Proceedings of 14º International Plant Nutrition Colloquiu

FÖLSTER, H.

SOMMER, R.

SÁ, T. D. A. de

VIELHAUER, K.

VLEK, P. L. G.

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W J. Horst et ai. (Eds), Plant nutrition - Food security and sustainability of agro-ecosystems. 1014-10 15. 200 J02001 Kluwer Academtc Publishers. Prtnted in lhe Netherlands.1014Water and nutrient balance under slash-and-burn agriculture in theEastern Amazon, Brasil - The role of a deep rooting fallow vegetationR. Sommer", ToD.Ao de Sá?), K Vielbauer" , P. L. G. Vlek1) and H. Fõlster"/) ZEF - Center of Development Research, University of Bonn, Walter-Flex-Str. 3. Bonn. D 53lJ3. Germany,r.sommertiiuni-bonn.deli EMBRAPA-CPATU Belém. DI". Eneas Pinheiro s/n, Belém, 66095-100. BrazilJI Institute ofSoi! Science and Forest Nutrition, University of Gôttingen. Büsgenweg 2. Gõttingen. 37075, GermanyKey words: leaching, nutrient recycling, secondary forest, shifting cultivation, subsoil nutrient accumulationAbstr8ctA 4-year-old fallow vegetation was able to noticeably deplete the soil water store down to 6 m depth. The annual uptakefrom 0.9 to 6 m soil depth reached 400 mm. An equivalent uptake of dissolved nutrients from this layer is Iikely. Thus, thefallow vegetation might purnp up nutrients, which are oui of reach for the shallow roots-system of the crops of a precedingcultívation phase, This feature, however, limits the scope ofprolonged cropping and any agriculture that reduces the vitalityof the fallow vegetation,IntroduetíonIn the- Bragantina region of the Eastern Amazon,shifting cultivation predominates, which is mostlypracticed by smallholders with properties smaller than ahundred hectare. After land preparation by slash-and-burn,maize tZea mays L.), beans (Vigna unguiculata L.) andcassava (Manihot esculanta CRANTZ) are sequentiallycultivated for a period of 1.5 to 2 years. In the subsequentfallow period li! woody secondary vegetation can re-establish. Nowadays, however, shifting cultivation is underchange, Demographic pressure on land leads to anintensified cultivatíon with shortened fallow periods (3-7years) and a move to semí-permanent crops Iike passionfruit (Passiflora edulis). This development endangers theessentiaJ function of the fallow period to restore soilfertility. As a long-term consequence soiJ fertilitydeteriorates and land degrades creating a need forencroaching 00 primary forest areas.Recent studies ofthe deeper subsoil using pits or augerrevealed a deep-extending root system for primary forestsin South-Pará (Nepstad et al., 1991) as well as for thefallow vegetation in the Bragantina region (Somrner et ai.,2000). In the south of Pará, Nepstad et ai. (1994) showedthat deep roots are necessary for a sufficient water supplyof the evergreen forest in the dry season. This might alsobe the case for the evergreen secondary vegetation in theBragantina region. To examine this assumption, wemonitored the deep soil water depletion under a fallowvegetation during lhe dry season in 1997.The feature of lhe fallow vegetatíon to establish a deeproot system might also be irnportant regarding the nutrientdynamics of shifting cultivation, If the root system wasimportant for deep-soil water uptake, an equivalemnutrient uptake might be likely. This would substantiallyinfluence the nutrient efficiency of this land-use systemand diminish leaching lesses. Therefore, leaching wasquantified under li slash-burned site during a cultivationphase,Materiais and methodsTo study the cultivation-induced leaching, a 7-year-oldfallow vegetation was selected. This was slashed andburned in December 1996 and a sequence of maize,cowpea (Vigna unguiculata) and cassava was planted inthe following 1.5 years. Concentrations of dissolvednutrients were determined in samples of soil solution takenbiweekly at 0.9 m, 1.8 m and 3 m soi! depth using suction-cup lysimeters (each time 6 repetitions). The nutrientconcentrations were combined with water fluxes derivedfrom a soil-water model applied on the cultívation site.To determine deep soil water uptake, the annualdynamics of the soil water pressure head at different depthin the soil profile were recorded with tensiorneters under a4-year-old faJlow vegetation starting in early April 1997.Based on this data set the soil water movement wasmodelled using iaboratory soil-water retention curves,pedo-transfer functions and the soil water model Hydrus-ID (Van Genuchten, 1987).Results-Due to EI Nino effects the dry season of 1997 wasexceptionaJl, pronounced, with only 148 mm rainfallbetween 281 of August and the onset of the rainy seasonon 8th January i998. In 1997 total annual precipitation was2104mm and in 1998 2545mm. According to results ofthesoil water model, the transpiration ofthe fallow vegetationamounted to 154 mm within the dry season (Table 1). Asmuch as 74 % of this transpiration water was extractedfrom the soil profile below 0.9 m depth. For the wholeyear} 400 mm and 427 mm of water originated from thesoil layer of 0.9 to 6> m depth in 1997 and 1998,respectively. Maize, cowpea and cassava with theirshallower root system did not deplete the soil water storagebelow 3 m depth and water from 0.9 to 3 m contributedless than 10% to the annual water uptak.e.Table i. Soi! water extraction under a 4-year old fallowvegetation from 22/8/1997 to 8/111998 and for the years1997 and 1998 considering different soil layers,Percentages refer to the extraction from O to 6 mSoil depth 22/8/97-8/1/98 -- 1997 --- --- 1998 ---(rn) (%) (mm) (%) (mm) (%)0- 0.9 m 40 26 730 65 853 670.9 - 3 m 41 27 199 18 223 183 - 6 m 73 47 202 18 204 !6................................... _ '.- _ _ _ _ .Sum 154 1131 1280Considerable amounts of nutrients were leached belowthe rooting zone of these crops. At 0.9 m depth, theseamounted to 77kg nitrate-N, i.3 kg P, 25 kg K, 149 kg Ca,34 kg Mg and 7 kg S per hectare over the two-yearcropping period, However, during further percolation,nitrate, K, Ca and Mg were strongly reduced, so that lossesat 3 m decreased to about 9 kg nitrate-N, 13 kg K, 45 kgCa and 10 kg Mg per hectare over the two years (Table 2).Table 2. Amounts of leached nutrients during the croppingperiod 1997-1998 considering reference soil depths ofO.9,1.8 and 3 mSoil depth Nitrate-N K Ca Mg-------------- [kg ha" 2a·l] -----------------O.9mIJ!m3m77 25 149 3428 5 87 219 13 45 10The decrease with depth in the amounts of dissolvednutrient was apparently related to the cation and anionexchange capacity, The effectivecation exchange capacity(ECEC) ofthe soilat 0.9 m depth was around 1 cmol.kg"and decreased to around 05 cmol.kg" at 3 m. Anurugsa(1998) studied the anion exchange capacity (AEC) of soilof the study region at 30-50 em depth. According to hissequential batch experiments AEC reached 0.08 cmol.kg"aí a soil-pH adjusted to 6.5 which increased to 0.4cmol.kg" at lipH of3J.Even if the ion exchange capacityof the deeper soil is rather low, the thickness of'theprofileallows that large amounts of exchangeable ions can be. retained.1015DiseussíonAbout two-third ofthe annually transpired water of'thefallow vegetation was taken up from the upper 0.9 m soil-layer. StiI! deep soil layers significantly contributed to thewater supply of the vegetation. This might help to explainwhy most of the fallow species are able to maintain anevergreen canopy during the dry season.The deeper soi! layers, however, appeared to be out ofthe reach for the roots of maize, cowpea, cassava andnewly introduced cash crops like passion fruit. Assumingan equivalent uptake of dissolved nutrients during waterextraction, a nutrient-pumping mechanism of the deep-rooting fallow vegetation seems Iikely. The retention ofnutrients beyond the time of cropping, therefore, providesthe opportunity for the regrowing fallow vegetation to takeup and to recycle these nutrients. It is likely that due to adecreasing ionic strength ofthe percolating soil water afterfallowing of the crop land, retained ions are released andmight be exported ifnot recycled by the fallow vegetation.OUI" results on the water balance of the 4-year-oldfallow vegetation show that even young secondaryvegetation is able to extract deep soil water, acharacteristic, so far only known for primary forest. Ifproperly integrated into a shifting cultivation system suchvegetation therefore provides a safety net against nutrientleaching. Intensified cultivation by semi-permanent orpermanent crops suppresses the rootstock of the fallowvegetation and, therefore delays or even prevents its re-establishment, Hence, the nutrient efficiency of thesesystems is strongly reduced in the long run endangeringagricultura! productivity and ecosystem functioning as awhole,ReferencesAnurugsa B 1998 101. Ph.D. Thesis. University ofGõttingen.Nepstad DC et ai. 1994 Nature 372,666-669.Nepstad De, Uhl C and Serrão EAS 1991 Ambio 20,248-255_Sommer R, Denich M and Vlek PLG 2000 Plant Soil 219,231-241,Van Genuchten MT 1981 Research Report No. 121. U.S.Salinity Laboratory, USDA, ARS, Riverside. CA.

Water and nutrient balance under slash-and-burn agriculture in the eastern Amazon, Brasil - the role of a deep rooting fallow vegetation.

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Water and nutrient balance under slash-and-burn agriculture in the eastern Amazon, Brasil - the role of a deep rooting fallow vegetation.

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