nutrient cycling and fertility management in temperate short rotation forest systems

NUTRIENT CYCLING AND FERTILITY MANAGEMENT

IN TEMPERATE SHORT ROTATION FOREST SYSTEMS

PAUL HEILMAN*{ and RICHARD J. NORBY$*WSU Puyallup Research and Extension Center, Puyallup, WA 98371, U.S.A.,

$Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422,U.S.A.

(Received 4 April 1997; revised 17 November 1997; accepted 2 December 1997)

AbstractÐUnder most conditions, fertilizers will be required to maintain production of short rotationforestry (SRF) plantations. Information from fertilizer trials together with knowledge of general soilfertility in an area permits approximation of fertilizer requirements. Re®ning those approximations forspeci®c plantations is important for the following three reasons: the need to assure high production; theneed to minimize production costs; and the desire to limit o�-site e�ects of fertilizer application. Tomeet those goals, requires understanding the behavior of fertilizer in soils including leaching, immobiliz-ation and, in the case of nitrogen, denitri®cation. Knowledge of nutrient cycling in SRF including nutri-ent removal at harvest, other nutrient losses, and natural inputs of nutrients, helps in achieving goodfertilizer practices. Cropping strategies that minimize fertilizer use can lower costs and reduce o�-sitee�ects of fertilizing. This review summarizes current knowledge of nutrient cycling, cropping strategiesand fertility management in temperate SRF plantations. # 1998 Elsevier Science Ltd. All rightsreserved

KeywordsÐNutrient removal in harvests; nitrate leaching; denitri®cation; fertilizer requirements; crop-ping strategies for SRF plantations; sustainability of SRF; waste disposal with SRF; soil pH and SRFplantations.

1. INTRODUCTION

The major environmental concerns with short

rotation forest (SRF) systems in relation to

plant nutrients include:

1. non-point source consequences of fertiliza-

tion and irrigation to surface water and to

groundwater;

2. changesÐeither depletion or accretion in

elemental content of the soil; and

3. disposal of ash or other wastes left after

burning or other processes using the bio-

mass.

It is felt that appropriate fertility and irriga-

tion management of SRF is fundamental for

minimizing the above environmental concerns.

Also, the authors' position is that nutrient el-

ements need to be applied to avoid both loss

of production and undue depletion of soil fer-

tility. Minimizing contamination of surface

waters requires proper use of nutrient amend-

ments and also minimizing runo� and soil ero-

sion. Avoiding damage to groundwater also

requires proper use of nutrient amendmentsand, where the crop is irrigated, appropriateirrigation practices. Proper and sustainableirrigation means that enough salts be leachedfrom the soil pro®le to avoid their buildup inthe soil to harmful levels. Disposal of wastecan be satisfactorily accomplished by returningthe material to the soil provided the sameprinciples to avoid nutrient overloading arefollowed as for other fertility amendments.

The rationale for writing the paper is theconviction that understanding nutrient cycling,speci®cally in SRF, is vital for developingguidelines for nutrition management that mini-mize environmental damage. The current stateof knowledge on nutrient cycling in SRF plan-tations is reviewed and cropping strategiesthat reduce the need for fertilization are con-sidered.

2. NUTRIENT CYCLING IN SRF PLANTATIONS

2.1. Nutrient removals in harvest

Much of the current concerns about en-vironmental e�ects of short rotation forestryrelate to nutrient removals in harvest (Table 1).

Biomass and Bioenergy Vol. 14, No. 4, pp. 361±370, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0961-9534/98 $19.00+0.00PII: S0961-9534(97)10072-1

{Author to whom correspondence should be addressed.

361

In a conventional forestry context, this con-

cern is entirely appropriate. The whole treeharvesting together with the short harvest

cycles, often with leaves intact, result in a

nutrient depletion rate that is far greater than

for conventional forest harvests. Yet, in the

context of agriculture, where most if not all

short rotation forestry systems are ®ndingtheir home, SRF seems to be an ordinary user

of nutrients.

Research studies show that quantities of

nutrients removed from SRF harvest vary

with yield, species of tree or clone and time of

harvest. In a study by the senior author withR. F. Stettler employing six black cottonwood

(Populus trichocarpa) clones, two TxD (P. tri-

chocarpa x P. deltoides) hybrids and the

Euramerican clone `Robusta', nutrient

removals in a harvest at age four (excluding

leaves) varied from 95 to 420 kg N haÿ1, 14±105 kg P haÿ1, and 80±288 kg Ca haÿ1

(Table 1).1 However, SRF trees are usually

harvested throughout the year, including when

leaves are present. If leaves are removed from

the site, nutrient removals will be signi®cantly

greater than these numbers. The quantity of

nutrients removed with leaves will varydepending on time in the growing season and

degree of removal of leaves from the site.

E�orts to retain leaves on the site seem war-

ranted.

Nutrient removal in the above study1 was

related to yield, but clones varied substantiallyin their e�ciency of nutrient use in terms of

unit of woody biomass produced per unit of

nutrient consumed. Black cottonwood clones

produced more wood per unit of N and P

than both hybrid types with `Robusta' using

much more N and Ca per unit of wood pro-duction than the other materials. High use of

nutrients by the Populus deltoides parentspecies (Baker and Blackmon, unpublisheddata reported in Hansen and Baker2) corre-sponds to the high use by the TxD hybridsinvestigated in this study. The quantity of Kin above ground woody tissues of 4-year-oldSRF Populus plantations ranged from 86 to118 kg haÿ1] (Table 1).2,3

2.2. Nutrient losses by leaching, denitri®cationand other processes

Few data have been reported on nutrientlosses in SRF plantations other than forremovals in harvest. Such losses can occurfrom leaching, runo�, erosion and in the caseof nitrogen, from denitri®cation. In SRF,most of these losses, except for denitri®cation,are likely to occur in early years of establish-ment and following harvest because of lack ofsoil protection during these periods and theabsence of nutrient uptake by the crop. Undernatural forest conditions, quantities of nutri-ents leached from soils are usually almost neg-ligible.5

However, inappropriate fertilization with Nfertilizer can result in high nitrate levels belowthe rooting zone of young, shallow-rootedSRF species. In an American sycamore(Plantanus occidentalis) plantation, nitrate con-centrations at 60 cm (below rooting depth)were strongly in¯uenced by age of the treeswhen fertilized.4,6 Nitrate at this depth wasalso in¯uenced by rate and frequency of ureaadditions. Following an application of ureathat was made when the trees were ®rstplanted, nitrate concentrations in soil waterexceeded 10 mg lÿ1 for several months in theplots receiving either 250 or 450 kg N haÿ1.With no further additions of fertilizer in sub-sequent years, the N concentration in soil

Table 1. Nutrient removals in SRF harvests

Nutrient content in above-ground boles and branches(kg haÿ1)

Material Age (years)Biomass(Mg haÿ1) N P K Ca References

Black cottonwood 4 29±72 95±233 14±34 Ð 80±205 1

Eastern cottonwood 4 27±34 101±128 18±22 92±118 203±262

Baker andBlackmon

(unpublished)in Refs2,3

Hybrid poplars 4 44±111 241±420 41±105 Ð 159±288 1

Hybrid poplarEuramerican

4 37 213 31 86 126 2

American sycamore 4a 28±43 112±197 Ð Ð Ð 4

aThe plantation was established with 1-year-old seedlings and hence, the plantation was 3 years old.

PAUL HEILMAN and RICHARD J. NORBY362

water quickly fell to control values. In theplots that received annual springtime additionsof urea, nitrate concentrations increasedrapidly and remained high throughout thesummer. As the trees got older and larger,there was less nitrate in soil water, probablyre¯ecting greater uptake of nitrate by thetrees. Based on nitrate concentration and esti-mates of monthly water ¯uxes through thesoil, ca 20±40% of the fertilizer leached below60 cm deep in the soil. Comparison of a singleapplication of fertilizer at a high rate withsplit applications giving the same total amountof fertilizer showed no signi®cant di�erence intotal leaching loss. However, the single heavydose caused much higher NOÿ3 concentrationsin soil water.

Depth of rooting is an important factorin¯uencing both the ability of SRF plantationsto access nutrients and soil water deep in thesoil pro®le and to prevent leaching of mobilenutrient ions such as nitrate. Whereas roots ofthe sycamore plantation were limited to thetop 60 cm, rooting depth of the hybrid cotton-woods exceeded 3.2 m.1,19 Rooting depth anddistribution is in¯uenced by soil conditions aswell as the plant. Knowledge of rooting depthon speci®c sites is important for managing fer-tility.

The nitri®cation potential of the soil is acritical factor governing the quantity of nitro-gen in soil water following application of ureaand ammonium sources of N. Soil incubationmeasurements suggested that the soil in thesycamore plantation had a large nitri®er popu-lation and a high nitri®cation capacity. Hence,the added urea was rapidly converted tonitrate. On a di�erent soil, the size and ac-tivity of the nitri®er populations was limitedby N substrate availability.7 In that soil,repeated annual applications of N signi®cantlyincreased the potential for nitri®cation. Abuildup of nitri®ers in response to urea fertili-zation caused higher soil nitrate levels follow-ing split applications (four times per year)than following a single (one per year) appli-cation of the same total quantity of N.8

Application of nitrogen to cropland, eitheras fertilizer or organic wastes, appears to sub-stantially increase denitri®cation.9 Only nitrateis subject to denitri®cation, and as much as70% of the applied nitrogen can be unac-counted for. Since denitri®cation requires anenergy source and anaerobic conditions, appli-cations of organic forms of nitrogen to wet

soils maximize denitri®cation losses.Maintaining high soil nitrogen levels, as hasbeen recommended for SRF willow,10 likelyresults in large losses from denitri®cationwhich, under some conditions, can be as highas 150 kg N haÿ1 yrÿ1.9

Without denitri®cation, leaching of nitrateto groundwaters from both nitrogen fertilizingand disposal of organic wastes would be amuch more frequent and serious problem. Forinstance, an experimental treatment that fea-tured weekly treatment of SRF willow withsmall quantities of dissolved fertilizer appliedwith irrigation resulted in nitrate levels as highas 34.1 mg N lÿ1 in soil leachate.11 However, afollow-up investigation at the site with 15N-labeled fertilizer indicated no leakage of thelabeled material to the ground water.12 Suchresults suggest that whether or not nitrate ac-cumulates in the groundwater in a given situ-ation probably depends as much ondenitri®cation activity as on nitrate loading.

2.3. Natural inputs of nutrients

Nutrients are added to soils in precipitation,dust and other depositions, weathering of soilminerals and, in the case of nitrogen, as aresult of biological nitrogen ®xation. Most ofthe latter occurs as a result of symbiotic nitro-gen ®xation by Rhizobium and Frankia spp.Such ®xation by stands of red alder (Alnusrubra) can be as high as 150 kg N haÿ1 yrÿ1.13

For discussion of nitrogen ®xing species andSRF plantations, see Section 3 of this paper.

The quantities added in precipitation varyconsiderably among nutrients and geographi-cal areas. Estimates of N deposition to NorthAmerican forest stands range from 4 to53 kg haÿ1 yrÿ1]. Nitrogen deposition inEurope is higher: 11±64 kg haÿ1 yrÿ1, with onereport from the Netherlands as high as115 kg haÿ1 yrÿ1.14

Clearly, the lower N levels in precipitationare insigni®cant for SRF production levels ashigh as 40±115 kg N haÿ1 yrÿ1, on the otherhand, may supply all the N that is needed forSRF production (see Section 4).

2.4. Annual lea�all and its nutrient content

Lea�all is the most obvious agent of nutri-ent cycling in perennial crops. In a study ofblack cottonwood and its hybrids, annual leaf-fall in the fourth year varied from 4.4 to6.6 Mg haÿ1 and was generally related to clo-nal productivity (Table 2). The high leaf pro-

Nutrient cycling and fertility management in SRFs 363

duction by the TxD hybrids is comparable tothe highest values reported for hybrid poplarin close-spaced short-rotation culture and inolder plantations.2 Highest lea�all productionby black cottonwood is comparable to that of23- to 30-year-old stands of red alder15 and isconsiderably higher than the 1.1±2.0 Mg (lea-ves) haÿ1 on 11-year-old eastern cottonwoodsfrom three sources.16 Mean concentrations ofN in the lea�all were relatively similar amongclones with only `Robusta' being substantiallyhigher, and one TxD hybrid substantiallylower than the others. Generally, nitrogenconcentration in the lea�all declined over thegrowing season (data not shown). However,the pattern of lea�all di�ered among clones.Loss of leaves began earlier in the black cot-tonwood with several of these clones havingboth an early and a late peak in the pattern ofloss. Leaf infection by Septoria andMelampsora in several of the black cotton-wood clones, but not evident in the hybrids,was likely partly responsible for the earlylosses. Since N concentration was higher inthe earlier-shed leaves, one e�ect of early leafloss was to increase mean concentration oflea�all N. Consequently, relative to their pro-ductivity, the black cottonwood clonesreturned more N in lea�all than did the TxDhybrids. `Robusta' lea�all was also high in Nas a result of the inherently high foliar nitro-gen concentration and perhaps the early leafabscission leaf characteristic of this clone.

2.5. Nutrient transfer from root mortality

A less obvious and very much more di�-cult-to-measure component of nutrient cyclingis the nutrients returned to the soil because ofroot mortality. In some forest systems, ®neroot turnover is considered to be a major com-

ponent of annual nutrient cycling.17 Similar

studies have not been done with short rotation

forestry. Tentative evidence from mini-rhizo-

tron observations of some poplar clonessuggest low rates of mortality for ®ne roots

with the studied clones (D. I. Dickmann,

Michigan State University, East Lansing, per-

sonal communication). Indications from a

study by Heilman et al.18 showed a life of a

few months for some of the initial roots on

poplar cuttings. In view of Dickmann's obser-

vations, the observed high mortality of initial

roots from cuttings may be a feature unique

to cuttings. Clearly, the potential signi®cance

of ®ne root mortality to soil fertility and con-

tent of organic matter in Populus and other

SRF species is unknown.

The magnitude of root biomass in SRF sys-

tems is of interest relative to mortality and as-

sociated nutrient cycling. In a study of coarse

and ®ne roots in 4-year-old TxD hybrids and

their parental species, dry weights of stumpsand attached coarse roots of eight clones aver-

aged from 12.3 to 29.6 Mg haÿ1.19 The lower

values were from the parental species and the

higher from the hybrids. The ratio of biomass

of stumps and coarse roots to above-ground

lea¯ess biomass averaged 0.22±0.33 with no

clear di�erences in the ratios between hybrids

and parental species (Table 3). Weights of

roots remaining in the soil were collected by

sampling with soil cores to a depth of 3.18 m.

The quantity of these roots varied from 6.6 to

11 Mg haÿ1 with no signi®cant di�erences

among the clones (Table 3). Weights of the

®nest roots only, that is those <0.5 mm in di-

ameter, varied from 4 to 6.5 Mg haÿ1 with sig-

ni®cant di�erences among clones beingpresent. The values for these 4-year-old trees

generally exceeded the 2.1±5.6 Mg haÿ1 of ®ne

Table 2. Nitrogen in lea�all from SRF1

Nitrogen in leaf fall

Materials Age (years)

Woody biomassproductiona

(Mg haÿ1 yrÿ1)Annual leaf fall(Mg haÿ1 yrÿ1)

Concentrationb

(Mg gÿ1) Weight (kg haÿ1)

Black cottonwoodLow yieldingc 4 7±14 4.4±5.1 1.29±1.43 62±73High yieldingd 4 16±18 5.0±5.6 1.38±1.44 69±80

TxD hybrids 4 27±28 5.9±6.6 1.21±1.42 80±84`Robusta' hybrid 4 11 4.9 1.66 82

aMean annual production over the ®rst 4 years.bWeighted average over the period of collection in the fourth growing season.cProductivity of these clones was not signi®cantly greater than productivity of `Robusta'.dProductivity of these clones was signi®cantly above `Robusta'.


roots reported for 40- to 170-year-oldDouglas-®r20, but that comparison is con-founded by the short-root morphology ofmycorrhizal Douglas-®r roots. Considerablevariation was evident among the clones in theratio of biomass of ®ne roots to above-groundbiomass. That ratio averaged ca 6.8% orsomewhat above the 5% reported for mostforest stands.20

Harvest without stump removal can leave inthe ground in the form of stumps and rootsfrom 34 to 42% of above-ground lea¯ess bio-mass (based on 4-year-old Populus).19

Consequently, they and their attached rootsrepresent a substantial pool of organic ma-terial and nutrients for the subsequent crop.However, their contribution is likely no morethan is obtained from the sod and grass rootswhen grassland is converted to SRF planta-tions.

2.6. Uptake of nutrients

Under most circumstances, N is the elementmost limiting to growth. Annual uptake of Nin SRF plantations is largely in proportion toabove ground productivity. In a study of 4-year-old short rotation Populus, annual uptakeof N excluding root tissues ranged from 95 to276 kg N haÿ1.1 The clone `Robusta' in thatstudy, with its high concentration of nitrogenboth in woody and leaf tissue, was consider-ably higher in uptake relative to productivitythan other clones in the study. Fertilizationwith N also can increase N content of woodybiomass.4 The above nitrogen uptake values in

above-ground tissues are comparable to Nuptake of many agricultural crops.21

Although ®gures were presented from astudy of root and stump biomass (Table 3), noestimates were made in that study of eithermean N concentration in roots and stumps orof the total N contained in those tissues.Without such data, it is not possible to esti-mate total annual uptake of N for the combi-nation of above- and below-ground tissues. Inview of the 34±42% of above-ground biomassrepresented by stumps and roots, and recog-nizing the likelihood that N concentration inbelow-ground tissues is likely less than inabove-ground tissues which includes leaves,annual uptake of N in all the tree's tissue isprobably not more than 130% of uptake byabove-ground tissues.

3. CROPPING STRATEGIES FOR MINIMIZING THENEED FOR NUTRIENT AMENDMENTS

Cultural options for most SRF speciesinclude a range of spacing distances andarrangements: rotation ages from 1 to 8 ormore years; management of standards (re-establishment after each harvest or as cop-pice); and plantations either as pure monocul-tures of clones or as mixtures of a number ofclones. Longer rotations probably help mini-mize fertilizer needs:

1. by increasing the harvest index, that is, theratio of bole wood to total biomassremoved, since bole wood is the componentof biomass with the lowest concentration ofnutrients; and

Table 3. Below- and above-ground (lea¯ess) dry weight of biomass of Populus at 4 years19

Below-ground biomass dry matter (Mg haÿ1) Root/shoot ratiosa

Clone No.Stumps andlarge roots

Small and ®neroots

Total stumpsand roots

Above-groundlea¯ess biomass

(Mg haÿ1)Stumps andlarge roots

Total stumpsand all roots

P. trichocarpa10±12 12.3 d 6.9 a 19.2 b 54.8 b 0.22 0.3580±083 19.4 bcd 6.6 a 26.0 ab 69.9 ab 0.28 0.37

P. deltoidesST 66 14.2 cd 8.1 a 22.3 b 54.9 b 0.26 0.41

TxD hybrids11±005 21.4 abc 7.2 a 28.6 ab 84.3 ab 0.25 0.3419±056 29.6 a 8.2 a 37.8 a 90.5 a 0.33 0.4244±136 25.1 ab 9.3 a 34.4 a 85.7 ab 0.29 0.4047±160 18.7 bcd 9.6 a 28.3 ab 67.8 ab 0.28 0.4255±258 24.4 ab 11.0 a 35.4 a 93.7 a 0.26 0.38

Note: means in columns not followed by the same letter are signi®cant at P = 0.05 according to Duncan's multiplerange test.

aRoot dry weights/dry weights of above ground lea¯ess biomass.


2. with less frequent harvests, the opportunityfor soil compaction, erosion, denitri®cationand leaching is less.

Soil compaction, by limiting rooting, likelyincreases the need for nutrient additions.Consequently, within the context of SRF,minimizing need for nutrient additions callsfor the longest possible rotation periods,which for the Paci®c Northwest of the UnitedStates, means rotations of 8±10 years orlonger. In turn, rotations of that length callfor somewhat wider spacings, up to 3�3 m ormore instead of the more common 1.5±2�3 m.

Another option to minimize fertilizer appli-cations is use of nitrogen ®xing plants. Somenitrogen ®xing tree species suitable for SRFplantations include Alnus spp., Robinia spp.and in semi-tropical areas, Albezia spp. Sinceyields from these trees are generally substan-tially less than for most non-N-®xing SRFspecies, strategies for their use include ro-tations with the more productive types anduse in mixed stands. Yields of alder andPopulus mixes are generally unsatisfactory.22±25 For instance, in a study of a 1:1 mixture ofPopulus hybrids and red alder, ®xation of Nby the alder became negligible after the secondyear when the alders became overtopped andshaded by the faster growing Populus.24 Adi�erent planting arrangement that would pro-vide less shading of the alders could prove tobe satisfactory. One mixture reported to elim-inate the need for N fertilizer without loss ofproduction combined Eucalyptus withAlbezia.26

Options that mix trees with agriculturalcrops, usually as interplantings between treerows, have been used in many countries; how-ever, they have not been strategies associatedwith SRF plantations in the U.S.A. They maywell, however, minimize use of nutrientamendments for the trees, since nutrientsapplied to the intercrop are likely partly uti-lized by the trees.

4. FERTILITY MANAGEMENT FOR SRF

An understanding of nutrient cycling pro-cesses can lead to environmentally sound man-agement in SRF plantations. Several optionsare available to address the question of nutri-ent losses connected with SRF productionwith the most important one being use of

nutritional amendments. Potential amend-ments in addition to commercial fertilizerinclude a variety of waste products, such asyard waste, animal manures, biosolids ande�uents of various kinds. The other optionsavailable to address the question of nutrientlosses involve strategies to minimize nutrientlosses. These include harvest during the dor-mant season when leaves are absent, returningbranches and bark (and leaves, if present) tothe soil, and planting species and clones thatare most e�cient in using nutrients for pro-duction. Probably the most important aspectof fertility management is to develop prescrip-tions for fertilizer use on speci®c SRF cropsand on speci®c sites. Unfortunately, determin-ing the quantity of nutrients supplied by a soilis di�cult. With SRF being a relatively newcropping system, information on fertility man-agement is still fairly limited. Some guidanceis available from the more abundant fertilityresearch in conventional forestry and agron-omy, but more study of nutrition managementfor SRF systems is needed.

Environmental consequences of fertilizeruse, particularly the e�ect on water quality,are becoming increasingly important.Generally, the species used in SRF plantationsdi�er somewhat in their requirements for soilnutrients, but all appear to need a fairly highsupply of nutrients to maintain high levels ofproduction. Ideally, several factors need to beconsidered in specifying the appropriate rateof fertilizers applied to a crop under a givensituation. These include nutrient requirementsfor the speci®c cultivar or species, soil fertilitystatus, moisture available to the crop, fertilizercost and environmental concerns. For lowvalue crops, such as trees, fertilizer costs arede®nitely of concern, but with high valuecrops, the cost of fertilizer can be consideredirrelevant. Soil testing guidelines developedthrough ®eld research are commonly used inagriculture, but soil testing has more limitedapplication to forest and SRF crops.

4.1. Fertilization strategies

Two contrasting approaches to fertilitymanagement for SRF crops are evident in theliterature. The more common approach andthe more conservative in the use of fertilizer isnot overly concerned about depletion of ferti-lity as long as productivity is not signi®cantlyreduced. Under this approach, fertilizers areapplied only when the diminished supply of a


nutrient begins to impact growth.27 Such a

course minimizes fertilizer use, at least in-itially. It also maximizes both e�ciency of

nutrient use by the crop and economic return

on the fertilizer investment. However, it does

not assure maximum yield and, thus, may not

give maximum pro®t with high value crops.

This appraisal also does not address the issue

of sustainability of SRF crop systems. Thisconservative approach to fertilization of

poplars was recommended by Dickmann and

Stuart.28 These authors advocated fertilizers

be used only when de®ciency symptoms

become evident. Furthermore, because of fre-

quent failures of poplar plantations to respond

to fertilizer applied the ®rst year after plant-

ing, these authors recommend that fertilizationbe delayed to the second or third year after

establishment.

The contrasting approach seeks to maintain

fertility at a high `steady state' level in order

to assure optimal nutritional status of the

crop. Under this method, high rates of fertili-

zer are required not only to supply nutrientsto the growing crop but also to steadily

increase soil fertility. This approach is more

likely to be used in higher value agricultural

crops, but was used in SRF willow plantations

by Ingestad and Agren.29 If soil fertility is

very low, high rates of fertilizer may be necess-

ary for several years before high soil fertilitystatus is achieved. When high fertility is

reached, the quantity of fertilizer applied is

reduced but not to the point that soil fertility

would begin to decline. Fertility management

for SRF plantations should, in most cases, fall

somewhere between the above two

approaches. Maintaining a high nitrogen ferti-lity level is costly and substantially increases

the potential for leaching and denitri®cation

losses. In view of today's environmental con-

cerns, the major advantage of conservative

approach is minimizing nutrient losses to the

environment. The drawback of the conserva-

tive approach is in the risk of reduced yieldssince fertilizer is not applied until nutrient de-

®ciency is approached, a condition that can be

di�cult to determine. For instance, in one ex-

periment with SRF Populus TxD hybrids, N

fertilization that increased the foliar N concen-

tration from 25 mg gÿ1, a level often con-

sidered adequate, to 32.5 mg gÿ1 resulted in a22% increase in bole dry weight yield at

4 years of age.30

Organic materials, such as animal manures,municipal sludge (biosolids), composts and avariety of liquid e�uents, can be used tosupply nutrients (and irrigation in the lattercase). In a comparison of growth responsebetween manure and chemical fertilizers, oneinvestigator reported that nitrogen in manurewas more e�ective for `Robusta' poplar.31

However, his claim seems doubtful since therecommended rates for the pig slurry of 30±80 m3 haÿ1 yrÿ1 with an N content of7 kg N mÿ3 would provide upwards of560 kg N haÿ1. Clearly, such N loading isbeyond most rates recommended for commer-cial fertilizer on SRF crops.

4.2. Soil pH

Nutrient availability is a�ected by the pH ofthe soil, and low soil pH can limit growth ofpoplars and willows used in SRF plantations.Soil pHs limiting growth have been reportedas follows: pH±H2O below 4.5±5.0;32,33 pH±KCl below 4.0±4.5 for several poplars.34,35

Optimal pH±KCl can vary among species andhybrids, being generally lower for certainTacamahaca and Tacamahaca x Aigeiros andAigeiros cultivars hybrids (pH 4.5±6.535 andbalsam and balsam poplar hybrids (5.0±5.5);than for some Euramerican hybrids (5.0±7.0).34 Optimum pH±H2O for the cultivarDTac 32 was reported to be 6.0±7.0.32 Poplarssensitive to high pH, showing both reducedgrowth and iron de®ciency chlorosis include`Oxford', a P. maximowiczii x P. berolinensishybrid35, and P. trichocarpa from westernOregon and western Washington (unpublisheddata). Maintenance of appropriate pH can bean important fertility management require-ment for SRF plantations. Additions of basicmaterials, such as ®nely ground limestone,basic slag and wood ash, can be used toincrease soil pH and improve tree growth.Application of municipal biosolids that havebeen heavily treated with lime can signi®cantlyincrease pH. For example, application of suchbiosolids to willow on peat ground increasedpH from 5.2 to 7.7.36

4.3. Fertilizer use and recommendations

Virtually all studies show that nutrientsapplied as fertilizer are substantially <100%utilized; consequently, more nutrients must beapplied than will be taken up by crops.37

Limited evidence shows that with nitrogen, asmall `priming e�ect' can occur from fertiliza-


tion, whereby availability and uptake of native

soil nitrogen is slightly increased.38,39

However, the magnitude of the priming e�ect

is trivial compared to the potential nitrogen

losses from denitri®cation9 discussed earlier.

Consequently, an application of nitrogen must

generally be over twice as much as the

removal of nitrogen in harvest40, since plants

obtain only from one-third to one-half of the

nitrogen applied as fertilizer.9 Recovery of

potassium by crops is similarly limited with

uptake by annual crops seldom exceeding

50%.41 E�ciency of use of phosphorus is even

lower being <20±30% according to Russell.9

Although speci®c data on e�ciency of nutrient

uptake by SRF species and clones may not be

available, generally satisfactory fertilizer pre-

scriptions can be developed, based on

removals in SRF harvest adjusted both for

nutrients supplied by the soil and for uptake

e�ciency of nutrients applied in fertilizer.

Although the above sounds simple enough,

determining the quantity of nutrients supplied

by the soil is very di�cult when the crop is as

deeply rooting as many Populus clones.

Consequently, fertilizer trials remain the most

reliable means of determining quantities of

nutrient elements needed for speci®c clones in

speci®c situations. Table 4 gives some rec-

ommendations for SRF plantation fertilization

which were derived from fertilization trials

with several SRF species.

The sometimes con¯icting requirements of

maximizing tree growth and fertilizer recovery

while minimizing nutrient leaching and cost

were evaluated in an American sycamore plan-

tation.4 Four di�erent N fertilization regimes

were evaluated, all of which involved the ad-

dition of 450 kg N haÿ1 as urea over a 3-year

period. These regimes were:

1. a one-time only application of450 kg N haÿ1 in the ®rst year;

2. three annual additions of 150 kg N haÿ1;3. nine additions (three per year) of

50 kg N haÿ1 or a ballooning regime with50, 150 and 250 kg N haÿ1 in the 3 years.

The balloon treatment was found to be opti-mum both in terms of maximizing stem bio-mass production and reducing potentialgroundwater contamination. Stem mass after3 years was 52% greater than in unfertilizedcontrol plots. There were fewer leaching losseswhen the fertilizer was added in three appli-cations each year compared to the sameamount added at the beginning of each grow-ing season; however, three applications peryear produced no measurable gain in biomassproduction. The single application of450 kg N haÿ1 in the ®rst year when the plantswere small was ine�cient because growth ben-e®ts were short-lived and there were excessiveleaching losses. The recovery of fertilizer N inabove-ground perennial tissue was calculatedas the di�erence in N content (per hectarebasis) between fertilized and control plotsdivided by the amount of N added. Recoverywas highest (19%) with the balloon treatmentincreasing fertilizer additions commensuratewith increasing tree size. The values for theother treatments ranged from 6.8 to 10.7%,which was within the range reported byBallard.45

The experience with hybrid poplar planta-tions throughout the Paci®c Northwest of theU.S.A. is that little fertilizer is actuallyrequired for maximum production. On mostalluvial sites west of the Cascade Mountains,response of plantations to fertilization andlime has been minimal. These are generallydeep soils, often having high organic mattercontents even in deeper layers of the soil pro-

Table 4. Fertilizers for SRF

MaterialsRecommendations(nutrient haÿ1 yrÿ1) Soil conditions References

SalixSeveral clones 150 kg N Irrigated sand soil, South Sweden 12

All 80±120 kg N General recommendation 42

30 kg P80 kg K

PopulusDxN hybrids 112±168 kg N Greatest need and response on sandy soil 43

Eucalpytus 10±75 kg N Old sugar cane land, Hawaii 44

General (all species) 50±70 kg N Fertilize at 3-year intervals 40

12±16 kg P30±50 kg K


®le. Consequently, even though the pH maybe low, fertility levels, including exchangeablecalcium, can be relatively high. East of theCascades where irrigation is required (in con-trast to westside-of-the-Cascades plantations),treatments with 50 kg N haÿ1 in the ®rst year,boosting to 75 kg N haÿ1 in the second yearand to 125 kg N haÿ1 by the fourth year (allapplied in the irrigation water) have been con-®rmed by foliar analysis to provide adequateN. Annual applications of 10±20 kg P haÿ1

and 2±5 kg Zn haÿ1 are also considered to ben-e®t the East of the Mountains plantations.

The lack of N fertilizer response with thesehybrids on the nutrient-rich westside (of theCascade Mountains) soils in Oregon andWashington and the relatively low applicationrates on the eastside (of the Mountains) illus-trate how fertility management strategies needto be tailored to local conditions. In manycases, much less fertilizer will be required thantraditional approaches might call for. Otherfactors, such as associated nitrogen ®xation,could provide surprises that require additionalresearch in fertility management of SRF plan-tations. Other areas needing more researchinclude, but are not limited to: rooting depthand distribution for important species andclones as a�ected by soils and managementpractices; use of N-®xing plants in SRF; deni-tri®cation in SRF plantations; diagnosing soilfertility status of soils for SRF plantations;and use of SRF plantations for their potentialfor waste and e�uent disposal, stream protec-tion and agricultural diversi®cation.

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nutrient cycling and fertility management in temperate short rotation forest systems

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