Humic substances of surface waters : Litter and soil throughflow relationships in two forested ecosystems

Analytrcu Chrmrcu Acta, 232 (1990) 3-10

Elsevter Science Publishers B.V , Amsterdam - Pnnted m The Netherlands

Humic substances of surface waters

Litter and soil throughflow relationships in two forested ecosystems

H A. ANDERSON *, A. HEPBURN, J.D MILLER, M STEWART, R C FERRIER and T.A B WALKER _^_ ,^

ikxslon ofsbris and soli MeiuOhiOgy, liiihcauiay Lund Use Reseurcit hstrtute, Crutgtebucicier, Aberdeen A~g9 &y(oreut Brrtuinj

(Received 12th November 1989)

ABSTRACT

Hurmc substances were isolated from the vegetatton throughfall, soil dramage waters and adJacent stream waters at

two forested sites m the Centrai Region of Scotland- Fiacttonatton on XAD-8 hydrophobic resin gave three humic

substance components and one hydrophthc acid fraction Norway spruce generally gave 2-3 times more dissolved

orgamc carbon in the litter drainage compared- with Sitka spruce, aiihough there was iittie quahtative difference m

organic acidity between the two htterflows. The humic substances m the dramage at ca 1 m depth (BC honzon)

showed considerable differences between sites, reflectmg the types of sods and sources of sod water. Phenohc acids

released on hydrolysis confirm the differences between sites and may indicate the sources of subsotl water and then

relationshtps with the stream outputs

Research on humic substances, of either ter- restrial or aquatic origin, has been dominated by attempts to characterize them chemically [l]. This has inevitably led to greater emphasis being placed on soil humus, gtven the relative ease of its isolation in amounts sufficient for a wide range of experimentation. While there remains considerable debate as to the similarities between soil and aquatic humus [2-51, the widespread adoption of isolation and fractionation procedures centred on the use of macroporous, non-ionic resins [6] should remove the future possibility of differences between data sets being created by the use of dissim- ilar extraction and fractionation techniques.

Improvements in analytical instrumentation have led to rapid advances in non-degradative approaches to attack structural problems in humic substance research [7]. However, degradative techniques usually have the advantage of allowing specific changes in composition to be followed

with time, or at various points within an ecosystem, and at levels of detail not otherwise available. Among degradative techniques, acid hydrolysis has

previously been used to study both the vegetable origin and the degrees of hurmfication of soil humic substances [8]. In tl-ns study, acid hydrolysis was used to quantify differences between humic substances isolated from water flowing through the litter layers and the subsoil BC horizons in two forested ecosystems during October 1985 to May 1987, and to compare these with then counter- parts in the adjacent streams. The objective was to quantify the organic acidity in the throughflows and to draw conclusions, where possible, concem- ing the source of the organic acidity.

Most attempts to detail soil organic acidity cycling have dwelt on soil reactions per se, without

reference to the “real-world” scenario of plants, soils and water exlstmg in a dynamic combmation. This failure is understandable, given the degrees

H A ANDERSON ET AL

of experimental difficulty mvolved; this study at- tempted to address this problem.

EXPERIMENTAL

Site characteristics As part of the Surface Water Acidification Pro-

gramme (SWAP), a series of experimental sites was established by this Institute (MLURI) to study the processes involved during the passage of pre- cipitation through vegetation and soils into streams. Two of these sites were established at the Loch Ard Forest, Central Scotland, in upland catchments receiving the same current levels of pollution inputs and with similar deposition patterns and trees of a similar age (35-40 yr), but one (Chon) being capable of sustaining a fish popula- tion, while the other (Kelty) was fishless. Follow- ing a detailed soil survey of both catchments [9,10], replicated hillslope plots were established on the dominant vegetation/soil combinations at each site, at Chon this being Norway spruce on a well drained humus-iron podzol, and at Kelty Sitka spruce on a peaty gley soil. The plots were instru- mented to allow the sampling of inputs concur- rently with vegetation and soil throughputs and associated stream waters (Table 1). The sites, their instrumentation and their hydrology and hydro- chemistry for the study period 1985-87 are dis- cussed in greater detail elsewhere [ll-131.

Sampling strategy In order to isolate sufficient dissolved organic

components for analysis, 20 1 aliquots of water were usually collected. Samples, especially those from vegetation and litter layers, were prone to biologically induced changes if they were allowed to stand for long periods, e.g., 7-14 days. Al- though the total dissolved orgamc carbon content was relatively unaffected, the distribution of humic and non-humic substances changed drastically (see below). This can be overcome by using in situ fractionation techniques, but for this study 20 1 samples were taken during ramstorm events. The holistic approach to site instrumentation adopted for the MLURI/SWAP sites allowed the “nor- mality” of such samples to be judged in the con-

TABLE 1

Samphng pomts wlthm the expenmental plots at Chon and

Kelty

(Each samphng pomt termmated m a tlppmg-bucket gauge

[poly(methyl methacrylate)] connected either to a composite

collector or to a 24-positlon event sampler [ll]. The tippmg-

bucket gauges were also connected to a data-loggmg system to

give throughput volumes. Sod temperatures and carbon dl-

oxide concentrations were also momtored [12])

Inputs Bulk preclpltatlon and mterceptlon

Vegetation Throughfall, stemflow and htterflow

SOllS Chon Kelty

Surface Surface

0 0

E Ahg Bs BCg BC

(Collectors below honzons except for Bs, BC

and BCg, where they were wlthm the honzon

depth as described m the sod survey)

outputs Streams

text of the detailed hydrological and hydrochemical responses for each site. This is especially important for event samples, where rapid chermcal changes, including dissolved organic carbon (DOC), occur throughout the hydrograph [14,15]. In this mstance, the 20 1 samples collected were a half “cut” of the litterflow, with the remaining part going to the event system, and similarly one third of the BC horizon flow, from the litter trays and lysimeter gutters, respectively.

Fractionation and isolation methods Water samples were analysed for a full range of

variables [13], including DOC, the latter by a UV-persulphate method. The 20 1 water samples were clarified by throughflow centrifugation (De Lava1 Gyro Tester centrifuge), filtered and acidified to pH 2.0 with redistilled 6 M hydrochloric acid before fractionation on XAD-8 resin into a sorbed humic component and an unretained hydrophilic acid fraction [16]. The solid hydrophilic acid was recovered by repeated flash- evaporation at < 35” C, adding deionized water until the hydrochloric acid had been removed, followed by freeze-drying. Back-elution of the resin column with 0.5 M sodium hydroxide solution and acidification of the coloured eluate to pH 1 with

HIJMIC SUBSTANCES OF SURFACE WATERS

redistilled 6 M hydrochloric acid allowed the sep- aration by centnfugation of any precipitated humic acid from the soluble fulvic acid, which was then

re-adsorbed on the XAD-8 resin. Unacceptable losses of DOC occurred in re-

moving chloride ion from the organics by back- elution of the resin column with deionized water at this stage. Accordingly, a DOC-exhaustive water back-elution was carried out prior to the normal back-elutton of the sodium fulvate with dilute

sodium hydroxide solution and recovery of the fulvic acid by treatment of the eluate with proton- ated cation-exchange resin [16]. Hydrochloric acid in the water eluate was removed by repeated evaporation, as for the hydrophilic acid. All components were recovered by freeze-drying. Hence, each water sample gave a maximum of four products, viz., humic, fulvic and hydrophilic acids and the water-soluble fulvrc sub-fraction, referred to hereafter as HA, FA, Hydro and H,O-sol, respectively. As will be seen below, the combined results for the fulvic and water-soluble fractions can gen-

erate “normal” fulvic acid data, where desired. Ash contents were determined after combustion

at 650° C for 16 h. Elemental analyses were per- formed on a Carlo Erba 1106 CHN analyser, and carboxylic and phenolic acidity were determined by titration [17].

RESULTS AND DISCUSSION

Hydrologwal and carbon budgets As can be seen in Table 2, the average rainfall

during the period was similar for both sites. Not shown here are the data indicating that Kelty has a greater input from mists and other aerosols [12]. The most obvious difference between the sites lies in the volumes of water passing through the soils. As might be expected when comparing a peaty gley soil with a freely drained podzol, Kelty is generally wetter than Chon, but both hillslope sites have similar general hydrology, i.e., the soils “wet-up” literally. At the start of soil drainage flow, water is seen first at the base of the profiles in both soils and, with increasing inputs, the wetting front moves upwards. Thts mdtcates a rapid Nerttcal drainage in both soils, probably by prefer-

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TABLE 2

Seasonal amounts of water through the sites durmg the study

penod

[Ramfall, throughfall and htterflow are expressed as mm mput (=l mm2 ), wlule sod water flows are as 1 m-’ contour;

np = honzon not present Wmter = Nov -Apnl mcluslve,

summer = May-Ott ]

Chon Kelty

Summer Winter Summer Winter

Ramfall 680 1313 782 1330

Throughfall 487 769 567 750

Litterflow 172 204 198 168

( Surface <l 2 24 18

so11 Orgamc 2 28 1500 2200

Water E 18 38 np np BS or Bn 1000 2700 16800 25 600

\ BC or B?g 10600 28200 10200 22000

ential flow paths, with lirmted contact times between the downward-moving water and the soil mass [18]. Detailed hydrological monitoring at Chon has shown that a wetting front moves down-

slope within the BC horizon, with a complex inter-relationship to the surface layer wetting [19].

It is inherently difficult to translate the rainfall and vegetation/litter throughputs, measured as mm inputs, mto the same units as the soil

throughflows, which are expressed as the outputs from 1 m soil face contours (for details of zero tension lysimeters, see [ll]). Whereas the former imply simple area measurements, the latter are representative of serial integrations of inputs upslope of the sampling points. However, attenua- tion of sampling frequency may help to resolve this problem by effectively allowing a database of soil responses to be built up. This study with seasonal differentiation represents the first at- tempt to study the inter-relationships of soil and soil-water humic substance dynamics in the field.

Organic matter in water, normally estimated as the aggregates in dissolved (DOC) and particulate (POC) organic carbon, is readily fractionated into operationally defined humic and non-humic sub-

stances. As can be seen in Table 3, there are major site, seasonal and fractional differences between Chon and Kelty throughputs. Carbon release mto

solution under Norway spruce during the summer period at Chon was twice that of the correspond-

6 HA ANDERSON ET AL

TABLE 3

Seasonal dtstnbutton of orgamc carbon and htumc substance fracttons for the study period

(Values are as mg C 1-t for each fractron, wtth the volume-weighted throughputs in parentheses. For htterflows, the latter are

expressed as g C m-‘, whrle the BC flows are as g C m-r contour [12,13])

Flow Parameter Chon Kelty

Summer Wmter Summer Wmter

Lttterflow H so-sol 9 (1 55) 4 (0.82) 2 (0.40) 2 (1.50) FA 6 (1.03) 8 (1.63) 3 (0.59) 3 (2 25) Hydro 12 (2.06) 1 4 (0 29) 6 (1.19) 1 (0 75)

BC flow HaO-sol 0 5 (5 30) 1.5 (42.3) 1 (10.2) 7 (154.0) FA 0.3 (3.18) 0 6 (16.9) 3 (30 6) 3 (66.0) Hydro 0 9 (9 54) 0 8 (22.6) 2.4 (24.5) 2 (44.0)

ing Kelty/Sitka combinatton, both in absolute concentration and in the volume-weighted throughputs. However, the winter period showed the opposite pattern, with a four-fold increase in

volume-weighted humic substance release at Kelty, but no similar seasonality at Chon. The greater release of humic substances during the winter period is repeated emphatically in the BC horizon

drainage at both sites. Reference has been made to the instability of

litterflow components on standing in the field. Table 4 shows the results of comparing bulk sample collection and storage in the field for 14 days and in situ fractionation of water. Details of pro- cedure and further results will be published elsewhere, but it is evident that, at Chon, humic

acid IS formed on standing, mainly at the expense of the hydrophilic acid fraction. A similar reduction in the hydrophilic acid fraction at Kelty coincides with an increase in the H,O-sol fraction.

This site-dependent difference may be a species-

related effect, and it is equally possible that the in situ fractionation system may not be free from similar changes in mixture composition.

Organic acidity budgets Organic acidity is often cited as a panacea for

cation-anion imbalance in hydrochemical budgets. However, few examples exist where direct measurements have been made of actual or potential anionic activity.

Major differences in organic acidrty fluxes, both seasonal and site-derived, can be seen in Table 5. Qualitatively, the carboxyl and phenolic acidities are sunilar in both Chon and Kelty litterflow H,O-sol and FA fractions, but the increased carbon flux from the Norway spruce litter ensures

that a threefold augmentation of organic acidity IS seen at Chon when compared with that of the Kelty Sitka litter drainage.

The qualitative organic acidity of the BC hon-

zon flows show gross similarities between sites,

Changes m iitteriTow compostnon on standmg m the tieid, compared with an m situ fracttonatton technique .

(1, Composne over 14 days, 2, m srtu AG MP 50H + XAD-8 tandem resin columns, 1 and 2 samphng same dramage water Hunnc

substance fractrons as mg 1-l ash-free yrelds)

Sue Date Throughput (1) HA H,O-sol FA Hydro

TABLE 4

Chon November 1988 (1) 18.5 48 54 10 51

(2) 1.6 19 29 29 78

August 1989 (1) 39 66 77 34 95

(2) 3.4 34 41 79 138

Kelty August 1989 (1) 4.2 02 57 23 33

(2) 2.1 20 11 53 77

HUMIC SUBSTANCES OF SURFACE WATERS 7

TABLE 5

Seasonal dlstnbutlon of carboxyhc acid (COOH) and phenohc hydroxyl (WOH), 1.e , potential orgamc acldlty m the water-soluble

and fulvlc acid fractions, expressed as meq me2 (htterflow) and meq m-l (BC flow)

Flow Season Parameter Chon

H,O-sol FA

Kelty

H,O-sol FA

LItterflow Wmter

Summer

mg fraction l- ’ peq mg-’ COOH

PhOH

meq mm2 COOH

PhOH

mg fraction 1-l

ceq mg-’ COOH

PhOH

meq mm2 COOH

PhOH

BC flow Winter

Summer

mg fraction 1 - ’ peq mg-’ COOH

PhOH

meq mm’ COOH

PhOH

mg fraction 1 - ’ wq mg -’ COOH

PhOH

meq m-’ COOH

PhOH

Stream Wmter

Summer

mg fraction 1 - ’ peq rng-’ COOH

PhOH

Output 1 a COOH

PhOH

Output 2 a COOH

PhOH

mg fraction 1-l

peq mg-’ COOH

PhOH

Output 1 a COOH

PhOH

Output 2 a COOH

PhOH

13 25 5 9

6.6 3.6 69 5.2

7.0 4.0 67 5.9

18 18 6 8

19 20 6 9

19 12 4 8

15 5.6 77 60

1.6 4.9 6.1 5.1

25 12 6 10

25 10 5 8

5 3 17 6

5.1 9.1 74 65

2.1 7.4 27 36

804 170 2768 858

381 626 1010 475

1 1 2 I

5.9 89 II 53

20 78 81 87

63 94 157 378

21 83 165 621

3 5 9 3

8.6 4.9 41 38

3.9 13 69 98

28 26 40 13

13 7 61 32

0.68 0.65 107 0 99

0.31 0.18 1 80 2 55

4 2 8 9

9.1 5.1 78 64

2.9 3.4 15 48

18 5 32 36

6 4 31 21

0.51 0.14 0 87 0 98

0.16 0.10 0 84 0 73

a Outputs based on stream flow records as gauged by the Forth kver Purlflcatlon Board Output 1 = meq me2 based on runoff

(total outflow equivalent 1707 mm Kelty, 1495 mm Chon) Output 2 = Meq per 6 months, based on annual volumetnc outputs

46 x lo8 1 at Kelty and 40 X lo8 1 at Chon

but with phenolic activity distributed more into the FA fractions. This broad pattern is not carried mto the streams, with only the Kelty winter stream chemistry showing a concentration of phenohc

activity in the FA fraction. The throughputs and outputs of organic acidity

are greater than required to offset any imbalance m the cation-anion budgets as shown elsewhere

[13], but not all of the potential acid functional groups will be neutralized by base cations, leaving a considerable proportion of organic acidity in

these highly organic surface waters. The organic acidity can perversely act almost m an amphoteric

form when reactions occur between the humic substances and alummium, since the formation of organoaluminium complexes reduces the available

8 HA ANDERSON ET AL

TABLE 6

Phenohc acids released from water hurmc substances on acid hydrolysis

(Unless m&cated othemse, values are as pg mg-’ ash-free fraction The “bulk” values are composites of H,O-sol and FA analyses,

1.e , those expected from a normal XAD-8 FA, 1.e , without the HzO-sol sub-fractlonatlon, as m [6], and are composite values

4HBA = 4-hydroxybenzolc, VAN = 4-hydroxy-3-methoxybenzolc, 3,4DHBA = 3,4-dtiydroxybenzolc acid, respectively RI =

VAN. 4HBA; R, = 3,4DHBA : VAN + 4HBA)

Site Flow Season Parameter 4HBA VAN 3,4DBHA Sum of RI R2

phenohcs

Chon Litterflow Winter

BC flow Winter

Stream Winter

Kelty Litterflow Wmter

BC flow Winter

Summer

Summer

Summer

Winter

H,O-sol

FA

Bulk FA

H ,O-sol

FA

Bulk FA

HA

H,O-sol

FA

Bulk FA

H,O-sol

FA

Bulk FA

H,O-sol

FA

Bulk FA

H ,O-sol

FA

Bulk FA

H,O-sol

FA

Bulk FA

H,O-sol

FA

Bulk FA

H ,O-sol

FA

Bulk FA

H,O-sol

FA

Bulk FA

0 98 2 96 2 07

0.78 3.09 2 38

0 62 6.93 171

0 06 0.07 127

0 39 126 108

0.38 2 76 0.46

0.21 1.40

0.23 040

0 30

0.90

0 12 0.24 0 31

0 28 0.73

0 54

162

0.29 0.69

18 40 02

80 29 06

60 3.0 05

5.5 2.7 0.7

7.5 70 06

63 40 06

93 112 02

06 12 11

2.2 -1 >2d

14 12 98

19 23 11

3.6 3.6 05

2.7 3.2 07

2.9 5.9 0.1

4.8 98 0.2

3.6 7.3 0.2

2.2 5.9 0.5

1.7 7.1 0 002

19 67 02

15 16 16

1.5 2.1 10

1.5 1.7 1.4

0.7 1.1 0.8

07 25 09

07 20 09

16 20 03

3.9 29 21

26 26 16

16 21 1.3

14 17 0.3

15 19 0.8

a Chon BC FA had sumlar trace amounts (< 0.005 gg mg-‘) of 4HBA and VAN

organic acidity at the same time as removing monomeric alumimum from solution. Similar reactions will occur with any polyvalent cations, and many of these can increase soil and water acidity by generating protons via hydrolysis reactions at the initial soil water pH.

Hydrolysis products and degree of humlfication Phenolic acids produced on acid hydrolysis of

the humic substance fractions are detailed in Ta- ble 6. These acids are assumed to arise from the

lignins of the parent plant materials [7], the acid hydrolysis conditions cleaving the simple struc- tures from the more complex humic molecules. This relates to the products identified after sodium amalgam reduction of humtcs [20] and the mix-

tures produced by alkaline hydrolysis of sedimen- tary humic matter [21], but in this instance the mixtures of acids produced from the acid hydrolysis are generally simpler than in these alternative methods. Identifying the source of the phenohc acids assumes (a) the derivation of 3,4-dihydroxy-

HUMIC SUBSTANCES OF SURFACE WATERS 9

benzolc acid either from 4-hydroxyphenyl residues by hydroxylation or from guaiacyl residues by demethylation and (b) that such transformations are representative not only of the route of the humification process from plant lignin but also of the degree of humification.

In the acid hydrolysates of most soil humic

substances, 3,4-dihydroxybenzoic acid (3,4DHBA) is the major phenolic product detected. In the present aquatic humic data, this is not the case, and this may be an illustration of the leachmg of humic substances at early stages in the humification cycle. The 1,2_dihydroxyphenyl structure 1s

particularly prone to both biotic and abiotic oxidatlve degradation [22], but the frequency of

occurrence of the dihydroxy acid in hydrolysates suggests that some protective reaction exists in sods and waters. In either case, this can be ascribed to complexing of metal cations by the rmg, giving products which could be antagonistic towards phenol-oxidizing systems.

Monocotyledonous plants, and the humic substances denved from them, give lignin degradation products dominated by 4-hydroxybenzoic acid

(4HBA) on acid hydrolysu, whereas vegetation associated with coniferous trees and heather moorland gves nse to 3-methoxy-4-hydroxybenzolc acid (VAN). Thus hydrolysates from humic substances formed under grasses usually contain these acids in the ratio VAN : 4HBA < 1.0, whereas the coniferous analogues have a ratlo > 2.0, the inter- mediate range of ratio deriving from deciduous sources. Likewise, the ratio 3,4DHBA : 4HBA + VAN can be used as an indicator of degree of humificatlon, with values < 0.66 indicating low humificatlon and > 1.0 being representative of well hurmfied organic matter. These assumptions were tested initially on vegetation, litter and soil organic fractions [7], but have subsequently been used with a wide range of soil-derived hurmc substances. This work 1s the first report of the apphcatlon of the technique to surface water humics.

In general, the FAs gave higher yields of pheno- lit acids than the H,O-sol fractions (Table 6). Litter-derived humics gave the highest yields, with

the Norway spruce producing between 50 and 400% more phenohcs than the Sitka, the latter

species also showing a significantly lower degree of humification, even though, as expected, all litter products were in the little humified category. The phenolic acids produced from the Chon litterflow humic acid are more abundant than those in the fulvic fractions, and confirm that the humic acid is the least well humified fraction of the litterflow humic substances. Both BC winter flows give

products in the highly humified category, with the Chon subsoil water being very distinctive m this respect. On entering the streams, the BC water is obviously diluted with litter-like inputs, remi-

niscent of the litter-derived inputs stressed by Likens et al. [23]. The Kelty stream has a closer relationship with the BC flow than that at Chon, and this is a similar conclusion to that drawn from the main hydrochemical studies [13]. As stressed above, the BC flows are necessarily aggregates of upslope processes, and it 1s tempting to speculate that, at Chon, the BC flow may be dominated by water derived from grassy moorland above the

forest, 300 m above the experimental site. This conclusion is adequately supported by the pheno- lit acid data from the winter period, with the VAN : 4HBA ratio approaching unity. However, both Chon and Kelty BC humics have a high degree of humification; this may indicate a fractionation of organic matter within the soil solution flow paths, or a longer residence time for the humic substances in the BC drainage. In view of the current interest m “old” and “new” soil water outputs, this aspect is under further study.

Conclwon

Seasonal differences have been established in the hurmc substance production and output from both forested sites. In the litter layers, there 1s a major difference between the production of humic substances under Norway and Sitka spruce, with Norway producing twice as much soluble carbon over the study period. The instability of the organic mixture eluted from the litters necessitated the use of spot sampling during rainstorm events, but these samples could be related to the annual mean budget chemistry for the sites. Potential organic acidity showed a definite seasonal pattern, with greater production of hurmc substances in the dramage waters during the winter penod.

10 HA ANDERSON ET AL

Acid hydrolysis of the isolated humic substances gave patterns of phenolic acids which demonstrated that, whereas the Kelty stream water bore a close resemblance to the soil BC horizon drainage, the subsoil water from Chon indicated a vegetation source some distance upslope, and a greater residence time for this water.

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D J Henderson and C G.B. Campbell, The North Chon

Catchment Area, Surface Water Actdiftcation Programme

Intenm Report, Macaulay Land Use Research Instttute,

Aberdeen, 1986.

G. Hudson and J.A Hipkm, Soil Survey of the Kelty Water

Catchment, Surface Water Actdificatton Programme In-

tenm Report, Macaulay Land Use Research Institute,

Aberdeen, 1986

J.D. Miller, A.W. Stuart and G.J. Gaskm, Bnttsh Geomor-

phologcal Research Group, Tech Bull., 37 (1990) in press.

J D. Miller, H.A Anderson, R C Ferrier and T.A.B.

Walker, Forestry, (1990) m press.

J.D Miller, H.A. Anderson, R.C. Ferner and T.A B

Walker, Forestry, (1990) m press

A C Edwards and M S. Cresser, Water Res , 21 (1987) 49.

G. Jacks, E. Olofsson and G Werme, Ambto, 15 (1986)

282.

The authors acknowledge the financial assistance from the Surface Water Acidification Pro- gramme in this study. They would also thank the Forestry Commission for permission to use the sites and for their assistance during the project.

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Documents

Humic substances of surface waters : Litter and soil throughflow relationships in two forested ecosystems