Mineralization of Nitrogen and Phosphorus from Soil Organic Matterin Southern Pine Plantations

P J. Polglase, N. B. Comerford,* and E. J. Jokela

ABSTRACTSustained weed control and/or annual applications of fertilizer have

accelerated stand development of young slash pine (Pinus elttottii En-gelm. var. elliottii) and loblolly pine (P. taeda L.) growing on Spo-dosols in Florida. Productivity of these plantations will depend largelyon the extent to which nutrients are recycled through soil organicmatter derived from decomposition of fresh residues. We attemptedto sample this newly formed soil organic matter and measured theeffects of weed control and fertilizer treatments on N and P miner-alization under laboratory conditions (42-d aerobic incubation). Se-quential in situ containments using interbed soil were used to provideinformation on field rates of mineralization. The laboratory studydemonstrated that specific N mineralization was little affected by cul-tural treatment. In contrast, specific P mineralization was consistentlyincreased by fertilizer application, both in the laboratory and in situ.This suggests that, in organic residues, the general composition of Psubstrates was altered by fertilizer application, while organic-N sub-strates remained unaffected. The effect of weed control inhibited spe-cific N and P mineralization in the laboratory, indicating that organicresidues of understory vegetation are a better supply of available Nand P than are organic residues of pine. In fertilized plots and for P,annual rates of in situ mineralization were large relative to demand(uptake) by vegetation. The recycling of P through organic residuesof fertilized plots has the potential to enhance long-term productivitybeyond the immediate benefits derived from fertilizer uptake.

COMMERCIAL PLANTATIONS of slash pine dominatethe flatwoods of northern Florida. This species

is particularly suited to the relatively infertile soils ofthis region because the trees have a low absolute re-quirement for nutrients (Gholz et al., 1985b). Evenso, it has been long known that these stands respondwell when nutrients, particularly N and P, are addedin fertilizers (Pritchett and Llewellyn, 1966; Bengt-son, 1979).

More recently, the Intensive Management PracticesAssessment Center (IMPAC, U.S. Forest Service) im-posed sustained weed control and complete fertilizertreatments (in factorial combination) in a long-termexperiment designed to establish the growth potentialof slash and loblolly pines. Four years after planting,aboveground biomass of loblolly pine in the weed-control treatment was equivalent to that in the fertil-ized treatment, and was eightfold greater than the un-treated reference (Colbert et al., 1990; Neary et al.,1990a). Biomass in the combination treatment (weedcontrol plus fertilizer) was 17-fold greater than thereference. Because water was not limiting in thesestands (Swindel et al., 1988), growth was controlledprimarily by nutrient availability (Neary et al., 1990a).

The short-term benefits derived from fertilizer in-

P.J. Polglase and E.J. Jokela, Dep. of Forestry, 118 Newins-Ziegler Hall, and N.B. Comerford, Dep. of Soil Science, 2169McCarty Hall, Univ. of Florida, Gainesville, FL 32611-0303.Contribution of the Florida Agric. Exp. Stn. Journal no. R-01448.Recieved 14 Mar. 1991. * Corresponding author.

Published in Soil Sci. Soc. Am. J. 56:921-927 (1992).

puts were immediate and obvious. Long-term benefitswill be determined largely by the rate at which N andP are cycled through organic residues discarded duringstand development (Jorgensen et al., 1980). Chrono-logically, these benefits (if any) will be evident firstin the release of N and P from decomposing litter,and then later in the release of N and P from soilorganic matter that has formed from the decomposi-tion of both litter and roots. Release of N and P fromlitter as affected by cultural treatment at the IMPACsite has been reported previously (Polglase et al., 1992a,c). The objective of this study was to examine theeffect of cultural treatments at the IMPAC site on thecycling of N and P through soil organic matter.

This study is predicated on the assumption that,although the stands were only 6 yr old at the com-mencement of sampling, decomposition of litterfalland roots will have been sufficient to add significantlyto the store of soil organic matter. The long-term po-tential of this organic matter to supply N and P canthen be determined by measuring N and P mineral-ization in the laboratory, and expressing the results asa percentage of the total pool of organic substrate pres-ent. Because this protocol provides information onpotential N and P mineralization only, our final ob-jective was to measure N and P mineralization in thefield, and to interpret these results in relation to cal-culated demand by vegetation.

Measuring the contribution of plant residues to Nand P mineralization is made difficult if fertilizer ispresent to confound soil analyses. McLaughlin et al.(1988) circumvented this problem in arable systemsby labeling plant residues with 33P and fertilizer with32P. Our study was made feasible by two importantfeatures of the IMPAC experiment. First, fertilizerwas applied in a narrow band (30-cm semicircle) onone side of each tree. Hence, soil could be collectedaway from these banded areas and analyzed withoutinterference from residual fertilizer. Second, the lackof adsorptive surfaces (Fox et al., 1990) in the sandyA horizon of these Spodosols enabled us to measureP mineralization directly as production of inorganic P(Pi).

METHODSSite Description

The study was located about 10 km north of Gainesville,FL. The climate is warm temperate-subtropical. Mean an-nual precipitation is 1350 mm, most of which occurs insummer and the least in fall and spring. The mean annualtemperature is about 24 °C (Dohrenwend, 1977). The soilsare poorly drained Pomona fine sands (sandy, siliceous,hyperthermic Ultic Haplaquods). A typical profile has aspodic horizon at 20 to 50 cm and an argillic horizon at 90to 120 cm. The A horizon consists of organic matter mixedwith quartz sand and very few, if any, primary or secondary

Abbreviations: P,, inorganic P; ANOVA, analysis of variance;ODW, oven-dry weight.

921

922 SOIL SCI. SOC. AM. J., VOL. 56, MAY-JUNE 1992

minerals are present. Hence, cation-exchange capacity islow (<5 cmolc kg-1) and sorption of added Pf is undetect-able. Some general properties of the surface soil at theIMPAC site are given in Tables 1 and 2.

A wildfire destroyed the previous slash pine plantationin 1981. In 1983, 1-yr-old loblolly and slash pine seedlingswere hand planted following site preparation of roller drumchopping and conventional bedding.

Experimental Design and TreatmentsThe IMPAC experiment consisted of three replicates of

a 2x2x2 factorial of species, sustained weed control, andannual fertilizer application arranged in a randomized split-plot (species) design (Swindel et al., 1988). Weed controlinvolved complete and sustained elimination of understoryand ground vegetation using sulfometuron methyl [methyl2;[[[[(4,6-dimethyl-2-pyrimidinyl) amino] carbonyl] amino]sulfonyl] benzoate), glyphosate [isopropylamine salt of N-(phosphonomethyl) glycine], and triclopyr(3,5,6-trichloro-2-pyridinyloxyacetic acid) applied at labeled rates. Com-peting vegetation not controlled by herbicides was con-trolled mechanically by rotary cutters.

The fertilizer included nearly all essential mineral ele-ments in two or three split applications per year and wasplaced in narrow bands around the base of each tree. Theannual applications (kg ha-1) were: N (60), P (24), K (50),Ca (20), S (13), Mg (10), Fe (0.50), Zn (0.5), Mn (0.5),Cu (0.06) and B (0.06).

Mineralization StudiesThe effect of cultural treatments in accelerating stand

development introduced a number of factors that could con-found results. A consideration of those factors that providedthe rationale behind our methodology is therefore war-ranted.

First, increased interception of light and water by a moredeveloped canopy in treated plots (Dalla-Tea and Jokela,1991) and the insulating effect of a thicker litter layer couldalter the microclimate of the forest floor and hence affectmineralization. Second, soil organic matter in these plotscan be considered as a mixture of old material (i.e., thatsoil organic matter present at the time of site preparation)and new material (i.e., that soil organic matter introducedthrough decomposition of organic residues discarded by thecurrent crop). Therefore, not only will the total amount ofsoil organic matter (and total N and P) vary among treat-ments, but so will the relative proportions of old and newmaterial, and thereby potentially bias mineralization results.

We accounted for microclimatic variation by incubatingsoil in the laboratory under standard conditions of temper-ature and moisture. To account for differences in total sub-strate quantity, we calculated N mineralization per unit oftotal N, and P mineralization per unit of total P. This spe-cific mineralization (Frazer et al., 1990) under laboratory

conditions provides a functional index of overall substratequality (Vitousek et al., 1983; Sollins et al., 1984). Finally,in order to collect soil containing the least old and the mostnew soil organic matter, we sampled from within furrowsfor the laboratory study. During site preparation, A horizonsoil is pushed up to form a new bed, thereby creating afurrow that, essentially, is an exposed E horizon. Soil or-ganic matter of the E horizon is low (< 0.5%), providing asmall background against which to measure mineralizationinputs from new soil organic matter.

We hypothesized that this protocol of collecting soil fromfurrows and incubating it in the laboratory to determinespecific mineralization offered the best possibility of de-tecting the effect of cultural treatments on the cycling of Nand P through soil organic matter. However, annual ratesof mineralization in the field are best determined by con-taining relatively undisturbed soil in situ (Adams et al.,1989). We used such a procedure in this study but, becausewe wanted the most realistic measures of mineralization insitu, soil was sampled from the interbed of each plot. Feederroots are prevalent here (Escamilla, 1990) and interbedsconstitute a substantial proportion of the total surface areaof the plot.

Spatial variability in soil nutrient pools dictates that sam-pling for in situ mineralization be reasonably intensive (Ad-ams et al., 1989). Therefore, we confined in situ sampling toonly one of the blocks so that the number of samples requiredfor analysis was kept to a logistically practical number. Allfour plots in loblolly pine were studied to determine a treat-ment effect. In slash pine, only the weed-control treatmentwas sampled, the results of which were compared with thecorresponding treatment of loblolly pine to determine a spe-cies effect. The weed-control treatments were chosen for thiscomparison in order to sample organic matter derived pri-marily from pine residues. The absence of fertilizer precludedany accidental contamination of samples.

We assumed that the substantial aboveground responsesto cultural treatments would be manifest in measurementsof mineralization in situ, and hence results would be self-evident even though plot replication was absent. As will beshown, the results for P mineralization support this conten-tion and we believe the lack of plot replication is compen-sated for by the intensive sampling scheme employed.

Laboratory IncubationsSoil was sampled from each plot on 12 Aug. and 29

Nov. 1989, and 31 Mar. 1990. On each of these dates, 20cores of surface soil (0-5 cm) were collected from randomlyselected positions in furrowed areas of each plot. Thesesamples were combined, sieved (< 2 mm to exclude coarseroots), and mixed thoroughly to provide one sample perplot.

For each bulked soil sample, mineralization potential wasdetermined by aerobic incubation. Specifically, a subsam-

Table 1. Selected chemical properties of soil collected fromfurrows and used for the laboratory incubations. The onlycultural treatment to affect any parameter was that of weedcontrol.

Loblolly pinet

W F WF

Slash pine_________ Main

W F WF effect*Total C(g kg-1) 10.6 16.9 13.6 15.0 10.8 15.0 11.5 15.9 ***Total N (mg kg-1) 462 699 564 676 451 683 447 689 ***Total P (mg kg-1) 25.8 35.4 36.7 37.8 26.9 34.1 25.9 37.4 **C/N ratio 22.9 24.2 24.1 22.2 24.0 22.0 25.7 23.1C/P ratio 411 477 371 397 401 440 444 425pH 3.9 3.8 3.8 3.8 4.0 3.9 3.9 3.9**,*** Significant at P < 0.01 and 0.001, respectively.t Treatment: R = reference; W = sustained weed control; F = annual

fertilizer application; WF = weed control plus fertilizer application.

Table 2. Selected chemical properties of soil collected frominterbed areas where mineralization in situ was measured.

Loblolly pinet

Total C (g kg-')Total N(mg kg-1)Total P(mg kg-1)C/N ratioC/P ratioMean pool of NO3-N (mg kg-1)Mean pool of total inorganic N(mg kg-1)

Mean pool of inorganic P(mg kg-1)

R24.7

82837.829.8

6530.080.49

0.12

W18.2

74237.824.5

4810.061.08

0.39

F31.0

105954.729.3

5670.040.96

1.46

Slash pineWF22.8

83342.127.4

5420.040.73

0.45

W18.1

73637.124.6

4880.081.28

0.30

t Treatment: R = reference; W = sustained weed control; F = annualfertilizer application; WF = weed control plus fertilizer application.

POLGLASE ET AL: NUTRIENT MINERALIZATION IN PINE PLANTATIONS 923

pie of soil was placed into a Nalgene filtering apparatusand adjusted to near field capacity (=0.01 MPa) by per-colating water under light suction. Twenty grams (freshweight) of this sample was placed in a 120-mL polyethylenebottle, which was capped and incubated at 38 °C for 42 d.Bottles were removed weekly for aeration, at which timemoisture contents were adjusted gravimetrically. Initialconcentrations of NH4-N, NO3-N, and Pf were determinedafter extraction in 0.1 M KC1 (soil/solution ratio of 1:5 [w/v], 1 h shaking time), and filtration through preleached0.45-|xm cellulose-acetate filter pads. Preleaching was nec-essary to remove NO3-N, the concentrations of which, al-though small in absolute terms, were large relative to thelow concentrations in soil. After 42 d of incubation, soilsamples were extracted with KC1 and final concentrationsof inorganic constituents determined. Subsamples of bulksoil were analyzed for total C, N, and P. Specific rates ofmineralization were calculated as the amount of N and Pmineralized during the 42-d incubation as a percentage ofthe organic substrate present initially.

A previous, unpublished time-course study of mineral-ization in these plots demonstrated that incubation for 42 dwas sufficient to detect significant differences among treat-ments. Furthermore, mineralization of P conformed to asimple exponential model (Stanford and Smith, 1972), withmineralization rate constants (k) similar to those recordedfor N mineralization (k between 0.031 and 0.045 [7 d]),which were also within the range reported for other N-mineralization studies (Stanford et al., 1973; Campbell etal., 1981; Poovarodom and Tate, 1988). This provides ad-ditional evidence that mineralized P was unaffected bysorption-desorption reactions.

We used 0.1 M KC1 to extract soil (instead of the usual1 or 2 M KC1) for convenience because salt concentrationhad no effect on the amounts of NH4-N and Pj extracted(data not shown). Again, this was due to the lack of ad-sorptive surfaces for these soils.In Situ Containments

In situ mineralization was studied using the method ofLemee (1967) as modified by Adams and Attiwill (1986).Beginning on 19 May 1990, ten polyvinyl chloride corers(5-cm i.d.) were inserted to 5-cm depth at random positionsin the interbed location of each plot. Root severance re-duced uptake of N and P by vegetation while direct leachinglosses were minimized by a test plug placed over the corer.The ten corers were collected after about 1 mo and the soilbulked into pairs to yield five samples for analysis. Thecorers were then reinserted into the soil to be left for another1-mo containment. Undisturbed soils (0-time samples) col-lected at the beginning of every containment period wereused to determine initial conditions. This procedure wasrepeated monthly for a period of exactly 1 yr.

For each sampling, soil was collected in the early morn-ing, sieved in the field (<2 mm) and returned to the lab-oratory for extraction as described in the laboratory-incubationprocedure. Filtered solutions were analyzed for NH4-N,NO3-N, and Pj either the same day of collection, or thefollowing day after storage at 4 °C. Monthly rates of net Nand P mineralization were summed to yield annual totals.The last four collections of 0-time samples were air driedand stored for analysis of total C, N, and P.

From 17 July onward, maximum and minimum soil tem-peratures were recorded for each month of containment by athermometer inserted to 5-cm depth in the center of each plot.Soil moisture contents were determined gravimetrically.Chemical Analyses

Total P and N in soil samples were measured after diges-tion in H2SOJtt2O2 at 340 °C. Inorganic constituents inthese acid digests, and those in KC1 extracts, were analyzedby manual colorimetric methods. Inorganic P was measured

by the procedure of Murphy and Riley (1962) and NH4-Nwas measured by the indophenol blue method (Keeney andNelson, 1982). Nitrate was measured as NO2-N followinghydrazine reduction (Mullin and Riley, 1955) using a boratebuffer (instead of phenol) as suggested by Best (1976). ThepH was determined in a 1:2 (w/v) soil/water slurry. Soilorganic matter was determined by the Walkley-Black method(Nelson and Sommers, 1982).

Statistical AnalysisFor the laboratory study, pools of nutrients and specific

N and P mineralizaton for each collection were tested forsignificant differences (P < 0.05) caused by the main ef-fects of species, weed control, and fertilizer application andfor second-order interactions by ANOVA using a split-plotdesign (Freund et al., 1986). Each plot was used as a rep-licate in this analysis because bulked samples (one sampleplot"1) were used for the laboratory incubations. The spe-cies x block interaction was used as the main error termwhen analyzing for the effect of species. The effect of timeon collection was tested for all variables by one-way AN-OVA. In the in situ study, lack of plot replication precludedthe use of statistical analyses.

RESULTSLaboratory Incubations

Concentrations of total C, N, and P in furrow soilswere significantly greater in soil collected from weed-control plots (P < 0.01) than where the understorywas present (Table 1). Increases in nutrient concen-trations were in proportion to those of organic matter,however, and so C/N and C/P ratios were not affected.The pH of these soils was about 3.9 (Table 1).

Concentrations of NO3-N in bulk soil were alwayslow (<0.08 mg kg-1 ODW), as were in situ rates ofnitrification. Rates of denitrification in other stands ofthis area are minimal (Boomsma, 1979), and leachinglosses of NO3-N are unaffected by harvesting and siteoperations such as burning of slash (Riekerk, 1981).Because of this unimportant role of NO3-N in eco-system N cycling, we present results for changes intotal inorganic N only (net N mineralization).

Specific rates of N mineralization varied little amongcultural treatments (Fig. 1). In the August sampling,the main effect of fertilizer application in increasingspecific N mineralization, although significant (P <0.01), was small and was not evident in the two sub-sequent samplings. In the August and March sam-plings, a fertilizer x weed control interactionsuppressed rates of mineralization (P < 0.05). Thatis, N mineralization in the combination treatments werelower than in the fertilizer treatments. The effect ofsampling time was significant (Fig. 1; P < 0.01).Specific N mineralization (averaged among all treat-ments) was 3.5% in August and 5.6% in Decemberand March.

Rates of P mineralization varied considerably (Fig.2). In the August collection, specific P mineralizatonwas only 0.31% in the reference plots. In all otherplots, specific P mineralization was 3.6%, but onlythe effect of fertilizer application was significant (P< 0.05). The stimulatory effect of fertilizer applica-tion was again evident in the subsequent collections,but was significant only in March (P < 0.05). For allthree collections, a fertilizer x weed control inter-action inhibited specific P mineralization. In March,

924 SOIL SCI. SOC. AM. J., VOL. 56, MAY-JUNE 1992

g'coN

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O

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8-

6-

4'

2-

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B-

4-

2-

F* August 1989FxW**

_

u.I r-- T1

November 1989

I

I

T

T

T

I FxW*

T TT | T

F* March 1990T SP**I FxW*

FxSP" T

T

--1

1

R W F WF R W F WF

SlashLoblollyTreatment

Fig. 1. Laboratory rates of specific N mineralization in relationto cultural treatment, species, and time of sampling. SpecificN mineralization is calculated as the total amount of Nmineralized during a 42-d aerobic incubation as a percentageof total N present initially. Treatments: R = reference; W= sustained weed control; F = annual fertilizer application;WF = weed control plus fertilizer. *,**,*** Main effects ofW, F, species (SP), and second-order interactions significantat P < 0.05, 0.01, and 0.001, respectively. Error barsrepresent one standard deviation.

specific P mineralization in fertilized plots of loblollypine was greater than in slash pine (P < 0.05).

In Situ ContainmentsThe strong weed-control effect on concentrations of

soil C, N, and P in furrows was not evident in soilcollected from interbeds (Table 2). Concentrations ofall these variables in interbeds were higher than in soilcollected from furrows. Concentrations of inorganicN in the bulk soil were extremely low, with NH4-N(averaged across 12 collections) being <1.3 mg kg"1

CoN

10)

C0)

"co'o8.

CO

R-

6-

4-

2-

o-8-

6"

4-

2-

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F** August 1989FxW

1

T

T

T

JL

I

T T T T

November 19

_ Ti i-

T

89

1

FxW* March 1990

1TIT JLl T

T

R W F WF

Loblolly

R W F WFI__________i

SlashTreatment

Fig. 2. Laboratory rates of specific P mineralization in relationto cultural treatment, species, and time of sampling. SpecificP mineralization is calculated as the total amount of Pmineralized during a 42-d aerobic incubation as a percentageof total P present initially. Treatments: R = reference; W= sustained weed control; F = annual fertilizer application;WF = weed control plus fertilizer. *,**,*** Main effects ofW, F, species (SP), and second-order interactions significantat P < 0.05, 0.01, and 0.001, respectively. Error barsrepresent one standard deviation.

ODW. The mean concentration of P; was 1.46 mgkg-1 ODW in the fertilizer treatment, but was <0.45mg kg-1 ODW in all other plots.

Specific annual rates of N and P mineralization inloblolly pine were greatly affected by cultural treat-ments (Fig. 3). The proportion of total N mineralizedwas 0.007% in the reference, about 0.55% in both thefertilizer and weed-control treatments, and 0.77% inthe combination treatment. Specific P mineralizationwas —0.64% in the reference (i.e., net immobiliza-tion), about 1.6% in the weed-control treatment, and8.2% in both fertilized treatments (Fig. 3).

POLGLASE ET AL: NUTRIENT MINERALIZATION IN PINE PLANTATIONS 925

^ 0.6-0z1| 0.4-0

^ 0.2-0^

O

min

eral

izati c

en o

o c

.y 4-'00 2H

COo-

-2-

N

P

' ——— '

R W F WF

Loblolly

W

Slash

TreatmentFig. 3. Annual rates of specific N and P mineralization in situ.

Specific mineralization is the amount of nutrient mineralizedas a percentage of the total present initially. Treatments: R= reference; W = sustained weed control; F = annualfertilizer application; WF = weed control plus fertilizer.

The patterns of areal mineralization rates were sim-ilar to those of specific mineralization (Fig. 4). In thesurface 5 cm of the reference plot, both N and P min-eralization were <0.1 kg ha-1 yr-1. The maximumrate of N mineralization was 4.5 kg ha-1 yr-1 in thecombination treatment, while P mineralization wasgreatest in both the fertilized treatments (mean of —2.1kg ha-1 yr"1). Both specific and areal rates of N andP mineralization in the two weed-control treatments(species comparison) were remarkably similar (Fig. 3and 4).

DISCUSSIONThe most notable feature of these results is the con-

sistent stimulatory effect of fertilizer application onspecific P mineralization. This suggests that some ofthe fertilizer P, having been taken up by plants andreturned to the soil via decomposition of residues, wasconverted into organic components that were readilymineralized. In contrast, specific N mineralization wasnot affected by cultural treatment in the laboratory,suggesting that the general composition of organic-Nsubstrates was unchanged. Our previous results fromthe IMPAC site support this conclusion, and help toexplain how the dichotomous response of N and P tofertilizer application might have evolved. Fertilizerapplication caused the concentration of total P in fresh

to

I.0

CD

5'

4'

3"

2"

1 •

o-1 -

I — |

__

• !

DN• P

PPB

W WF W

Loblolly SlashTreatment

Fig. 4. Annual areal rates of N and P mineralization in situ.Treatments: r = reference; W = sustained weed control; F= annual fertilizer application; WF = weed control plusfertilizer.

needle litter (Oi horizon) to nearly double. This dou-bling was accounted for by an increase in the propor-tion of total P that was inorganic. Hence, P; was about40% of total P in needles from nonfertilized plots andabout 75% of total P in fertilized plots (Polglase etal., 1992b). Cold water readily leached this P; fromintact needles of fertilized plots, but not from needlesof nonfertilized plots (Polglase et al., 1992a). After 1yr of decomposition in the field, about 50% of theinitial P content of Oi horizon needles had been lost,and again this depletion was accounted for by loss ofPI (Polglase et al., 1992c). On an absolute basis, lossof P from Oi horizon needles in fertilized plots wasnearly twice the loss from needles in nonfertilized plots.On the basis of this evidence, it is clear that P is verymobile, due largely to accumulation and loss of P, inresidues, and that this inorganic component is readilymanipulated by fertilizer additions. Presumably, thismobility extends into the soil profile where the excessP in residues of fertilized plots is synthesized into estercompounds that are readily mineralized.

Fertilizer application did not cause an increase inconcentrations of N in Oi horizon needles. No inor-ganic N was detected, and only about 20% of total Ncould be extracted by 0.30 M trichloroacetic acid (Pol-glase et al., 1992b). As a result, no N could be leachedfrom intact needles (Polglase et al., 1992a) and N wasimmobilized during the initial stages of decomposition(Polglase et al., 1992c). These results, together withthose of this study, demonstrate the inability of fer-tilizer to elicit a change in the chemistry or turnoverrates of N throughout the decomposition profile. Note,this does not infer that N was not limiting in this site.Because of covalent bonding, N tends to maintain aclose and somewhat constant association with C (McGilland Cole, 1981); therefore, N dynamics are not aseasy to manipulate as are P dynamics.

Another striking result was that, in the laboratorystudy, a fertilizer (F) x weed control (W) interactionconsistently inhibited specific rates of N mineraliza-tion (Fig. 1) and, particularly, P mineralization (Fig.2). In weed-control treatments, inputs of organic mat-

926 SOIL SCI. SOC. AM. J., VOL. 56, MAY-JUNE 1992

ter are derived solely from decomposition of pine res-idues. In the other treatments, inputs of organic matterare derived from decomposition of both pine and un-derstory residues. Therefore, the negative F x W in-teraction strongly suggests that, with respect to Pnutrition, residues of understory vegetation are of greatersubstrate quality than are residues of pine trees. Theunderstory community in these plots is dominated bywiregrass (Aristida stricto Michx.), chalky bluestem(Andropogon capillipes Nash.), panic grasses (Pani-cum spp. and Dichantelium spp.) sawtooth palmetto[Serenoa repens (B.) Small.] and gallberry [Ilex gla-bra (L.) Gray]. Presumably, the residues of these sub-ordinate plants are less recalcitrant than those of pine,and hence encourage more rapid rates of decomposi-tion and nutrient turnover. The beneficial effects de-rived from decomposition of understory residues must,however, be balanced against the negative effect ofthis vegetation in competing with pine roots for avail-able resources. Understory plants are most abundantin the first few years following site preparation (Gholzet al., 1985b), and at this time competitive effects aredetrimental to pine trees (Conde et al., 1987; Nearyet al., 1990b). During stand development, the over-story canopy closes and understory vegetation be-comes suppressed. At that time, the beneficial effectsderived from the relatively rapid turnover of unders-tory residues is undoubtedly greatest.

No in situ mineralization method can ever yieldabsolute and accurate measures of field rates. This isdue to the vagaries of the methods used, all of whichsever plant roots. Thorough discussions of the in situmethodology have been presented elsewhere (Adamset al., 1989; Vitousek et al., 1989). Nonetheless, insitu studies can provide valuable information whencompared with other components of the N cycle inforests (Pastor et al., 1984), and in this context theresults from our study (Fig. 4) can be usefully com-pared with calculated rates of uptake at the IMP ACsite.

We calculated annual aboveground P uptake as thesum of P accumulated into the standing crop, thatdiscarded in litterfall, and that removed in throughfall(Cole and Rapp, 1980). Incremental gain was esti-mated by applying diameter and height growth mea-sured during the study period to allometric regressionsdeveloped for these trees at age 4 yr (S.R. Colbertand E.J. Jokela, 1991, unpublished data). The acces-sion of nutrients in litterfall was quantified by Dalla-Tea (1990), while the results of Gholz et al. (1985a)were used to estimate canopy leaching. We confinedour calculations to those plots where the understorywas absent.

In loblolly pine, annual aboveground P uptake was6.7 kg ha-1 in the weed-control treatment, and 11.7kg ha""1 in the combination treatment. In slash pine(weed-control treatment), P uptake was 3.6 kg ha-1.Allowing for the uncertainties in these measurementsand those associated with the in situ core method, itis still clear that, in the surface 5 cm of soil (the depthto which mineralization was measured), supply of Pin nonfertilized plots (<0.7 ha-1 yr-1) was far lessthan demand. In contrast, P mineralization in the fer-tilized plots ( = 3 kg ha-1) was relatively high. Given

that P often limits productivity of stands in this region(Pritchett and Llewellyn, 1966; Neary et al., 1990a),the rapid recycling of P through organic residues offertilized plots has the potential to enhance long-termproductivity beyond the immediately beneficial effectsderived from fertilizer uptake.

ACKNOWLEDGMENTSFinancial support was provided primarily by the industrial

members of the Cooperative Research in Forest Fertilization(CRIFF) program at the University of Florida. Additionaltechnical assistance was provided by the USDA Forest Service(Dr. D.G. Neary), and by Dr. A.C. Mace, Jr. We thank M.McLeod for laboratory assistance and S. Stearns-Smith andF. Dalla-Tea for statistical analyses.

IMO & TIMMER: NITROGEN UPTAKE AT CONVENTIONAL AND EXPONENTIAL FERTILIZATION SCHEDULES 927