Results and Discussion

5.7.4.1 Carbon Isotopic Signature in the Sorghum Plants

Figure 5.2 shows the average isotopic signature of various parts of the sorghum plant, as evaluated at the end of the vegetative cycle. Generally, data indicated that the roots were more 13C-enriched than the grain and leaves, and in the first 15 cm of roots appeared more 13C-enriched than at deeper soil depths. Our results are consistent with previous studies on isotope ratios in plant biomass which showed that roots are generally more 13C-enriched than leaves (Schweizer et al. 1999; Brugnoli and Farquhar 2000). It is to be noted that our d13C value for the stem was not statistically different from that of roots (Fig. 5.2).

5.7.4.2 Initial Variability of Soil d13C

The average value of total d13C value in the field site before the experiment was —23.003%, with a low variability within the soil profile (Table 5.8). We observed a low d13C variability in the 0-60 cm layer with values ranging from —24.48 to

5"C

Grain Mean ±o

Leaves

Stem

Root

Depth Mean ±c

30-60cm -9.7 1.1 Fig. 5.2 Distribution of the carbon isotopic signature in sorghum plants

Table 5.8 Distribution of the d13C (%) isotopic signature in the soil profile

Soil layer (cm)

d13C organic carbon

d13C total carbon

Mean

±a

Mean

±a

0-15

-26.89

1.63

-23.01

0.66

15-30

-26.95

1.49

-23.18

0.60

30-60

-26.65

1.59

-22.83

0.97

—20.14%. As for total carbon, we observed a low d13C variability in the layer and within the soil profile (Table 5.8). The uniformity of isotope signature represents an important advantage in field experiments, since even small variations in time can be detected in the isotopic signature of SOM. Average values of organic d13C found at the beginning of the experiment are typical of soils, where C3 plants have prevalently grown in the past (Boutton et al. 1998).

5.7.4.3 Variation in d13C Values of Soil Organic Carbon

Treatment differences in isotopic signature of SOC after three experimentation years are shown in Fig. 5.3. Generally, our results indicated that d13C values of SOC were affected by the input of both new C4-sorghum material and applied compost. This holds true for all treatments (CPT1, CPT2, TRA and 0-N), which were more significantly enriched in 13C than the initial SOC at all soil depths. As expected, variations were larger for plots where no organic matter was applied, except for sorghum residues (TRA and 0-N) which were more enriched in 13C than plots amended with compost, and especially for the CPT2 double-dose plot. The latter treatment showed a dilution of the root effect in the isotope ratio of SOC. This was due to the large amount of organic carbon applied with compost and characterized by a d13C value of — 16%, and it was found to be true at all soil depths. However, the effects of compost application on the soil isotopic signature need to be discussed in more complex terms since a stimulating effect of compost on root production has also been found (data not shown).

The surface d13C of SOC showed the maximum difference between initial SOC (for TRA and 0-N of 4.02%, 4.01%, respectively) (Fig. 5.4).

The roots subsoil deposition was examined through the isotopic signature of soil organic carbon. Figure 5.3 shows the time-frame of the isotopic signature in the TRA treatment (mineral fertilization), where 13C values of SOC were affected by only the input of new C4-sorghum residues. Results are related to the experiment start and to the 3 year crop cycles. Overall, the data indicate that after each year SOC was more 13C-enriched than for the experiment start at all sampling depths. A significant increase was recorded after the first and second years. Liang et al. (2002) reported that SOC d13C values during one cycle of maize growth in pots varied from —27.2 to —25.4% and explained the net signature increase (1.8%) by the input of roots and root exudates. These authors found that C4-derived SOC during maize growth varied from 1.3 to 12.3%, which accounted for 1.3-14.5 g C pot—1. After the first year of sorghum cultivation, our results were very similar to those of Liang et al. (2002) (Fig. 5.3). Conversely, stabilization around the value of —23% was observed in the third year.

Figure 5.4 highlights the effects of 3 years of sorghum cropping on the isotopic signature of organic carbon in the TRA (mineral) and 0-N plots. The mineral fertilizer treatment (TRA) shows more 13C-SOC enrichment than the 0-N treatment. If we compare this finding with the root biomass data reported above, we find

Year

Year

a -a

-0- 0-15 cm 15-30 cm -■- 30-60 cm

Fig. 5.3 Average d C (%) values in the three top layers of the TRA treatment from the beginning of the experiment (0) throughout the end of the sorghum cycle in September 2009 (III). Bars indicate standard deviation of four replicates

Fig. 5.3 Average d C (%) values in the three top layers of the TRA treatment from the beginning of the experiment (0) throughout the end of the sorghum cycle in September 2009 (III). Bars indicate standard deviation of four replicates

Soil depth

0-15 cm 15-30 cm 30-60cm

Soil depth

0-15 cm 15-30 cm 30-60cm

Fig. 5.4 Average d13C (%) values before the experiment start (T0) and at the end of the sorghum cycle in September 2009 in three soil depths in plot without compost application and in 0-N plot (vertical bars represent standard deviations)

that the larger effect of TRA on soil isotope signature corresponds to a lower root biomass (data not shown). This may indicate that in our experiment the large N supply provided in TRA did not increase total root production but corresponded to a greater fine-root production and turnover (Nadelhoffer 2000). Even a small net root biomass would be compatible with a large root input to SOC and result in a greater 13C signature in TRA than in 0-N.

Figures 5.5 and 5.6 show the profile distribution of d13C as a function of time in the TRA and 0-N treatments. The d13C values show that the modification of the isotope ratio due to root deposition in soil was limited to the top 1 m in the first year, and then gradually extended to deeper soil horizons. In TRA, the effect was slower especially in deep layers, where the top layers' d13C values were reached after 2 years, whereas they were below 1.2 m only in the third year.

Therefore, it only took 1 year of C4 species cropping to significantly affect the isotopic signature of SOC in the ploughed layer and immediately below that. Moreover, it took only from 2-3 years to 13C-enrich the soil profile down to 210 cm to reach the same values as those found at soil surface where root density is much larger.

The d13C values of vegetation are the major factor in controlling the isotopic signature of soil organic carbon (Boutton 1996). In fact, plant-derived organic carbon is delivered to soil by either roots exudation and its metabolic bioproducts, or dead plant residues (Yoneyama et al. 2006), and both inputs to SOC affect soil isotopic signature. Furthermore, plant-derived organic carbon and soil microbial metabolites are further 13C-enriched by approximately 1% as a result of isotopic fractionation during SOM mineralization (CO2 release) (Boutton 1996), though this effect is negligible in short-term experiments (Shang and Tiessen 2000). In the case of soil 13C-enrichement by dead plant residues, it has been found that roots influence by up to 2%, as compared to the plant leaf d13C values (Yoneyama et al. 2006). Although no much information is available on the amount of persistent

^-^

\ ^

v^^K * ■

. t *

—-•—- 0 time —O— I year

—□— III year

Fig. 5.5 Average d C (%) values in the soil profile of the TRA treatment plot down to 2.10 m depth before the experiment start and throughout the three following years of the sorghum cycle

Fig. 5.5 Average d C (%) values in the soil profile of the TRA treatment plot down to 2.10 m depth before the experiment start and throughout the three following years of the sorghum cycle

Soil depth (m)

Fig. 5.6 Average d13C (%) values in the soil profile of the 0-N treatment plot down to 2.10 m depth before the experiment start and throughout the three following years of the sorghum cycle sorghum roots in organic matter at soil depths, these are more 13C enriched (Fig. 5.2) than aboveground sorghum tissues (Vonfischer and Tieszen 1995; Hobbie and Werner 2004). Moreover, root turnover is likely to contribute to the increase of isotopic signatures in deeper soil layers, especially in coarsely textured soils where rooting is characteristically deep (Schenk and Jackson 2005).

The fraction of soil C derived from new sorghum residues (/new) calculated with the isotopic mixing model (Leavitt et al. 1994) is presented in Table 5.7. One year

Soil depth (m)

— •- -0 time —o—I year —■—II year

\

Fig. 5.6 Average d13C (%) values in the soil profile of the 0-N treatment plot down to 2.10 m depth before the experiment start and throughout the three following years of the sorghum cycle after shifting from the C3 vegetation to the sorghum cultivation (2007), the proportion of sorghum-derived carbon was between 8.24% and 21.29%, while 27.95% was reached after 2 and 3 years. At the end of 2008 and 2009 crop cycles, these percentages were slightly larger for TRA than for 0-N. The sorghum-derived carbon in the overall SOC decreased with depth, and more rapidly below 30 cm. These results agreed with previous works that indicated that C4-derived SOC was evenly distributed in the upper 30 cm (Angers et al. 1995).

However, our experimental results are difficult to compare with most field data in literature since they refer to a short-time span (3 years). In comparison with literature findings, the percent enrichment found in our experiments was generally large, though it may be due to the low soil organic carbon content of our plots soil.

With regard to longer-time experiments, Gregorich et al. (1995) estimated that 25-35% of maize-derived carbon contributed to total organic carbon in the Ap horizon of a clay loamy soil after 25 years of continuous maize cropping in Canada. Flessa et al. (2000) in Germany found that only 15% of total carbon content in a loamy sandy Ap horizon was maize-derived after 37 years of maize cultivation. Liang et al. (2002) reported that C4-derived SOC during maize growth varied from 1.3 to 12.3%, accounting for 1.3-14.5 g C pot-1. In France, silty loamy maize soils contained 44% of maize-derived carbon after only 23 years of cultivation (Puget et al. 1995).

It is estimated that 10-40% of C fixed by photosynthesis by arable crops may be lost by roots respiration or released to soil by rhizodeposition (Martin and Merckx 1992), being such large variation in estimates dependent on plant species, growth stage, nutrient status and other environmental conditions. However, other factors affecting the contribution of maize-derived C to SOC is (1) the time elapsed from the C3 vegetation shift to maize cultivation; (2) removal of crop residues instead of their use in filed mulching. With respect to literature, aboveground biomass was removed for silage use by Flessa et al. (2000) as we did in our experiments, whereas Gregorich et al. (1996) left residues in the field. Nevertheless, our 13C-enrichment results are in the same order of magnitude as those reported in the literature.

5.8 Conclusions and Recommendations

Our results from field experiments within the MESCOSAGR project, suggest some recommendations for the management of agro-systems aiming to improve C and N sequestration in soil and efficiency of nutrients uptake. It was shown that compost amendments may be a good strategy to increase N sequestration in soils. In particular, compost-derived nitrogen that was fixed in soil due to repeated amendments was found to gradually decrease, thus changing the 15N signature mostly toward a reduction in soil macro-aggregates and an increase in fine soil particles. These findings suggest that an evaluation of soil macro-aggregate percentage may become a suitable means to assess short-term changes in agro-ecosystems. Conversely, long-term changes may be usefully monitored by measuring the content of micro-aggregates, which showed lesser sensitivity to yearly variation of compost quality.

In the first experimentation year, it was found that the OM mineralization rate depended on compost maturity and composition. This implies that it would be possible to regulate nitrogen availability for plants by modifying the quality and composition of compost. On the other hand, in the first year, the contribution of compost to plant nutrition was about 20% of the applied N, while this percentage decreased progressively in the following two experimentation years. Thus, it appears plausible to recommend that a steady N availability to plants over the years should be attained with a yearly supply compost that accounts for the different mineralization rate in diverse years.

Furthermore, we showed that carbon coming from sorghum residues reached about 28% of soil C after 3 years. Sorghum roots turnover contributed also in silt clay soils to increase the carbon isotopic signatures in deeper soil layers, most probably because of the sorghum deep root system. This finding suggests that the inclusion of sorghum in crop rotations may considerably increase carbon sequestration in the whole soil profile and be beneficial to the overall soil quality.

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