FACTORS DETERMINING CARBON STORAGE CAPACITY IN DRYLAND SOILS

5.6.1 Quantity of Crop Residues

Crop residues are the main source of SOC and improve the chemical, physical, and biological properties of the soil. According to Power and Legg (1978), degraded soils of North Africa are a good example of the effect of off-field residues exports on SOM. Ayanlaja and Sanwo (1991) showed that SOC content is proportional to the amount of residues recycled. Soudi et al. (2000) reported that the amount of residues returned to the soil in arid zones of Morocco are low (Figure 5.4), because off-field exports are used for livestock feed during the dry season. This is a typical situation in smallholders’ mixed cropping in Africa (FAO 2005). Naman and Soudi (1999) and Traore et al. (2007) underlined that improving management of crop residues is the best way to regulate the SOC levels.

5.6.2 Nature of Crop Residues

According to Mustin (1987), the resistance to biodegradation of SOM is correlated to the biochemical nature of plant cell tissues. Similar findings were reported by studies carried out in irrigated drylands in Morocco (Naman et al. 2015; Naman et al. 2018). The biochemical fractionation of the OM returned to the soils from seven crop residues revealed that their tissues contain more cellulose, hemicellulose, and soluble fractions than lignin (Figure 5.5). Wheat residues are characterized by the highest concentration of cellulose (40.6%) compared to the other residues. Sorghum (Sorghum) and maize (Zea mays) contained moderate amounts of lignin. The hemicellulose concentration varies between 10.0% for tomato residues and 23.5% for wheat residues. The highest lignin concentration was found in sorghum residues (9.6%). In terms of soluble fractions, tomato residues had the highest fraction (65.3%), and those of wheat had the lowest fraction (29.8%). Crop residues with

FIGURE 5.4 Quantities of residues returned by different crops in arid irrigated areas in Morocco. (From Soudi, B. et al., Proble'matique de gestion de la matiere organique des sols: Cas des pe'rimetres irrigue's du Tadla et des Doukkala, in Seminaire “Intensification agricole et qualite des sols et des eaux," Rabat, 2-3 Novembre 2000, B. Soudi (ed.), pp. 25-30, 2000.)

FIGURE 5.5 Biochemical fractions of crop residues in irrigated dryland soils in Morocco. (Adapted from Soudi, B. et al., Proble'matique de gestion de la matiere organique des sols: Cas des pe'rimetres irrigue's du Tadla et des Doukkala. In Seminaire “Intensification agricole et qualite des sols et des eaux," Rabat, 2-3 Novembre 2000, B. Soudi (ed.), pp. 25-30, 2000.)

high concentrations of fiber generate more stable C in the soil. Lignin is degraded almost entirely to phenolic compounds with polymerization properties leading to stable humic substances (Datta et al. 2017). Organic residues decompose differently depending on their biochemical composition (Guggenberger 2005; Bouajila et al. 2014). Residues rich in lignin are difficult to decompose due to the recalcitrance of these macromolecules. Cellulose, the main constituent of the cell wall structure, is easily biodegradable, but becomes less biodegradable if coated with hemicellulose, which has a higher degree of polymerization. When hemicellulose becomes encrusted in lignin, it also becomes protected from rapid decomposition (Guggenberger 2005).

5.6.3 C/N Ratio: A Still-Practical Indicator for Organic Matter Decomposition

The C/N ratio is an indicator of the humification potential of returned OM (plant residues, manure, etc.) to the soil. It is unanimously accepted that the higher the C/N ratio of organic residues incorporated into the soil, the slower is the decomposition and the more stable the humus produced (Waksman 1924; Jensen 1929; Allison 1955; Fog 1988). The slow decomposition is essentially attributed to the highly polymerized humus macromolecules that are difficult to degrade (Swift et al. 1979; Monties 1980).

To better understand the processes of humification and mineralization of the SOM and the consequent supply of mineral N potentially absorbed by the crops, it is essential to consider the interactions between the C and N cycles. In fact, a high C/N ratio favors the production of stable C in humus, whereas a low C/N ratio favors the mineralization of OM and the production of C0₂, mineral-N, and other mineral constituents. The breakpoint value that drives the dominance of one process over the other is related to the assimilation needs of the microorganisms present in the soil (to satisfy the C/N ratio of their tissues), and varies with their population type (Janssen 1996). C/N breakpoints varying from 20 to 25 were reported by Tate (1995) and Bengtston et al. (2003). Vigil and Kissel (1991) reported a higher value of 40. Chen et al. (2014) ranked data for C/N in relation to mineralization-immobilization for residues from various crops that showed that mineralization is dominant for C/N values less than 22.7, that concurrent mineralization- immobilization occurs for C/N values exceeding 30, while immobilization becomes dominant for high C/N values (>78).

5.6.4 Mineralization versus Humification in Drylands

In drylands of Africa, the low OM maintenance in the soils, as a result of low restitution of crop residues, is often aggravated by rapid and intense mineralization because of the high temperatures favoring microbial activity. The OM mineralization rate is amplified in irrigated soils in semiarid and arid areas because of the combined effect of temperature and moisture content. Estimated rates of OM mineralization under Mediterranean conditions in Morocco range from 1.9% to 3.3% yr¹ (Soudi et al. 2000).

Carbon fluxes resulting from the decomposition of fresh OM added to the soil follow concurrently the process of humification with a rate of k_h and that of mineralization with a rate of k_m. The later involves primary mineralization (k_ml) and secondary mineralization (k_m2) (Figure 5.6).

In drylands, although secondary mineralization is slower compared to primary mineralization given the molecular complexity of the humic compounds, both processes greatly exceed, in terms of intensity, the humification rate. Furthermore, in these areas, the rate of mineralization is amplified when the soil moisture content is greater than 50 of the soil water-holding capacity, particularly during the warm rainy season or in the case of irrigation (Flowers and Chalagha; Soudi et al. 1990; Li 1990).

Applying equation (4) of Henin and Dupuis (1945) to the degradation of OM:

where C is organic carbon, k_h is the humification rate, ЮС is the fresh organic carbon in the crop residues, k_m2 is the secondary mineralization, and Cs is the stable SOC; and using the example of data reported by Soudi (2000) and Naman et al. (2015) for Mediterranean conditions, mainly (1) an average quantity of residues returned of 1.5 Mg ha^-1, (2) an average humification rate (k_h) of about 15% for most crop residues, (3) an average annual OM mineralization rate (k_m2) of about 2.5%, and (4) an average stable SOC content of about 0.7%, the apparent annual deficit of SOC is estimated to be about 0.3 Mg ha^-1.

This clearly shows that current farming systems in drylands are part of a trend of SOC losses that are not expected to be counterbalanced by stable C production. A study by Naman et al. (2015) showed that the rate of compensation of C depleted by mineralization in the case of arid irrigated

FIGURE 5.6 Soil organic carbon evolution in the soil (k_h: rate of humification; k_m): rate of primary mineralization; k_m2: rate of secondary mineralization).

FIGURE 5.7 Compensation rate of the mineralized organic carbon by humification for selected crop residues in three different soil types. (Adapted from Naman, F. et al., J. Mater. Environ. Sci., 6, 3574-3581, 2015.)

areas in Morocco varies from 4% to 32% depending on soil type and the nature of residue returned (Figure 5.7). Wheat (with the highest C/N ratio) showed the highest compensation rates among the five crops, and the calcium-rich Mollisol showed the highest values among the three soil types.

5.6.5 Effect of Soil Texture

As noted above, after going through a partial or total humification process, SOM develops close bonds with clays. Ladd et al. (1996) and Hassink et al. (1994) showed that the decomposition of fresh crop residues and the mineralization of native SOM are very fast in sandy soils compared to clay soils. This is partly attributed to a strong physical or physiochemical protection of SOM compounds by adsorption on the clay minerals surface or by their encapsulation in the very small pore of aggregates that are inaccessible to microorganisms (Elliott and Coleman 1988). In addition, the protecting effect depends on the kind of clay minerals. High-charge 2:1 clays have more bounding forces than 1:1 clays. Martin and Haider (1986) reported that smectites are very effective protectors of organic compounds, whereas kaolinites are rather weak. This protective effect is generally related to the importance of the charge, the location of the charge, and the swelling-shrinking properties. Theng et al. (1986) stated that SOM structures can be incorporated in the interlayers of clay minerals. Nguyen (1982) showed that extracellular enzymes are adsorbed by the clays and therefore are not able to reach their C substrates in the micropores. A characterization study of C contents in different particle size fractions conducted in irrigated semiarid soils in Morocco (Naman et al. 2002) showed that the three particle-size fractions, sands, silts, and clays, comprised 15% to 37%, 19% to 40%, and 24% to 66% of the total carbon, respectively.

Zech et al. (1997) studied correlations among the C stock and various soil parameters in different regions of Africa. They reported that in semiarid soils of Senegal, soil C stock was significantly correlated to the N reserves (r = 0.93***), P reserves (r = 0.63***), fine earth fraction (r = 0.51*), potential cation exchange capacity (CEC) (r = 0.8***), and clay content (r = 0.79***). Eliminating variables that were auto- correlated, the authors found that the regression can be simplified to contain only clay content as follows:

with C and clay contents in 10⁶ g ha^-1 in 1 m soil depth.

5.6.6 Carbon Status and Intensive Cultivation in Dry Irrigated Areas

The history of the farming system and the degree of intensive cultivation have marked effects on the dynamics of SOC. Unsound agricultural intensification can cause negative effects on the environment. However, if intensification is accompanied by sustainable management practices (sustainable intensification), positive effects can be obtained, among which is carbon sequestration. Lai (2002) reported that in China, agricultural intensification and the adoption of a set of recommended management practices on cropland, forest land, and grazing land have a potential to sequester 59-106 Tg C yr¹, with 25-37 Tg C yr¹ in the croplands.

Comparing two vertic calcixerolls under arid Mediterranean conditions in Morocco that have undergone similar pedogenesis, Soudi et al. (2000) observed that the soil under irrigated intensive agriculture (Tadla) is poorer in organic C, labile organic N forms, and clay-fixed (nonexchangeable) ammonium than that under rainfed agriculture (Chaouia) (Table 5.2). These differences were attributed mainly to the intensive cultivation practices that are not accompanied by adequate management of crop residues. Indeed, in most irrigated arid zones, temperature and irrigation ensure optimum thermal and moisture conditions for the mineralization process. This phenomenon is amplified by frequent tillage that increases the accessibility of OM to biodegradation. The low' levels of chemically hydrolyzable-N and amino acids-N in the irrigated soil showed a tendency to deplete readily biodegradable organic-N forms. In fact, management failures of crop residues do not allow' replenishment of these pools of SOM. The low nonexchangeable ammonium content in the irrigated soil, compared to that in rainfed soil, is also a worthy indicator of the effect of intensive cultivation. In fact, the intense nitrification process under irrigation and the high crop mobilization of mineral N shift the equilibrium toward the release of the clay fixed-ammonium. These results confirm that soil type alone cannot explain the trends in SOM evolution and that the degree of agricultural intensification and soil use have a significant impact.

Other studies have reported opposite trends with agricultural intensification when residues are appropriately managed. Liao et al. (2015, 2016) reported that intensification in northern China engendered a significant increase of SOC. Their results indicate that from 1982 to 2011, SOC content and C stock in the surface (0-20 cm) layer of the cropland increased from 7.8 to 11.0 g kg^-1 (41%) and from 21 to 33.0 Mg ha^-1 (54%), respectively. The estimated SOC stock (0-20 cm) of the farmland for the entire county increased from 0.75 to 1.2 Tg (59%). Correlation analysis revealed that under intensification conditions SOC was increased significantly with crop residues, w'hile it was decreased with increased mean annual temperature. This show's that unless proper management practices are adopted, intensification will cause a decline of SOC.

TABLE 5.2

Comparison of Some Dynamic Parameters of Organic Matter in Topsoil Layer (0-10 cm) in Two Contrasting Arid Zones of Morocco: Chaouia (Rainfed) and Tadla (Intensive Irrigated)

Source: Soudi, B. et al., Probl6matique de gestion de la maticre organique des sols: Cas des pcrimotres irrigues du Tadla et des Doukkala. In S'eminaire "Intensification agricole et qualite des sols et des eaux,” Rabat, 2-3 Novembre 2000, B. Soudi (ed.), pp. 25-30,2000.