Fine root mineralization, soil organic matter and ... - Science Direct

Agriculture Ecosystems & Enwronment ELSEVIER

Agriculture, Ecosystems and Environment 59 (1996) 191-202

Fine root mineralization, soil organic matter and exchangeable cation dynamics in slash and burn agriculture in the semi-arid northeast of Brazil Armando S.N. Lessa *, Darwin W. Anderson, Jackie O. Moir Department of Soil Science, Universityof Saskatchewan, Saskatoon, Sask. S7N OWO,Canada Accepted 13 March 1996

Abstract The objective of this study was to understand the causes of crop productivity decline on a soil cultivated by the slash and bum method. The contribution of ashes, fine roots, and soil organic matter (SOM) mineralization to the pool of available nutrients of a nutrient-poor Haplustox of the semi-add zone of northeastern Brazil was documented. Ashes were the most important input of nutrients to the soil. The burning of the vegetation debris produced 11 Mg ha-~ of ash containing considerable quantities of Ca and K, and some N, Mg and P. The ashes, in general, contained more Ca and Mg, and less N and K, than the estimated requirements of the cassava (Manihot esculenta) crop during the cultivation cycle, whereas the P in ashes was equivalent to the crop P uptake. About 65% of the fine roots from the native vegetation decomposed in the soil during the first rainy season after the slash and burn, contributing Ca, N, and Mg to stores of available nutrients, with limited supply of K and P. The SOM content decreased with cultivation. The losses ranged from 4 to 16%, 6-18%, and 10-20%, for C, N and organic P, respectively. The sum of exchangeable cations, base saturation and pH increased after the bum, whereas the exchangeable Al and A1 saturation strongly decreased, promoting better growing conditions for the cassava crop, particularly during the first years of the cultivation cycle. The soil properties reverted to pre-burn conditions within two or three crop years, productivity declined and the field was abandoned to natural fallow.

Keywords: Slash and bum; Ashes; Fine roots; Dynamics of exchangeable cations; Soil organic matter; Oxisol soils

1. Introduction Traditional shifting cultivation through periodic slashing and burning o f small plots for agricultural purposes has been the usual practice in the s e m i - a d d Brazilian northeast. The cleared plots are cropped with initial low productivities for a few years, crop

* Corresponding author. Tel.: (306)966-6827; fax: (306)9666881.

productivity declines to critical levels, and the plots are then abandoned to natural regrowth. The most c o m m o n crops grown b y the local peasants and small holders are cassava, maize (Zea mays) and cowpea (Vigna sp.). The amount of ash produced in slash and burn agriculture and its nutrient concentration varies greatly due to differences in vegetation, climate, and soils o f each particular site, and due to the characteristics and conditions of the pre-burn residues and

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A.S.N. Lessa et al. / Agriculture, Ecosystems and Environment 59 (1996) 191-202

factors associated with the fire (Harwood and Jackson, 1975; Sanchez, 1976; Seubert et al., 1977; Raison, 1979; Ewel et al., 1981; Stromgaard, 1984; Andriesse and Schelhaas, 1987). Ashes are an important component of slash and burn agriculture capable of improving soil fertility. Fine roots may be an important source of nutrients after slashing and burning due to the intense decomposition that may be stimulated by the effect of ashes in the soil and consequently, better environmental conditions. The release of nutrients from labile SOM is considered to be quite important to subsistence farming in the tropics (Mueller-Harvey et al., 1985). The rate of decomposition of SOM in tropical savannas is reported to range from 0.5 to 1.2% per year (Nye and Greenland, 1960; Sanchez, 1976), with levels of 1-4% per year in cultivated savannas (Sanchez, 1976). The clearing and cultivation of savanna soils may increase rates of SOM decomposition because of a general increase in soil temperature, better soil aeration and more intense microbial activity. Nevertheless, changes in the concentration of organic C over the comparatively short cultivation cycle in traditional shifting cultivation systems can be small and difficult to detect (Sanchez, 1976). Understanding the reasons why soil fertility declines and the mechanisms of nutrient availability over the cultivation cycle may help in developing or adopting alternatives for slash and burn agriculture. The composition of the ashes produced and their effect on fertility, the mineralization of fine roots in the soil, and the mineralization of SOM are important factors. Several hypotheses can be formulated with regard to the declining crop productivity in

slash and burn agriculture. It may be caused by (a) the using up of nutrients contained in ashes and fine roots, (b) reduced supply of nutrients from SOM mineralization, (c) erosion and soil compaction, (d) breaking down of the tight nutrient cycling originally present in the native ecosystem and leaching of nutrients out of the root zone, (e) pest, disease and weed infection, or (f) a combination of all these factors. The objectives of this study are: (a) to evaluate the effect of ashes on soil fertility, (b) to examine the contribution of fine root mineralization, and (c) to examine the changes in the concentrations of SOM and exchangeable cations upon cultivation.

2. Materials and methods

2.1. Description of the area The research site is located at the Experimental Station of the Institute of Agronomic Research (IPA) on nearly level tableland referred to locally as 'Chapada do Araripe' (40°20'W, 7°35'S), in Pernambuco State, northeastern Brazil, at an altitude of 820 m above sea level, and 750 km from the Atlantic coast. The region has a BSWH (KSppen classification) climate with highly variable annual rainfall between 235 and 1146 mm and a mean of 790 mm over the last 50 years. Most of the rainfall occurs between December and April (Fig. 1). The average annual air temperature is 24°C, with a difference between mean summer and mean winter temperatures of less than 5°C. The local mean annual poten-

90~

16 80 0 1

100

"200

80

s

70 50 40 30 2010 £ 0 . J

~

Temperature

Ralafall

140 ~

. ~ v.~ .

. F

. . M A

M

,

.

~

~

',

100 80 "60 "40 "20 0

J J A S O N D Months Fig. 1. Mean monthly air temperature and rainfall at Amripina, based on records from 1968 to 1988.

A.S.N. Lessa et al. ~Agriculture, Ecosystems and Environment 59 (1996) 191-202

193

240 m

I

I . . . . . . . .

1 9 8 8 - native (burned in

August)

Om

Pl~t

1989 and 1990 - cropped

1 9 8 8 - abandoned

I

I I I I I

1987 - burned (burned in September) 1 9 8 8 , 1 9 8 9 and 1990 - cropped

I

I

60 m

I

I

60 m

120 m

I

Fig. 2. Plot m a p at Araripina research site.

tial evapotranspiration, according to the Laboratory of Meteorology of the IPA Research Station, is 1127 mm, as calculated by the Thornthwaite method. The soils at the study site are Oxisols (Xanthic Haplustox, fine-loamy, kaolinitic, isohyperthermic) (Soil Survey Staff, 1987), developed in situ from underlying Cretaceous sandstone of the Exu Formation (Buerlen, 1962). The soils have a well developed fine granular structure. The land is level (less than 1% slope), well drained and without evidence of soil erosion. A typical soil profile is described in Embrapa (1972); the soils at the study site are part of soil association LVd9, which occupies about 2000 km 2 of the Chapada do Araripe.

covered with native vegetation during the first half of 1988, which was slashed by June. Plot 2 had been abandoned to natural fallow in April 1988, following the harvest of the third crop grown on that soil, completing a 5-year cultivation cycle. Plot 3 had been burned in September 1987, cassava and beans were growing together in the field, and ashes from the previous year's burning were still visible on the soil surface in June 1988. Ashes were collected from the soil surface of Plot 1 with a spatula from eight randomly selected areas of 0.40 m × 0.40 m, immediately after the burning of the Native Plot in August 1988. The ashes were placed in plastic bags, later sieved through 35 mesh (0.42 mm), and weighed without correction for moisture content. Bulk soil samples were taken at 0-5, 5-20 and 0-20 cm depths every 10 m along three parallel transects 10 m apart in Plots 1 and 3 (Fig. 2), in April of 1988, 1989 and 1990. Plot 2 was sampled

2.2. Plot layout and sampling The three plots studied were adjacent and of approximately the same size (Fig. 2). Plot 1 was

Table 1 N u m b e r o f sample replicates for each depth o f sampling on Plots 1, 2 and 3 in 1988, 1989, and 1990 at Araripina Year

1988 1989 1990 a Soil depth.

Plot 1

Plot 2

Plot 3

0-5 cm a

5 - 2 0 cm

0 - 2 0 cm

0-20 cm

0-5 cm

5 - 2 0 cm

0-20 cm

-

-

36 33

36 -

13 18 18

-

36 33

21 -

39 -

18 18

194

A.S.N. Lessa et al. / A gricultur e, Ecosystems and Environment 59 (1996) 191-202

only in 1988. The number of samples taken from each plot in different years varied (Table 1). Soil samples were air-dried, transported to the laboratory at the University of Saskatchewan and stored for analyses. All soils sampled in 1988 and subsequent years were sieved (less than 2 mm sieve) and fine roots were removed by hand-picking. The fine roots were oven-dried (60°C), ground in a cyclone mill and digested for total element analyses. Two approaches were used in data analysis: (a) a comparison of key soil properties in a particular plot in successive years; (b) a comparison of such key properties in different plots during the same year. Thus, the analysis shows the changes that occur with time within different periods of the cultivation cycle on the same soil, and differences between different phases of the cycle. Non-replicated core samples were taken in 1988 in all plots to measure bulk density. The cylinders used were 4.9 cm in diameter, and 5.0 cm in height. Soil samples were oven-dried at 105°C, weighed, and the bulk density estimated.

2.3. Analytical methods The total elemental analyses of ashes and fine roots were carried out by digesting the samples with concentrated H2SOg//H202, followed by analysis of N and P on a Technicon autoanalyzer (Thomas et al., 1967). Cations in the ash and roots were analyzed by plasma emission spectroscopy. All existing soil carbon (C) was considered to be organic C as carbonate minerals are unlikely to be present in Oxisols. Total C in soil was estimated by dry combustion and the two-endpoint titration method (Tiessen et al., 1981). Total N and P in soil were determined by autoanalysis on the concentrated H2SO4/H202 digest (Thomas et al., 1967). Soil Po was obtained by the summation of P fractions sequentially extracted (Hedley et al., 1982). The pH of soils and ash was determined in water (1:2 soil:water ratio). The cations (A1, Fe, Ca, Mg, K) were extracted using an unbuffered 1 M NH4C1 extractant (1:5 soil:solution ratio), the extraction was repeated twice and the extracts were combined. The soil was shaken for 30 min with the extractant and centrifuged at 10000 rev min - t for 10 rain, before filtration of the supematant (under 0.45 ~m). The

elements were determined by plasma emission spectroscopy. The sum of exchangeable cations (except H ÷ which was not determined) in the NH4CI extract was calculated without corrections for soluble cations.

2.4. Statistical analyses Frequency distribution diagrams were examined to assess the normality of the populations. The soil properties evaluated were normally distributed, allowing the use of parametric methods. The large number of samples that were taken in each plot compensates for the lack of replication of plots, which was not possible under the conditions of this study. In addition, the variability of many soil properties at the site was not known and, hence, many samples were included in the evaluation. Analysis of variance (ANOVA) or t-tests where appropriate were carried out to segregate variances and test the hypothesis of no effect by testing for equality of the population means. All analyses were performed at the 5% probability level, unless otherwise specified. The computer software Statview SE (1988) was used for statistical analyses.

3. Results and discussion

3.1. Slash and burn of the native vegetation After slashing the vegetation on Plot 1, the trunks were removed from the field and the other debris was left to dry, and then burned in August, prior to the rainy season in November 1988. The ash produced was a mixture of particles of different sizes and varied in color because the fire pattern and intensity varied from one point to another in the field. The large amount of ashes (estimated at 11 Mg h a - l ) produced by the burn (Table 2) indicated incomplete combustion of the slash because of the high percentage of ash in relation to the mass of pre-burn residues (61 Mg ha - l ) (Lessa, 1995). The presence of charcoal and wood fragments, and the comparatively high N content of the ash further indicate incomplete volatilization. The return of ash

195

A.S.N. Lessa et a l . / A g r i c u l t u r e , Ecosystems and Environment 59 (1996) 1 9 1 - 2 0 2

Table 2 Air-dry weight, nutrient content, pH and coefficients of variation (CVs) of the ash (under 35 mesh) produced on Plot 1 at Araripina after the slash and burn in 1988 Ash a m g g - I dry ash Mean

c v (~)

p

N

Ca

Mg

K

Fe

1.1 88

3.1 62

30.8 125

4.2 125

5.4 101

12 84

35 91

348 105

47 III

A1

18.8 17

pH

30.7 19

10.2 10

kg ha -1 Mean

11294 30

cv (%)

61 95

178 45

347 47

a Mean of eight replications.

has been reported to vary from 2 to 9% for wood and 13-20% for pasture grasses (Harwood and Jackson, 1975; Raison, 1979). It is also possible that the amount of ashes was overestimated because of some contamination with soil during ash collection. Partial volatilization of N during the burn is consistent with results reported for other environments (Sanchez, 1976; Seubert et al., 1977; Stromgaard, 1984). About half the N present in the litter of the native vegetation on Plot 1 remained in the ash (Lessa, 1995). In semi-arid Sri Lanka, more than 90% of the N from burning plant debris was volatilized (Andriesse and Schelhaas, 1987). The probable contamination of the ash with soil during collection makes doubtful any comparison of elemental concentrations with other results. It is evident, however, that the ash contributes significant

amounts of P, Ca, Mg and K to the cultivated system (Table 2). 3.2. Rate o f f i n e root mineralization

The fine root content at 0 - 2 0 cm depth was estimated at 2.27 mg g-~ soil on Plot 1 in 1988 and had decreased to 0.84 mg g-~ soil by 1989 (Table 3). The difference is equivalent to about 3.7 Mg ha -~ of fine roots mineralized, and probably an important source of N and Ca to crops. We estimate (from Table 3) that between 50 and 60% of the N and P, and between 80 and 90% of the Ca, Mg and K, were released from decaying roots during the first year of cultivation. Contribution of fine roots through mineralization to P availability appears to be of little importance. The reduced content of fine roots in 1989 indi-

Table 3 Mass and nutrient content of f'me roots. Soil samples were taken in 1988 ( 0 - 2 0 cm depth) and 1989 (0-5 and 5 - 2 0 cm depth) on Plot 1 a Year

Fine roots b

p

N

Ca

Mg

K

440a 20 439a 19

15960b 12 18939a 19

6662a 49 3216b 58

993a 44 303b 54

2266a 33 1103b 48

Ix g g - t f i n e roots

Mean CV (%) Mean CV (%)

1988 1989

2.27a 60 0.84b 38

kgha -/ Mean Mean

1988 1989

6084 2402

2.7 1.1

97.1 45.5

a See Fig. 2. b Expressed as mg roots g - l soil, except for rows 5 and 6, which are in kg h a - i. Values followed by different letters are significantly different at P < 0.05.

40.5 7.7

6.0 0.7

13.8 2.6

196

A.S.N. Lessa et a l . / Agriculture, Ecosystems and Environment 59 (1996) 191-202

cates that intense decomposition began after the slash and burn of the vegetation, coincident with the first rainfall. Intense root decomposition may be stimulated by higher soil temperatures in the cleared plot and an initially higher soil moisture content due to a higher percentage of the rainfall that reaches the soil on cleared plots and reduced transpiration in the absence of a dense vegetation cover. In addition, microbial activity may have increased after the burn because of the increase in base cations and in soil pH. Root decomposition appears to be an important contributor to N and Ca availability early in the cultivation cycle. Except for N and K, the ashes contained approximately the estimated amounts of nutrients needed by the cassava crops that grew in the plot at low and decreasing levels of productivity over the years of cultivation. However, ashes and fine roots together contributed an estimated 88 kg N ha -~ . An N input of this magnitude or higher may explain why much of the soil N after the burn was considered readily available (Christensen and Muller, 1975; DeBano et al., 1979) and why response to N fertilizers by crops growing in fields just after burning may not be noticeable (Vlamis and Gowans, 1961; DeBano and Conrad, 1974). 3.3. SOM changes after clearing and cultivation Concentrations of soil C, N and Po in SOM (Table 4) decreased owing to the clearing and type of management carried out at the site. These changes occurred in the first years of cultivation, and then decomposition processes tended to balance or level off after the initial losses. Losses ranging from 4 to 16%, 6-18%, and 10-20% for C, N and Po, were responsible for the release of amounts varying from 1000 to 4000 kg, 123-383 kg, and 6 - 1 0 kg, respectively, of C, N and P in the soil. Although the losses were relatively small, they appear to be of considerable biological importance for the nutrient-poor soil of the research site. The amount of N made available during the first years indicate that factors other than N appear to be more important in limiting cassava productivity in the initial phases of the cultivation cycle. Small amounts of P appear to have been released through SOM mineralization, consistent with the large C:Po ratios,

Table 4 Concentrations of C, N and Po at two depths sampled at different times at Araripina Plot

Year

C (mg g-l)

N (~g g-l)

Po (l~g g-l)

C:N:Po

1988 a

10.3a 9.6a 9.3a

746a 728a 674a

25a 21b 23ab

412:30:1 457:35:1 404:29:1

1b

1988 1989 1990

10.3a 10.7a 9.1b

746a 656b 626b

25a 20b 23a

412:30:1 535:33:1 396:27:1

3b

1988 1989 1990

728a 594b 640b

21a 19b 19b

457:35:1 426:31:1 468:34:1

O - 20 cm depth 1 3 2

O-5 cm depth 1b 3b

5 - 20 cm depth 1b

3b

9.6a 8.1b 8.9ab

1989 1990

9.2b 10.6a

886a 792b

26a 26a

354:34:1 408:30:1

1988 1989 1990

13.8a 9.6b 9.6b

988a 718b 759b

42a 23b 25b

329:24:1 417:31:1 384:30:1

1989 1990

10.9a 8.5b

579a 571a

18b 22a

605:32:1 386:26:1

1989 1990

7.3b 8.6a

552a 600a

18a 16b

406:31:1 537:37:1

Means followed by different letters differ significantly ( P < 0.05). Within each soil depth, comparisons are a between plots or b within a plot between years. Bulk densities are 1.34 Mg m -3 for Plot 1 in 1988, and 1.43 Mg m -3 in 1989 and 1990; for Plot 3, 1.43 Mg m -3 in 1988, 1989, and 1990.

and the low level of P in the soil (Lessa, 1995). The slight changes in SOM with cultivation suggest that most of the SOM may be well-humified material complexed with mineral colloids and not important to nutrient cycling and soil fertility in the short tenn. However, factors related to the natural infertility of the soil and constricted microbial activity may explain the lack of more dramatic reductions in the SOM content. The results of SOM decomposition on Plot 1 point to a higher decrease in the concentration of Po over C or N from 1988 to 1989. Total N and Po losses were 12% and 20% of their initial contents,

A.S.N. Lessa et a l . / Agriculture, Ecosystems and Environment 59 (1996) 191-202

releasing 123 kg ha - l and 10 kg ha - l of N and Po, respectively (Table 4). The higher decrease in Po concentration than for C and N in the 0 - 2 0 cm depth from 1988 to 1989 is probably due to a higher demand for P just after the initiation of the cultivation process. The higher rate of decomposition for Po in relation to C and N is possible because of the nature of the P bonds in SOM, which may allow for the release of P through a biochemical mineralization pathway (McGill and Cole, 1981). Mineralization of Po is more responsive to pH increase than either C or N (Thompson et al., 1954; Harrison, 1982), because hydroxyl competition with phosphate for organic or metal-organic bonding sites may release P into the soil solution (Hingston et al., 1972; Mattingly, 1975). It is probable that the amount of available P and other nutrients released from the ashes, an adequate energy supply as fine roots decompose, coupled with the pH increase, would all favor more intense microbial activity, and promote the release of Po from soil organic components, increasing available P (Halstead et al., 1963). Organic P accumulated at the 5 - 2 0 cm depth on Plot 1 from 1989 to 1990 (Table 4). Despite the limitations in using the C:Po ratios to infer C decomposition or movement in soil due to the different pathways of mineralization that C and Po undergo, based on the gains of Po with depth in 1990 and ratios of C:Po varying from 386 to 605, the movement of SOM down to 5 - 2 0 cm depth may have amounted to 3000-5000 kg C ha -1 from 1989 to 1990. The sequence of events on Plot 3 (Table 4) represents a more advanced phase of the cultivation cycle. The data indicate more consistent changes, and that the more intense reactions occurred at 0 - 5 cm depth. There were slightly higher rates of SOM decomposition in this plot. Carbon, N and Po decreased by 16%, 18% and 10%, with losses of 4290 kg ha -~, 383 kg ha - l , and 6 kg ha - l , respectively, at 0-20 cm depth from 1988 to 1989. The rates of mineralization for C, N and Po were higher at 0-5 cm depth, where SOM is thought to be more labile and readily decomposable, compared with SOM in lower soil depths, which is known to be more strongly associated with mineral particles (Martin and Haider, 1986). Soil organic matter in upper soil layers contains a higher proportion of the light fraction and

197

Table 5 Nutrient uptake and removal by the cassava crop (based on Howeler, 1981), and nutrient release from ash and free root mineralization (from Tables 2 and 3) (all values expressed in kg ha - t ) N Total plant uptake t - t 4.91 of fresh root harvested Removal t - ~ of 2.33 fresh roots harvested Removal over the 5 year 70 cultivation cycle (30 Mg h a - i fresh roots) Ash content 35 Fine roots mineralization 52

P

Ca

Mg

K

1.08

1.83

0.79

5.83

0.52

0.61

0.34

4.11

16

18

10

123

12 2

348 33

47 5

61 11

macroorganic matter, fractions which are more responsive to management practices (Janzen et al., 1992; Biederbeck et al., 1994; Bremer et al., 1994). Decomposition rates at 0 - 5 cm depth were 30%, 27%, and 45% for C, N, and Po, respectively, at 0-5 cm depth from 1988 to 1989. Organic matter decomposition in the upper soil depths was probably stimulated by the higher pH and the increase in available Ca and P added to the soil in ashes. The estimated removal of P by harvest (Table 5) over the 5 year cultivation cycle is about equal to the P contained in ashes plus that released by fine root mineralization. The inputs of P in wet and dry deposition can be assumed to be negligible (Sanchez, 1976; Jordan, 1985). Other possibilities, such as the input of P in pollen from adjacent savanna, or added as soluble P in runoff from adjacent fields can also be assumed to be negligible. These data suggest that the only source of P to the cassava was the ashes and as P in ashes was depleted, yields decreased. Sampling of Plot 3 soils was carried out at the end of the rainy season in 1988, and it is possible that some of the added N from ashes and fine roots had already been lost by leaching or denitrification. Additions of N in the tropical savannas via rainfall and dust are reported to vary from 4 to 8 kg ha-~ and amounts fixed by nonsymbiotic and symbiotic Nfixation are reported to vary from 0 to 45 kg ha- ~ on an annual basis (Sanchez, 1976). These processes may have contributed some N to the soil. Assuming that the inputs of N by fixation were about 30 kg ha-t annually, that would represent 150 kg N ha-

198

A.S.N. Lessa et al. / A griculture, Ecosystems and Environment 59 (1996) 191-202

during the cultivation cycle. The gains in the concentration of soil C at 5 - 2 0 cm depth from 1989 to 1990 on Plot 3 (Table 4) indicate an amount of 2800 kg C h a - ~. The overall magnitude of changes generally support the idea that organic matter made a modest contribution to soil fertility, despite the considerable amount of N mineralized upon cultivation. At Araripina, cassava is generally harvested three times during the complete cultivation cycle (5 years). The estimated yields are 15 Mg h a - ~ of fresh roots in the first harvest, which is reduced by 30% in the second harvest, and by another 30% in the third harvest. Nitrogen output through cassava harvest is estimated to be about 70 kg ha - l for the total cultivation cycle. Thus, the inputs of N from ashes (Table 2) and mineralization of fine roots (Table 3) (total estimated 87 kg ha - l ) exceeded the amounts of N taken up by the crop to produce 30 Mg h a - 1 of fresh roots (Table 5). Losses of N from the soil may

have occurred, particularly in the early stages of the cultivation cycle. It is a common practice at Araripina, as in other shifting cultivation systems, to intercrop cassava with legumes, usually beans, which may also be of low productivity. The annual yield of beans is generally lower than 300 kg h a - l and the residue from the crop is left on the field after harvest. It may have contributed some additional N to the soil.

3.4. C h a n g e s in s o i l e x c h a n g e a b l e saturation and pH values

cations,

base

Exchangeable cations, base saturation, and pH are important agronomic properties on acidic soils. The data from the plots sampled in 1988 and from Plot 3 sampled over time, consistently indicated that exchangeable A1 was reduced markedly and that the

Table 6 Exchangeable AI and base cations, sum of cations (cmol kg- ~ soil), base saturation(%) and pH values at two depths sampled over time at Araripina Plot

Year

AI

Fe

Ca

Mg

K

Sum cations

Base sat.

pH

1 3 2

1988 a

0.47a 0.12c 0.30b

0.17b 0.38a 0.22b

0.59b 1.01a 0.79b

0.21b 0.32a 0.18b

0.11b 0.14a 0.12b

1.55b 1.97a 1.61b

56 75 69

4.9b 5.3a 4.9b

1b

1988 1989 1990

0.47b 0.55a 0.52b

0.17a 0.13b 0.03c

0.59a 0.67a 0.67a

0.21a 0.19a 0.19a

0.1 la 0.1 la 0.09a

1.55a 1.65a 1.50a

59 59 63

4.9a 5.0a 5.0a

3b

1988 1989 1990

0.12c 0.39b 0.53a

0.38a 0.25a 0.03b

1.01a 0.54b 0.58b

0.32a 0.16b 0.18b

0.14a 0.09b 0.08b

1.97a 1.43b 1.40b

75 55 60

5.3a 4.9b 4.9b

1b

1989 1990

0.04b 0.13a

0.06a 0.01b

2.15a 1.75a

0.51a 0.42a

0.21a 0.12b

2.97a 2.43a

97 94

6.1a 5.8b

3b

1988 1989 1990

0.03b 0.08b 0.23a

0.02b 0.12a 0.02b

3.28a 1.56b 1.18b

0.81a 0.36b 0.34b

0.20a 0.13b 0.12b

4.34a 2.25b 1.89b

99 91 87

6.5a 5.6b 5.5b

1b

1989 1990

0.72a 0.65b

0.15a 0.03b

0.18b 0.31a

0.08a 0.11a

0.08a 0.08a

1.21a 1.18a

28 42

4.6a 4.6a

3b

1989 1990

0.49a 0.63a

0.29a 0.03b

0.20b 0.39a

0.08b 0.13b

0.07a 0.07a

1.14a 1.25a

32 47

4.6a 4.7a

0-20 cm soil depth

0-5 cm soil depth

5-20 cm soil depth

Means followed by different letters differ significantly(P < 0.05). Within each soil depth, comparisonsare a between plots or b within a plot between years.

A.S.N. Lessa et a l . / Agriculture, Ecosystems and Environment 59 (1996) 191-202

ashes contributed to a higher level of exchangeable base cations (particularly Ca), base saturation, and higher pH values in the first year of the cultivation cycle (Table 6). Exchangeable A1 increased as cultivation progressed, whereas the level of exchangeable base cations decreased, along with declines in pH and base saturation. The proportion of exchangeable A1 to total cations (A1 saturation) is considered a better parameter than exchangeable AI concentration to indicate the potential toxicity of AI to plants (Lopes and Cox, 1977). Aluminum saturation was 30% in the soil on Plot 1 (Native Plot) in 1988 (Fig. 3), a value considered to

g

100 90 80 70 60

[] K

rnMg BCa life DAI

50 40 30 2o '4

10 0 1989

1988

1990

100' 90 80' 70' 60 50 40 30 20 10 0 1990

1989

100 90 80 70 60" 50" 40" 30"

1989

Years

1990

Fig. 3. Effect o f cultivation on cation saturation on Plot 1.

199

be the critical level for most crops (Abruna et al., 1964, Brenes and Pearson, 1973). The native vegetation is probably more tolerant to A1 than domestic crops. Nevertheless, the strong initial decrease in exchangeable AI (Table 6) is an important consequence of slashing and burning, creating an environment comparatively enriched in bases at the onset of the crop cycle. In general, the values for exchangeable bases and sum of cations were lower than expected, based on the nutrient content of ashes. As an example, the Ca added to the soil from ash and root mineralization, an estimated amount of 380 kg ha -~, would be enough to increase the level of exchangeable Ca at 0 - 2 0 cm depth by 0.66 cmol Ca kg -l. Yet, there is the possibility that substantial amounts of exchangeable Ca, Mg and K have been lost by leaching, that the annual sampling interval was unable to detect. The increase in pH in Plot 3 is a likely response to the addition of ashes to the soil. The pH was lower in Plot 2 at the end of the cultivation cycle. The decrease in pH on Plot 2 suggests a rapid loss of base cations from the soil, probably by leaching. Overall, as for the SOM, changes were small. Changes were more evident on Plot 3 and levels of exchangeable base cations, base saturation and pH were always higher at 0 - 5 cm than 5-20 cm depth. The exchangeable AI (Table 6) and AI saturation (Fig. 4) values were extremely high in the lower depths and considered toxic to most crops. Exchangeable Fe behaved differently from exchangeable AI and generally decreased with cultivation. The decrease in exchangeable Fe with cultivation tended to follow decreases in pH and SOM content. Lower pH values cause an increase in the net positive charge at the surface of Fe compounds, thus increasing the quantities of anionic organics adsorbed to mineral surfaces at higher bonding energies (Schwertmann et al., 1986). The base cation saturation was high at 0 - 5 cm depth, particularly for Ca (Fig. 4). The amount of Ca estimated to have been removed by harvesting the crop is about 18 kg ha(Table 5). Estimated crop uptake represents less than 5% of the Ca added to the soil (Tables 2, 3 and 5). Calcium from the ashes may have been complexed with anions such as phosphates and some Ca may have been leached below the 0 - 2 0 cm depth of soil. The concentration of Ca present in soil after the

200

A.S.N. Lessa et al./Agriculture, Ecosystems and Environment 59 (1996) 191-202

cultivation cycle, indicated by its value on Plot 2, was about the original value o f the Native Plot (Plot 1 in 1988). Input o f M g from ashes and fine roots accounted for the increase in exchangeable M g in Plot 3. The estimated amount o f M g exported by harvesting is about 10 kg ha - l (Table 5), whereas inputs from ashes and root mineralization contributed an estimated 50 kg h a - 1. Estimated input of K from ashes and fine roots was about 70 kg ha-1 (Table 5), contributing to the initial increase in exchangeable K on Plot 3. However, the estimated K harvested by the crop is about 123 kg ha-~ (Table 5). It appears

[] K

nMg ICa liFe DAI

1988

1989

100 90 80 70 60 50 40 30 20 10 0 1988

100 90 80 70 60 50 40 30 20 10 0

1989

1990

1990

Table 7 Inventory for some of the nutrients in soils on Plot 1 from 1988 to 1990 (all values in kg ha- 1) N

Contained in the 0-20 cm soil depth in 1988 a Est. input from fine root mineralization Est. input from ashes Total (a) Contained in the 0-20 soil depth in 1989 (b) Difference by the following year ((b) - (a)) Contained in the 0-20 soil depth in 1990 (c) Difference accounted for by leaching losses and crop removal ((c)- (a))

P

Mg

K

1999 322 316

68

115

52

2

5

11

35 2086 1876

12 348 336 697 340 383

47 120 65

61 187 123

-55

-64

1790 349 383

65

100

-296

-55

-87

-210 4

13

Ca

33

-314

-314

a Bulk density: 1.34 Mg m -3 for 1988, and 1.43 Mg m -3 for 1989 and 1990.

that K was the most limiting nutrient for cassava growth at the site. It may have also leached below the 0 - 2 0 cm depth. The inventory for some nutrients in soils on Plot 1 is shown in Table 7. The recovery in S O M levels towards the end of the cultivation cycle m a y be due to the competition between main crop and weeds during the last crop cycle, when the yields were low and weeds were not removed from the plot. Weeds may play an important role in re-establishing nutrient cycling. Because they are able to continue dry matter production under declining soil moisture, they can accumulate a large percentage of available N, P and K in dry matter than other c o m m o n crops and thus, can limit losses of nutrients, as demonstrated for the cropping phase of milpa agriculture in Central America (Lambert and Arnason, 1989).

4. C o n c l u s i o n s

1989

Years

1990

Fig. 4. Effect of cultivation on cation saturation on Plot 3.

Burning the slashed vegetation produced about 11 Mg ha - l of Mr-dried ash and contributed 12, 35, 348, 47 and 61 kg h a - l o f P, N, Ca, M g and K, respectively, to the pool o f plant-available nutrients in soil. The amounts of Ca and Mg in ashes were greater than estimated crop requirements, but the

A.S.N. Lessa et al./ Agriculture, Ecosystems and Environment 59 (1996) 191-202

amounts of N and K were only 70% of these. The P content of ashes was equivalent to the estimated P uptake by the crop. The ashes were strongly alkaline, with a pH of about 10. The base cations in the ashes contributed to the increase in soil pH, sum of exchangeable cations, and base saturation after the slash and burn. Considerable amounts of fine roots were present (6 Mg ha -t ) at 0 - 2 0 cm depth and their rapid decline in Plot 1 during the early stages of the cultivation cycle suggests that fine root mineralization is a source of nutrients that is quickly used up. It also indicates that the rate of decomposition of fresh residues in the soils of semi-arid agroecosystems may be higher, particularly early in the cultivation cycle, than the rates reported to date (Sanchez, 1976). The amount of nutrients released in the soil through fine root mineralization represented an estimated 52, 2, 33, 5, and l l kg h a - t of N, P, Ca, Mg and K, respectively. Fine root mineralization contributed a considerable amount of N to the cycle of cultivation. Except for N, ashes were a much more important source of nutrients during cultivation than the mineralization of fine roots. SOM decreased upon cultivation with rates ranging from 12 to 16%, 12-18%, and 10-20% and the losses ranged from 1.6 to 4.3 Mg, 123-383 kg and 6 - 1 0 kg ha -l for C, N and Po, respectively, based on Plots 1 and 3 during the earliest years of cultivation. Organic P may have mineralized earlier than C and N. The greatest losses in organic components occurred at 0 - 5 cm depth, with indications of possible migration of soluble organic components below 5 cm depth. The addition of base cations in the ash increased the levels of exchangeable Ca, Mg and K in the soil after the burn. Soil properties such as base saturation, pH, and sum of cations followed similar trends, and later decreased as the cultivation cycle progressed. Nutrient uptake by the cassava crop and probably leaching decreased the store of available nutrients. Exchangeable A1 behaved quite dynamically during the cultivation cycle. The initial decrease in AI created, most probably, better soil conditions for microbial activity, fine root and SOM mineralization, P availability, and cassava growth. The later increase in AI followed the reduction in pH, the using up of ash-P and base cations, when

201

crop productivity was low and the plot abandoned for the natural fallow. At this stage, the weed canopy had most probably substantially counteracted losses by leaching and nutrient cycling may have been favoured. The establishment of such conditions would contribute to a more effective nutrient cycling, recreating the original soil conditions which are more appropriate to the native vegetation, as evident from the data on Plot 2, just after the crop cycle.

Acknowledgements We wish to thank Drs. I.H. Salcedo, E.V.S. Sampaio, H. Tiessen, and M.C. Santos for sampling and discussions, and the OAS and CNPq (Brazil) for providing part of the financial support for the research and preparation of this document.

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