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    230   C.P. Kushwaha et al. / Applied Soil Ecology 16 (2001) 229–241

    practices has increased from 15 to 38million ha

    over the last decade (CTIC, 1994) because of the

    potential of this practice to reduce soil erosion, con-

    serve soil moisture and improve soil structure. In

    semi-arid conditions conservation tillage may reduce

    evaporative water loss and increase crop biomass

    thereby favouring organic matter accumulation from

    the higher input of residues (Campbell and Janzen,

    1995).

    Soil structure plays a key role in the ability of soil

    to store organic matter (Balabane, 1996). Soil organic

    matter can be physically protected from microbial

    attack within soil aggregates, and contributes to the

    productivity and physical well-being of soils (Camp-

    bell et al., 1996). The proportions of water-stable

    aggregates in a soil often change very rapidly when

    tillage practices and crop rotations are modified

    (Angers et al., 1992). In addition to soil type and

    climatic variables, the magnitude of these effects de-

    pends on the intensity of cultivation, in particular, thetype and frequency of tillage and the quantity and

    quality of fertilizers and organic residues returned to

    the soil (Rasmussen and Collins, 1991). Conventional

    tillage practices disrupt soil aggregates, exposing

    more organic matter to microbial attack (Beare et al.,

    1994). Organic matter can be protected from mi-

    crobial attack by adsorption to clay minerals (Ladd

    et al., 1985) and the formation of microaggregates

    (Gregorich et al., 1989), by isolation in micropores

    (Foster, 1981) and by physical protection within sta-

    ble macroaggregates (Elliott, 1986; Gupta and Ger-

    mida, 1988). In conventional agricultural production

    practices, tillage and residue removal increase theloss of soil organic matter (Dalal and Mayer, 1986),

    and reduce the proportion of macroaggregates (Tis-

    dall and Oades, 1980). This is mainly due to loss

    of the labile organic matter (i.e. microbial biomass)

    which is responsible for the formation of macroaggre-

    gates. In cultivated soils, the organic matter content

    of macroaggregates is considerably greater than that

    of microaggregates. Application of organic matter in

    the form of crop residues has long been known to

    improve the properties of soils (Blevins and Frye,

    1993). The only shortcoming of residue application

    is the immobilization of nutrients during the early

    phase of decomposition because of its wider C/Nratio. This can be overcome by the use of chemical

    fertilizers in addition to crop residue. It has been

    reported that straw incorporation may increase the

    formation and stability of soil aggregates (Smith and

    Elliott, 1990).

    The role of conservational tillage and residue re-

    tention in aggregate formation is poorly documented

    in tropical agroecosystems. Such information is lack-

    ing from dryland (rainfed) agroecosystems of India

    which account for ca. 70% of the cultivated land in

    the country. An understanding of the effect of various

    management practices on the soil aggregation may

    help develop sustainable crop production systems in

    dryland agroecosystems. The main aim of this study

    was to determine the changes in both soil organic

    matter (organic and microbial C) and water-stable

    aggregates under different tillage systems and residue

    retention/removal conditions. This study addresses the

    following questions with respect to tropical dryland

    agroecosystems:

    1. What is the impact of tillage reduction and residue

    manipulation on the physico-chemical properties of soil?

    2. Are the changes in water-stable soil aggregates

    associated with the levels of soil organic mat-

    ter under different tillage and residue retention

    practices?

    3. To what extent do macro- and microaggregates dif-

    fer with respect to their organic C and total N ac-

    cumulations?

    4. What is the relationship between soil microbial

    biomass and water-stable soil aggregates under dif-

    ferent tillage systems and residue conditions?

    2. Materials and methods

    2.1. Study site

    These experiments were conducted on the dry-

    land farm of the Institute of Agricultural Sciences,

    Banaras Hindu University at Varanasi (at 25◦18N

    latitude and 83◦1E longitude, 76 m above the msl).

    The region has a tropical moist subhumid climate,

    characterized by strong seasonality with respect to

    temperature and precipitation. The year is divisi-

    ble into a warm rainy season (July–September), acool winter (November–February), and a hot sum-

    mer (April–June). October and March constitute

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    Fig. 1. Temperature and rainfall data for the study site.

    transitional months between seasons. The summer is

    dry and hot with temperatures ranging between 35

    and 45◦C during the day. Warm conditions (25–35◦C)

    and high relative humidity (70–91%) prevail dur-

    ing the rainy season. The rainy season starts with

    the onset of heavy monsoons in the end of June

    and continues till mid-October, over 85% of the an-

    nual rainfall being received within 4 months. The

    long-term average annual rainfall is about 1100 mm.

    During the study period (1997–1998), annual rainfallwas 1287mm with 60 rainy days and an extended

    dry period of about 9 months. During the winter sea-

    son, day temperature falls to between 10 and 25◦C

    with occasional light rain received from the retreating

    western monsoon. Mean monthly values of temper-

    ature and rainfall are plotted in Fig. 1. The rainy

    and winter seasons are the major cropping seasons

    in this region. Rice was selected as the rainy sea-

    son test crop and barley as the winter season test

    crop.

    The soil of the Banaras Hindu University campus

    which includes the agriculture farm is characterized

    as Banaras type III by Agarwal and Mehrotra (1952).It is an inceptisol with a flat topography, pale brown

    colour, and sandy loam texture.

    2.2. Experimental design

    The experiment was designed to vary the degree

    of soil disturbance using different tillage practices

    (such as conventional tillage, minimum tillage and

    zero tillage), and the amount of organic input through

    residue retention from the previous crop to the next

    crop. Rice was grown during the rainy season in 1997

    in a conventionally tilled field of ca. 3000 m2. When

    rice matured (November 1997), six treatments withthree replicate 9 m × 10 m plots were established in a

    randomized block design. A 1 m uncultivated strip was

    left to separate the treatment blocks. An assessment of 

    the initial soil fertility status, carried out in November

    1997, showed that the differences in organic C and

    total N contents between different replication blocks

    were not significant; the means(±S.E.) for the three

    blocks were: organic C 7.85 ± 0.08gkg−1, total N

    0.872 ± 0.008gkg−1.

    The treatments were: (1) conventional tillage

    (disked twice, cultivated once to 20 cm depth), and

    residues removed   (CT − R); (2) conventional tillage

    (as above), and residues retained  (CT + R); (3) min-imum tillage (disked once, cultivated once to 10 cm

    depth), and residues removed (MT−R); (4) minimum

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    Table 1

    Chemical composition and quantities of crop residues retained in different treatments; values are means ± S.E.

    Crop residue Carbon (gkg−1)a Nitrogen (g kg−1)b C/N ratio Quantity of residue (kg ha−1)

    Rice 491 ± 5.2 6.02 ± 0.25 81.56 6825 ± 358 (in all residue retained treatments)

    Barley 489 ± 5.3 6.22 ± 0.20 78.61 5831 ± 495 (in CT + R), 6687 ± 308

    (in MT + R), 3750 ± 508 (in ZT + R)

    a Determined as: loss in ignition/2 (McBrayer and Cromack, 1980).b Micro-Kjeldahl method (Jackson, 1958).

    tillage (as above), and residues retained (MT+R); (5)

    zero tillage (no cultivation other than the disturbance

    caused by the planting tines), and residues removed

    (ZT  − R); (6) zero tillage (as above), and residues

    retained (ZT +R). In residue removed plots, harvest-

    ing of rice was done at the ground level, leaving no

    standing aboveground rice residues. In the residue re-

    tained plots, harvesting of rice was done 20 cm above

    the ground, leaving a portion of the rice aboveground

    residue standing in the plot. This was incorporated

    into the soil during the tillage operations (either

    conventional or minimum tillage) carried out before

    sowing of barley (October 1997). In the case of zero

    tillage plots, the standing rice residue was chopped at

    the ground level and left on the soil surface at the time

    of first tillage operation in conventional and minimum

    tillage plots. When barley matured (April 1998), it

    was harvested in the same way as the preceding rice

    and the same six treatments with barley aboveground

    residue were again established for the succeeding

    rice crop. The chemical composition and the amounts

    of rice and barley residues retained are given in

    Table 1.NPK chemical fertilizers (80 kg N ha−1, 40 kg

    P ha−1 and 30kgKha−1 for rice and 60kgNha−1,

    40kgPha−1 and 30kgKha−1 for barley) were ap-

    plied in all treatments.

    2.3. Soil sampling

    Soil samples were collected once after the rice har-

    vest (November 1998) for physico-chemical and ag-

    gregate analyses. Three sub-samples from the upper

    10 cm depth were collected by using 10×10×10cm3

    monoliths from each replicate plot, and field moist

    samples were gently crumbled manually and sieved(4.75, 2.0–4.75, 0.5–2.0, 0.3–0.5,

    0.053–0.3, and   0.3 mm were bulked as macroaggre-

    gates and those 

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    2.6. Estimation of soil microbial biomass

    Microbial biomass C (MBC) was measured by de-

    termining the organic C in chloroform fumigated and

    non-fumigated samples by dichromate digestion as de-

    scribed by Vance et al. (1987). Soil MBC was then

    estimated from the equation: MBC = 2.64EC   (Vance

    et al., 1987), where   E C   is the difference between C

    extracted from the K2SO4   extract of fumigated and

    non-fumigated soils. With the same K2SO4   soil ex-

    tracts, microbial biomass N (MBN) was determined as

    total N using Kjeldahl digestion procedure (Brookes

    et al., 1985). The flush of total N (K2SO4   extractable

    N from non-fumigated soil subtracted from that of fu-

    migated soil) was divided by a K N (fraction of biomass

    N extracted after chloroform fumigation) value of 0.54

    (Brookes et al., 1985). All the results are expressed on

    oven dry soil (105◦C) basis.

    2.7. Data analysis

    Statistical analysis was carried out using the SPSS

    package. Significance of differences between treat-

    ment means was examined by an LSD range test pro-

    cedure at the 5% level of significance. Data were anal-

    ysed by correlation and regression analysis to evaluate

    relationships between the different soil parameters.

    Table 2

    Changes in soil properties following various treatments; the values are means ± S.E.; treatment codes: CT − R, conventional tillage and

    residues removed; CT+R, conventional tillage and residues retained; MT−R, minimum tillage and residues removed; MT+R, minimum

    tillage and residues retained; ZT − R, zero tillage and residues removed; ZT + R, zero tillage and residues retained

    Soil properties Treatments LSD

    CT − R CT + R MT − R MT + R ZT − R ZT + R

    pH 6.85 ± 0.018 6.76 ± 0.014 6.84 ± 0.016 6.75 ± 0.016 6.75 ± 0.007 6.72 ± 0.008 0.04

    Bulk density

    (gcm−3) 1.27 ± 0.006 1.24 ± 0.007 1.29 ± 0.007 1.25 ± 0.006 1.42 ± 0.004 1.40 ± 0.004 0.04

    WHC

    (gkg−1) 412 ± 2.6 423 ± 1.9 416 ± 3.2 431 ± 0.9 410 ± 0.6 414 ± 2.4 9.10

    Organic C

    (gkg−1 soil) 7.8 ± 0.21 9.8 ± 0.23 8.2 ± 0.15 11.1 ± 0.17 8.1 ± 0.15 8.6 ± 0.15 0.68

    (kgha−1)a 9906 ± 264 12152 ± 286 10578 ± 197 13875 ± 216 11502 ± 217 12040 ± 214 885

    Total N

    (gkg−1 soil) 0.87 ± 0.017 1.18 ± 0.013 0.92 ± 0.006 1.33 ± 0.017 0.89 ± 0.005 0.98 ± 0.019 0.05

    (kgha

    −1

    )

    a

    1105±

    22 1461±

    18 1180±

    7 1663±

    22 1262±

    7 1367±

    26 70C/N ratio 8.96 8.30 8.95 8.34 9.10 8.80

    a 0–10cm depth.

    3. Results

    3.1. Physico-chemical properties

    The soil was almost neutral in reaction (pH 6.8 in

    CT−R, which can be regarded as the control, Table 2).

    The maximum tillage reduction along with residue re-tention (ZT+R) decreased the pH slightly. The tillage

    practices affected the bulk density considerably (for

    instance, it increased from 1.27 g cm−3 in CT − R to

    1.40gcm−3 in ZT + R) and the effect of residue re-

    tention alone was small. The water holding capacity

    increased marginally (5%) in the MT + R treatment

    compared to the CT − R treatment.

    Within a short time, tillage reduction in association

    with residue retention (MT+R, ZT + R) significantly

    increased soil organic C and total N contents in the

    0–10 cm soil layer compared to the CT −R treatment

    (Table 2). Compared to CT  − R, the maximum in-

    crease occurred in the MT + R treatment (organic C42% and total N 53%). Residue retention alone also

    increased organic C (by 26%) and total N (by 36%)

    in the CT + R treatment. The minimum increases of 

    organic C (10%) and total N (12%) were recorded in

    the ZT + R treatment. In the residue removed treat-

    ments, the changes in the concentrations of organic

    C and total N were relatively small. The amounts of 

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    Table 3

    Per cent soil distribution in aggregate size classes and their MWD in different treatments, values are means ± S.E.; treatment codes as in

    Table 2

    Treatments Size classes (mm) MWD (mm)

    >4.75 2.0–4.75 0.5–2.0 0.3–0.5 0.053–0.3   4.75 mm size class in all treat-

    ments (Table 3). The maximum increase was recorded

    in the MT + R treatment, and less marked increase

    in the 2.0–4.75 mm size class. However, the propor-

    tion of soil in the lower size classes (

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    Fig. 2. Distribution of water-stable soil macroaggregates (>0.3 mm) and microaggregates (

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    Fig. 4. Amount of total N in macro- and microaggregates under different treatments in a tropical dryland agroecosystem; values are means,

    LSD is shown at   p

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    Table 6

    Correlation and regression parameters ( A, intercept;   B, regression coefficient;   r , correlation coefficient) reflecting relationships between

    different soil parameters and micro- and macroaggregates   (n = 18); MBC, microbial biomass carbon; MBN, microbial biomass nitrogen;

    SOC, soil organic carbon; STN, soil total nitrogen

    Parameters   A B   S.E. of   B r 2a

    y x

    Macroaggregatesb

    vs. MBCc

    24.93 0.080 0.012 0.73Macroaggregatesb vs. MBNc 32.76 0.530 0.110 0.59

    Macroaggregatesb vs. SOCd 18.35 0.004 0.001 0.52

    Macroaggregatesb vs. STNd 25.42 0.024 0.006 0.50

    Microaggregatesb vs. MBCc 75.25   −0.080 0.012 0.70

    Microaggregatesb vs. MBNc 67.34   −0.526 0.115 0.57

    Microaggregatesb vs. SOCd 82.32   −0.004 0.001 0.52

    Microaggregatesb vs. STNd 74.91   −0.024 0.006 0.49

    a All values for   r 2 are significant at   p

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    reduction) in combination may prove better than either

    practice used alone to elevate the soil nutrient levels.

    Varying reports are available with respect to the

    fraction of C in organic input conserved as soil or-

    ganic C. For instance, Vandermeer (1995) reported

    the incorporation of 20–40% of plant residue C into

    soil biological material and release of the rest as CO2,

    and Jenkinson (1988) approximated 70% respiratory

    loss during residue decomposition (the remaining con-

    served in soil). In contrast, low incorporation of only

    4–7% of crop residue C in soil organic C was reported

    in Danish soils by Powlson et al. (1987), and slightly

    higher retention of ca. 14% of plant residue was re-

    ported by Saffigna et al. (1989). In the present case,

    a large fraction of the C contained in crop residues

    in different treatments (ca. 36–41%, computed from

    Tables 1 and 2) was reflected by an increase in soil

    organic C 1 year after the setting up of the experi-

    ment. Such high incorporation of the residue C in soil

    may be related to the high decomposition rate of thecrop residues in dry tropical agroecosystems (Singh

    and Shekhar, 1989a,b). In a tropical grassland (under

    similar climatic conditions), where litter decomposi-

    tion was rapid, Gupta and Singh (1982a,b) showed that

    ca. 35% of the decomposed plant debris was annually

    conserved as new soil organic matter.

    In different treatments, expected results were ob-

    tained with soil pH, bulk density and water hold-

    ing capacity. In the present study, soil pH decreased

    slightly in zero tillage, similar to that observed by

    Blevins et al. (1977). Elimination of soil mechanical

    loosening caused by tillage operations is responsible

    for the increase in soil bulk density in zero tillagealone  (ZT − R)  and in combination with residue re-

    tention   (ZT + R). Similar to this study, Genter and

    Blake (1978) in Minnesota found higher bulk den-

    sity in zero tillage treatments than in ploughed treat-

    ments. Lal (1989) also reported that upland soils in the

    tropics may be more susceptible to compaction than

    equivalent soils in the temperate zone, due to the an-

    tecedent low level of organic matter, lack of ameliora-

    tive effect of freezing and thawing, and predominance

    of low-activity clays. Tillage reduction in association

    with residue retention significantly increased the wa-

    ter holding capacity in the present study, mainly due

    to large increase in soil organic C in residue retainedtreatments (MT+R, CT+R and ZT+R). The nature

    and quantity of soil organic C has been shown to con-

    trol water holding capacity (Piper, 1966). Similarly,

    Singh and Singh (1993) reported significant increase

    in water holding capacity in straw treated plots in the

    dryland agroecosystem of Indian tropics.

    Cultivation tends to increase the rate of organic

    matter loss in soils primarily by accelerated micro-

    bial decomposition (Seybold et al., 1999), and alters

    structural stability of soil and reduces the amount of 

    soil organic matter (van Veen and Paul, 1981). Loss

    of organic matter reduces the proportion of macroag-

    gregates in cultivated soils (Tisdall and Oades, 1980).

    Cultivation also reduces the proportion of water-stable

    macroaggregates and increases the proportion of mi-

    croaggregates (Monreal and Kodama, 1997). Further,

    it has been reported that the reduction in tillage will

    improve the stability of soil aggregates and the in-

    creased amount of organic matter is responsible for

    improved soil structure. Residue retention in associa-

    tion with tillage reduction (MT+R and ZT+R) in the

    present study considerably increased the proportion of macroaggregates in the soil. The enhanced level of or-

    ganic matter in residue retained treatments and reduc-

    tion in soil mechanical disturbance are responsible for

    increases in the proportion of water-stable macroag-

    gregates.

    In the agroecosystem studied tillage reduction and

    residue retention both increased aggregate stability

    (MWD), the effect of the latter being greater than

    the former. The greatest increase in MWD was in the

    MT+R treatment followed by the ZT+R and CT+R

    treatments. This may be due to increases in organic

    C and microbial biomass in these treatments, as both

    of these play significant roles in aggregate stability(Chaney and Swift, 1984). Studies performed under

    green house conditions have shown that the stability

    of soil aggregates can be modified after only a few

    months of plant growth (Dufey et al., 1986; Monroe

    and Klavidko, 1987). Angers and Mehuys (1988), in

    a field experiment, reported that within two growing

    seasons the aggregate content of a marine clay soil

    was increased by up to 50% under barley and alfalfa

    compared to a fallow control or corn. They concluded

    that the growth of barley and alfalfa for a short period

    significantly increased both the macroaggregates size

    and stability of clay soil compared to a fallow control

    and to initial conditions.Microbial management through residue manage-

    ment and conservation tillage has great potential for

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    the management of organic matter and nutrients in

    dryland agroecosystems (Singh and Singh, 1993).

    Conservation tillage is reported to lead to greater and

    better quality soil organic matter and higher micro-

    bial biomass (Elliott and Coleman, 1988) through

    increased number and diversity of soil organisms and

    their interactions among biota. Generally, the growth

    and functioning of microbial biomass is limited by C

    (Smith and Paul, 1990; Singh and Singh, 1993). When

    flushes of C are supplied to the soil in the form of 

    crop residues, the microbial biomass increases in size

    until the substrate is depleted. In the present study,

    residue retention and tillage reduction both increased

    the level of soil microbial biomass, the maximum ef-

    fect on microbial biomass being recorded in MT +R,

    either alone or in combination. It has been reported

    that use of minimum and zero tillage retained more

    crop residue C as soil organic C and soil MBC com-

    pared to conventional tillage (Salinas-Gracia et al.,

    1997). Singh and Singh (1993) stated that microbialgrowth due to the application of organic matter such

    as straw is mainly dependent on the availability of 

    C in the soil; they reported 77% increases for MBC

    and MBN under straw + fertilizer, and 51 and 84%

    increases under straw treatment for MBC and MBN,

    respectively. Saffigna et al. (1989) reported smaller

    increases in soil MBC and MBN (10–23%) following

    the incorporation of straw into Australian soils. Al-

    though the quantity of microbial biomass is mainly

    related to C inputs, other mitigating factors can reg-

    ulate the growth and activity of the native microflora

    (Smith and Paul, 1990).

    Soil aggregate disruption by tillage may be coun-tered by an increase in the stability of aggregates due

    to enhanced microbial activity by the addition of crop

    residues following cultivation (Molope et al., 1987;

    Beare et al., 1994). Incorporation of organic materials

    into soil promotes the aggregation of soil particles

    (Smith and Elliott, 1990). Cultivation results in loss

    of labile organic matter which binds microaggregates

    into macroaggregates and the inter-microaggregates

    organic matter is responsible for the long-term fertil-

    ity of native soils. In general, the stability of aggre-

    gates increases with increased soil microbial biomass

    (Lynch, 1984), although the relationship between

    microbial biomass and stabilizing effect depends onthe microbial species. Since the most labile fraction

    of soil organic matter is reflected in soil microbial

    biomass, the negative impact of cultivation on the

    microbial content was one of the reasons for reduced

    macroaggregation in cultivated soil (Gupta and Ger-

    mida, 1988). Because the microbial biomass plays an

    important role in the metabolism of transient binding

    organic matter, such as plant and microbial derived

    polysaccharides, the increase in microbial biomass

    will positively affect the soil aggregation in agro-

    ecosystems. Although the increases in organic C,

    MBC and MBN were higher in CT+R treatment than

    in ZT + R treatment, the percentage of macroaggre-

    gates was higher in the ZT +R, which may be due to

    the combined effect of soil organic matter enrichment

    and complete elimination of soil disturbances in the

    zero tillage. Several studies have shown that reduced

    tillage practices can result in greater aggregation and

    higher standing stocks of soil organic matter compared

    with conventional tillage practices (Doran, 1980;

    Lamb et al., 1985; Bruce et al., 1990; Havlin et al.,

    1990).In the present study, the concentration of organic C

    and total N were the highest in the minimum tillage

    residue retained treatment for both macro- and mi-

    croaggregates. In all treatments, concentration of or-

    ganic C and total N were higher in the macroag-

    gregates than the microaggregates. Since macro- and

    microaggregates of the residue retained treatment in

    the present study had more organic C and total N

    than macro- and microaggregates from residue re-

    moved treatment, they were more stable. This is be-

    cause of the role of soil organic matter in the stabiliza-

    tion of soil structure as exhibited by significant posi-

    tive correlation between organic C and aggregate sta-bility (Chaney and Swift, 1984). Greater susceptibility

    of macroaggregates compared to microaggregates has

    been reported to the disruptive forces of cultivation

    and to the dispersion that results from rapid wetting

    or raindrop impact (Tisdall and Oades, 1980, 1982;

    Beare et al., 1994). Some reports suggest that fungi are

    dominant in macroaggregates and bacteria dominate in

    microaggregate (Tisdall and Oades, 1982; Gupta and

    Germida, 1988). In the present study, the C/N ratio

    of macroaggregates was greater than that of microag-

    gregates. Since fungi have a considerably greater C/N

    ratio than bacteria, it is possible that macroaggregates

    may have greater C/N ratios than microaggregates dueto the dominance of fungi in the former and bacteria

    in the latter.

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    We found strong positive correlations of soil mi-

    crobial biomass, total N and organic C with macroag-

    gregates, and strong negative correlations between the

    above soil parameters with microaggregates (Table 6).

    This suggests a strong influence of organic matter

    addition through residue manipulation on MBC level

    in the soil. The microbial biomass and its metabolite

    products bind microaggregates which subsequently in-

    creases the proportion of macroaggregates in the soil.

    Better correlations of MBC with macroaggregates than

    microaggregates suggest a strong effect of MBC on

    the formation of macroaggregates in soil. Thus, it may

    be suggested that decrease in the mechanical disrup-

    tion of the soil aggregates by tillage reduction along

    with an increase in active and labile soil organic matter

    may prove more effective in improving aggregation

    than changes in total soil organic matter. Water-stable

    aggregates are responsible for maintaining the soil

    structure and organic matter (Table 6). Besides several

    beneficial effects, tillage practices generally decreasesoil aggregation. For sustained crop production, we

    recommend retention of 25–40% post-harvest above-

    ground crop residue from the previous crop and its in-

    corporation into soil through minimum tillage before

    sowing the next crop. In low organic matter containing

    soils in tropical dryland agroecosystems, management

    practices involving manipulation of crop residues

    and optimum tillage reduction can improve and sus-

    tain soil fertility by increasing soil organic matter,

    water-stable soil aggregates and microbial biomass.

    Acknowledgements

    We thank the Head and the Programme Co-ordinator

    CAS in Botany for providing laboratory facilities and

    Prof. S.R. Singh, Institute of Agricultural Sciences,

    Banaras Hindu University for providing experimental

    plots and other field facilities in the dryland farm.

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