seasonal variations of soil organic matter in a long-term agricultural experiment

Plant and Soil 160: 225-235, 1994. © 1994 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 3245

Seasonal variations of soil organic matter in a long-term agricultural experiment

PETER LEINWEBER, HANS-ROLF SCHULTEN and MARTIN KORSCHENS tlnstitute for Structural Analysis and Planning in Areas of Intensive Agriculture (ISPA), University of Osnabriick, P.O. Box 1553, Driverstrafle 22, 49364 Vechta, Germany, ZFachhochschule Fresenius, Department of Trace Analysis, Dambachtal 20, 65193 Wiesbaden, Germany and 3UFZ- Umweltforschungszentrum Halle-Leipzig GmbH, Department of Soil Research, Hallesche Str. 44, 06246 Bad Lauchstiidt, Germany

Received 15 June 1993. Accepted in revised form 10 December 1993

Key words: fertilization, field experiment, organic carbon, particle-size fractions, pyrolysis-mass spectrometry, seasonal variations, soil organic matter

Abstract

Seasonal variations of soil organic matter (SOM) were studied in the unfertilized plot (U) and in the NPK+farmyard manure plot (NPK+FYM) of the 88-year-old 'Static Experiment' at Bad Lauchst~idt (Germany). Decreases in the C concentrations by 0.24% (U) and 0.43% (NPK+FYM) between June and August were observed which were significant at the p < 0.01 level. The largest differences in N concentrations were 0.035% (U: August vs. September) and 0.029% (NPK+FYM: April vs. May). The C/N ratios were lowest in July and August (~12).

The seasonal variations of SOM contents were reflected in significant differences in the C concentrations of organo-mineral particle-size fractions. The proportions of soil C, associated with clay increased from 56% and 38% in April to 69% and 48% in October in the untreated and NPK+FYM- treated plot, respectively.

Pyrolysis-field ionization mass spectra of whole soil samples taken in June and August showed larger differences in the molecular composition of SOM in the untreated plot than in the NPK+FYM plot. On the basis of thermograms for six important compound classes of SOM, seasonal variations in (a) their amounts and (b) their incorporation in thermally different stable humic and/or organo-mineral bonds were visualized. Within four weeks of a net mineralization of SOM, portions of phenols, lignin monomers, lignin dimers, alkylaromatics, lipids, N-containing compounds and carbohydrates reached a higher thermal stability, which can be explained by advanced crosslinking. These results represent the first application of this novel methodology to the subtle and difficult problem of seasonal SOM variations.

Introduction

Seasonal variations of organic carbon and total nitrogen concentrations in soils have been observed by Sauerlandt and Tietjen (1971), Flaig (1974), van Veen and Paul (1981), K6rschens (1982) and Parton et al. (1987) and others.

Often, the C concentrations reached a first maximum in spring, followed by a decrease during summer and a second, lower maximum in autumn. However, detailed information on qualitative changes of soil organic matter (SOM) during the seasons is still lacking (Andreux et al. 1990).

226 Leinweber et al.

Physical separations of SOM, such as particle- size and density fractionations, are useful in studies of dynamic processes in soils rather than the classical humic substance fractionations (Oades, 1989). Recently, this topic was treated very throughly by Christensen (1992). So far the role of organo-mineral size-fractions in seasonal SOM variations has not been investigated.

Pyrolysis-field ionization mass spectrometry (Py-FIMS) has been successfully applied to studies of humic substances, whole soils (Schul- ten and Leinweber, 1993a) and organo-mineral particle-size fractions (Leinweber and Schulten, 1992; Schulten and Leinweber, 1991, 1993b). The analytical potential of Py-FIMS for studies of long- medium- and short-term SOM dynamics recently has been demonstrated (Leinweber and Schulten, 1993a).

The objectives of the present study were to investigate - seasonal variations in the soil organic matter contents in two different fertilization treatments of an 88-year-old agricultural experiment, and - qualitative changes of SOM during the seasons by particle-size fractionations and Py-FIMS of whole soil samples.

M a t e r i a l s a n d m e t h o d s

Field experiment

The samples were obtained from two treatments of the 'Static Experiment' at Bad Lauchstfidt (Sachsen-Anhalt, Germany). This experiment was established in 1902. A comprehensive de- scription of the different treatments and princi- pal results has been given by K6rschens and Eich (1990). The soil, developed on loess loam, contains 21% clay, 67% silt and 12% sand. It is classified as Haplic Chernozem (USDA: Hap- loboroll). The mineralogy of the clay and fine-silt fractions was as follows: fine+medium clay (<0.63/zm): 37% illite, 20% illite/smectite and 20% smectite; coarse clay (0.63-2/.~m): 43% illite, 8% illite/smectite and 11% smectite; fine silt (2-6.3/xm): 32% mica and 4% illite/smectite and smectite (differences from 100% were due to increased amounts of quartz and feldspars with

increases in particle-size) (Reuter and Lein- weber, 1989).

The fertilization treatments chosen for the present study were 'Unfertilized' (U: without any fertilization since 1902, plot 18) and 'NPK+ farmyard manure' (NPK+FYM, plot 1). The crop rotation includes sugar beets, summer barley, potatoes and winter-wheat. Detailed information on the mineral fertilizer applications can be obtained from K6rschens and Eich (1990). About 30t ha -1 of FYM were spread to sugar beets and potatoes so that on average 15 t FYM ha-~ a-~ were applied in this crop rotation. The stubble and the roots of barley and wheat are generally incorporated into soil after harvest. The organic matter input from each of these two grain crops was about 1.04tha -1 stubble, 0 .66tha -1 roots and 1.91tha -1 stubble, 0 .82tha -1 roots in the untreated and in the NPK+FYM plot, respectively (measurements of two years). Winter wheat has been cropped in the sampling year.

The soil temperatures and moisture contents during the sampling period in 1990 are shown in Figure 1. The decrease of moisture during the spring period was due the water uptake of the growing wheat plants. The following steep increase from the 21st to 23rd week was the result of strong precipitation events of 10 to 40 mm in the 21st and 22nd week.

Soil sampling

The soil samples were collected weekly between April 4th (13th week) and September 18th (40th week). At four regularly distributed sampling locations on each plot, 100 cm3-soil cores were taken using a 3.5-cm diam. steel coring bit (4 cores for each plot). A 2.5-m distance from the edges was established to avoid edge-effects. The exact sampling points at the four locations were randomly distributed. The samples were taken within the homogeneous ploughed A-horizon between 5 and 15 cm depth.

Immediately after sampling the cores were weighed and dried (105 °C, over night) and re- weighed. This sample treatment allowed later calculations of moisture contents (Fig. 1) and wet and dry bulk density. The dry samples were sieved (<2mm) and stored. Plant fragments

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Fig. 1. a Soil moisture content and 5 soil temperatures (10 cm depth, two measurements per day) in the untreated (D) and NPK+FYM-treated plot (E) during the sampling period in 1990 ('Static Experiment' at Bad Lauchstfidt).

>2ram have been discarded. Subsamples for chemical analyses were obtained by careful divi- sion using a Laborette 10-sample divider (Fritsch, Idar-Oberstein, Germany). Powdered soil samples were used for C, N and Py-FIMS analyses.

Organic carbon and total nitrogen analyses

Organic carbon contents were determined by dry combustion using the W6sthoff-Carbon Analyzer (Bochum, Germany). Total nitrogen was determined by the Kjeldahl-method. Two to three replicates were carried out for each individual sample so that at least 8 values were obtained for each week and treatment.

Particle-size fractionations

The whole soil samples from the 13th, 24th, 33rd and 39th week (U), and from the 13th, 16th, 24th, 29th and 38th week (NPK+FYM) were bulked and subdivided using a sample-divider

Seasonal variations of soil organic matter 227

(see above). Then 50 g of subsample were dis- persed in 150mL of deionized water using a Branson 450 probe4ype sonifier. The instrument was set at 80% power output (360 W), working in 1:1 operating/interruption intervals for 15 min. (840 W mL 1 suspension), Sand (>63/~m) was collected by wet-sieving. The remaining suspension was fractionated into clay (<2 gm), fine silt (2 to 6.3~m), medium silt (6.3 to 20 p~m) and coarse silt (20 to 63 p~m) by repeated sedimentation/decantation (Schulten et al., 1993). The size fractions were freeze-dried and weighed. The coefficients of variation of the fraction amounts in two replicates were 0.02 to 19.0% (mean 1.9%). The recovery of soil weight ranged from 97 to 100%.

Pyrolysis-field ionization mass spectrometry

For temperature-resolved Py-FIMS about 5 mg of the samples was thermally degraded in the ion-source of a Finnigan MAT 731 mass spec- trometer. The temperature controlled direct introduction system with electronic temperature- programming (IGT Instrumente- and Gerfite- Technik GmbH, 5203 Much, Germany), ad- justed to the +8 kV potential at the ion-source, was used. The samples were heated in high vacuum from 50 to 700 °C at a heating rate of approximately 0 .5Ks J. About 60 magnetic scans were recorded for the mass range 16 to 1000 Daltons (single spectra). For each scan, the ion intensities of marker signals for important classes of chemical compounds in SOM were calculated. These marker signals were chosen on the basis of extensive studies of plant materials, extracted humic substances, particle-size fractions and whole soil samples of different origin and composition, using Py-FIMS, high-resolution Py-FIMS and Py-GC/MS (Hempfling et al., 1988, Hempfling and Schulten, 1990, Schulten and Schnitzer, 1992). After averaging the ion intensities for the various substance classes from the replicated measurements, these were plotted versus the pyrolysis temperature, resulting in characteristic thermograms of each compound class. All Py-FIMS data were normalized per mg sample to enable relative quantitative compari- sons between the different soil samples. Three to five replicate samples have been analysed. The


mean coefficient of variation for mass signals >0.2% relative abundance of the summed spectra was ~6% (n = 5) (Schnitzer and Schulten, 1992). Detailed statistical evaluations of sample weight and residue, volatilized matter and total ion intensities (Sorge et al. 1993) and descrip- tions of the Py-FIMS methodology have been given (Schulten, 1987, 1993).

Results and discussion

Organic C and total nitrogen concentrations

The organic C concentrations (Fig. 2) showed pronounced seasonal variations in the two treatments. The trends were somewhat different between the treatments. In the NPK+FYM plot the C concentrations decreased during April and increased during August. In the untreated plot changes in C concentrations were less pronounced during these time-periods. The largest C concentrations were observed in June. The

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most drastic change was the clear decline between June and August by approximately 0.4% C (NPK+FYM) and by 0.2% C (U), For the total N concentrations (Fig. 2) the curves showed peaks in the 19th and 32rid week (NPK+ FYM) and in the 18th, 25th, 30th and 39th week (U). Very clear decreases in N concentrations occured between mid August to September by 0.014% (NPK+FYM) and by 0.035% (U). Ac- cordingly, the C/N ratio differences (Fig. 3) were largest during May to June and in Sep- tember, and lowest in July and August. The most marked differences between the treatments were the initial decrease in C/N ratios during April to May and the lower amplitudes in the NPK+FYM treatment (Fig. 3).

The standard deviations of the replicate analyses (n = 8) were in the range 0.006 to 0.218% for C and 0.002 to 0.023% for N determinations. Thus all calculated differences were significant at the p < 0.05 level. The variations in the C and N concentrations, in particular the C losses between spring and summer, confirm corresponding results from other field experiments in Germany, e.g. -0 .5% C between July and August (Kleinhempel and Knappe, 1966), -0.1 to -0 .2% C between April and October (Sauer- landt and Tietjen, 1971), -0.05% C between May and July (Flaig, 1976) and -0.124% C between June and August (Reichelt 1990). Kfrschens (1982) reported seasonal changes of the C concentrations by 0.2 to 0.7% in this

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Fig. 3. Seasonal variations of C/N ratios in differently treated plots of the 'Static Experiment' at Bad Lauchstiidt (A, U; • NPK + FYM).


experiment, with the highest values for grass. The pronounced decreases of SOM contents between spring and summer are probably due to decomposition of fine roots and previous-years plant residues which passed the 2-mm sieve and thus contributed to the C contents of the spring- time samples. In addition, the unfavourable ecological conditions such as increased temperatures (see the temperature peak in the 24th week in Fig. lb), dryness (Fig. la) and reduced porosi- ty (Fig. 4) may have caused decreased abun- dances of soil organisms in summer. For in- stance, mesofauna species might have left the sampled layer by downward, vertical movement. Another interesting aspect to explain the sudden drop in C contents should be considered. The increased dry bulk density of 1.11gcm -3 (U) and 1.15 g cm -3 (NPK+FYM) in the 24th week to 1.60gcm -3 (U) and 1.40gcm -3 (NPK+ FYM) (Fig. 4) indicate considerable compaction which could have reduced the thickness of the Ap horizon from originally 30 cm after ploughing to a minimum of 22 cm (U) and 25 cm (NPK+ FYM). Approaching a constant sampling depth, possibly some material with lower C content has been collected during summer. This should be significant, in particular, when a horizontal distribution of the C contents is occurring during the growing and harvesting period, which has not been reported so far.

Furthermore, it is suggested that the variations of the C and N concentrations are also accompanied by changes in the molecular composition of SOM. This is indicated by the clear seasonal differences in the N concentrations. It should be

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noted that the strong decrease of N concentrations by 0.035% within six weeks between July and mid of September in the untreated plot (Fig. 2) was only lower by a factor of 0.5 than the average difference of N concentrations between the untreated and NPK+FYM-treated plots, which were under different management for 88 years. This decrease in soil N concentration corresponds to a loss of 157 kgN ha -1 in the sampled layer (5 to 15 cm depth) when the changes in soil bulk density (Fig. 4) are considered. Since no significant N uptake by plants occurred during this time period, an incorporation into N-forms which were not digested by hot sulphuric acid, gaseous N losses due to denitrifi- cation and vertical translocations can be the reasons for this strong decrease in soil N. The latter two effects could have been promoted by a number of precipitation events between the 29th and 35th week and increased soil moisture contents (Fig. 1) which exceeded the field capacity.

Organo-mineral particle-size fractions

On average of all fractionations, 21.5% clay, 7.0% fine silt, 17.9% medium silt, 43.4% coarse silt and 7.4% sand were obtained. The coefficients of variation were 4 to 5% for clay and silt and 19% for sand. Since the clay and silt contents were equal to those obtained by the standard pipette method (K6rschens and Eich, 1990) the results of particle-size fractionations showed the adequate dispersion of whole soil by the ultrasonic energy applied and the good repro- ducibility of the fractionation procedure.

The organic carbon concentrations of the size- fractions (Table 1) followed the general order clay >fine silt > sand > medium silt > coarse silt. All size-fractions from the NPK+FYM treatment had significantly larger organic C concentrations than those from the untreated plot. Seasonal variations were indicated by differences in C concentrations, which, however were some- times insignificant at the p < 0.05 level.

In the untreated plot, the increase in whole soil C between April and June (Fig. 2) was reflected in slightly increased C concentrations in clay and sand. The drastic decrease in soil C during summer was due to decreases in C concentrations in all size fractions with significant

230 L e i n w e b e r et al.

Table 1. Seasonal variations of the organic C concentrations in five organo-mineral particle-size fractions from the untreated and the NPK+FYM-treated plots of the 'Static Experiment' at Bad Lauchst/idt (% w/w, standard deviations in parenthesis, underlined values of consecutive sampling dates indicate differences significant at the p < 0.05 level, except for NPK + FYM, 16th and 24th week, medium silt)

Treatment Week Clay Fine silt Medium silt Coarse silt Sand

u 13th 4.67 (0.03) 3.54 (0.06) 0.73 (0.01) 0.10 (<0.01) 1.02 (0.09) U 24th 4.70 (0.18) 3.55 (0.17) 0.73 (0.06) 0.10 (<0.01) 1.10 (0.05) U 33th 4.59 (0.12) 3.31 (0.10) 0.69 (0.01) 0.09. (<0.01) 0.65 (0.09) U 39th 4.75 (0.05) 3.58 (0.08) 0.77 (0.10) 0.09 (<0.01) 1.09 (0.02) NPK+STM 13th 5.18 (0.11) 5.50 (0.20) 1.87 (0.05) 0.25 (0.01) 3.03 (0.08) NPK+STM 16th 5.29 (0.12) 5.01 (0.04) 1.71 (0.04) 0.21 (0.01) 2.47 (0.02) NPK+STM 24th 5.22 (0.07) 5.13 (0.06) 1.73 (0.03) 0.24 (0.02) 2.60 (0.12) NPK+STM 29th 5.21 (0.03) 5.19 (0.19) 1.81 (0.03) 0.24 (0.02) 2.53 (0.10) NPK+STM 38th 5.28 (0.12) 5.18 (0.08) 1.73 (0.01) 0.23 (0.01) 2.92 (0.22)

differences for fine silt, coarse silt and sand. Despi te the weak increase in whole soil C between August and October , significant increases were observed for the C concentrations in clay, fine silt and sand.

In the N P K + F Y M treatment , the initial decrease in soil C was due to decreased C concentrations in fractions > 2 / z m , whereas clay showed a somewhat enlarged value (Table 1). The following weak increase in soil C was due to C in silt and sand. The strong decrease in soil C between the 24th and the 29th week was only reflected by the C concentrations in clay and sand. Moreover , increased C concentrations were observed in fine silt and medium silt during this t ime period. Between the 29th and 38th week, a slight increase of C concentrations in clay and sand corresponded to the increased values of the whole soil samples.

The recovery of whole soil C in the sum of all fractions was 81 to 83% for the untreated and 80 to 92% for the N P K + F Y M - t r e a t e d samples. Differences to 100% are explained by losses of soluble organic carbon and of fraction material during fractionation. This can be also concluded f rom the recovery of whole soil which increased f rom 96.9 and 96.6% in the 13th week to 100.0 and 98.0% in the 39th/38th week in the un- t reated and in the N P K + F Y M - t r e a t e d samples, respectively. The distribution of soil C in the various size fractions also showed seasonal variations (Fig. 5). It was surprising that the strong decrease in soil C between June and August was not clearly reflected by the C distribution. The most obvious differences were the increased C proport ions in clay, fine silt and medium silt.

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This tendency seemed to continue later in the year, except for clay in the N P K + F Y M treatment . The general order of C distribution, clay > fine silt > medium silt > sand > coarse silt was disturbed neither by the long-term different fertilization nor by seasonal variations. In the untreated soil, more C was associated with clay whereas in the N P K + F Y M treatment the size fractions > 6 . 3 / x m contained larger proport ions of whole soil C (Fig. 5).

The different behaviour of size fractions f rom the untreated and N P K + F Y M - t r e a t e d plot during the rapid SOM decrease in summer corresponds to results of C mineralization studies through incubation of isolated fractions: The

larger decrease in C concentration in fine silt and coarse silt from the untreated plot is in line with results of Gregorich et al. (1989) who observed a larger C mineralization in silt than in clay. On the other hand, Christensen (1987) found the range sand > clay > silt for the decomposability of C, which agrees with the decreased C concentrations in sand and clay in the NPK+FYM treatment. In studies of long-term SOM dynamics in particle-size fractions it was often observed that the relative amounts of clay-associated organic matter increased in situations of a net- mineralization of SOM (see Christensen, 1992). This was also the general trend for the seasonal changes of C distribution on size fractions (Fig. 5).

Pyrolysis-field ionization mass spectrometry

The Py-FI mass spectra of samples from the 24th and 33rd week (U) (Fig. 6) and from the 25th and 29th week (NPK+FYM) (Fig. 7) showed the presence of carbohydrates (m/z 72, 82, 84, 96, 98, 110, 126, 132, 144 and 162), N-containing compounds (m/z 59, 67, 79, 81, 93, 95,103, 109, 117, 123 and 145), phenols and lignin monomers (m/z 94, 108, 110, 122, 124, 126, 138, 140, 152, 154, 164, 166, 168, 178, 180, 182, 194, 196, 208, 210 and 212), lignin dimers (m/z 246, 260, 270, 272, 274, 284, 286, 296, 298, 300, 310, 312 and 330), alkylaromatics (m/z 92, 106, 120, 134, 148, 162, 176, 190, 204, 206, 218, 220, 232, 234, 246, 260, 274, 288 and 302) and lipids (m/z 256, 270, 284, 298, 312, 326 and 340). In the spectrum of samples from the NPK+FYM treatment the mass-signals covered a wider mass range, due to larger SOM contents and the more abundant lignin dimers and lipids. The slight differences according to sampling time were indicated by reduced intensities of signals from carbohydrates and N-containing compounds in the sample from the 33th week (U) (Fig. 6).

Thermograms were plotted for the thermal evolution of carbohydrates, phenols, lignin monomers, lignin dimers, alkylaromatics, lipids and N-containing compounds (not shown). The thermograms of carbohydrates reached a first maximum at 370°C and a second at 480 to 500 °C. Gaussian-like curves with peaks at 490 to 500°C were observed for phenols and lignin


monomers, at 520 °C for lignin dimers and lipids, and at 520°C for alkylaromatics. The largest evolution of N-containing compounds was observed between 430 and 490 °C.

Since all data were normalized to ion counts per mg sample, the seasonal variations in quan- tities and thermal properties of these compound classes can be visualized by plotting difference- thermograms (thermograms for the samples from the 24th week minus those from the 33rd week (U) and from the 24th week minus those from the 29th week (FYM+NPK)). In these difference-thermograms, the intensity differences >0 indicate losses, and those <0 gains, of the ion intensities during the eight-week (U) and four-week (NPK+FYM) period of rapid SOM decline from spring to summer (Figs. 8 and 9). In the untreated plot, the decreased ion intensities of phenols, lignin monomers, alkylaromatics and lipids resulted from losses of molecular subunits which were volatilized at 400 to 420°C and at 470 to 500°C (Fig. 8). For carbohydrates and N-containing compounds the difference-thermograms were more diffuse, but losses were observed even at lower temperatures (300 to 400°C). The corresponding curves for samples from the FYM+NPK-treatment had similar shapes, but the differences were generally larger (Fig. 9). In the two treatments negative differences of ion intensities were observed at pyrolysis temperatures >500°C. These indicate relative enrichment of molecules, which are volatile only at high pyrolysis temperatures, during the absolute decline in SOM content between spring and summer.

The clear peaks in the difference-thermograms indicate that organic substances of different thermal stability were mineralized. In the untreated plot, the major losses of phenols, lignin monomers, alkylaromatics and lipids can be assigned to the decomposition of medium stable humic substances as shown by the peaks at 450 to 520 °C in Figure 8. The less pronounced peaks at lower temperatures indicate the mineralization of carbohydrates, N-containing compounds, alkylaromatics, phenols and lignin monomers from plant materials or soil organisms in the untreated plot. This distinction is based on studies of grass roots, stems and leaves (Schulten et al. 1992), straw and farmyard manure (Lein-

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weber and Schulten, 1993a) and microbial biomass (Leinweber and Schulten, 1993b). In the NPK+FYM plot, the larger intensity differences (factor 2 to 3) show a larger turnover of the various SOM compartments. In particular the losses of plant, manure or biomass-derived ther- molabile carbohydrates, aromatic and N-containing compounds reached a relatively higher level than in the untreated plot.

Most interesting were the gains in ion intensity of all six compound classes at >500 to 550 °C, visualized by the negative intensity differences in Figures 8 and 9. These show that some portion of the organic molecules required higher temperatures for their volatilization during pyrolysis. When we take into consideration that SOM constituents in clay required higher thermal energy for the evolution during Py-FIMS (Schul-

ten and Leinweber, 1993b), the gains of ion intensity at >500°C in the difference-thermograms correspond well to the larger contribution of clay-associated organic matter to SOM (Fig. 5). We suggest, that organic molecules have been gradually incorporated into humic or organo- mineral bonds of greater stability. In the case of intermolecular bonds in humic macromolecules this can be explained by advanced crosslinking. For organo-mineral bonds, interlayering of aliphatic molecules in expandable clay minerals could be possible. Although no direct evidence supporting these suggestions exists at present, the investigated thermal stabilization during annual SOM dynamics is in very good agreement with some other observations: Leinweber and Schulten (1993c) showed a similar trend during 20 years of SOM maturation in a pot experi-


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hi, + L, k.,hbn HII. I lld.t,,llll,,h ,,I.N,L,.I, h., ,l, l,1. ,k, ,. .L ,.,. ,l,,., l.. l.l,t I,IJ,I,,+,,I,LI,,,.,.L,.,,,,,., .......... , . . . . i i i i i i i I i 1 i i i J l i i . . . . i . . . . . . . . . i " ~ I + i + - I I I ' I ' ' ' I +

150 200 250 .300 .350

10 I

b)

1 3 2 146 I i 186

ld t 230 244 2 5 8

J I Jl, L L ,It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' l ' ' ' ' I ' ' ' '

O0 150 200 250 .300 .350 m / z ~-

I 400

't 400

Fig. 7. Py-FI mass spectra of soil samples from the NPK+FYM plot of the 'Static Experiment" at Bad Lauchstfidt, taken (a) in the 25th week and (b) in the 29th week.

ment. In these samples the gains at larger pyrolysis temperature, expressed by the number of ion counts shifted towards >550°C, were larger, on average, by a factor of 26 than in the present study• Possibly, the repetition of annual organic matter stabilization lead to the larger value due to the summation over the 20 years of the experiment. Furthermore, Andreux et al. (1990) reported from French studies that alkali- solubility of organic matter was lowest in summer. They suggested a progressive polymeriza- tion of humic macromolecules during the dry season. This view is supported by the results of the present study since the moisture contents decreased from 22.0% w/w (23rd week, NPK+ FYM) and 20.0% w/w (23rd week, U) to 6.5% w/w (NPK+FYM) and 9.6% w/w (U) in the 29th week (Fig. 1), which correlated to the

enhanced thermal stability of some SOM constituents.

Conclusions

The large differences in the C and N concentrations during the vegetation period reached values which were lower by factors of only 0.3 to 0.6 compared to the average differences between the 88-years contrastingly managed soils. This points to the fundamental importance of poorly understood seasonal changes for SOM dynamics in agro-ecosystems, including the nitrogen cycle. For example, the large loss of 157 kg N per ha from the sampled 10-cm-layer in the Ap-horizon (U) during four weeks needs more detailed investigations.


200

I00

°

-100

g

~5 >,

Pheno l s , l ign in m o n o m e r s 200 " ~

1oo : : , i

0

-1°°1 : i i :

L ign in d imers

200

100

-100

A l k y l a r o m o t i c s L i p i d s

• ~ . ] • t~i • 100

i ' ' ~ 0

I ' -I00

. v

C a r b o h y d r a t e s

: . o g : o # • ~o , 4o •

o --~ ,-~- ~ : ~ - i o o : • : ' o .

• i - io •

200 300 400 500 6gO

T e m p e r a t u r e [ ' C ]

N i t r o g e n c o m p o u n d s

200

100

0

- I 0 0

200 300 400 500 600

Fig. 8. Difference-thermograms for the evolution of important compound classes of S a M from the untreated plot (thermograms of the sample from the 24th week minus thermograms of the sample from the 33th week).

E

m "E

o~

~5 >,

P h e n o l s , l i go in m o n o m e r s

600 • : #~tK

450 . . . . . w • . . • . • .

3oo : : : :• i

- • - e @ .

-15

Alky lo romat ics

6OO

I, o ,:L 500

-150

Carbohydrates eoo] . i • ~ .

450 1 i : : ~ i

,6oi : 2 :

- x s o J 200 300 400 500 600

Lig nin d imers

600 ] i . i • i

450 t " i i i i 300 i i i i i

o . • • x x •

-150

L i p i d s

600 • • •

. . . , . 450 i i i i i

5i1 J, i i i i .

Nitrogen compounds

1 :! : ! 45o : : 9 a a ® :

300 : : o. [] io :

: ~ o ~io%:

-150 200 300 400 500 600

T e m p e r a t u r e [ ' C ] . . . . . . .

Fig. 9. Difference-thermograms for the evolution of important compound classes of SOM from the N P K + F Y M plot (thermograms of the sample from the 25th week minus thermograms of the sample from the 29th week).

The physical fractionation of SaM was shown to be useful for the monitoring of seasonal changes. From these results it can be concluded that not only long-term and medium-term but also very short-term variations of SaM are accompanied by redistributions of C between size fractions.

Among the various methods for the evaluation of the Py mass-spectral data tested in several SaM studies, the computing of thermograms for important compound classes was most sensitive to observe and visualize chemical and structural changes of SaM during the vegetation period. Larger thermal energies for the evolution of SaM compounds were related to the strengths of humic and organo-mineral bonds in size fractions (Schulten and Leinweber, 1993b). The mineralization of coarser-sized plant fragments from silt and sand along with enhanced crosslinking within the humic macromolecules appears the best explanation for the clear seasonal variations of SaM between spring and summer. Possibly, this could be promoted by the drying and shrink- ing of the colloidal organo-mineral soil particles, by higher soil temperatures during summer, and by effects of the mineral matrix.

Acknowledgements

This work was financially supported by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg (projects Schu 416/3 and 416/12-3). The technical assistance with preparation of particle-size fractions and C and N analyses, supplied by Miss H Reddig, University Os- nabriick, is gratefully acknowledged.

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Secrion ediror: R Merckx

seasonal variations of soil organic matter in a long-term agricultural experiment

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