Nutrient Changes in Decomposing Loblolly Pine Forest Floor1

J. R. JORGENSEN, C. G. WELLS, AND L. J. METZ2

ABSTRACTIn two loblolly pine stands (Pinus taeda L.), 11 and 32 years

of age at study inception, nutrients in litterfall and throughfallwere measured, and declines in nutrient content were observedas forest floor layers decomposed for up to 8 years. The forestfloors up to 8 years of age in the two stands had similar ratesof nutrient release. After 8 years of decomposition, 67% ofthe C, 27% of the N, 63% of the P, 91% of the K, 67% ofthe Ca, 79% of the Mg were released from the forest floor ma-terials. In the younger stand, release from the forest flooraccounted for 34, 59, 47, and 65%, respectively, of the N, P,K, Ca, and Mg assimilated annually into the abovegroundbiomass. In the older stand, the forest floor release accountedfor 86, 104, 73, 72, and 71%, respectively, of the N, P, K, Ca,and Mg, indicating that as the stand matures, the importanceof the forest floor increases in the nutrient cycling process.Nitrogen content of the forest floor appeared to increase throughthe third year of decomposition and then to decrease whenneedles and branches were assumed to be the only forest floorinputs. However, when fine particles and throughfall wereincluded as forest floor inputs, no increase in forest floor Nwas observed.

Additional Index Words: nitrogen release, nitrogen fixation,nutrient release, nutrient cycling, litterfall, throughfall.

Jorgensen, J. R., C. G. Wells, and L. J. Metz. 1980. Nutrientchanges in decomposing loblolly pine forest floor. Soil Sci. Soc.Am. J. 44:1307-1314.

ONE OF THE INTERDEPENDENT PROCESSES that Controlnutrient cycling in a forest ecosystem is nutrient

release from the decomposing forest floor. Switzerand Nelson (1972) estimated that the forest floor ina 20-year-old loblolly pine (Pinus taeda L.) stand pro-vided from a maximum of 54% for Ca to a minimumof 16% for K of the nutrients the trees assimilated intotheir aboveground biomass. Other workers have shownthat the forest floor is a major factor in stand nu-trition for a variety of plant species, stand ages, andenvironments. Their data, however, have often beengathered from a series of plantations of different agesor from short-term decomposition and nutrient re-lease studies. Data gathered over a long period arerequired to make firm conclusions about organic mat-ter decomposition and rates of nutrient release fromthe forest floor. In this paper, we present such datafor two loblolly pine plantations on the PiedmontPlateau in North Carolina.

We report changes in nutrient content and concen-tration of organic matter (OM), C, N, P, K, Ca, andMg in the forest floor over a period of 8 years. Fromthese data, we estimate the quantity of nutrients ex-pected to become available for tree growth.

METHODSStudy Sites

Two loblolly pine plantations, 11 and 32 years old at the startof the study in 1963 and referred to as the younger and olderplantations in this report, were located on Duke Forest nearDurham, N.C. At the time of final data collection in 1971, theplantations were 19 and 40 years old. The older plantation wasthinned at ages 17, 21, and 26. The younger plantation was

thinned at age 16, after the study had been in progress for5 years. Thus, the forest floor in the younger stand decomposedunder two conditions during the study period—5 years under adense closed canopy and 3 years after the canopy had beenopened. Only information on litter that fell and had begundecomposing before the thinning is included in this report.Nutrient cycling information related to the thinning at age 16was reported by Wells and Jorgensen (1975). Characteristicsof the plantations are listed in Table 1.

The two plantations were established on old fields within ayear or two after the land had been last cultivated. By age 10,the forest canopy had closed, herbaceous cover had been almosteliminated, and a forest floor had developed. In both planta-tions loblolly pine was dominant; hardwoods occurred only in theunderstory.

Litterfall and Forest Floor SamplingA single 0.059-ha plot, with five randomly placed litter traps,

was established in each plantation in 1963. The litter traps, 1X 1 m, with bottoms made of fiberglass mesh with about sevenopenings/cm were set 15 cm off the ground. These traps caughtthe coarse litter but allowed moisture (called throughfall) andfine materials to pass through. In 1964, a second plot withseven traps was established adjacent to the first, and two addi-tional traps were added in the original plot. For simplicity,8 years are mentioned throughout the paper as the collectionperiod, but the period on half of the plots was only 7 years.

Litter that accumulated in the traps was collected the firstof each month beginning in November 1963. It was separatedinto needles, branches, hardwood leaves, and a miscellaneousfraction that consisted of bark, fruit, flowers, and other minerals.

The amount of coarse litter reaching the forest floor was cal-culated from data obtained from the monthly collections. InDecember 1968, and continuing for approximately 1 year, twotraps from each plot were replaced by plastic buckets with open-ing areas of 0.1 m2. The primary purpose of these bucketswas to collect throughfall and fine litter not caught on the m2

traps, although they also caught coarse litter on screens locatednear the bucket openings. Fine litter consisted of pollen, in-sect parts and frass, dust, and small parts of plants. Composi-tion varied with season.

Throughfall and both fine and coarse litter were collectedfrom buckets at approximately monthly intervals. No preserva-tives were added to the collected throughfall. Throughfall wasbrought to the laboratory and separated into liquid and resi-due. Since we collected throughfall and fine litter for only 1year, we had to estimate the amounts for the remaining 7 years.We assumed that throughfall was constant throughout the study.Nutrient input from the fine litter for the 7 years, however,was estimated by correlating it with the nutrient input fromthe coarse litter.

Annual accumulations of forest floor were separated by layinga section of 1 X 1 m fiberglass screening, with three openingsper cm, over the litter material that had accumulated by 1October of each year. One "sandwich," made up of alternatinglitter and fiberglass screening layers, was adjacent to each offive traps in each plot. In 1971, two sections 30 X 30 cm inthe older stand were cut through each of the five sandwichesdown to the mineral soil. Thus, individual forest floor layersthrough 8 years and composite samples of the older forest floorbetween the bottom screen and the mineral soil were obtained.A preliminary sampling of the older plantation was made in1968 when five annual forest floor layers within the sandwicheswere collected. The forest floor of the younger plantation wassampled in 1971 by randomly selecting and then collecting allmaterial from two sandwiches on each of the two plots and thensampling the older floor beneath these two sandwiches.

We tried to separate each forest floor layer into needles,branches, hardwood leaves, and miscellaneous, as had been done

1 Contribution from the Southeastern Forest Exp. Stn., U.S.Dep. of Agric., For. Serv., Asheville, NC 28802. Received 21Jan. 1980. Approved 19 Aug. 1980.

2 Research Soil Scientists, Southeastern Forest Exp. Stn., For-estry Sciences Lab., Research Triangle Park, NC 27709.

1307

1308 SOIL SCI. SOC. AM. J., VOL. 44, 1980

Table 1—Characteristics of the loblolly pine plantations.Plantation

age(years)

11

Soil

Creedmore si,

Age of trees at Domi-measurement Trees/ Average riant BA/

(years) ha diameter height ha

cm m m1

16 2,243 16 15 49Aquic hapludult

32 Appling si, 37Typic hapludult

462 30 27 34

with the freshly fallen litter. However, needles became increas-ingly difficult to separate as decomposition proceeded, andonly the concentration and release of nutrients associated withall materials in each layer are reported here.

Chemical and Physical AnalysesLitterfall and forest floor samples were dried at 70°C and

ground before analysis. Organic samples were ashed in a muf-fle furnace in preparation for the determination of K, Ca, andMg by atomic adsorption and P by a method involving thevanadomolybdophosphoric yellow color. Total N in the litterfalland forest floor was determined by macro-Kjeldahl and N inthroughfall by an automated Technicon autoanalyzer procedure.Carbon content of the forest floor was estimated as: C = 3.61+ .499 (% loss on ignition). This equation, obtained by analysisof representative samples in a carbon-induction furnace, ac-counted for 97% of the observed variation in C. Nutrient con-centrations are reported on an ash-free basis to avoid sampleweight variations caused by mineral soil contamination.

Statistical AnalysisData were subjected to several statistical procedures before

the selection of appropriate transformations and general pre-diction equations. In most instances, plots were made of theappropriately transformed data, along with values predicted bythe equations and their residuals. These plots, in addition tostatistical evaluation, led to the selection of two general equa-tions—one to predict percentages of elements remaining and asecond to predict their concentrations for up to 8 years of forestfloor decomposition. For the equations used to predict per-centage of element remaining, a logarithmic transformation ofdata was generally beneficial, whereas data transformation topredict element concentration was not.

After logarithmic transformations of data, percentage of P,K, and Mg remaining in the forest floor were predicted by thegeneral equation

In Y = a + b In (A + 0.1) + e

where Y = the percentage of element remaining, A = years of

decomposition, a, b = constants derived from data, and e =error term assumed to follow a normal distribution with mean0 and variance <?. Percentages of OM, C, N, and Ca remainingwere predicted by similar equations except (A + 1.0) was sub-stituted for (A + 0.1). Because the logarithm at 0 is undefined,constants were added to age, with 1 and 0.1 being the mostappropriate of those examined. To correct a bias in the meanestimate of Y, the antilogs of the predicted values were multi-plied by a correction factor, exp(8*/2), the residual error of InY as suggested by Baskerville (1972).

Element concentrations in the forest floor were predicted withthe general equation

y = a + bA + cA> + dA> + ewhere A is years of forest floor decomposition. Terms not sig-nificant at the 10% level were not included in the final equa-tions.

Percentage of N remaining was also predicted from untrans-formed data on coarse litter, coarse -I- fine litter, and coarse +fine litter + throughfall. These regressions were designed toshow the relationship of each input to elemental gain or loss inthe forest floor.

Regression equations based upon the elements in coarse +fine litterfall were used to calculate the percentage of eachelement remaining in the forest floor of each plantation andthen for both plantations combined. Predictions were improvedby combining data from both plantations, thus only the equa-tions resulting from this combination are presented. The pre-diction of concentration of elements in the forest floor, how-ever, was not improved by combining data from both planta-tions. This lack of improvement was due, in part, to initialdifferences between the two plantations in the concentrationsof nutrients in their forest floors. In any case, concentrationpredictions reported here are based upon the more comprehen-sive data set, that of the older plantation. The throughfall por-tion of nutrient input was excluded from the prediction equa-tions because it was estimated for 7 of the 8 years. Inclusionof the throughfall constant would have changed the equation'sintercept but not its form.

RESULTSForest Floor Input

During the 5 years before thinning in the youngerstand, total annual litterfall—coarse and fine materials—increased from 7,100 kg/ha of ash-free organic matterto over 8,800 kg/ha. Annual litter production in theolder stand varied from year to year, with a range of4,200 to 7,500 kg/ha and an average of 6,100 kg/haover the 8-year period. Ice storms increased litterfallone year to the detriment of the next, but the reason

Table 2—Average annual input to the forest floor.

Element

Ash-free or-ganic matter Carbon Nitrogen Phosphorus Potassium Calcium Magnesium Total ash

Plantation age (years)

Source

NeedlesTrashBranchesLeavesFine materialThroughfall

Total

NeedlesMisc., branches, and leavesFine materialThroughfall

11

6,202221774

9508t

7,711

80.4. 13.0

6.6-

32

4,38656579762

284t

6,094

72.023.34.7-

11

3,084112401

4254t-

3,855

80.013.46.6-

32

2,18128541331

142f-

3,052

71.523.94.6-

11

35.01.72.70.1

13.89.3

62.6

55.97.2

22.014.9

32

23.83.32.30.57.33.7

40.9

58.214.917.89.1

11

6.50.10.20.10.80.58.1

80.23.79.96.2

32 11

- kg/ha per year -3.90.20.20.10.80.3

12.20.30.60.10.8

11.75.5 25.6

— % of total —70.9 47.79.1

14.55.5

3.53.1

45.7

32

9.61.01.00.20.76.6

19.1

50.311.43.7

34.6

11

22.70.52.10.10.94.5

30.8

73.78.82.9

14.6

32

15.11.32.10.80.66.5

26.4

57.215.92.3

24.6

11

5.60.10.10.10.31.87.9

70.92.53.8

22.8

32

3.80.20.20.20.41.86.6

57.69.06.1

27.3

11 32

250.6 177.29.7 24.7

15.5 15.90.7 5.1

21.lt H.8tt t

297.6 234.7

84.2 75.58.7 19.57.1 5.0-

t Carbon and ash content of fine material estimated at 48 and 4%, respectively, of collected oven-dry material.I Dissolved organic matter and ash not measured.

JORGENSEN ET AL.: NUTRIENT CHANGES IN DECOMPOSING LOBLOLLY PINE FOREST FLOOR 1309

for much of this variation is not known.In both stands, needles made up approximately three

fourths, and branches and associated bark about onetenth of the litterfall. Two miscellaneous components,trash and fine materials, each contributed 5 to 10%.Hardwood leaves made up 1% or less of the totals.Bioinput—total litterfall + throughfall—to the forestfloor in the young stand was about 25% greater thanin the older one. Needles accounted for most of the

difference-approximately 1,800 kg/ha more needlesper year in the young stand.

Largely because of the difference in coarse and finelitterfall and throughfall, greater quantities of everynutrient reached the forest floor in the young thanin the old stand (Table 2). Significant inputs of Nand K in the young stand came from either or boththe fine material and throughfall in addition to in-creased quantities of the elements in coarse litter.

120

oi so

uito60

uiuKUIO.

30

ORGANIC MATTER• ———^— PERCENTAGE REMAINING

1 2 3 4 5 6 7PERIOD OF AGING (YEARS)

120

90

60

30

0

CARBON• -^——^ PERCENTAGE REMAININGo —— —— —— CONCENTRATION OU

Sr —

1 2 3 4 5 6 7 8PERIOD OF AGING (YEARS)

70

"1zg50 I

40

100

80

ui<Eui

z 40uio

20

120

Z 90<

UJ

1uoUI0,

60

30-

POTASSIUM• PERCENTAGE REMAINING°—————-CONCENTRATION O)

lOOi

1 2 3 4 5 6 7 8PERIOD OF AGING (YEARS)

CALCIUM- ————— PERCENTAGE REMAININGo — — — CONCENTRATION «)

0.3u

02 §

80

0.12 g

0.10 2 602

0.08 zuio0.06 §' 40"

20

(20

90

60

30

PHOSPHORUS• ^———— PERCENTAGE REMAININGo — — — — CONCENTRATION W

0.15

0.12

0.09 oezUI

0.06 zoo0.03

1 2 3 4 5 6 7PERIOD OF AGING (YEARS)

MAGNESIUM> ———•—— PERCENTAGE REMAININGo ————— CONCENTRATION (10

0.10

0.08 FCL

0.06uoo

I 82 3 4 5 6 7 8 0 1 2 3 4 5 6PERIOD OF AGING (YEARS) PERIOD OF AGING (YEARS)

Fig. 1—Percentages of OM, C, P, K, Ca, and Mg in the forest floor after up to 8 years of decomposition as based upon initial contentsin coarse and fine litter input and elemental concentrations in the forest floor after up to 8 years of aging.

1310 SOIL SCI. SOC. AM. J., VOL. 44, 1980

160

120

UJCEUJ3UlOo:UJa.

80

40

0

NITROGENPERCENTAGE REMAINING

1 CONCENTRATION K)

120

2.2i

1.8 •

<1.4 o:

oz

UJcrzUJ

Oo:

100-

u. 80

1.0oo

0.6

oUJ

60

COARSE LITTER

COARSE + FINE LITTER * THROUGHFALL

2 3 4 5 6PERIOD OF AGING (YEARS)

2 3 4 5 6PERIOD OF AGING (YEARS)

Fig. 2—Percentage remaining and concentration of N after up to 8 years of decomposition in the forest floor. (A) Percentage of Nremaining calculated from fine and coarse litter input, data transformed. (B) Percentage of N remaining calculated from coarselitter, coarse + fine litter, and coarse + fine litter + throughfall, jiot data transformed.

The additional P transferred within the young standwas due to the greater litterfall and an increase inthe concentration of P from 0.09% in the older standto 0.1% in the younger stand.

Forest Floor Decomposition and Nutrient ReleaseOrganic matter decomposition and nutrient release

from the litter were greatest for all elements duringthe first year litter was on the ground. However, ele-mental net decrease during this initial period and insubsequent periods varied from element to element(Fig. 1 and 2, Table 3). The element with the slowestnet decrease was N. During the first year of litter de-composition, the net decrease was only 10%, and thenet decrease over 8 years was only 27%.

The basis on which N input was estimated greatlyinfluenced the shape of the loss-rate curve (Fig. 2B,Table 4). When only coarse litter was considered asan input, as is done in most studies, the best-fittingprediction curve showed that forest floor N increasedthrough the third year of decomposition and thendeclined. When the N contents of coarse and finelitter and throughfall were considered, an annual re-lease rate of 4 to 5% of the initial N was revealed,and an initial increase was lacking. The differencebetween these two curves indicates that the increasein forest floor N when coarse litter only is considered

is due primarily to enrichment by fine litter andthroughfall.

During the first year of litter decomposition, aboutone fourth of the OM and C was lost. Subsequentlosses were smaller, but after 8 years about two thirdsof the initial C and OM was lost.

Since C was released more rapidly than N, the C/Nratio decreased from approximately 77:1 in new-falienmaterial to 34:1 at age 8. This later value was cal-culated from equations predicting the percent of Nand C remaining (Fig. 1 and 2, Table 5). Concentra-tion of N more than doubled during this period, in-creasing from 0.76% in 1-year-old forest floor materialto 1.84% in 8-year-old material, whereas C concentra-tion increased from only 53 to 62.5% in the same time.

In contrast to N and C, K release from the forestfloor was rapid. After the first year, only about 30%of the initial K remained and at the end of the secondabout 20%. At the end of the eighth year, only 10%of the initial amount of K remained in the forest floor.Concentration of K declined from 0.2% in litterfallmaterial to less than 0.1 % 1 year later. Concentrationfurther decreased through the third and fourth years,after which it increased. By the seventh year, concen-tration was similar to that found in the 1-year-oldforest floor.

About half of the P in litter remained after the firstyear of decomposition. Subsequent, annual losses were

Table 3—Elements present in the forest floor of loblolly pine plantations 8 years after study inception.

Forest floor age group (years)

2-3 4-8 El-8 >8 Total forest floor

Plantation age (years)t

Element 32 lit 32 11 32 11 32 11 32 11 32

—————————————————————————————— ———— kg/ha ———————————————————————————————————Organic matterCarbonNitrogenPhosphorusPotassiumCalciumMagnesium

5,4342,826

484.43.9

19.62.7

5,1752,742

342.93.4

16.62.6

7,6504,234

897.34.9

29.33.9

5,2282,795

574.43.8

18.02.4

11,8897,195

18014.97.2

56.77.7

10,0675,653

14710.26.6

35.05.0

24,97314,255

31726.616.0

105.614.3

20,47011,190

23817.513.869.610.0

4,0622,498

8010.34.3

24.24.5

12,3938,322

17818.915.148.410.8

29,03516,753

39736.920.3

129.818.8

32,86319,512

41636.428.9

118.020.8

t Age at study inception.t Estimated from average litterfall input and appropriate regression of percentage remaining.

JORGENSEN ET AL.: NUTRIENT CHANGES IN DECOMPOSING LOBLOLLY PINE FOREST FLOOR 1311

Table 4—Regression coefficients, R', and a*, obtained by predictingthe percentages of elements remaining in the forest floor as

deposited by coarse and fine litterfall using the generalequation \nY= a + b\n(A + Q, where A - years

of decomposition and C is a constant.InY

OMCNPKCaMg

4.6904.6704.5934.0643.3394.6663.779

-0.647-0.561

'-0.146-0.230-0.546-0.535-0.370

1110.10.110.1

0.890.860.310.780.880.820.83

0.02790.02640.02480.03140.08890.03310.0577

about 3% of the initial amount. Phosphorus concen-tration in the forest floor, after a first-year decrease,approximately doubled, going from 0.067% at the endof the first year to 0.123% 7 years later. The increasein concentration, like that of N, was caused by therapid loss of OM from the forest floor and by theimmobility of the residual P rather than by an in-crease in the total amount of P present.

After the first year of forest floor decomposition,75% of the Ca and 43% of the Mg remained. By theeighth year only 33% of the original Ca and 21% ofthe Mg were found in the forest floor. Concentrationof Ca throughout the decomposition period remainedabove or only slightly below that of the initial littermaterial. Concentration of Mg fell to half of itsinitial value by the fourth year, but after four moreyears of decomposition, concentration was similar tothat in fresh material.

Nutrient Transfer From the Forest FloorNutrient transfers from the eight forest floor layers

formed during the period of study were computed bydetermining annual nutrient inputs over the 8-yearsampling period and then applying the nutrient lossequations.

Average annual rates of nutrient release for theforest floor deposited before the study was establishedwere also calculated. For the older stand, these valueswere based upon average rates of litterfall and nutrientrelease observed in that stand. In the younger stand

Table 5—Regression coefficients, R' and a* obtained by predictingthe concentrations and percentage remaining of elements in

the forest floor using the general equation Y = a + bA+ cA* + dA*, where A = years of forest

floor decomposition.y

(cone.)

CNPKCaMg

a

52.0380.4820.0580.150NS0.067

6

1.3560.2480.008

-0.070NS

-0.014

C

-0.470-0.012.NS

0.017NS0.002

d

0.058NSNS

-0.001NSNS

R'0.970.860.560.46-

0.38

<?

0.3880.02030.0002500.000553

-0.000169

Y(% remaining)N,tNJ:N,§

99.78494.88791.011

7.885-3.366-4.291

-1.306NSNS

---

0.260.320.45

216.604147.975145.278

t Based upon coarse litterfall.t Based upon coarse and fine litterfall.§ Based upon coarse and fine litterfall and throughfall.

a different basis for calculation was used because theamount of litterfall increased from the time of plantinguntil sometime after the stand closed. As a basis forcalculation in this stand we assumed that forest floordevelopment began at age 4 and that through age 11litterfall and its nutrients increased linearly. Basedupon these premises and with the nutrient loss equa-tions, we calculated nutrient release from the forestfloor material laid before 1963. These calculationsgave us estimates of the nutrients that were releasedfrom the forest floors present at study establishmentthrough their eighth year of decomposition in bothstands. These losses, through the eighth year, werethen deducted from the nutrients in the forest floorswhen the study was first established in 1963. Thisremainder was finally substracted from the pre-1963forest material as measured in 1971. The differencesin the two values were used to estimate nutrient re-lease from materials over 8 years of age. Results areshown in Table 6.

We estimated that 28.8 kg of N, 5.7 kg of P, 12.7 kgof K, 17.4 kg of Ca, and 4.6 kg of Mg were .releasedannually from the entire forest floor in the olderplantation (Table 7). In the younger plantation, net

Table 6—Net change over an 8-year period in the nutrient content of forest floor material 8 years and older in plantations11 and 32 years of age at tune of study inception.

Elements

NitrogenPhosphorusPotassiumCalciumMagnesium

NitrogenPhosphorusPotassiumCalciumMagnesium

Forest floorat inception

131.510.39.3

58.210.0

353.939.233.9

112.221.2

Calculatedlosst

28.94.07.0

30.73.7

26.84.27.9

30.73.9

Calculated Actual remainderremainder} after 8 years

——————————— kg/haPlantation age 11

102.66.32.3

27.56.3

Plantation age 32327.135.026.081.917.3

80.310.34.3

24.24.5

178.418.915.148.410.8

Net change inmaterial olderthan 8 years§

-22.3+4.0+ 2.0-3.3-1.8

-148.7-16.1-10.9-33.5-6.5

Annual changes^

-2.79+0.50+0.25-0.41-0.22

-18.59-2.01-1.36-4.19-0.81

t Calculated nutrient losses through the eighth year of decomposition for forest floor materials aged 1 to 8 years.t Calculated residual nutrients in the forest floor material after deduction for decomposition losses after the first and through the eighth year.§ Loss (-) or gain (+) of nutrients in the forest floor older than 8 years compared with the calculated remainder.1 (-) indicates nutrient release, (+) indicates no nutrient release.

1312 SOIL SCI. SOC. AM. J., VOL. 44, 1980

Table 7—Annual nutrient release from the forest floor as calculated by regression and average litterfall inputin two loblolly pine plantations.

Element

N P K Ca MgPlantation age

Source

Forest floor

Net totalThroughfallTotal -

Age of litter

years0-11-22-33-44-55-66-77-80-8>817.5

11

5.12.81.91.41.10.90.80.7

14.72.8

17.59.3

26.8

32

3.61.91.31.00.80.60.50.5

10.218.628.83.7

32.5

11

3.80.60.30.20.20.10.10.15.4

(0.5)4.90.55.4

32

2.60.40.20.10.10.10.10.13.72.05.70.36.0

11 32

kg/ha released (accumulated)10.01.20.50.30.20.20.10.1

12.6(0.2)12.411.724.1

9.01.00.50.30.20.10.10.1

11.31.4

12.76.6

19.3

11

6.73.82.21.51.10.90.70.6

17.50.4

17.94.5

22.4

32

5.12.91.71.20.80.60.50.4

13.24.2

17.46.5

23.9

11

3.40.60.30.20.10.10.10.14.90.25.11.86.9

32

2.70.40.20.10.10.10.10.13.80.84.61.86.4

release was 17.5 kg of N, 4.9 kg of P, 12.4 kg of K,17.9 kg of Ca, and 5.1 kg of Mg. These amounts areequivalent to 6.9% of the N, 15.7% of the P, 43.9%of the K, 14.7% of the Ca, and 22.1% of the Mg inthe total forest floor of the older plantation and 4.4%of the N, 13.3% of the P, 61.1% of the K, 13.8% ofthe Ca, and 27.1% of the Mg in the total forest floorof the younger plantation.

With the exception of N, over half of each elementreleased annually came from litterfall deposited inthe past 2 years. In the slowly accumulating forestfloor of the older stand, nearly two thirds of the Nreleased came from forest floor materials older than8 years of age, and only one fifth came from the forestfloor that was 2 years old or less. However, in theyounger stand, when the forest floor was accumulatingrapidly, less than 20% of the N released came frommaterials older than 8 years, and nearly 50% camefrom materials 2 years old or less.

Approximately three fourths of the K released fromthe forest floor of both stands was from litter decom-posing 1 year or less. Our estimate of K release fromforest floor materials more than 8 years of age in theyounger stand (0.2 kg/ha annually) is small and rela-tively unimportant in the nutrient cycling of theplantation. The corresponding value in the olderstand (1.4 kg/ha of K annually) was about 10% ofthe total from the forest floor.

Calcium was released at a steadily declining rateas the forest floor decomposed. For materials agedfor 8 years, one third was released from the 1-year-oldmaterial, another third from the 2- and 3-year-oldmaterial, and the final third from material 4 to 8years old. In the older stand, the forest floor over 8years old supplied about one fourth of the Ca releasedfrom the forest floor, but it supplied a negligibleamount in the younger stand.

About half of the P and Mg in litter of the olderplantation was released during the first year of decom-position. In the younger plantation, three fourthsof the released P and two thirds of the Mg came fromthe forest floor that was 1 year old. The forest floorover 8 years old in the younger plantation released noP and only an insignificant amount of Mg. However,in the older plantation the forest floor older than 8

years contributed important amounts of these ele-ments to tree growth (Table 7).

Throughfall was an important source of nutrients.As a proportion of the total throughfall + forest floorrelease or loss, throughfall provided 35% of the N,9% of the P, 49% of the K, 20% of the Ca, and26% of the Mg in the younger plantation and 11%of the N, 5% of the P, 34% of the K, 27% of the Ca,and 28% of the Mg in the older stand.

DISCUSSIONReliability is a major consideration with any nu-

trient-cycling information. In another study, Wellsand Jorgensen (1975) estimated nutrient release fromthe forest floor of the younger stand by taking 800.093 m2 samples of the forest floor on each of twodates. Estimates of annual release of N, P, K, Ca, andMg from the forest floor and in throughfall in thatstudy were 25.2, 4.0, 20.6, 24.1, and 4.7 kg/ha, respec-tively. In the present study, our estimates of the samevalues are quite similar-26.8, 5.4, 24.1, 22.4, and 6.4kg/ha per year. Switzer and Nelson (1972) reportedlower release for all nutrients except for N in a 20-year-old loblolly pine stand. However, their releaserates appear to be associated with a forest floor con-taining only about 25 to 80% of the nutrients foundin our 11-year-old and 32-year-old stands.

Although net C loss was related to the decline inforest floor weight, C concentration increased gradual-ly for the first 4 years and then more rapidly for thenext four. The slow initial increase was similar tothat reported by Hayes (1965) for needles, but Hayesalso monitored an initial 4-month period in which Cconcentration decreased. The rapid initial loss in litterweight and the accompanying increase in C concen-tration suggested to Hayes that molecules relativelylow in C (simple sugars, pectins, and organic acids)are most susceptible to leaching or to microbial at-tack. Loss of these compounds increases the concen-tration of materials high in C, such as lignin, whichare resistant to decomposition.

An absolute increase in N in decomposing litter hasbeen reported, but the source of the N is uncertain(Will, 1967; Hayes, 1965; Gosz et al., 1973; Lemee and

JORGENSEN ET AL.: NUTRIENT CHANGES IN DECOMPOSING LOBLOLLY PINE FOREST FLOOR 1313

Bichaut, 1973). The increases have been ascribed totranslocations or fixation by microflora (Lemee andBichaut, 1973; Hayes, 1965), rainfall and insect activity(Bocock, 1963), or additions by flowers, pollen, budscales, and other fine residues (Gosz et al., 1973).In our study, N appeared to be accumulating whenonly coarse litter was considered. When fine ma-terials were included, a nearly linear decrease in Nin the forest floor was revealed. We conclude thatnet accretion of N in the forest floor due to fixationand translocation by flora and fauna is small comparedwith input from fine particulates. Nutrient transloca-tions by organisms or nutrient replacement by through-fall may be substantial and important in ecologicalsuccession, but they cannot be estimated in this study.

In agriculture, C/N ratios are usually related tothe potential for N release. In our study, release wasnot well correlated with this ratio. In both planta-tions, over 10% of the total N was released duringthe first year of decomposition when the C/N ratioaveraged 77:1. During the 7 years that followed, theC/N ratio was halved, but N released rate declinedto only 2 to 3% annually. Nitrogen release from theforest floor over 8 years of age appears to be positivelycorrelated with its average age and C/N ratio. Inthe 32-year-old plantation, residual N was releasedat an average annual rate of 10% from forest floormaterial over 8 years old that had a C/N ratio of 47:1.In the 11-year-old plantation, in which the forestfloor over 8 years of age was only beginning to de-velop, the C/N ratio was 31:1, and only 3% of the Nwas released annually. Obviously, several processes areinvolved in N release under varying forest floor con-ditions, but these processes do not appear to be directlyrelated to gross C/N ratio. Initially, the more readilymineralized or soluble N compounds found in insectfrass and fresh litter are released. Thereafter, N maybe released by microbial action on the more resistantplant and microbial materials, or by the transloca-tion of N to and from the forest floor by roots andtheir mycorrhizal associates and by free-living fungi.Stark (1972) has shown that fungi have the ability toconcentrate N and other nutrients. By concentration,protoplasmic movement, and autolysis, fungi may beable to move nutrients from one layer of soil to anoth-er. Due to these translocations and additions fromthroughfall, nutrient availability as strictly definedcannot be determined in this study. Instead, onlynet changes in nutrient status of the various forestfloor layers can be measured.

In an earlier study, Wells and Jorgensen (1975) esti-mated the rates of nutrient assimilation by all plantparts (other than roots less than 1 cm in diameter)for the younger stand at age 16. The proportions ofthese needs that could have been met by nutrient re-lease from the forest floor are 84, 59, 47, 74, and 65%,respectively for P, K, N, Ca, and Mg. Over half ofthe P, K, and Mg and about one third of the Ca andN could have come from litterfall residing in the forestfloor for 1 year or less. In this younger stand, forestfloor material older than 8 years contributed onlyminor amounts of all nutrients, except N, for treegrowth. Throughfall nutrients could account for 44%of the K but only 6% of the P assimilated by growingtrees.

In the older plantation, the nutrients released bythe forest floor were sufficient to satisfy much of the

stand's nutrient demand. Based upon calculated re-lease, the forest floor could supply 86, 104, 73, 72, and71% of the stand's annual uptake of N, P, K, Ca, andMg.3 If nutrients in throughfall are added to thosefrom the forest floor, these two sources could supply97, 109, 110, 99, and 98% of the N, P, K, Ca, and Mgtaken up annually. Forest floor materials less than 8years old were the main source of the soluble cations,whereas the forest floor older than 8 years of agesupplied over half of the N and a third of the P re-quired by the trees.

A comparison of the two plantations shows thatradical changes take place as a stand develops. Inthe early stages of plantation growth, throughfallcontributes substantial amounts of N and K. As astand ages, the amount of these elements in through-fall decreases. The most dramatic change, however,is the replacement of the mineral soil by the forestfloor as the principal nutrient source in older planta-tions.

In a study of a loblolly pine plantation, beginningat age 5 and ending 10 years later, Wells and Jorgen-sen (1975) reported a decrease in mineral soil nu-trients, other than P, through the surface 60 cm. Inan older plantation, Wells and Jorgensen (1975) re-ported no significant changes in nutrients in the top75 cm of mineral soil during the 5-year period be-tween ages 34 and 39. These data corroborate thoseof the present study. During plantation developmentthe source of nutrients for growth is gradually shiftedfrom the mineral soil to the forest floor. At age 16,for example, the forest floor could supply only 34%of the N requirements, whereas between the ages of30 and 40 the forest floor could supply 86% of the re-quirements. When nutrients in througfall are added,the figures are 52 and 97%, respectively.

The restructuring of source, although important forall elements, has special implications for N. If theforest floor is removed periodically by fire, as throughcontrolled burning, most of the N that could be re-cycled from the forest floor is lost to the atmosphere(DeBell and Ralston, 1970). The length of time thatthe mineral soil can contribute 30 to 40 kg/ha of Nannually for tree growth and maintenance is prob-lematic.

Because nitrogen is released slowly from the forestfloor after the first few years of litter decomposition,any adverse effects of the destruction of the forestfloor may not be noticed for some time, especially onfertile soils. On poor soils, especially those with coarsetexture and excessive drainage, the loss of the forestfloor as a source of N may cause a rapid decline inthe growth and yield of succeeding generations of trees.Better soils often contain a reserve of nutrients thatcan be drawn upon by trees, especially during standestablishment (Wells and Jorgensen, 1975). The min-eral soil reserve is maintained or augmented at cer-tain periods in tree rotations by nutrients from theforest floor, throughfall, or decomposing roots. Thus,the effects\ of forest floor removal on site nutrientstatus and tree grwth depends on the nutrient contentof the mineral soil reserve and the mineralization orrelease rate of the nutrients in this reserve.

8 Quantity based upon nutrients in average needlefall and nu-trients required for the annual growth of wood and needles barkassociated with the bole, branches, and large roots.

1314 SOIL SCI. SOC. AM. J., VOL. 44, 1980