REGULAR ARTICLE
Impact of pre-harvest burning versus trash conservation
on soil carbon and nitrogen stocks on a sugarcane plantation
in the Brazilian Atlantic forest region
Érika Flavia Machado Pinheiro&Eduardo Lima&
Marcos Bacis Ceddia&Segundo Urquiaga&
Bruno J. R. Alves&Robert M. Boddey
Received: 3 December 2009 / Accepted: 4 February 2010 / Published online: 2 March 2010
#Springer Science+Business Media B.V. 2010
Abstract Owing to the increased demand for ethanol biofuel from sugar cane, the area planted to this crop in Brazil has increased from 4.8 to 9.5 Mha since 2000. At the same time there has been pressure from environ-mental groups and others to cease the pre-harvest burning of cane, and today over 40% of the crop is harvested without burning, thus conserving the trash on the soil surface. While most trash decomposes during the year, it is generally assumed that this transition from burning to trash conservation will have benefits for cane productivity and increase soil carbon stocks. To investigate the possible benefits of this change of practice an experiment was carried out in the state of Espírito Santo, south-eastern Brazil, to investigate the long-term effects of the practice of pre-harvested burning compared to trash conservation on soil fertility and soil C and N stocks. The results showed that over a 14-year period, trash conservation marginally decreased soil acidity and significantly increased soil C and N stocks in 0–10 cm depth interval. Although the trash
conservation treatment accumulated 13 Mg C ha−1
more than the burned treatment, this difference was not statistically different. However, the stocks of N to 100 cm depth were 900 kg ha−1 higher under the trash
conservation treatment and this difference was statis-tically significant. The 13C abundance data suggested that where trash was conserved, more soil C was derived from the sugar cane than from the original native vegetation.
Keywords 13C . Carbon accumulation .
Green manure . Pre-harvest burning . Soil organic matter . Sugarcane . Trash conservation
Introduction
Brazil is the world’s largest producer of sugarcane. In
2008 a total of 8.2 Mha of cane fields were harvested for the production of over 27 billion litres of ethanol for fuelling light vehicles and 30.8 million tonnes (Tg) sugar. Since the year 2000 cane production has increased from 255 Tg to over 653 Tg in 2008 (projected to be 660 Tg in 2009), and taking into account the fields recently planted, this crop nowa-days covers just over 9.5 Mha (IBGE-SIDRA 2009; UNICA2009). The large increase has been stimulated by the international interest in bio-ethanol as a vehicle fuel (ethanol exports reached 5.4 billion litres in 2008) and the introduction of FlexFuel (Otto cycle) motors which can run on any mixture of hydrous DOI 10.1007/s11104-010-0320-7
Responsible editor: Elizabeth M. Baggs.
É. F. Machado Pinheiro
:
E. Lima:
M. B. CeddiaDepartamento de Solos, Instituto de Agronomia da UFRRJ, BR 465, km 7,
23890-000 Seropédica, RJ, Brazil
S. Urquiaga
:
B. J. R. Alves:
R. M. Boddey (*)Embrapa–Agrobiologia,
Rodovia BR 465, km 7, Caixa Postal 74505, 23890-000 Seropédica, RJ, Brazil
(95%) ethanol and Brazilian gasoline (a mixture of approximately 76% gasoline and 24% of anhydrous [99.5%] ethanol). Light vehicles with FlexFuel motors were launched on the Brazilian market in 2003 and over 8.3 million had been sold until October 2009 (ANFAVEA2009).
The State of São Paulo is responsible for over 60% of the cane production and the crop occupies approximately 4.5 Mha, 18% of the whole area of the State. The air pollution caused by burning the cane at harvest is thought to have significant detrimental effects on human health, especially respiratory problems for young children and the elderly (Godoi et al. 2004; Arbex et al. 2007). In 2003 this led to legislation being passed in this State that mandates all pre-harvest burning of cane in São Paulo must be phased out by the year 2022. Only on land that has greater than a 12% slope, where machine harvesting is non-viable, will burning be allowed until 2032. Today approximately 40% of Brazil’s
sugar-cane is not subject to pre-harvest burning (green sugar-cane harvesting), and most of this area is in São Paulo, but the change in practice is increasing steadily.
It is natural to assume that the preservation of the cane trash (usually between 10 and 15 Mg ha−1
crop−1, Resende et al. 2006; Mello et al. 2006) will
lead to the accumulation of soil carbon (Vallis et al. 1996). The trash is on the soil surface but much decomposes during the crop cycle, and thus it is expected that only a small proportion becomes integrated into the soil. At the experimental site described by Resende et al. (2006) in the Agreste region of Pernambuco, at harvest, 12 Mg ha−1of trash
were deposited on the soil surface when the cane (yield, 120 Mg stems ha−1) was not burned, and after
12 months at the time of the next harvest, only 500 kg ha−1 remained (Resende 2003). These authors found
that trash conservation increased cane yields over a 16 year period by 25%, but that soil C only increased by 2.5 Mg C ha−1 over the whole period or a mean
annual gain of only 156 kg C ha−1.
From short-term studies (4 years) in São Paulo, other authors have suggested that the change from pre-harvest burning to trash conservation would promote a mean soil C accumulation of 1.62 Mg C ha−1 year−1 (Cerri et al. 2004; Mello et al. 2006).
However, this estimate was based on data taken from fields where the cane crop was not replanted, which in São Paulo generally occurs every 5 to 6 years. The
deep ploughing and harrowing involved in replanting stimulates soil organic matter (SOM) decomposition and probably causes the loss of a large proportion of this C.
The energy balance of bio-ethanol from sugarcane has been recently computed by Macedo et al. (2008) and Boddey et al. (2008). This is the ratio of fossil energy used to produce 1 litre of ethanol in comparison with the total energy produced when the ethanol is used as fuel. Both studies concluded that the balance was approximately 9:1 and Boddey et al. (2008) reported that for a 100 km journey in a FlexFuel family car there is a 79% offset in greenhouse gas emissions compared to the same vehicle travelling the same distance running on pure gasoline. If the change from burned cane (which facilitates manual harvesting) to machine harvesting of “green”cane leads to significant soil C
sequestra-tion, this will have an impact on the greenhouse gas emission offset. Hence the question of the magnitude of this soil C change is relevant to any assessment of the overall environmental impact of bio-ethanol production from sugarcane.
The objective of this study was to compare the soil C stocks under sugar cane subject to pre-harvest burning, or trash conservation over a 14 year period, with a view to contributing reliable data to evaluate the impact of the conversion to trash conservation on the mitigation of greenhouse gas emissions.
Material and methods
Site and experimental layout
The experiment was established in the experimental area of the LAGRISA “Usina”(distillery/cane
facto-ry), Linhares, Espírito Santo (19°18′ S, 40°19′ W),
situated in the Atlantic Forest region. Mean monthly temperatures range from 20.5°C in July to 26°C in February. Mean annual rainfall was 1226 mm over the 14 years of the experiment. The soil is classified as a Haplic Acrisol (Abruptic, Hyperdytric– FAO
Classi-fication) or an Argissolo Amarelo by the Brazilian classification. Soil granulometric analysis data are displayed in Table1.
cane experiment was established in 1989 with a randomized block design with five replicates and just two treatments (A) with, or (B) without, pre-harvest burning of the cane.
The 10 plots consisted of 6 rows of sugarcane of 95 m in length spaced at 1.2 m between rows. The variety of sugar cane was RB 73-9735. Basal fertilization at the start of the experiment was 500 kg ha−1dolomitic lime,
81 kg ha−1of K as potassium chloride, and 55 kg ha−1
of P as single super phosphate, applied in the furrow at planting. In the following years (1990–2002), soon
after harvest, the ratooning cane crop was fertilised with 400 kg/ha of compound fertiliser (25-00-20) and 150 kg N in the form of urea.
Sugar cane was planted after deep ploughing and harrowing in May 1989. The cane was all manually harvested, the consequence being that almost no machinery entered the experimental area. Machines, especially the heavy cane harvesters, compact the soil which often leads to progressively decreasing yields and the cane plantations are usually replanted every 5 to 6 years. This experiment and the surrounding commercial area were not replanted until 2003, just after the soil sampling reported here.
Cane harvest
The first harvest was made after 18 months in late 1990 and then the next 13 ratoon crops harvested at 12-month intervals. All unburned plots were cut first as well as the border area (two rows on each side) around each plot. The trash on the soil surface of these plots was then saturated with water from a tanker truck to prevent it being burned, and the remaining plots then burned off for harvesting the following day.
The aerial tissue of the plants was manually harvested and separated into fresh stems, trash (un-burned) and flag leaves. These materials were weighed fresh and then sub-sampled for evaluation of dry weight. All plant material was dried (65°C for >72 h) and subsequently ground using a Wiley mill (<0.85 mm).
Soil sampling
Soil samples (0–5, 5–10, 10–20, 20–30, 30–40, 40–
60, 60–80 e 80–100 cm depth) were taken after the
14th harvest in October 2003, by opening one trench per plot (1.2 m square) for the evaluation of soil bulk density of each depth interval (one replicate from each of the four sides of the trench from the centre of each depth interval). The soil samples were taken with Kopeck rings (4.5 cm i.d. and total internal volume of 101 cm3). Separate soil samples for chemical analyses were taken from the whole depth interval, were air dried, then passed through a 2 mm sieve. Bulk density samples were removed from the Kopeck rings, dried at 105°C, weighed and then discarded.
Analysis of soil and plant material
Soil samples taken in 2003 were analyzed for pH (in water), were extracted with a solution of potassium chloride (1M) to evaluate exchangeable Al, Ca and Mg using standard techniques (Embrapa 1997), and for available K and P by extraction in a mixture of dilute sulphuric (0.025M) and hydrochloric (0.05M) acids (Mehlich I extractant) as described in the Embrapa soil analysis manual (Embrapa 1997).
For the analyses of total N and C, and the C isotopic abundance, sub-samples were further ground to a fine powder (<0.15 mm) using a roller mill similar to that described by Arnold and Schepers (2004). Total N concentration was determined on aliquots of 1.0 g of soil using semi-micro-Kjeldahl digestion in heated
Table 1 Physical properties of the Yellow Ultisol under the long-term sugarcane experiment at the LAGRISA cane factory, Linhares, North Espirito Santo, Brazil)
Depth (cm) Sand
(g kg−1) Clay(g kg−1) Silt(g kg−1)
Experimental plots
0–5 890 100 10
5–10 890 100 10
10–20 890 100 10
20–30 880 120 0
30–40 800 200 0
40–60 760 200 40
60–80 750 230 20
80–100 700 280 20
Forest
0–5 700 280 20
5–10 720 260 20
10–20 850 140 10
20–30 790 180 30
30–40 660 260 80
40–60 610 330 50
60–80 590 310 100
aluminium digester blocks (36 samples, two standards and two blanks) followed by steam distillation using a Tecator Kjeltec model 3100 (Tecator, Höganäs, Swe-den) automatic titration/distillation unit as described by Urquiaga et al. (1992). Total C analysis was performed on∼150 mg aliquots of the samples using a total C and N analyser (LECO model CHN 600, Leco Corp., St. Joseph, MI). The 13C isotopic abundance of the soil samples was determined on aliquots containing be-tween 200 and 400 mg total C using a continuous-flow isotope-ratio mass spectrometer (Finnigan DeltaPlus, Bremen, Germany) in the “John Day Stable Isotope
Laboratory”of Embrapa Agrobiologia.
Calculations
Soil C and N stocks
The total C and N stocks in the soil were estimated using the procedure recommended by Ellert and Bettany (1995). If in one plot the soil is compacted more than in another, the profile of former to any specific depth will contain a greater mass of soil. It was assumed that any differential soil compaction between plots was most significant in the surface layers of the profiles so that the C and N stocks were calculated by subtracting the total C and N content of the extra weight of soil in the deepest (either 5–10,
30–40 or 80–100 cm for calculation of the stocks to
10, 40 or 100 cm, respectively) layer sampled in each profile as described by Neill et al. (1997).
The reference profile used in this study was the treatment without pre-harvest burning. As proposed by Balesdent et al. (1990) any one treatment (preferably that with the lowest soil mass in the profile) can be used to correct the others.
Estimation of the proportion of soil C derived from original native vegetation
The proportion of C originating from sugar cane and the C originating from forest was determined accord-ing to the equation described by Cerri et al. (1985):
%CFo¼
d13Csample d13C4
d13C3 d13C4
100
whereδ13Csampleis the13C abundance of sample from
sugar cane plots. These samples contain C derived
from native vegetation (Forest - C3) and C input from the sugar cane (C4);
δ13C3 13C abundance of soil organic matter C from
native vegetation (C3) in the same depth interval
δ13C4 13C abundance of C from crop root residues
(C4)=−10.7‰
%CFo percentage contribution of C derived from C3 vegetation (original soil C under the forest) in relation to a mixture of C derived from C3 and C4 vegetation.
Statistical analyses
Comparisons between burned and unburned plots were made using standard analysis of variance (ANOVA) based on the values of ‘F’. No statistical
comparison was attempted between the experimental plots and the neighbouring forest remnant, as the soil texture of the soil under the forest was considerably higher than that of the soil under the plots.
Results
Sugarcane yield
In this region rainfall varies widely in distribution and total from year to year (Fig. 1). Despite the lower rainfall in the year following planting, this first crop gave higher yields than any subsequent ratoon crop, probably mainly because the growing period was 18 months compared to 12 months for the ratoons. Surprisingly, in the first few years of the experiment there seemed to be little relationship between total annual rainfall and cane yield. However, after 1996 there was a strong tendency for yields to be positively related to annual rainfall. Unfortunately the individual replicate data were lost for this study in Linhares in a computer malfunction, but over the whole 14 crops the mean yields were very similar, 78 and 80 Mg fresh stems ha−1 for the burned and unburned cane,
respectively. These yields are close to the 2009 national average yields (79.5 Mg ha−1 -
IBGE-SIDRA 2009)
true for the sucrose concentration (Ceddia et al. 1999). Data for later harvests are not available.
Soil chemical and physical properties
The preservation of trash for 14 years showed a tendency to increase soil pH, lower available Al3+and conserve Ca2++ Mg2+down to at least 40 cm depth, compared to the burned treatments, although this effect was only statistically significant for pH from 20 to 30 cm depth and for the Ca+Mg in the top 5 cm of soil (Table2).
As forest areas were not included in the experi-mental design, it was not possible to make a rigorous statistical comparison between the soil chemical and physical properties between the experimental plots and the area of native forest. However, it is clear from the soil bulk density data that to a depth of 60 cm the soil under the experiment was compacted although
there was no discernable effect in the 0–5 cm depth
interval (Fig.2). This compaction can be attributed to the first few years of pasture (cattle trampling), to the ploughing and/or to the machinery used to distribute setts, fertilisers and spray herbicides etc.
Total soil C and N
To a depth of 30 cm the soil C and N concentrations under the native forest were generally higher than under the sugarcane experiment (Fig.3). Only in the most superficial depth interval (0–5 cm) was there a
statistically significant higher concentration of C and N under the conserved trash compared to the burned treatment.
The soil under the area of forest neighbouring the experimental plots was very different in colour and in texture (Table 1) from that under the experimental plots. For this reason we did not use this area to
Year
1990 1992 1994 1996 1998 2000 2002 2004
Mean cane y
ield (Mg ha
-1 )
0 20 40 60 80 100 120 140 160 180
Rainf
all (
mm)
0 500 1000 1500 2000
Trash conserved Burned cane Annual rainfall (mm)
Fig. 1 Total annual rainfall and mean cane yield on trash conserved and burned cane after 14 years of sugarcane (1989–2003) at
Usina LAGRISA (Linhares, Espírito Santo, Brazil)
Depth (cm) pH (H2O) Al (meq/100 cm3) Ca+Mg (cmolc/dm3) K (g/dm3) P (g/dm3)
TC BC TC BC TC BC TC BC TC BC
0–5 5.62 5.70 0.00 0.10 3.56a 2.08b 131.4 119.8 6.80 9.60
5–10 5.38 5.10 0.24 0.34 1.32 0.92 56.4 70.6 2.80 3.40
10–20 5.34 4.98 0.26 0.40 0.96 0.84 25.6 54.6 2.00 3.00
20–30 5.48a 4.92b 0.18 0.40 1.18 0.84 19.0 53.8 2.40 3.30
30–40 5.64a 5.02b 0.18 0.34 1.24 1.00 19.6 33.4 3.20 2.75
40–60 5.70 5.30 0.12 0.24 1.22 1.18 11.8 15.6 1.00 2.06
60–80 5.54 5.22 0.24 0.24 1.14 1.04 13.2 5.6 1.40 1.52
80–100 5.48 4.94 0.24 0.44 0.90 0.68 7.80 4.8 2.80 1.38
Table 2 Soil fertility parameters under the sugar-cane harvest systems after 14 years of cultivation under trash conserved (TC) and burned cane (BC)a
aValues are means of five
replicates
correct the soil C stocks for equal mass of soil (see Material and methods). However, to compare the stocks under the experimental plots, the quantity of soil C under the burned treatment was adjusted so that the total C was quantified in the same mass of soil present under the unburned treatment.
The soil under the area of native forest was much higher in clay content than that under the experimental plots (Table 1), and it is therefore not surprising that the concentration of C was consider-ably higher in the depth intervals from 0–30 cm
under the forest compared to the soil under the sugar
cane (Fig. 3). The large quantity of surface trash in the unburned treatment was almost certainly respon-sible for the significantly higher concentration of C in the 0–5 cm depth interval compared to the burned
plots. However, only in this depth interval was there any significant difference in soil C concentration between treatments.
The pattern of distribution of soil N was very similar to that of the soil C (Fig. 4) and this was manifested in the similarity of the soil C:N ratio within each depth interval (data not shown).
After 14 years of trash conservation, the stock of soil C in the 0–10 cm depth interval was
approxi-mately 4 Mg ha−1(significant atP=0.05) greater than
under the burned plots (Table3). This difference was increased to 13 Mg ha−1 when the stocks were
evaluated to 100 cm, although this difference was not significant at P<0.05. The differences in soil N stocks between burned and unburned plots were significant for both the 0–5 and the 0–100 cm depth
intervals (Table 3). This difference in soil C stock is equivalent to an increase in soil C stocks of 0.93 Mg C ha−1year−1.
Carbon from native vegetation and from cane residues
The vegetation in the forest of this region is entirely composed of C3species and at the soil surface the13C natural abundance was close to−28‰as reported in other studies in this region (Flexor and Volkoff1977;
Soil C concentration (g kg-1)
0 2 4 6 8 10 12 14 16 18 20 22 24
Depth (cm)
0
20
40
60
80
100
Burned cane Trash conserved Forest
Fig. 3 Total carbon (g C kg soil−1) of soil to a depth of 100 cm under the forest neighboring the sugarcane experiment. Means of five replicate profiles. Error bars for forest data indicate standard errors of the means, and for burned and unburned sugarcane, LSD (Studentp<0.05)
Soil Bulk Density (g cm-3)
1.25 1.30 1.35 1.40 1.45 1.50
Depth (cm)
0
20
40
60
80
100
Burned cane Trash conserved Forest
Fig. 2 Soil bulk density in the profiles (0–100 cm) of the soil
below the plots managed under trash conserved or burned cane and the neighboring forest. Error bars for forest data indicate standard errors of the means, and for burned and unburned sugarcane, LSD (Studentp<0.05)
Soil N concentration (g kg-1)
0.0 0.5 1.0 1.5 2.0
Depth (cm)
0
20
40
60
80
100
Burned cane Trash conserved Forest
Tarré et al. 2001). The 13C abundance increased slightly with depth (Fig.5).
There was no significant difference in13C abundance between the soil under the burned cane and that under the unburned cane at any depth interval (Fig. 5). However, even in the 80 to 100 cm depth interval, the soil C under the experimental plots was 1.6‰higher
than under the forest suggesting some deposition of C4C even at this depth.
While it was evident from the soil texture (Table1) and bulk density data (Fig. 2) that the soil under the neighbouring forest was not identical to that under the experimental plots at the time the land was cleared, the13C abundance of each depth interval was used to calculate the proportion of C derived from the forest (Fig.6). These data show that there was a higher total C content of the soil under the unburned cane compared to the burned treatment in the 20 to 30 cm depth interval, and a reverse situation in the 30 to 40 cm interval. However, these differences were not significant (P<0.05) and are probably just evidence of the variability in the depth at which soil clay content suddenly increases in the different plots (Table1).
The data show that in the 0–10 cm depth interval
the majority (a mean of 56%) of the soil C was derived from cane over the 14 years of the experiment (or the previous 3 years of Brachiaria pasture), but this proportion gradually decreased with depth.
Discussion
Our earlier study in the State of Pernambuco showed a very strong relationship of annual rainfall with cane yield, and also significantly higher cane yields were
Table 3 Influence of the pre-harvest burning (trash conserva-tion) on C and N stocks in different depth intervals, after 14 years of sugarcane (1989–2003), at Usina Lagrisa, Linhares,
Espírito Santo
Harvest systems 0–10 cm 0–40 cm 0–100 cm
Soil carbon stocka,b(Mg C ha−1)
Trash conserved 14.8 a 49.6 a 113.8 a
Cane burned 10.9 b 43.5 a 100.8 a
aValues of treatments are means of five replicates
bMeans in the same column followed by the same lower case
letter are not significantly different atP<0.05
δ13C (‰)
Fig. 5 13C natural abundance of soil to a depth of 100 cm under the forest neighboring the different sugarcane harvest system. Means of five replicate profiles. Error bars for forest data indicate standard errors of the means, and for burned and unburned sugarcane, LSD (Studentp<0.05)
recorded where trash was conserved in years with lower rainfall (Resende et al. 2006). This can be attributed to the conservation of soil moisture by the soil cover provided by the trash. One possible reason for the lack of a positive response to trash conserva-tion in this present study was that the 150 kg ha−1of
N fertiliser (urea) was added to the surface of the soil (burned treatment) or the surface of the trash. Several studies have shown large losses of N fertiliser via ammonia volatilisation from the surface of the trash (Freney et al.1992,1994) so it is expected that there would be a lower N fertiliser use efficiency in the conserved trash treatment, and this may be responsi-ble for the lack of higher yields where trash was conserved. That this effect was not seen in the study of Resende et al. (2006) may be due to the much lower quantity of N fertiliser used (half the plots received 80 kg N ha−1, the remainder none).
Since 1980 the Australian sugar industry has gradually changed from pre-harvest burning of cane to trash conservation. While there seems that no long-term experiments were established to examine the impact of this change of management on SOM levels, Vallis et al. (1996) used the Century model (Paustian et al.,1992) to simulate changes in C stocks in 0–20 cm
depth interval of soil under conditions of Northern Queensland (Ingham). The simulations indicated that soil C content would increase by approximately 40% over after 60 to 70 years, and about half this increase (5 Mg C ha−1) would occur in the first 20 years. This
is close to our estimate of an increase of between 4 and 6 Mg C ha−1over 14 years at Linhares (Table3).
However, the soils at the Ingham site were finer texture (∼600 g sand kg−1) compared to the Linhares site (890 g sand kg−1), annual rainfall was higher
2280 mm yr−1(versus 1220 mm at Linhares) and far
more seasonal (∼74% in the months from December to March), and the simulation included a replanting of the crop every 6 years.
As was to be expected, soil C and N decreased with depth, although the increase in clay content with depth of this Acrisol (Table1) meant that the decrease with depth was attenuated compared to other tropical soils with uniform clay+silt content down the profile such as Ferralsols (Sisti et al. 2004; Diekow et al. 2005) or even another Acrisol (Haplic Profondic) from the same region (Tarré et al. 2001). The steep decrease in soil C with depth generally seen in most soil types was not apparent in this soil until depths
below 40 cm, and even then it was only gradual. This may be explained by the fact that the clay content doubled between 20 and 40 cm depth and clay (or silt+clay) concentration is a major controlling factor of soil organic matter content (Feller and Beare1997). As has been observed in other studies, soil 13C abundance under the forest (solely C3 vegetation) increased with depth (e.g. Flexor and Volkoff 1977; Cerri et al. 1985; Tarré et al. 2001). This increase is considered to be partly due to the fact that SOM deeper in the profile is more humified, and during humification 12CO2, is lost more rapidly than 13CO2 (Blair et al. 1985). Since the industrial revolution in the 1800s the burning of fossil fuels has not only increased the concentration of CO2in the atmosphere but also led to atmospheric CO2to become increas-ingly depleted in13C. Hence more recent soil organic matter, which is in a larger proportion in the surface layers, contains C of lower 13C abundance (Von Fischer and Tieszen 1995). This is a further mecha-nism which contributes to the observed increase in soil 13C abundance with depth. However the differ-ence between the surface layers and the lowest depth interval samples was only∼1.0‰, much less than the difference of 2.6‰observed by Tarré et al. (2001) at
a site on a similar soil approximately 400 km N of Linhares.
The difference of 1.6 ‰ between the 13C
abun-dance of the soil in the 80 to 100 cm depth intervals under the forest vegetation and the sugarcane exper-iment suggests that even in the 80 to 100 cm depth interval 12% of the soil C was derived from C4 vegetation, suggesting that the sugarcane, or the previous Brachiaria, roots reached this depth and deposited considerable quantities of C4–C. This contrasts considerably with the results of Tarré et al (2001) who found no significant C derived from Brachiaria at depths greater than 40 cm even after approximately 20 years of pasture establishment.
In southern Brazil, Feller et al. (2001) reported that an average of 0.32 Mg C ha−1 yr−1 was
accumulated in 12 years in the first 20 cm depth of an Oxisol by omitting burning. Other estimates exist, however, for short periods of non-burning. Luca et al. (2008) reported increases ranging from 2.0 to 3.1 Mg C ha−1and 4.8 to 7.8 Mg C ha−1(1.2 to 1.95 kg
ha−1year−1), respectively for the top 5 cm and 40 cm
Sugar cane is typically replanted every 5–6 years
and heavy tillage practices are commonly used which stimulate SOM loss. However, in this present study the plantation was not renewed during the 14 years between initial and final soil sampling, which sug-gests that this difference would be lower if the crop had been replanted in accordance with general practice. The simulated and measured data of Vallis et al. (1996) clearly indicate a large reduction in soil C during this process of replanting even though in the long term their simulations suggested soil C would continue to accumulate.
In this present study cane was harvested manually in both systems (trash and burned cane), while the actual management change occurring in Brazil is from manual harvesting of burned cane to mechanical harvesting of green (unburned cane). There is consid-erable concern that the heavy harvesting machines and high capacity trailers used in the mechanical harvesting may result in increased soil compaction compared to manual harvesting of burned cane. As compaction may reduce root growth, and it is C derived from roots that is responsible for the great majority of C inputs into the soil (except in the first few cm of soil), it is even possible that the changeover to trash conservation could lead to reduced inputs of soil C. No rigorous long-term comparisons of the change from manually harvested burned cane to mechanically harvested green cane seem to have been performed.
Our results that indicate that the change from pre-harvest burning to trash conservation increased soil C stocks to a depth of 100 cm by almost 1 Mg ha−1
year−1. However, the fact that the crop was not
replanted during this period, and machinery was not used to harvest the unburned cane, suggests that this estimate is likely to be considerably higher than for the regions where manual burned-cane harvesting is being replaced by machine harvesting of unburned cane.
Acknowledgements The authors express their gratitude to Altiberto M. Baêta and Roberto G. de Souza for the total N and 13C abundance analyses, respectively. The first author EFMP
gratefully acknowledges National Research Council (CNPq) for a PhD Research Fellowship and the authors SU, BJRA and RMB for“productivity”fellowships from CNPq and from the program
Cientista de Nosso Estado of the Rio State Research Foundation (FAPERJ). We are grateful to Lagrisa Linhares Agropecuária S/A in Espírito Santo for maintaining the experiment from 1989 onwards. The work was funded by Embrapa, CNPq, FAPERJ and the Universidade Federal Rural do Rio de Janeiro.
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