Appendix
to
PERSPECTIVES
ON SOIL CARBON.
And a
simple model to get it: Cpersp.
Quantification
of parameters and influencing factors.
Background
to the values used.
Inputs
of organic matter to soils.
Present
guidelines used in Cpersp, expressed as total organic matter input, dry matter.
Cereals:
1.5* grain yield, of which 1/3 is straw.
Oilseeds: 2
* seed yield, of which ½ is straw.
Beets and
potatoes: 0.1 * root or tuber yield.
Leys: a
flat figure 6000 kg per year, to which “extra” not harvested growth should be
added.
For a wheat
yielding 8 tons this means an input of 12 tons organic dry matter. This may
seem high. It includes 4 tons of straw, maybe 2 ton of stubble etc, 2-3 tons of
visible roots. This adds up to at most 9. The rest is rhizosphere deposition,
the sum of fine roots, dead roots, root exudation, microorganisms feeding on
the root system during the vegetation period.
Berglund et
al (2002) have for Swedish conditions
given harvest residue and root data for different crops. A further calculation
based on these data give the following figures for total input of organic matter in relation to yields: winter wheat
factor 2, winter rape 3, barley 1.6. The topsoil deposition will be somewhat
lower.
Wilts et al
(2004), quote from abstract, SC= total source C input): “The SC input from
unharvestable sources was 1.8 times more than SC from aboveground stover when N
was added …”
Johnson et
al (2006) give a comprehensive review. The importance of the rhizodeposition is
stressed. Based on their extensive litterature review they state “ ..estimates
are that 2.5 to 6 times the amount of C incorpored into root biomass may be
represented as rhizodeposition”. Based on their calculations of crop C
turnover, the figures expressed as “input of organic matter to field/ grain
yield” will be for wheat 2.6 and for barley around 3. These should for
comparisons with topsoil soil C be reduced for the C deposited below topsoil.
Nevertheless, the figures given above for Cpersp seem low and conservative in
this perspective.
For leys
the connection between harvested vegetation and organic matter input is less
clear. ”. For instance, Skinner et al (2006) found no relation between soil C
and canopy photosynthesis. The use of a constant value of 6000 kg organic
matter (d.m.) per ley year fits well with experimental data, but there is of
course a close interrelation with the factor “soil resting months”.
Certainly,
these quantifications need to be checked and revised when necessary.
“Soil
resting months.”
Currently
there are environmental programs stimulating leaving the soil undisturbed
after harvest and over the winter
season. The objective is to reduce mineralization of soil nitrogen. But of
course this will also reduce the mineralization of soil carbon. Somehow or other
this has to be considered in calculations of soil C.
Let us see “soil resting months” as a preliminary
try to include and quantify these aspects., which are of practical significance in the agriculture of today.
Present
system in Cpersp: since it has a Scandinavian origin 3 winter months are
supposed to give near zero mineralization and consequently there are 9 active
months. The background for the “normal” mineralization figure is autumn tillage
after harvest. If the soil is not disturbed we have a “soil resting month”,
which reduces the annual mineralization by 1/9. The soil of overwintering leys
is not disturbed at all, which should mean 9 resting months and no
mineralization at all. However, when comparing with experimental results it was
evident that this is too extreme, and the present guideline is 6 resting months
for overwintering leys.
The
guidelines given should be interpreted
with common sense and local knowledge. In practice this factor is very
important. The positive effects of leys cannot be explained by their input of
organic matter only.
A problem
is that longterm experiments where this factor can be put to test only can be
found for leys. Uhlen (1991) describes rotations with different ley
combinations: 6 years with 0, 2 or 4 years of ley. Good agreement with soil C
development is achieved with 6 soil resting months.
Extension
to cover crops and tillage.
Cover
crops.
Cover crops
work in two ways: they contribute with organic matter and they may mean that
the soil is left longer undisturbed.
Guidelines
for cover crops are given in “Humusbilanzierung”. The factors to use in Cpersp
are organic matter input and “soil resting months
Cover crops
is a fairly recent development. Experimental quantification of their carbon
effect cannot yet be found. Some circumstantial results from longterm
experiments with strong undergrowth of weeds exist from Rothamsted.
Tillage
effects.
Tillage
effects on soil carbon are well documented but there are some conflicting
results.
West et al
(2002) in a review of experimental data,
calculate net carbon differences between tillage systems. Both soil
fluxes and emissions from fuels etc are considered. In average they found that
conversion to minimum tillage could save 370 kg C per hectare and year.
In
their review Johnson et al (2006)
summarize “ ..minimum source C inputs to maintain soil organic carbon
….averaged 2.5 +-1 Mg/ha based om mouldboard plow sites (n=13) and 1.8+- 0.44 Mg/ha (n=5) based on no-till
and chisel plow sites.” This means, indirectly, that in conservation
tillage the soil C losses where about
70% of those in mouldboard plow systems. Note that this is expressed as input
of organic residue C, whereas normally in this report the resulting “stable
soil C” is used.
“Conservation
tillage” is a composite term for several practices. Some of them:
No-till
Shallow
tillage (harrow or disc)
Chisel plow
Shallow
mouldboard plow.
Reicosky
(1999) measured carbon losses as carbon dioxide after use of different tillage
implements. These direct losses were considerable, amounting to hundreds of kg
C per hectare during a few weeks. Compared to mouldboard plow, the losses from
chisel plow and disc implement were around 30% and from undisturbed soil 7%. It
is concluded that we should minimize the volume of soil disturbed and leave
residues on the soil surface.
One way to
handle the tillage question in the calculations is to reduce the mineralization
in relation to the soil volume left undisturbed. For a preliminary test this
relation was included in Cpersp, and below the outcome is compared to research
reports.
Borresen et
al (1993) reported from a Norwegian experiments: 13 years, spring cereals, 25 cm mouldboard plow compared to 10 cm
rotavator. Soil C data are given only for 0-5 cm (2.3 and 2.8 %C respectively)
and 10-20 cm (2.3 and 2,3%). Weighed together for 0-25 cm this will be 2.3 and
2.4 % C respectively. At start 1976
soil C was 2.3%. Cpersp gives
2.25 and 2.42% C respectively in year 13 at an assumed mineralization of 1%
when the cultivation depths 25 and 10 respectively were used.
Another
report by Borresen et al (1994) gives data from an experiment started 1939 with
different plowing depths, 12,18 and 24 cm. The soil C is very high, at start
about 9% (recalculated from a diagram). Soil C data from 1975, 1981 and 1987
are given per layer. Based on these the soil C in the layer 0-24 are
recalculated and used for comparison with Cpersp. After 42 years the soil C
accornding to measurements are 6.9% and 7.5% respectively for 24 and 12 cm plowing
depth. Cpersp gives the values 6.7 and 7.6.
1.2% mineralization and 6 resting months för leys were used. Obviously,
at these high soil C contents leys also loose soil C. Cpersp overestimated the
difference somewhat. But if the real disturbance depth was 15 instead of
12 there had been good accordance.
Puget and Lal (2005) performed a metaanalysis of published results om MT versus conventional. 56 pairs could be included and the average gain of soil C for MT was 330 kg per hectare compared to conventional plowing.
Jarecki and
Lal (2005) describe two experiment in Ohio with different tillage, a silt loam
soil (Charleston, 41 year experiment)
and a clay soil (Hoytville. 16 years). At Charleston there was an MT
effect of about 180 kg C annually but “the clay soil did not respond to tillage
treatments”. However, the measurements at Hoytville are compatible with a
mineralization of 1.5% and an advantage for MT of 300- 400 kg C annually. The C
stock is high and the time short.
Wright et
al found a soil C increase for NT of
about 250 kg C annually in a 20 year experiment. All increase was in the top 5
cm, deeper layers where not affected.. Aggregate stability was higher for NT.
No starting C content is given so the dynamics cannot be explored.
Slepetiene et al (2005) have studied a 40 year experiment in Lithuania: conventional plowing, shallow plowing and minimum tillage. Additional treatment and yield information
has been given (Slepetiene et al 2006). Shallow plowing gave insignificant differences in layer 0-30 but higher in 0-20. MT increased soil C in 0-30. Overall, at start the soil C was low 1.2%), and soil C increased in all treatments. Cpersp could describe the development well, with the parameters mineralization 1.7% and MT disturbance 5 cm.
In addition to these effects on soil C gross content, MT improved the humus quality.
Dersch et al (2001) summarized experiments in Austria. They conclude about minimum and reduced tillage that in 10 years these practices resulted in 4.7 and 3,2 tons of soil C compared to conventional plowing.
There are more critical reports. Dolan et al carefully investigated a tillage experiment in Rosemount, Minnesota. There were clear increases of soil carbon for reduced tillage in the layer 0-20 cm, but “the summation of soil organic carbon over depth to 50 cm did not vary among tillage treatments”. They stress the need for correct sampling protocol with bulk density determinations down to sufficient depth. It is suggested that in conventional plowing both rhizodeposition and downward transport of breakdown substances can increase soil C below 30 cm. To some extent this work casts a shadow of doubt over optimistic reports on carbon sequestration. However, on closer look at the results and probing with Cpersp it appears that the variation in the experiment does not allow conclusions about effects smaller than 800 kg C per year. So – a question mark remains.
Other profile studies show consistent increases in total C stock for reduced tillage (Puget, Lal and Izzuralde et al, 2005), in average an advantage of 280 kg C per hectare and year for no till..
Koch and Stockfisch (2006) introduce another question mark: what happens if a field with conservation tillage occasionally is plowed? They conclude that the accumulated gain in soil C rapidly disappears. A 9 year period of reduced tillage was followed by plowing and the development of soil C after the plowing operation was followed. Within 3 years a decline of 4400 kg C/ha was measured. It seems that this should cancel the possible gains during the 9 year period of reduced tillage, although this comparison cannot be made. Also in this experiment important changes occur in the layer below the plowing depth 30 cm, a layer which was fairly high in soil C in this loess soil.
References:
Berglund, Kerstin, Örjan Berglund och Anna Gustafson Bjuréus 2002. Markstrukturindex - ett sätt att bedöma jordarnas fysikaliska status och odlingssystemets inverkan på markstrukturen. SLU, Institutionen för Markvetenskap, Avdelningen för lantbrukets hydroteknik, Avdelningsmeddelande 02:04.
Borresen,
T. And Njös, A., 1994. The effect of ploughing depth and seedbed preparation on
crop yields, weed infestation and soil properties from 1940 to 1990 on an loam
soil in eastern Norway. Soil and Tillage Research 32, 21-39.
Borresen,
T. and Njös, A. 1993. Plowing and rotary cultivation for cereal production in a
long-term experiment on a clay soil in southeastern Norway. 1. Soil properties.
Soil and Tillage Research 28,97-108.
Carlgren, K och Mattsson, L 2001. Swedish Soil
Fertility Experiments. Acta Agr. Scand., Sect. B, 2001, 51, 49-78.
Dersch, G and
Böhm, K 2001. Effects
of agronomic practices on the soil carbon storage potential in Austria.
Nutrient Cycling in Agroecosystems, 60, 1-3, 49-55.
Dolan, M S,
Clapp, C E, Allmaras, R R, Baker, J M and Molina, J A E 2006. Soil Organic Carbon and Nitrogen on a
Minnesota soil as related to tillage, residue and nitrogen management. Soil
& Tillage Research 89, 221-231.
Jarecki, M.
K. and Lal, R., 2005. Soil organic carbon sequestration rates in two long-term
no-till experiments in Ohio. Soil Sci. 170,4,289-291.
Johnson, J. M-F., Allmaras, R.R. and Reicosky, D. C., 2006. Estimating source carbon from Crop Residues, Roots and Rhizodepositits Using the National Grain-Yield Database. Agron. J. 89:622-636.
Koch, H-J
and Stockfisch, N 2006. Loss of soil organic matter upon ploughing under a
loess soil after several years of conservation tillage. Soil & Tillage
Research 86, 1, 73-83.
Mattsson, Lennart och Larsson, Hans 2005. Att föra bort eller bruka ner halmen påverkar mullhalt, daggmaskar och skadedjur. Inst. för Markvetenskap, Avd. för Växtnäringslära, Rapport 210.
Mattsson, Lennart 1991. Nettomineralisering och rotproduktion vid odling av några vanliga lantbruksgrödor. Inst. för Markvetenskap. Avd. för Växtnäringslära, Rapport 182.
Persson, Jan 2004. Kortsiktiga och långsiktiga
markbiologiska processer med speciell hänsyn till kvävet. Kungl. Skogs- och
Lantbruksak. Tidskr. 143, 12,
67-94.
Puget, P.
and Lal, R., 2005. Soil organic carbon and nitrogen in a Mollisol in central
Ohis as affected by tillage and land use. Soil and Tillage Research, 80, Issues
1-2. 201-213.
Puget, P,
Lal, R, Izzuralde, C, Post, M and Owens, L 2005. Stock and distribution of
total and corn-derived organic carbon in aggregate and primary particle
fractions for different land use and soil management practices. Soil Sci. 170,
4, 256-279.
Reicosky,
D. C., 1999. Effect on Conservation
Tillage on Soil Organic Carbon Dynamics: Field experiments in the U. S. Corn
Belt. In: (editors) Stott, D. E., Mohtar, R. H. and Steinhardt,
G. C., 2001. Sustaining the global farm. Selected papers from 10th International Soil Conservation Organization
Meeting, May 1999 at Purdue University.
Sauerbeck, D.,
R., 2001. CO2 emissions
and C sequestration by agriculture – perspectives and limitations. Nutrient
Cycling in Agroecosystems, 60, 253-266.
Six, J.,
Elliott, E. T. and Paustian, K., 1999. Aggregate and Soil Organic Matter
Dynamics under Conventional and No-Tillage Systems. Soil Sci. Soc. of Amer. J.
63:1350-1358.
Skinner, R. H., Sanderson, M. A., Tracy, B. F. and Dell, C. J., 2006. Above- and belowground productivity and soil
carbon dynamics of pasture mixes. Agr. J., 98,320-326.
Slepetiene,
A. and Slepetys, J., 2005. Status of humus in soil under various long-term
tillage systems. Geoderma, 127, issues 3-4, 207-215.
Slepetiene,
A., 2006. Pers. Comm.
Uhlen, G., 1991. Long-term effects of fertilizers, manure, straw and crop rotation on total N
and total C in soil. Acta Agric. Scand. 41,119-127.
West, T.,
O. and Marland, G., 2002. Net carbon flux from agricultural ecosystems:
methodology for full carbon cycle analyses. Environmental Pollution 116,
439-444.
VDLUFA
2004. Humusbilanzierung - Standpunkt. www.vdlufa.de
Wilts, A.
R., Reicosky, D. C., Allmaras, R. R. and Clapp, C. E., 2004. Long Term Residue
Effects. Soil Sci. Soc. of Am. J. 68:1342-1351.
Wright, A.
L. and Hons, F. M., 2005. Tillage impacts on soil aggregation and carbon and
nitrogen sequestration under wheat cropping systems. Soil and Tillage Research,
84, 67-75.