Plant and Soil 252: 195–205, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Organic matter transformations and soil fertility in a treed pasture insemiarid NE Brazil

H. Tiessen1,4, R.S.C. Menezes2, I.H. Salcedo2 & B. Wick3

1Institute for Crop & Animal Production in the Tropics Universität Göttingen, Grisebachstr. 6, 37077 Göttingen,Germany. 2Department of Nuclear Energy, Umversidade Federal de Pernambuco, Recife, Brazil. 3Institute forTropical Forestry, Universität Göttingen, Germany. 4Corresponding author∗

Key words: 13-C isotope abundance, C-4 and C-3 metabolism, Cenchrus ciliaris, Prosopis juliflora, spatialvariability map, Spondias tuberosa soil phosphorus, Ziziphus joazeiro

Abstract

Planted silvo-pastoral systems are formed by sparing selected native trees when land is cleared for pasture estab-lishment, or by planting selected species – often known agroforestry species – into the establishing pasture. Isolatedtrees within pastures and savannas are often associated with ‘resource islands’, characterized by higher fertility andorganic matter levels under the tree canopies. We here examine the processes underlying the differences in fertilityand organic matter in a buffel grass (Cenchrus ciliaris L.) pasture that contained two tree species (Ziziphus joazeiroMart., Spondias tuberosa Arruda Cam.) preserved from the native thorn forest and a planted agroforestry species(Prospois juliflora Swartz D.C). The objective is to distinguish effects of soil variability from those induced bythe presence of trees or the planting of pasture. The δ13C signatures of the original (largely C3) vegetation, thepreserved and planted trees, and the planted C4 grass were used to distinguish the provenance of organic matterin the top soil (0–15 cm). This allowed the conclusion that all trees maintained C3 derived C at the original thornforest level, while lower levels under pasture were due to mineralisation of organic matter. The net rates of forest-derived C loss under pasture varied with soil type amounting to between 25 and 50% in 13 years after pastureestablishment. Only on Alfisol, C inputs from the pasture compensated for the C3-C losses. Analysis of organicand inorganic P fractions indicated Z. joazeiro and P. juliflora enriched the soil under their canopy with P, whereasS. tuberosa had no positive effect on fertility. A combination of ANOVA and spatial analysis and mapping wasused to show vegetation effects.

Introduction

In agroforestry and silvopastoral systems, trees areincluded in cultivated fields or pastures in order tomaintain soil fertility (Garcia-Miragaya et al., 1994),cycle nutrients (Young, 1997), improve microclimate(Weltzin and Coughenour, 1990), manage water tables(Sala et al., 1989) and improve overall system pro-ductivity (Rhoades, 1997). Traditionally, landuse sys-tems which combine trees with pastures are created byselective cutting of native forest or savanna vegetationduring which trees that are judged valuable by the landusers are left standing. In newer agro-silvopastoralinitiatives, selected tree species, often fast growing

∗ E-mail: TIESSEN@SASK.USASK.ca

legumes with a history of agroforestry use, are planted– sometimes at quite high densities. Studies in NEBrazil on the optimum tree density within pastures in-dicated better plant biomass and animal production attree densities corresponding to 30% of ground cover(Araújo Filho, 1990; Silva et al., 1995).

Isolated trees in grasslands or savannas are asso-ciated with ‘resource islands’ or ‘islands of fertility’(Belsky et al, 1993; Halvorson et al., 1995; Kellman,1979; Menezes and Salcedo, 1999; Reynolds et al.,1990; West, 1981). Improved fertility under isolatedtrees can result from litterfall or dung inputs fromsheltering animals (Belsky et al., 1993; Farrel, 1990;Rhoades, 1995; Tiedemann and Klemmedson, 1973;Weltzin and Coughenour, 1990).

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In a study by Menezes and Salcedo (1999), theresource island effects of three tree species (Ziziphusjoazeiro, Spondias tuberosa, and Prosopis juliflora)in a pasture of buffel grass (Cenchrus ciliaris) wereexamined using radial transects extending from thetree trunk to open grass. Higher levels of soil total C,N and P, and exchangeable Ca, Mg and K under thecanopies of Z. joazeiro and P. juliflora compared toopen grass areas were found. Under the canopies ofS. tuberoa , only soil Mg and organic P levels werehigher. Herbaceous understory biomass was signific-antly lower under the canopy of S. tuberosa and P.juliflora (107 and 96 g m−2, respectively), relatively toopen grass areas (145 and 194 g m−2). No herbaceousbiomass differences were found between Z. joazeirocanopy and open grass areas. Wick et al. (2000) foundhigher microbial biomass and enzyme activities un-der the trees of the tree-grass association than undergrass alone at the same site. Both these studies docu-mented differences in soil quality due to the presenceof trees in a planted pasture but did not investigate theprocesses that led to these differences.

Trees enrich surface soils by recycling leaf litterthat contains nutrients which may have been taken upat depth or nitrogen which may have been fixed. Forestsoils commonly have a steep gradient of organic mat-ter and associated nutrients with depth. When tropicalforests are replaced by pastures, soil organic mat-ter and associated nutrient levels may change. Forsites in Mexico (Garcia-Oliva et al., 1994, 1999) andCosta Rica (Veldkamp, 1994) losses of organic matterwere reported. Buschbacher et al. (1988) reported nochange and Cerri et al. (1991) reported organic mat-ter gains on Amazonian sites converted to pastures.Normally only net changes in organic matter and as-sociated nutrients are distinguished without evaluatingactual turnover of soil organic matter.

Organic matter turnover can be studied using Cisotope composition of the soil. The organic matterderived from grasses with C4 metabolic pathway canbe distinguished from that derived from predominantlyC3 forest vegetation based on δ13C abundance (Bales-dent et al., 1987, Cadisch and Giller, 1996; Mariotti,1991; Skjemstad et al., 1994). When one vegetationcover replaces the other, and if the time of initiationof the replacement is known, the decay rate of or-ganic matter derived from previous vegetation and theaccretion rate of new organic matter can be calcu-lated (Gregorich et al., 1995; Vitorello et al., 1989;Veldkamp, 1994).

Soil P fractions also have been used to study soil-vegetation processes (Lajtha and Schlesinger, 1988;Tate and Salcedo, 1988; Tiessen, 1993; Walbridgeand Richardson, 1991). Phosphorus is useful in thiscontext because total P is inherited from the parentmaterial, and is enriched in the top soil by recyc-ling of vegetation P (Sanchez, 1995). There are onlynegligible net gains of P in stable natural soil pro-files, while losses are slow (Letkeman et al., 1996).Different organic and inorganic P fractions thereforerepresent the results of in situ biological processes ofimmobilisation and mineralisation (Tiessen, 1993).

The site studied by Menezes and Salcedo (1999)and Wick et al. (2000) was cleared 13 years priorto sampling. The purpose of the present work isto use δ13C composition of soil organic matter andsoil P fractions together with the site history in or-der to examine the processes that have led to thepresence of different soil organic matter and fertilitystatus under different vegetation cover across the land-scape. The combination of C analyses and P fractionswas used to distinguish plant-related differences fromthose determined by soil variability.

Materials and methods

Soils were sampled on a beef cattle ranch near Cus-todia, Pernambuco, in the semiarid interior of NEBrazil. The native vegetation of the region is a semi-deciduous thorn forest known as Caatinga (Mors,1994). Traditional land uses are extensive grazingand browsing of Caatinga or slash-and-burn shift-ing cultivation followed by extensive browsing ofthe regenerating fallow vegetation (Sampaio, 1995).Mean annual temperature is 26 ◦C. Average rainfall of740 mm y−1 falls from January to May and the dryseason between June and December is nearly rainless(Sampaio, 1995). Rainfall is unreliable and extendeddroughts occur once or twice every decade (Eiten,1992). This has limited the planting of improvedpastures to relatively few large properties which canmanage the climatic risk. The soils in the landscapeare classified as Haplustalf on the uplands and slopes,and Ustorthent (alluvial/colluvial) in the lower partsof the landscape. Both soil types have an average sandcontent of 75%. Slopes are between 0 and 4%.

In 1984, 3000 ha of mature Caatinga vegetationwere cleared on the ranch with chains, and buffelgrass (Cenchrus ciliaris L.) was seeded. In the follow-ing year, the exotic leguminous tree Prosopis juliflora

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Swartz D.C. was planted into the pasture on a 10 by 10m grid. At the time of sampling in 1997, the tree plant-ation was therefore 12 years old. Caatinga regrowth inthe buffel grass and around the Prosopis was slashedevery other year.

On another 3000 ha of the ranch, two native Caat-inga species were preserved during land clearing in1984. Ziziphus joazeiro Mart, is a non-deciduous treethat is frequently left standing by farmers in the regionsince it is valued for its shade, edible fruit, wood andbrowse material (Noel, 1995). Spondias tuberosa Ar-ruda Cam. is deciduous and is commonly preserved inslash-burn sites because it produces a marketable fruit(Machado et al., 1997; Mors, 1994). At this site, thepreserved mature trees naturally occur at an averagespacing of some 30–40 m apart. The preserved treeswere estimated to be at least 50 years old.

After the initial clearing, no fire was used in themanagement of the areas, and no fertilizer was everapplied. Animal density on the ranch is managed near0.17 animal ha−1, well below the normal density onbuffel grass pastures in the region. The combinationof low animal density and high tree density in the pas-tures resulted in a negligible impact of animals on theshade areas under the trees. No significant amountsof dung were found and animals were usually widelydispersed on the ranch.

The same three species as used in the previousstudies by Menezes and Salcedo (1999) and Wick etal. (2000) were used here. Three individuals each wererandomly selected and soil properties compared relat-ive to canopy position for each individual. Observedvariability was mapped and statistically examined us-ing the three replications. Soils were sampled to 15 cmdepth under and around three random trees of eachspecies on grids of 89 samples centred on each treein the following way: a central sample was taken nearthe trunk. Eight samples were taken around the trunkin North-South, East-West and intermediate directionsat 1/4 of the canopy radius. These were followed bya further eight samples at 1/2 the canopy radius, andmore samples extending outward at distances equiv-alent to 1/2 canopy radius from there. The samplingscheme is illustrated in the C data plots in which eachmeasured value is mapped and isolines were calcu-lated (Figure 1). Sometimes, the outer samples enteredthe zone of influence of another tree and this was re-corded. All samples were classified as ‘under canopy’,‘canopy edge’ or ‘outside canopy’. Soils were alsoclassified as Entisols or Alfisols. Although sampleswere taken based on tree selection, an equal number

of Alfisols and Enfisols happened to be collected in thegrid samples. In addition, 32 samples were taken fromnative Caatinga areas that had remained interspersedwith the pastures on the ranch.

Prior to analysis, coarse and floatable organic mat-ter was removed from soil samples in a water sus-pension with mild sonication (50W, 3 min.). Total Cand 13C natural abundance (δ13Cv.pDB) of soils andplant materials were measured on an isotope ratiomass spectrometer (Europa Scientific 20–20, Crewe,UK). Phosphorus was fractionated using a sequentialextraction simplified by omitting the bicarbonate ex-tract from the method of Tiessen and Moir (1993).Fractions were reported as available (resin extract-able) Pi, resistant Pi (NaOH and hot acid extractable)and Po (the sum of the organic fractions extracted byNaOH and hot acid). Total N and P were determinedby autoanalyzer following digestion with concentratedH2SO4 and H2O2.

Contour maps of soil properties were generatedusing the moving weighted least squares griddingmethod of MacGridzo (Rockware, Inc.). This methodreduces the influence of individual samples on thecontours. These maps therefore provide a visual as-sessment of the spatial distribution of soil properties.More formally, statistical differences between differ-ent sample groupings were evaluated using ANOVA(Statworks SE + graphics).

Results and discussion

Spatial variability

Soil properties varied across the landscape, and thismay determine plant distribution. This spatial variab-ility needs to be taken into account before the effectsof vegetation and vegetation change on soil proper-ties can be examined. At the study site, Alfisols andEntisols formed a patchwork within the landscape,with Entisols in the lower areas and Alfisol in theupper areas, often with <50 cm elevation difference.Soil properties between these soil types differed andmust be analysed independent of vegetation effects.Of the trees inherited from the native vegetation, Z.joazeiro (like several of the Ziziphus spp. of the Sahel)showed a preference for the lower landscape positionsand occurred almost exclusively on the Entisols. Thispreference of Ziziphus joazeiro for Entisols is likelyto be related to moisture availability since it is evid-ent in the dry regions of the Sertão, while in more

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Figure 1. Distribution of C3 and C4 derived carbon (numbers give mg g−1 at the grid points) in the grids centred on examples of Z. joazeiro,S. tuberosa and P. juliflora. Isolines were calculated and drawn using moving weighted least square gridding (MacGridzo).

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humid regions, Z. joazeiro occurs throughout the land-scape. Spondias tuberosa occurred equally on Alfisolsand Entisols. Prosopis juliflora was planted only onAlfisols.

At the present site, canopy effects resulting fromthe tree must therefore be separated from plant-independent soil effects that are due to the trees’ (orplanters’) preference for specific soil types.

The origin of soil organic C was evaluated based onthe δ13C values determined for leaf material from C.ciliaris (–12.6‰), and an average value of –26.6‰forthe C-3 plants, since differences in the δ13C of leavesfrom Z. joazeiro (–27‰), S. tuberosa (–26.6‰), andP. juliflora (–26.3‰) were relatively small. Other C-3 plants were present in small amounts both underthe canopy and in the open grass areas (Menezes andSalcedo, 1999). Assuming that the slight enrichmentin 13C that occurs during the incorporation of litterinto soil organic matter (Mariotti, 1991) applies toall residue materials, we used the average differencebetween the C-4 and C-3 leaf materials of 14.0‰ tocalculate contributions of these metabolic plant typesto soil organic C concentration as:

C3-derived C = (measured δ + 12.6) /

(–14.0) ∗ [organic C]

C4-derived C = (measured δ + 26.6) /

(14.0) ∗ [organic C]

In addition to the pasture sites we measured or-ganic C and δ13C on 32 samples from four separateareas of native Caatinga vegetation interspersed withthe pastures of the study sites. Organic C content inone of the Caatinga sites (Site mean 10 mg g−1) whichwas predominantly on Entisols was significantly lowerthan in the other three Caatinga areas on Alfisols(mean 16 mg g−1). Based on measured δ13C this cor-responds to 13.7 mg g−1 of C3-C and 2.4 mg g−1 ofC4-C under native Caatinga in three of the Caatingas,and to 9.1 and 1.0 mg g of C3-C and C4-C in thepredominantly Entisol area (Table 1). The differenceof 11% C4-C in Entisols vs. 18% C4-C in Alfisols isnot significant because of the high variability in theCaatinga, i.e. δ13C for Caatinga showed no signific-ant differences with an over-all mean of –24.7±0.9.Over all Alfisols and Entisols combined there were86±7% C3 derived soil C under Caatinga, based onthe above benchmarks and calculations. Caatinga con-tains a small proportion of plants with C-4 as wellas some with crassulacean metabolism. This mixedcomposition accounts for part of the C4-C content

we calculate. In addition, there is a slight positiveshift in δ upon incorporation of organic matter intothe soil (Mariotti, 1991) which we did not consider.The 2 δ-unit higher values found in the soils than inthe leaves of the three species analysed are thereforein part true C4 contributions, and in part the result ofdiscrimination during OM transformations. The fol-lowing discussion is based only on relative shifts inδ values upon land use change, and is therefore notaffected by this uncertainty.

The two tree species inherited from native veget-ation were at least 50 years old and C amounts andcomposition were likely to be near steady state, sinceorganic C turnover in soils of the region occurs attime scales below 100 years (Shang and Tiessen, 2001;Tiessen et al., 1994).

Under pasture, total organic C in the Entisol aver-aged 7.7 mg g−1, significantly (p=0.000l) lower thanin the Alfisol with 17.1 mg g−1. The proportion of C-3 derived C was also significantly (p=0.0001) differentbetween the two soil types, with 66% and 73% in theEntisols and Alfisols, respectively. This means thatcomparisons of C and δ13C (Table 1) between treesmust be restricted to samples from the same soil type.It also indicates that plant productivity and or stabilisa-tion of soil organic matter are less in the Entisols thanthe Alfisols, and that C4 grasses may be slightly moreimportant for the organic matter budget in the lowerlandscape positions. This supports the, albeit non sig-nificant, difference seen in the Caatinga sites above.Since canopy and non-canopy samples were taken inthe same proportions on both soil types, the comparis-ons between soil types based on all samples taken arenot biased by the sampling for canopy effects.

Plant effects

Ziziphus joazeiro occurred only on Entisols where itmaintained twice as much C3-C under its canopy thanwas found outside the canopy (Table 1, Figure 1).The C3-C level under the canopy is similar to that ofCaatinga on the mainly Entisol site, indicating thatthe difference is due to a loss of C3-C outside thecanopy when the Caatinga surrounding the Z. joazeirowas cleared and replaced by grass. The loss of C3-C from the cleared areas surrounding Z. joazeiro ofabout 50% after 13 years points to a very rapid organicmatter turnover in this environment, similar to resultsby Shang and Tiessen (1998). Since the C4-C wassimilar under and outside the Z. joazeiro canopy, theintroduction of grasses did not significantly increase

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Table 1. C3 and C4 derived carbon at different canopy positions of native Z. joazeiro and S.tuberosa, planted P. juliflora, and in native caatinga

Trees Soil type Canopy position C3-C C4-C

mg g−1 mg g−1

Z. joazero Entisol outside 3.7 ± 1.3 a1 a2 2.6 ± 1.0 na3 A

Z. joazero Entisol intermediate 4.7 ± 2.0 b 2.7 ± 1.1 ns

Z. joazero Entisol under 7.5 ± 3.1 c 2.3 ± 0.8 ns

S. tuberosa Both outside 11.1 ± 5.7 b B 3.3 ± 1.0 c B

S. tuberosa Both intermediate 10.9 ± 4.8 ab 2.8 ± 1.1 b

S. tuberosa Both under 8.5 ± 3.9 a 2.0 ± 1.0 a

S. tuberosa Alfisol only outside 16.0 ± 3.7 b B 3.7 ± 0.8 bB

S. tuberosa Alfisol only intermediate 14.9 ± 2.5 b 3.5 ± 1.0 b

S. tuberosa Alfisol only under 11.1 ± 2.3 a 2.7 ± 0.3 a

S. tuberosa Entisol only outside 6.2 ± 1.9 a B 3.0 ± 1.1 b B

S. tuberosa Entisol only intermediate 7.0 ± 2.0 ab 2.1 ± 0.6 a

S. tuberosa Entisol only under 8.0 ± 1.8 b 1.6 ± 0.7 a

P. juliflora Alfisol outside 10.9 ± 5.1 a B 4.8 ± 1.9 a C

P. juliflora Alfisol intermediate 10.9 ± 4.2 a 4.9 ± 1.8 a

P. juliflora Alfisol under 14.4 ± 4.1 b 6.2 ± 2.8 b

Caatinga Alfisol only closed canopy 13.7 ± 3.2 b 2.4 ± 1.0 b

Caatinga mostly Entisol closed canopy 9.1 ± 2.2 a 1.0 ± 0.7 a

1numbers within a block of data (for a species and C origin) followed by different small lettersare significantly different (p=0.05).2Capital letters designate significient differences (p=0.05) between samples from differenttrees and soils outside the canopy.3 ns – ANOVA for the data block is non-significant (p=0.05).

the C4-C, i.e. little sequestration of new C had oc-curred in the soil. Ziziphus joazeiro thus maintainedan island of higher tree-derived soil C content, whilethe surrounding cleared areas had lost C.

Spondias tuberosa occurred on both soil types. OnEntisol it had only slightly higher C3-C below thanoutside the canopy, and had significantly less C4-Cbelow than outside the canopy (Table 1, Figure 1). Alow grass biomass under the canopy was reported byMenezes and Salcedo (1999). The higher C3-C andlower C4-C contents under the canopy balance so thattotal C was similar under and outside the S. tuberosacanopy. Total C (both C3 and C4 contributions) inEntisol surrounding S. tuberosa was 30% higher thanthat in Entisol surrounding Z. joazeiro (Table 1). Itis probable that this reflects slight micro-relief dif-ferences with S. tuberosa positioned in slightly lessleached locations that are transitional to the Alfisols

on higher relief. If the lower C3-C outside the canopyof S. tuberosa is attributed to mineralisation since theintroduction of pasture, and if the level under the S.tuberosa canopy is taken as a reference, C mineral-isation corresponds to a 22% loss over 13 years. If thenative Caatinga level of C3-C on Entisols (9.1 mg g−1)is used as a reference, the mineralisation loss amountsto 32%. On the other hand, C4-C is nearly doubleoutside the canopy than under S. tuberosa, and threetimes the level seen in Caatinga on alluvial sites point-ing to a substantial C sequestration during the 13 yearsof pasture. This is in contrast to the lowest landscapepositions around Z. joazeiro.

On Alfisol, both C3-C and C4-C were greater out-side than under the canopy of S. tuberosa (Table 1).Comparison with native Caatinga levels (Table 1)shows that conversion to pasture increased C sequest-ration by grasses on this soil, and that S. tuberosa

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maintained slightly lower C levels under its canopythan the average native vegetation. This indicates thatfor the Alfisol situation, the average Caatinga refer-ence should be used and a C3-C loss of 32% may haveoccurred under pasture. The differences in C balancebetween the two soil types emphasise that plant ef-fects must be examined in dependence on soil type.Analysis distinguishing only species for both soilscombined will not show these effects (Table 1).

Prosopis juliflora was planted after complete clear-ing of the Caatinga and shows both higher C4-C andC3-C levels under the canopy (Table 1, Figure 1).Twelve years after planting in this semiarid environ-ment, P. juliflora were much smaller trees than themature inherited species, and their canopies were veryopen, so that grasses are relatively productive underthe tree, although total herbaceous biomass was onlyhalf that outside the canopy (Menezes and Salcedo,1999). Slight shading and nutrient inputs from thetree may have aided soil organic matter sequestra-tion under the tree relative to the open grass land.The Prosopis juliflora canopy therefore generated aresource island attributable to the tree effect, althoughC4-C represented the same proportion of organic C(30%) both under and outside the canopy. C3-C con-tent under P. juliflora (14.4 mg g−1) was similar tothat of the Caatinga on Affisols (13.7 mg g−1) andidentical to levels seen in the Caatinga remaining ad-jacent to the Prosopis/pasture site. This indicates thatthe small trees have maintained C3-C levels of thenative forest, and that the levels outside the canopyrepresent a mineralisation loss of 24% over 13 years.This mineralisation loss was compensated by C4-Csequestration outside the canopy, while under the can-opy nearly 4 mg g−1 C4-C may have been sequesteredabove native levels.

When all data for P. juliflora and Z. joazeiro arepooled (omitting S. tuberosa, which maintains low Cunder its canopy relative to the Caatinga average), thereduction in C3-C outside the canopies is 3.7 mg g−1

on Entisols, and 3.6 mg g−1 on Alfisols. This rep-resents mineralisations of 50% and 25% in C3-C forEntisols and Alfisols, respectively, in the cleared areas.When, instead of the specific trees, average nativeCaatinga C levels are used as a reference, reductions of41% and 25% are calculated for Entisols and Alfisols,respectively.

Tree effects can also be observed in the C mapsfor three example trees (Figure 1): the location of Z.joazeiro is clearly visible in the C3-C map, but invis-ible in the C4-C map. S. tuberosa on the Entisol shows

a slight depression for the C4-C values, but this is notwell centred on the tree position. The C3-C imprintof S. tuberosa is not clearly visible. Despite high fieldvariability of both C3 and C4 derived carbon, the po-sition of P. juliflora is clearly visible for both isotopemaps. High C3-C levels on the edges of the map aredue to the narrow spacing between trees and the influ-ence of the next individual. These outer samples wereclassed as ‘intermediate’ canopy position for the abovestatistical analysis.

Soil nutrients

Regressions between C and N in soil organic matterare usually very close; over all samples the regressionwas C = 12.5 N - 0.5 (r2 = 0.92). Accordingly, thetrends seen in organic C are reflected in N contents(Table 2). Nitrogen fixation in this semiarid environ-ment is likely to be highly unreliable. There was noevidence for N contributions from the legume P. juli-flora. A C:N ratio of 12.6 under the canopy showedthat there was no enrichment with N relative to themean value for the ranch (12.5). Since C4 plants arenot symbiotic N fixers, a regression of the ratio ofC3-C / C4-C against soil N might amplify evidenceof other N fixation in the system. The regression washighly significant although with a low r2 = 0.18. Onthe other hand, a similar regression against organicC had an r2 =0.14, indicating that C3 plants simplyhad a greater contribution to overall soil organic mat-ter content, and that any link to N fixation remainsunconfirmed.

Organic P similarly was correlated to organic C(r2=0.59). Organic P content was higher under Z.joazeiro canopies than in surrounding soil, similar un-der P juliflora and lower under S. tuberosa (Table 2).Z. joazerio and P. juliflora maintained higher availablePi and total P levels under their canopies. These twospecies therefore represent islands of P fertility andorganic matter in the pastures.

The δ13C data discussed above indicate that the el-evated organic matter contents under the trees largelyrepresent C levels inherited from the Caatinga. Thesame can be assumed for the N enrichment, since Nis closely related to C. Organic P is too variable to per-mit clear conclusions about the reasons for its elevatedlevels: organic P could be inherited from the Caatinga(under the tree) while it is mineralised in the clearedpasture, or it could have accumulated under the trees(Table 2). It is clear though that S. tuberosa maintainslow P levels under its canopy. Both Z. joazeiro and

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Table 2. Soil organic matter and phosphorus fractions as a function of canopy position

P. juliflora Z. joazero S. tuberosa

organic C outside 15.6 ± 6.6 a2 6.3 ± 1.9 a 14.4 ± 6.3

(mg g−1) intermediate 15.8 ± 5.5 a 7.4 ± 3.1 b 13.8 ± 5.6

under 20.7 ± 5.8 b –25

total N outside 1242 ± 363 a 545 ± 160 a 1237 ± 599

(µg g−1) intermediate 1304 ± 335 a 652 ± 285 b 1162 ± 528

under 1646 ± 476 b –24% 856 ± 287 c –36% 1016 ± 302

organic P outside 75 ± 18 33 ± 8 a 103 ± 49

(µg g−1) intermediate 75 ± 16 37 ± 11 a 85 ± 48 ab

under 85 ± 34 ns 49 ± 12 b –23% 74 ± 31 a +39%

available Pi outside 4.7 ± 2.5 a 7.7 ± 6.2 a 4.7 ± 2.0

(µg g−1) intermediate 5.3 ± 2.4 a 10.3 ± 10.3 a 4.7 ± 1.4

under 10.8 ± 8.3 b –56% 21.0 ± 15.3 b –63% 5.0 ± 2.4 b ns

resistant Pi outside 92 ± 21 a 78 ± 17 a 101 ± 30 ab

under 125 ± 47 b –26% 101 ± 28 b –19% 102 ± 18 a +24%

total P outside 272 ± 65 a 224 ± 108 b 274 ± 103

(µg g−1 intermediate 276 ± 53 a 229 ± 74 ab 264 ± 92

under 329 ± 55 b –17% 267 ± 84 a –16% 260 ± 69 ns

1Percent values give the difference (if significant at p=0.05) between under and outside canopy positions using ‘under’as the reference.2Small letters indicate significant (p=0.05) differences in data blocks.3ns – ANOVA for the data block is non-significant (p=0 05).

P. juliflora have elevated available Pi, resistant Pi andtotal P levels under their canopies. Resistant Pi or totalP would not be lost upon conversion of Caatinga topasture. Unlike C3 derived C, the elevated level un-der the tree therefore must represent an accumulationunder the tree rather than a loss in the pasture. Inaccordance with these data, both Z. joazeiro and P.juliflora are clearly visible islands of fertility on themaps of total, organic and available P (Figure 2).

An average total P enrichment of some 40 µg g−1

under Z. joazeiro may be explained because the tree islarger than most Caatinga species, has a dense canopyand is long-lived. Z. joazeiro can therefore accumulatelitter-recycled P over many years. Can an enrichmentof nearly 50 µg P g−1 under the 12 year old P.juliflora relative to the surrounding pasture be attrib-uted to accumulation by biocyding under the trees?During biocycing, Po recycled from tree litter en-riches the topsoil with mineralised Pi. The canopy ofthese young P. juliflora covered about 20–30 m2. Themeasured soil P enrichment therefore represents some200–300 g P per tree to the sample depth of 15 cm

(bulk density 1.3 g cm−3). We measured a P contentin the Prosopis litter of 1.8 mg g−1. Some 100–160 kgof litter would therefore be required to accumulate P tothe observed levels above those inherited by the Caat-inga. This P would have to be taken up by the tree frombelow the top 15 cm over the 12 years since planting.if one discounts the first half of this period becausethe trees were small, an approximate 130 kg litterper 6 years and tree correspond to a litter productionof 2100 kg ha−1 yr−1, given the 10×10 m plantinggrid. Studies from other tropical areas showed a litterproduction for P. juliflora between 6100 and 7400 kgha−1 yr−1. (Gark, 1992; Gark and Jam, 1992). Eventhe relatively small and open-canopy trees could there-fore have produced enough litter to account for theobserved enrichment in the soil. The average 130 kglitter per 6 years per 25 m2 correspond to 800 g m−2

per year. This is higher than the average 300 g m−2

per year that Barth and Klemendson (1986) recordedunder P. juliflora ‘shrubs’. These comparisons indicatethat the present P. juliflora are in between published

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Figure 2. Distribution of total, organic and resin extractible P (numbers give µg g−1) in the grids centred on examples of Z. joazeiro, S.tuberosa and P. juliflora. Isolines were calculated and drawn using moving weighted least square gridding (MacGridzo). Outer points are notshown to simplify presentation.

values, and it is reasonable to attribute the observedsoil enrichments to these small trees.

It can thus be concluded that both Z. joazeiro andP. juliflora have enriched the topsoil under their can-opies with total P to about 16% above the surroundingland by bio-cycling of P from below the top soil. Theproportionally much greater doubling in available P islikely due to a greater activation of the P cycle (Wick

et al., 2000) and to the constantly mineralising organicP inputs under the trees.

The percentage of total P that is in organic formwas not affected by canopy position, probably becauseof the complex interactions of organic matter levels,organic matter turnover, and P levels. A significantdifference was seen between the Entisols and Alfisolswhich contained 20% and 32%, respectively, of their

204

P in organic form. This is related to both the lower Clevels and lower stability of C in the Entisols, seen inthe δ13C data.

In summary, the analysis of 13C abundance and Pforms has permitted to qualify the findings of nutri-ent enrichment by Menezes and Salcedo (1999). Theobserved islands of P fertility under two of the treespecies are attributable to biological enrichment bythe trees under their crowns. The cation enrichment(Menezes and Salcedo, 1999) likely follows similarprocesses. Elevated organic C and N contents on theother hand are likely inherited from native Caatinga,and the trees (both inherited and planted) simply pre-vented the organic matter mineralisation that occurredin the cleared pasture. This does not imply static Cbalances under the trees, but a maintenance of el-evated levels trough a balance of C (and N) inputsand outputs. Importantly, and in an extension of find-ings in the same region by Shang and Tiessen (1998,2001), organic matter in this tropical semiarid regionis turning over and mineralising under disturbance atan extremely rapid rate. Previous work had pointed toa mean residence time well below 100 and possiblynear 50 years. Considering all the possible comparis-ons developed above, a reasonable best estimate forC mineralisation rates is bounded between 25 and50% loss in 13 years, giving a half life of soil carbonbetween 13 and 30 years in this untilled pasture. Evid-ence from several tropical sites had already indicatedthat turnover and mineralisation of C in tropical soilsis very rapid, with half-lives below 100 years underdisturbance (Trumbore, 1993; Tiessen et al., 1994).No resistant fraction, which might be expected fromstudies on temperate soils, was found in two differ-ent soils from NE Brazil, (Shang and Tiessen, 1997,1998). The present data provide further evidence forthe extreme lability of tropical soil carbon, even un-der minimal disturbance, and point to the importanceof managing organic matter inputs and plant cover intropical land use systems.

Acknowledgements

This study was financed by the Inter American Insti-tute for Global Change Research (IAI) grant CRNOO1. We are grateful to Agropecuaria Jaçana for accessto the sites and support in the field.

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