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Soil Science Society of America Journal 66:845-847 (2002)
© 2002 Soil Science Society of America

DIVISION S-3—NOTES

Maize root-induced change in soil organic carbon pools

B. C. Liang*,a, X. L. Wangb and B. L. Mab

a Greenhouse Gas Division, Pollution Data Branch, Environment Canada, 19th Floor, 351 St-Joseph Blvd., Hull, QC, K1A 0H3 Canada
b Eastern Cereal and Oilseed Research Centre (ECORC), Agriculture and Agri-Food Canada, Central Experimental Farm, Ottawa, ON, K1A 0C6 Canada

* Corresponding author (Chang.Liang{at}EC.GC.CA)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 REFERENCES
 
Quantification of root or root-induced changes in soil organic C (SOC), water-soluble organic C (WSOC), and microbial biomass C (MBC) is important for understanding processes of soil C storage. A greenhouse study was conducted on a Bainsville loamy sand (Typic Hapludoll) to evaluate root or root-induced quantitative and compositional changes in various SOC pools by growing corn (Zea mays L.), a C4 plant, on a historically C3 soil. Significant shifts in 13C of SOC pools, most noticeably in WSOC and MBC were observed. During the course of a growing season, the proportion of C4-derived C varied from 0 to 12.3% of whole SOC, from 0 to 30.7% of WSOC, and from 0 to 52% of MBC, indicating a major contribution of root or root-induced C to various soil C components, especially WSOC and MBC. The amount of C4-derived C in the entire soil estimated by the 13C natural abundance ({delta}13C) was remarkably consistent with the amount of C4-C retained in the soil microbial biomass, WSOC, and corn roots, suggesting that measurements of 13C of the entire soil following the shift of C3 to C4 plants can be used as an indirect measure of root or root-induced C pools during the growing period.

Abbreviations: MBC, microbial biomass C • SOC, soil organic C • WSOC, water-soluble organic C


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 REFERENCES
 
Q UANTIFICATION OF ROOT or root-induced changes in SOC, WSOC, and MBC is important for understanding processes of soil C storage through soil microbial biomass. Plant allocation of photosynthate to roots is a very important source of available C for soil microorganisms. However, quantification of root-derived C available for soil microbial biomass and total root-derived C has challenged soil scientists for decades. Microbially respired C or C incorporated in microbial biomass is known to be an indicator of soil available C (Haller and Stolp, 1985; Wang et al., 1989). Gregorich et al. (2000) reported that a dramatic shift in {delta}13C of soil microbial biomass occurred during the early stages of a shift from soil grown with C3 plants to soil grown with C4 plants, even though as much as 74 to 96% of SOC originated from C3 plants. These results are also consistent with the findings of Ryan and Aravena (1994), who found that 5 yr of continuous corn resulted in 55% of soil microbial biomass being derived from C4-C. Bruulsema and Duxbury (1996) reported that 23% of soil microbial biomass was derived from corn over the course of a single growing season.

Qian and Doran (1996) estimated the contribution of C4 root-derived C to soil microbial biomass by measuring the {delta}13C change in microbial biomass during crop growing season, and reported 88, 283, and 402 kg C ha-1 of root-derived available C from corn at 4, 8, and 16 wk, respectively. They concluded that the {delta}13C in microbial biomass could provide a practical and reliable way to assess available C released from plant roots during growth.

The objective of this study was to evaluate root-induced changes in the quantity and isotopic composition of SOC, WSOC, and soil MBC during corn growth in a greenhouse study using 13C natural abundance approach.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 REFERENCES
 
A pot experiment with complete randomized block design was carried out in a greenhouse to study allocation of plant C in soil C pools. The pot had a dimension of 20-cm i.d. by 35-cm height, and contained 10.4 kg of Bainsville loamy sand (Typic Hapludoll). The soil was collected from the 0- to 15-cm layer of native pasture land predominantly with C3 grass. The soil was passed through a 5-mm sieve before use, and contained 0.7 g kg-1 of soil total N, and 9 g kg-1 of SOC, with a {delta}13C of -27.2{per thousand}.

Three corn seeds were planted in each pot, but only one plant was kept shortly after emergence. During corn growth, 800 ml of nutrient solution with a concentration of 0.25% N, containing 20-9-17 N-P-K (20-20-20 of N-P2O5-K2O) (Plant Prod, Ritchie Feed & Seed), was applied to each pot twice (i.e., 1 mo after emergence and at tasselling stage). Water was applied to near field capacity once a week during early corn growth and twice a week after stem elongation. The light period was set at 16 h daily from 06.00 to 22.00. Day time temperature was 25°C and night time temperature was 15°C. During the growing period corn was harvested five times, at 15, 28, 67 (tasselling), 84(silking), and 110 (maturity) d after emergence. At each sampling time, six pots were harvested. The aboveground portion of the plant was cut from the soil surface, dried at ~70°C for 4 d, and weighed. Corn roots were carefully washed on a 0.85-mm sieve, collected, dried, and weighed for three pots. Fresh soil samples were taken for measurements of MBC on the other three pots. Then, the soil and roots were air-dried. Larger roots were removed by hand, dried in an oven, and ground to pass through a 2-mm sieve. The air-dried soil was also ground to pass through a 2-mm sieve. The ground roots were thoroughly mixed with the soil. A small representative sample was taken and ground to pass through a 0.25-mm sieve for measurements of SOC and {delta}13C. It should be noted that there is no distinction between corn-root C and SOC in this study. Analysis of SOC and {delta}13C involved pretreating the soil with phosphoric acid in a silver capsule to remove inorganic C, then drying the sample for 16 h at 75°C prior to analysis of C. The stable C isotope ratios were measured using the method of Knight et al. (1994), by combusting ~10 mg of soil samples on a RoboPrep Tracer Mass Spectrometer (Europa Scientific, Crewe, UK) equipped with a single inlet and triple collectors, described by Swerhone et al. (1991).

Soil MBC was determined by the fumigation-extraction method (Voroney et al., 1993) with modifications (Gregorich et al., 2000). Approximately 50 g of fresh soil was used for extraction and measurements of MBC, and its subsequent {delta}13C. Ultra-pure water (Compact Milli-Q and Milli-RQ Water Systems, Millipore Corp., Bedford, Massachusetts) was used to extract soluble organic C with and without prior fumigation by shaking for 1 h on a mechanical shaker. Soluble organic C in the extracts was obtained by centrifuging at 1000 x g (10000 rpm) for 10 min. and filtering through a 0.45-µm glass fibre filter using a suction funnel under a tension of -7 kPa. Soluble OC was determined on a Total Organic Carbon Analyzer (Shimadzu TOC-5050, Tokyo, Japan). Microbial biomass C was calculated as the difference in soluble organic C concentrations between fumigated and nonfumigated samples divided by 0.35 (Voroney et al., 1993).

After determination of soluble organic C in the extract, samples were acidified to pH 3.7 with 0.05 M H2SO4 to remove inorganic C in the solution and then freeze-dried. The stable C isotope ratios were measured by combusting ~1 mg of freeze-dried material on an ANCA-MS (Europa Scientific, Crewe, UK). The natural abundance of heavy isotopes was expressed as parts per thousand relative to the international standard PDB using delta units ({delta}). The {delta}13C value was calculated from the measured C isotope ratios of the sample and standard gases as:

where R is the mass 45/44 ratio of the sample or standard gas.

The {delta}13C ({per thousand}) of MBC was estimated as the {delta}13C of the C extracted from the fumigated sample in excess of that extracted from the nonfumigated sample, as follows:

[2]
where Cf and Cnf were the amounts of C extracted from the fumigated and nonfumigated samples (mg C kg-1), and {delta}13Cf and {delta}13Cnf were the {delta}13C of the fumigated and nonfumigated extracts ({per thousand}), respectively. The soluble organic C for the nonfumigated sample in this study was considered to be WSOC.

The proportion of C4-derived C in SOC, WSOC and MBC was calculated as follows:

[3]
where {delta} equals {delta}13C value of SOC, WSOC, or MBC extracted from any particular sampling date, {delta}cr is the mean {delta}13C value of corn roots at maturity (-11.9{per thousand}), {delta}r represents the {delta}13C value of SOC (-27.2{per thousand}), {delta}r equals the {delta}13C value of WSOC (-24.2{per thousand}), or {delta}r is the {delta}13C value of MBC extracted from the initial soil (-24.4{per thousand}). At maturity, corn shoot and root contained 43.0 and 43.7% of C, respectively.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
REFERENCES
 
The aboveground dry-matter accumulation (derived from plant shoot C; Table 1) generally followed a sigmoid pattern, i.e., a slower rate of accumulation in early and late growing stages and a faster rate of accumulation in the middle (Table 1). At maturity, the total aboveground dry-matter was on average 234 g pot-1 with a harvest index of 50%. At 28 d after emergence, the total amount of plant dry-matter was approximately equally allocated between above and belowground. After that, the aboveground dry-matter accumulation increased exponentially while the rate of dry matter accumulation in the roots was relatively low. The highest amount of root dry-matter was observed at the tasselling and silking stages, and decreased slightly at maturity. At maturity, root recovery was ~10% of the aboveground plant dry matter (Table 1).


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  		Table 1. Plant shoot C, measured root C, changes in amounts and isotopic composition of soil organic C (SOC), water soluble organic C (WSOC), and microbial biomass C (MBC) during corn growth in a greenhouse study.

 
The amount of MBC was generally higher during the tasselling and silking stages of growth, coinciding with the highest root biomass accumulation and a greater amount of substrate available for MBC as indicated by a higher amount of WSOC (Table 1). During corn growth, the {delta}13C values of the extracted fumigated soluble organic C varied from -24.4 to -19.0{per thousand}, while the {delta}13C values of the extracted nonfumigated soluble organic C varied from -24.2 to -20.4{per thousand} (Table 1). Consequently, the calculated {delta}13C values of MBC varied from -24.4 to -17.8{per thousand}. The proportion of C4-derived C in the soil microbial biomass increased dramatically, and reached to ~52% at silking stage, and then decreased slightly to ~48% at maturity (Table 1). The enrichment of {delta}13C in the soil microbial biomass and progressive increases in the proportion of C4-derived C in the microbial biomass following corn growth indicated a greater contribution of root-derived C to the soil microbial biomass than to other soil C pools.

The {delta}13C values of SOC during corn growth varied from -27.2 to -25.4{per thousand}, a net increase of 1.8{per thousand} (Table 1). This change in the composition of SOC was because of the input of corn roots and root exudates. The proportion of C4-derived SOC during corn growth varied from 1.3 to 12.3%, which accounted for 1.3 to 14.5 g C pot-1 (Table 1). The proportion of C4-derived C in soil microbial biomass varied from 4.8 to 9.2% of the measured root C, and from 3.4 to 8.5% of the total C4-derived C in the soil. Similarly, the proportion of C4-derived C in WSOC varied from 0.6 to 1.5% of the measured root C, and from 0.5 to 1.2% of the total C4-derived C in the soil. Based on the amount of MBC and the proportion of C4-derived C in the soil microbial biomass, we calculated the amount of C4-derived C in the soil microbial biomass to vary from 0.1 to 0.7 g C pot-1 with large values after tasselling (Table 1).

The amount of available C4-derived C utilized by microorganisms can be estimated with an appropriate substrate utilization efficiency, ranging from 20 to 60%, with an average of 40% under greenhouse conditions (Gilmour and Gilmour, 1985; Leite et al., 1989; Molina et al., 1983; Paul and Juma, 1981). Assuming a uniform substrate utilization efficiency of 40% (Qian and Doran, 1996), we estimated that bioavailable C4-derived C either retained in or respired by MBC was 0.25, 0.50, 1.75, 1.75, and 1.26 g C pot-1, at 18, 28, 67, 84, and 110 d after emergence, respectively. These results were generally consistent with the findings of Qian and Doran (1996). Since the average turnover time of MBC in the soil is not known, the total amount of bioavailable C4-derived C cycling through the microbial biomass during the entire growing period cannot be accurately estimated for this study. Nevertheless, the total amount of bioavailable C4-derived C cycling through the microbial biomass would be at least 500 kg C ha-1, accounting for ~13% of the total root C.

Even though the proportions of C4-derived C in the SOC, WSOC, and MBC generally followed the same patterns during corn growth, the proportions of C4-derived C in the microbial biomass and WSOC were considerably higher than that in the SOC (Table 1). This uneven distribution of C4-derived C in the SOC, WSOC, and MBC indicates a greater bioavailability of C4-derived C than that of C3-derived native SOC. The ratios of the proportion of C4-derived C in the microbial biomass to the proportion of C4-derived C in WSOC were ~2.6, 3.0, 1.6, 1.7, and 2.1 at 18, 28, 67, 84, and 110 d after emergence, respectively. If the flow of C through water soluble components is assumed to supply substrate for soil microbial biomass turnover (McGill et al., 1986), greater ratios of the proportion of C4-derived C in MBC to that of C4-derived C in WSOC implied that WSOC derived from C3 or C4 source is not equally available for soil microbial biomass, and a portion of the WSOC is more readily mineralizable and that a large share of this portion is derived from recently deposited crop residues.

The amount of C4-derived C calculated from the SOC and from the sum of C4-derived C in the WSOC, MBC, and roots were 1.3, 2.8, 13.4, 12.9, and 14.5, and 1.2, 2.9, 13.8, 13.9, and 13.1 g C pot-1, respectively at 18, 28, 67, 84, and 110 d after emergence (Table 1). The amount of C4-derived C estimated by {delta}13C in the entire soil was remarkably consistent with the amount of C4-C retained in the microbial biomass, WSOC, and corn roots. This suggests that the measurement of {delta}13C of the entire soil following the shift of C3 to C4 plants can be used as an indirect measure of root or root-induced C during the growing period. This information is important for understanding the role of belowground biomass in the storage and dynamics of SOC.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 REFERENCES
 
ECORC Contribution No. 11717.

Received for publication June 11, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 REFERENCES
 




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