Agroforestry Systems 35: 255-268, 1997. © 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Cardamom, mandarin and nitrogen-fixing trees in agroforestry systems in India's Himalayan region. II. Soil nutrient dynamics

R. S H A R M A 1, E. SHARMA l and A. N. PUROHIT 2 i G.B. Pant Institute of Himalayan Environment and Development, Sikkim Unit, P.O. Tadong, Sikkim 737 102, India; 2 High Altitude Plant Physiology Research Centre, H.N.B. Garhwal University, Srinagar (Garhwal), U.P.-246 174, India

Key words: land use transformation, N-mineralization, nutrient dynamics, P-fractions, seasonal variations

Abstract. A study on seasonal soil nutrient dynamics was made in large cardamom (Amonum subulatum) and mandarin (Citrus reticulata) agroforestry systems in the Sikkim Himalaya, India. Alnus nepalensis was the N2-fixing associate in the large cardamom system, and Albizia stipu- lata in the mandarin agroforestry system. Sites without N2-fixing species in both agroforestry types comprised native non-symbiotic mixed tree species. Soil was acidic in the cardamom agro- forestry and slightly acidic to neutral in the mandarin agroforestry system. Total-N in soils was the highest in the forest-cardamom stand and the lowest in the mandarin-based agroforestry systems. Soil ammonium-N and nitrate-N concentrations were highly seasonal, and the ratio of seasonal maximum and minimum varied up to six times. The C/N ratio was higher in cardamom agroforestry indicating lower N availability than in the mandarin agroforestry. Cardamom stand with Alnus showed a relatively narrower C/N ratio. N2-fixing species help in maintenance of soil organic matter levels with higher N-mineralization rate as land use change from natural- forest system to agroforestry systems with sparse tree populations. Ratios of inorganic-P/total- P were lower in cardamom agroforestry than the mandarin agroforestry. Seasonal fluctuation in Ca-PO4, AI-PO4 and Fe-PO4 contents regulated the availability of phosphates to some extent for plant uptake.

Introduct ion

Most of the natural undisturbed ecosystems have synchrony between nutrient release and plant uptake. But the majority of land management systems such as agroforestry systems suffer from disruption of such synchrony. Mineralization of decomposing residues is a major source of plant nutrients in highly weathered soils with little inherent mineral fertility (Brown et al., 1994; Sanchez et al., 1989).

Details on large cardamom and mandarin plants were presented in paper I (Sharma et al., 1997). The selected stands in this study present the scenario of gradual agroforestry land use transformation in the region (Sharma et al., 1994, 1995). The synchronization of nutrient release and plant uptake in man- agement is a difficult task but can be achieved if the site specific background information on soil nutrient dynamics are available. This study envisages to provide detailed information on soil nutrient levels and seasonal dynamics

256

particularly of different forms of nitrogen and phosphorus in four types of land use transformation representative agroforestry systems in temperate and subtropical zones of the Sikkim Himalaya, India.

Methods

Location and climate of the study sites and descriptions on agroforestry stands were presented by Sharma et al. (1994, 1995). Soil nutrient dynamics and standing states were studied in the marked sample areas of each of Alnus-cardamom and forest-cardamom in temperate, and Albizia-mandarin and mandarin agroforestry systems in subtropical zone. Soil samples of 0-15 and 15-30 cm depths from 0.5, 1.0 and 1.5 m from the base of three trees in each stand were collected in replicates (n -- 3) in autumn, winter, spring and rainy seasons. The data are presented together for temperate and subtropical agroforestry sites to show the impact of land-use transformation from near natural forest situation to intensively practiced agriculture, and thus should not be considered as a mere comparison between two contrasting agroforestry systems and sites.

Soil analyses were conducted as follows: soil moisture and pH in fresh samples; organic carbon by a modified Walkley-Black method (Anderson and Ingram, 1989); total nitrogen by a modified Kjeldahl method (Anderson and Ingram, 1989); ammonium-nitrogen by KC1 extraction and using modified Kjeldahl method (Allen, 1989); nitrate-nitrogen by phenoldisulphonic acid method in water extract (Allen, 1989); nitrogen mineralization by aerobic incu- bation (Andersen and Ingram, 1989); total phosphorus using hydrogen peroxide oxidized acidified ammonium fluoride extract by chlorostannous reduced molybdophosphoric blue colour method (Jackson, 1967); inorganic phosphorus using dilute HC1 and ammonium fluoride extract by chlorostan- nuous reduced molybdophosphoric blue colour method (Jackson, 1967); avail- able phosphorus using sodium bicarbonate extract by colorimetric method (Anderson and Ingrain, 1989); aluminium-phosphate, iron-phosphate, calcium- phosphate and occluded iron-phosphate by P-fractionation method (Jackson, 1967).

Per cent stone in soil from all the four stands of agroforestry systems was determined. Organic carbon, total nitrogen and total phosphorus in different seasons in each horizon (0-15 and 15-30 cm) of soil were estimated from bulk density, soil volume, and nutrient concentration values. The amounts of nutrients estimated in both horizons were summed to obtain total content down to 30 cm depth.

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Results

Moisture

Soil moisture in the different agroforestry systems varied significantly between stands and seasons (Table 1). Overall, soil moisture was higher in forest-cardamom stand than in Alnus-cardamom stand in all the seasons. Soil moisture in both the Alnus-cardamom and forest-cardamom agroforestry systems showed near saturated to saturated levels in all seasons. Its levels in Albizia-mandarin and mandarin stands remained above field capacity level in winter and rainy seasons, while it decreased below the field capacity level in autumn and spring.

pH

The soil in the different agroforestry systems varied significantly between stands, seasons and depths. Overall, the pH value was the lowest in the forest-cardamom and the highest in the mandarin stand. In the Alnus- cardamom stand, the lowest pH values were recorded in spring and rainy seasons, and the highest in autumn (Table 1). In the forest-cardamom the lowest pH was also observed in rainy season. The pH in mandarin-based agro-

Table 1. Seasonal variation in soil moisture and soil pH of cardamom and mandarin agroforestry systems in Sikkim, India.

Parameter Agroforestry Soil depth Spring Rainy Autumn Winter systems (cm)

Soil moisture (%)

pH

Forest-cardamom 0-30 31.25 34.67 38.28 38.02 Alnus-cardamom 0-30 27.45 30.88 27.86 28.95 Albizia-mandarin 0-30 13.12 23.23 13.47 22.43 Mandarin 0-30 15.57 24.97 9.73 20.45

Forest--cardamom 0-15 5.37 4.78 5.17 4.83 15-30 5.18 4.69 5.28 4.96

Alnus-cardamom 0-15 4.93 5.01 5.75 5.60 15-30 5.07 4.94 5.87 5.65

Albizia-mandarin 0-15 5.55 6.22 6.37 5.96 15-30 5.54 6.65 6.46 5.95

Mandarin 0-15 6.93 5.68 6.85 6.01 15-30 7.08 5.89 6.92 6.37

Values are pooled for soil samples collected at different distances from tree base (pH n = 9) and also top 30 cm soil depth (moisture n = 18). ANOVA: soil moisture - stand F3. 62 m 685, P < 0.005; s e a s o n F3, 62 ~ 1 0 5 , P < 0.005; stand × s e a s o n F9, 62 = 3 0 , P < 0.005; depth and other interactions are not significant; LSD (0.05) = 0.82; soil pH - stand F3 ,62 = 578, P < 0.005; s e a s o n F3,62 = 86, P < 0.005; depth F j . 6 2 = 10.65, P < 0.005; stand x s e a s o n F9, 62 = 5 7 , P < 0 . 0 0 5 ; other interactions are not significant; LSD (0.05) = 0.07.

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forestry was slightly acidic and towards neutral. In the Alb&ia-mandarin stand the lowest pH values were obtained in the spring season, and the highest in the rainy and autumn seasons. On the contrary, in the mandarin stand the lowest pH values were recorded in the rainy season and the highest in the spring and autumn seasons. The soil in the cardamom-based agroforestry was more acidic, while it was slightly acidic to neutral in the mandarin-based agroforestry system.

Organic carbon

Soil organic-C in the different agroforestry systems varied significantly between stands, seasons and depths (Figure 1). Organic-C was lower at 15-30 cm depth compared with 0-15 cm for all seasons and stands. The highest values were recorded in the forest-cardamom stand followed by the Alnus- cardamom stand, and lower values in both the Albizia-mandarin and mandarin stands. Pooled soil organic-C was always higher in both depths at all seasons in the forest-cardamom stand compared with the Alnus-cardamom stand. In the case of the mandarin agroforestry system it was higher in both depths during all seasons except spring in the Albizia-mandarin stand compared with the mandarin stand. Soil organic-C contents up to 30 cm depth in different seasons ranged from 67-87 Mg/ha in the Alnus-cardamom, 78-121 Mg/ha in the forest-cardamom, 24-38 Mg/ha in the Albizia-mandarin and 24-45 Mg/ha in the mandarin stand (Table 2).

Nitrogen

Total nitrogen. Soil total-N in the different agroforestry systems varied sig- nificantly between stands, seasons and depths (Figure 1). It was always higher at 0-15 cm compared with 15-30 cm depth in all seasons and stands (Figure 1). Highest total-N was recorded in the forest-cardamom stand followed by Alnus-cardamom, Albizia-mandarin and lowest in the mandarin stand. The C/N ratio was narrower in the mandarin- than in cardamom-based agroforestry systems. The C/N ratio in the forest-cardamom and mandarin stands having non-Nz-fixing trees was lower at 0-15 cm depth compared with 15-30 cm depth in autumn (Table 4). Total-N content in the soil up to 30 cm depth in different seasons ranged from 6-8 Mg/ha in the Alnus-cardamom, 7-11 Mg/ha in the forest-cardamom, 2.9-5.2 Mg/ha in the Albizia-mandarin and 2.9-4.6 Mg/ha in the mandarin stand (Table 2).

Ammonium-N. Soil ammonium-N in the different agroforestry systems varied significantly between stands and seasons (Table 3). The variation between the depths was not significant. In different depths and seasons, it ranged from 0.67-2.93 rag/100 g soil in the Alnus-cardamom stand, 0.73-4.27 mg/100 g soil in the forest-cardamom stand, 0.53-4.80 mg/100 g soil in the Albizia- mandarin stand and 0.68-4.93 mg/100 g soil in the mandarin stand.

259

(A)

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(B)

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ly

0,8 0.7 0,6 0.5 0,4 0.~ 0.i 0.

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M 15.s0 A u t u m n

Figure 1. Seasonal var iat ion in soi l organic carbon and soi l total n i t rogen in cardamom and mandar in agrofores t ry sys t ems in S ikk im , India. FC = f o r e s t - c a r d a m o m ; AC = Alnus-cardamom; A M = Albizia-mandarin; M = mandar in . (A) o rgan i c -C (%) and (B) to ta l -N (%) at two dep ths

( 0 - 1 5 and 15 -30 cm), Va lues are poo led for soi l s amp le s co l l ec t ed at d i f fe ren t d i s t ances f rom

tree base (n = 9). ANOVA: Soil o rgan ic ca rbon - s tand F3.62 = 714, P < 0 .005; season F3, 62 =

36, P < 0.005; dep th Fi,62 = 228, P < 0 .005; s tand × season F9,62 = 10.67, P < 0 .005; s tand × dep th F3.62 = 31, P < 0 .005; s ea son × dep th F3,62 = 4.63, P < 0.01; s tand x s ea son × dep th

F9. 62 = 2.33, P < 0.05; L S D (0.05) = 0.20: soi l total n i t rogen - s tand F3.62 = 403, P < 0.005;

s ea son F3, 62 = 79, P < 0 .005; dep th F~, 62 = 192, P < 0 .005 ; s t and x s ea son Fg. 62 = 6.6,

P < 0 .005; s tand x dep th F3.62 = 23, P < 0 .005; s eason × dep th F3.62 = 18, P < 0.005; s tand ×

season x depth F9,62 = 2.72, P < 0.01; L S D (0.05) = 0.02.

260

Table 2. Seasonal variation in soil organic carbon, total nitrogen and total phosphorus in different agroforestry systems in Sikkim, India.

Nutrients (Mg/ha in 30 cm depth)

Seasons Agroforestry systems

Forest- Alnus- Albizia- Mandarin cardamom cardamom mandarin

Organic-C Spring 120.72 86,38 34.15 45.13 Rainy 78.14 79.77 23.94 25.23 Autumn 105.80 86,79 37.77 23.85 Winter 106.38 66.67 27.75 24.86

Total-N Spring 10.03 7.95 4.45 4.51 Rainy 7.25 7.84 3.43 2.86 Autumn 10.62 8.15 5.20 4.61 Winter 7.59 5.88 2.88 2.86

Total-P Spring 1.739 1.752 1.825 3.544 Rainy 1.514 1.129 1.201 1.026 Autumn 1.825 1.941 1.971 1.924 Winter 1.149 1.278 1.717 1.504

Table 3. Seasonal variation in soil ammonium-nitrogen and nitrate-nitrogen of cardamom and mandarin agroforestry systems in Sikkim, India.

Soil nutrients Agroforestry systems Spring Rainy Winter

Ammonium-N Forest-cardamom 2.58 0.89 2.75 (rag/100 g soil) Alnus-cardamom 2.32 0.75 1.38

Albizia-mandarin 1.85 0.71 3.70 Mandarin 3.41 0.76 3.41

Nitrate-N Forest-cardamom 0.61 1.57 1.93 (mg/100 g soil) Alnus-cardamom 0.50 1.26 1.34

Albizia-mandarin 0.99 1.50 1.98 Mandarin 3.92 1.58 1.52

Values are pooled for soil samples collected at different distances from tree base and top 30 cm soil depth (n = 18). ANOVA: soil ammonium-nitrogen - stand F3, 46 = 8.33, P < 0.005; season F:, 46 = 73.53, P < 0.005; stand x season F6. 46 ~ 7.23, P < 0.005; depth and other interactions are not significant; LSD (0.05) = 0.36; soil nitrate-nitrogen - stand F3, 46 ~ 39.87, P < 0.005; stand x s e a s o n F6. 46 = 39.52, P < 0.005; season, depth and other interactions are not significant; LSD (0.05) = 0.22.

A m m o n i u m - N w a s the l o w e s t in the r a i n y s e a s o n in a l l the a g r o f o r e s t r y

sy s t ems . It was the h i g h e s t in w i n t e r in al l a g r o f o r e s t r y s tands e x c e p t fo r the

Alnus -cardamom s tand that s h o w e d the h i g h e s t v a l u e s in spr ing .

Nitrate-N. Soi l n i t r a t e - N in the d i f f e r e n t a g r o f o r e s t r y s y s t e m s v a r i e d s ign i f i -

c an t ly b e t w e e n s tands o n l y (Tab le 3). T h e soi l n i t r a t e - N in d i f f e r e n t dep ths

and s e a s o n s r a n g e d f r o m 0 . 3 6 - 1 . 6 1 m g / 1 0 0 g so i l in the A l n u s - c a r d a m o m

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stand, 0.55-3.13 mg/100 g soil in the forest-cardamom stand, 0.70-2.56 mg/100 g soil in the Albizia-mandarin stand and 1.18-4.35 mg/100 g soil in the mandarin stand.

Mineralization. N-mineralization under in situ moisture levels when incubated at 26 + 2 °C revealed the highest rate in the forest-cardamom (54 p,g N/g soil/14 days) stand followed by the Alnus-cardamom stand (35 ILtg N/g soil/14 days), Albizia-mandarin (19 ~tg N/g soil/14 days) and the lowest in the mandarin stand (8 ~tg N/g soil/14 days). The data on N-mineralization rates of all the sites were pooled, and regression analysis was done to see the relationships of soil moisture and soil organic matter on mineralization rates. Strong positive relationships of both soil moisture and organic matter against N-mineralization were observed. Higher moisture and higher organic matter in the soil resulted into greater rates of N-mineralization (Figure 2).

80 f 70

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~ , ~ . 50

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I I I ! I I 10 20 30 40 50 60

80

70

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10

S o i l M o i s t u r e (%)

, / I O , ~ ~ ' i I I i I I 1 2 3 4 5 6 7 8

Soil O r g a n i c M a t t e r (%)

Figure 2. Relationships of soil moisture and soil organic matter with nitrogen mineralization rates. Values of all the agroforestry types are pooled. (N-mineralization = - 1 . 3 1 2 + 1.91 soil moisture, r = 0 .937 , P < 0 .01: N - m i n e r a l i z a t i o n = 0 .076 + 9.25 soil organic matter, r = 0 .844 ,

P < 0.01) .

262 (A)

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180 150 120

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210

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125 100

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9 8 9

8 7 8 7

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Figure 3. Seasonal variation in soil total phosphorus, inorganic phosphorus and available phos- phorus in cardamom and mandarin agroforestry systems in Sikkim, India. FC = forest- cardamom; AC = Alnus--cardamom; AM = Albizia-mandarin; M = mandarin. (A) total-P, (B) inorganic-P and (C) available-P at two depths (0-15 and 15-30 cm). Values (in mg/100 g soil) are pooled for soil samples collected at different distances from tree base (n = 9). ANOVA: soil total phosphorus - stand F 3 , 6 2 = 51, P < 0.005; s e a s o n F 3 , 6 2 = 453, P < 0,005; depth FL62 = 56, P < 0.005; stand x season F9, 62 = 44, P < 0.005; stand x depth F3.62 = 3.69, P < 0.025; season x depth F3, 62 = 3.34, P < 0.025; stand x season × depth F9. s2 = 1.09, not significant; LSD (0.05) = 4.9: soil inorganic phosphorus - stand F3.62 = 346, P < 0.005; season F3.62 = 106, P < 0.005; depth Fi.62 = 14, P < 0.005; stand x season F9.62 = 97, P < 0.005; stand x depth F3.6~ = 5.25, P < 0.005; season x depth F3.62 = 3.42, P < 0.025; stand x season x depth F9.62 = 1.82, not significant; LSD (0.05) = 4.3: soil available phosphorus - stand F3.46 = 195, P < 0.005; season F2,4~ = 171, P < 0.005; depth FL, ~ = 5.93, P < 0.025; stand x s e a s o n F 6 , 4 6 = 114, P < 0.005; other interactions are not significant; LSD (0.05) = 0.27.

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Phosphorus

Total phosphorus. Soil total-P in the different agroforestry systems varied significantly between stands, seasons and depths (Figure 3). The soil total-P in different depths and seasons ranged f rom 46-112 mg/100 g soil in the A lnus-cardamom stand, 43-145 mg/100 g soil in the fores t -cardamom stand, 52-174 mg/100 g soil in the Albizia-mandarin stand and 53-208 mg/100 g soil in the mandarin stand. Highest values of total-P were recorded in autumn in all agroforestry stands except the mandarin stand where it was the highest in spring. The total-P was higher in the mandarin-based agroforestry systems compared with the cardamom-based agroforestry systems (Figure 3).

Inorganic phosphorus. Soil inorganic-P in the different agroforestry systems varied significantly between stands, seasons and depths (Figure 3). Soil inorganic-P was recorded to be the highest in the mandarin stand followed by the Albizia-mandarin and much lower in the Alnus-cardamom and fores t - cardamom stands (Figure 3). Ratio of inorganic-P to total-P in the cardamom agroforestry was relatively lower than in the mandarin agroforestry (Table 4). Mostly the ratio was higher in lower soil depth (15-30 cm) in the cardamom agroforestry. In the Albizia-mandarin stand the ratio was higher at 15-30 cm depth compared with 0-15 cm in the autumn, spring and rainy

Table 4. Seasonal variation in ratio of organic carbon and total nitrogen, and inorganic phosphorus and total phosphorus in soil samples from different agroforestry systems in Sikkim, India.

Agroforestry systems Soil depth Seasons (cm)

Spring Rainy Autumn Winter

Organic-C : total-N Forest-cardamom 0-15 11.58 11.60 9.89 14.22

15-30 12.71 9.79 10.09 13.71 Alnus-cardamom 0-15 11.20 11.15 10.76 11.67

15-30 10.50 9.08 10.47 10.96 Albizia-mandarin 0-15 7.92 8.52 8.72 10.19

15-30 7.39 4.43 5.33 9.07 Mandarin 0-15 10.08 9.39 4.94 9.06

15-30 9.91 7.92 5.65 8.29

Inorganic-P : total-P Forest-cardamom 0-15 0.243 0.381 0.093 0.221

15-30 0.172 0.501 0.137 0.365 Alnus-cardamom 0-15 0.223 0.204 0.176 0.295

15-30 0.180 0.316 0.192 0.295 Albizia-mandarin 0-15 0.432 0.338 0.376 0.610

15-30 0.345 0.391 0.494 0.485 Mandarin 0-15 0.752 0.543 0.711 0.485

15-30 0.620 0.442 0.692 0.389

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seasons. In the mandarin stand the ratio was higher in the upper soil horizon in all seasons (Table 4).

Available phosphorus. Soil available-P in the different agroforestry systems varied significantly between stands, seasons and depths (Figure 3). In different seasons and depths, it ranged from 0.51-1.61 mg/100 g soil in the Alnus- cardamom stand, 0.41-1.41 mg/100 g soil in the forest-cardamom stand, 1.06-4.59 mg/100 g soil in the Albizia-mandarin stand, and 1.07-9.27 mg/100 g soil in the mandarin stand. The available-P was the highest in the mandarin stand followed by Albizia-mandarin, Alnus-cardamom and the lowest in the forest-cardamom stand (Figure 3). Pooled available-P for the rainy season for both depths showed that it was higher by 1.14 times in the Alnus-cardamom stand compared with the forest-cardamom stand, and higher by 2.37 times in the mandarin stand compared with the Albizia-mandarin stand.

Fractionated phosphorus. P-fractions such as Ca-PO4, A1-PO4, Fe-PO4 and occluded F e - P O 4 w e r e estimated (Table 5). Pooled values in each season are presented as there were no significant differences between depths. Significant

Table 5. Seasonal variation in fractionated phosphorus (mg/100 g soil) in soil samples from different agroforestry systems in Sikkim, India.

Agroforestry systems Seasons Calcium Aluminium Iron Occluded iron phosphate phosphate phosphate phosphate

Forest-cardamom Spring 8.4 5.6 9.4 465 Rainy 11.3 6.9 13.1 365 Winter 6.7 9.7 12.3 370

Alnus-cardamom Spring 9.8 6.1 10.7 478 Rainy 10.3 5.0 8.0 383 Winter 8.8 9.3 14.5 348

Albizia-mandarin Spring 14.8 6.6 9.4 477 Rainy 38.0 17.9 18.8 238 Winter 40.0 17.3 19.8 313

Mandarin Spring 18.2 7.7 15.3 504 Rainy 48.4 28.4 23.4 208 Winter 42.1 21.6 22.9 324

ANOVA P-values a Stand 0.005 0.005 0.005 0.005 Season 0.005 0.005 0.005 0.005 Stand x season 0.005 0.005 0.005 0.005 LSD (0.05) 3.9 1.6 1.6 35.2

a Beneath each column, P values associated with an analysis of variance (ANOVA) are given, with LSD values (P = 0.05) as the interaction is significant. Actual estimates were made at two soil depths (0-15 and 15-30 cm) and three distances from the base of the trees (0.5, 1.0 and 1.5 m), but differences due to these factors are not significant and hence presented pooled values.

265

variations between stands, seasons, and stand and season interaction were recorded in all the fractionated forms (Table 5). Occluded Fe-PO4 was very high in both the Alnus-cardamom and forest-cardamom stands. In these stands Fe-PO 4 was more than Ca-PO4, and A1-PO4 was the lowest (Table 5). In the mandarin-based agroforestry occluded Fe-PO 4 was also very high. Ca-PO 4 was much higher than A1-PO 4 and Fe-PO4 in the mandarin-based agroforestry stands (Table 5). Ca-PO4 was significantly higher in the mandarin stand than in the Albizia-mandarin stand. Its variations between the cardamom- and mandarin-based stands were highly significant. Interestingly, comparison of occluded F e - P O 4 between stands of each cardamom and mandarin agroforestry type in a season was not statistically significantly; however, if comparisons between all four stands are made, then statistically significant differences were recorded in some of the seasons and stands (Table 5).

Discussion and conclusion

Soil is highly acidic in the cardamom agroforestry system. In the mandarin- based agroforestry it is slightly acidic to neutral. Mixed stands with N2-fixing trees commonly have lower pH than non-N2-fixing stands (Binkley and Sollins, 1990; Franklin et al., 1968). In the present study, the effect of N2- fixing Alnus for lowering soil pH was difficult to assess as it was compared with the forest-cardamom stand having an old mixture of natural trees whose soil was highly acidic. However, in the mandarin agroforestry a lower pH was observed in some of the seasons in the stand with N2-fixing Albizia.

The cardamom-based agroforestry systems mainly depend on recycling of the litter and cardamom residue for inorganic nutrients, whereas the mandarin- based agroforestry system depends on litter, crop residue and external inputs of organic nutrients. Soil organic carbon/matter levels are good indicators of soil fertility status (Tiessen et al., 1994). The sequence of land management transformation from forest-cardamom (comprising mixed natural trees) to Alnus-cardamom (having planted Alnus trees) and to mandarin-based agro- forestry systems have apparently resulted in the reduction in soil organic carbon levels.

Binkley and Vitousek (1989) have elaborately described N-availability in different ecosystem types. The total-N in the forest-cardamom stand was the highest and decreased in the Alnus-cardamom stand, and much lower values were recorded in the mandarin-based agroforestry systems. The high values of total-N in the forest-cardamom stand have resulted due to accumulation over the years with lower fluxes. The soil total-N showed annual fluctua- tions that seem very small relative to the total pool. Therefore, to see more of the seasonal dynamics the inorganic-N in the form of ammonium-N and nitrate-N were estimated, although they were small in quantity relative to the annual fluxes. Ammonium-N concentration was highly seasonal, showing very low values in the rainy season in all agroforestry stands. The smaller values

266

of ammonium-N in the rainy season are indicative of greater plant uptake in spite of high mineralization rates in the season. Nitrate-N also showed seasonality with variation up to six-fold in the ratio of maximum to minimum concentration. Low nitrate-N was recorded in the spring in the forest- cardamom stand and in Alnus-cardamom and Albizia-mandarin stands with N2-fixing species. The higher C/N ratios in the cardamom agroforestry indicate lower N-availability in these stands than the mandarin agroforestry. In the cardamom agroforestry, the Alnus-cardamom stand showed a relatively lower C/N ratio indicating higher N-availability than the forest-cardamom stand.

The organic carbon/matter and moisture were higher in the cardamom-based agroforestry than the mandarin-based agroforestry. The organic matter levels decreased from the forest-cardamom stand to the Alnus-cardamom stand, Albizia-mandarin and mandarin stands; consequently, the rate of N-mineral- ization from the forest-cardamom stand decreased by seven-fold in the mandarin stand. The presence of N2-fixing species in cardamom and mandarin while changing land-use from the natural forest tree mixture to sparsely tree- based agroforestry with traditional crops have helped in maintained the soil organic levels and relatively higher N-mineralization rate, which show poten- tial of sustenance with these species as associates.

In the soil total-P of all the agroforestry stands, most of the phosphorus was in the organic form except in spring in the mandarin stand that showed higher inorganic-P mainly because of intensive cultivation activity in this stand. Ratios of inorganic-P/total-P were lower in cardamom-based agro- forestry than mandarin-based agroforestry, indicating relatively higher inor- ganic-P available for plant uptake in the latter. Availability of phosphorus is highly pH dependent, and it was quite evident in this study. A shift of one pH unit may change phosphorus solubility by 10-fold (Binkley and Vitousek, 1989). Lower pH values in all the seasons in the cardamom agroforestry could have resulted in low levels of available-P in soil. In contrast, the mandarin agroforestry system seemed to have slightly acidic to neutral pH, consequently showing higher available-P especially in the rainy season. In spite of seasonal fluctuation, considerable amounts of phosphates are not available to plants as they are fixed in the occluded iron-phosphate form in all the stands, quite characteristic of soils in the Himalayan region. However, certain amounts of phosphates are fixed by calcium, aluminium and iron, and their seasonal fluctuations in contents and exchange to soluble form are expected to regulate the availability of phosphates for plant uptake. Higher C a - P O 4 and A1-PO 4 in the mandarin than cardamom agroforestry stands explain the higher available- P in the former.

The land use transformation from forest to agriculture has been a common phenomenon in the last few decades in the eastern Himalaya. Agricultural land use has increased by about 13% in the present study sites of the Mamlay watershed during the last four decades (Sharma et al., 1992). The land-use transformation from forests to agriculture provides a series of agroforestry situations from natural mixed tree species in the forest-cardamom to planta-

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tion of N2-fixing trees in Alnus-cardamom to more of horticulture with mixed tree species including N2-fixing Albizia in Albizia-mandarin to horticulture with mixed tree species in the mandarin stand. Such land-use transformation is expected to cause changes in soil nutrient levels and fertility, irrespective of systems prevailing either in temperate or subtropical conditions. N2-fixing species are used in the management of such agroforestry systems, and this study has evaluated their role in the maintenance of soil fertility. Soil moisture and acidity decreased from the near forest situation in the forest-cardamom to Alnus-cardamom, and least in the Albizia-mandarin and mandarin agro- forestry stands. The organic matter levels also decreased from forest- cardamom to Alnus-cardamom, Albizia-mandarin and mandarin stands, and consequently rates of N-mineralization decreased. The presence of N2-fixing species in Alnus-cardamom and Albizia-mandarin have helped in maintaining the soil organic levels, and the relatively higher N-mineralization rate shows the potential of sustenance with these N2-fixing species as associates. Soil total-N was high in the near forest situation and decreased with intensifica- tion of agriculture; however, N-availability for plants was more with N2-fixers especially in the Alnus stand. Available-P was higher in the Alnus-cardamom stand than the forest-cardamom, denoting more P-availability for plant growth in the presence of N2-fixing Alnus. Soil moisture, pH, organic carbon, total- N, ammonium-N, nitrate-N, total-P, inorganic-P, available-P and fractionated- P showed high seasonal variability in different agroforestry stands. Seasonal fluctuations in phosphates fixed by calcium, iron and aluminium regulate to some extent the phosphorus availability for plant uptake.

Acknowledgements

G.B. Pant Institute of Himalayan Environment and Development provided the necessary facilities. This work was a part of the project funded by the Department of Science and Technology (New Delhi) in Tropical Soil Biology and Fertility (TSBF) - coordinated programme initiated by South Asian Regional Network, New Delhi. Dr R.C. Sundriyal, Dr S.C. Rai, Dr Y.K. Rai, Ms Manju Sundriyal, Mr L.K. Rai, Ms Sabita Krishna and Mr J. Dhakal helped during the field experimentation, laboratory analyses and in the preparation of the manuscript.

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