Biol Fertil Soils (1999) 29 :38–45 Q Springer-Verlag 1999

ORIGINAL PAPER

A. K. Patra 7 S. C. Jarvis 7 D. J. Hatch

Nitrogen mineralization in soil layers, soil particlesand macro-organic matter under grassland

Received: 17 November 1997

S. C. Jarvis (Y) 7 D. J. HatchInstitute of Grassland and Environmental Research, NorthWyke Research Station, Okehampton, Devon, EX20 2SB, UKe-mail: steve.jarvis6bbsrc.ac.uk,Tel: c44-1837-82558, Fax: c44-1837-82139

A. K. PatraIndian Grassland and Fodder Research Institute,Jhansi 284003, India

Abstract A study was conducted to determine miner-alization rates in the field and in different soil layersunder three grassland managements (viz. a reseededsward, a permanent sward with a conventional N man-agement, and a long-term grass sward with 0 N (0-N)input). Potential mineralization rates of soil particles(sand, silt and clay) and macro-organic matter fractionsof different sizes (i.e. 0.2–0.5, 0.5–2.0 and 12 mm) werealso determined in the laboratory. In the reseededplots, net mineralization was unchanged down to 40 cmdepth. In the undisturbed conventional-N swards, mi-neralization rates were substantially higher in the toplayer (0–10 cm) than in the deeper layers. In plotswhich had received no fertilizer N, mineralization wasconsistently low in all the layers. There was more ma-cro-organic matter (MOM) in the 0-N plots (equivalentto 23 g kg–1 soil for 0–40 cm) than in the two fertilizedplots (i.e. conventional-N and reseeded) which con-tained similar amounts (ca. 15 g kg–1 soil). C and Ncontents of separated soil particles did not differamongst the treatments, but there were large differ-ences with depth. Potential mineralization in the bulksoil was greatest in the 0–10 cm layers and gradually de-creased with depth in all the treatments. Separatedsand particles had negligible rates of potential mineral-ization and the clay component had the highest rates inthe subsurface layers (10–40 cm). MOMs had high po-tential rate of mineralization in the surface layer anddecreased with soil depth, but there was no clear pat-tern in the differences between different size fractions.

Key words Grassland 7 Nitrogen 7 Mineralization 7Macro-organic matter 7 Soil particles

Introduction

In N cycle modelling, accurate prediction of soil miner-al-N (NminpNH4

ccNO3–) dynamics is important be-

cause this component is influenced by inputs frommany sources as well as soil and climatic factors. To de-fine the inputs, the following simple equation can bepresented (Jarvis et al. 1996):

NminpSOMcRcMcFcRN

where SOM is the N derived from native soil organicmatter, R is from the crop residues (from previouscrops, senescing roots and shoots etc.), M from animalmanures and excreta, sewage sludge or other addedwaste materials, F that supplied from inorganic fertiliz-er and RN the residual nitrate (NO3

–) or ammonium(NH4

c) already present in the soil.According to Monaghan and Barraclough (1995),

the sources of mineral soil N can be divided into fourdistinct, measurable pools: current plant residues, thesoil microbial biomass (including that following normalgrowth patterns or in resting phases and the productsfrom both populations), the light fraction organic mat-ter and, by difference, the more stabilized humic mate-rials. Depending on the soil and environmental condi-tions, the quantity of total soil N to maximum rootingdepth (1 m) has been estimated to range from 1 to10 t ha–1, much of which is in the top 15 cm or so. Forfertile arable soils in Britain, the average figure for soilN has been reported to be 4 t ha–1 (Anon 1983): othershave suggested that undisturbed older pastures accu-mulate up to 15 t N ha–1 through returns in plant resi-dues, roots and stubbles etc. in the top 10 cm (Ryden1984). Recent measurements using field incubationtechniques (Hatch et al. 1991; Gill et al. 1995) haveshown that in long-term grassland, the annual releaseof mineral N from this source is considerable. These

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and other similar measurements have concentrated onthe top 10 cm of the soil; there is also considerable po-tential for the organic materials in the lower layers tosupply N into the mineral pool. This release could beimportant because if the source is located below themain rooting depth, there could be a major contribu-tion to leaching losses.

The extent of release of N from native soil organicmatter (SOM) could therefore have both economic andenvironmental implications (Jarvis et al. 1996). Of thedifferent fractions of SOM, macro-organic matter(MOM) comprises a significant component of the ‘light’fraction of organic matter. MOM is generally thoughtto consist of dead fibrous materials in a state of partialdecomposition, including that from roots, but not in-cluding living materials (Whitehead et al. 1990). War-ren and Whitehead (1988) suggested that the MOMfraction in grassland soils may contribute substantiallyto the available N. This conclusion was derived fromthe observation that plant uptake of N was significantlyincreased in soils where MOM was returned, comparedwith uptake where MOM had been removed. The mi-neral constituents of the soil matrix also form close as-sociations with organic materials which will exert im-portant controls over mineralization (Hassink et al.1993). Studies have shown that there is a close interac-tion between the decay products and microbial activityin clay soils (Gregorich et al. 1991). Texture plays animportant role therefore in determining mineralizationrate (Hassink 1992), but there is little information onhow this would be influenced by any changes in the na-ture of the soil organic materials which may occur withdepth.

In recent years, N mineralization from SOM, parti-cularly in grassland soils, has been the subject of muchresearch. An understanding of N release from this pooland extension of the existing data base is essential inorder to be able to include this component of N cyclingin improvements of fertilizer N recommendations andthe general efficiency of N utilization (Jarvis 1993; Jar-vis et al. 1996). The present study therefore aimed toprovide more information about net and potential mi-neralization in grassland soils at different depths in dis-turbed and undisturbed grassland soils. Additionally,effects of depth and disturbance on SOM associatedwith different soil components (including MOMs anddifferent particle size fractions) and their potential torelease N was also determined.

Materials and methods

Soil/site characteristics

The experiment was carried out on Rowden Moor at the Instituteof Grassland and Environmental Research, North Wyke, Devon.Annual precipitation at this site averages 1035 mm, of which twothirds fall between October and March (Tyson et al. 1993). Thesite has been under permanent grassland management for over 40years and is on a poorly drained clayey non-calcareous pelosol of

the Hallsworth series which is derived from the Culm measures.Over the last 12 years the site has been under experimental man-agement with approximately 1 ha sized plots grazed with beefsteers. Half of the plots are drained (with lateral field drains at85 cm depth and mole drains at 55 cm) and the remainder retaintheir original poorly drained status. A number of N fertilizermanagements were imposed on the plots in March 1992 (see Ty-son et al. 1993; Gill et al. 1995) and for our measurements wecompared plots with a permanent grass sward under a conven-tional fertilizer (280 kg N ha–1) management (Con-N), a recentlyreseeded area with a similar N management background (Res-N),and a grass sward which had received no fertilizer N inputs overthe last 30–40 years (0-N). The reseeded area had been cultivated(to a depth of 10 cm) after destruction of the existing sward dur-ing the autumn prior to measurements and resown with a peren-nial ryegrass seed mixture soon afterwards. All of the treatmentsused for this study were on undrained areas and had been pre-viously grazed, except the reseeded area which had not beengrazed since the resowing in 1995. The swards were grass based(i.e. they contained little or no white clover) with approximately100%, 40% and 20% perennial ryegrass (Lolium perenne L.) inthe Res-N, Con-N and 0-N treatments, respectively. The weatherdata during the present measurement periods are given in Table 1and some important soil properties are shown in Table 2. Furtherdetails of the site and climatic conditions can be found elsewhere(Tyson et al. 1993).

Soil sampling

Measurements were based on four pseudo-replicated areas withinlarge (1 ha) field treatment areas. Four replicate soil samples, tak-en at random from within each experimental area, were obtainedon 21 May 1996 using a 3.5-cm-diameter auger and were dividedinto 0–10, 10–20, 20–30 and 30–40 cm depths. The samples werebulked together, crumbled, and stones 16 mm discarded. Eachbulk sample was then extracted by dispersing 50 g fresh soil in100 ml 2 M KCl followed by shaking for 1 h. The extract was fil-tered and analysed for NH4

c and NO3– contents using standard

autoanalyser methodology. On the same day, samples were alsotaken for in situ field measurement of net N mineralization andquantification of MOM and soil particles. Soils were also sampledon 5 August 1996, in this case for in situ net N mineralizationmeasurements only.

Net mineralization: field incubation

The method for field measurement of net mineralization usingsoil cores, with acetylene (C2H2) added to inhibit nitrification, hasbeen described fully elsewhere (Hatch et al. 1990). Instead of fourcores in the incubation vessel, a single core was used on this occa-sion. In brief, the technique involved soil sampling at randomfrom within each experimental area. PVC pipes of 37 mm internaldiameter and 3 mm thickness were hammered into the soil to40 cm depth. After removal from the soil, the intact core (still re-tained within the PVC sleeve) was cut into 10 cm lengths. To al-low gaseous exchanges with the soil core, 4!1 cm diameter holeswere made in each 10 cm length of pipe. Each core was placed ina 1 l glass jar which was sealed by a poly-acetyl lid fitted with asilicone rubber gasket and held in place by a screw cap. Acetylene(20 ml) was added to the jar to give approximately 2% (v/v) in thehead space. There were three or four replicates for each depthfrom each of the three treatments at the first sampling and threereplicates at the second sampling. The jars containing the coreswere then placed in holes in the ground (12 cm diameter, 15 cmdepth) adjacent to the experimental areas for an incubation peri-od of 7 days. After incubation, the soil from each jar was ex-tracted with KCl, as described above and the extract analysed formineral N contents. The difference in NH4

c–N contents beforeand after incubation provided the measure of net mineraliza-tion.

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Table 2 Potential mineralization (mg N gP1 material) of wholesoil and separates from permanent pastures at different depths asdetermined by anaerobic laboratory incubations. Values are

means (with SE) for three replicate samples; final column indi-cates significance of difference between samples at differentdepths

Soil separate Soil depth (cm) Sig. diff.(P~0.05)

0–10 10–20 20–30 30–40 LSD

Whole soil 251.5 113.6 29.7 10.0 15.5 *(5.47) (6.62) (2.81) (2.88)

Macro-organic matter12 mm 1684 1583 1194 63 881 *

(309.9) (429.7) (78.5) (62.7)

0.5–2 mm 1266 1207 967 248 756 *(139.3) (392.8) (135.9) (148.6)

0.2–0.5 mm 1344 1086 586 64 1229 NS(251.8) (625.6) (317.6) (100.3)

Sand 5.0 ~1.0 ~1.0 ~1.0 3.01 *(1.7) (0.5) (0.2) (0.2)

Silt 56 16 3 ~1.0 13.7 *(1.7) (7.4) (3.4) (0.6)

Clay 65 17 5 1 21.9 *(11.5) (6.5) (2.1) (0.7)

Table 1 Some properties of the soils from the experimental sites, before commencing measurements

Depth(cm)

Soil properties [on air-dry (35 7C) basis]

Moist.(%)

pH(1 :2)

C(%)

N(%)

C/Nratio

NH4c

N mgkgP1

NO3P

N mgkgP1

Sand(%)

Silt(%)

Clay(%)

Reseeded-N0–10 57 5.8 5.30 0.52 10.3 1.08 0.63 9.7 52.0 25.1

10–20 42 5.9 3.70 0.40 9.3 1.12 0.90 14.2 46.7 32.620–30 29 6.2 1.24 0.21 5.9 0.98 0.75 18.9 35.6 36.830–40 28 6.5 0.57 0.16 3.6 0.67 0.98 11.0 28.0 67.6Conventional-N0–10 49 5.6 4.66 0.47 10.0 3.9 4.90 7.1 59.5 36.2

10–20 39 5.9 2.20 0.27 8.1 1.4 1.50 7.7 50.1 37.520–30 34 6.0 0.90 0.22 4.1 0.8 1.35 8.7 31.9 48.930–40 29 5.9 0.53 0.18 3.0 3.0 0.93 16.3 18.4 73.6Zero-N0–10 66 5.4 5.06 0.46 10.9 1.9 1.00 4.9 63.0 25.6

10–20 46 5.5 2.87 0.32 9.1 1.1 0.84 8.9 53.3 36.620–30 37 5.4 1.00 0.19 5.4 0.5 0.84 8.5 35.0 52.230–40 33 5.2 0.56 0.16 3.5 0.5 0.72 10.3 9.0 71.2

Soil separates

Earlier preliminary studies indicated that there was no significanteffect of field treatment on laboratory measurements of potentialmineralization, but there were effects of soil depth. For the pres-ent study therefore, potential mineralization was determined inwhole soil and various separates from samples at different depths.For this purpose a single bulked soil sample (16 cores) from eachof the three field treatments was used to provide three replicatesoil samples for examination.

The bulked soil samples from each field treatment (providingthree replicate samples of 1 kg fresh soil for each depth, i.e. 0–10,10–20, 20–30 and 30–40 cm) were collected on day 0 of the firstincubation period (May 21). After bulking, each separate layerwas crumbled, thoroughly mixed and stones and obvious pieces oforganic materials removed. Sub-samples were taken for oven dry-ing (at 105 7C) for moisture content determination or were airdried (30 7C) for C and N analyses. Samples of fresh soil weretaken for particle separation and were pre-soaked in de-ionized

water for 1 h and the wet soil was then washed gently through asuccession of sieves (2.0, 0.5 and 0.2 mm) until the water ranclear. This gave three MOM fractions: 12.0 mm (consisting most-ly of roots), 0.5–2.0 mm (containing some fine root particles) and0.2–0.5 mm (without any identifiable root material). Each organicmatter fraction was washed off the sieve by flotation to separate itfrom the sand which remained on the sieve. The individual MOMfractions were then blotted with absorbent tissue paper to removeexcess water and the total fresh weight recorded. Samples werestored in a refrigerator at 4 7C until required for incubation stud-ies.

The sand which was retained on the sieves was recovered andalso stored at 4 7C and all the washings from the sieves were keptfor separation of the silt and clay fractions and their associatedorganic matter. The total volume of the washings associated withsieved material from each soil sample was noted and a sub-sampleof 5 l was obtained from the original washings, after stirring vigor-ously. After noting the temperature of the sub-sample, the sus-pension was thoroughly mixed again and allowed to stand for a

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Fig. 1 In situ net mineralization rates (kg N ha–1 day–1) of soil ni-trogen at different depths under different grassland managementsystems on two occasions: a during 21–28 May and b during 5–12August. Data are the mean of four replicates with standard errorsshown as bars for three grassland swards: conventional, fertilizedlong term sward (CON N) (L), cultivated and reseeded sward(RES-N) (lUuUu), and a long-term sward receiving no fertilizer N (0-N) (l)

fixed time (according to Stoke’s Law) to allow the silt fraction(0.02–0.2 mm) to settle. The remaining suspension, containingclay materials (~0.02 mm), was siphoned off. A standard volumeof water was added to the settled contents which were then re-shaken thoroughly. The suspension was then allowed to standagain for a fixed time, after which the suspended clays were si-phoned off and added to the previous siphoning. This procedurewas repeated several times until all the clay had been removed.The separated silt and clay fractions were centrifuged at 2000 rpmto remove excess water and the wet weight of materials separatedfrom the original 5 l was recorded: a sub-sample of each superna-tant was retained for use as an inoculum for the incubations. Sub-samples of sand, silt and clay were oven-dried for 24 h at 105 7C.The remaining moist samples of all the fractions collected werestored at 4 7C until incubation.

Potential net N mineralization of soils, MOMs and soil particles

Air-dried soils from each depth of the three different grasslandssystems were used for this study following the procedure de-scribed by Lober and Reeder (1993). Briefly, 5 g soil were addedto a syringe body (60 ml) with two 2 mm glass beads (to assistmixing). The syringe plunger was inserted to ca. 30 ml positionand the syringes were then clamped in an upright (i.e. needle endup) position, and 13 ml of deionized water injected. Entrapped airwas eliminated by gently vibrating the soil/water mixture andcarefully pushing the plunger further into the syringe body untilthe soil slurry reached the Luer-Lok tip. Finally, the syringes andtheir contents were placed needle end down and incubated for 7days at 37 7C.

After incubation, 37 ml of 2.7 M KCl were injected into eachsyringe to give a 1 :10 soil : 2 M KCI extraction ratio (Bremner1965). The plungers were partially withdrawn to provide a head-space, and the syringes were then vigorously shaken on a wrist-arm shaker for 1 h. The suspensions were then filtered withWhatman GF/A glass filters and the filtrates analysed for NH4

c

and NO3–. The NH4

c and NO3– contents of representative sam-

ples of the soils were also determined prior to incubation. Miner-alization was calculated as the difference between post- and pre-incubation NH4

c-N concentrations: NO3– contents were low

throughout all samples and did not change significantly during in-cubation.

Anaerobic incubation of sand, silt and clay separates was con-ducted in the same way as for the bulk soil samples except thatappropriate supernatants were used instead of deionized water(to provide inocula) during the incubation. Further, in the case ofroots and MOMs, acid washed sand was used to make up theweight to 5 g because only small quantities of sample were availa-ble. Calculation of mineralization rates was based on the dryweight of materials taken for incubation.

Total C and N

The total C and N contents of the dried roots, MOMs and sam-ples of the sand, silt and clay particles (gently ground to break upthe solid mass formed during drying) were determined with aCarlo-Erba (Erba Science UK) C and N Analyser. The sand frac-tion was passed through a 0.5 mm sieve after being ground; thesilt and clay particles were not sieved.

All data were analysed by standard analysis of variance meth-ods.

Results

In situ net N mineralization

Rainfall during the two experimental periods was simi-lar (24.3 and 27.8 mm between 21–28 May and 5–12

August, respectively: soil temperatures at 10 cm weregreater during August (14.8 7C, on average) than inMay (10.7 7C, on average).

There were three distinct patterns of net N minerali-zation rates with depth under the three different grass-land management systems (Fig. 1) and these patternswere similar on both sampling dates. Net mineraliza-tion was generally higher on the second sampling occa-sion, but the difference between samplings was onlysignificant (P~0.05) in the 30–40 cm layer of the Con-N treatment. The greatest mineralization rates werefound in the 0–10 cm layer of the Con-N treatments onboth occasions: this was significantly greater (P~0.05)

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Fig. 2 Soil moisture content (%), on oven dry weight basis, atdifferent depth under different grassland management systems,measured on two occasions: a during 21–28 May and b during5–12 August, during in situ net N mineralization studies. Data arethe mean of four replicates with standard errors shown as bars forthree grassland swards: CON N (L), RES-N (lUuUu), and 0-N (l)

than that for the comparable layer of both other treat-ments on the second sampling and for the 0-N treat-ment on the first occasion. In the Con-N soil, the ratesdecreased in the lower layers (P~0.05), and there wasa trend for this to continue with depth on the first occa-sion: on the second occasion, the only difference wasbetween the 0–10 and the remaining layers. There wasstill substantial mineralization occurring even at30–40 cm. Overall, the lowest rates of net mineraliza-tion occurred in the 0-N treatment and there were nodifferences between depths in this case: there was someevidence on the first sampling date that net immobiliza-tion had occurred in the 0–10 cm layer.

Net rates in the Res-N treatment were again similaron both occasions and there was no significant effect ofdepth. Cultivation and reseeding had reduced thegreater rates observed in the undisturbed (Con-N)treatment. Mineralization rates were therefore more orless even across all depths in this treatment.

A comparison of the mineralization rates with soilmoisture contents (Fig. 2) shows that on the first occa-sion soil moisture content was always greatest in the0–10 cm layer, and then decreased with depth. This pat-tern was most distinct in the Con-N treatment. On thesecond occasion (5 August) moisture contents werelower and the effect of depth had disappeared so thatvalues were similar for all soil layers in all treatments.The data indicate that net mineralization occurred inthe soil even though soil moisture contents were rela-tively low, but that there were substantial differencesbetween treatments on the same soil with a similarmoisture status.

Potential mineralization

Preliminary studies had indicated that there were nodifferences between the field treatments in their poten-tial to release N. This perhaps may have been suggestedby the similarity in the C :N ratios of the bulk soils (Ta-ble 2). The present measurements indicate that therewere, however, substantial effects of depth (Table 3),(P~0.05) so that the potential net mineralization(PNM) decreased by approximately 25 times betweenthe 0–10 and the 30–40 cm layers. There were similarsignificant (P~0.05) patterns of decreasing PNM withdepth for each of the separated soil fractions (Table 3).The maximum PNM rates were found with the MOMmaterials and relatively low rates with the mineral frac-tions (especially the sand). There was a greater rate ofPNM for the 12 mm than for the other MOM fractionsin the upper layers, but the effects were not signifi-cant.

N and C contents of all three MOM materials weresimilar (Table 3) and although there was a trend for Ccontents to decrease with depth, this effect was not sig-nificant: C :N ratios did not differ substantially withdepth. However, the C :N ratio of the 0.2–0.5 mm

MOM fraction was always lower than those for the oth-er two size fractions.

The mineral separates had much lower C, N andC:N values than the MOMs. The trends of decreaseswith depth were more distinct than with MOMs andwere apparent for values of C, N and C:N. For all threevariables, the values decreased in the order:silt1clay1 sand. Calculations based on the known par-ticle size distribution for the soil (Table 1) indicatedthat the mineral component of the soil could have con-tributed up to 21% of the N released during the anae-robic incubation measurements, with little significantcontribution from the sand fraction.

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Table 3 Carbon and nitrogen (% dry matter) contents of soilseparates at different depths used in assessment of potential mi-neralizaton (see Table 2). Values are means (with SE) for three

replicate samples; final column indicates significance of differencebetween samples at different depths

Soil separates Soil depth (cm) LSD Sig. Diff.(P~0.05)

0–10 10–20 20–30 30–40

Macro-organic matter12 mm %C 37.6(0.03) 37.8(0.20) 35.4(1.34) 34.1(4.62) 7.86 NS

%N 1.85(0.00) 1.85(0.003) 1.84(0.007) 1.81(0.035) 0.058 NSC:N 20.3 20.4 25.3 18.8

0.5–2 mm %C 36.2(0.24) 36.1(0.15) 33.8(0.50) 37.5(0.61) 1.36 *%N 1.77(0.003) 1.77(0.00) 1.76(0.015) 1.75(0.009) 0.029 NSC:N 20.5 20.4 19.2 21.4

0.2–0.5 mm %C 34.4(0.09) 33.8(1.39) 29.5(2.86) 25.5(4.37) 8.82 NS%N 2.17(0.00) 2.18(0.007) 2.19(0.033) 1.79(0.395) 0.648 NSC:N 15.9 15.5 13.5 14.2

Sand %C 3.02(0.343) 2.13(0.128) 1.31(0.105) 0.93(0.071) 0.63 *%N 0.27(0.035) 0.19(0.021) 0.15(0.009) 0.13(0.008) 0.075 *C:N 11.2 11.2 8.7 7.2

Silt %C 6.53(0.355) 3.08(0.462) 1.18(0.200) 0.49(0.018) 1.01 *%N 0.61(0.026) 0.33(0.036) 0.18(0.023) 0.13(0.009) 0.083 *C:N 10.7 9.3 6.6 3.8

Clay %C 5.09(0.247) 2.98(0.448) 1.04(0.098) 0.56(0.062) 0.86 *%N 0.57(0.018) 0.37(0.047) 0.23(0.007) 0.18(0.003) 0.082 *C:N 8.9 8.1 4.5 3.1

Discussion

This study has shown that the pattern of N mineraliza-tion is influenced by past history of management prac-tices and by soil depth. During 21–28 May the soil tem-perature was lower and the soil moisture was higherthan during 12–18August and greater rates of N miner-alization were observed during the latter occasion, pre-sumably due to higher temperatures. Previous esti-mates of net mineralization on the same soil type usingthe field incubation technique found that there was apositive correlation with temperature, but less sensitivi-ty to soil water content (Hatch et al. 1991; Gill et al.1995). Of the previous management practices exam-ined, the past background fertilizer input had a signifi-cant influence on the mineralization process in the0–10 cm horizon (Gill et al. 1995), indicating that therewas a pool of readily mineralizable N created by theprevious fertilizer regime. Where fertilizer was appliedto a previously unfertilized sward, there was an imme-diate increase in the annual net mineralization rate(Gill et al. 1995). The extent of mineralization thus ap-parently depends on the build up of readily mineraliza-ble materials. Background management had a markedeffect on the measurements of net mineralization onthe two occasions in the present study although themeasured characteristics of the soils did not indicatethe potential for significant effects. This may at leastpartially reflect the different natures of the sward com-position on each treatment.

In grassland, the quantities of both roots and MOMsincrease with the age of the sward (Garwood 1967).

Cultivating the present old sward for reseeding in-fluenced the pattern of net mineralization. Mechanicaldisruption of the soil structure increases aeration andmakes previously protected SOM available for degrad-ation and increased rates of mineralization have beenobserved in disturbed soils (Balesdent et al. 1990) de-pending on factors such as age and composition ofsward, fertilizer application, temperature and soil mois-ture. The increased NH4

c contents in the lower layersmight have also arisen for the same reason. Significantmineralization has been shown to occur in deeper partsof soil profiles (Hadas et al. 1989; Weir and MacRae1993). This has important implications for leaching ofNO3

–, especially where soils have been drained andhave improved aeration (Scholefield et al. 1993). Anenhanced supply of NO3

–, possibly originating from mi-neralization at depth, may increase the potential for de-nitrification deeper in the profile and especially if thisalso provides a supply of available C (Jarvis and Hatch1994). In the present soil, cultivation apparentlychanged the pattern of mineralization compared withthe undisturbed soil, with lower net rates being ob-served on both occasions in the top 10 cm of the culti-vated soil. This was probably due either to a changingimmobilization/mineralization balance or to movementof mobile organic materials to greater depth. Boththese possibilities have implications for losses and themovement of N and C (and other nutrients) into wa-ters.

Although moisture contents were much reduced onthe second occasion, moisture was not a limiting factorfor mineralization. Semb and Robinson (1969) andWild (1972) observed that mineralization took place

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even in dry seasons and at moisture tensions 115 bar.A possible explanation for this is the presence of sub-stantial amounts of water present at high tensions inwell–aggregated soils. Interactions with soil tempera-ture are also important in determining mineralizationrates: other studies on the same soil showed that a ma-jor proportion of the variability in net rates could beaccounted for by variation in soil temperature (Gill etal. 1995). Recent work (Clough et al. 1998) has indi-cated that accumulated soil temperatures may providean effective predictor of mineralization.

There were major similarities between treatments inthe PNM in any one layer in all the present soils; condi-tions of the incubation experiment must have maskedany differences created in SOM quality created by thebackground managements under investigation. Thelower net PNM values below 10 cm confirm the generaltrend of distinct changes with depth in the amounts ofbiologically active fractions of SOM, although a changein the overall ‘quality’ of the material as indicated by Cand N contents was not always evident. This demon-strates the difficulty in defining overall relationships inwhich measures of potential mineralization can be re-lated to effects in the field.

Of the soil separates, sand, as expected, had a negli-gible PNM because of the inherently low sorption ca-pacity for organic materials. The silt and clay fractionsboth had high PNM rates and, in some instances, parti-cularly in the surface layers of the soil, silt had highervalues than clay. The reason for the higher rate is notclear, but clay particles may hold organic materialsmore tightly on their surfaces than silt (Hassink 1993).An alternative explanation may be that this effect wasbecause of incomplete dispersion of the soil which re-sulted in microaggregates in the silt fraction whichcould have been eliminated by, for example, ultrasoni-cation. Although a relatively mild separation procedurewas used, separation of the different materials mayhave introduced changes in the potential to be accessedby microorganisms to differing extents. Other studieshave indicated that silt makes the largest contributionto the total soil PNM (Gregorich et al. 1989; Catrouxand Schnitzer 1987). Decreasing C :N contents with soildepth is to be expected as organic materials becomemore and more degraded. These effects have been de-monstrated in soils from the same area in previousstudies (Kerley and Jarvis 1997). In the present studythe changes with depth in C :N ratio indicate differen-tial microbial activities associated with different materi-als. All C :N ratios of the materials associated with themineral fractions narrowed with depth: that of the siltmaterials to a greater extent than in the sand, and to alesser extent, than in the clay also. This change in or-ganic matter materials, coupled with a differential dis-tribution with the different components of the soil atdifferent depths will have implications for their furtherbreakdown. Further, different types of organo-mineralcomplexes may differentially affect the microenviron-ment created in the immediate vicinity of particle sur-

faces, again with potential impact on N release: clays,for example may provide a more favourable environ-ment for decomposer organisms than minerals such asquartz (Christensen 1992).

These effects and changes with depth in C :N werenot displayed in the separated MOM fractions whichexcluded most of the stubble materials. MOMs isolatedfrom other grassland soils were composed largely ofroot residues (Warren and Whitehead 1988). The C:Nratio of ryegrass roots is generally in the range of 25–45(Whitehead 1970); this compares with 12–22 in theMOMs in the present study, indicating that these hadundergone at least partial humification. Mineral N wasreleased at high rates from all MOMs, indicating thatmineralization from these fractions could occur evenduring a short decomposition period, probably becausethese materials would also carry a greater complementof mineralizing organisms than the other separates. Wefound, however, little evidence to support the view thatthe release of N from MOMs was influenced by thebackground pasture managements. This may require afiner definition of the materials because there wereonly relatively small differences between the three cate-gories of MOMs used and their C and N status. Despitelittle difference in quality (as defined by C :N), therewere again some effects of soil depth on potential mi-neralization with an average decrease of over 17 timesbetween 0–10 and 30–40 cm. Warren and Whitehead(1988) also incubated MOMs in moist conditions (notanaerobic) for 28 days at 25 7C and found rates of re-lease of ca. 600 mg N g–1 (and in one instance up to2348 mg g–1) from materials isolated from ten differentsoils. The highest values in their study were associatedwith a low C:N ratio (13.4) of the MOM and a N con-tent of 3.78%. In contrast, our measurements (over 7days, but in anaerobic conditions at a higher tempera-ture of 37 7C) produced values of 110000 mg g–1 inMOMs from the 0–10 cm layer. Interestingly, althoughthe C :N ratios of the MOM fractions ranged from 13 to25, there was no clear relationship with PNM values.There is an obvious need for more detailed studies todetermine the quantitative contribution of these soilfractions to gross and net mineralization under fieldconditions. If relationships can be established betweenrelease of N from specified materials and external con-ditions, this may offer the basis for developing im-proved prediction of the supplies of N from SOM.

Conclusions

In this study, there were clear effects of soil depth onthe distribution of organic materials and actual and po-tential rates of mineralization. There were also indica-tions that disturbance of long-term swards changed thedistribution of these effects which may have implica-tions for mineralization at depth in the soil and the po-tential for loss of NO3

–. There was a large potential ofthe soil to release large amounts of N from organic ma-

45

terials which appeared to be especially related to sepa-rated MOMs. Release of N from organic materials as-sociated with mineral components displayed differenttrends to those of the MOMs. The MOM materials de-serve further examination so that effects of their chem-ical composition on their contribution to net and poten-tial mineralization of N in grassland and other soils canbe determined and more explicitly included in modelpredictions.

Acknowledgements This work was undertaken whilst A.K.P.was on a study visit from IGFRI, Jhansi, India. The financial sup-port provided by the British Council on behalf of the ODA (UK)funded Indo-UK Project on Forage Production and Utilization isgratefully acknowledged. A.K.P. is also grateful to Professor R.J.Haggar (IGER) and Dr. Bhag Mal (IGFRI), for their supportduring this study visit, to Michael Davies, IGER, North Wyke, fortechnical assistance and to A. W. Bristow for analytical advice.The work forms part of a MAFF, London, commission: IGER issupported by the BBSRC.

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