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Carbon contents and aggregation related to soil physical and biological properties under a land-use sequence in the semiarid region of central Argentina
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Soil & Tillage Research 99 (2008) 179–190 www.elsevier.com/locate/still
Carbon contents and aggregation related to soil physical and biological properties under a land-use sequence in the semiarid region of central Argentina Elke Noellemeyer *, Federico Frank, Cristian Alvarez, German Morazzo, Alberto Quiroga Facultad de Agronomı´a, Universidad Nacional de La Pampa, C.C. 300, RA-6300 Santa Rosa, La Pampa, Argentina Received 25 June 2007; received in revised form 11 December 2007; accepted 8 February 2008
Abstract Land-use change affects vast areas of the semiarid region of central Argentina, where agriculture becomes predominant over mixed farming systems, and large areas of permanent pastures (PAS) are being converted to agricultural land. This land-use change causes loss of soil structure, but very little is known about the effect of changes in aggregate size distribution on soil physical, chemical and biological properties. We decided to use dry sieved aggregates since this technique is commonly used in semiarid regions. The study was carried out at Anguil, La Pampa, Argentina. The soil was a sandy loam Entic Haplustoll with a carbonatefree A-horizon. The PAS site had been under weeping love grass for more than 40 years. Parts of this PAS were turned to cultivation in 1989 (CULT14) and in 2001 (CULT2). Sampling was carried out at 0.6 m intervals to 0.18 m depth. Bulk density (BD), organic carbon (OC), and water holding capacity and infiltration were determined on these samples. Dry aggregate size distribution and OC content of the size fractions were determined on large undisturbed samples. Samples of pooled aggregate size fractions >4, 1–4, and <1 mm, as well as corresponding samples of non fractionated soil were incubated and respiration was measured by CO2 evolved. The soil of CULT2 had 29% lower contents of large (>4 mm) and 37% higher contents of very small (<1 mm) aggregates than PAS. The intermediate size aggregates were not affected by the short-term effect of tillage. OC loss in CULT2 was 16% regarding PAS. Longer term effects of cultivation were characterized by 30% loss of intermediate size aggregates, 22% increase of bulk density, 74 and 19% decrease in water infiltration and water retention, respectively of CULT14 compared to PAS. A 32% decrease of OC was observed after 14 years of cultivation. Intermediate size aggregates had highest OC contents and no difference between treatments was found, except for a lower value of large aggregates in CULT14. Respiration rates and total CO2 evolved was related to OC contents of fractions; however, PAS respired more from its small aggregates than expected from their OC content. The results showed that OC turnover and loss of aggregation was very fast in this soil, but soil hydraulic properties were affected in the longer term. Dry aggregates were found to useful for studying soil degradation, and they showed similar trends as those indicated in the literature for water stable aggregates. # 2008 Elsevier B.V. All rights reserved. Keywords: Land-use change; Semiarid Argentina; OC turnover; Dry aggregate size changes; Physical properties; Respiration rates
1. Introduction * Corresponding author. Tel.: +54 2954 433093x104. E-mail address: [email protected] (E. Noellemeyer). 0167-1987/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2008.02.003
In the semiarid region of central Argentina agriculture becomes increasingly dominant over the traditional mixed farming and animal husbandry system, as large
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areas of permanent pastures (PAS) are being converted to cropland. Soil carbon losses due to cultivation and tillage on virgin soils are reported to be in the order of 10–55% (Brown and Lugo, 1990; Burke et al., 1989). This ample range might be due to climate and texture differences, with highest losses in sandy soils in semiarid arid regions (Balesdent et al., 2000). Various authors reported soil C losses in different semiarid regions ranging from 35 to 56% (Zach et al., 2006; Elberling et al., 2003) after 3–5 years of cultivation. Cultivation also changes the soils’ aggregate size distribution and stability, which has been related to soil organic matter and microbial activity in numerous studies (Balesdent et al., 2000; Paustian et al., 2000; Six et al., 2000a, 2000b; Beare et al., 1994; Dinel et al., 1992). In general, water stable aggregates smaller than 250 mm obtained by wet sieving have been used in these studies, and conceptual frameworks about aggregate hierarchy, related pore sizes and the relationship between aggregate turnover and C dynamics have been developed (Six et al., 2004). However, the soil’s surface in its natural state is formed by dry aggregates ranging in size from less than 1 mm to more than 8 mm. This is specifically true for sandy loam and loam soils in semiarid regions that developed under grassland vegetation. Pena Zubiate et al. (1980) described the soil structure in the semiarid Pampa as predominantly medium to large sub-angular blocks. Dry sieved aggregates could therefore be considered to represent more truly the actual state of aggregation and soil structure, and differences in size of these aggregates have been associated with the effect of different tillage practices (Hevia et al., 2007). They are obtained by gentle hand manipulation followed by sieving (Douglas and Goss, 1982; Chepil, 1953) as in contrast to water stable aggregates fractionated by suspension and wet sieving. Dry sieved aggregates have mainly been used in wind erosion studies (Diaz Zorita et al., 2002; Zobeck, 1991) and few references exist about their use to evaluate the effect of management on soil aggregate stability (Eynard et al., 2004; Martens, 2000) and on C sequestration in these aggregates (Holeplass et al., 2004). In the context of the semiarid Pampa of central Argentina the most common land-use change is the cultivation of very old PASs (mainly Eragrostis curvula) for cash cropping. The climatic variations that occurred during the past three decades, characterized by higher rainfall during the summer (Sierra et al., 2001), induced farmers to increase the area of summer crops at the expense of these long established PASs. Based on the concepts developed for water stable aggregates we supposed that in the sandy to sandy loam soils of this region, the conversion of PAS into
cultivated lands would cause rapid change of dry aggregate size distribution, carbon loss and associated deterioration of soil physical properties such as the soil’s pore system, water dynamics and susceptibility to erosion. We also hypothesized that C losses would affect the biological activity of the soil. The objective of the present study therefore was to evaluate short and longer term effects of cultivation on soil structure and related physical properties, carbon stocks and their distribution in different size dry aggregates. 2. Materials and methods 2.1. Study area The study was carried out in 2003 on three adjacent fields at INTA (National Institute for Agricultural Technology) Experimental Station in Anguil (368350 1900 S and 638570 4600 W), in the center of the semiarid Pampa of Argentina (Fig. 1). The soil was a sandy loam Entic Haplustoll with a carbonate-free Ahorizon of approximately 25 cm depth. The PAS site had been under weeping love grass for more than 40 years. A 25-ha part of this PAS was ploughed in 1989 (CULT14) and then cultivated with a rotation of wheat (Triticum aestivum); oats (Avena sativa); sunflower (Elianthus annuus) and alfalfa (Medicago sativa) PAS with conventional tillage (disk plough). This field had been under alfalfa PAS for the last 3 years when the sites were sampled. Another 12 ha part of the original PAS was ploughed in 2001 and cultivated with one crop of forage oats and one crop of sunflower, both with conventional tillage before sampling (CULT2). The three treatments thus reflected a land-use time sequence under the same soil and climatic conditions. Soil properties of the three sites are shown in Table 1. 2.2. Soil sampling and analysis All soil samples were collected at six points at 20 m distance each, along an N–S linear transect at each site. This sampling method was chosen in order to create pseudo-replicates of each treatment, since true replicates were impossible to obtain given that only one field represented each treatment. Sampling was carried out with cylinders (471.24 cm3 volume) at 0–0.06, 0.06– 0.12 and 0.12–0.18 m depth, all within the limits of the A-horizon and within the tillage depth of the disk plow. Samples were air dried, passed through a 2 mm sieve and weighed, and bulk density (BD) of the soil was calculated. Organic carbon (OC) was determined by wet
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Fig. 1. Map of La Pampa, Argentina.
oxidation with sodium dichromate and sulphuric acid at 120 8C and titration of the CO2 trapped in NaOH (Snyder and Trofimov, 1984). Water infiltration was also measured at the six sampling points in each treatment using the double ring method as described by Ferna´ndez et al. (1971). Infiltrated water was determined after 1, 10, 20, 30, 40, 50 and 60 min, and the results were expressed as accumulated infiltration (mm h1). Initial soil water contents of the treatment sites were 10.5, 10.2 and 12.3% in the first 0.20 m for PAS, CULT2 and CULT14, respectively. Water holding capacity was determined in 1 m2 plots in each treatment, at the same points where Table 1 Basic soil properties of the A-horizon in the three adjacent fields corresponding to the three treatments studied
1
C (g kg ) N (g kg1) P (mg kg1) Ca (mmol p+ kg1 soil) Mg (mmol p+ kg1 soil) K (mmol p+ kg1 soil) Na (mmol p+ kg1 soil) CEC (mmol p+ kg1 soil) Base saturation (%)
CULT14
CULT2
PAS
12.3 b 0.86 b 10.4 a 99 b 30 a 3.7 a 29 a 191 b 84.5 a
12.8 b 0.93 b 4.8 b 122 a 23 a 2.6 b 23 b 200 a 87.7 a
16.5 a 1.1 a 6.7 b 131 a 30 a 3.7 a 23 b 203 a 90.3 a
infiltration was measured. After the infiltration assay, when the soils were saturated to at least 0.40 m depth, the soil surface was covered with polyethylene to prevent evaporation. Samples were collected after 3, 6, 8 and 12 days and their volumetric water content was determined by weighing moist and oven-dried (60 8C) samples. Large undisturbed samples to a depth of 0.20 m (1413.72 cm3 cylinders) were collected at six points along the sampling transect for aggregate size distribution determination. Air-dried samples were manually disaggregated with very gentle pressure through rupture across the natural planes of weakness (Arshad et al., 1996). The same technician skilled in this procedure processed all samples. Then the samples were shaken through a battery of 8, 4, 3, 2 and 1 mm diameter sieves during 30 min. Soil mass retained by each sieve was weighed and C content of these aggregate size fractions was determined by the same method as for OC. 100 g samples of pooled aggregate size fractions >4, 1–4, and <1 mm, as well as corresponding samples of non fractionated (entire) soil were incubated in closed recipients in a growth chamber at 24 8C and at 80% of their water holding capacity. The respired CO2 was trapped in 1N NaOH and the excess was titrated with 0.1N HCl. Determination of CO2 was realized daily at
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Table 2 Dry aggregate size distribution Size classes (mm)
CULT14 CULT2 PAS
Dry aggregate size distribution (g kg1 soil) >8
4–8
3–4
2–3
1–2
<1
281.4 a 160.4 b 226.9 a
156.4 a 114.6 b 147.6 b
85.8 b 116.7 ab 147.4 a
68.4 b 84.1 ab 95.6 a
76.9 b 113.9 a 122.6 a
331.1 b 409.6 a 259.8 b
References: soil mass (g kg1) in the aggregate size classes. Different letters indicate significant differences ( p <0.05) within a column (n = 6).
the beginning of the experiment and less frequently when respiration rates slowed down. When rates had stabilized no further measurements were taken. 2.3. Statistical analysis Mean values of the six replicates of all variables were compared using one-way analysis of variance and separated by the Fisher LSD test at the 95% confidence level. CO2 incubation data were logtransformed when necessary, and regression lines were compared according to the method proposed by Sokal and Rohlf (1968). 3. Results 3.1. Soil physical properties Dry aggregate size distribution (Table 2) differed according to the history of land-use of the soil. The soil with a short period of cultivation (CULT2) had lower contents of large (>4 mm) and higher contents of very small (<1 mm) aggregates than both the PAS and longterm cultivated (CULT14) soil. The intermediate size aggregates were very little affected by the short-term
effect of tillage, since values for CULT2 and PAS were similar. However, CULT14 showed significantly lower values, which might have been due to the longer term effect of tillage on aggregate size distribution. The decrease in medium size aggregates in this site was considerably higher than that of CULT2, with losses of 42, 29 and 37% for the 3–4, 2–3 and 1–2 mm size classes, respectively. The relatively high values of large aggregates in CULT14 may be attributed the effect of the alfalfa PAS, which covered this soil for the last 3 years. The highest water infiltration rate was found in CULT2, whereas CULT14 had a considerably lower rate than both other treatments (Fig. 2), even during the first 10 min. Accumulated infiltration of CULT14 was 74% lower than that of PAS, and CULT2 showed a 38% increase (Table 3) with respect to the latter. Soils reached steady state of water movement between 8 and 12 days, when no further water drainage occurred due to gravity forces. Soil water content at this moment (Table 3) can be considered as the water holding capacity as measured in field conditions (Fuentes Yagu¨e, 1996). Water holding capacity was 22.5% (w/w) in CULT14, whereas both CULT2 and PAS had significantly higher values of 25.5 and 27.6%, respectively. However, when these values were converted to volumetric water contents, due to the differences in BD between CULT14 and both other treatments, no significant difference was found (27.4%, 25.5% and 28.7% for CULT14, CULT2 and PAS respectively).
Table 3 Soil physical properties
CULT14 CULT2 PAS Fig. 2. Accumulated water infiltration in the field (mm) after 1 h. References: error bars represent standard error of means (n = 6).
BD (Mg m3)
WC 12 (%, w/w)
I (mm h1)
1.22 a 1.00 b 1.04 b
22.5 b 25.5 a 27.6 a
100.7 c 540.5 a 391.3 b
References: bulk density (BD), water content 12 days after saturation (WC 12) and infiltration (I). Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
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Table 4 Organic carbon (C) Depth (cm)
CULT14 CULT2 PAS
C content (g kg1)
C mass (Mg ha1)
0–6
6–12
12–18
0–6
6–12
12–18
11.5 c 14.3 b 17.0 a
10.4 a 12.0 a 12.8 a
8.5 a 9.4 a 9.7 a
8.4 b 8.6 b 10.6 a
7.4 a 7.6 a 9.1 a
6.0 a 6.2 a 6.4 a
References: total C content (g kg1) and volumetric C mass (Mg ha1) of the soil. Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
Table 5 Organic carbon (C) of different aggregate size fractions Size classes (mm)
CULT14 CULT2 PAS
C content (g kg1)
C mass (Mg ha1)
>4
1–4
<1
>4
1–4
<1
8.6 b 11.7 a 12.1 a
23.6 a 16.9 a 20.7 a
10.9 a 12.3 a 12.4 a
9.2 a 6.4 b 9.2 a
13.1 a 10.6 b 15.4 a
8.8 b 10.1 a 6.6 a
References: total C content (g kg1) and volumetric C mass to a depth of 20 cm (Mg ha1) of the different aggregate size fractions. Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
3.2. Organic matter Organic C in the uppermost depth layer (0.00– 0.06 m) decreased from 17.0 g kg1 in PAS to 14.3 g kg1 in CULT2 and 11.5 g kg1 in CULT14 (Table 4). Carbon losses in this layer due to cultivation were 16% for CULT2 and 32% for CULT14. However, there were no significant differences among treatments in the two deeper layers (0.06–0.12 and 0.12–0.18 m). When these values were transformed to mass per hectare, OC losses were 4.3 and 3.7 Mg ha1 for CULT14 and CULT2, respectively. The OC content of aggregate size fractions ranged from 8.6 to 12.1 g kg1 in >4 mm aggregates; from 16.9 to 23.6 g kg1 in 1–4 mm aggregates, and from 10.9 to 12.4 g kg1 in <1 mm aggregates (Table 5). There were no significant differences in OC content of aggregate size fractions among treatments, with the exception of the lower value of >4 mm aggregates in CULT14. The highest OC contents were found in the 1–4 mm size class in all treatments, with an average value of 20.4 g kg1. The two other size classes had very similar average values of 11.9 and 10.8 g kg1 for <1 and >4 mm, respectively. We then calculated OC mass per hectare contained in these aggregate fractions by multiplying the OC concentration in each fraction by its weight proportion, considering the sampling depth of 0.20 m and the average bulk density: OC (kg Mg1) weight of aggregate fraction (kg Mg1) 2000 m3 ha1 average bulk density (Mg m3). The results are also shown in Table 5. This way
of expressing the data made long- and short-term effects more evident: CULT2 had the highest amount of OC in the <1 mm fraction and least in the largest aggregates. On the other hand, CULT14, showed a similar distribution of aggregate size classes as PAS. 3.3. Biological properties In order to characterize the biological activity and availability of C pools contained in the different size Table 6 Accumulated CO2 production after 17 days Aggregate size class
Treatment
CO2-17 (mg C kg1 soil)
>4 mm
CULT14 CULT2 PAS
177.0 b 178.2 b 302.3 a
1–4 mm
CULT14 CULT2 PAS
565.4 a 377.8 b 479.5 ab
<1 mm
CULT14 CULT2 PAS
182.0 b 211.0 b 466.1 a
Entire soil
CULT14 CULT2 PAS
351.4 a 414.7 a 386.6 a
References: accumulated CO2 production after 17 days of incubation (mg C kg1 soil). Different letters indicate significant differences ( p < 0.05) within a column and within an aggregate size fraction (n = 6).
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Fig. 3. Respiration of different aggregate size classes and entire soil. References: values represent the average of six replicates.
fractions incubation assays for pooled fractions of large (>4 mm), intermediate (1–4 mm) and small (<1 mm) aggregates were carried out. We expected that soil fragments would show different respiration rates according to their difference in C contents. The total amounts of CO2-C evolved after 17 days of incubation (Table 6) confirmed this hypothesis. The intermediate (1–4 mm) aggregate size fraction showed the highest values with an average of 474.2 mg C kg1 soil for all three treatments, while >4 and <1 mm size classes had averages of nearly half that value (219.2 and 286.4 mg C kg1 soil, respectively). CULT2 had the lowest CO2-C production (377.8 g C kg1 soil) in the intermediate aggregate size fraction while both CULT14 and PAS had similar and higher values. This trend coincided with the respective aggregate size OC contents of the treatments. No such trend was observed in the largest size fraction where PAS had significantly higher CO2-C production (302.3 mg kg1) than CULT14 (177.0 mg kg1) and
CULT2 (178.2 mg kg1), whereas the OC contents of this fraction were similar between PAS and CULT2. In the <1 mm fraction PAS also produced significantly more CO2-C (466.1 mg kg1) than both other treatments, which also could not be predicted from its OC contents. Accumulated CO2 respiration from the entire soil did not differ among treatments, and the values were considerably lower compared to the sum of aggregate size fraction CO2-C respiration for each treatment. These sums were 924, 766 and 1247 mg C kg1 for CULT14, CULT2, and PAS, respectively, while the values for entire soil of these treatments were 351, 414, and 386 mg C kg1, respectively. Within each site, the aggregate size fractions also differed in their respiration rates (Fig. 3). A steep curve and very rapid depletion of evolved CO2 was observed in the >4 mm class. On the contrary, <1 mm aggregates showed the lowest slope and also the longest period of respiration.
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Fig. 4. Regression lines for CO2 production of entire soil and different aggregate size fractions.
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Within each fraction, we also found different slopes for PAS, CULT2 and CULT14 (Fig. 4). With the exception of >4 mm aggregates of CULT2 and CULT14, all data sets had to be log-transformed in order to obtain regression lines that adjusted to a linear model with R2 ranging from 0.85 to 0.96. Statistical comparison of these regression lines revealed that only >4 mm aggregates of CULT2 and CULT14 had comparable regression lines ( p = 0.60), with similar slopes ( p = 0.87) that could be considered superposed ( p = 0.85). All other regression lines were different between treatments. The contribution of total CO2-C produced in 17 days by each aggregate fraction was calculated considering their weight proportion and CO2-C evolved during incubation (Fig. 5). The 1–4 mm aggregate fraction contributed most to respiration in all treatments, ranging from 42% in PAS to 49% in CULT14. Larger aggregates contributed similar amounts in PAS and CULT14 (28 and 29%, respectively) while in CULT2 this fraction made up only 19%. These trends were expected considering the OC content and weight proportion of these fractions. On the other hand, smaller than 1 mm aggregates decreased in their contribution according to land use from 30% in PAS to 22% in CULT14. This was not related to their OC content and aggregate size distribution. The total CO2-C production was calculated by the sum of aggregates’ contributions (Fig. 5), considering the partial contributions of each class, and it showed great differences among treatments. PAS had significantly higher values than both cultivated soils, and there were no significant differences between the latter. When compared to the entire soil, only PAS had similar values of aggregates’ sum and undisturbed (entire) sample. On the other hand, the CO2-C released by CULT14 and
Fig. 5. Contribution of different aggregate size classes to total CO2 (mg C kg1 soil) produced in 17 days, compared to the entire soil sample.
CULT2’s entire soil was higher than their sum of aggregates’ respiration. 4. Discussion Land-use change from a permanent PAS to cultivation caused important changes in the size distribution of aggregates. After 2 years of tillage, large aggregates had decreased by 29% compared to the PAS. At the same time, small aggregates (<1 mm) increased by 37%. The losses of aggregates in the intermediate size classes ranged from 21, 12 and 7% for 3–4, 2–3 and 1–2 mm aggregates, respectively. This suggested that larger soil fragments contributed more to the loss of soil structure, and that cultivation produced an immediate increase of small soil particles, especially those that are susceptible to wind erosion (<0.84 mm). The longer term effect of cultivation on this soil was more related to intermediate size aggregates in the 1 to 4 mm size classes, which were significantly lower in CULT14 than in the permanent PAS, whereas the proportion of largest and smallest aggregates was similar between these treatments. The relatively high values of large aggregates in CULT14 may be attributed the effect of the alfalfa PAS, which covered this soil for the last 3 years. A recuperation of soil structure under PAS and under no-till agriculture has often been observed. For instance, Paustian et al. (2000) and Six et al. (2000b) found increases in water stable aggregates and soil OC in soils that were not tilled. Our data indicated that the same might hold true for dry sieved aggregates. The distribution of aggregate size classes among our landuse sequence treatments was very different since the size fraction which represented the smallest amount of soil mass was different in each treatment: PAS had 26% of small aggregates, CULT2 had 28% of large aggregates and 23% of the soil under CULT14 was found in intermediate ones. It has been reported by various authors that tillage destroys water stable aggregates and specifically affects the larger soil aggregates, which are considered to be less stable (Beare et al., 1994; Kay, 1990; Elliott, 1986; Tisdall and Oades, 1982; Van Veen and Paul, 1981). This could be applied to our results as a short-term consequence of cultivation. However, the longer term effect on intermediate size aggregates we observed could not be compared with results obtained with water stable aggregates. There are few studies on the dynamics of dry sieved aggregates to be found in the literature. For instance, Eynard et al. (2004) found smaller mean weight diameters in dry aggregates of perennial PASs on Ustolls and Uderts of South Dakota
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than in corresponding no-till and tilled plots. Drury et al. (2004) also found higher proportion of larger aggregates in continuous corn than under rotation corn in a clay loam, typic Argiaquoll. The apparent contradiction with our results might be explained by the climatic regime and clay contents that favor high carbon stocks and corresponding fine granular structures in Ustolls and Argiaquolls under land-use systems that have relatively high inputs of plant residues. In Udolls of the semiarid Pampa large sub-angular blocks are formed as predominant structural units under PASs and native vegetation, while carbon losses due to cultivation lead to predominance of smaller less stable aggregates and soil compaction (Quiroga et al., 1998). Thus, the effect of texture and environmentally determined carbon stocks of the soil have to be considered when comparing the results of dry sieved aggregate size distributions and these have to be related to the structure type conditioned by soil genesis. Drury et al. (2004) already suggested that their results showed that indigenous soil properties exerted a greater influence on aggregate size distribution than cropping history. After 14 years of cultivation, soil water holding capacity decreased by nearly 19%. This change considerably affects the soils potential for crop productivity, especially in a semiarid region where long drought periods are common. The loss of water retention might be related to the significantly lower proportion of aggregates in the 1–4 mm size class in CULT14. Following Elliott and Coleman’s (1988) theory of hierarchical pore categories that correspond to aggregate size classes as their mirror images, this would indicate that the intermediate size aggregates have associated pore sizes which play an important role in defining water retention. Dexter (2004) showed that soil microstructure is responsible for most physical soil properties and that structural porosity is directly related to water holding capacity. In the case of the sandy loams of the semiarid Pampa, water holding structural porosity is apparently more related to medium size aggregates (1–4 mm). The decrease of structural porosity under cultivation (CULT14) was also confirmed by a 22% increase of bulk density in CULT14 relative to PAS and CULT2. For the case of water infiltration, the short-term effect of cultivation was positive, facilitating water capture and translocation in the soil. The longer term effect of agricultural use however, greatly diminished the soil’s capacity to infiltrate water and could lead to runoff losses and decreased water use efficiency. Thus, water retention, aggregate size class, bulk density and
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water infiltration data revealed significant reduction of structural porosity due to 14 years of cultivation. On the other hand, the short-term effect of cultivation on structural porosity and soil hydraulic properties was very small, despite the considerable loss of large and increase of very small aggregates that are most susceptible to wind erosion. Organic C losses were 4.3 and 3.7 Mg ha1 for CULT14 and CULT2, respectively. About half of this decrease was due to the OC loss in the uppermost 0.06 m soil layer (2.2 and 2.0 Mg ha1 for CULT14 and CULT2, respectively). The apparent OC loss rate during the first 2 years of cultivation was 1.85 Mg C ha1 year1, while the average rate after 14 years of cultivation under a rotation with alfalfa PAS was 0.31 Mg C ha1 year1. Considering 26.1 Mg ha1 as a stable OC content in this soil its average half life would be approximately 42 years, but during the first few years of cultivation a significantly lower half life of 7 years was found. These data confirmed that OC turnover rates in soils of the semiarid Pampa are high, with half lives of approximately 10 years and no evidence of long-term stabilized C (Zach et al., 2006). Apparently, most of the C was lost during a short period following cultivation of the PAS, and even under rotations that include alfalfa (as in CULT14) a further decrease was observed. Angers et al. (1992) reported an increase in OC content upon conversion of continuous corn to alfalfa PAS, which might indicate that under continuous cropping, C degradation would have been even more severe than that observed in CULT14. Carbon contents in aggregates size fractions were very similar among treatments, and only CULT14 had significantly lower OC contents in the largest aggregate fraction. While our results did not show a clear effect of land use on OC contents of aggregate fractions, Drury et al. (2004) reported higher OC contents in all >0.25 mm aggregate fractions under rotation, compared with corn monoculture, and Holeplass et al. (2004) also found higher OC concentration in these aggregate sizes under grain – PAS rotation than under all – grain production systems. The highest OC content among aggregate fractions was found in the pooled 1–4 mm size class in all treatments. For water stable aggregates, Holeplass et al. (2004) found a trend of increasing OC concentration with decreasing aggregate size, while Saroa and Lal (2003) reported that OC increased with increasing aggregate size. This trend reflects the concept of aggregate hierarchy proposed by Tisdall and Oades (1982). Our results, however, did not agree with this theory, since intermediate size aggregates had higher OC contents than both other fractions. Zotarelli et al.
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(2005) explained the lack of aggregate hierarchy as shown by a similar OC content across aggregate size fractions as the effect of principal binding agents other than organic matter. In their study on water stable aggregates in low activity clay oxisols under different cropping systems, these authors conclude that the OC loss from natural vegetation to conventional tillage can only partly be explained by the loss of C-rich macroaggregates and an increase in C-poor microaggregates, since both fractions did not differ in their OC concentrations. Thus, several authors reported divergences from the aggregate hierarchy model under different soil conditions even for aggregates separated by wet sieving. Our data suggested that for the dry sieved aggregate size classes we studied, the most important fraction for OC stabilization would be the intermediate class (1–4 mm), due to its higher OC concentration and the observed decrease of this aggregate size fraction after prolonged agricultural use. The lower OC contents of >4 and <1 mm fractions (not significant) in CULT14 would suggest that in these dry sieved aggregate classes the conceptual model of Six et al. (1998), which states that in tillage agroecosystems less C is sequestered due to higher macro aggregate turnover that lowers the rate of micro aggregate formation within macro aggregates also could be valid. Incubation assays showed that on the average the OC content of the aggregate size fraction was related to the amount of CO2-C evolved by the fraction. The intermediate size class had highest OC contents and also produced most CO2-C. Both for OC contents and CO2-C production this fraction showed values almost twice as high as both >4 and <1 mm fraction. These results do not agree with those of Drury et al. (2004), who also used dry sieved aggregates to evaluate the impact of aggregation on biological processes under rotation and continuous cropping. They found an increase of CO2 production with decreasing aggregate size, but they also reported a strong relation between CO2 respiration and POC contents, especially under rotation. Our results however did not show a clear effect of treatment on the OC content of the aggregate size fractions, nor on respiration. Whereas the low CO2-C production of CULT2 in the 1–4 mm fraction could be associated to its low OC content, this was not true for CO2-C in the >4 and <1 mm fractions. In both cases OC contents were similar between CULT2 and PAS, while CO2-C production was similar for CULT14 and CULT2. This might indicate a similar nature of OC in these fractions between CULT14 and CULT2. Short and
longer term effects of cultivation on the biological behavior of the aggregate size fractions therefore could not be distinguished. The comparison between the CO2-C production of the entire soil and the sum of the aggregate size fractions revealed that size fractions produced far more CO2-C. The smallest difference was found in CULT2 (414.7 and 767.0 mg kg1, for entire soil and aggregate sum, respectively, which represents 1.85 times the amount), while in the case of CULT14, CO2-C production of aggregate sum was 2.63 times that of the entire soil; and for PAS this values was 3.23 times. The lower CO2-C production of entire soil could be attributed to protection of OC due to the undisturbed soil matrix, while aggregate fractionation by dry sieving might have released otherwise protected organic material. Similarly, Drury et al. (2004) found that grinding the aggregates reduced the differences of CO2 production between aggregate size classes and generally enhanced CO2 production. In an attempt to evaluate the contribution of each aggregate size fraction to soil respiration we calculated these amounts pondered by the aggregates’ size fraction’s weight proportion and found that only PAS had similar values between the sum of pondered aggregate contribution and entire soil CO2 production. Both cultivated treatments had higher entire soil CO2 production than the pondered sum of aggregates production. This result was astonishing, since the sum of CO2 production of separate aggregate fractions was considerably higher than entire soil respiration. However, taking into account their contribution to total soil mass this trend is reversed and no explication for this apparent contradiction could be adventured. A steep curve and very rapid depletion of evolved CO2 was observed in the >4 mm class, which might be related to a more labile nature of C. On the contrary, <1 mm aggregates showed the lowest slope and also the longest period of respiration. Zhang et al. (2007) studied the respiration rates of different OC fractions and found that labile fractions showed steeper slopes than heavy C fraction and whole soil OC. The difference of respiration rates of aggregate size fractions among treatments that became evident in the comparison of regression lines therefore could indicate differential substrate availability and/or differences in microbial community. The major differences were between PAS and both cultivated treatments, suggesting that despite the positive effect of the alfalfa PAS on OC contents and aggregate size distribution in CULT14, the biological activity had not been recovered.
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5. Conclusions Land-use change from permanent PAS to arable agriculture greatly affected OC stocks and all tested physical and biological soil properties. Cultivation caused a decrease of structural porosity, which was related to higher bulk density and decreased infiltration rate. This process might be divided into a shortterm disruption of large aggregates, while the longterm effect appeared to be the decrease of intermediate size ones. The latter was reflected in an important reduction of soil’s water retention and infiltration, which could have a negative impact on soil quality and productivity. OC losses were similar between short- and long-term cultivation, which indicated a very fast rate of C turnover in the studied soil. The intermediate size aggregates had the highest OC contents, and also contributed most to total respiration. Differences in land-use history only became evident in aggregate size fractions, while entire soil samples did not show different respiration rates or total CO2 production. Our results showed that dry sieved aggregates provide meaningful fractions for studies of the impact of land use on soil physical and biological properties. Acknowledgement This work was financed by the Interamerican Institute for Global Change Research (IAI) within the collaborative research networks CRN 001 and 2031. References Angers, D., Peasent, A., Vigneux, J., 1992. Early cropping-induced changes in soil aggregation, organic matter, and microbial biomass. Soil Sci. Soc. Am. J. 56, 115–119. Arshad, M.A., Lowery, B., Grossman, B., 1996. Physical test for monitoring soil quality. In: Doran, J.W., Jones, A.J. (Eds.), Methods for Assessing Soil Quality. Soil Science Society of America, Madison, WI, pp. 123–141. Balesdent, J., Chenu, C., Balabane, M., 2000. Relationships of soil organic matter dynamics to physical protection and tillage. Soil Till. Res. 53, 215–230. Beare, M., Cabrera, M., Hendrix, P., Coleman, D., 1994. Aggregateprotected and unprotected organic matter pools in conventional and no-tillage soils. Soil Sci. Soc. Am. J. 58, 787–795. Brown, S., Lugo, A., 1990. Effects of forest clearing and succession of the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands. Plant Soil 124, 53–64. Burke, I., Yonker, C., Parton, W., Cole, C., Flach, K., Schimel, D., 1989. Texture, climate and cultivation effects on soil organic matter content in US Grassland soils. Soil Sci. Soc. Am. J. 53, 800–805.
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Chepil, W.S., 1953. Field structure of cultivated soils with special reference to erodibility by wind. Soil Sci. Soc. Am. Proc. 26, 4–6. Dexter, A.R., 2004. Soil physical quality Part I. Theory, effects of soil texture, density and organic matter and effects on root growth. Geoderma 120, 201–214. Diaz Zorita, M., Perfect, E., Grove, J.H., 2002. Disruptive methods for assessing soil structure. Soil Till. Res. 64, 3–22. Dinel, H., Le´vesque, P., Jambu, P., Righi, D., 1992. Microbial activity and long-chain aliphatics in the formation of stable soil aggregates. Soil Sci. Soc. Am. J. 56, 1455–1463. Douglas, J.T., Goss, M.J., 1982. Stability and organic matter content of surface soil aggregates under different methods of cultivation and in grassland. Soil Till. Res. 2, 155–175. Drury, C.F., Yang, X.M., Reynolds, W.D., Tan, C.S., 2004. Influence of crop rotation and aggregate size on carbon dioxide production and denitrification. Soil Till. Res. 79, 87–100. Elberling, B., Toure´, A., Rasmussen, K., 2003. Changes in soil organic matter following groundnut-millet cropping at three locations in semi-arid Senegal, West Africa. Agric., Ecosyst. Environ. 96, 37–47. Elliott, E.T., 1986. Aggregate structure and carbon, nitrogen and phosphorus in native soils and cultivated soils. Soil Sci. Soc. Am. J. 50, 627–633. Elliott, E.T., Coleman, D.C., 1988. Let the soil work for us. Ecol. Bull. 39, 23–32. Eynard, A., Schumacher, T.E., Lindstrom, M.J., Malo, D.D., 2004. Aggregate sizes and stability in cultivated South Dakota prairie Ustolls and Usterts. Soil Sci. Soc. Am. J. 68, 1360– 1365. Ferna´ndez, P., Luque, J., Paoloni, J., 1971. Ana´lisis de la infiltracio´n y su aplicacio´n para disen˜os de riego en el valle inferior del Rı´o Colorado. Revista de Investigaciones Agrarias (Argentina), serie 3, vol. 8, pp. 1–29. Fuentes Yagu¨e, J.L., 1996. Te´cnicas de Riego. Mundi Prensa Madrid, p. 471. Hevia, G.G., Me´ndez, M., Buschiazzo, D.E., 2007. Tillage affects soil aggregation parameters linked with wind erosion. Geoderma 140, 90–96. Holeplass, H., Singh, B.R., Lal, R., 2004. Carbon sequestration in soil aggregates under different crop rotations and nitrogen fertilization in an inceptisol in southeastern Norway. Nutr. Cycl. Agroecosys. 70, 167–177. Kay, B., 1990. Rates of change of soil structure under different cropping systems. Adv. Soil Sci. 12, 1–52. Martens, D.A., 2000. Management and crop residue influence soil aggregate stability. J. Environ. Qual. 29, 723–727. Pena Zubiate, C., Maldonado Pinedo, D., Martinez, H., Hevia, R., 1980. Suelos. In: Inventario Integrado de los Recursos Naturales de la Provincia de La Pampa. INTA – UNLPam – Gobierno de La Pampa. Santa Rosa, L.P., Argentina. Paustian, K., Six, J., Elliott, E.T., Hunt, H., 2000. Management options for reducing CO2 emissions from agricultural soils. Biogeochemistry 48, 147–163. Quiroga, A.R., Buschiazzo, D.E., Peinemann, N., 1998. Management discriminant properties in semiarid soils. Soil Sci. 163, 591–597. Saroa, G.S., Lal, R., 2003. Mulching effects on aggregation and carbon sequestration in a Miamian soil in central Ohio. Land Degrad. Dev. 14, 481–493. Sierra, E., Pe´rez, S., Casagrande, G., Vergara, G., 2001. Efectos del ENSO sobre las precipitaciones del trimestre noviembre-enero (1921/1998) en el centro-este de la provincia de La Pampa
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(Argentina). Revista Argentina De Agrometeorologı´a (Argentina) 1, 83–87. Six, J., Elliott, E.T., Paustian, K., Doran, J.W., 1998. Aggregation and soil organic matter accumulation in cultivated and native grassland soils. Soil Sci. Soc. Am. J. 62, 1367–1377. Six, J., Elliott, E., Paustian, K., 2000a. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem. 32, 1099–2103. Six, J., Elliott, E.T., Paustian, K., Combrink, C., 2000b. Soil structure and soil organic matter: I. Distribution of aggregate size classes and aggregate associated carbon. Soil Sci. Soc. Am. J. 64, 681–689. Six, J., Bossuyt, H., Degryze, S., Denef, K., 2004. A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics. Soil Till. Res. 79, 7–31. Snyder, J.D., Trofimov, J.A., 1984. A rapid accurate wet oxidation diffusion procedure for determining organic and inorganic carbon in plant and soil samples. Commun. Soil Sci. Plant Anal. 15, 587–597.
Sokal, R., Rohlf, F., 1968. Biometrı´a. Principios y me´todos estadı´sticos en la investigacio´n biolo´gica. H. Blume Ediciones, Madrid, Espan˜a. Tisdall, J., Oades, M., 1982. Organic matter and water stable aggregates in soils. Soil Sci. 33, 141–161. Van Veen, J., Paul, E., 1981. Organic carbon dynamics in grasslands soils. Background information and computer simulation. Can. J. Soil Sci. 61, 185–201. Zach, A., Noellemeyer, E., Tiessen, H., 2006. Carbon turnover and 13C natural abundance under landuse change in the semiarid La Pampa, Argentina. Soil Sci. Soc. Am. J. 70, 1541–1546. Zhang, J., Song, C., Wenyan, Y., 2007. Tillage effects on soil carbon fractions in the Sanjiang Plain, Northeast China. Soil Till. Res. 93, 102–108. Zobeck, T.M., 1991. Soil properties affecting wind erosion. J. Soil Water Conserv. 46, 112–118. Zotarelli, L., Alves, B.J.R., Urquiaga, S., Torres, E., Santos, H.P., Paustian, K., Boddey, R.M., Six, J., 2005. Impact of tillage and crop rotation on aggregate-associated carbon in two oxisols. Soil Sci. Soc. Am. J. 69, 482–491.
Soil & Tillage Research 99 (2008) 179–190 www.elsevier.com/locate/still
Carbon contents and aggregation related to soil physical and biological properties under a land-use sequence in the semiarid region of central Argentina Elke Noellemeyer *, Federico Frank, Cristian Alvarez, German Morazzo, Alberto Quiroga Facultad de Agronomı´a, Universidad Nacional de La Pampa, C.C. 300, RA-6300 Santa Rosa, La Pampa, Argentina Received 25 June 2007; received in revised form 11 December 2007; accepted 8 February 2008
Abstract Land-use change affects vast areas of the semiarid region of central Argentina, where agriculture becomes predominant over mixed farming systems, and large areas of permanent pastures (PAS) are being converted to agricultural land. This land-use change causes loss of soil structure, but very little is known about the effect of changes in aggregate size distribution on soil physical, chemical and biological properties. We decided to use dry sieved aggregates since this technique is commonly used in semiarid regions. The study was carried out at Anguil, La Pampa, Argentina. The soil was a sandy loam Entic Haplustoll with a carbonatefree A-horizon. The PAS site had been under weeping love grass for more than 40 years. Parts of this PAS were turned to cultivation in 1989 (CULT14) and in 2001 (CULT2). Sampling was carried out at 0.6 m intervals to 0.18 m depth. Bulk density (BD), organic carbon (OC), and water holding capacity and infiltration were determined on these samples. Dry aggregate size distribution and OC content of the size fractions were determined on large undisturbed samples. Samples of pooled aggregate size fractions >4, 1–4, and <1 mm, as well as corresponding samples of non fractionated soil were incubated and respiration was measured by CO2 evolved. The soil of CULT2 had 29% lower contents of large (>4 mm) and 37% higher contents of very small (<1 mm) aggregates than PAS. The intermediate size aggregates were not affected by the short-term effect of tillage. OC loss in CULT2 was 16% regarding PAS. Longer term effects of cultivation were characterized by 30% loss of intermediate size aggregates, 22% increase of bulk density, 74 and 19% decrease in water infiltration and water retention, respectively of CULT14 compared to PAS. A 32% decrease of OC was observed after 14 years of cultivation. Intermediate size aggregates had highest OC contents and no difference between treatments was found, except for a lower value of large aggregates in CULT14. Respiration rates and total CO2 evolved was related to OC contents of fractions; however, PAS respired more from its small aggregates than expected from their OC content. The results showed that OC turnover and loss of aggregation was very fast in this soil, but soil hydraulic properties were affected in the longer term. Dry aggregates were found to useful for studying soil degradation, and they showed similar trends as those indicated in the literature for water stable aggregates. # 2008 Elsevier B.V. All rights reserved. Keywords: Land-use change; Semiarid Argentina; OC turnover; Dry aggregate size changes; Physical properties; Respiration rates
1. Introduction * Corresponding author. Tel.: +54 2954 433093x104. E-mail address: [email protected] (E. Noellemeyer). 0167-1987/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2008.02.003
In the semiarid region of central Argentina agriculture becomes increasingly dominant over the traditional mixed farming and animal husbandry system, as large
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areas of permanent pastures (PAS) are being converted to cropland. Soil carbon losses due to cultivation and tillage on virgin soils are reported to be in the order of 10–55% (Brown and Lugo, 1990; Burke et al., 1989). This ample range might be due to climate and texture differences, with highest losses in sandy soils in semiarid arid regions (Balesdent et al., 2000). Various authors reported soil C losses in different semiarid regions ranging from 35 to 56% (Zach et al., 2006; Elberling et al., 2003) after 3–5 years of cultivation. Cultivation also changes the soils’ aggregate size distribution and stability, which has been related to soil organic matter and microbial activity in numerous studies (Balesdent et al., 2000; Paustian et al., 2000; Six et al., 2000a, 2000b; Beare et al., 1994; Dinel et al., 1992). In general, water stable aggregates smaller than 250 mm obtained by wet sieving have been used in these studies, and conceptual frameworks about aggregate hierarchy, related pore sizes and the relationship between aggregate turnover and C dynamics have been developed (Six et al., 2004). However, the soil’s surface in its natural state is formed by dry aggregates ranging in size from less than 1 mm to more than 8 mm. This is specifically true for sandy loam and loam soils in semiarid regions that developed under grassland vegetation. Pena Zubiate et al. (1980) described the soil structure in the semiarid Pampa as predominantly medium to large sub-angular blocks. Dry sieved aggregates could therefore be considered to represent more truly the actual state of aggregation and soil structure, and differences in size of these aggregates have been associated with the effect of different tillage practices (Hevia et al., 2007). They are obtained by gentle hand manipulation followed by sieving (Douglas and Goss, 1982; Chepil, 1953) as in contrast to water stable aggregates fractionated by suspension and wet sieving. Dry sieved aggregates have mainly been used in wind erosion studies (Diaz Zorita et al., 2002; Zobeck, 1991) and few references exist about their use to evaluate the effect of management on soil aggregate stability (Eynard et al., 2004; Martens, 2000) and on C sequestration in these aggregates (Holeplass et al., 2004). In the context of the semiarid Pampa of central Argentina the most common land-use change is the cultivation of very old PASs (mainly Eragrostis curvula) for cash cropping. The climatic variations that occurred during the past three decades, characterized by higher rainfall during the summer (Sierra et al., 2001), induced farmers to increase the area of summer crops at the expense of these long established PASs. Based on the concepts developed for water stable aggregates we supposed that in the sandy to sandy loam soils of this region, the conversion of PAS into
cultivated lands would cause rapid change of dry aggregate size distribution, carbon loss and associated deterioration of soil physical properties such as the soil’s pore system, water dynamics and susceptibility to erosion. We also hypothesized that C losses would affect the biological activity of the soil. The objective of the present study therefore was to evaluate short and longer term effects of cultivation on soil structure and related physical properties, carbon stocks and their distribution in different size dry aggregates. 2. Materials and methods 2.1. Study area The study was carried out in 2003 on three adjacent fields at INTA (National Institute for Agricultural Technology) Experimental Station in Anguil (368350 1900 S and 638570 4600 W), in the center of the semiarid Pampa of Argentina (Fig. 1). The soil was a sandy loam Entic Haplustoll with a carbonate-free Ahorizon of approximately 25 cm depth. The PAS site had been under weeping love grass for more than 40 years. A 25-ha part of this PAS was ploughed in 1989 (CULT14) and then cultivated with a rotation of wheat (Triticum aestivum); oats (Avena sativa); sunflower (Elianthus annuus) and alfalfa (Medicago sativa) PAS with conventional tillage (disk plough). This field had been under alfalfa PAS for the last 3 years when the sites were sampled. Another 12 ha part of the original PAS was ploughed in 2001 and cultivated with one crop of forage oats and one crop of sunflower, both with conventional tillage before sampling (CULT2). The three treatments thus reflected a land-use time sequence under the same soil and climatic conditions. Soil properties of the three sites are shown in Table 1. 2.2. Soil sampling and analysis All soil samples were collected at six points at 20 m distance each, along an N–S linear transect at each site. This sampling method was chosen in order to create pseudo-replicates of each treatment, since true replicates were impossible to obtain given that only one field represented each treatment. Sampling was carried out with cylinders (471.24 cm3 volume) at 0–0.06, 0.06– 0.12 and 0.12–0.18 m depth, all within the limits of the A-horizon and within the tillage depth of the disk plow. Samples were air dried, passed through a 2 mm sieve and weighed, and bulk density (BD) of the soil was calculated. Organic carbon (OC) was determined by wet
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Fig. 1. Map of La Pampa, Argentina.
oxidation with sodium dichromate and sulphuric acid at 120 8C and titration of the CO2 trapped in NaOH (Snyder and Trofimov, 1984). Water infiltration was also measured at the six sampling points in each treatment using the double ring method as described by Ferna´ndez et al. (1971). Infiltrated water was determined after 1, 10, 20, 30, 40, 50 and 60 min, and the results were expressed as accumulated infiltration (mm h1). Initial soil water contents of the treatment sites were 10.5, 10.2 and 12.3% in the first 0.20 m for PAS, CULT2 and CULT14, respectively. Water holding capacity was determined in 1 m2 plots in each treatment, at the same points where Table 1 Basic soil properties of the A-horizon in the three adjacent fields corresponding to the three treatments studied
1
C (g kg ) N (g kg1) P (mg kg1) Ca (mmol p+ kg1 soil) Mg (mmol p+ kg1 soil) K (mmol p+ kg1 soil) Na (mmol p+ kg1 soil) CEC (mmol p+ kg1 soil) Base saturation (%)
CULT14
CULT2
PAS
12.3 b 0.86 b 10.4 a 99 b 30 a 3.7 a 29 a 191 b 84.5 a
12.8 b 0.93 b 4.8 b 122 a 23 a 2.6 b 23 b 200 a 87.7 a
16.5 a 1.1 a 6.7 b 131 a 30 a 3.7 a 23 b 203 a 90.3 a
infiltration was measured. After the infiltration assay, when the soils were saturated to at least 0.40 m depth, the soil surface was covered with polyethylene to prevent evaporation. Samples were collected after 3, 6, 8 and 12 days and their volumetric water content was determined by weighing moist and oven-dried (60 8C) samples. Large undisturbed samples to a depth of 0.20 m (1413.72 cm3 cylinders) were collected at six points along the sampling transect for aggregate size distribution determination. Air-dried samples were manually disaggregated with very gentle pressure through rupture across the natural planes of weakness (Arshad et al., 1996). The same technician skilled in this procedure processed all samples. Then the samples were shaken through a battery of 8, 4, 3, 2 and 1 mm diameter sieves during 30 min. Soil mass retained by each sieve was weighed and C content of these aggregate size fractions was determined by the same method as for OC. 100 g samples of pooled aggregate size fractions >4, 1–4, and <1 mm, as well as corresponding samples of non fractionated (entire) soil were incubated in closed recipients in a growth chamber at 24 8C and at 80% of their water holding capacity. The respired CO2 was trapped in 1N NaOH and the excess was titrated with 0.1N HCl. Determination of CO2 was realized daily at
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Table 2 Dry aggregate size distribution Size classes (mm)
CULT14 CULT2 PAS
Dry aggregate size distribution (g kg1 soil) >8
4–8
3–4
2–3
1–2
<1
281.4 a 160.4 b 226.9 a
156.4 a 114.6 b 147.6 b
85.8 b 116.7 ab 147.4 a
68.4 b 84.1 ab 95.6 a
76.9 b 113.9 a 122.6 a
331.1 b 409.6 a 259.8 b
References: soil mass (g kg1) in the aggregate size classes. Different letters indicate significant differences ( p <0.05) within a column (n = 6).
the beginning of the experiment and less frequently when respiration rates slowed down. When rates had stabilized no further measurements were taken. 2.3. Statistical analysis Mean values of the six replicates of all variables were compared using one-way analysis of variance and separated by the Fisher LSD test at the 95% confidence level. CO2 incubation data were logtransformed when necessary, and regression lines were compared according to the method proposed by Sokal and Rohlf (1968). 3. Results 3.1. Soil physical properties Dry aggregate size distribution (Table 2) differed according to the history of land-use of the soil. The soil with a short period of cultivation (CULT2) had lower contents of large (>4 mm) and higher contents of very small (<1 mm) aggregates than both the PAS and longterm cultivated (CULT14) soil. The intermediate size aggregates were very little affected by the short-term
effect of tillage, since values for CULT2 and PAS were similar. However, CULT14 showed significantly lower values, which might have been due to the longer term effect of tillage on aggregate size distribution. The decrease in medium size aggregates in this site was considerably higher than that of CULT2, with losses of 42, 29 and 37% for the 3–4, 2–3 and 1–2 mm size classes, respectively. The relatively high values of large aggregates in CULT14 may be attributed the effect of the alfalfa PAS, which covered this soil for the last 3 years. The highest water infiltration rate was found in CULT2, whereas CULT14 had a considerably lower rate than both other treatments (Fig. 2), even during the first 10 min. Accumulated infiltration of CULT14 was 74% lower than that of PAS, and CULT2 showed a 38% increase (Table 3) with respect to the latter. Soils reached steady state of water movement between 8 and 12 days, when no further water drainage occurred due to gravity forces. Soil water content at this moment (Table 3) can be considered as the water holding capacity as measured in field conditions (Fuentes Yagu¨e, 1996). Water holding capacity was 22.5% (w/w) in CULT14, whereas both CULT2 and PAS had significantly higher values of 25.5 and 27.6%, respectively. However, when these values were converted to volumetric water contents, due to the differences in BD between CULT14 and both other treatments, no significant difference was found (27.4%, 25.5% and 28.7% for CULT14, CULT2 and PAS respectively).
Table 3 Soil physical properties
CULT14 CULT2 PAS Fig. 2. Accumulated water infiltration in the field (mm) after 1 h. References: error bars represent standard error of means (n = 6).
BD (Mg m3)
WC 12 (%, w/w)
I (mm h1)
1.22 a 1.00 b 1.04 b
22.5 b 25.5 a 27.6 a
100.7 c 540.5 a 391.3 b
References: bulk density (BD), water content 12 days after saturation (WC 12) and infiltration (I). Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
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Table 4 Organic carbon (C) Depth (cm)
CULT14 CULT2 PAS
C content (g kg1)
C mass (Mg ha1)
0–6
6–12
12–18
0–6
6–12
12–18
11.5 c 14.3 b 17.0 a
10.4 a 12.0 a 12.8 a
8.5 a 9.4 a 9.7 a
8.4 b 8.6 b 10.6 a
7.4 a 7.6 a 9.1 a
6.0 a 6.2 a 6.4 a
References: total C content (g kg1) and volumetric C mass (Mg ha1) of the soil. Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
Table 5 Organic carbon (C) of different aggregate size fractions Size classes (mm)
CULT14 CULT2 PAS
C content (g kg1)
C mass (Mg ha1)
>4
1–4
<1
>4
1–4
<1
8.6 b 11.7 a 12.1 a
23.6 a 16.9 a 20.7 a
10.9 a 12.3 a 12.4 a
9.2 a 6.4 b 9.2 a
13.1 a 10.6 b 15.4 a
8.8 b 10.1 a 6.6 a
References: total C content (g kg1) and volumetric C mass to a depth of 20 cm (Mg ha1) of the different aggregate size fractions. Different letters indicate significant differences ( p < 0.05) within a column (n = 6).
3.2. Organic matter Organic C in the uppermost depth layer (0.00– 0.06 m) decreased from 17.0 g kg1 in PAS to 14.3 g kg1 in CULT2 and 11.5 g kg1 in CULT14 (Table 4). Carbon losses in this layer due to cultivation were 16% for CULT2 and 32% for CULT14. However, there were no significant differences among treatments in the two deeper layers (0.06–0.12 and 0.12–0.18 m). When these values were transformed to mass per hectare, OC losses were 4.3 and 3.7 Mg ha1 for CULT14 and CULT2, respectively. The OC content of aggregate size fractions ranged from 8.6 to 12.1 g kg1 in >4 mm aggregates; from 16.9 to 23.6 g kg1 in 1–4 mm aggregates, and from 10.9 to 12.4 g kg1 in <1 mm aggregates (Table 5). There were no significant differences in OC content of aggregate size fractions among treatments, with the exception of the lower value of >4 mm aggregates in CULT14. The highest OC contents were found in the 1–4 mm size class in all treatments, with an average value of 20.4 g kg1. The two other size classes had very similar average values of 11.9 and 10.8 g kg1 for <1 and >4 mm, respectively. We then calculated OC mass per hectare contained in these aggregate fractions by multiplying the OC concentration in each fraction by its weight proportion, considering the sampling depth of 0.20 m and the average bulk density: OC (kg Mg1) weight of aggregate fraction (kg Mg1) 2000 m3 ha1 average bulk density (Mg m3). The results are also shown in Table 5. This way
of expressing the data made long- and short-term effects more evident: CULT2 had the highest amount of OC in the <1 mm fraction and least in the largest aggregates. On the other hand, CULT14, showed a similar distribution of aggregate size classes as PAS. 3.3. Biological properties In order to characterize the biological activity and availability of C pools contained in the different size Table 6 Accumulated CO2 production after 17 days Aggregate size class
Treatment
CO2-17 (mg C kg1 soil)
>4 mm
CULT14 CULT2 PAS
177.0 b 178.2 b 302.3 a
1–4 mm
CULT14 CULT2 PAS
565.4 a 377.8 b 479.5 ab
<1 mm
CULT14 CULT2 PAS
182.0 b 211.0 b 466.1 a
Entire soil
CULT14 CULT2 PAS
351.4 a 414.7 a 386.6 a
References: accumulated CO2 production after 17 days of incubation (mg C kg1 soil). Different letters indicate significant differences ( p < 0.05) within a column and within an aggregate size fraction (n = 6).
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Fig. 3. Respiration of different aggregate size classes and entire soil. References: values represent the average of six replicates.
fractions incubation assays for pooled fractions of large (>4 mm), intermediate (1–4 mm) and small (<1 mm) aggregates were carried out. We expected that soil fragments would show different respiration rates according to their difference in C contents. The total amounts of CO2-C evolved after 17 days of incubation (Table 6) confirmed this hypothesis. The intermediate (1–4 mm) aggregate size fraction showed the highest values with an average of 474.2 mg C kg1 soil for all three treatments, while >4 and <1 mm size classes had averages of nearly half that value (219.2 and 286.4 mg C kg1 soil, respectively). CULT2 had the lowest CO2-C production (377.8 g C kg1 soil) in the intermediate aggregate size fraction while both CULT14 and PAS had similar and higher values. This trend coincided with the respective aggregate size OC contents of the treatments. No such trend was observed in the largest size fraction where PAS had significantly higher CO2-C production (302.3 mg kg1) than CULT14 (177.0 mg kg1) and
CULT2 (178.2 mg kg1), whereas the OC contents of this fraction were similar between PAS and CULT2. In the <1 mm fraction PAS also produced significantly more CO2-C (466.1 mg kg1) than both other treatments, which also could not be predicted from its OC contents. Accumulated CO2 respiration from the entire soil did not differ among treatments, and the values were considerably lower compared to the sum of aggregate size fraction CO2-C respiration for each treatment. These sums were 924, 766 and 1247 mg C kg1 for CULT14, CULT2, and PAS, respectively, while the values for entire soil of these treatments were 351, 414, and 386 mg C kg1, respectively. Within each site, the aggregate size fractions also differed in their respiration rates (Fig. 3). A steep curve and very rapid depletion of evolved CO2 was observed in the >4 mm class. On the contrary, <1 mm aggregates showed the lowest slope and also the longest period of respiration.
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Fig. 4. Regression lines for CO2 production of entire soil and different aggregate size fractions.
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Within each fraction, we also found different slopes for PAS, CULT2 and CULT14 (Fig. 4). With the exception of >4 mm aggregates of CULT2 and CULT14, all data sets had to be log-transformed in order to obtain regression lines that adjusted to a linear model with R2 ranging from 0.85 to 0.96. Statistical comparison of these regression lines revealed that only >4 mm aggregates of CULT2 and CULT14 had comparable regression lines ( p = 0.60), with similar slopes ( p = 0.87) that could be considered superposed ( p = 0.85). All other regression lines were different between treatments. The contribution of total CO2-C produced in 17 days by each aggregate fraction was calculated considering their weight proportion and CO2-C evolved during incubation (Fig. 5). The 1–4 mm aggregate fraction contributed most to respiration in all treatments, ranging from 42% in PAS to 49% in CULT14. Larger aggregates contributed similar amounts in PAS and CULT14 (28 and 29%, respectively) while in CULT2 this fraction made up only 19%. These trends were expected considering the OC content and weight proportion of these fractions. On the other hand, smaller than 1 mm aggregates decreased in their contribution according to land use from 30% in PAS to 22% in CULT14. This was not related to their OC content and aggregate size distribution. The total CO2-C production was calculated by the sum of aggregates’ contributions (Fig. 5), considering the partial contributions of each class, and it showed great differences among treatments. PAS had significantly higher values than both cultivated soils, and there were no significant differences between the latter. When compared to the entire soil, only PAS had similar values of aggregates’ sum and undisturbed (entire) sample. On the other hand, the CO2-C released by CULT14 and
Fig. 5. Contribution of different aggregate size classes to total CO2 (mg C kg1 soil) produced in 17 days, compared to the entire soil sample.
CULT2’s entire soil was higher than their sum of aggregates’ respiration. 4. Discussion Land-use change from a permanent PAS to cultivation caused important changes in the size distribution of aggregates. After 2 years of tillage, large aggregates had decreased by 29% compared to the PAS. At the same time, small aggregates (<1 mm) increased by 37%. The losses of aggregates in the intermediate size classes ranged from 21, 12 and 7% for 3–4, 2–3 and 1–2 mm aggregates, respectively. This suggested that larger soil fragments contributed more to the loss of soil structure, and that cultivation produced an immediate increase of small soil particles, especially those that are susceptible to wind erosion (<0.84 mm). The longer term effect of cultivation on this soil was more related to intermediate size aggregates in the 1 to 4 mm size classes, which were significantly lower in CULT14 than in the permanent PAS, whereas the proportion of largest and smallest aggregates was similar between these treatments. The relatively high values of large aggregates in CULT14 may be attributed the effect of the alfalfa PAS, which covered this soil for the last 3 years. A recuperation of soil structure under PAS and under no-till agriculture has often been observed. For instance, Paustian et al. (2000) and Six et al. (2000b) found increases in water stable aggregates and soil OC in soils that were not tilled. Our data indicated that the same might hold true for dry sieved aggregates. The distribution of aggregate size classes among our landuse sequence treatments was very different since the size fraction which represented the smallest amount of soil mass was different in each treatment: PAS had 26% of small aggregates, CULT2 had 28% of large aggregates and 23% of the soil under CULT14 was found in intermediate ones. It has been reported by various authors that tillage destroys water stable aggregates and specifically affects the larger soil aggregates, which are considered to be less stable (Beare et al., 1994; Kay, 1990; Elliott, 1986; Tisdall and Oades, 1982; Van Veen and Paul, 1981). This could be applied to our results as a short-term consequence of cultivation. However, the longer term effect on intermediate size aggregates we observed could not be compared with results obtained with water stable aggregates. There are few studies on the dynamics of dry sieved aggregates to be found in the literature. For instance, Eynard et al. (2004) found smaller mean weight diameters in dry aggregates of perennial PASs on Ustolls and Uderts of South Dakota
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than in corresponding no-till and tilled plots. Drury et al. (2004) also found higher proportion of larger aggregates in continuous corn than under rotation corn in a clay loam, typic Argiaquoll. The apparent contradiction with our results might be explained by the climatic regime and clay contents that favor high carbon stocks and corresponding fine granular structures in Ustolls and Argiaquolls under land-use systems that have relatively high inputs of plant residues. In Udolls of the semiarid Pampa large sub-angular blocks are formed as predominant structural units under PASs and native vegetation, while carbon losses due to cultivation lead to predominance of smaller less stable aggregates and soil compaction (Quiroga et al., 1998). Thus, the effect of texture and environmentally determined carbon stocks of the soil have to be considered when comparing the results of dry sieved aggregate size distributions and these have to be related to the structure type conditioned by soil genesis. Drury et al. (2004) already suggested that their results showed that indigenous soil properties exerted a greater influence on aggregate size distribution than cropping history. After 14 years of cultivation, soil water holding capacity decreased by nearly 19%. This change considerably affects the soils potential for crop productivity, especially in a semiarid region where long drought periods are common. The loss of water retention might be related to the significantly lower proportion of aggregates in the 1–4 mm size class in CULT14. Following Elliott and Coleman’s (1988) theory of hierarchical pore categories that correspond to aggregate size classes as their mirror images, this would indicate that the intermediate size aggregates have associated pore sizes which play an important role in defining water retention. Dexter (2004) showed that soil microstructure is responsible for most physical soil properties and that structural porosity is directly related to water holding capacity. In the case of the sandy loams of the semiarid Pampa, water holding structural porosity is apparently more related to medium size aggregates (1–4 mm). The decrease of structural porosity under cultivation (CULT14) was also confirmed by a 22% increase of bulk density in CULT14 relative to PAS and CULT2. For the case of water infiltration, the short-term effect of cultivation was positive, facilitating water capture and translocation in the soil. The longer term effect of agricultural use however, greatly diminished the soil’s capacity to infiltrate water and could lead to runoff losses and decreased water use efficiency. Thus, water retention, aggregate size class, bulk density and
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water infiltration data revealed significant reduction of structural porosity due to 14 years of cultivation. On the other hand, the short-term effect of cultivation on structural porosity and soil hydraulic properties was very small, despite the considerable loss of large and increase of very small aggregates that are most susceptible to wind erosion. Organic C losses were 4.3 and 3.7 Mg ha1 for CULT14 and CULT2, respectively. About half of this decrease was due to the OC loss in the uppermost 0.06 m soil layer (2.2 and 2.0 Mg ha1 for CULT14 and CULT2, respectively). The apparent OC loss rate during the first 2 years of cultivation was 1.85 Mg C ha1 year1, while the average rate after 14 years of cultivation under a rotation with alfalfa PAS was 0.31 Mg C ha1 year1. Considering 26.1 Mg ha1 as a stable OC content in this soil its average half life would be approximately 42 years, but during the first few years of cultivation a significantly lower half life of 7 years was found. These data confirmed that OC turnover rates in soils of the semiarid Pampa are high, with half lives of approximately 10 years and no evidence of long-term stabilized C (Zach et al., 2006). Apparently, most of the C was lost during a short period following cultivation of the PAS, and even under rotations that include alfalfa (as in CULT14) a further decrease was observed. Angers et al. (1992) reported an increase in OC content upon conversion of continuous corn to alfalfa PAS, which might indicate that under continuous cropping, C degradation would have been even more severe than that observed in CULT14. Carbon contents in aggregates size fractions were very similar among treatments, and only CULT14 had significantly lower OC contents in the largest aggregate fraction. While our results did not show a clear effect of land use on OC contents of aggregate fractions, Drury et al. (2004) reported higher OC contents in all >0.25 mm aggregate fractions under rotation, compared with corn monoculture, and Holeplass et al. (2004) also found higher OC concentration in these aggregate sizes under grain – PAS rotation than under all – grain production systems. The highest OC content among aggregate fractions was found in the pooled 1–4 mm size class in all treatments. For water stable aggregates, Holeplass et al. (2004) found a trend of increasing OC concentration with decreasing aggregate size, while Saroa and Lal (2003) reported that OC increased with increasing aggregate size. This trend reflects the concept of aggregate hierarchy proposed by Tisdall and Oades (1982). Our results, however, did not agree with this theory, since intermediate size aggregates had higher OC contents than both other fractions. Zotarelli et al.
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(2005) explained the lack of aggregate hierarchy as shown by a similar OC content across aggregate size fractions as the effect of principal binding agents other than organic matter. In their study on water stable aggregates in low activity clay oxisols under different cropping systems, these authors conclude that the OC loss from natural vegetation to conventional tillage can only partly be explained by the loss of C-rich macroaggregates and an increase in C-poor microaggregates, since both fractions did not differ in their OC concentrations. Thus, several authors reported divergences from the aggregate hierarchy model under different soil conditions even for aggregates separated by wet sieving. Our data suggested that for the dry sieved aggregate size classes we studied, the most important fraction for OC stabilization would be the intermediate class (1–4 mm), due to its higher OC concentration and the observed decrease of this aggregate size fraction after prolonged agricultural use. The lower OC contents of >4 and <1 mm fractions (not significant) in CULT14 would suggest that in these dry sieved aggregate classes the conceptual model of Six et al. (1998), which states that in tillage agroecosystems less C is sequestered due to higher macro aggregate turnover that lowers the rate of micro aggregate formation within macro aggregates also could be valid. Incubation assays showed that on the average the OC content of the aggregate size fraction was related to the amount of CO2-C evolved by the fraction. The intermediate size class had highest OC contents and also produced most CO2-C. Both for OC contents and CO2-C production this fraction showed values almost twice as high as both >4 and <1 mm fraction. These results do not agree with those of Drury et al. (2004), who also used dry sieved aggregates to evaluate the impact of aggregation on biological processes under rotation and continuous cropping. They found an increase of CO2 production with decreasing aggregate size, but they also reported a strong relation between CO2 respiration and POC contents, especially under rotation. Our results however did not show a clear effect of treatment on the OC content of the aggregate size fractions, nor on respiration. Whereas the low CO2-C production of CULT2 in the 1–4 mm fraction could be associated to its low OC content, this was not true for CO2-C in the >4 and <1 mm fractions. In both cases OC contents were similar between CULT2 and PAS, while CO2-C production was similar for CULT14 and CULT2. This might indicate a similar nature of OC in these fractions between CULT14 and CULT2. Short and
longer term effects of cultivation on the biological behavior of the aggregate size fractions therefore could not be distinguished. The comparison between the CO2-C production of the entire soil and the sum of the aggregate size fractions revealed that size fractions produced far more CO2-C. The smallest difference was found in CULT2 (414.7 and 767.0 mg kg1, for entire soil and aggregate sum, respectively, which represents 1.85 times the amount), while in the case of CULT14, CO2-C production of aggregate sum was 2.63 times that of the entire soil; and for PAS this values was 3.23 times. The lower CO2-C production of entire soil could be attributed to protection of OC due to the undisturbed soil matrix, while aggregate fractionation by dry sieving might have released otherwise protected organic material. Similarly, Drury et al. (2004) found that grinding the aggregates reduced the differences of CO2 production between aggregate size classes and generally enhanced CO2 production. In an attempt to evaluate the contribution of each aggregate size fraction to soil respiration we calculated these amounts pondered by the aggregates’ size fraction’s weight proportion and found that only PAS had similar values between the sum of pondered aggregate contribution and entire soil CO2 production. Both cultivated treatments had higher entire soil CO2 production than the pondered sum of aggregates production. This result was astonishing, since the sum of CO2 production of separate aggregate fractions was considerably higher than entire soil respiration. However, taking into account their contribution to total soil mass this trend is reversed and no explication for this apparent contradiction could be adventured. A steep curve and very rapid depletion of evolved CO2 was observed in the >4 mm class, which might be related to a more labile nature of C. On the contrary, <1 mm aggregates showed the lowest slope and also the longest period of respiration. Zhang et al. (2007) studied the respiration rates of different OC fractions and found that labile fractions showed steeper slopes than heavy C fraction and whole soil OC. The difference of respiration rates of aggregate size fractions among treatments that became evident in the comparison of regression lines therefore could indicate differential substrate availability and/or differences in microbial community. The major differences were between PAS and both cultivated treatments, suggesting that despite the positive effect of the alfalfa PAS on OC contents and aggregate size distribution in CULT14, the biological activity had not been recovered.
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5. Conclusions Land-use change from permanent PAS to arable agriculture greatly affected OC stocks and all tested physical and biological soil properties. Cultivation caused a decrease of structural porosity, which was related to higher bulk density and decreased infiltration rate. This process might be divided into a shortterm disruption of large aggregates, while the longterm effect appeared to be the decrease of intermediate size ones. The latter was reflected in an important reduction of soil’s water retention and infiltration, which could have a negative impact on soil quality and productivity. OC losses were similar between short- and long-term cultivation, which indicated a very fast rate of C turnover in the studied soil. The intermediate size aggregates had the highest OC contents, and also contributed most to total respiration. Differences in land-use history only became evident in aggregate size fractions, while entire soil samples did not show different respiration rates or total CO2 production. Our results showed that dry sieved aggregates provide meaningful fractions for studies of the impact of land use on soil physical and biological properties. Acknowledgement This work was financed by the Interamerican Institute for Global Change Research (IAI) within the collaborative research networks CRN 001 and 2031. References Angers, D., Peasent, A., Vigneux, J., 1992. Early cropping-induced changes in soil aggregation, organic matter, and microbial biomass. Soil Sci. Soc. Am. J. 56, 115–119. Arshad, M.A., Lowery, B., Grossman, B., 1996. Physical test for monitoring soil quality. In: Doran, J.W., Jones, A.J. (Eds.), Methods for Assessing Soil Quality. Soil Science Society of America, Madison, WI, pp. 123–141. Balesdent, J., Chenu, C., Balabane, M., 2000. Relationships of soil organic matter dynamics to physical protection and tillage. Soil Till. Res. 53, 215–230. Beare, M., Cabrera, M., Hendrix, P., Coleman, D., 1994. Aggregateprotected and unprotected organic matter pools in conventional and no-tillage soils. Soil Sci. Soc. Am. J. 58, 787–795. Brown, S., Lugo, A., 1990. Effects of forest clearing and succession of the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands. Plant Soil 124, 53–64. Burke, I., Yonker, C., Parton, W., Cole, C., Flach, K., Schimel, D., 1989. Texture, climate and cultivation effects on soil organic matter content in US Grassland soils. Soil Sci. Soc. Am. J. 53, 800–805.
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