Scientific paper

Regeneration Procedures ofPinus radiata in theSouthern Cape Province

Part III: Changes in Organic Matter Loads inResponse to the Experimental Treatments

J.B. Zwolinskil , D.G.M. Donald- and A. van Laar"

1 M. White Smith Hall, School ofForestry, Auburn University, Auburn, Al 36849-5418 USA2 Forestry Faculty, University ofStellenbosch, Private Bag X5018, 7599 Stellenbosch.

SYNOPSIS

The present paper is part 3 in the series ofpublications dealing with the regeneration ofPinus radiata inthe Southern Cape Province. The objective of the study was to compare the organic carbon contents in thesoils and the surface organic matter loads among the experimental treatments, before and afterapplication, and one year after plantingand to estimate the total organic matter content in different above­and below-ground strata. The paper discusses the methods being used and evaluates the results.

Keywords: site preparation, organic matter.

INTRODUCTION

The importance ofhumus and of an ample supply oforganic matter as an indicator of good "radiata sites"(Pinus radiate) has been accepted since the firstplantations were established in the southern Cape(Sherry, 1938; Poynton, 1979b). However, recent site­growth studies failed to support the correlation be­tween site index of the commercial pine species andorganic carbon content in the soils, despite the factthat soils with a wide range of organic carbon (0,2 %to 18,5 % in A horizon) were investigated (Grey,1989a and b; Schafer, 1988 a and b).

Soil physical and chemical properties are affectedby organic matter more than by any other singlefactor. Brady (1974) summarised the influence ofsoilorganic matter on soil properties as follows: 1)changessoil colour to brown or black, 2) ameliorates soilphysical properties by improving granulation, reduc­ing plasticity, and increasing water-holding capac­ity, 3) increases cation adsorption capacity by two to30 times compared to mineral colloids, 4) improvessupply and availability of nutrients by providingtheir organic forms and by extraction of elementsfrom minerals exposed to acid humus.

Plant tissue is the original source of soil organicmaterial accumulate on the ground surface (foliage,

* To whom the correspondence should be addressed.

branches and stumps) and in the soil (roots). As thesematerials are decomposed and digested by micro­organisms, they are incorporated into underlyinghorizons by infiltration or through physicaltranslocation by soil fauna. The quantity and qualityof plant material and the rate of its decompositionaffect organic matter contents in the soils. Tillage isknown to have an effect on soil organic matter con­tent and rate ofmineralisation through translocationof plant material from the ground surface to themineral soil, disruption ofaggregates and increase inaeration.

The aim of this study was 1) to compare organiccarbon contents in the soils and the surface organicmatter (SOM) loads among the experimental treat­ments before and after application, and one yearafter planting, and 2) to estimate total organic mattercontent in different above- and below-ground strata.

METHODS

Post-harvest slash (stems and branches thicker than20 mm) was uniformly dispersed over the experimen­tal area at Blueliliesbush. Mass of dry slash wasassessed before soil cultivation and one year afterplanting. The amount reported "after" application oftreatments was not assessed but is presented as themean of the two values. The mean of four randomplots (each o£1OO m-) was taken as representative forall treatments before site preparation. Slash was

Suid-Afrikaanse Bosboutydskrif- nr. 167, Desember 1993 21

removed from plots prepared for ripping and plough­ing. Therefore no slash was recorded in these plotsone year after planting. At Kruisfontein above-groundbiomass of trees was removed after felling and noslash remained in the experimental area.

Five random samples of SOM (any dead organicmatter thinner than 20 mm found above the mineralsoil consisting oflitter, fermented litter and humus)were drawn in each subplot, bulked and dried toconstant mass. Organic carbon was determined bytreating the soil with a hot mixture of potassiumdichromate and sulphuric acid. Excess dichromatewas backtitrated with iron ammonium sulphatehexydrate. Reduced dichromate was assumed to beequivalent to organic carbon in the sample (Walkleyand Black, 1934). Total organic matterwas estimatedby multiplying the figures for organic carbon by theconventional "Van Bemmelen factor" ofl,724 (Allison,1965). Organic matter calculated by this method isprobably substantially underestimated because 1)only oxidisable carbon is detected with the Walkley­Black method, and 2) the conversion factor is re­ported to be much higher, especially for subsoils(Allison, 1965). The method is, however, applied as astandard procedure and for this reason was used inthis study.

The amount of organic matter in the soil wascalculated for an area of 1 ha, assuming that samplesfrom specific soil depths are representative of adja­cent soil layers of proportional thickness (i.e. 50-100mm for 0-150 mm, 200-250 mm for 150-300 mm,

350-400 mm for 300-450 mm soil strata at Blue­liliesbush, and 50-100 mm for 0--175 mm, 250-300mm for 175-350 mm soil strata at Kruisfontein). Thevalues of soil bulk density used in these calculationswere reported by Zwolinski (1992).

The data were subjected to an analysis ofvariance,the means between specific treatment levels beingtested with Tukey's HSD test. Repeated measuresanalysis of variance was applied to compare themeans derived from repeated measurements. Moredetailed informationregardingthe experiments, sam­pling and analysis is presented by Zwolinski et al.(1993).

RESULTS AND DISCUSSION

In the experiment at Blueliliesbush the oven-dryslash amounted to 37,40 Mg/ha after harvesting.Eighteen months later only 29,42 Mg/ha ofthe slashremained. The succession oftimber decomposers wasinitiated by insects feeding between bark and wood.The most common species were Orthotomicus erosus(Scolytidae) , usually found under the thicker bark,andPissodes nemorensis (Curculionidae), more com­mon on stem tops and thicker branches. Activity ofthese insects resulted in the loosening and peeling offof the bark. In the decomposition of wood the firststage was the cerambycoidal stage, with Delochilusprionides (Cerambycidae) being the most commonspecies. Occasionally the timber was invaded by ants,and then a formicoidal stage could be distinguished.Fruiting bodies of Pycnoporus cinnabarinus

TABLE 1. Mass ofsurface organic matter at Blueliliesbush and Kruisfontein before sitepreparation, atplanting,and one year after planting!

Location Treatment Mass of litter (Mg/ha)

Before At planting One year latercultivation

C12 Soil cultivation:augering 73,923 10,2IB2 4,71 c1

disking 74,333 1,78A2 0,85A1

pitting 79,73 3 9,66B2 5,19c1

ripping 58,963 8,47B2 2,94B1

Weed control:standard 73,27 3 7,382 3,031

total 68,713 7,692 3,821

B2 Soil cultivation:bedding 10,303 3,691 5,622

mounding 9,56 3 3,491 5,522

pitting 7,83 4,33 5,82

Phosphogypsum:0 Mg/ha 8,8IS 3,221 5,112

12 Mg/ha 9,65 3 4,451 6,202

1 Significant differences (p<O,05) between the treatment means are indicated with letter indexes while number indexes indicatesignificant (p<O,05) differences between the assessment means.

22 South African Forestry Journal- No. 167 December 1993

(Polyporaceae) were found frequently on the decom­posing wood.

Results of SOM surveys are presented in Table 1for the experiments at Blueliliesbush and Kruis­fontein.

In the study area at Blueliliesbush, SOM loadswere estimated at 27,67 Mg/ha in a mature stand ofPinus pinaster (De Ronde, 1992). After harvestingthe amount of SOM increased on average by 159 %.This increase resulted from incorporation of foliage,cones, thin branches and bark ofthe felled trees on tothe forest floor. Decomposition of the SOM was veryfast. Within a six-months period the amount ofSOMin augered and pitted plots decreased on average to12,9 %. This reduction occurred in SOM undisturbedby the soil cultivation treatments (SOM in the plant­ing spots was incorporated into the mineral soil). Theamount of SOM in the ploughed plots decreased toonly 2,4 % as a result of pre-treatment removal ofslash, mechanical incorporation of organic matterinto the soil during ploughing and decomposition.After ploughing the amount of SOM recorded dif­fered significantly from the other treatments. Oneyear after planting the amounts ofSOM decreased to6,5 % for augering and pitting, 5,0 % for ripping and1,1 % in ploughed plots. In the ripped plots thedecomposition rate was slightly faster, probably be­cause weeds were suppressed by the bulldozer duringripping and a better environment for decompositionwas created. Differences between the weed controltreatment means were not significant. Unexpectedlythere was a tendency for a slower decomposition inplots which received total weed control. This couldresult from an additional litter load after weed con­trol operations and the impact of herbicides on theactivity of micro-organisms responsible for litter de­composition. Apothecia of Cyclaneusma minus andpycnidia of Sphaeropsis sapinea were common on

dead pine foliage in the experimental area. Thesepathogenic fungi are also recognised as saprophytesin South Africa (Crous et al., 1990), and they clearlycontributed to early decomposition of litter in theexperimental plots.

At Kruisfontein, much less SOM accumulatedunder the young stand ofP. pinaster." The amount ofSOM was reduced after treatment because it wasincorporated into the mineral soil in beds and mounds.There were no significant differences between thetreatment means. After 14 months the amount oflitter increased mainly because of dieback of theforest floor vegetation disturbed by the soil cultiva­tion treatments. A substantial error in this type ofassessment could result from confusing partiallydead plant material with litter.

Contents of organic carbon in the soils of theexperiments at Blueliliesbush and Kruisfontein areshown in Tables 2 and 3.

After harvesting the amount of organic carbon inthe A horizon in the experiment at Blueliliesbushexceeded the mean of 2,65 % reported for the regionby Schafer (1989a and b). The amplitude of changesin organic carbon, as a result of treatment applica­tion, was largest in the 50 to 100 mm depth, where asubstantial post-treatment increase was followed bya decrease in organic carbon content one year afterplanting. At 200 to 250 mm depth an initial moderateincrease was maintained after one year. The changesreported in shallower horizons resulted in higherlevels oforganic carbon at any stage compared to thepre-treatment level. However at the 350 to 400 mmdepth the content of organic carbon decreased ini­tially but still exceeded the pre-treatment levels oneyear after planting. It is suggested that this "delayed"increase in a deeper horizon results from the timeneeded for the organic matter to infiltrate from theupper horizons as well as a slower rate ofroot decom-

TABLE 2. Organic carbon content in the soils at Blueliliesbush before site preparation (before), at planting(after), and one year after planting (later)!

Treatment Depth: 50 . 100 mm Depth: 200 • 250 mm Depth: 350 • 400 mm

Before Mter Later Before Mter Later Before Mter Later

Soilcultivation:augering 2,69 3,78 3,26 2,19 2,32 2,40 1,93 1,56 2,05disking 3,081 4,382 3,461 2,091 3,031 3,082 1,63 1 1,40 1 2,512pitting 2,64 3,72 2 1,441 2,461 2,00 1 1,21 1,07 1,77ripping 3,61 3,38 3,00 1,941 2,182 2,461 1,31 1,511 1,83

3,96 2,46 1 2,422

2,461

Weed control:standard 2,861 4,042 3,611 1,911 2,321 2,51 1,39 1 1,381 2,112total 3,18 3,61 2" 1,97 1 2,461 2,44 1,691 1,511 1,972

3,23 2,691

21 Significant differences (005) between the assesment means are indicated by number indexes,

Suid-Afrikaanse Bosboutydskrif- nr. 167, Desember 1993 23

cause 1) only oxidisable carbon was reported in thisstudy, and 2) mass of stumps and roots was notincluded in the assessment. Nevertheless, the in­crease in the soil organic matter corresponds well to

FIGURE 2. Estimated amount of organic matter atKruisfontien, above ground level (surface organicmatter (SOM)) and in the soil at depths: 0-175 mmand 175-350 mm, before cultivation (1), at planting(2), and one year after planting (3).

3

2 3

Pitting

~ 0-150 mm

Mounding

1 2 3

Assessment

2 3 1 2 3

Assessment

_ Slash

_ 300-450 mm

2 3

_ SOM

_ 150-300 mm

_ SOM ~ 0-175 mm _ 175-350 mm

3

Organic matler (Mg/ha)

60

30 ~ Bedding

3:~

Organic matter (Mg/hal

150

100

50

o50

100

150

200

250

300350 -'---,-,------.-----.----.--,--==;=--,--,---,-,---.------,----,-.--'

FIGURE 1. Estimated amount oforganic matter atBlueliliesbush, above ground level (slash and surfaceorganic matter (SOM)) and in the soil at depths: 0-150mm, 150-300 mm, and 300-450 mm, before cultivation(1), at planting (2), and one year after planting (3).

position in deeper soils. The differences between thetreatment means were not significant but in generaldisking followed by ripping yielded the highest levelsof organic carbon. Lower levels of organic carbonafter one year in plots receiving total weed controlresulted from the slower decomposition of organicmatter reported already for 80M.

The pre-treatment levels of organic carbon in theKruisfontein experiment were very low, but theyimproved as a result of the soil cultivation treat­ments. Erecting ofbeds on top ofthe ground resultedin a doubling of80M covered with soil ofthe organichorizon. Mineral soil from 250 mm depth was depos­ited on top of the beds and therefore this part wasrelatively poor in organic carbon (1,51 %). Moundswere also erected on top of the ground, and consistedof mixed 80M and soil from the organic horizon.Usually very little of the soil from deeper horizonswas added to mounds and therefore mounds hadhigher organic carbon contents than beds at 50 to 100mm depth. Pitting improved carbon levels at 50 to100 mm depth by incorporation of 80M into themineral soil but improvement at the 250 to 300 mmdepth was considerably smaller than after the othertreatments. After treatment application the organiccarbon content was increased above 2 % for most ofthe observationsbut one yearafter plantingit droppedbelow this level. The differences in organic carboncontent between the soil cultivation treatments weresignificant at 50 to 100 mm depth after treatmentapplication and at 250 to 300 mmdepthone year afterplanting. There were no significant differences be­tween the phosphogypsum treatments. The depthsreported here refer to the position of planted trees;they do not correspond to the same strata in theground before and after bedding or mounding.

Estimated amounts of organic matter for below­and above-ground strata are presented in Figures 1and 2 for the experiments at Blueliliesbush andKruisfontein.

In the experiment at Blueliliesbush, the amount oforganic matter in the soil was underestimated be-

TABLE 3. Organic carbon content in the soils at Kruisfontein before site preparation (before), atplanting (after),and one year after planting (later)'

Treatment Depth: 50 - 100 mm Depth: 250 - 300 mm

Before Mter Later Before Mter Later

Soil Cultivation:bedding 1,21 1,51A 1,11 0,45' 2,32 2 1,67B2

mounding 1,291 2,50 B3 1,642 0,51 ' 2,182 1,36AB2

pitting 1,25 ' 2,02AB2 1,702 0,48 ' 1,222 0,92 A2

Phosphygypsnm:0 Mglha 1,25 1 1,982 1,632 0,47 1 2,413 1,402

12 Mglha 1,25 ' 2,04 2 1,33 1 0,50 ' 1,322 1,232

1 Significant differences (p<0,05) between the treatment means are indicated with letter indexes while number indexes indicatesignificant «0,05) differences between the assesment means.

24 South African Forestry Journal- No. 167 December 1993

the amountoforganic matter infiltratingor mechani­cally incorporated into the soil from the ground sur­face. All treatments resulted in an immediate in­crease of organic matter in the soil to 300 mm depth.One year after planting the organic matter contentdecreased in the 0 to 150 mm depth, remained un­changed between 150 to 300 mm depth, and in­creased in the deeper horizon with a slight overallincrease. It seems likely that these changes resultedfrom mineralisation, decomposition and movementof organic matter to the deeper soil horizons.

At Kruisfontein large amounts of organic matterwere accumulated in mounds and beds by trans­locating SOM and top-soil from surrounding areas.These high levels of organic matter refer only to theplanting spots or lines (mounds or beds) despite thembeing recalculated and presented for 1 ha areas. Asthe roots of the growing trees penetrate outside thebeds or mounds they will grow into soils with a lowerorganic matter content. On average, the whole-plotorganic matter budgets for bedding or mounding areprobably similar to the pitting treatment becausethere was no external input of organic matter. Afterpitting, there was a substantial increase in organicmatter, far greater than could be explained by apossible input from SOM. Dieback and decomposi­tion of roots ofthe disturbed fynbos vegetation seemsto be the only explanation for this increase. One of themost abundant species, Watsonia knysnana, is knownto produce large underground reserves (Jan Vlok,personal communication, Saasveld FRC, 1990). Oneyear after planting, the rate of decrease of organicmatter content was- smaller in the pits than in thebeds or mounds. This was probably a result of fastermovement of organic matter from the loose soil ofbeds or mounds to the deeper soil horizons, and abetter environment for mineralisation (higher tem­perature and good aeration).

ACKNOWLEDGEMENTS

This study was funded by the Department of WaterAffairs and Forestry and conducted by the Division ofForest Science and Technology of the CSIR.

REFERENCES

ALLISON, L.E., 1965. Organic carbon. In: Black, C.A. (ed.),Methods ofSoil Analysis, Part 2: Chemical and Microbiologi­cal Properties, American Society of Agronomy, Inc., Pub­lisher, Madison, Wisconsin, pp. 1367-1378.

BRADY, N.C., 1974. The Nature andProperties ofSoils. MacMillanPublishing Co., Inc., New York, 639 pp.

CROUS, P.W., WINGFIELD, M.J., and SWART, W.J., 1990.Shoot and needle diseases ofPinus spp. in South Africa, SouthAfrican Forestry Journal 154: 60-66.

DE RONDE, C., in prep. The impacts of management on nutrientcycling in plantation forestry in the Southern Cape. Unpubl.Ph.D. thesis, University of Stellenbosch.

GREY, D.C., 1989a. Environmental factors and diameter distri­bution in Pinus radiata stands. South African Forestry Jour­nal 149: 36-43.

GREY, D.C., 1989b. A site-growth study of Pinus radiata in thesouthern Cape. South African Forestry Journal 150: 32-39.

SHERRY, S.P., 1938. The rate of growth and health of thesouthern pines in the Midland Conservancy. South AfricanForestry Journal 1: 30-40.

POYNTON, R.J., 1979b. Tree Planting in Southern Africa, Vol­ume 1: The Pines. Department of Forestry, Pretoria, 576 pp.

SCHAFER, G.N., 1988a. A site growth model for Pinus elliottii inthe southern Cape. South African Forestry Journal 146: 12­17.

SCHAFER, G.N., 1988b. A site growth model for Pinus pinasterin the southern Cape. South African Forestry Journal 146:18-22.

WALKLEY, A., and BLACK, C.A., 1934. Method of determina­tion of organic carbon in soil. Soil Science 37: 29.

ZWOLINSKI, J.B., 1992. Regeneration procedures and mortalityof Pinus radiata D. Don in the southern Cape Province. Ph.D.thesis, University of Stellenbosch, xxiv + 257 pp.

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