Recha et al. Nut Cycl Agr Jan2013

14 Pages·2013·0.45 MB·English

NutrCyclAgroecosyst
DOI10.1007/s10705-013-9554-0
ORIGINAL ARTICLE
Stream water nutrient and organic carbon exports
from tropical headwater catchments at a soil degradation
gradient
John W. Recha • Johannes Lehmann •
M. Todd Walter • Alice Pell • Louis Verchot •
Mark Johnson
Received:23July2012/Accepted:15January2013
(cid:2)SpringerScience+BusinessMediaDordrecht2013
Abstract Carbonandnutrientlosseswerequantified watershed OC losses, even 50 years after the forest
fromfoursmallheadwatercatchmentsinwesternKenya wasremoved.Flow-weightedstreamwaterconcentra-
intheyear2008.Theyincludeaforestedcatchmentand tionsofDOCandcoarseparticulateOC,allNspecies,
three catchments under maize continuously cultivated totalP,KandNasignificantly(P\0.05)increasedin
for5,10and50 yearsfollowingforestconversion.The streams after forest conversion and long-term cultiva-
CisotopiccompositionofdissolvedorganicC(DOC)in tion. Solute concentrations increased despite the fact
stream discharge suggested that soil organic C (SOC) that soil contents decreased and total water flow
derived from the original forest rather than OC from increasedindicatingmobilizationofCandN,PandK
maize may have contributed to a large extent to from soil with progressing cultivation. In contrast, Ca
andMgconcentrationsinstreamwaterdidnotsystem-
aticallychangeafterdeforestationandcultivation,and
may be controlled by geochemical weathering rather
J.W.Recha(cid:2)J.Lehmann(&) thanbychangingwaterflowpathsortopsoilcontents.
DepartmentofCropandSoilSciences,Cornell
All OC and nutrient exports increased with longer
University,Ithaca,NY,USA
e-mail:[email protected] cultivation over decadal time scales (P\0.05) to the
same or greater extent than through deforestation and
M.T.Walter
the first years of cultivation. Fluvial OC and total N
DepartmentofBiologicalandEnvironmental
losseswere2and21 %oftotalSOCandtotalNdecline,
Engineering,CornellUniversity,Ithaca,NY,USA
respectively,inthetop0.1 mover50 years.FluvialOC
A.Pell losses therefore played a minor role, and SOC losses
DepartmentofAnimalSciences,CornellUniversity,
were mainly a result of microbial mineralization.
Ithaca,NY,USA
ResultingtotalNlossesbystreamdischarge,however,
L.Verchot were large with 31 kg ha-1 year-1 after 50 years of
CenterforInternationalForestryResearch,Bogor, continuous cropping in comparison to fertilization of
Indonesia 40 kg N ha-1 year-1. Most (91 %) of the N losses
occurred as NO -. In contrast, P losses by stream
M.Johnson 3
InstituteforResources,EnvironmentandSustainability, discharge were negligible in comparison to plant
Vancouver,Canada uptake.Waterlossesshouldbemanagedtoreducesoil
fertility declines especially through large N export
M.Johnson
from agricultural headwater catchments. However,
DepartmentofEarthandOceanSciences,University
ofBritishColumbia,Vancouver,Canada stream concentrations of both P (0.01–0.15 mg L-1)
123
NutrCyclAgroecosyst
and N (0.4–4.8 mg L-1) were moderate or low with gathered in both temperate (Nair and Graetz 2004;
respecttopossibleconsequenceforhumanhealthandnot Schipper et al. 2007) and tropical regions (Pandey
responsibleforeutrophicationobservedinLakeVictoria. etal.2010).However,studiesonnutrientlossesfrom
entirecatchmentshavemostlybeenconfinedtoforests
Keywords Carbon(cid:2)Cultivation(cid:2) rather than to different agricultural watersheds with
Degradationgradient(cid:2)Forestconversion(cid:2) increasingperiodofcultivation.StudiesfromMalay-
Headwatercatchment(cid:2)Nutrients siaandtheAmazonexaminedimpactsofdisturbances
(Zulkifli 1990), pasture (Thomas et al. 2004) and
forestclearing(Lesack1993;Malmer1996;Williams
Introduction andMelack1997;Williamsetal.1997).Itisnotclear,
to what extent continuous cultivation and accompa-
The impact of land use change on nutrient and soil nyingsoildegradationovertimeaffectfluvialnutrient
organic carbon (SOC) dynamics in the tropics is lossesinadditiontotheeffectsoflossofforestcover,
particularlyseveresincesoildegradationismorerapid andhowthesefluviallossescomparetonutrientinput
than in temperate zones (Spaans et al. 1989; Malmer andcropuptake.
and Grip 1990; Grip et al. 2004; Hartemink et al. Not only the relationship between agricultural
2008). Continuous cultivation and land tillage cause nutrient input and fluvial export is unclear, but also
rapid loss of SOC (Mann 1986; Davidson and what these exports mean for pollution of waterways.
Ackerman 1993; Tilman et al. 2002; McLauchlan Lake Victoria, the largest tropical lake in the world,
2006;Solomonetal.2007)duetothedisruptionofthe is significantly contaminated with DOC and nutri-
physical, biochemical, and chemical mechanisms of ents, notably P, posing a threat to people, the fish
soilorganicmatter(SOM)stabilization.However,the industry and biodiversity (Verschuren et al. 2002). It
reductioninSOCstocksmaynotbesolelyattributed is widely believed that the contamination originates
to increased mineralization losses to CO , but could fromlandusechangeinducedbyagriculture(Scheren
2
includeasignificant,yetpoorlyquantifiedproportion et al. 2000; Hecky et al. 2010). This is a common
of stream water losses (Davidson and Ackerman attributionandnon-pointsourcepollutionwithNand
1993). On a basin and headwater catchment scale of P by agriculture is indeed observed in many parts of
forests, stream water OC losses appear to be a minor the world (Carpenter et al. 1998). Yet, fertilizer use
fraction of total SOC losses compared to mineraliza- in tropical smallholder agriculture is typically much
tionofCO (Richeyetal.2002;Johnsonetal.2006b). lower than in industrialized countries where detailed
2
However, the importance of fluvial OC losses during assessments are available. No data are available that
thefirstyearsofconversionfromforeststocultivation, investigate the nutrient and OC loads of agricultural
which arethe mostimportant intermsofSOClosses headwaters in Africa and the contribution that
(Davidson and Ackerman 1993) have not been well deforestation by burning and progressive cultivation
quantified. While mineralization losses typically would make.
decrease with greater period of cultivation (Kimetu The dynamics of nutrient and C losses over the
et al. 2009), the stream water discharge usually course of the year can provide insight into the
increases (Williams et al. 1997; Germer et al. 2010; mechanism and pathways of nutrient and C in
Recha et al. 2012), potentially increasing the impor- watersheds (Markewitz et al. 2004; Johnson et al.
tanceoffluvialOClossesascultivationprogresses. 2006a).Typically,weexpectadilutionofnutrientand
Soil organic matter decline leads to rapid nutrient Cconcentrationsduringtherainyseason(Likensand
releaseofNandPaswellasreducedcationexchange Bormann 1995). However, under certain circum-
capacity resulting in diminished nutrient retention of stances of highly weathered soils, concentrations
the soil (Lal 2006). However, surprisingly little may increase during the wet season as a result of
information is available on how soil degradation greatersurficialflowpathsthatmobilizesolutesfrom
following land use conversion affects nutrient losses nutrient and C-rich topsoils (Markewitz et al. 2004).
from watersheds. On a plot level, information on It is not clear whether forest conversion and soil
accelerateddecreaseinsoilnutrientcontentsresulting cultivationmaychangetherelationshipbetweenwater
from various types of agricultural practices has been and nutrient concentrations and how these dynamics
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NutrCyclAgroecosyst
are modified in tropical soils rich in weatherable The site has a mean elevation of 1,800 m above sea
minerals (Davidson et al. 2004; Yusop et al. 2006) level, a maximum daily temperature of 26 (cid:3)C and a
withhighexchangeablebasecationcontents(Kimetu minimumof11 (cid:3)C(Glenday2006).
et al. 2008). The fluvial DOC losses may be a The Kakamega-Nandi forest in Western Kenya is
significant fraction of C losses and of total solute the country’s only remaining tropical rain forest.
losses in watersheds (Selva et al. 2007). In forest Massive deforestation has taken place to create land
ecosystems,themajorityofthefluvialorganicClosses for settlement and farming. About 16 % of forest
occurred as DOC rather than coarser organic C cover was lost between 1986 and 2001 (Awiti et al.
fractions (Johnson et al. 2006b; Selva et al. 2007). 2008). The forest forms the easternmost relic of the
DespitethelowerimportanceofparticulateorganicC, Guinean-Congolian rainforest belt, which once
mostoftheorganicCwasmobilizedfromthetopsoil spanned from East to West Africa. The area around
litterlayer(Johnsonetal.2006a,b).Whetherthesame thisforest isamongthemostdenselypopulatedrural
istrueforagriculturalsoils,orwhethersubsoilorganic areasintheworld.Ithadapopulationdensityof778
CmaygaininimportanceforsoilsdepletedinSOCby persons per km2 in 2009 compared to 73 persons per
long-termcultivationisnotclear. square kilometer for the entire country (Kenya
Thepurposeofthisstudywastoquantifythefluvial National Bureau of Statistics 2010). Consequently
nutrient and OC losses from headwater catchments theforestisunderhighanthropogenicpressure,which
separating the effects of soil degradation and con- ismirroredbythedecreasingnaturalforestcoverand
comitant losses of soil organic matter contents from intensive cultivation (deGraffenried and Shepherd
thoseofforestclearingintheuppercatchmentsofthe 2009; Swallow et al. 2009). Past deforestation rates
LakeVictoriabasin.Itishypothesizedthat(1)stream intheKakamega-Nandiforestindicatedadecreaseof
waterOClossesincreaseinimportancewithlengthof forest area and an increase in the fragmentation of
cultivation; (2) long-term cultivation is as or more natural,old-growthforest(LungandSchaab2006).
important than deforestation and short-term cultiva- Soils in the Kapchorwa catchment are kaolinitic
tion for increasing nutrient and OC losses; and (3) Acrisols (FAO-UNESCO-ISRIC 1988), which are
headwaters with small holder agriculture are not classified as Ultisols in the US soil taxonomy (Soil
important sources of N and P to rivers and lakes. SurveyStaff2003).Theparentmaterialofthesesoils
The effects of continuous agricultural land use on is principally granite, with some inclusions of
nutrientswereinvestigatedbycomparing threehead- Precambriangneisses,whichsupportsLuvisols(Wer-
water catchments following forest conversion to ner et al. 2007) and other undifferentiated basement
continuous maize production for either 5, 10, or system rocks at higher elevations (Jaetzold and
50 yearsinWesternKenyawithacatchmentthathas Schmidt 1983). Soils in the catchment have
remainedforested. 45–49 % clay, 15–25 % silt, and 26–40 % sand
(Kimetuetal.2008).
The forest section of the Kapchorwa catchment is
Methods part of the Kakamega-Nandi forest composed of
tropical rainforest species. It is largely an indigenous
Thestudysite forest with a 30-m closed canopy dominated by
evergreen hardwood species. The most common
The field measurements were done in Kapchorwa species are Funtumia africana, Ficus spp, Croton
within Nandi County in Kenya, which is located in spp, and Celtis spp (Glenday 2006). Other species
EasternAfrica.Thesiteislocated60 kmnortheastof include Aningeria altissima (A. Chev.), Milicia ex-
Lake Victoria at longitude 35(cid:3)00000 E and latitude celsa (Welw.) C.C. Berg, Antiaris toxicaria (Lesch.)
0(cid:3)100000N(Rechaetal.2012).Theregionbelongstothe andChrysophyllumalbidum(G.Don),Oleacapensis
sub-humidecologicalzonecharacterizedbyabimodal (L.) and Croton megalocarpus (Hutch.) (Kinyangi
rainy season with a mean annual precipitation of 2008). The above and below ground net primary
2,000 mm(Awitietal.2008).The‘‘longrainseason’’ productionoftrees ina tropical forest is estimated at
is from April to June (*1,200 mm) and the ‘‘short 15.2 Mg ha-1 year-1 (Hertel et al. 2009). The agri-
rain season’’ from August to October (*800 mm). cultural catchments have maize as the sole crop, and
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NutrCyclAgroecosyst
have been under maize cultivation since conversion Stream stage was recorded using water capacitance
from forest cover. The maize grain yields without probes (Odyssey Dataflow Systems Pty Ltd., New
fertilizer input inthe 5-,10- and 50-year old agricul- Zealand) installed at the weir. The probes were
turalcatchmentsare6.5,5.5and2.5 Mg ha-1 year-1, programmed to give a reading of the average stream
respectively(Ngozeetal.2008). stage at 2 min. Data from these probes were down-
loadedbiweekly.Theweirratingsweredeterminedat
Hydrologicinstrumentationandfieldwork lowandintermediateflows.Theweirratingcorrelation
coefficientswerer2 = 0.944,r2 = 0.915,r2 = 0.926,
Inordertoidentifytheheadwatercatchments,thetime r2 = 0.931fortheforest,5-,10-and50-yearconver-
whennativeforestswereconvertedtoagriculturewas sions, respectively. The stream hydrographs were
determined from historical community settlement normalizedbycorrespondingcatchmentsizestoallow
patterns over the last century (Bleher et al. 2006; comparisonbetweenresponsesforthe4catchments.
Rechaetal.2012).Specificyearsofconversionwere Streamwatersamplingwasdonebiweeklyattheweir
verified from data available from records of the outlet, at the beginning and mid of every month. The
Kenyan government from the Department ofForests, watersampleswerefilteredthrough0.45-lmpore-size
theMinistryofAgriculture,aswellasfrominterviews glass-fiber filter, into two separate 50-mL centrifuge
with officials of local institutions and from county vials.Weaddedthymolintothefirst50-mLcentrifuge
council records. Within each site, population settle- vial that was used for determination of calcium (Ca),
mentpatternsweredistinctandnewlyacquiredfields magnesium (Mg), sodium (Na), potassium (K), total
were excised from sections of the native forest for dissolvedphosphorus(TDP),nitratenitrogen(NO –N),
3
agriculture.Allfourheadwatercatchmentsarelocated ammonium nitrogen (NH ?-N) and total dissolved
4
within an area of 6 km2 (Fig. 1) and represent a soil nitrogen(TDN).Weadded10 %HClintothesecond
degradation gradient that corresponds to years under 50-mLcentrifugevialthatwasusedfordeterminationof
maize cultivation that has been used in several other dissolved organic carbon (DOC) and the DOC stable
studies(Kimetuetal.2008,2009;Ngozeetal.2008; isotopicratioof13Cto12C.The0.45-lmpore-sizeglass-
KimetuandLehmann2010;Rechaetal.2012).Such fiberfilterpaperwithsedimentwasairdriedandkeptfor
chronosequencessubstitutetimeforspaceandhaveto the determination ofcoarseparticulateorganiccarbon
be carefully selected to assure similar properties (CPOC), the CPOC isotopic ratio of 13C to 12C, and
before the change (Huggett 1998). Also hydrological coarseparticulatenitrogen(CPON).
differencesbetweencatchmentshavetobeconsidered Five soil samples from 0 to 0.1 m depth were
(Elsenbeer,2001;Johnsonetal.2006b)andonlyclear collected randomly by stratifying for location within
trendsacrosstheentiresetofthefourcatchmentsare slope and plateau of each catchment, after carefully
interpretedhere. removing the litter layer (capturing only the mineral
Hydrologic instrumentation was installed in 2006 horizon).TheywereanalyzedforSOM,SOC,totalN,
(Rechaetal.2012)andcatchmentsmonitoredcontin- available P, Ca, K, Mg and Na. Similarly, four sites
uously. The boundaries of each catchment were were randomly selected by stratifying for location
determinedusingaGlobalPositioningSystem(GPS) within slope and plateau of each catchment and
onthelandscapearoundeachspringuptotheplateau, additional soilsampled at depths of0.1–0.3, 0.3–0.9,
using an average of five to eight independent assess- 0.9–1.5, 1.5–2.4 m. Bulk density was determined by
ments. The GPS data were then used to generate a thesoilcoremethod;weusedfivesoilsamplesfroma
Geographical Information System (GIS) output and depth of 0–0.1 m in each catchment (Campbell and
map of the area. The sizes of the catchments were Henshall1991).Soilporosity(ø)wascomputedfrom
12.8 ha for the forest, 14.4 ha for the 5-year old bulk density (q ) and particle density (q ) using the
b p
conversion, 9.1 ha for the 10 year conversion and formula ø = 1 - (q /q ). The use of organic and
b p
10.0 haforthe50-yearconversion.Therewereatotal inorganic fertilizers was assessed in all studied
ofone,six,andelevenhouseholdslivinginthe5-,10-, agricultural watersheds by a full survey of all
and 50-year conversion catchments, respectively. households using interviews. Farmer interviews in
A standard V-notch weir was constructed at each the watersheds confirmed information provided
catchment outlet for determining stream discharge. by the studies of Kinyangi (2008), Kimetu (2009),
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NutrCyclAgroecosyst
Fig.1 Mapofthestudy
areaoftheKapchorwa
headwaterchronosequence
ofcatchmentsinwestern
Kenya,anditsapproximate
locationwithinKenya(inset
showsKenyaandits
districts).Theweirpositions
areshownbydotsineach
catchment.Lettersindicate
landusehistoriesofthe
catchments:forested
catchment(a),maize
cultivationfor5years(b),
10years(c)and50years
(d)followingforest
conversion
and Moebius-Clune et al. (2011). The soils received Plasma Atomic Emission Spectroscopy (ICP-AES,
nil, or only very little inorganic fertilizer since forest ARCOS, Germany).TDNwasanalyzedusing Shima-
clearing,noanimalmanure,andwerealwayscropped dzu’sTotalNitrogenModule,TNM-1thatuseschemi-
with maize. From the interviews, only di-ammonium luminescence. Dissolved organic nitrogen (DON) was
phosphate (DAP, N:P:K of 18:46:0) fertilizer was computed using the formula DON = TDN-NO - ?
3
used at an application rate of6 and9 kg ha-1 within NH ?. DOC analysis was carried out on a Shimadzu
4
the 10 year and 50 year watersheds, respectively, for Total Organic Carbon-Visionary Series (TOC-V )
CSH
theyear2008.Thefarmersinthe5 yearwatersheddid analyzerfollowingtheproceduredescribedbyQianand
notuseanyfertilizer.TheNandPadditionwas1.02 Mopper(1996).NO –NandNH ?-Nweredetermined
3 4
and 2.76 kg ha-1, respectively, for the 10 year on a Seal AQ2-Automated Discreet Analyzer (Seal
watershed. The addition for the 50 year watershed Analytical,England).
was 1.62 and 4.14 kg ha-1 of N and P, respectively.
Animal manure was notused in anyhousehold. Statisticalanalyses
Laboratorymeasurements Statistical analyses were done using JMP Version 8
(SASInstituteInc,Cary,NC,USA)forsoilproperties
SoiltotalCwasdeterminedbydrycombustionafterfine andstreamwatersolutesofthefourcatchments.Aone
grinding soil using a Mixer Mill (MM301, Retsch, way ANOVA was used for the soil properties, and a
Germany).SampleswereanalyzedfortotalCcontents repeated measure analysis controlling for date of
withaEuropaANCA-GSLCNanalyzer(PDZEuropa sampling was performed for the stream water solutes.
Ltd., Sandbach, UK). Soil organic matter (SOM) was Both analyses were followed with posthoc multiple
determinedbylossonignition(Storer1984)andsoilpH comparisons using a Tukey correction for multiple
(inwater)atthew/vratio1:2.5usingaglasselectrode comparisonswhenthecatchmenteffectwassignificant.
(ThermoScientific,Beverly,MA,US).Filterscontain- ComparisonswereconsideredsignificantatP\0.05.
ing CPOC were ground using a Mixer Mill (MM301,
Retsch,Germany)andanalyzedfortotalC,theisotopic
ratioof13Cto12C,andCPONusingacoupledEuropa Results
20–20continuousisotoperatiomassspectrometry(PDZ
EuropaLtd.,Sandbach,UK).TheMehlich3extraction Soilproperties
procedure (Mehlich 1984) was used for the available
soil P, Ca, Mg, K and Na. The Ca, Mg, K, Na and P Within the top 0.1 m, the SOM and SOC concentra-
concentrations were obtained by Inductively Coupled tions significantly decreased by 59 % and 75 %,
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NutrCyclAgroecosyst
Table1 Soilpropertiesinthetop0.1m
Forest 5yearconversion 10yearconversion 50yearconversion
Soilorganicmatter(%) 17.08a 13.67b 8.53c 7.00d
Soilorganiccarbon(gkg-1) 108.3a 68.8b 36.4c 27.5c
Soilorganiccarbon(tha-1) 86.6a 62.6b 37.5c 32.2c
Soilnitrogen(gkg-1) 11.2a 6.9b 3.4c 2.3c
Soilnitrogen(tha-1) 8.96a 6.28b 3.50c 2.69c
Soilbulkdensity(gcm-3) 0.80c 0.91b 1.03ab 1.17a
Totalporosity 0.70a 0.66b 0.61bc 0.56c
Moistureat0.33bar(%v/v,fieldcapacity) 34.7a 34.1a 24.9b 23.0b
pH(water,1:2.5) 7.39a 6.48b 6.23b 5.81c
AvailableP(gkg-1) 0.011a 0.007a 0.003a 0.006a
AvailableK(gkg-1) 0.69a 0.58ab 0.23b 0.44ab
AvailableCa(gkg-1) 6.25a 4.98a 1.77b 2.91b
AvailableMg(gkg-1) 0.73a 0.45b 0.20c 0.33bc
AvailableNa(gkg-1) 0.026a 0.020a 0.010a 0.011a
MeanswithinarowfollowedbythesamelettersarenotsignificantlydifferentfromeachotheratP\0.05(n=5)
respectively,andthesoilNcontentsby79 %fromthe trendsatdepth.Similarly,thed13Cvaluesbecameless
forest to the 50-year old agricultural catchment negative deeper in the profile for all the catchments
(Table 1). The forest SOC and soil N were signifi- indicating a slight enrichment in the heavier isotope
cantly (P\0.05) higher than the 5-year conversion. comparedtothetopsoil.
Soilsinthe10-and50-yearconversiondidnotdiffer.
TheSOM,SOCandNlosswasveryrapidinthefirst Streamwaterchemistry
10 yearsofconversiondecreasingby50,66and70 %,
respectively. Available soil P, K and Na did not The d13C composition of CPOC (Table 2) did not
change as a result of forest clearing and soil use change from the forest (-27.52 %) to the 10 year
between the studied watersheds. In contrast, extract- conversion (-27.28 %). The 50-year old catchment
able Ca and Mg significantly (P\0.05) decreased hadaslightlyhigherd13Cvaluecomparedtotheother
overtime(Table 1).Similarly,pHvaluessignificantly catchments.ThestreamwaterDOCd13Ccomposition
decreasedfromforest(7.39)toagriculture(5.81).
Thesoilq increasedrapidlyby28.8 %inthefirst
b Delta 13C (‰)
10 years of cultivation from 0.80 to 1.03 g cm-3
-28 -26 -24 -22 -20 -18 -16 -14 -12
(Table 1). Overall, the soil qb increased by 46.3 % 0.0
forest
from 0.8 to 1.17 within 50 years of cultivation. The
5 year
rapid increase in qb in the first 10 years after forest 0.5 1500 yyeeaarr
conversion was followed by a 12 % drop in the total
porosityfrom0.69to0.61inthesameperiod. m) 1.0
In the 0–0.1 m depth, the soil d13C values were h (
pt
-26.07 ± 0.68 %, -25.37 ± 0.84 %, -21.21 ± e 1.5
D
0.87 % and -19.18 ± 0.44 % for the forest, 5-,
10- and 50-year conversion catchments, respectively 2.0
(Fig. 2). This indicates enrichment in the heavier
isotope from the forest to the catchment under 2.5
cultivationfor50 years.Thed13Cvaluesatthetopsoil
Fig.2 Longtermshiftintheisotopiccompositionofd13Cof
(0–0.1 m) increased linearly over time from -26.07
thesoilsatvariousdepthsinthewatersheds.n=4.Barsare±1
to -19.18 % (r2 = 0.95; P\0.0001), with similar standarddeviationofthemeanforeachdepth
123
NutrCyclAgroecosyst
Table2 Theisotopic
Forest 5year 10year 50year
compositionofd13Cofthe
conversion conversion conversion
CPOCandDOC,andthe
flowweightednutrient
Sedimentandfilteredstreamwater
concentrationsofthe
CPOCd13C(%) -27.52b -27.29b -27.28b -26.40a
headwatercatchments
DOCd13C(%) -28.59c -28.25c -26.79b -22.75a
Streamflowconcentrations
DOC(mgL-1) 1.31a 1.48a 1.47a 1.52a
CPOC(mgL-1) 1.25a 1.43a 1.49a 1.65a
CPON(mgL-1) 0.12a 0.14a 0.15a 0.16a
TDN(mgL-1) 0.49b 0.67b 4.83a 4.64a
NO -(mgL-1) 0.40b 0.58b 4.71a 4.52a
3
Meanswithinarow DON(mgL-1) 0.10a 0.09a 0.10a 0.12a
followedbythesameletters TDP(mgL-1) 0.03b 0.01b 0.05b 0.15a
arenotsignificantly
K(mgL-1) 0.36c 0.80b 0.95a 1.18a
differentfromeachotherat
P\0.05(n=24for Ca(mgL-1) 7.23a 5.55b 5.48b 7.16a
CPOCd13C,n=4for Mg(mgL-1) 3.01a 2.04c 1.61d 2.54b
DOCd13C,andn=12for Na(mgL-1) 2.99c 3.70c 4.65b 5.61a
allothers)
followed the same trend as the sediment but with a ThetotalCexportsincreasedafterforestconversion
significantly greater magnitude of enrichment in the andwithsubsequentcultivation(Table 3).TheDOCand
heavierisotopefromtheforestandrecentconversion CPOC export doubled in the first 5 years following
tothe10-yearoldconversion.Dischargeinthe50-year forestconversion.Theincreasecontinuedbyafurther43
conversionhadasignificantlyhighervaluethanthatof and 70 % from the 5- to the 50-year conversions,
allotherheadwaters(-22.75 %).Theflow-weighted respectively. Amounts of CPON, TDN and NO -
3
averageconcentrationsofDOC,CPOCandCPONdid exported were also twice the first 5 years after forest
not change with forest conversion and duration of conversion. Upon longer cultivation between 5 and
cultivation (Table 2). TDN concentrations in the 10 years, CPON export increased 3.5 times, while the
discharge from forest and 5-year catchments did not TDNandNO -exportincreasedeightfold.Between10
3
differ, but increased with longer cultivation. NO - and 50 years of continuous cultivation, the amount of
3
was the dominant form of dissolved N (91 %) in the CPONexportdoubledwhilethatofTDNandNO -did
3
Kapchorwawatershedfluvialecosystem,followedby notincreasefurther.ThemagnitudeofTDPexportwas
DON (9 %), whereas NH ? was below our detection farlessthanthatofCandallothernutrients.Theamounts
4
limit.The10-and50-yearconversionshadninetimes ofTDPexporteddidnotchangeafterforestconversion
higherTDNconcentrationscomparedtotheforestand (\0.03 kg ha-1), but increased with subsequent culti-
most recent conversion. Stream TDP concentrations vationup to the0.98 kg ha-1. The cations(K, Ca Mg
wereatleastoneorderofmagnitudelowercompared andNa)exhibitedasimilartrendwherebytheamounts
to TDN, Ca, K, and Mg. TDP concentrations of the exported increased from the forest to the 5-, 10- and
forest, 5- and 10-year conversions were threefold 50-yearconversions.Amongthemeasuredbasecations,
lowerthanthatofthe50-yearconversion(P\0.05). KhadthelowestexportwhileCahadthehighest.
StreamwaterKandNaconcentrationsbothincreased Therewasnotaclearseparationofthehydrograph
upon forest conversion and subsequent cultivation. between the long rainy (April-June) and short rainy
Overall,KconcentrationsincreasedthreefoldandNa (August-October) seasons in the Kapchorwa catch-
concentrationstwofoldfollowing50 yearsofcultiva- ment in2008. The stream flow response toprecipita-
tion after forest conversion. In contrast to all other tioneventsincreasedwithlongercultivation(Fig. 3).
solutes, Ca and Mg concentrations did not show a Therewerenorelevantchangesintheconcentrations
consistenttrendfromforesttocultivation. of DOC, TDN, TDP, Ca, and Mg across the whole
123
NutrCyclAgroecosyst
Table3 Streamwater carbon and nutrient exports (kgha-1 fluvial C export is low compared to other studies.
year-1)fromtheKapchorwaheadwatercatchments Mooreetal.(2010)observedafluvialtotalorganicC
removal of 12 % by a Bornean blackwater river in a
Forest 5year 10year 50year
conversion conversion conversion peat-coveredarea ofIndonesia.CumulativefluvialN
lossesoverthefirst50 years(usinglinearinterpolation
DOC 3.87 6.85 8.46 9.79
of the sum of TDN and CPON) were 1.3 t ha-1
CPOC 3.94 6.60 8.57 10.63
compared to the total soil N losses of 6.3 t ha-1
CPON 0.33 0.64 0.87 1.03
(representing21 %)inthefirst0.1 m(calculatedfrom
TDN 1.45 3.12 27.76 29.93
Table 1).SimilartofluvialClosses,watershedsatour
NO - 1.18 2.68 27.09 29.16
3 siteshowedlowerexportthanatotherlocations.
DON 0.27 0.44 0.67 0.77
Both fluvial CPOC and DOC were mainly mobi-
TDP 0.09 0.03 0.29 0.98 lized from the topsoil, as d13C values in stream
K 1.06 3.68 5.44 7.61
discharge were lower than in the topsoil and soil
Ca 21.4 25.7 31.5 46.2
valuesincreasedwithdepthasalsoreportedbyKrull
Mg 8.93 9.42 9.23 16.38 etal.(2002)atanearbysite.Similarconclusionswere
Na 8.86 17.08 26.70 36.22 drawnbyFranketal.(2000)andRaymondandSaiers
(2010) who found most of the DOC losses to be
derived from the topsoil in catchments in central
year.TheKandNaconcentrations,however,decreased
Switzerland and eastern United States, respectively.
slightly after the long rainy season in August and
Alternatively, the low d13C values in our study may
remainedlowfortheshortrainyseason.Overall,there
alsoindicatethatOClossesoriginatedfromolderSOC
was no relationship between the solute concentrations
fractions characterized by small size and association
anddischarge(Fig. 4).
with minerals that showed lower d13C values in the
same chronosequence (Kinyangi 2008). The isoto-
Discussion piccompositionofthetopsoilcorrelatedstronglywith
the DOC d13C (r2 = 0.84; y = 1.132x ? 7.1478)
SourcesoffluvialOCandnutrientlosses as well as with the CPOC d13C (r2 = 0.83; y =
18.25x - 49.523),butwithalowslopeforCPOC.
EventhoughannualfluvialOCexportsincreasedmore
thanthree-fold(ourstudy)andCO evasionfromSOC Nutrientlosses,cropproductionandstream
2
mineralization decreased (Kimetu and Lehmann pollution
2010),annualDOCandCPOClossesbystreamwater
of22 kg ha-1fromthehighlydegradedsoilswerestill TotalN(TDN ? CPON)lossesof31 kg ha-1year-1
low with only 2 % mineralization losses of the inthehighlydegradedsoilswererelevantcomparedto
1,200 kg ha-1 determined by direct measurements crop N uptake of maize at the same sites with
from the soil surface using static vented chambers. 90 kg N ha-1 year-1 (Kimetu et al. 2008). There
Similarly, cumulative fluvial organic C losses were wasnofertilizerapplicationintherecentconversion,
0.9 t ha-1 (2 %) over the first 50 years (using linear whileapplicationsinthe10-and50-yearconversions
interpolation of the sum of DOC and CPOC) com- were about 40 kg N ha-1 (average from all farms in
pared to SOC losses of 54 t ha-1 in the first 0.1 m thestudiedcatchments).Thetypicalapplicationrates
(calculatedfromTable 1;similarSOClossesreported to maize by most farmers in the entire region are
bySolomonetal.2007;KimetuandLehmann2010). 20 kg N ha-1 (Ngoze 2008), less than the stream
It is possible that inorganic C and particularly waterlosses reportedhere.Evenmoredramatic were
dissolved CO may significantly contribute to total the fluvial Ca and Mg losses of 46 and 16 kg ha-1
2
fluvial C losses from watersheds as shown in the year-1, respectively, which were greater than uptake
Amazon(Johnsonetal.2008).However,evenwitha by a single maize crop of 20 and 10 kg ha-1,
possibly larger inorganic than organic C loss, micro- respectively (Kimetu et al. 2008). In contrast, annual
bialmineralizationfromthesoilstillexceeds90 %of fluvial P losses of 1 kg ha-1 were low compared to
total SOC losses over the 50 years. The quantified plant uptake of 10 kg ha-1 (Kimetu et al. 2008) and
123
NutrCyclAgroecosyst
8 8
Forest 5 year
4 4
10
-1mg L) 6 3 -1day) -1mg L) 6 3-1day)
Concentration ( 24 12 Discharge (mm Concentration ( 24 12 Discharge (mm
0 0 0 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Day Day
8 8
10 year 50 year
4 4
-1) 6 1) -1) 6 1)
L - L -
mg 3 day mg 3 day
Concentration ( 24 12 Disharge (mm Concentration ( 24 12 Discharge (mm
0 0 0 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Day Day
DOC TDP TDN Calcium Potassium Magnesium Sodium Discharge
Fig.3 Bi-weeklydischargeandconcentrationsofDOC,Na,Ca,KandMgoftheKapchorwaheadwatercatchments
8 the watershed was not measured and stream water
losses are only a lower boundary of actual losses of
-1)L 6 soilP.Nonetheless,bothCaandMgaswellasPlosses
g by stream water may not be as serious as N losses
m
s ( 4 because available P, Ca, and Mg in soil decreased
n
o muchlessthantotalN(Table 1;similarforthesame
ati
ntr 2 sites reported by Ngoze et al. 2008; Kinyangi 2008),
e and crops were found to respond less to P than to N
c
n
o additions (Ngoze et al. 2008). In addition, TDN
C 0
concentrations and total losses by stream discharge
increased the most by forest conversion and contin-
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
uous cultivation compared to any other nutrient
Discharge (mm day-1 ha-1) studiedhere.
DOC Calcium Magnesium Despite the significant N losses from stream
TDN Potassium Sodium
discharge for agricultural productivity, the observed
TDP
concentrations in the studied headwater catchments
Fig.4 TherelationshipbetweendischargeandDOC,Naand are not an environmental concern for aquatic envi-
nutrientconcentrationsinthe50yearwatershed
ronments. Average annual N concentrations at or
fertilizationofabout7.5 kg P ha-1(sumofinorganic below 3 mg L-1 and maximum concentrations of
and organic fertilizers averaged for all farms) in the 6 mg L-1 during the dry season of December to
oldestconversion.However,redistributionofPwithin March (Fig. 2) do not pose a risk for human health
123
NutrCyclAgroecosyst
(Rubio-Arias et al. 2010) and are not expected to The increase in streamwater concentrations of
cause eutrophication (Hill et al. 2011; King and TDN, NO - and DON with longer duration of
3
Balogh 2011). Similarly, P concentrations of cultivation can be attributed to three sources. (1)
0.01–0.15 mg P L-1 (Fig. 2) in the stream discharge MineralizationofSOMledtoaccumulationofmineral
are moderate but not a cause for significant environ- N and especially nitrate (as seen from stream water
mental concern (Auer et al. 1986; King and Balogh concentrations) which is weakly adsorbed to soil
2011;Yuetal.2011).Valuesupto6.7 mgPO -PL-1 mineralsandeasilyleached.(2)Thegreaternumberof
4
havebeenfoundintheRiverThamescatchmentinthe households on older conversions presumably led to
UK(Youngetal.1999)andupto10 mgPO -PL-1in higher inputs of farm manure from the cattle kept by
4
the Spree River in Germany (Lewandowski and the farmers. The manure was shown to consist of
Nutzmann 2010). Also DOC concentrations are sig- 22 g N kg-1(Kimetuetal.2008).Halfofthefarmers
nificantlylower inthestudiedheadwatersthaninthe in the older conversions applied inorganic fertilizer
lower Yala and Lake Victoria (Lalah and Wandiga equivalentto40 kg N ha-1.(3)Asstatedabove,only
2006).Therefore,thewelldocumentedcontamination farmers with fields that have been cultivated for
ofthelowerYalaRiver(Aloo2003)andLakeVictoria 10 yearsorlongerapplymineralfertilizers.Fieldsthat
withDOC,NandPwhichhasbeensuspectedtocause havebeenrecentlyclearedatthesamesitesarerarely
the observed eutrophication (Scheren et al. 2000; fertilizedduetotheirhighNmineralizationratesand
LalahandWandiga2006;Heckyetal.2010)doesnot theirlowresponsetomineral(Ngozeetal.2008)and
seem to be caused by agriculture in the headwater organicNapplications(Kimetuetal.2008).
catchments,butmustoccurfurtherdownstream. The increase in fluvial K and Na concentrations
with longer cultivation may be explained by lower
Carbonandnutrientlossesafterlandusechange SOC contents and the soil mineralogy being domi-
nated by kaolinite, both resulting in low cation
Our DOC concentrations and total losses were in the retention (Kimetu and Lehmann 2010). Krull et al.
same range as reported by Lesack et al. (1984) from (2002) reported mineralogy dominated by quartz,
theGambiaRiverintheWestAfricansavannahwitha kaolin and mica, with minor components associated
DOC concentration of 1.98 mg L-1 and DOC export with feldspars and oxides in the adjacent Kakamega
of 2.67 kg ha-1 year-1. Similar DOC concentrations part of the forest. They observed an abundance of
but 2–10 times greater export were found by Cairns quartz, kaolinite, muscovite and microcline with a
etal.(2009)intheforestsoftheOregonCascadesand slight increase in goethite with increasing soil depth.
of southern Amazonia by Johnson et al. (2006b), yet The observed greater proportion of surface runoff of
values are within the range reported for a variety of 4–10 %ofrainfallfromforestcomparedtolong-term
watershedselsewhere(Lesacketal.1984;Selvaetal. cultivatedfields(Rechaetal.2012)mayalsohaveled
2007; Stutter et al. 2008; Waldron et al. 2009). The toagreatermobilizationofKandNanearthesurface.
CPOC and CPON concentrations and export were in Extractable K concentrations did not decrease with
the same order of magnitude as those of DOC and forest conversion and cultivation (Table 1; compare
DON. This finding differs from that of Vidal-Abarca withKinyangi2008).SimilartoN,agreaternumberof
etal.(2001)whofoundDOCtobethemostimportant households in the older conversion watersheds may
fraction (98 %) of organic C flowing in a saline and haveaddedmorekitchenwastethatarerichinKand
semi-arid stream in Spain. Similarly greater propor- Na such as ash from firewood cookstoves and farm
tions of DOC were also reported by Johnson et al. yard manure. Kimetu et al. (2008) documented
(2006a, b) from southern Amazonia. Therefore, par- contentsof23.2 g K kg-1inthemanure.
ticulateOClossesaregreaterthaninotherstudies,but The lack of change or even slight decrease in Ca
theobservedfluvialOClossesfromtheforestseemto andMgstreamwaterconcentrationsmaybearesultof
berepresentativeofthosefoundinotherstudies.The adominanceofweatheringastheprimarysourceofCa
examined change of solute losses as a response to and,toacertainextent,Mg.Inaddition,thesignificant
landuse change may therefore be indicative of losses decrease in extractable Ca in the topsoil (Table 1) in
inotherregions. combination with the rather decreasing stream water
123