Seasonal changes in soil microbial biomass and mineralizable c and n in wheat management systems

Soil Biol. B&hem. Pergamon

0038-0717(94)00105-7

Vol.

26, No. I I,

pp.

1469-1475,1994

Copyright $_>1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-0717194 $7.00 + 0.00

SEASONAL CHANGES IN SOIL MICROBIAL BIOMASS AND MINERALIZABLE C AND N IN WHEAT MANAGEMENT SYSTEMS A. J. Department

FRANZLUEBBERS*,

F. M. HONS

and D. A.

ZUBERER

of Soil and Crop Sciences, Texas Agricultural Experiment Station, College Station, TX 77843-2474, U.S.A.

Texas A&M University,

(Accepted 3 May 1994) Summary-Crop management strategies can affect the short-term dynamics of the active C and N pools of soil organic matter (SOM) by altering the timing, placement, quantity, and quality of crop root and residue input, as well as nutrient status and environmental conditions (i.e. soil temperature and water content). Our objectives were to quantify seasonal changes in soil microbial biomass (SMB) and mineralizable C and N in continuous wheat (Triticum uestivum L.), continuous wheat-soybean [GIycine max (L.) Merr.], and wheattsoybeansorghum [Sorghum hicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with or without N fertilization. Soil classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas was sampled shortly after planting, during flowering, and following harvest of wheat. Soil microbial biomass C (SMBC) increased by 18% and mineralizable C increased by 37% from planting to flowering when averaged across crop sequence, tillage, and N fertilization regimes. At harvest, SMBC and mineralizable C had returned to the amount at planting in all crop sequences, except in continuous wheat, in which decomposition proceeded without C input during the long fallow. Mineralizable C was 64, 28, and 15% greater at flowering compared to planting under NT and 45, 38, and 29% greater under CT at depths of 0 to 50, 50 to 125, and 125 to 200 mm, respectively. The greater increase in mineralizable C near the surface may be related to the abundance of crop roots, rhizosphere products, and more optimal air-filled porosity. With N fertilization, mineralizable N followed the same seasonal pattern as SMBC and mineralizable C. Without N fertilization, mineralizable N did not change during the growing season, despite increased SMBC and mineralizable C at flowering, indicating greater immobilization of N at flowering. Seasonal inputs of crop roots, rhizosphere products. and crop residues significantly altered SMB and mineralizable C and N of this soil, illustrating the dependence of N dynamics on short-term C inputs. Seasonal changes in the active C and N pools of SOM depended upon (i) crop sequence for the quantity, quality, and frequency of C input, (ii) tillage for the depth distribution of added substrates, and (iii) N fertilization

for the quantity and quality of substrates. conservation.

These seasonal changes can alter N availability and

INTRODUCTION

Soil N availability for crop growth varies widely depending upon soil type (Stanford and Smith, 1972) long-term crop management (Beauchamp et al., 1986), organic amendment (Bonde et al., 1988: Boyle and Paul, 1989), tillage (Carter and Rennie, 1982) and N fertilization (El-Haris et al., 1983). Soil microbial biomass plays an important role in soil N cycling as both the transformation agent and source-sink of N (Bonde ef al., 1988; Duxbury et al., 1991). Biological cycling of N in soil is strongly linked to the dynamics of C. It has been suggested that each soil has an inherent potential to mineralize N under standard conditions (Stanford and Smith, 1972). This hypothesis suggests that short-term changes in C input or environmental conditions (i.e. seasonal or within a cycle of a crop rotation sequence) have little influence on N mineral*Author for correspondence

ization when subjected to standard laboratory conditions. In contrast to this hypothesis, however, Bonde et al. (1988) found that mineralizable N in a long-term barley (Hordeum distichum L.) experiment in Sweden was 40-70% greater in April than in August, depending upon crop management. A decrease in mineralizable N between sampling dates suggested that sources of mineralizable N in the soil decreased. Important seasonally-dependent sources of mineralizable N may include soil microbial biomass (SMB), crop roots and residues, as well as biologically, chemically, or physically altered soil organic matter (SOM). Seasonal changes in soil moisture, soil temperature, and C input from crop roots, rhizosphere products (i.e. root exudates, mucilage, sloughed cells, etc.), and crop residues can have a large effect on SMB and its activity (Ross, 1987) which in turn, affect the ability of soil to supply nutrients to plants through SOM turnover (Bonde and Rosswall, 1987). Limited data, however, exist on the magnitude of seasonal changes 1469

1470

A.

J.

FRANZLUEBBERS Ed al

in SMB and mineralizable C and N in different agricultural ecosystems to ascertain the practical significance of these fluctuations on crop growth and nutrient conservation. We determined the amounts of SMB and mineralizable C and N in several long-term wheat sequences that were managed under different tillage and N fertilization regimes at planting when crop root and residue additions were minimal, at flowering when crop root additions were maximal, and at harvest following crop residue addition. MATERIALS AND METHODS

Crop management and site characteristics A long-term field experiment was begun in 1982 in the Brazos River floodplain in southcentral Texas (30”32’ N, 94’26’ W). Long-term differences in soil organic, microbial biomass, and mineralizable C and N due to differences in crop sequence, tillage, and N fertilization in this study occurred (unpublished data), but are not the focus of our paper. Wheat was managed under conventional (disk) tillage (CT) and no tillage (NT) in continuous wheat (CW), continuous wheat-soybean (CWS), and rotated wheat-soybean-sorghum (RWS) crop sequences. Continuous wheat produced one crop each year, CWS produced two crops each year, and RWS produced three crops every 2 yr. Cropping intensity was defined as the fraction of the year when a crop was growing in each crop sequence. This was 0.5 for continuous wheat, 0.67 for RWS, and 0.88 for CWS. Nitrogen fertilizer (NH,NO,) was broadcast on wheat at 0 and 6.8 g N.rn-* during late winter or early spring. Soybean did not receive N fertilizer, while sorghum received 0 and 9 g N.rn-* banded preplant. A split-split plot within a randomized complete block design was established with crop sequence as the main plot, tillage as the split plot, and N fertilizer rate as the split-split plot. Split-split plots measured 4 x 12.2 m. Treatments were replicated four times. The soil was classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) and contained an average of 12% sand, 45% silt, 31% clay, 9% CaCO,, 0.8% organic C, and a pH of 8.2 (1: 2, soil : water). Annual temperature averages 20°C and rainfall averages 978 mm. Soil sampling Sampling periods (SP) included shortly after wheat planting in November 1991 (SPl), during wheat flowering in March 1992 (SP2), and following wheat harvest in late Mayearly June 1992 (SP3). Soil samples consisted of 10 composited cores (19 mm dia) per split-split plot that were divided into depth increments of O-50 mm, 50-125 mm, and 125-200mm. Soil was sieved (5 mm, with visible pieces of crop residues and roots removed) and stored in a moist condition at 4°C. Soils at depths of 50-125 mm and 1255200mm collected at planting

and harvest were near saturation at the time of sampling and were dried for 14 h at 23°C to obtain a more friable sample before sieving. Chemical and physical analyses Pre-incubation samples were dried (6O”C, 48 h) 2 days before incubation. Dried soil was screened (2 mm) and analyzed for initial NH,-N and NO,N concentration following extraction by shaking for 30 min in 2 M KC1 (I : 4, w/v) using an automated salicylic acid modification of the indophenol blue method (Technicon Industrial Systems, 1977a) and cadmium reduction method (Technicon Industrial Systems, 1977b), respectively. Soil bulk density was calculated from the soil dry weight (60°C 48 h) and volume of the coring device. This was used to convert chemical and biological units of measurement from a mass to volume basis. Biological analyses Soil microbial biomass C and N were estimated using the chloroform fumigation-incubation method (Jenkinson and Powlson, 1976) with the following modifications. Moist soil (ca 30 g dry weight equivalent) was placed in 50 ml beakers, fumigated, brought to a water potential of approximately -0.03 MPa (30% gravimetric moisture content) with deionized water and incubated in l-1 air-tight canning jars in the presence of 10 ml of 0.5 M KOH at 25°C for 10 days. The quantity of CO,+ absorbed in the alkali was determined by titration (Anderson, 1982). Soil microbial biomass C (SMBC) was determined from the following equation: SMBC = (mg CO,-C.kg-’

soil. 10 dayss’)rumlgated/kC

where, kc = 0.41 (Voroney and Paul, 1984). Following the 10-d incubation, soil was dried at 60°C for 24 h and sieved (2 mm). A 7-g portion was extracted in 28 ml of 2 M KC1 for 30 min. The soil extract was analyzed for NH,-N as described above. Soil microbial biomass N (SMBN) was determined from the following equation: SMBN = [(mg NH.,-N.kg-’

soil. 10 days-‘)rumlgated

- (mg NH,-N . kg- ’ soil),,i,i,,]/kN where, k, = 0.41 (Carter and Rennie, 1982). No significant increase in NO,-N was observed in fumigated samples. Mineralizable C and N were estimated from the quantity of CO,-C and NH,-N + NO,-N, respectively, mineralized from an unfumigated sample during a 10-d incubation at 25°C and a soil water potential of -0.03 MPa (Campbell et al., 1991). Specific respiratory activity of SMBC and SMBN was estimated by dividing the net potential activity (i.e. mineralizable C and N, respectively) by the size of the microbial pool (i.e. SMBC and SMBN, respectively) (Campbell et al., 1991).

Seasonal

Statistical

microbial

biomass

and mineralizable

C and N

anal.yses

Treatment differences in chemical and biological indices were analyzed with the GLM procedure of SAS (SAS Institute Inc., 1985). Sources of variation included crop sequence, tillage, N fertilization, soil depth, sampling period, and their interactions. Additional orthogonal contrasts were used to test for significant differences in response indices among sources of variation with more than two levels, including crop sequence, soil depth, sampling period, and their interactions. Crop management variables were considered repeated measures when analyzed with soil depth and sampling period (Steel and Torrie, 1980). Separation of means by LSD at P I 0. I, P 5 0.01, and P < 0.001 was used only when an orthogonal contrast was significant. Correlations between SMBC, SMBN, and the C to N ratio of SMB were determined on mean values of the four replications.

.

Soil microbial biomass C and N Soil microbial biomass C increased significantly from planting to flowering in all crop sequence and tillage regimes at all depths, except at the 0 to 50 mm depth in RWS under CT (Fig. 1). At harvest, SMBC had returned to the amount at planting in CWS and RWS, except at the 125 to 200 mm depth in RWS under both tillage regimes. This seasonal pattern of SMBC is probably a result of increased C input from rhizosphere products (Xu and Juma, 1993) to the soil before and during flowering, especially during fluctuating spring temperatures (Martin and Kemp, 1980). A similar seasonal pattern of SMBC, with increased amounts near wheat flowering and lower concentrations at planting and harvest was reported in Oregon (Collins et al., 1992). In CW, however, SMBC was greater at harvest compared to planting under both tillage regimes at all depths. In CW, the fallow period before planting of wheat was 6 months, while the fallow period was 3 months in RWS, and 1 month in CWS. A longer period for decomposition without C input in CW probably contributed to the lower amount of SMBC at planting compared to harvest than in more intensive crop sequences. Similarly, lower SMBC concentrations were observed in wheat-fallow sequences compared to continuous wheat (Campbell et al., 1991; Collins et al., 1992), in which maintenance of a larger SMB was dependent upon C input to balance decomposition. Soil microbial biomass N ranged from 14 to 122 mg.kg-‘, depending upon crop management system, soil depth, and sampling period and was highly related to SMBC [Fig. 2(a)]. Despite this strong overall relationship, SMBN did not increase significantly from planting to flowering as was observed for SMBC. This resulted in greater C-to-N ratios of SMB during flowering and at harvest com-

Soil depth

LSD(P
O-50

114 61

mm

0 50-125 mm

r=

0

/--& &..’

--.

. .... ..’

CT Rotated Nheatkoybea,”

600 RESULTS AND DISCUSSION

1471

CT Continuous \ wheat/soybean

c $%G

I

NT Rotated wheatkoybea ” 1 Sampling

2

3

m NT Continuous 1wheat/soybean

I

2

3

period

Fig. 1. Seasonal changes in soil microbial biomass C averaged across N fertilization regimes as affected by crop sequence, tillage, and soil depth. Sampling periods are (1) planting (early November), (2) flowering (early March), and (3) harvest (late May). CT= conventional tillage and NT = no tillage.

pared to planting. Average C-to-N ratios of SMB were 11.1 at planting, 12.8 during flowering, and 13.4 at harvest. Greater C-to-N ratios of SMB at flowering and harvest coincided with greater addition of organic C substrates (i.e. rhizosphere products, crop roots, and crop residues) and may have been a result of significant reimmobilization of N during the incubation following fumigation (Voroney and Paul, 1984). The relationship between the C-to-N ratio of SMB and SMBN was non-linear [Fig. 2(b)]. Soil microbial biomass N was lowest at the 125 to 200mm depth, which was below the cultivation zone. High C-to-N ratios of SMB at low concentrations of SMBN may be indicative of (i) a bio-physical response to decreased N availability in undisturbed soil horizons or more likely (ii) reimmobilization of N during the IO-day incubation following fumigation. It has been suggested that fungi are capable of remobilizing N from less active to more active tissues to maintain growth (Cowling and Merrill, 1966) such that the C-to-N ratio of less active tissue increases. It is possible that some microbial populations at lower depths remained viable but were less active because of reduced substrate availability or poor aeration. Average monthly water-filled pore space at the

1472

FRANZLIJEBBERS~~ al

A.J.

the C-to-N ratio of newly formed SMB due to a more active physiological status and different species composition (Ridge, 1976). The k, used for calculation of SMBN, therefore, was adjusted in order to obtain the same C-to-N ratio of SMB at all concentrations of SMBN according to the equation:

0

SMBC = 108 + 9.13 (SMBN) 0.938

r2 =

k, = k, x 10.22/(C, : NE)

C/N = 10.2 + 44.9 x e-0-“86 (SMBN) RMSE = 1.7

0

30

60

Soil microbial

biomass

90

120

N (mg.kg-’

150

soil)

Fig. 2. Relationship of soil microbial biomass N to (a) soil microbial biomass C and (b) the C/N ratio of soil microbial biomass for all sampling periods. soil depths, and crop sequence, tillage. and N fertilization regimes.

125-200 mm depth exceeded 70% during the entire year under both tillage regimes in CW (unpublished data). Higher C-to-N ratios of SMB at the 75-150 mm depth compared to the O-75 mm depth have been reported for several wheat crop sequences in Saskatchewan (Campbell et al., 1991). In addition, reimmobilization of N during the IO-day incubation following fumigation could have increased the “apparent” C-to-N ratio of SMB if the in situ C-to-N ratio of SMB were greater than

Table

where, k, is the efficiency factor used for calculation of SMBC (i.e. 0.41), 10.22 is the basal plateau derived from the non-linear equation [Fig. 2(b)], and C,:N, is the C-to-N ratio of the fumigation-incubation flush. Voroney and Paul (1984) have previously suggested using a floating k, value that depended upon the C-to-N ratio of the fumigation-incubation flush. The quantity of N immobilized in the SMB from planting to flowering was 2.2, 1.8 and 1.4 g N.rn~‘.0.2rn~’ in CW, RWS, and CWS, respectively, when averaged across tillage and N fertilization regimes (Table 1). The decrease in SMBN from flowering to harvest released mineral N that was potentially available to the maturing wheat crop. The decrease in SMB from harvest to the next planting season, especially in CW with a long summer fallow, could have left mineralized N vulnerable to leaching or gaseous losses. However, the quantity of N released from SMBN from harvest to planting was always less than or equal to the NO,-N accumulated from harvest to planting (Table l), suggesting that little of the mineralized N in CW during the summer fallow was lost. Losses of mineral N under typical climatic conditions for this soil would be expected to be greatest during winter and spring when plant growth is minimal and rainfall is abundant.

I, Estimated

mineralization (+) or immobilization (-) of N from soil microbial biomass between sampling periods and difference in NO,-N content of soil between planting and harvest Mineralization-immobilization of N from soil microbial biomass?

Tillage

CKm sequencef

N fertilizer

SP2-SP3

SPI-SP2$ g.m

CT CT CT CT CT CT NT NT NT NT NT NT LSD.,,,,,

cw RWS cws cw RWS cws cw RWS cws CW RWS cws

0 0 0 6.8 6.8 6.8 0 0 0 6.8 6.8 6.8

-

2.0 -2.0 - I.5 -1.9 -1.3 ~ 1.6 -2.2 -2.1 -0.9 -2.5 -1.9 - 1.4 1.4

‘.0.2m- ’ -0.6 1.0 0.7 0.3 0.6 0.9 0.8 I.2 0.6 0.9 0.4 0.5 1.9

NO,-N

SP3-SPI 2.7

I .a 0.8 1.5 0.7 0.7 I .4 0.9 0.3 1.6 I.5 0.9 I.7

SPI -SP3 4.5 16 2.2 4.6 2.6 2.5 2.3 I.5 I.0 3.3 17 I.8 I.1

tMineralization and immobilization of N calculated from difference in soil microbial bmmass C between sampling periods assuming a C-to-N ratm of soil microbial biomass of 10.2. sequences are continuous wheat (CW), continuous wheat-soybean (CWS), and rotated SCrop wheat-soybean-sorghum (RWS). BSampling periods are planting (SPI), flowering (SP2), and harvest (SP3). SPI-SP2 is the change in SMBN from planting to flowering, where positive values indicate mineralization and negative values indicate immobiliration.

Seasonal microbial biomass and mineralizable C and N

Soil

.

30

depth

O-50 mm 50-125 mm

4.5 1.7

.

125-200

1.4

mm

20

T

-i P z w?

filled pore space increased with depth (unpublished data). Seasonal changes in mineralizable C reflected the availability of easily-decomposable substrates, which is supported by greater specific respiratory activity of SMBC during flowering compared to planting and harvest (Table 2). Mineralizable C was highest during flowering, possibly due to deposition of rhizosphere products (Xu and Juma, 1993; Swinnen et al., 1994). At crop maturity (i.e. 3 to 4 weeks before harvest), these substrates probably diminished, although mature roots and fresh crop residue inputs provided new substrates at harvest (Bottner et al., 1988). The elevated amount of mineralizable C in CW at harvest compared to planting (similar to that observed for SMBC) diminished during the 6 months of fallow without C input. Mineralizable C at a depth of O-200 mm averaged across tillage and N fertilization regimes was 47% greater in CW, 38% greater in RWS, and 27% greater in CWS at flowering compared to planting. The decrease in mineralizable C from flowering to harvest, however, was similar for all crop management systems, averaging 21 f 3%. The inverse relationship between cropping intensity and the magnitude of increase in mineralizable C from planting and flowering was probably a result of the lower mineralizable C concentration at planting in crop sequences having a longer fallow. In England, maximum seasonal changes of SMBC and mineralizable C in continuous wheat were ca 20 to 30% (Patra et al., 1990). The seasonal pattern of mineralizable N was similar to that of mineralizable C only at the &50 mm depth with N fertilization, but deviated from the pattern of mineralizable C at the two lower depths and without N fertilization (Fig. 4). An increase in mineralizable N during flowering only at the &50 mm depth with N fertilization may have been an indirect result of greater mineralizable C due to increased deposition of rhizosphere products in the surface layer (Recous er al., 1988). Greater mineralizable C during flowering and harvest with unchanged mineralizable N in unfertilized soil and at lower depths indicates that more N was immobilized, perhaps due to decreased substrate quality (i.e. higher C-to-N ratio). Roots and rhizosphere products can be low in N concentration and high in mineralizable C,

LSDfP
0

-

a--$- -

10

;

‘.

3 :

Continuous wheat

Rotated vheatlsoybean

Continuous wheat/soybean

0

U 2 c, : ”

30

e x ._ E 20

10

Continuous wheat 0 1

2

1

3

2

3

1

2

3

Sampling period Fig. 3. Seasonal changes in mineralizable C averaged across N fertilization regimes as affected by crop sequence, tillage, and soil depth. Sampling periods are (1) planting (early November), (2) flowering (early March), and (3) harvest (late May). CT = conventional tillage and NT = no tillage.

Mineralizable

C and N

Seasonal changes in mineralizable C followed the same pattern as SMBC, but were more pronounced (Fig. 3). Mineralizable C during flowering compared to planting averaged 64, 28, and 15% greater under NT and 45, 38, and 29% greater under CT at depths of O-50, 50-125, and 125-200 mm, respectively. The greater relative increase in mineralizable C near the soil surface during the wet spring months between planting and flowering may have been related to greater root growth in soil with highest nutrient concentration and most optimum air-filled porosity. During this same period, soil bulk density and water-

Table

2. Specific during

respiratory

the wheat

activity

growing

of soil microbial

season averaged

Soil microbial

1473

biomass

across crop

Specific

respiratory

biomass

C

N fertilizer

C and N affected sequence activity

regimes

of

Soil microbial rate (g.m-

by N fertilization

and tillage

biomass

N

I)

Sampling period

0 mg CO,-C.g-

Planting

21.4

Flowering

24.0

Harvest

21.4

LSD rp
6.8

’

I.1

0

6.8

SMBC.d-’

mg inorganic

N.g

-’

SMBN.d

21.2

22.4

21.6

25. I

16.4

21.7

21.3

17.3

20.7 4.5

’

A. J. FRANZLUEBBERS et al.

1474 4 r

O-50

mm

T

SO-125 mm

T 125-200 mm1

Bonde T. A., Schniirer J. and Rosswall T. (1988) Microbial biomass as a fraction of ootentiallv mineralizable nitroeen in soils from long-term ‘field expe>iments. Soil Biology & G. (1988) Root activity and Biology and Fertility of Soils

BoGle M. and Paul E. A. (1989) Carbon and nitrogen mineralization kinetics in soil previously amended with sewage sludge. Soil Science Society of America Journal 53, YT--L”,.

n

J-qP
2

3

I

2

3

1

2

3

Sampling period Fig. 4. Seasonal changes in mineralizable N averaged across crop sequence as affected by tillage, N fertilization and soil depth. Sampling periods are (1) planting (early November), (2) flowering (early March), and (3) harvest (late May).

Lafond G. P. (1991) Effect of crop rotations and cultural practices on soil organic matter, microbial biomass and respiration in a thin Black Chernozem. Canadian Journal of Soil Science 71, 363-376. Carter M. R.and Rennie D. A. (1982) Changes in soil quality under zero tillage farming systems: distribution of microbial biomass and mineralizable C and N potentials. Canadian Journal of Soil Science 62, 587-597.

leading to significant N immobilization (Mary et al., 1993). Lower specific respiratory activity of SMBN during flowering and harvest compared to planting in

soil without N fertilization (Table 2) supports the hypothesis of decreased substrate quality, which led to a greater amount of mineralized N that was recycled through the SMB. Mineralizable N decreased an average of 19% from flowering to harvest with N fertilization and from planting to harvest without N fertilization when averaged across tillage and crop sequence regimes. Bonde and Rosswall (1987) observed maximum seasonal changes in potentially mineralizable N of

40-70% in various cropping systems in Sweden. A decrease in mineralizable N from one sampling to another may imply (i) a net release of mineral N that can be available for crop growth or environmental loss or (ii) a decrease in substrate quality, such that more N is immobilized. Seasonal changes in SMB and mineralizable C and N of this soil illustrated the dependence of seasonal N dynamics on short-term substrate availability from cron roots. rhizosnhere nroducts, and cron residues. Seasonal changes’ in the active C and* N pools of SOM depended upon (i) crop sequence for the quantity, quality, and frequency of added substrates, (ii) ._ tillage - for the depth distribution of substrates, and (iii) N fertilization-for the quantity and quality of substrates. REFERENCES Anderson J. P. E. (1982) Soil respiration. In Methods of Soil Analvsis. Part 2. 2nd Edn (A. L. Paee et al.. Eds). ,

pp. 837871. Agronomy Monograph 9, American Societv

of Aeronomv and Soil Science Societv, of America. Madison. ’ Beauchamp E. G., Reynolds W. D., Brasche-Villeneuve D. and Kirbv K. (1986) Nitrogen mineralization kinetics with different soil pretreatments and cropping histories. Soil Science Society of America Journal 50, 14781483. Bonde T. A. and Rosswall T. (1987) Seasonal variation of potentially mineralizable nitrogen in four cropping systerns. Soil Science Society of America Journal 51, 1508-1514.

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- __^ _._

8,2OY-213. Martin J. K. and Kemp J. R. (1980) Carbon loss from roots of wheat cultivars. Soil Biology & Biochemistry 12, 551-554.

Mary B., Fresneau C., Morel J. L. and Mariotti A. (1993) C and N cycling during decomposition of root mucilage, roots and glucose in soil. Soil Biology & Biochemistry 25, 1005-1014.

Patra D. D., Brookes P. C., Coleman K. and Jenkinson D. S. (1990) Seasonal changes of soil microbial biomass in an arable and a grassland soil which have been under

uniform management for many years. Soit Biology & Biochemistry 22, 739-742.

Recous S., Fresneau C., Faurie G. and Mary B. (1988) The fate of labelled 15N urea and ammonium nitrate applied to a winter wheat crop. I. Nitrogen transformations’& the

soil. P/ant and Soil i12, 205-214. Ridge E. H. (1976)Studies on soil fumigation. Il. Effects on bacteria. Soil Biology & Biochemistry 8, 2499253. Ross D. J. (1987) Soil microbial biomass estimated by the fumigation-incubation procedure: seasonal fluctuations and influence of soil moisture content. Soil Biology & Biochemistry 19, 397404.

SAS Institute Inc. (1985) SAS User’s Guide: Statistics. Version 5 Ed. Cary. Stanford G. and Smith S. J. (1972) Nitrogen mineralization potentials of soils. Soil Science Society af America Proceedings 36, 465472.

Steel R. G. D. and Torrie J. H. (1980) Principles and Procedures af Statistics: A Biometrical Approach..2nd Edn McGraw-Hill. Swinnen J., Van Veen J. A. and Merckx R. (1994) Rhizosphere carbon fluxes in field-grown spring wheat: model calculations based on 14C partitioning after pulselabelling. Soil Biology & Biochemislry 26, 17lLl82.

Seasonal microbial biomass and mineralizable C and N Technicon Industrial Systems (1977a) Determination of Nitrogen in BS Digests. Technicon Industrial Method 334-74 W/B. Technicon Industrial Systems, Tarrytown, NY. Technicon Industrial Systems (1977b) Nitrate and Nitrite in Soil Extracts. Technicon Industrial Method 487-774. Technicon Industrial Systems, Tarrytown, NY. Voroney R. P. and Paul E. A. (1984) Determination of k,

1475

and k, in situ for calibration of the chloroform fumigation-incubation method. Soil Biology & Biochemistry 16, 9-14. Xu J. G. and Juma N. G. (1993) Above- and belowground transformation of photosynthetically fixed carbon by two barley (Hordeum uulgare L.) cultivars in a typic cryoboroll. Soil Biology & Biochemistry 25, 1263-1272.