Mineralization and microbial assimilation of
14
C-labeled straw in
soils of organic and conventional agricultural systems
Andreas Flieûbach*, Paul MaÈder, Urs Niggli
Research Institute of Organic Agriculture (FiBL), Ackerstrasse, Postfasch, CH-5070 Frick, Switzerland
Abstract
An incubation experiment on straw decomposition was carried out with soils from a long-term ®eld trial at Therwil, Switzerland. Two conventional agricultural systems, one with (CONFYM) and one without manure, an organic system managed according to bio-dynamic farming practice (BIODYN) and an unfertilized control were compared. CONFYM received stacked manure and an additional mineral fertilizer. BIODYN received composted farmyard manure and no mineral fertilizers. Both systems received the same amount of manure based on 1.4 livestock units haÿ1
. The aim of the investigation was to explain the large dierences in soil microbial biomass and activity between the systems, especially between the manured soils. Dierences in microbial C-utilization eciency were suggested to be the main reason. We followed the decomposition of 14C-labeled plant material over a period of 177 days under controled incubation conditions. Prior to incubation, microbial biomass was 75% higher andqCO2up to 43% lower in the BIODYN soil than in the conventional soils. At the end of the incubation period, 58%
of the applied plant material was mineralized to CO2in the BIODYN soil compared to 50% in the other soils. This dierence
became signi®cant 2 weeks after application of plant material and is suggested to be due to decomposition of more recalcitrant compounds. After addition of plant material, the increase of microbial biomass in the unmanured systems was higher than in the manured systems, but with a higher loss rate thereafter. The amount of 14C incorporated into C
mic as related to 14CO2
evolved was markedly higher in the BIODYN soil. The results support the hypothesis that agricultural measures applied to the BIODYN system invoke a higher eciency of the soil microbial community with respect to substrate use for growth.7 2000 Elsevier Science Ltd. All rights reserved.
Keywords:Organic farming; Long-term ®eld trial; Mineralization; Assimilation; Microbial biomass; Energy use eciency
1. Introduction
Almost 0.5% of European agricultural land (80,000 farms) is under organic management, promoted mainly by direct or indirect governmental subsidies and the increasing demand for organically grown food. Between 1990 and 1997, the area managed organically in Europe increased from 250,000 ha to almost 2,000,000 ha (Lampkin, 1997). In addition to crop ro-tation, organic fertilization and rejection of chemically
produced fertilizers and pesticides, organic farming systems aim to keep the nutrient ¯ow in a closed cycle on the farm (IFOAM, 1996). The European Commu-nity has released a directive on organic farming ((EWG) No. 2092/91) to assure comparable pro-duction standards and several organic farming systems are certi®ed and/or controled by their label organiz-ations. Bio-dynamic farming diers from other organic farming systems mainly in the speci®c amendments made to crops, soils and manure (Table 1) based on anthroposophic philosophy (Koepf et al., 1976).
Soil organic matter responds to changes in land use according to the quantity and quality of organic ma-terial entering the soil (Jenkinson and Ladd, 1981; SoÈchtig and Sauerbeck, 1982). Manure and plant
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* Corresponding author. Tel.: 7225; fax: +41-62-865-7273.
due transformation processes are most important for organic farmers, who do not use mineral fertilizers and so rely entirely on biological nutrient cycling processes (KoÈpke, 1997). These are sensitive indicators of soil quality (Elliott et al., 1996). Organic farming systems have the potential to minimize some of the negative impacts of conventional agriculture (e.g. NO3-losses to
the groundwater, soil erosion, eects of pesticides on non-target organisms, loss of crop genetic diversity) (Tilman, 1998), and often lead to improvement of soils in terms of biological and chemical properties and physical stability (Drinkwater et al., 1998; MaÈder et al., 1996; Reganold et al., 1987). Soils from bio-dynamic farms are similar to conventionally farmed soils with regard to soil structure but have a higher production potential (Droogers and Bouma, 1996). On bio-dynamic farms in New Zealand, the total C and N clearly increased during pasture and decreased during cropping phases, which was not the case on conven-tional farms (Murata and Goh, 1997).
Soil microbial biomass and activity are signi®cantly increased by crop rotation compared to monoculture as well as by addition of organic manures (Anderson and Domsch, 1989). The biologically active pool rep-resents only a small fraction of the total soil organic matter, but it is often increased in organically managed soils, as revealed by microbial biomass measurements (Flieûbach and MaÈder, 1997; Franzluebbers et al., 1996; Oberson et al., 1993; von LuÈtzow and Ottow, 1994; Zelles et al., 1992), and the physical and chemi-cal properties of soil organic matter (Flieûbach and MaÈder, 2000; Wander and Traina, 1996). Microbial biomass and microbial activities involved in phos-phorus dynamics are also enhanced in biologically managed systems (Oberson et al., 1996), emphasizing the key position of soil microbial activity in transform-ation of nutrients for the crop.
An active soil micro¯ora which rapidly decomposes plant litter and manure is important in the mineraliz-ation of plant nutrients. Lack of synchronicity with plant demand, with mineralization coinciding with periods of low plant requirement, however, may be a serious problem in soils with high biological activity. On the other hand, a considerable part of the decom-posing material will be used to form new microbial biomass, and labile as well as stabilized organic com-pounds in the soil (Jenkinson et al., 1987; Parton et al., 1987). Catabolic and anabolic soil processes may ®nally reach a steady state under continuous manage-ment, with the mean annual C input equaling the amount of C respired.
The microbial respiration to microbial biomass ratio (qCO2), which is related to soil development, decreases
with succession and increases with environmental stress and indicates the energy needed for maintenance of microbial biomass (Anderson and Domsch, 1993; Insam and Haselwandter, 1989; Wardle and Ghani, 1995). A high qCO2 in agricultural soils indicates that
nutrients are recycled quickly (Insam et al., 1991). Thus, qCO2 and processes related to the build-up and
turnover of microbial biomass may be useful indicators of changes due to dierences in soil management and farming practice.
In the present study, we compared soils from or-ganic and conventional agricultural systems with respect to C mineralization and immobilization. Soil biological properties have altered considerably in the 19 years of the experiment. Within an identical crop rotation, microbial biomass, qCO2, microbial
func-tional diversity, and microbial P dynamics, all indi-cated the greater importance of microbial processes in organically managed soils (Flieûbach and MaÈder, 1997; Oberson et al., 1996). We propose that this is due to C utilization eciency, as indicated by qCO2
Table 1
Main dierences between the treatments of the DOC-farming systems experiment (2nd and 3rd crop rotation period)
Bio-dynamic (BIODYN) Fertilized with composted farmyard-manure (FYM) (é 881 kg C haÿ1
yÿ1
; C/N = 7.9), slurry and amended with mineral and herbaceous preparationsaaccording to bio-dynamic farming. Mechanical weed control
Conventional (CONFYM) Fertilized with stacked, anaerobically rotted FYM (é 990 kg C haÿ1yÿ1; C/N = 11.4) and additional
mineral fertilization according to ocial norm, integrated plant protectionb
Mineral NPK (CONMIN) Unfertilized from 1978 until 1985, but then fertilized with mineral fertilizers according to ocial norm (é 90 kg N haÿ1to winter wheat), integrated plant protectionb
Unfertilized (NOFERT) Unfertilized since 1978, but amended with mineral and herbaceous preparationsaaccording to
bio-dynamic farming. Mechanical weed control
aThe bio-dynamic preparations (P) consist of the following: P 500: cow-manure fermented in a cow horn; P 501: silica fermented in a cow
horn, which were amended at rates of 250 and 4 g haÿ1, respectively. Composting additives are yarrow ¯owers (P 502,Achillea millefolium, L.),
camomile ¯owers (P 503,Matricaria recutita, L.), stinging nettle (P 504,Urticaria dioica, L.), oak bark (P 505,Quercus robur, L.), dandelion ¯owers (P 506,Taraxacum ocinale, Wiggers) and valerian ¯owers (P 507,Valeriana ocinalis, L.). A decoct of shave-grass (Equisetum arvense, L.) is applied once during vegetative growth to wheat and potatoes as a protective agent against plant diseases at rates of 1.5 kg haÿ1
. For further details see Reganold and Palmer (1995); or Koepf et al. (1976).
b
Herbicides (1±2 treatments yÿ1
) and fungicides (2±3 treatments yÿ1
and dierences in light fraction organic matter (Flieûbach and MaÈder, 2000). This hypothesis was tested by examining microbial mineralization and im-mobilization of14C-labeled plant material.
2. Materials and methods
2.1. The DOC farming systems experiment
We investigated soils from a long-term experiment at Therwil (Switzerland), begun in 1978 as a collabora-tive project of the Swiss Federal Research Station for Agroecology and Agriculture (FAL, Reckenholz) and the Research Institute of Organic Agriculture (FiBL, Frick), the main aim of which was to compare pro-ductivity and plant nutrient dynamics in organic and conventional systems. The soil is an haplic luvisol on deep deposits of alluvial loess. The site has a documen-ted history as an arable soil with a ley-rotation since 1957. After 14 years of plot management physical, chemical and biological parameters of the soils were investigated as indicators of soil fertility (Besson and Niggli, 1991; MaÈder et al., 1997).
The average particle size distribution was: clay 16%, silt 70%, sand 14% (MaÈder et al., 1993). Long-term annual mean temperature was 9.08C and annual rain-fall averaged 872 l mÿ2 (310 m above sea-level). The third crop rotation (1992±1998) included potatoes, winter wheat 1, beet roots, winter wheat 2, and three consecutive years of grass-clover in all systems. The treatments diered in fertilization practice and plant protection strategy. Mechanical weed control implied minor dierences in soil cultivation. The ®eld exper-iment was designed as a Latin square with plot sizes of 520 m, separated by grass strips of 5 m width to
avoid interactions. The main dierences between the farming systems are given in Table 1.
Soil samples from the unfertilized (NOFERT) and the minerally fertilized (CONMIN) treatment as well as the high intensity conventional (CONFYM) and bio-dynamic (BIODYN) treatments were collected in March, 1997 under winter wheat (Triticum aestivum, L.) from each of the four replicate ®eld plots (15 3 cm
cores/plot, 0±20 cm depth). In the laboratory, soils were stored at 48C for 6 weeks, before sieving (<2 mm) and adjusting water to 50% water holding ca-pacity (around 24% H2O of dry matter). Data on soil
pH (0.1 M KCl, 1/10; w/v), organic C and N (CHN analyser, LECO) are given in Table 2
2.2. Preincubation
Replicate 100 g (dry matter) subsamples from each plot were incubated at 208C in glass beakers, which were placed in 2.5 l plastic jars together with a CO2
-trap (10 ml 2N NaOH) and water at the bottom of the jar to minimize water loss from the sample. The alkali traps were exchanged after 5 and 11 days and aliquots were analysed by gas chromatography to determine the amount of absorbed CO2.
2.3. C-mineralization
After preincubation, subsamples were each mixed with 180 mg of ground (<500 mm) uniformly 14 C-labeled straw from shoots of the needle grass Stipa capensis (34% C; 4.1% N) with a speci®c activity of 1991 MBq gÿ1 C. The plants were grown from the
seedling stage in a closed cabinet supplied with 14CO2
at a constant speci®c activity (Flieûbach et al., 1995). Each sample was returned to its glass beaker and recompacted to 100 ml to ensure a close contact between the 14C straw and the soil particles. Sucient replicates were set up to allow destructive sampling. C-mineralization was then followed by trapping14CO2in
alkali as described above. One ml of the NaOH± Na214CO3 solution was mixed with 14 ml of a
scintil-lation cocktail (IRGA Safe Plus, Packard) and measured in a liquid scintillation counter (Tricarb, Packard). Residual 14C in the dried samples at day 30 resulted in a 98.8%21.7% recovery of the applied
14
C.
2.4. Soil microbial biomass
Soil microbial biomass estimation was performed after preincubation (day 0) and on days 10, 30, 70 and 177 following 14C-addition. Soil microbial biomass car-bon (Cmic) was estimated by
chloroform-fumigation-extraction (CFE) according to Vance et al. (1987). CFE was carried out on 10 g (dry matter) subsamples extracted with 40 ml 0.5 M K2SO4. Total organic
car-bon (TOC) in soil extracts was determined by infrared spectrometry after combustion at 8508C (DimaTOC, Dimatec, Essen). 14C was measured by liquid scintil-lation counting in 1 ml aliquots as described above. Cmic and
14
Cmic were calculated using a kEC factor of
0.45 (Joergensen and Mueller, 1996).
Table 2
2.5. Statistics
All data were analysed by ANOVA and, where sig-ni®cant eects were observed, Tukey's HSD test was applied for comparison of treatment means (JMP, SAS Institute, Cary, NC). Spatial eects were considered by including row and column structures of the ®eld trial design.
3. Results
3.1. Soil microbial biomass and respiration
Soil respiration rates during preincubation declined after the initial 5-day period. A 33% decrease was observed after a subsequent 5-day incubation. The decrease was slightly higher in the manured (CON-FYM, BIODYN) than in the unmanured (CONMIN, NOFERT) soils. Soil microbial biomass carbon (Cmic)
in the bio-dynamic treatment was roughly 70% higher than in the conventional, mineral and unfertilized sys-tems. Biomass speci®c respiration (qCO2) was
calcu-lated using respiration rates of the second preincubation period. qCO2 was higher in soils of the
mineral fertilizer system than in the bio-dynamic sys-tem (Table 3).
3.2. Straw mineralization
Cumulative evolution of 14CO2followed a sigmoidal
curve shape (Fig. 1(a)). By day 10, 14CO2 evolution
was signi®cantly lower in the conventional soil than in all the other soils. After the rapid initial decomposition phase (Fig. 1(a)), 29% (CONFYM) to 31% (BIO-DYN) of the applied 14C-straw was found as 14CO2.
After day 10, 14CO2-evolution declined in the
unma-nured and conventional soils (NOFERT, CONMIN, CONFYM) resulting in signi®cantly lower C mineral-ization rates as compared to the bio-dynamic soils, where mineralization remained at a higher level. After 177 days of incubation, 58% of the 14C in the added straw had been mineralized in the bio-dynamic treat-ment, whereas in all the other treatments 14 C-mineral-ization was about 50%. Total CO2-evolution rates
increased signi®cantly after straw application but declined to the initial values after 25 days. At the end of the experiment, there were signi®cant dierences (p = 0.05) in the amount of total CO2 evolved
between the manured (BIODYN, CONFYM) and the unmanured treatments (NOFERT, CONMIN) (Fig. 2).
3.3. Microbial biomass during straw-decomposition
Ten days after the initial14C-straw amendment, Cmic
increased by 20% in the manured (CONFYM, BIO-DYN), and 28% in the unmanured systems (NOFERT, CONMIN). Throughout the experiment the relative order of microbial biomass remained the same among the treatments: BIODYN > CONFYM > NOFERT > CONMIN. The decline back to the in-itial values was faster in CONMIN and CONFYM soils than in BIODYN and NOFERT soils (Fig.3(a)). Cmicloss between the 10th and the 177th day was 26%
in the bio-dynamic soil and 42% in the other soils. Ten days after 14C-straw application, microbial bio-mass in CONMIN, CONFYM and NOFERT soils had each incorporated about 73 mg 14C gÿ1soil,
whereas incorporation in BIODYN soil was signi®-cantly higher at 98mg 14C gÿ1(Fig. 3(b)). With
decreas-ing Cmicafter day 10,14Cmicalso decreased.
During straw mineralization, qCO2 in NOFERT,
CONMIN and CONFYM soils increased by almost the same order of magnitude until the 10th day, whereas in the bio-dynamic soil the qCO2increase was
signi®cantly less (Fig. 4). After 30 days of incubation,
qCO2in all soils had returned to the initial values. The
dierences in qCO2between the BIODYN soil and the
two conventional ones (CONFYM, CONMIN) were signi®cant throughout the experiment.
4. Discussion
4.1. Straw mineralization
After 3 days of incubation, 11±12% of the input14C from the applied plant material had been mineralized. Assuming that soluble straw 14C is mineralized in the early stages of decomposition, only the CONFYM soil
Table 3
CO2-evolution during the preincubation period and CmicandqCO2at starting time of the incubation experimenta
CO21±6 days (mg CO2±C gÿ1hÿ1) CO27±11 days (mg CO2±C gÿ1hÿ1) Cmic(mg gÿ1soil) qCO2(mg CO2±C mgÿ1Cmichÿ1)
NOFERT 0.224 b 0.159 b 210.1 a 0.767 ab
BIODYN 0.356 a 0.223 a 372.1 b 0.620 b
CONFYM 0.327 ab 0.209 ab 237.5 a 0.878 ab
CONMIN 0.273 ab 0.193 ab 187.6 a 1.088 a
a
showed a slightly retarded decomposition after 7 days compared to the other soils. The rates and periods of decomposition in the dierent soils were compared according to Ladd et al. (1995). After 10 days, the rate of 14CO2 evolution in mineral and unfertilized soils
approached that of conventional soil, whilst in bio-dynamic soil the rate remained higher. The large dier-ence between the bio-dynamic and the other treatments was almost entirely due to the decomposition period between day 16 and 86, with no dierences being found in later decomposition periods (Fig. 1(b)).
Initial mineralization rates were not related to the size of the microbial biomass, presumably because the population growing on the easily available straw com-pounds consisted mainly of rapidly-growing r-strate-gists. This is also indicated by a signi®cantly lower speci®c activity of 14C within the microbial biomass of the bio-dynamic soils. At day 10, only 22% of Cmic in
the BIODYN soil was straw-derived, compared with 29% in CONMIN and NOFERT soils. This dierence may re¯ect a dilution eect, or be due to a smaller fraction of the markedly higher Cmic in the BIODYN
soil being metabolically active as compared to CON-MIN and NOFERT soils. However, after 30 days of incubation accumulated 14CO2correlated well with
in-itial Cmicvalues.
At the end of the incubation 14CO2 accounted for
58% of the 14C supplied to the BIODYN soil, 52% in CONFYM and 50% in the two unmanured soils (CONMIN, NOFERT). Associated with a low total CO2 evolution 14CO2 represented 46% of total CO2
evolved in the unfertilized soil, which was signi®cantly higher than in the other three soils (p = 0.05). This soil has not been fertilized for 19 years and probably used a greater proportion of the applied straw for res-piration because of C limitation.
Fig. 1. (a) Cumulated percentage of14CO
2evolved from the applied14C-labeled straw over an incubation period of 177 days at 208C. NOFERT:
4.2. Microbial turnover
The eciency of straw 14C utilization by soil mi-crobes, as expressed by the ratio of14Cmic to the sum
of 14Cmic and 14CO2, was signi®cantly higher in the
bio-dynamic than in the other soils at all sampling times. After 10 days of incubation, substrate use e-ciency (ranging from 0.27 to 0.34) was markedly lower than values reported by Chotte et al. (1997) after 3 days of incubation. Obviously, turnover had occurred after 10 days in our experiment. The decrease in utiliz-ation eciency with incubutiliz-ation time resulted in values ranging between 0.09 and 0.13 at the end of the exper-iment. The faster decrease in the two conventional soils (CONFYM, CONMIN) indicated an increased turnover of14C through the biomass, compared to the unfertilized and the bio-dynamic soil. This conclusion is also supported by the higher qCO2 in the
conven-tional soils. These results are in accordance with those of von LuÈtzow and Ottow (1994) for Cmic values
measured on two farms, where turnover times of mi-crobial biomass were two times faster in conventional than in bio-dynamic systems.
Microbial biomass loss between day 10 and day 177 was the same for all treatments and varied from 103± 123 mg Cmic and 37±41 mg 14Cmic gÿ1 soil. At day 10,
the relative loss of Cmic was signi®cantly lower in the
BIODYN soil than in the two conventional soils.
Assuming microbial turnover depends on the amount of existing biomass, following ®rst order kinetics, the relative dierences in microbial biomass loss indicate a faster turnover in conventional soils.
4.3. Eciency of substrate utilization
The yield in 14Cmic per unit 14CO2 released after 10
days of incubation was signi®cantly higher in soils managed according to bio-dynamic farming than in the other soils (Table 4). A higher value may be due to greater protection of the new microbial biomass de-rived from the applied labeled substrate as argued by Ladd et al. (1995). These authors found higher values in soils with higher clay content. In our treatments, there was no dierence in clay content, but Corg was
higher in bio-dynamic soils. The ratio of Cmic-to-Corg,
Fig. 3. Changes in total soil microbial biomass (Cmic) (a) and14C
labelled biomass (14C
mic) (b) after amendment with straw over a
period of 177 days at 208C. For treatments see Fig. 1 (average of four ®eld replicates; standard error bars shown).
Fig. 2. Total CO2and straw-derived14CO2evolved during 177 days
which is frequently applied as an indicator of organic matter quality (Insam et al., 1989), was closely corre-lated to the yield in14Cmic per unit14CO2(r2= 0.73),
supporting the view that Corg as a structural soil
com-ponent protects microbial biomass.
The microbial population in the bio-dynamic soil has a higher mineralization potential together with a higher potential for assimilation. Similar eects have been observed in soils from a long-term ®eld trial in Bavaria, Germany (Haider and GroÈblingho, 1990) although in this case extremely dierent agricultural treatments (bare fallow, potato monoculture, organic farming) were compared and the dierences in 14 C-straw mineralization were rather small compared to those reported here. Nevertheless, microbial assimila-tion of straw 14C decreased markedly with increasing
management intensity in their soils, as con®rmed by our results. C-utilization was found to be less ecient in intensively managed soils, and the authors conclude that intensive management leads to a less active humus concomitant with a decrease in aromatic and phenolic compounds in soils and humic substances.
Microbial biomass C of the conventional soil was only 64% that of the bio-dynamic soils. This dierence is remarkable, since the total C input via organic man-ure was similar (Table 3) (Flieûbach and MaÈder, 2000). Moreover, during the ®rst two crop rotation periods, crop yields in the BIODYN system were about 18% lower than in CONFYM (MaÈder et al., 1999). These results suggest that the input of organic matter originating from above-ground plant residues and roots is also lower. Organic fertilization in the conventional treatment had little eect on microbial biomass, thus, the system eect re¯ected by changes in microbial biomass indicates dierences in the quality of organic matter input and plant protection strategy. C and nutrient cycling processes also appear to be aected.
P®ner (1993) reported a distinctly lower earthworm biomass and abundance in the two conventional treat-ments of the same ®eld trial, which the author attribu-ted mainly to the toxic eects of agrochemicals used in the conventional plots. However, modern pesticides applied in customary amounts have rarely been found to show signi®cant direct long-term eects on soil mi-crobes (Domsch, 1992). Pohl and Malkomes (1990) found side-eects on soil organisms to be caused merely by the reduction in weediness due to either her-bicide application or mechanical eradication. There-fore, uncoupling of metabolic processes or interactions between soil biota are likely to play a major role in such side-eects.
As reported by Flieûbach and MaÈder (1997), mi-crobial functional diversity in soils from the ®eld trial has been higher in the bio-dynamic than in the conven-tional soils. An inverse relationship was found between
qCO2 as an indicator of microbial maintenance
requirements and functional diversity based on meta-bolic capabilities. The present study supports the hy-pothesis that qCO2 indicates the eciency of substrate
use in microbial populations.
Acknowledgements
We are grateful to Prof. A. Wiemken (Botanical Institute, University of Basle) for use of the isotope laboratory and for his valuable co-operation. The long-lasting collaboration with the Swiss Federal Research Centre for Agroecology and Agriculture in maintaining the DOC-®eld experiment is gratefully acknowledged. We thank Dr. R. Martens (Institute for
Fig. 4. Biomass-speci®c respiration (qCO2) directly before (day 0)
and during the time course after addition of straw. For treatments see Fig. 1 (average of four ®eld replicates; standard error bars shown).
Table 4
14CO
2 evolved during 10 days of incubation after straw amendment
and the respective coecient of 14C
mic/14CO2 and the Cmic-to-Corg
ratio as an overall indicator of soil organic matter qualitya
14CO 2±C
(mg14CO 2±C gÿ1)
14C
mic Cmic-to-Corgratio
(mg gÿ1) 14CO
2±C
NOFERT 185 ab 0.418 a 16.8 bc
BIODYN 196 c 0.513 b 20.7 a
CONFYM 179 a 0.422 a 17.4 ab
CONMIN 187 b 0.373 a 13.4 c
a
Soil Biology, FAL Braunschweig) for14C-labeled plant material and Kurt Ineichen and Martin Koller for their help in the laboratory and skillful technical assist-ance. This work is part of a research project funded by the Swiss National Science Foundation.
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