Agriculture, Ecosystems and Environment 132 (2009) 126–134

Soil aggregation and organic matter in a sandy clay loam soil of the IndianHimalayas under different tillage and crop regimes

R. Bhattacharyya *, Ved Prakash, S. Kundu, A.K. Srivastva, H.S. Gupta

Vivekananda Institute of Hill Agriculture, Indian Council of Hill Agriculture, Almora 263 601, Uttarakhand, India

A R T I C L E I N F O

Article history:

Received 17 December 2008

Received in revised form 12 March 2009

Accepted 16 March 2009

Available online 14 April 2009

Keywords:

Conservation tillage

Crop rotation

Soil organic C storage

Aggregate stability

Aggregate associated C and N

Indian Himalayas

A B S T R A C T

In agricultural systems, maintenance of soil organic C (SOC) has long been recognized as a strategy to

reduce soil degradation. Management practices, such as conservation tillage and legume-based cropping

sequences, have the potential to enhance SOC and total soil N (TSN) content and improve soil

aggregation. We examined the effects of three tillage systems [conventional tillage (CT), minimum

tillage (MT) and zero tillage (ZT)] and three crop rotations [soybean–wheat (S–W), soybean–lentil (S–L)

and soybean–pea (S–P)] on SOC and TSN storage and their distribution within aggregate-size fractions in

Indian Himalayas, where almost all above-ground crop residues were removed. A field experiment was

conducted on a sandy clay loam soil (Typic Haplaquept; Eutric Cambisols) from 1999 to 2003 near

Almora, India. Results indicate ZT significantly (P < 0.05) increased SOC and TSN storage over CT in the 0–

15 cm depth by 10.2 and 17.2%, respectively. Plots under S–L and S–P (continuous leguminous cropping)

had 10.7 and 13.1% higher SOC content than S–W plots in the surface soil layer (0–15 cm depth).

However, both tillage and crop rotation had no impact on the sub-surface (15–30 cm) soil layer or 0–

30 cm soil profile as a whole. On an equivalent initial soil mass basis, SOC storage to about 15 cm after

four years was 26.0 Mg ha�1 in continuous NT plots, but just 23.9 Mg ha�1 in continuous CT. Zero tillage

increased bulk density, mean weight diameter and the proportion of macroaggregate fractions (2–

4.75 mm, 0.25–2.0 mm) in soil compared with CT in the surface soil layer, but not in the sub-surface soil

layer. In the surface soil layer, ZT significantly (P < 0.05) increased SOC concentration compared with CT

in all aggregate-size fractions. Crop rotation had no effect on aggregate associated-SOC in both soil

layers. Plots under S–P and S–L rotations had higher SOC concentration only for whole soil (<4.75 mm

fraction) than S–W rotation in the 0–15 cm soil depth. Impacts of ZT included a greater proportion of

macroaggregate (2.0–4.75 mm size fraction)-associated TSN than MT and CT and higher aggregate-

associated TSN in ZT plots than CT within all aggregate-size fractions (except<0.053 mm size fraction) in

the surface soil layer (0–15 cm) only. Plots under S–L and S–P rotations increased TSN concentration

compared with S–W by about 19 and 21% for whole soil (<4.75 mm), in the 0–15 cm soil. Thus, short-

term conservation tillage and continuous leguminous cropping under rainfed conditions improved SOC

and TSN storage in the soil surface in the Indian Himalayas.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

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1. Introduction

Carbon sequestration using innovative soil and crop manage-ment practices is needed both to augment soil C storage to mitigate

* Corresponding author. Current address: School of Applied Sciences, University

of Wolverhampton, Wulfruna Street, Wolverhampton WV1 1LY, UK. Tel.: +44

7852609026; fax: +44 1902 322680.

E-mail addresses: ranjan_vpkas@yahoo.com, Ranjan.Bhattacharyya2@wlv.ac.uk

(R. Bhattacharyya).

Abbreviations: SOC, soil organic carbon; TSN, total soil nitrogen; CT, conventional

tillage; MT, minimum tillage; ZT, zero tillage; S–W, soybean–wheat rotation; S–L,

soybean–lentil; S–P, soybean–pea; SOM, soil organic matter; MWD, mean weight

diameter; C, carbon; N, nitrogen; P, phosphorus; K, potassium; BNF, biological N2

fixation; LSD, least significant difference; WSA, water stable aggregates; SEY,

soybean equivalent yield.

0167-8809/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.03.007

greenhouse gas emission, and improve soil quality and cropproductivity (Sainju et al., 2008). Soil organic matter (SOM) isimportant in maintaining several soil properties (Gregorich et al.,1994) and can be controlled by management practices, includingchoice of cropping, management of crop residues and methods andintensity of tillage (Voroney and Angers, 1995). Tillage mixes SOCin the surface layers and may increase its decomposition. Loss ofSOM leads to poorer soil tilth with subsequent loss of soilproductivity (Lal et al., 1989; Ouedraogo et al., 2007).

Soil organic matter sequestration may be achieved by adoptingconservation tillage systems. On agricultural soils with low tomedium clay content, loss of SOM can be minimized with the use ofZT/MT that allows crop residues to remain on the soil surface andminimizes soil disturbance (Arshad et al., 1990; Ismail et al., 1994;Erenstein and Laxmi, 2008).

Table 1Initial (1999) soil properties of the experimental site.

Soil properties Soil depth (cm)

0–15 15–30

pH (soil:water, 1:2.5) 5.9 (�0.07)a 5.8(�0.11)

Bulk density (Mg m�3) 1.33 (�0.03) 1.37 (�0.04)

Total soil organic C (g kg�1) 11.24 (�0.15) 7.63 (�0.24)

Total soil organic C (Mg ha�1) 22.42 (�0.73) 15.23 (�0.34)

Total soil N (g kg�1) 0.85 (�0.04) 0.54 (�0.06)

Total soil N (Mg ha�1) 1.70 (�0.08) 1.07 (�0.15)

Sand (g kg�1) 585 (�8) 598 (�11)

Silt (g kg�1) 197 (�5) 209 (�9)

Clay (g kg�1) 218 (�8) 193 (�16)

a Data in parentheses indicate standard deviation (n = 4).

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134 127

In the sub-temperate areas of the Indian Himalayas, rainfedcrop production is limited by insufficient and inadequate rainfalldistribution. So, adoption of conservation tillage is very importantbecause of concerns over soil erosion, water conservation anddecreasing profit margin with CT. In this region, informationregarding the effects of different tillage practices and croprotations on SOC sequestration is scanty. Although the effects oflong-term tillage and rotation on different soil properties havebeen studied widely throughout the world, the short-term effectsof tillage practices on SOC and TSN sequestration have not beenextensively studied (McCarty et al., 1998; VandenBygaart and Kay,2004), especially in hill slope agro-ecosystems under rainfedconditions.

Most evidence of the effects of crop rotations on soilaggregation and SOC storage comes from studies on fine-textured(clayey or silty clay) soils producing annual row crops on cool andhumid soils of USA (Wright and Hons, 2004), Eastern Canada(Angers and Carter, 1996; Whalen et al., 2003) and Europe (Martin-Rueda et al., 2007; Lopez-Fando et al., 2007; Alvaro-Fuentes et al.,2008a,b). There is limited information on whether sandy clay loamsoils producing combinations of two leguminous crops a yearrespond as rapidly to conservation tillage systems, even afterremoval of almost all above-ground crop residues. Hence, wehypothesized that soils under ZT, MT and CT would behavedifferently in terms of bulk density, aggregate stability and SOCand TSN storage. Furthermore, continuous leguminous croppingsystems would improve SOC/TSN stocks in soils compared tolegume–cereal rotations, as the former would add more leaf-fall,nodule and root biomass to soils. Therefore, the major objective ofthis research was to evaluate the effects of tillage systems and croprotations on soil aggregation and total SOC and TSN storage afterfour years of rainfed cropping in the Indian Himalayas, wherealmost all above-ground crop residues were removed. Specificobjectives of this study were to determine short-term effects oftillage and crop rotation on aggregate-size distribution andaggregate-associated C and N.

2. Materials and methods

2.1. Site details

The experiment was initiated in 1999 on a sandy clay loam soilat the experimental farm of the Vivekananda Institute of HillAgriculture located at 298360N, 798400E, and 1250 m above meansea level. The local geological formation consists of tectonicallydisturbed tertiary crystalline rocks (Gairola and Singh, 1995). Soilcharacteristics based on analysis of initial soil samples taken in1999 are given in Table 1. The climate of the region is sub-temperate. The average daily maximum and minimum airtemperatures ranged from 31.7 to 20.6 8C in June and 17.8 to

Table 2Weather parameters of the site during the experimental period and crop calendar.

Crops and year Rainfall

(mm)

Average maximum

temperature (8C)

Average minimum

temperature (8C)

A

(

Soybean, 1999 538.3 28.5 18.0 3

Soybean, 2000 870.5 27.9 17.9 3

Soybean, 2001 464.0 29.3 17.8 3

Soybean, 2002 643.1 28.8 17.1 3

Wheat, 1999–2000 307.6 22.6 4.7 2

Wheat, 2000–2001 130.1 23.6 5.3 2

Wheat, 2001-02 238.4 23.4 4.2 2

Wheat, 2002–2003 353.3 23.3 4.2 2

Lentil and Pea, 1999–2000 250.4 21.4 3.8 2

Lentil and Pea, 2000–2001 98.0 22.8 3.7 2

Lentil and Pea, 2001–2002 182.1 22.4 3.5 2

Lentil and Pea, 2002–2003 306.3 22.4 3.4 2

1.1 8C in January. The mean (30 years) annual rainfall is 1058 mm,with approximately 70% of the total precipitation falling during therainy season (June–September). Prior to the initiation of thisexperiment, the site was used for testing the performance of maize(Zea mays L.) varieties under mono-cropping for several years.Maize crop varieties were grown with recommended mineralfertilization. In the past and during other times of the year, the sitewas a native grassland that was continuously cut.

2.2. The experiment

The experiment was laid out in split-plot design with threetillage management practices (zero, minimum and conventional)in main plots (9 m � 3 m size) and three sequential cropping[soybean (Glycine max (L.) Merr.)–wheat (Triticum aestivum L.Emend. Flori & Paol), soybean–lentil (Lens culinaris Medicus) andsoybean–field pea (Pisum sativum L. Sensu Lato) in sub-plots(3 m � 3 m size) with three replications. Under ZT, every year theseeds were sown in furrows with the help of a hand pulled furrowopener. Whereas, under MT sowing was performed after a singletillage operation by spade (�15 cm soil depth) and under CTsowing was done following two tillage operations (�15 cm soildepth) made with a spade at seven-day intervals (Bhattacharyyaet al., 2006).

The details of the sowing and harvesting time of the crops andthe detail of the weather parameters during the period of theexperimentation are given in Table 2. Before sowing of soybeanand winter crops, weeds were controlled with the application ofgramaxone (1, 10-dimethyl 1-4, 40-bipyridylium) @ 1.0 kg activeingredient (a.i.) ha�1 under ZT plots. Weeds in the MT and CT plotswere controlled by pre-emergence spray of alachlor [2-chloro-N-(2, 6-diethylphenyl)-N-(methoxy-methyl) acetamide] @ 2.0 kga.i. ha�1 in soybean followed by one hand weeding at 45 days aftersowing. Similarly, during winter season weeds were controlled inCT and MT systems by spraying isoproturon [3,-(4-isopropylphe-

verage evaporation

mm d�1)

Date of sowing Date of harvesting

.3 02.06.1999 03.10.1999

.0 01.06.2000 03.10.2000

.4 06.06.2001 05.10.2001

.0 05.06.2002 06.10.2002

.3 10.10.1999 03.05.2000

.2 09.10.2000 04.05.2001

.2 12.10.2001 07.05.2002

.5 12.10.2002 08.05.2003

.0 22.10.1999 and 22.10.1999 16.04.2000 and 25.04.2000

.0 20.10.2000 and 20.10.2000 15.04.2001and 26.04.2001

.4 23.10.2001 and 23.10.2001 18.04.2002 and 28.04.2002

.2 25.10.2002 and 25.10.2002 19.04.2003 and 27.04.2003

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134128

nyl)1, 1 dimethyl urea] @ 1.0 kg a.i. ha�1 at 35 days after sowing inwheat and pendimethalin [N-(1-ethylpropyl)-3, 4-dimethyl-2, 6-dinitrobenzenamine] @ 1.0 kg a.i. ha�1 (as pre-emergence) in fieldpea and lentil followed by one hand weeding as and whenrequired.

Soybean variety ‘VL Soya 2’ was sown in 1st fortnight of Junewhereas, winter crops viz., wheat cv. ‘VL Gehun 616’, field pea cv.‘VL Matar 1’ and lentil cv. ‘VL Masoor 4’, were sown during 2ndfortnight of October in each year of the study. Soybean(80 kg seed ha�1) was sown in rows 40 cm apart to a depth of4–5 cm by hand. Wheat was sown (100 kg seed ha�1) in rows22.5 cm apart to a depth of about 5–6 cm by hand. The seed rate forlentil was 30–40 kg ha�1 and spacing was 25 cm between rows. Forpea, seed rate was 75–80 kg ha�1 and spacing was 30 cm betweenrows. Both lentil and pea were sown manually at about 5 cm soildepth. Recommended doses of fertilizers, which were applied todifferent crops were, 20 kg N + 34.9 kg P + 33.3 kg K ha�1 to soy-bean, 60 kg N + 13.1 kg P + 16.7 kg K ha�1 to wheat, 20 kg N+ 17.5 kg P + 33.3 kg K ha�1 to field pea and 20 kg N + 17.5 kg P +16.7 kg K ha�1 to lentil. Fertilizers used were urea for N, singlesuper-phosphate for P and murate of potash for K. Full amount of N,P and K in pulses and half amount of N along with full amount of Pand K in wheat were applied at the time of sowing. The remaininghalf amount of N was top-dressed in wheat after winter rains inFebruary. Recommended doses of fertilizers, which were applied todifferent crops were: 20 kg N + 34.9 kg P + 33.3 kg K ha�1 to soy-bean, 60 kg N + 13.1 kg P + 16.7 kg K ha�1 to wheat, 20 kg N +17.5 kg P + 33.3 kg K ha�1 to field pea and 20 kg N + 17.5 kg P +16.7 kg K ha�1 to lentil. Full amount of N, P and K in pulses and halfthe amount of N, along with full amounts of P and K in wheat, wereapplied at the time of sowing. The remaining half amount of N wastop-dressed in wheat after winter rains in February. At maturitythe above-ground portion of all crops were harvested leavingapproximately 5 cm high stubble in the field.

2.3. Soil sampling and analysis

Initial soil samples (0–15 cm) were collected on 15 May 1999.In 2003, on 8 May 2003 (after the winter crop harvest), triplicatesoil samples were collected from each plot using a core sampler(15 cm high and 7.6 cm diameter) from 0 to 15 and 15 to 30 cmdepths. Depth-wise samples were mixed to produce a compositesample from each plot and replicated under tillage systems andcrop rotation. After sieving all soil samples with a 4.75-mm sieve,we observed that no aggregates were>4.75 mm. In the laboratory,samples were dried at 50 8C for seven days, and a portion waspassed through a 2.0 mm sieve and the other portion was passedthrough a 4.75 mm sieve. The <2.0 mm size fraction of the soilsamples were ground to pass a 0.2 mm sieve and analyzed for totalSOC and TSN using a CHN analyzer (model FOSS Hareus, CHNORapid), while the other fraction was used for aggregate-sizedistribution and MWD. Additional triplicate samples were takenusing the core sampler (15 cm high and 7.6 cm diameter) for bulkdensity measurements from both soil depths. Total SOC and TSNstorage within bulk soil (<2.0 mm) were calculated using bulkdensity, soil depth and SOC and TSN concentrations. We assumedbulk soil (<2.0 mm) under a particular soil depth and treatmenthad the same bulk density of whole soil (<4.75 mm) to calculateSOC and TSN storage. The SOC and TSN sequestered in each plotafter four years of the respective management practices wascalculated as the mathematical difference between the 1999 and2003 SOC storage (in the <2.0 mm size fraction) values.

A wet sieving procedure was modified for determination ofaggregate-size distribution, and aggregate stability indices (Kem-per and Rosenau, 1986). Following capillary wetting of exactly 50 goven-dried soil aggregates (<4.75 mm), samples were immersed in

water on a nest of sieves (2.0, 1.0, 0.5, 0.25 and 0.053 mm) for10 min before wet-sieving. The sieve nest was then clamped andtransferred to the drum securely. The sieve assembly wasoscillated up-and-down by a pulley arrangement for 20 min at afrequency of 30–35 cycles min�1 with a stroke length of 4 cm insalt free water inside the drum. The aggregates remaining on eachsieve were washed onto a preweighed filter, oven-dried at 105 8Cfor 24 h, and weighed. The soil samples that pass through0.053 mm sieve were also collected and oven-dried at 105 8C for24 h, and weighed for mean weight diameter (MWD), SOC and TSNmeasurement. The aggregates were then suspended in 50 mL of0.01 M Na4P2O7, shaken for 16 h, and filtered through a 0.053 mmsieve and the suspension poured through a sieve with the samemesh size as the one from which the aggregates were collected. Thesand remaining on each sieve was washed onto a preweighed filter,oven-dried at 105 8C for 24 h, and weighed. The mass of stable,sand-free aggregates was the difference between the mass of totalaggregates (non-dispersed) and the mass of sand collected on eachsieve. The proportion of water stable aggregates (WSA) in each sizefraction (WSAi) was calculated from the following expression(Whalen et al., 2003):

WSAi ¼Totali � Sandi

fSoil=ð1þMoistureÞg �P

Sandi(1)

where i is the ith size fraction (4.75–2.0 mm, 2.0–1.0 mm, 1.0–0.5 mm, 0.5–0.25 mm, 0.25–0.053 mm); Total is the oven-dry massof total, non-dispersed aggregates collected on each sieve; Sand isthe oven-dry mass of sand collected on each sieve; Soil is the oven-dry mass of the remoistened, sieved (<4.75 mm) soil; and Moistureis the gravimetric moisture content of the remoistened, sieved(<4.75 mm) soil.

Soil aggregate indices were calculated as follows:

Aggregate ratio ¼ percentage of macroaggregates

percentage of microaggregates(2)

where percentage of macroaggregates are the summation of soilaggregate-size fractions >0.25 mm and percentage of microag-gregates are the summation of soil aggregate-size fractions<0.25 mm. The MWD was calculated taking into account thesand-content in each aggregate-size fraction, using the followingequation (van Bavel, 1949):

MWD ¼X

XiWSAi (3)

where i is the ith size fraction (4.75–2.0 mm, 2.0–1.0 mm, 1.0–0.5 mm, 0.5–0.25 mm, 0.25–0.053 mm and <0.053 mm) and X isthe mean diameter of each size fraction, based on the meanintersieve size. Thus, six aggregate-size fractions were obtained: S1(2–4.75 mm), S2 (1.0–2.0 mm), S3 (0.5–1.0 mm), S4 (0.25–0.5 mm), S5 (0.053–0.25 mm) and S6 (<0.053 mm).

Sub-samples from each aggregate-size fraction were thenground to pass a 0.2 mm sieve and analyzed for total SOC and TSNusing a CHN analyzer. Whole soil samples (<4.75 mm) that werenot used for aggregate-size fractionation were also ground to passa 0.2 mm sieve and analyzed for SOC and TSN. Since we determinedthat the samples were free of inorganic C (carbonates), the total Cmeasured was taken to be equivalent to the quantity of organic C inthe soil samples.

We also calculated treatment impacts on C and N sequestrationin soil based on values expressed in terms of equivalent initial soilmass because the conventional method (concentration � soil bulkdensity � soil layer thickness) is believed to be inadequate, as itdoes not take into full consideration the impact of soil mass on Cand N storage (Hooker et al., 2005; Six et al., 2006). The equivalentmass procedures have been shown to recover greater C mass andare more sensitive in detecting differences of C mass among

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134 129

treatments (Ellert and Bettany, 1995; Ellert et al., 2002). Ellert andBettany (1995) found that the actual value selected as equivalentmass was less important than the need for using the samereference soil mass for comparisons of nutrient storage. Wedesignated the mass of the initial soil as the equivalent soil mass tocompare treatment effects. The reference values of 1995 and 2055Mg soil for 0–15 and 15–30 cm soil depths, respectively, wereconsidered. Briefly, equivalent mass of C and N was determinedusing the following equation (Six et al., 2002)

Mc ¼ ½ðConcc � Db � depthÞ � ConccðMsoil �Msoil;equivÞ� � 10 (4)

where Mc equals equivalent SOC or TSN mass per unit area(Mg ha�1); Concc equals SOC or TSN concentration (kg Mg�1); Db

equals soil bulk density (Mg m�3); depth equals horizon depth(m); Msoil equals soil mass (Mg m�2); Msoil, equiv equals equivalentsoil mass (Mg m�2). Total SOC and TSN mass on equivalent basiswere then calculated by summing the values to reach approxi-mately 30 cm soil depth.

2.4. Grain yield and weed biomass

Soybean, wheat, lentil and field pea grain yields were measured at14, 12, 12 and 12% moisture basis, respectively. Soybean equivalentyield (SEY) was calculated based on the minimum support price fixedfor farmers inaparticularyear.Toquantifytheweeddensity,dryweightof weeds were recorded in 2003 after harvest of winter crops in a 1 m2

area in 1 m� 1 m quadrant placed randomly on each plot. Weedspresent in the quadrant were removed, washed to remove the soilparticles and air-dried. Weed dry weight was recorded after drying(65 8C) the weeds to constant weight. In all plots, monocot (grasses)weeds were mainly present and Bermuda grass (Cynodon dactylon L.)was the dominant monocot weed.

Fig. 1. Effect of tillage and crop rotation on: (a) soil bulk density, (b) mean weight

diameter (MWD) and (c) aggregate ratio after four years of cropping.

[CT = conventional tillage, MT = minimum tillage, ZT = Zero tillage, S–

W = soybean–wheat, S–L = soybean–lentil and S–P = soybean–pea. Bars with

same letters within the tillage systems and within the crop rotations are not

significantly different (Tukey’s HSD tests, P < 0.05)].

2.5. Statistical analyses

Statistical analyses were performed using Analysis of Variance(ANOVA) for split-plot design (Gomez and Gomez, 1984). Tillagetreatment served as the main plot, cropping sequence was thesplit-plot. The treatment means were compared at the P < 0.05level using the LSD for all the parameters. Data were tested forseparation of means using Tukey’s Honestly Significant Difference(HSD) procedure. Where transformations were required to meetnormality assumptions, data were power-transformed.

3. Results

3.1. Soil bulk density

Soil bulk density was significantly higher under ZT and MT inthe soil surface (0–15 cm soil depth) compared with CT soil(Fig. 1a). There were no significant variations in soil bulk densityvalues due to tillage management in the 15–30 cm soil depth.Cropping system had no significant effect on soil bulk density indifferent soil layers. Hence, the interaction effects of tillage � crop-cropping on soil bulk density were non-significant (Fig. 1a).

3.2. Aggregate distribution and MWD of soil

Aggregate-size distribution was dominated (54–58%) bymicroaggregates in the 0–15 cm soil layer. The MWD in CT plotswas significantly smaller than that of ZT and MT plots (Fig. 1b).Percent macroaggregates (>0.25 mm) was greater under ZT andMT plots than under CT in the surface soil layer. The aggregate ratiowas 9% greater under MT than under CT in the 0–15 cm soil depth(Fig. 1c), indicating conservation tillage had a significant effect onaggregate stability of surface soil even after four years ofcultivation. Both tillage and rotation had little effect on proportionof water stable aggregates under different size fractions in the 0–15 and 15–30 cm soil layers and, hence, data are not shown. Plotsunder ZT had 24 and 11% higher large sized water stable aggregates(2.0–4.75 mm) than MT and CT, respectively, only in the 0–15 cmsoil depth. Plots under CT had 13% higher S5 (0.1–0.25 mmfraction) than ZT in the same soil depth. In the 15–30 cm soil layer,MT plots had higher proportion of S3 and lower proportion of S5than CT and ZT plots. Crop rotations had no significant effect onaggregate stability under different size fractions in the 0–15 and15–30 cm soil layers.

3.3. Soil organic C content on equivalent depth and initial

soil mass basis

Zero tilled plots stored approximately 10.2% higher equivalentdepth based-SOC than CT (23.8 Mg ha�1) after four years ofexperimentation in the 0–15 cm soil layer (Table 3). Difference inSOC storage between MT and CT plots was significant only in thesurface soil layer (0–15 cm depth). There were no statisticaldifferences in SOC stocks between ZT and CT in the 15–30 cm soildepths (Table 3). Similarly, crop rotations had significant impactson SOC storage in the surface soil layer only. However, theinteraction effects of tillage � cropping on SOC storage were non-significant in both soil depths (Table 3). After four years ofsoybean-based cropping with different tillage systems, there wasan increase in SOC storage in the 0–30 cm depth over the initial(1999) soil. In the 0–30 cm soil layer, SOC sequestration (calculatedas the mathematical difference between the 1999 and 2003 SOCstorage values) in the plots under CT was 0.41 Mg ha�1 yr�1.Sequestration of SOC over initial soil was 1.01 Mg ha�1 yr�1 withadoption of ZT and 0.99 Mg ha�1 yr�1 with continuous S–Pcropping.

Table 3Impact of tillage and crop rotation on total SOC and TSN storage after four years of cropping in a sandy clay loam soil of the Indian Himalayas.

Treatmentsa SOC storage (Mg ha�1) (soil depth (cm)) TSN storage (Mg ha�1) (soil depth (cm))

0–15 15–30 Total (0–30) 0–15 15–30 Total (0–30)

Tillage

CT 23.79 15.51 39.30 1.85 1.10 2.95

MT 25.89 14.45 40.34 2.12 1.11 3.23

ZT 26.23 15.44 41.67 2.17 1.13 3.30

LSD (P < 0.05), tillage 2.07 NS NS 0.18 NS NS

Cropping system

S–W 23.44 15.19 38.63 1.89 1.12 3.01

S–L 25.95 15.12 41.07 2.08 1.10 3.18

S–P 26.51 15.09 41.60 2.17 1.12 3.29

LSD (P < 0.05), cropping system 2.17 NS NS 0.17 NS NS

LSD (P < 0.05), tillage � cropping system NS NS NS NS NS NS

a CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–W = soybean–wheat, S–L = soybean–lentil and S–P = soybean–pea. LSD and NS indicate least

significant difference and not significant, respectively.

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134130

To account for unequal soil masses or densities, we calculatedorganic C and N stored on an equivalent mass basis of 1995 and2055 Mg soil ha�1 to approximately reach 15 and 30 cm soildepths, respectively (Table 4). At an equivalent soil mass of1995 Mg ha�1, ZT plots had a significantly higher SOC storagecompared to CT (Table 4). However, unlike SOC storage on anequivalent depth basis, plots under CT and MT had similar SOCstorage at an equivalent soil mass of 1995 Mg ha�1 (approximately15 cm soil layer). Total SOC accumulated at an equivalent soil massof 4050 Mg ha�1 were similar among tillage treatments. Similarly,at an equivalent soil mass of 1995 Mg ha�1, plots with S–Wrotation had significantly less SOC storage than S–L and S–P plots.Conversely, total SOC accumulated at an equivalent soil mass of2055 Mg ha�1 were similar among plots with different rotationaltreatments. Crop rotations had no impact on SOC storage at anequivalent soil mass of 4050 Mg ha�1. However, rotations withcontinuous leguminous crops had notable effects on SOCsequestration over the initial soil. Within approximately 30 cmsoil depth, SOC sequestration values on an equivalent mass basis inthe plots under S–L and S–P were 0.83 and 0.94 Mg ha�1 yr�1,respectively (Table 4).

3.4. Total Soil N content on equivalent depth and initial soil mass basis

Total soil N storage showed almost similar responses to tillageand crop rotation as SOC on the basis of both equivalent depth

Table 4Total soil organic carbon (SOC) and total soil nitrogen (TSN) storage with cumulative equ

after four years of cropping in a sandy clay loam soil in the Indian Himalayas.

Treatmentsa SOC storage (Mg ha�1) on equivalent

mass of

1995 Mg

soil

2055 Mg

soil

4050 Mg

soil

Tillage

CT 23.90 15.43 39.33

MT 25.74 14.38 40.12

ZT 25.97 15.41 41.38

LSD (P < 0.05), tillage 1.97 NS NS

Cropping system

S–W 23.32 15.14 38.46

S–L 25.94 15.02 40.96

S–P 26.33 15.06 41.39

LSD (P < 0.05), cropping system 2.12 NS NS

LSD (P < 0.05), tillage � cropping system NS NS NS

a CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–W = soybean

significant difference and not significant, respectively. Initial SOC storage values on equi

Initial TSN storage values on equivalent mass of 1995 and 2055 Mg soil were 1.70 and

(Table 3) and mass (Table 4). At an equivalent depth basis of 0–15 cm soil layer, plots under ZT had the highest TSN stock(2.2 Mg ha�1), which was similar to MT. In that soil layer, MT plotshad 14.6% higher TSN storage than CT (Table 3). Similarly, after fouryears of continuous cropping plots, S–L and S–P had 10.1 and 14.8%higher TSN stock, respectively, than S–W plots (1.9 Mg ha�1).Tillage and crop rotation had no detectable effect on TSN content inthe 15–30 cm depth. Interaction effects of tillage � cropping onSOC storage were non-significant (at P < 0.05) in both depths(Table 3). Management practices also increased TSN storage in the0–30 cm soil layer over the initial (1999) soil. Total soil Nsequestration in the plots under ZT was 0.53 Mg ha�1 after fouryears in that depth. Likewise, plots under S–P had sequestered0.52 Mg ha�1 over initial soil. At an equivalent mass basis of4050 Mg ha�1, TSN storage after four years in the ZT plots was0.58 Mg ha�1 over initial soil and those in the plots under S–P andS–L were 0.46 and 0.60 Mg ha�1, respectively (Table 3).

3.5. Distribution of SOC among aggregate-size fractions

To understand if management practices affected SOC withinparticular size fractions, SOC concentrations in different aggregate-size fractions were calculated. In the 0–15 cm soil layer, SOCconcentrations were significantly greater under ZT plots than CTwithin all size fractions (Fig. 2). In the same soil layer within all sizefractions, SOC concentrations in the plots under ZT were similar to

ivalent mass depth to 4050 Mg ha�1 and C/N ratio as affected by tillage and rotation

TSN storage (Mg ha�1) on equivalent

mass of

C/N ratio

1995 Mg

soil

2055 Mg

soil

4050 Mg

soil

0–15 cm

soil

15–30 cm

soil

Mean

(0–30 cm)

1.86 1.09 2.95 12.85 14.10 13.47

2.11 1.11 3.22 12.20 13.02 12.61

2.15 1.13 3.28 12.09 13.66 12.88

0.17 NS NS NS 1.07 NS

1.88 1.11 2.99 12.42 13.56 12.99

2.07 1.09 3.16 12.48 13.75 13.12

2.17 1.13 3.30 12.24 13.47 12.85

0.16 NS NS NS NS NS

NS NS NS NS NS NS

–wheat, S–L = soybean–lentil and S–P = soybean–pea. LSD and NS indicate least

valent mass of 1995 and 2055 Mg soil were 22.42 and 15.23 Mg ha�1, respectively.

1.07 Mg ha�1, respectively.

Fig. 2. Effect of tillage and crop rotation on total soil organic C concentration in

different aggregate-size fractions after four years of cropping at 0–15 cm soil layer.

[CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–W = soybean–

wheat, S–L = soybean–lentil and S–P = soybean–pea. S1 = 2.0–4.75 mm, S2 = 1.0–

2.0 mm, S3 = 0.5–1.0 mm, S4 = 0.25–0.5 mm, S5 = 0.053–0.25 mm and

S6 < 0.053 mm. Bars of a single parameter with same letter within the tillage

systems and within the crop rotations are not significantly different (Tukey’s HSD

tests, P < 0.05); all interaction effects of tillage � cropping system were not

significant at P < 0.05].

Fig. 3. Effect of tillage and crop rotation on total soil N concentration in different

aggregate-size fractions after four years of cropping at 0–15 cm soil layer.

[CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–

W = soybean–wheat, S–L = soybean–lentil and S–P = soybean–pea. S1 = 2.0–

4.75 mm, S2 = 1.0–2.0 mm, S3 = 0.5–1.0 mm, S4 = 0.25–0.5 mm, S5 = 0.053–

0.25 mm and S6 < 0.053 mm. Bars of a single parameter with same letter within

the tillage systems and within the crop rotations are not significantly different

(Tukey’s HSD tests, P < 0.05); all interaction effects of tillage � cropping system

were not significant at P < 0.05].

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134 131

MT plots. The largest SOC concentration in this depth occurred inthe S1 fraction under ZT and the least in the S2 fraction under CT.However, in the sub-surface soil layer, aggregate-associated SOCwas unaffected by tillage and hence, data are not presented. Croprotation had no impact on microaggregate-associated SOC con-centration in both 0–15 and 15–30 cm soil layers.

To compare C concentration within whole soil (<4.75 mm sizefraction) and different aggregate-size fractions, SOC concentrationdata of whole soil was presented in Fig. 2. Whole soil Cconcentrations were notably less than those observed for S1 sizefraction and were in between S2 and S3 fractions. Tillage effects on Cconcentrations within whole soil were similar to SOC within S1 inthe surface soil layer. However, unlike SOC within S1, plots under S–Land S–P had higher SOC within whole soil than S–W plots (Fig. 2).Soil organic C concentrations within S1 and S2 were notably higherthan that within whole soil under both soil layers, irrespective oftillage and crop rotation. Tillage and rotation had no effect on SOCconcentration within whole soil in the sub-surface soil layer.

3.6. Distribution of TSN among aggregate-size fractions and C:N ratios

Averaged across cropping sequences, TSN concentrationswithin all size fractions (except within S6) in the 0–15 cm soillayer were significantly greater under ZT plots than CT plots(Fig. 3). In the 15–30 cm soil layer, no differences in TSN storagevalues were noted among tillage systems. Aggregate-associatedTSN concentrations were not affected by crop rotation in both soillayers. Notably higher TSN concentrations were observed inmacroaggregate fractions than in microaggregates in the 0–15 cm soil depth. However, in the 15–30 cm soil layer, the reversewas true. There was no significant tillage � rotation interactionamong aggregate associated TSN concentrations within differentsize fractions. Tillage and rotation effects on TSN concentrations

within whole soil followed the same trend (Fig. 3) as SOC withinwhole soil.

The soil C/N ratios were calculated from SOC and TSNconcentrations for different aggregate-size fractions and the bulksoil. Bulk soil C/N ratios were not impacted by both tillage androtations, except that in the plots under MT were less than CT plotsin the 15–30 cm soil layer (Table 4). There were no consistentdifferences in C/N ratios of any aggregate-size class amongrotations and tillage treatments and, hence, data are not presented.The C/N ratio under S1 fraction was affected by tillage only for the2.0–4.75 mm size fraction in the 0–15 cm soil depth, but notaffected by crop rotations within different size fractions indifferent soil depths. The mean C/N ratio in that soil depth ofthe macroaggregates (the four largest size fractions, C/N = 12.5)was significantly (P < 0.05) less than that of the microaggregates(the two smallest size fractions, C/N = 14.5). The C/N ratio of S1fraction was 8% lower in the plots under ZT than CT in the 0–15 cmsoil layer.

4. Discussion

4.1. Soil bulk density

The significantly higher soil bulk density values underconservation tillage in the surface soil layer might be due tonon-disturbance of the soil matrix, which resulted in lower totalporosity than tilled plots. Lower bulk densities under CT in thesurface soil layer were caused by loosening of soils by tillageimplements and the mixing of crop residues into the plough layer(Hussain et al., 1998). Comparisons of bulk density values amongtillage systems have produced conflicting results. Several studieshave reported higher bulk density under ZT in the soil surfacecompared with tilled soil (Hill, 1990; Wu et al., 1992; Salinas-Garcia et al., 1997). In contrast, Azooz et al. (1996) reported slightor no differences in bulk density values between CT and ZT on theCanadian Prairies.

4.2. Aggregate stability

Higher SOC content in the surface layer of ZT system may lead tomore and stable aggregation in large macroaggregates (Lal et al.,1994). Mikha and Rice (2004) observed that CT significantly(P < 0.05) reduced macroaggregates (>2.0 mm and 0.25–2 mm)with a concomitant increase in microaggregates (<0.25 mm) after10 years of study in a US silt loam soil. The decline in the size ofaggregates with CT could be credited to mechanical disruption ofmacroaggregates, which might have exposed SOM previouslyprotected from decomposition (Six et al., 2000a). Favourableeffects of ZT on soil structural properties may be partly also due tohigher earthworm activity and more microbial biomass than in CTand MT treatments (Mahboubi and Lal, 1998). We observed thataggregate-size distribution was impacted by tillage only in the 0–15 cm soil depth for S1 and S5 size fractions. Residues incorporatedinto sub-surface soils by tillage might have increased aggregation,countering the destructive impacts of tillage on aggregation(Wright and Hons, 2005). Similar to the results of this study,several authors (Angers and Carter, 1996; Bissonnette et al., 2001;Whalen et al., 2003) reported that improvements in aggregationcan occur within 2–3 years of establishing conservation practices.

4.3. Soil organic C content in bulk soil (<2.0 mm)

Soil organic C contents improved from the start of theexperiment, even in the CT plots. This was mainly due to thefact that the whole plot before the start of this experiment wasunder continuous maize/corn (Z. mays L.) mono-cropping and

Table 5Mean grain yield of soybean and soybean equivalent yield (SEY) of winter crops as influenced by tillage practices in soybean based cropping systems during 1999–2000 to

2002–2003.

Treatmentsa Mean grain yield (Mg ha�1) of soybean Mean SEY (Mg ha�1) of winter crops Mean SEY (Mg ha�1) of the system

Tillage

CT 1.87 0.67 1.27

MT 2.31 1.00 1.66

ZT 2.36 1.07 1.71

LSD (P < 0.05), tillage 0.17 0.09 0.13

Cropping system

S–W 2.12 1.09 1.61

S–L 2.26 0.94 1.60

S–P 2.16 0.71 1.43

LSD (P < 0.05), cropping system NS 0.07 0.12

LSD (P < 0.05), tillage � cropping system NS 0.17 NS

a CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–W = soybean–wheat, S–L = soybean–lentil and S–P = soybean–pea. LSD and NS indicate least

significant difference and not significant, respectively.

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134132

during other times of the year it was a native grassland that wascontinuously cut. In this study, two crops per year not onlymaintained more roots and rhizodeposition in the plots, but alsothere was considerable leaf-fall biomass from soybean (even in theplots under unfertilized control) each year. It was estimated thatsoybean leaf-fall biomass in plots under unfertilized control was124 kg ha�1 (Kundu et al., 2007) in a sandy loam soil of the sameregion. Like CT management, soils under maize also needed twosets of manual tillage per year, one before maize sowing and theother for earthing up operations (35 days after sowing). All thesefactors were collectively responsible for higher SOC content in theplots under CT over initial SOC.

Since CT plots had higher crop productivity (mean of four years)than ZT plots (Table 5) and as almost all aboveground biomass wasremoved, SOC sequestration under ZT is believed to result fromreduced decomposition of SOC because of a less aerobic environ-ment and better physical protection of SOC within aggregates(Beare et al., 1994). Moreover, tillage increases the rate of SOMdecomposition and C and N mineralization (Blevins et al., 1977;Doran and Smith, 1987). Lupwayi et al. (1999) observed ZT systemscontributed less to atmospheric CO2 than CT and SOM accumulatedmore under ZT. Like us, short-term increases in SOC under ZT areseldom observed in sub-surface soils.

Soil organic C contents were also significantly (P < 0.05) alteredby crop rotations in the surface soil layer. After four years, meanSEY under S–W rotation was 11 and 16% higher than those underS–L and S–P rotations, respectively (Table 5). Assuming the amountof root biomass returned to soils were proportional to above-

Table 6Dry weed biomass (Mg ha�1) as affected by tillage and crop rotation after four years

of experimentation in the Indian Himalayas.

Treatmentsa Weed biomass at harvest

of winter crops in 2003

Tillage

CT 0.19

MT 0.21

ZT 0.47

LSD (P < 0.05), tillage 0.04

Cropping system

S–W 0.4

S–L 0.29

S–P 0.34

LSD (P < 0.05), cropping system 0.03

LSD (P < 0.05), tillage � cropping system 0.07

a CT = conventional tillage, MT = minimum tillage, ZT = zero tillage, S–W = soy-

bean–wheat, S–L = soybean–lentil and S–P = soybean–pea. LSD indicates least

significant difference.

ground biomass yields, it would be expected that the plots underS–W rotation would have the highest SOC content, since it had thehighest grain yields. However, this was not the case for�30 cm soildepth in two depth increments. The reasons for this inconsistenceeffect of S–W on SOC contents are not obvious. Possible causesinclude the quality of the residues, differential presence of rootbiomass in different soil depths and addition of leaf-fall and nodulebiomass by the lentil and pea crops. Furthermore, it was alsoobserved that plots under S–W had least weed biomass recordedafter four years of cropping. Higher added weed belowgroundbiomass in the S–L/S–P plots might also play a role in higher SOCcontent in those plots than the plots under S–W. Dry biomass(recorded at the end of four years) was �21 and 42% higher in theS–L and S–P plots, respectively, than S–W (0.24 Mg ha�1) plots(Table 6). The leguminous crops must have supplied N, organicmaterials and recycled leached nutrients. Furthermore, rotationsthat include soybean as the only legume gain little or no N in thesoil/plant system, due to the large export of N from this crop in theharvested grain (LaRue and Patterson, 1985). Consequently, plotswith S–W had no significant gain in SOC/TSN storage at both soillayers. In S–L and S–P rotations the other N2-fixing green-manurecrops (lentil and pea) were included instead of wheat. It, therefore,seems reasonable to conclude that N, leaf-fall and nodule biomassinput by lentil and pea along with increased weed biomass inputwere the key factors to the observed SOM accumulation orconservation under S–L and S–P plots.

We found the equivalent mass-based method provides a moreaccurate way to assess tillage and rotation impacts on SOC storagethan depth-based measurements. Generally, equivalent mass-based measurements show that total SOC accumulated to about30 cm soil profile as a whole under ZT was similar to CT and MT.Thus, tillage events for four years homogenized SOC through theprofile and resulted in no overall significant (P < 0.05) change ofequivalent mass-based SOC. Similarly, Yang and Kay (2001)reported that effect of tillage on SOC accumulation expressed onan equivalent mass basis was restricted to the upper 10 cm. Sinceaboveground biomass after crop harvest has been removed in thisexperiment, it was unexpected that changes in organic C and totalN would occur just in four years in this rainfed farming system.However, in this study, four manual diggings along with two handweeding per year using a ‘kutla’ (a small local digging instrumentused for uprooting weeds) in the plots under CT might be the majorcause for less SOC storage on equivalent mass basis to approxi-mately 15 cm soil depth than that under ZT plots even after fouryears. Moreover, ZT plots might have higher added belowgroundweed biomass than CT/MT plots due to higher weed growth inthese plots. Dry biomass (recorded at the end of four years) was�124 and 147% higher in the ZT plots than MT (0.21 Mg ha�1) and

R. Bhattacharyya et al. / Agriculture, Ecosystems and Environment 132 (2009) 126–134 133

CT (0.19 Mg ha�1) plots, respectively (Table 6). Added root biomassin the ZT and S–L/S–P plots as compared to CT and S–W plots,respectively, might be much higher as the dominant weed (C.

dactylon L.) is known to have a strong rooting pattern in the surfacesoil. Similar to the observed results, weed biomass, composedmainly of annual grasses, was shown to increase in ZT maize–soybean rotations compared to CT (Wrucke and Arnold, 1985).

4.4. Total soil N content in the bulk soil (<2.0 mm)

Like SOC storage, the difference in both equivalent mass anddepth based TSN storage among tillage systems occurred in thesurface soil layer only. Most accumulated soil N is derived fromroot residues along with biological N2 fixation (BNF), as above-ground biomass for all crops was removed leaving approximately5 cm high stubble. Furthermore, larger TSN storage values in theplots under ZT than CT imply that N might be incorporated inmicrobial biomass near the soil surface, leaving less N available formineralization or leaching (Bessam and Mrabet, 2003). Increasedsurface TSN storage, both on equivalent depth and mass basis inthe plots under the continuous leguminous cropping system, wasmainly due to BNF and shallow rooting patterns of the lentil andpea crops. Jensen (1997) found an average N benefit of about20 kg N ha�1 from peas in a crop rotation. Under CT this input wasnot apparent either because BNF input was reduced by soil mineralN release. Plots with S–P/S–L had higher TSN than S–W plots,probably due to notable reduction of nitrogenous emissions withpeas compared to wheat (Charles and Gosse, 2002).

4.5. Soil organic C concentration within aggregate-size fractions

Substantially higher SOC concentrations in macroaggregates inthe plots under ZT than CT and S–P than S–W could be due to lowerdecomposable SOM associated with these aggregates and thedirect contribution of SOM to the stability of macroaggregates. Theeffect of ZT on total SOC and TSN concentrations were morepronounced in macroaggregates than in microaggregates, indicat-ing greater sensitivity of macroaggregates to this managementpractices (Mikha and Rice, 2004). Similar observations of SOC inaggregate-size fractions in the plots under ZT and CT wereobserved for agricultural soils in Georgia (USA) (Bossuyt et al.,2002), in Texas (Wright and Hons, 2005) after 20 years of cropping,and in Kansas (Mikha and Rice, 2004) after 10 years of cropping.However, short-term improvement of macroaggregate-SOC in ZTand MT plots without manure application was seldom reportedearlier.

4.6. Total soil N within aggregate-size fractions and C/N ratios

Notably greater proportion of TSN was associated withmacroaggregates than microaggregates, indicating macroaggre-gates have higher C and N mineralization potentials thanmicroaggregates (Elliott, 1986). The decrease in C and Nconcentrations in the surface soil layer within a macroaggre-gate-size class with tillage precludes direct linkage between C andN accumulation and aggregation as proposed by Six et al. (2000a)in temperate soils. Similar SOC and TSN concentrations withinaggregate-size classes under different tillage and cropping systemsin the sub-surface soil suggests that apart from organic matter,aggregate formation was impacted by differences in residue inputand the predominant effect of mineral–mineral binding processes(Six et al., 2000b).

The lower C/N ratios of macroaggregate fractions under reducedtillage systems than CT were expected, as soils under ZT and MTmight typically contain more labile undecomposed SOM havinglower C/N ratios (Wright and Hons, 2004). Under CT, the smallest

size fraction probably contained SOM from disturbed anddisintegrated aggregates, which were only recently broken downduring tillage. The interaction effects of tillage � rotation for all soilproperties were not significant at P < 0.05 and require that sucheffects of soil management practices in the Indian Himalayas beevaluated in longer term studies. Field experiments are continuingsince 2003 with seasonal tillage alterations enabling evaluation oflong-term effects.

5. Conclusions

This study indicates management strategies, such as reducedtillage (ZT and MT) and continuous leguminous croppingsequences, played significant roles in total SOC and TSNsequestration soil in the rainfed hilly agroecosystem, wherealmost all above-ground crop residues were removed undersoybean based cropping. Short-term ZT and MT under continuoussoybean based cropping had higher equivalent depth based-SOCand TSN storage in surface soil (0–15 cm depth) only comparedwith CT in a sandy loam soil. Plots under continuous leguminouscropping (S–W and S–P) had higher SOC and TSN storage in thesurface layer than soybean–wheat plots in the surface soil layer.Reduced tillage improved surface soil aggregation, increased thepercentage of macroaggregates and aggregate-associated C and Nfor all size fractions over CT in the surface soil layer. However,although continuous leguminous crop rotations improved totalSOC and TSN storage, they had no impact on aggregate-sizedistribution and aggregate-associated C and N concentrations.Hence, further studies detailing the major factors and temporalchanges of soil surface aggregation under continuous leguminouscropping sequences with and without tillage are necessary toprovide better understanding of the long-term potential of thesemanagement practices to sequester C and N under IndianHimalayas.

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