

IIN~ 10

0 Bf 1

Bf 4

Bf 8 Bf 10 C A T

CRP

Fig. 5. Nitrate and ammonia levels in native, cropped and different bush fallow plots. Letters refer to significant differences for (NOs + NH4 ). TABLE 2 Median values of soil P (:¢g g - ' ) in various fractions in different plots Plot

Inorganic P fractions (Pi)

Organic P fractions (Po)

Residue Total soil P

Resin

Bicarbonate NaOH Acid

Total Bicarbonate NaOH Total

CRP BFI BF4 BF8

1.24 a 0.48 be

8.01 a 6.86 ~

24.0 a 20,3 b

3.86 a 3.00 ab

38 ~ 31 °b

3.9 ab 4.1 ~

19.8 b 14,6c

24 bc 19d

92.9 ~ 85.4b

155~ 137b

0.29 ~ 0.67 b

5,00 b 4,9 b

16,6 ~ 19.6 b

2.36 b~ 3.14 ab

24 ~ 28 bc

3.1 b 3.6 ~b

19.2 b 22.5 ab

22 ~d 26 ab

91.4 ~ 97.9~

137b 152a

BFI0

0,48 br 0,80 ab

4.90 b 5,90 a~'

19,0 b 13.1"

3,14 ~b 1.70 ~

28 bc 22 c

4.7 ~ 5,2 a

24.4 a 22.2 ~b

29 ~ 27 ab

92.5 ~ 75.5 b

149a 123 b

CAT

Median values within each column with the same superscript do not differ significantly at P < 0.05.

The depletion of Po from CAT to BFI levels indicates that, together with C and N, Po is mineralised during the cultivation cycle. Organic P then recovers during the subsequent bush fallow to the initial CAT levels (Table 2). Losses of Po in BF 1 amount to 30% of the Po content in CAT. For carbon these losses are 33% (about 10.4 t ha-I ) which leaves the C:Po ratio of BF 1 and CAT almost unmodified at 430: 1. Organic C and extractable Po were significantly correlated across all fallow areas (R = 0.80). This indicates that overall organic matter (C) mineralisation was the main driving force for Po mineralisation. A new fractionation procedure, currently under development, indicated that an additional 10%-20% Po was contained in the residual P fraction. Total Po levels would therefore have to be revised to between 30 llg g- ~ (in BFI ) and 48/zg g- ~ (in BF ! 0 ). This does not change trends in the relative C:Po ratios between different plots but actual C:Po ratios are likely to be closer to about 250. In addition to the dynamics of soil organic matter, the root biomass from

SOIL FERTILITY AND SHIFTING CULTIVATION IN BRAZIL

147

native vegetation decomposes upon cultivation and contributes to the carbon and nutrient pools. Initial estimates (A. Lessa, unpublished data, 1991 ) indicate a total dry matter of the root biomass between 8 and 12 t ha- l which is five times higher than that found under a high yielding tropical crop like sugar cane (Sampaio et al., 1987). Carbon in this pool would amount to approximately 4 t h a - ~and thus represent an additional 40% above the contributions from soil organic carbon. Similar to the N results, Po was not significantly depleted in the CRP plot which received high doses of fertiliser. It appears that a supply of external P and N from fertilisation supports the maintenance or accumulation of organic N and P in the soil to a greater extent than that of organic carbon. This site, therefore, had the lowest C:N:Po ratios. Immobilisation of Pi was observed by Hedley et al. (1982) in an incubation experiment during which both carbon and inorganic phosphorus were added to the soil. However, carbon additions during that experiment amounted to 7 mg g- ~which is considerably more than expected from crop residues and decomposing root biomass of the sorghum crop grown on the CRP. However, a similar enrichment with Po was observed in a heavily fertilised field of sugar cane in the humid coastal region of Pernambuco. There, the 0-5 cm layer of the topsoil was enriched by 13/zg Po g- I after 15 years of cultivation, despite a concomitant decrease of carbon from 19 to 15 mg g- ' (Araujo et al., 1989). The observations presented here confirm that land clearing and cultivation in the semiarid tropics induce C, N and Po mineralisation as has been shown for the humid tropics (Mueller-Harvey et al., 1985). Organic matter losses during cultivation were largely recovered after approximately 10 years of bush fallow, even though the fallow vegetation had not nearly attained the height, density or composition of the native vegetation. Accretions of total C during fallow are probably the result of slow but continuously increasing inputs of organic matter from litter fall and turnover of root biomass under fallow vegetation. The relatively slow rates of additions of organic matter as well as the slow rates of nutrient uptake by the re-establishing native vegetation are more effective in building up and maintaining soil organic matter levels than the periodic additions of crop residues under cultivation without fertilisation. Nutrient limitations on the cultivated sites prevent organic matter maintenance or increase, unless carbon inputs are accompanied by substantial fertilisation as on site CRP. Several native leguminous species such as Mimosa spp., Indigophora spp., Cassia excelsa Schard and Bauhinia cheillantha Stevd. are involved in the recovery of N levels during the bush fallow period. The study also points to the role of the Po fraction in providing P for plant growth through mineralisation in tropical soils (Adepetu and Corey, 1976; Tiessen et al., 1984).

148

H. TIESSEN ET AL.

Phosphorus transformations The buildup of Po during bush fallow differs from that of C and N because it does not correspond to a net increase in total P. The sum of all P fractions in the four fallow areas, although statistically different, differ in the most extreme case (BF1 vs. BF8 ) by 9.8% (Table 2), probably as a result of limited fertiliser additions during the cropping cycle. The difference in Po content between those two plots, however, is 28% and can thus not be explained by changes in total P. The net transfers are between Pi and Po (Table 2 ), with a 30% higher level of Po in CAT, relative to BFI, matching a 30% decrease in Pi. The important role of Po under native vegetation is confirmed by a 20% higher level of Po than Pi in the uncultivated CAT area. Po is, therefore, a major component of the P cycle in tropical soi~ (Stewart and Tiessen, 1987; Tate and Salcedo, 1988 ). The changes in Po contents are mostly observed in the hydroxide extractable fraction while the bicarbonate extractable fraction is always smaller and remains relatively constant (Table 2). Bicarbonate Po has been shown to be labile and available to plants and microbes in laboratory incubations (Bowman and Cole, 1978a) and comparative field studies (Tiessen et al., 1984). Hydroxide Po is more stable and turns over more slowly in the field (Bowman and Cole, 1978b), although it can serve as a source for microbial P uptake during short term incubations ifPi is limited (Chauhan et al., 1981 ). In a temperate soil containing 50/~g g- ' bicarbonate Po and 170/zg g- ~hydroxide Po, both fractions were depleted equally by 30% after 60 years of cultivation (Tiessen et al., 1983). In the present case about 6 years of cultivation ( ~ F l ) under trnpical conditions were sufficient to reduce the hydroxide Po content by 30% from its original level of 22/~g g-~, while bicarbonate Po levels remained constant at 4-5/zg g - ' . Similarly, the more heavily fertilised cultivated site (CRP) showed significantly higher levels of hydroxide Po but not of bicarbonate Po compared with BFi. An increase in Po as a result of Pi fertilisation has been documented for temperate soils by Sadler and Stewart ( 1975 ). In the present case this buildup is only reflected in the hydroxide Po fraction. In a survey of 11 P-limited top soils from Per~ambuco with an average total P content of 150 /~g g- ~bicarbonate Po levels were 6.9 +_2.5/~g g- ~with lowest values around 4/~g g- ~. Hydroxide Po levels were between 3 and 5 times higher (H. Tiessen, unpublished data, 1988). Even at low hydroxide Po levels, bicarbonate Po level was never below about 4 ~tg g- ~, which thus seems to represent a minimum size of this pool for vegetated top soils of the region. The bicarbonate Po pool, v~hich has been shown to be relatively labile and actively cycling in studies on temperate soils, appears to be at a constant, minimal level regardless of the cropping history of the site in the present study. In contrast, the hydroxide Po pool, which is usually more stable, reflects the

SOIL FERTILITY AND SHIFTING CULTIVATION IN BRAZIL

149

overall changes in soil organic matter and Po levels when the soil is stressed by cultivation and net P export. This pool thus may represent a relatively active reservoir (source or sink) of P in shifting cultivation under tropical conditions. Fertiliser additions over 12 years (comparing CRP vs. CAT levels) resulted in an increase in total P of 32/lg g- i p. The additional P was recovered as inorganic P in the acid (2 gg g- ' ), NaOH ( 11 gg g- i ) and residual ( 17 gg g- i ) fractions. The residual fraction also appears to act as a sink for P, mobilised when the natural 'closed' P cycle is disrupted by the cultivation and subsequent fallow periods. The sites under 4, 8 and 10 years ofbushfallow contain 15-20 gg g- I more P in the residual fraction than the native CAT. The trend is also shown in BF l, although the increase of l 0 gg P g- I (from CAT levels) after 6 years of cropping and organic matter mineralisation is not significant (Table 2 ). The increase in residual P may represent a withdrawal of P from active nutrient cycling as a result of a shifting cultivation cycle that is only slowly (if at all) reversible. The NaOH Pi fraction showed similar trends, although changes were smaller, and reflected fertiliser additions to a greater extent, suggesting a more 'active' fraction. CONCLUSIONS

Land under shifting cultivation in the semi-arid NE of Brazil usually sustains about 5 years of cropping, during which yields decline until farming is uneconomical and the land is abandoned. The useful lifespan of a cleared site depends on nutrient availabilities. In the case of P, the net nutrient export by a crop like cassava amounts to only 2-3/lg P g- ' of soil, and does not significantly change the soil's nutrient budget. However, during the cropping cycle, organic matter is mineralised, and organic P pools which play a vital role in plant nutrition are depleted. A concomitant increase in inorganic P does not sustain labile P or crop production levels in the oxisols studied because of P fixation. In addition, the reduction in organic matter contents may cause deficiencies in mineralisable N and S. Production can be maintained by substantial fertilisation, possibly in combination with liming (to reduce P sorption ). Fertilisation is usually neither economical nor available to subsistence farmers in a semi-arid environment where crop production is severely limited by moisture availability. It is therefore important that the constraints on the availability of natural soil nutrients under shifting cultivation be understood, in order to design viable regimes of managem~nt and supplementary fertilisation. No solutions can be offered, but from the above results and observation of local practices some suggestions may be made. ( l ) It is vital to slow the decomposition of soil organic matter, forest litter

150

H. TIESSEN ET AL.

and roots, so that nutrients are released by mineralisation at rates comparable to crop uptake. This will minimise losses or fixation of mineralised nutrients. (2) Slower growing crops with slower nutrient acquisition rates may be successful for longer periods. Some of the native and regrowth species have economic value for wood and fuel production. Selective clearing, and managed fallows may allow the persistance of these slow growing plants. (3) The inclusion of crop legumes appears a sensible option for maintaining N in the soil, but, in practice, sites with legume intercropped cassava are also abandoned after 4 years. Several of these options are currently being investigated in a system based on zero-till manual planting and low intensity burning that allows the resprouting of larger tree stumps. It appears that organic matter dynamics under these conditions are quite different, but it is likely that only a combination of intercropping and crop and managed fallow rotations together with some fertilisation will allow permanent land use in the area. ACKNOWLEDGEMENTS

This research was supported by the Canadian International Development Agency, NSERC grant no. OGP 2274, and the Brazilian CNPq. This is contribution no. R 622 of the Saskatchewan Institute of Pedology.

REFERENCES Adcpetu, J.H. and Corey, R.B., 1976. Organic phosphorus as a prcdictor of plant available phosphorus in southern Nigeria. Soil Sci., 19: 65-80. Araujo, M,S.B., Salcedo, I.H. and Sampaio, E.V.S.B., 1989. Efeito da fertiliza~o numa cronoseqU~ncia de cultivo sobre a distribui~io de P no solo. (Fcrtiliser effects in a cultivation chronosequence on the distribution of P in the soil ) Resumo, XXII Congresso Brasileiro de Ci6ncia de Solo, July 1989, Recife, Brazilian Soc. Soil Sci., p. 172. Bowman, R.A. and Cole, C.V., 1978a. Transformations of organic phosphorus substances in soils as evaluated by NaHCO3 extraction. Soil Sci., 125: 49-54. Bowman, R.A. and Cole, C.V., 1978b. An exploratory method for fractionation oforganic phosphorus from grassland soils. Soil Sci., 125: 95-101. Chauhan, B.S., Stewart, J.W.B. and Paul, E.A., 1981. Effects of labile inorganic phosphate status and organic carbon additions on microbial uptake of phosphorus in soils. Can. J. Soil Sci., 61: 373-385. Hayashi, 1. and Numata, M., 1976. Structure and succession ofcaatinga vegetation in the Brazilian North East. Tokyo Geogr. Pap., 20: 23-44. Hedley, M.J., Stewart, J.W.B. and Chauhan, B.S., 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci. Soc. Am. J., 46: 970-976. Hollander, M. and Wolfe, A.D., 1973. Nonparametric Statistical Methods. Wiley, NY, 503 pp. Janzen, H.H,, 1987. Soil organic matter characteristics after long-term cropping to various spring wheat rotations. Can. J. Soil Sci., 67: 845-856.

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