Drying of soil was linearly related to time, soil volume decreased and ratio of air within the soils increased. Respiration was related with decreasing humidity, storage of CO 2 in soil water results in RQ < 0.5 in the larger soil items at least for a while. Rate of drying decreased in the second part of the process. RQ increased as the CO 2 stored was aerated when its solvent-water evaporated and access of air into the soil increased; eventually RQ = 1.0 in the last days of the experiment. Respiration of the experimental soil stopped when GWC reached 0.15. ΣRQ for the whole process is about 0.7, a bit higher in experiments with less soil suggesting less anoxia.
In the present contribution our original idea was to study process of drying more in detail. Soils in natural settings are changing; this may be approached not only in field work, but also in lab experiments, where the conditions may be set more accurately and less rely on unpredictable weather etc. and show how they are reflected in the process, particularly in soil respiration. During the work we realized however, that some concepts are generally not quite clear or understood and we propose some clarification, both by pointing to literature not frequently referred in soil science and/or through experimental design.
Respiration was/is considered as a general measure of soil activity (somewhat mystical statement, preferably of soil organisms), recirculation of nutrients from plant residues, while drying of soil is associated with wet and dry periods, seasons, hence more recently with global changes.
Soil respiration was studied many times; the literature on it is quite extensive [
The sequence/wording “anaerobic respiration” (meaning reduction of e.g. nitrate, nitrite or sulfate replacing dioxygen as proton acceptor) is used quite frequently, though it is not particularly accurate, nitrate and sulfate reduction or proton acceptance would be more to the point, but may be perhaps tolerated.
Respiratory production of carbon dioxide occurs largely in the tricarboxylic acids cycle, is associated through oxidative phosphorylation with oxygen consumption, their ratio in terms of absolute values of moles or volumes is the respiratory quotient (RQ = [production CO2]/[consumption O2]).
Introduction of reliable IR non-dispersive spectrophotometers ( In fact already 1850 paper based on CO2) enabling measurements of small concentrations and changes of carbon dioxide concentrations to soil science provoked some shifts in terminology; soil biological activity―measured as production of carbon dioxide―is now quite frequently referred as soil respiration e.g. [
In soil literature retention of CO2 in soil water due its high solubility (distribution coefficient air/water = approx 1 compared to dioxygen 0.03) is mentioned only exceptionally. In soils with pH > 4 dissolved bicarbonate becomes the dominating form of the carbonate system [
Being aware of interferences in measurements is particularly important in studies of processes, like various kinds of acclimation to changing temperature and humidity or in the budget of a process.
We want to analyze particular parts of the drying process, not only desiccation, but also changes of soil volume, access of air/oxygen to soil aggregates, storage capacity for CO2 within the soil etc. and their impact on measured respiration data. This applies both to laboratory and field setups and involves size of soil items in both. Our soil is to some extent exceptional in high SOM content (around 50% of dry weight), many relationships studied in less organic soils need not to be applicable, also some of our conclusions are necessarily not of general use.
Drying of soils is a natural part of regular seasonal events, in observations and/or experiments many papers tend to limit themselves to just momentarily flashes or steady states; we tried however to estimate the budget of the drying process to go through different situations, states of the soil, get rid of many obstacles in getting real metabolic data. Incidentally, we came across low rates, their statistical treatment and time scales.
The location is on the northern outskirts of the city of Warsaw (Poland). Geographic coordinates are: 52˚20'212 N, 020˚51'260 E, elevation 81 m. Refer to Blazka and Fischer [
(GWC―gravimetric water content, WHC―water holding capacity, dm―dry mass).
Our procedure was based essentially on Grace et al., Priha and Smolander [
SOM was estimated as loss of weight in ignition to constant weight (LOI) at 500˚C on selected samples. Mean densities of SOM (ρ = 1.3) and of the mineral fraction of dry weight (ρ = 2.65), are from Ilsted et al. [
Organic matter = LOI was between 0.45 and 0.50.
ρ(soil) = (LOI * 1.30 + (1 − LOI) * 2.65 = 1.96.
(OCD―oxygen consumption, PCO2―production of CO2, RQ―respiratory quotient).
We used the volumetric method with the Scholander type respirometer to get both the oxygen consumption and carbon dioxide production. The capacity of its cylindrical flasks is 50 ml (inside radius r = 2 cm, V = 4 cm2) and we refer data as μl∙g−1(dry mass)∙h−1. Each experimental series consisted of two parts. In the first, without hydroxide in the flasks, changes in the volumes were recorded mostly for 5 hours, this gave the sum of CO2 changes and O2 consumption, in the second part after placing filter paper soaked with NaOH solution into the respirometric chamber, we recorded the oxygen consumption (OCD) for another 5 hours interval, readings each 30 minutes. This involves assumption of slow rates of metabolic changes. The bath temperature was 17.5˚C. Sensitivity of the volumetric respirometer is effectively not much worse (<0.1 μl gas∙g−1∙hour−1) than of the IR meters, though rates of providing data are not comparable.
While data of the second part were the OCD, PCO2 was calculated as first reading (without hydroxide) + second reading (with hydroxide), respecting the signs in readings and calculations. We then plotted the results against time of readings, the slope coefficient of the regression (β1) was the rate (O2 consumption-OCD or PCO2); the respective regression coefficients were all r > 0.95, they were later corrected for pressure and temperature, multiplied by the calibration factor and related per gram dry soil and hour. Weights of respiration vials with soil were taken regularly; tare was subtracted to get rates of drying.
Later in Results and discussion we will consider to what extent the data measured might be biased by diffusion or other transport problems within the soil items, the measured data may then be classified as apparent data (AOCD, APCO2 and ARQ).
Wet soil was gently pressed before start, there were no fissures between soil and wall of respirometric flask, they appeared however later during drying. We measured them by a series of blades (used in fine mechanics to measure narrow crevices, range 0.05 till 2 mm) on four sides of the vial, their mean was subtracted from r = 2 cm to get the radius of the base. The height (he) of the cylinders was estimated using a plate put on the top of the open vial and fixed to center by 4 screws projecting down on the outside of the flask, there were 5 evenly distributed holes on the plate, through which the distance from the top of the plate to soil top was measured by a rod, read by caliper and subtracted from the distance bottom of flask-top of plate. Mean of the 5 readings was height of the soil (he). Final calculation as cylinder volume:
Ws is mass of wet soil.
Changes of GWC in experiment are included as background for other parameters of the soil studied. We refer four principal experiments, which were preceded by several preliminary checks or pilot experiments, all done on one and the same kind of soil collected from one spot. All were aimed towards analysis of changes in soil respiration (OCD, PCO2 and RQ) in drying of the soil. The main difference between the experiments was in the amount of soil in flasks and therefore were coded as LS (less soil) and MS (more soil) respectively. Outline of the experiments is in
After each 10 days WHC was estimated in 10 of the vials in the first series (MS1, LS1). In experiment MS2 and LS2 we estimated WHC at the beginning and at termination of the experiment.
Additionally we made a 14-days drying experiment (MP―mud pies) starting at GWC = 1.0 and finished at
LS1 | LS2 | MS1 | MS2 | |
---|---|---|---|---|
dry mass [g] mean; se | 3.8; 0.0294 | 3.26; 0.025 | 15.22; 0.0751 | 14.61; 0.147 |
days of experiment | 24 | 13 | 52 | 42 |
starts of experiment | Aug 25, 14 | Feb 23, 15 | Nov 11, 14 | Feb 23, 15 |
1st linear section [d] | 24 | 13 | 32 | 29 |
˚C air | 17.2 - 19.0 | 18.8 - 19.7 | 17.0 - 18.8 | 18.4 - 19.7 |
β1 GWC rate a [m] | −1.845 | −4.43 | −1.632 | −1.257 |
β1 GWC rate b [m] | x | x | −0.721 | −0.412 |
mud-pies formation [d] | 8 | 6 | 16 | 20 |
humidity air % | 50 - 65 | 40 | 45 - 62 | 40 - 45 |
GWC start | 2.4 | 1.54 | 2.34 | 1.58 |
GWC end | 0.644 | 0.12 | 0.15 | 0.16 |
GWC estimates N | 7 | 5 | 15 | 15 |
GWC replications | 12 | 6 | 12 | 6 |
GWC rates are β1-coefficients in linear regressions per month, a stands for the first, b for the second linear section. In the LS was one section. LS1 was finished prematurely.
OCD | PCO2 | GWC | replicates | determinations | |
---|---|---|---|---|---|
MS1 | 0.2919 | 0.3581 | 0.1275 | 12 | 14 |
MS2 | 0.1373 | 0.1401 | 0.0511 | 6 | 13 |
GWC = 0.13 at the respiration stop. There were two modes of soil drying, each in six respirometers: a) soil (350 g dry weight) dried freely distributed on a plate, aliquots were taken for each respirometric measurements (per 6 * 5 g dry weight), returned to the plate and mixed; b) In the second mode wet soil was pressed by hand into the respirometric vials (per 5 g to each vial), it was left to dry freely between measurements each second day.
We also checked respiration in strict anoxia (SA) comparing respiration of a fresh collected soil with respiration after flushing the respirometers by pure dinitrogen (Air products.com, purity 5.2).
Was rather basic, it involved averages = means (m), standard deviations (s), coefficients of variation (cv) and standard errors of the mean (se) calculated for all series of data obtained, referred here or not. Two sided t-test was applied for checks of differences of starting and closing data of series and for checks of differences in neighboring data within series. In final evaluation principal (
This came out from the discrepancy in evaluation of rates in changes in GWC, Vs, Vpo related to day which was our working time unit. We were unable to reject the 0-hypothesis (slope-rate of change of GWC is not different from zero―(from horizontal plane line), whereas paired T-test showed significant differences not only between start and end of experiment (p < 0.001), but also between most of neighboring data in the series (
Results are largely based on the four drying experiments specified in Methods, Experimental,
MS2 | β1 (slope)/day | N = 14 all p | slopes/month | p | T-test of pairs start-end | T-test of neighboring days | |||
---|---|---|---|---|---|---|---|---|---|
p > 0.05 | p < 0.05 | p < 0.01 | p < 0.001 | ||||||
GWC | −0.0361 | >0.05 | −1.083 | <0.005 | p < 0.001 | 0 | 0 | 14 | |
Vs | −0.4255 | >0.05 | −12.76 | <0.001 | p < 0.001 | 6 | 5 | 1 | 2 |
OCD | −0.0733 | >0.05 | −2.319 | <0.001 | p < 0.001 | 3 | 1 | 10 | |
LS2 | N = 5 for GWC and Vs; N = 4 for OCD | ||||||||
GWC | −0.1138 | >0.05 | −3.41 | <0.005 | p < 0.001 | 0 | 0 | 0 | 5 |
Vs | −0.3963 | >0.05 | −11.89 | <0.001 | p < 0.001 | 0 | 0 | 2 | 3 |
OCD | −0.5904 | >0.05 | −17.71 | <0.001 | p < 0.001 | 0 | 1 | 0 | 3 |
Significance of slopes related to day and month, compared to 2-sided T-test for data pairs start-end of experiment and neighboring pairs during the experiment.
sandy like aggregates and demonstrate CO2 production in strict anoxia. Also we assured us that the hard dry mud pies are not just gypsum or carbonate.
Relationships of humidity (GWC) are therefore the first to be examined more in detail. Its changes related to time in terms of days[d] proved to be controversial; this resulted in use of month [m] as time reference unit in some selected relationships (
GWC decreased during the experiments linearly with time for approximately 2/3 of the course, towards the end in the MS experiments, there was a significant drop in rate (
So far cracks in dry soils were often referred from the field in the literature, but there was/is much less interest in experiments on changes of soil volume in lab. There are more recent works on the impact of repeated drying and rewetting cycles on characteristic of soils resulting so far in non-complete understanding of these processes. We registered shrinking of soil during LS1 and MS1 experiments and estimated it with assumptions of linearity; nevertheless these estimates for MS1 and LS1 are in good agreement with the data from the second series of experiments. All four experiments have shown significant shrinking and changes of pores (porosity). The LS2 experiment is less successful due to high rate of drying and GWC decrease, therefore it was short and only few data were obtained (
Soil humidity (GWC) appears to be the governing parameter in shrinking of soil and respiration of soil biota. Shrinking [soil volume Vs, cm3] and volume of calculated pores, both Vpo and Va, are linearly related with GWC, only Va in LS2 is not significant (
Formation of “mud-pies”, hard stone―like concretions of dried soil, occurred in all four experiments. In MS1 and MS2 it begun approximately in 1/4 of the drying time, resulted from shrinking of soil, fissures appeared between soil and the walls of respirometric flasks, thus more of the soil surface was opened to the gas phase in
LS1 | LS2 | MS1 | MS2 | |
---|---|---|---|---|
Vs; start | 9.79 (d8) | 8.042 | 37.7 (d16) | 32.735 |
N; se | 9; | 9; 0.33 | 13; | 24; 0.509 |
Vs end | 8.17 (d20) | 3.035 | 25.4 | 15.987 |
se | x | 0.222 | x | 0.245 |
Vs-GWC; β1 | 1.57 | 3.74 | 7.45 | 11.73 |
−r | 0.944 | 0.977 | 0.941 | 0.990 |
p< | 0.025 | 0.001 | 0.001 | 0.001 |
%Vpo in Vs start | 84.2 | 83.2 | 87.5 | 86.38 |
%Vpo in Vs end | 62.3 | 56.5 | 50.1 | 58.32 |
Vpo-GWC; β1 | 2.1775 | 3.15 | 10.696 | 13.252 |
−r | 0.955 | 0.9947 | 0.995 | 0.8767 |
p< | 0.001 | 0.001 | 0.001 | 0.001 |
%Va in Vs start | 15.18 | 16.13 | 10.3 | 15.13 |
%Va in Vs end | 36.20 | 41.79 | 45.01 | 68.75 |
Va-GWC; β1 | −1.13 | −0.139 | −3.572 | −1.468 |
−r | 0.983 | 0.347 | 0.964 | 0.877 |
p | <0.001 | >0.05 | <0.001 | <0.001 |
Va did not change significantly in drying, ratio to Vs changed just due to decrease of Vs.
the flasks. The rate of drying was also related to the amount of soil in the flasks. In the MS1 and MS2 series there is a higher soil layer, longer diffusion path and supposed humidity zonation. In the LS series drying and course of the experiments were faster (
All these processes result in increasing access of air into the soil preparation, in a much better ventilation of the dried soils than is in the wet soils and contributed to formation of the “mud pies” during the drying experiments.
It’s a common knowledge, that wet soils are soft, pastry; easily adopt form of container under only very slight pressure. When dried they become hard, stone-like, if slightly pressed together before drying; they resist pressures approximately like wall nuts, larger “mud pies” are more resistant than small ones. Sieved soil loosely distributed on a plate acquires in drying a sandy character; its total surface is much greater than that of the “mud pie”.
To check the character of the hardiness of the mud pies we chopped off 5.0 g material from the top of the 25 g “mud pie” and took an aliquot of the same weight from the sandy like dried soil. Both were suspended in 50 ml of pure water and extracted for 2 hours and 30 minutes. Both soils looked hydrophobic at the beginning, after approx. 45 minutes they turned more polar. Some air bubbles were attached to dark, presumably light organic soil particles. The extracts were separated from the rests of soils by filtration through Macherey-Nagel filter paper MN 640 m. Both filtrates were yellowish brown, the “mud pie” extract was distinctly darker, but not brown, both very slightly colloidal. Most of the residues remained in the Erlenmeyer’s with the heavy sand fraction, much less on the filters. We measured pH, conductivity and redox potential in both extracts: pH = 4.48 and 4.53, conductivity = 54 and 116 μS∙cm−1 in “sand” and “mud pie” respectively, redox was E = 320 mV in both. Addition of 1 ml 0.2 molar solution of BaCl2 to each of the extracts did not result in formation of any precipitate. We conclude, that the top of the “mud pie” probably contained more organic matter than “the sandy” fraction (more intense humic coloration of the extract), there was certainly no carbonate nor gypsum formed in the drying soils, could not been responsible for formation of the “mud pies”. The two major components of our soil are the postglacial sand, which however remained unchanged in all the procedures as a separate and non-reactive fraction. The second major component is SOM.
We want suggest replacement of H-bonds (water bridges) in wet soil by some other weak interactions (non- covalent bonds) during drying, possibly by hydrophobic weak bonds. A slight pressure and greater mass of wet soil brings soil aggregates closer together and enables formation of other particle interactions when O-(water-) bridges are abolished in drying.
For considerations of bonding within soil aggregates a change in scale of the aggregates might be useful. Remind that there are 6E23 single molecules per each mole (Avogadro number), thus in 1 μg of SOM 6E14 molecules of organic compounds of mean molecular mass 1000 may be accommodated. Soil science does not seem to be generally ready for thinking in such scales (unfortunate
Relationships of gas exchange (OCD, PCO2) with time and GWC are more complex, the respective regression coefficients are lower compared to regressions of GWC and time; the overall tendency of respiration (OCD and PCO2) is a decrease with progressing drying as water is a primary essential-even substrate-of life (
r/GWC | LS1 | LS2 | MS1 all | MS1 after d2 | MS2 all | MS2 after d3 |
---|---|---|---|---|---|---|
OCD β1 | −5.104 | −3.612 | −1.970 | −2.560 | −2.000 | −2.950 |
r | 0.833 | 0.956 | 0.837 | 0.936 | 0.712 | 0.949 |
PCO2 β1 | −3.704 | −0.877 | −0.816 | −0.978 | −1.149 | −1.351 |
r | 0.735 | 0.953 | 0.732 | 0.765 | 0.842 | 0.881 |
For MS experiments coefficients are for all data and also for dates after d2 and d3 due to the delayed Birch effect.
ning of experiments in both LS, or after three or five days in the MS series. There is a more or less stable respiration in the central part of the course in MS1 and MS2 (
Distribution coefficient between gas and water for CO2 is very close to one at the temperature of the experiments, storage capacity for CO2 in soil water is therefore equal to GWC * dry mass of soil. It is the volume of water in respirometer (microcosm), about 20 ml in the first days of the MS experiments or 5 ml in the both LS; apparent production in two or three days.
Another estimate of CO2 stored may result from the difference between AOCD and APCO2 during the flush of respiration when soil is still wet, in MS1 days 3 to12. Assuming real RQ = 1.0 it is 2.81 ml CO2 stored in 28.8 ml H2O, the storage capacity is thus used in less than 10%.
Similar data for the MS2 experiment is for days 5 to 12 and show 2.79 ml CO2 stored in 12.7 ml water, storage capacity used in approximately 22%. Assuming real RQ = 0.7 result in 1.97 ml CO2 stored in MS1 and 1.95 ml CO2 in MS2 respectively. Details of calculations are not shown.
The storage capacity for CO2 was falling in experiments with progressing drying. Angert et al. [
To show gas exchange in strictly anoxic soil we placed 21 g fresh collected sieved wet soil (7.3 g dm) into each of four respirometers. Two of the respirometers were flushed with pure dinitrogen 50 ml∙min−1, for 10 minutes; we registered O2 consumption and CO2 production [with and without NaOH trap inside]. The other pair of respirometers was a control to see regular soil respiration. There were 3.83 and 3.89 μl∙g−1(dm)∙h−1 respectively gas produced in the N2 flushed respirometers without the hydroxide trap. There was however no gas production, nor consumption when hydroxide trap was placed in both respirometers; this also shows that the gas produced without the trap is absorbed by hydroxide, it is most probably CO2 and not CH4. RQ cannot be determined as shown in Introduction. The regular respiratory control in the second pair of respirometers provided 8.92 and 8.56 μl O2∙g−1(dm)∙h−1 respectively and 4.51 μl CO2∙g−1(dm)∙h−1 in both respirometers, this is consistent with data in
Another pilot experiment aimed to differences of respiration in highly dried soils as “mud pies” or small sandy like aggregates. In the “mud pies” both OCD and PCO2 are some 20% lower than in the sandy sieved small aggregates. Fast drops of simultaneous decrease of OCD and PCO2 by approximately 0.6 - 0.8 μl∙g−1∙h−1 occurred at 0.63 GWC in “mud pies”, at 0.43 in “sand”. Drying in “mud pies” and associated decrease of respiration is slower in “mud pies”. There is no contradiction to
High respiration data occurring after changed conditions (sieving, watering…) are referred as the Birch effect [
High flushes of respiration following change of conditions are debated for last 50 years [
Summed OCD and PCO2 for the whole drying experiment showed in all four experiments RQ around 0.7 (
Studies of some process which take into account all data, characterizing the variable soil condition during a process may provide more information, than short measurements or flashes of soil metabolism in ± constant conditions. Type of instrumentation may be of secondary importance only?modern methods of O2 estimation are used only exceptionally [
Nevertheless if there is O2 consumption, anoxia is not in the total mass of soil, possibly in a limited area in center of the soil bloc.
There is a small, but significant peak of respiration in at the beginning of the decreasing phase at d 26 in MS1; in MS2 at d 26. A similar peak was registered by Thomson et al. [
LS1 | LS2 | MS1 | MS2 | |
---|---|---|---|---|
ΣOCD | 7.67 | 15.14 | 42.53 | 33.13 |
ΣCOP | 6.31 | 13.14 | 23.97 | 22.41 |
ΣRQ | 0.82 | 0.87 | 0.56 | 0.68 |
ΣRQ is the ratio of the two sums. Duration of experiments in
which did not continue to a respiration stop. We propose to interpret as increased respiration due to increasing access of air to the drying soil, in our experiments shortly overrun but proceeding desiccation of the soil and confirmed by fast drops afterwards.
All four series (including the prematurely finished LS1) end up with RQ = 1 when GWC is approximately = 1 in LS experiments, but GWC ≈ 0.6 in MS (
CO2 retained in the first section of experiments in water is completely aerated from the soil towards the end when soil is very dry; when respiration seizes at GWC ca 0.15, there is no measurable PCO2, as is no OCD.
All drying experiments were terminated when GWC dropped to 0.15 and respiration stopped. The experiment LS1 was in fact finished prematurely, nevertheless its final RQ = 1. Also in a preliminary experiment not reported here (summer 2014) respiration (both OCD and PCO2) seized at GWC = 0.15 and RQ = 1. The respiration limit at GWC = 0.15 need not be a universal value for many soils, Chowdhury et al. [
There is a distinct peak of respiration rate at the beginning of the experiment (Birch effect); it occurred with some delay in the larger soil blocks. The effect was not associated with increased humidity, rather with increased availability of oxygen within the soil; therefore we suggest interpreting as reaction of the enzymatic systems of the soil organisms. The respiration rate decreased linearly with decreasing humidity of the soil; RQ dropped below 0.5 in both larger soil blocks for some time; reason is evidently in storage of CO2 produced in soil water. Rate of respiration registered is also related to consistency of soil; hard “mud pies” respire some 20% less than the small aggregates of sandy character.
Respiration ceased when GWC dropped down to 0.15. RQ was increasing in the second part of the experiments; a few days before the end it reached 1.00 and remained so till the last moment. We think it comes from CO2 stored in the earlier parts of the drying process which is aerated when amount of water within the soil decreased and air in soil got higher. ΣRQ for the whole budget of the experiments is about 0.7; it is somewhat higher for the smaller soil items (LS) suggesting less hypoxia and may be consistent with partial use of fermentation products like ethanol (RQ = 0.57).
ZofiaFischer,PavelBlažka, (2015) Soil Respiration in Drying of an Organic Soil. Open Journal of Soil Science,05,181-192. doi: 10.4236/ojss.2015.59018